ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT
VOLUME 5
EQUI PMENT OPERATI NG & MAI NTENANCE 5. 2
NIREAS VOLUME 5 [5.2] 1
5.2 HYDRAULIC EQUIPMENT 5.2.1 Pumps 5.2.1.1 5.2.1.2 5.2.1.3 5.2.1.4 5.2.1.5 5.2.1.6 5.2.1.7 5.2.1.8 5.2.1.9
5.2.2 5.2.2.1 5.2.2.2 5.2.2.3 5.2.2.4 5.2.2.5 5.2.2.6 5.2.2.7
5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.2.3.5 5.2.3.6 5.2.3.7 5.2.3.8 5.2.3.9 5.2.3.10 5.2.3.11 5.2.3.12
5.2.4 5.2.4.1 5.2.4.2 5.2.4.3 5.2.4.4
5.2.5 5.2.5.1 5.2.5.2
5.2.6 5.2.6.1 5.2.6.2
5.2.7 5.2.8
Pumping stations Types of pumps Impellers Pumping station heads Power Pumps best operating efficiency Electrical motors Calculations Operation & Maintenance44
Pump Control systems Float control system Electrode control system Sonar control system Motor controllers Protective instrumentation Temperature detectors Vibration sensors
Valves Ball valves Gate valves56 Globe valves58 Needle valves Butterfly valves Check valves Quick opening valves Diaphragm valves Relief valves Pressure-reducing valves Air Relief valves Valve operators
Mixers Continuous rapid mixing Continuous mixing Calculations Operation & maintenance
Aeration systems Diffused-air aeration systems Mechanical aerators
Other equipment – operation & maintenance Dismantling joints Settling tank’s skimmer
Glossary Questions & Answers
NIREAS VOLUME 5 [5.2] 2
5.2 HYDRAULIC EQUIPMENT 5.2.1 Pumps Pumping is a unit operation that is used to move fluid from one point to another. This section discusses various topics of this important unit operation relevant to the physical treatment of water and wastewater. These topics include pumping stations and various types of pumps; total developed head; pump scaling laws; pump characteristics; best operating efficiency; pump specific speed; pumping station heads; net positive suction head and deep-well pumps; and pumping station head analysis.
5.2.1.1 Pumping stations The location where pumps are installed is a pumping station. There may be only one pump, or several pumps. Depending on the desired results, the pumps may be connected in parallel or in series.
In parallel connection, the discharges of all the pumps are combined into one. Thus, pumps connected in parallel increases the discharge from the pumping station.
On the other hand, in series connection, the discharge of the first pump becomes the input of the second pump, and the discharge of the second pump becomes the input of the third pump and so on. Clearly, in this mode of operation, the head built up by the first pump is added to the head built up by the second pump, and the head built up by the second pump is added to the head built by the third pump and so on to obtain the total head developed in the system.
Thus, pumps
connected in series increase the total head output from a pumping station by adding the heads of all pumps. Although the total head output is increased, the total output discharge from the whole assembly is just the same input to the first pump.
NIREAS VOLUME 5 [5.2] 3
Plan and section of a pumping station showing parallel connections
(Arcadio P. Sincero Sr. D.Sc. P.E., Gregoria A. Sincero M. Eng. P.E., 2003)
Pumps connected in series
5.2.1.2 Types of pumps
NIREAS VOLUME 5 [5.2] 4
Pumps are separated into two general classes: the centrifugal and the positive-displacement pumps.
Two basic categories of pumps are used in wastewater operations: velocity pumps and positivedisplacement pumps. Velocity pumps, which include centrifugal and Vertical turbine pumps, are used for most wastewater distribution system applications. Positive-displacement pumps are most commonly used in wastewater treatment plants for chemical metering.
Centrifugal pumps are those that move fluids by imparting the tangential force of a rotating blade called an impeller to the fluid. The motion of the fluid is a result of the indirect action of the impeller.
Displacement pumps, on the other hand, literally push the fluid in order to move it. Thus, the action is direct, positively moving the fluid, thus the name positive-displacement pumps.
Additional subcategories of main pump types are given bellow :
a. Radial-flow centrifugals b. Axial-flow and mixed-flow centrifugals Positive-displacement pumps c. Reciprocating pistons or plungers d. Diaphragm pumps e. Rotary screws Air pumps f. Pneumatic ejectors g. Air-lifts
NIREAS VOLUME 5 [5.2] 5 Efficiencies Efficiencies range from 85% for large capacity centrifugals (types a and b) to below 50% for many smaller units. For type c, efficiency ranges from 30% up depending on horsepower and number of cylinders. For type d, efficiency is almost 30%, and for types e, f, and g, it is below 25%.
Materials For water using type a or b pumps, normally bronze impellers, bronze or steel bearings, stainless or carbon steel shafts, and cast iron housing; for domestic waste using type a, b, or c pumps, similar except that they are often cast iron impellers; for industrial waste and chemical feeders using type a or c pumps, a variety of materials depending on corrosiveness; type d similar except that the diaphragm is usually rubber; types e, f, and g normally steel components
Pump selection Next table shows appropriate type of pump to be selected, depending on different parameters; the liquid to be pumped, the flows and the total head.
Pump selection
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 5 [5.2] 6
Types of pumps and pumping stations – schematic illustration
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 5 [5.2] 7 Types of pumps and pumping stations - sections
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 5 [5.2] 8 Typical submersible lift station
(Jensen Engineered Systems, 2012)
NIREAS VOLUME 5 [5.2] 9 Typical submersible lift station – 3D rerpesantation
NIREAS VOLUME 5 [5.2] 10
a. Radial-flow centrifugals Radial-flow pumps throw the liquid entering the center of the impeller or diffuser out into a spiral volute or bowl. The impellers can be closed, semiopen, or open depending on the application. Closed impellers have higher efficiencies and are more popular than the other two types. They can be readily designed with non clogging features. In addition using more than one impeller can increase the lift characteristics. These pumps can have a horizontal or vertical design.
Major components of a radial-flow centrifugal pump
(Frank R. Spellman, 2011)
NIREAS VOLUME 5 [5.2] 11 Overview of a Radial-Flow Centrifugal Pump
NIREAS VOLUME 5 [5.2] 12
Radial-Flow Submersible Sewage Pump
(Jensen Engineered Systems, 2012)
NIREAS VOLUME 5 [5.2] 13 Overview of a Radial-Flow Submersible Pump
(Jensen Engineered Systems, 2012)
NIREAS VOLUME 5 [5.2] 14
MORE Recessed impeller or vortex pumps A sub category of radial-flow centrifugal pumps is the recessed impeller or vortex pump. This type of pump uses an impeller that is either partially or wholly recessed into the rear of the casing. The spinning action of the impeller creates a vortex or whirlpool. This whirlpool increases the velocity of the material being pumped. As in other centrifugal pumps, this increased velocity is then converted to increased pressure or head. The recessed impeller or vortex pump is used widely in applications where the liquid being pumped contains large amounts of solids or debris and slurries that could clog or damage the impeller of the pump. It has found increasing use as a sludge pump in facilities that withdraw sludge continuously from their primary clarifiers. The major advantage of this modification is the increased ability to handle large materials that would normally clog or damage the pump impeller. Because the majority of the flow does not come in direct contact with the impeller, the potential for problems is reduced. Because of the reduced direct contact between the liquid and the impeller, the energy transfer is less efficient. This results in somewhat higher power costs and limits application of the pump to low to moderate capacities. Objects that might have clogged a conventional type of centrifugal pump are able to pass through the pump. Although this is very beneficial in reducing pump maintenance requirements, it has, in some situations, allowed material to pass into a less accessible location before becoming an obstruction. To be effective, the piping and valving must be designed to pass objects of a size equal to that which the pump will discharge.
Recessed or vortex impeller pump
(Frank R. Spellman, 2011)
Recessed or vortex impeller pump
NIREAS VOLUME 5 [5.2] 15
(Garr M. Jones, PE, 2006)
The most obvious visual difference between the vortex pump and radial flow models is that its semiopen impeller resides completely out of the volute. This feature offers the sewage pump designer three distinct advantages. First, the throughlet size can easily be made to equal that of the pump’s inlet. Therefore any solid that can enter the inlet can traverse the throughlet. Second, since the pumpage traverses the throughlet via vortex action, its solids seldom come in contact with the impeller. This reduces the possibility that solids, especially stringy ones, will become entangled or clog it. For the very same reason impeller wear is minimized. Finally, due to its location above the volute, unbalanced radial forces are almost nonexistent. This allows the vortex impeller to run continuously at or near shut off head without damage.
NIREAS VOLUME 5 [5.2] 16 b. Axial-flow centrifugals Axial-flow propeller pumps, although classed as centrifugals, do not truly belong in this category since the propeller thrusts rather than throws the liquid upward. Impeller vanes for mixed-flow centrifugals are shaped to provide partial throw and partial push of the liquid outward and upward. Axial- and mixed-flow designs can handle large capacities but only with reduced discharge heads. They are constructed vertically.
MORE Turbine pumps A sub category of axial-flow centrifugal pumps is the turbine pump. The turbine pump consists of a motor, drive shaft, discharge pipe of varying lengths, and one or more impeller-bowl assemblies. It is normally a vertical assembly, where water enters at the bottom, passes axially through the impeller-bowl assembly where the energy transfer occurs, then moves upward through additional impeller-bowl assemblies to the discharge pipe. The length of this discharge pipe will vary with the distance from the wet well to the desired point of discharge. Due to the construction of the turbine pump, the major applications have traditionally been for pumping of relatively clean water. The lineshaft turbine pump has been used extensively for drinking water pumping, especially in those situations where water is withdrawn from deep wells. The main wastewater plant application has been pumping plant effluent back into the plant for use as service water. The turbine pump has a major advantage in the amount of head it is capable of producing. By installing additional impeller-bowl assemblies, the pump is capable of even greater production. Moreover, the turbine pump has simple construction and a low noise level and is adaptable to several drive types— motor, engine, or turbine. High initial costs and high repair costs are two of the major disadvantages of turbine pumps. In addition, the presence of large amounts of solids within the liquid being pumped can seriously increase the amount of maintenance the pump requires; consequently, the unit has not found widespread use in any situation other than service water pumping.
NIREAS VOLUME 5 [5.2] 17 Turbine pumps
(Garr M. Jones, PE, 2006)
NIREAS VOLUME 5 [5.2] 18
Typical flow paths in centrifugal pumps, (a) Radial flow, vertical; (b) mixed flow; (c) radial flow, horizontal; (d) axial flow
(Garr M. Jones, PE, 2006)
NIREAS VOLUME 5 [5.2] 19 c. & d. Reciprocating pistons, plungers and diaphragm pumps Almost all reciprocating pumps used in wastewater treatment facilities are metering or power pumps. Frequently, a piston or plunger is used in a cylinder, which is driven forward and backward by a crankshaft connected to an outside drive. Adjusting metering pump flows involves merely changing the length and number of piston strokes. A diaphragm pump is similar to a reciprocating piston or plunger, but instead of a piston, it contains a flexible diaphragm that oscillates as the crankshaft rotates. Plunger and diaphragm pumps feed metered amounts of chemicals (acids or caustics for pH adjustment) to a water or waste stream. They also pump sludge and slurries in waste treatment plants.
e. Rotary screw pumps In this type, a motor rotates a vaned screw or rubber stator on a shaft to lift or feed sludge or solid waste material to a higher level . Archimedes screw lift is suitable for handling flows of the order of 20.000 m3 / h and manometric heights up to 10 meters. The advantages of the Archimedes screw are: •
They can handle increased presence of solids without clogging risk.
•
Are highly efficient, even at partial loads (25% - 75% of rated).
•
Low operation speeds, (between 10 and 100 rpm), prevent premature wear and contribute to durability.
•
The maintenance is economical and simple due to the easy access of the open bolt.
Rotary screw pumps are almost always installed in the entrance pump –lift station, for medium to large WWTP’s.
NIREAS VOLUME 5 [5.2] 20
Screw pump, an example of a positive-displacement pump
(Garr M. Jones, PE, 2006)
NIREAS VOLUME 5 [5.2] 21 f. Pneumatic ejectors In this pumping method waste flows into a receiver pot, and an air pressure system then blows the liquid to a treatment process at a higher elevation. A controller is usually included, which keeps the tank vented while it is being filled. When the tank is full, the level controller energizes a three-way solenoid valve to close the vent port and open the air supply to pressurize the tank. The air system can use plant air (or steam), a pneumatic pressure tank, or an air compressor. With large compressors, a capacity of 30 m3/hr with lifts of 15 m can be obtained. This system has no moving parts in contact the waste; thus, no impellers become clogged. Ejectors are normally more maintenance free and operate longer than pumps.
Pneumatic ejection system and associated equipment & piping
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 5 [5.2] Pneumatic ejector
22
(Garr M. Jones, PE, 2006)
g. Airlifts Airlifts consist of an updraft tube, an air line, and an air compressor or blower. Airlifts blow air into the bottom of a submerged updraft tube. As the air bubbles travel upward, they expand (reducing density and pressure within the tube) and induce the surrounding liquid to enter. Flows as great as 300 m3/hr can be lifted short distances in this way. Airlifts are used in waste treatment to transfer mixed liquors or slurries from one process to another.
NIREAS VOLUME 5 [5.2] 23 Airlift pump
(Garr M. Jones, PE, 2006)
5.2.1.3 Impellers Next figure shows various types of impellers that are used in centrifugal pumps. The one in (a) is used for axial-flow pump. Axial-flow pumps are pumps that transmit the fluid pumped in the axial direction. They are also called propeller pumps, because the impeller simply propels the fluid forward like the movement of a ship with propellers. The impeller in (d) has a shroud or cover over it. This kind of design can develop more head as compared to the one without a shroud. The disadvantage, however, is that it is not suited for pumping liquids containing solids in it, such as rugs, stone, because these materials may easily clog the impeller. In general, a centrifugal impeller can discharge its flow in three ways: by directly throwing the flow radially into the side of the chamber circumscribing it, by conveying the flow forward by proper design of the impeller, and by a mix of forward and radial throw of the flow. The pump that uses the first impeller is called a
NIREAS VOLUME 5 [5.2] 24
radialflow pump ; the second, as mentioned previously, is called the axial-flow pump; and the third pump that uses the third type of impeller is called a mixed-flow pump . The impeller in (c) is used for mixed-flow pumps. Various types of pump impellers: (a) axial flow; (b) open type; (c) mix-flow type; and (d) shrouded impeller
(Arcadio P. Sincero Sr. D.Sc. P.E., Gregoria A. Sincero M. Eng. P.E., 2003)
NIREAS VOLUME 5 [5.2] 25 Types of centrifugal pump impellers. A. Closed impeller; B. Semiopen impeller; C. Open impeller; D. Diffuser; E. Mixed flow impeller; F. Axial flow impeller
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 5 [5.2] 26
Open impellers Open impellers do not have a front or a rear shroud, so it allows debris that might foul the impeller to be dragged along and rubbed against the front and rear stationary wear plates, thus grinding down the particulate to a small enough size to pass through the impeller. This works well with soft particulates, but generally causes too much abrasion on both the impeller and the wear plates if the compound of the particulate is harder than that of the impeller. Another disadvantage of this open style is the need for the impeller vanes to be fairly thick. They must have the mechanical strength to support themselves under the stress of pumping the liquid. This added thickness results in a decrease in the flow area. Additionally, leakage in the impeller is caused by the clearances at the front and rear of the blade (where the hub and shrouds would be on a closed impeller). This leakage is very dependent on the clearances between the impeller and the wear plates. As the pump wears over time, these clearances become larger, further increasing the leakage losses, degrading the pumps efficiency and, in many cases, flow and head levels. An advantage of open impellers is that they develop almost no axial hydraulic thrust loads due to the lack of shrouds. Without cores they are also easy to manufacture – making them less expensive.
Open impeller
(Jensen Engineered Systems, 2012)
Closed impellers
NIREAS VOLUME 5 [5.2] 27 Closed impellers (also called enclosed impellers) have shroud and hub surfaces attached. The surfaces have several advantages. They eliminate the leakage losses across the vanes. They provide strength and stability allowing the vane thickness to be reduced, which increases the flow area through the impeller. The two shrouds also provide an axial thrust surface from which the pressure differential can be balanced. The obvious disadvantage of closed impellers is that any debris entering the vanes that is too large to pass through the impeller becomes stuck and must be removed by hand. This cleaning process, often referred to as de-ragging in the wastewater industry, requires time consuming and costly disassembly of the pump.
Closed impeller
(Jensen Engineered Systems, 2012)
Semi-open impellers
Semi-open impellers have only one shroud on either the front or the back. They have some of the advantages of each of the other styles, and their own set of drawbacks. Since fluid has only one leakage path over the blade, leakage losses are reduced making them more efficient than fully open impeller designs. Having one face of the impeller open allows particulate to pass that would clog many closed impellers. Their major disadvantage is the fact that they have only one shroud that fluid pressure builds upon. The differential pressure across the impeller can
NIREAS VOLUME 5 [5.2] 28 Semi open impeller
(Jensen Engineered Systems, 2012)
MORE Figure bellow shows various impellers used for positive-displacement pumps and for centrifugal pumps. The figures in d and e are used for centrifugal pumps. The figure in e shows the impeller throwing its flow into a discharge chamber that circumscribes a circular geometry as a result of the impeller rotating. This chamber is shaped like a spiral and is expanding in cross section as the flow moves into the outlet of the pump. Because it is shaped into a spiral, this expanding chamber is called a volute - another word for spiral. In centrifugal pumps, the kinetic energy that the flow possesses while in the confines of the impeller is transformed into pressure energy when discharged into the volute. This progressive expansion of the cross section of the volute helps in transforming the kinetic energy into pressure energy without much loss of energy. Using diffusers to guide the flow as it exits from the tip of the impeller into the volute is another way of avoiding loss of energy. This type of design is indicated in d , showing stationary diffusers as the guide. The figure in a is a lobe pump, which uses the lobe impeller. A lobe pump is a positivedisplacement pump. As indicated, there is a pair of lobes, each one having three lobes; thus, this is a three-lobe pump. The turning of the pair is synchronized using external gearings. The clearance between lobes is only a few thousandths of a centimeter, thus only a small amount of leakage passes the lobes. As the pair turns, the water is trapped in the “concavity� between two adjacent lobes and along with the side of the casing is positively moved forward into the outlet.
NIREAS VOLUME 5 [5.2] 29 Various types of pump impellers, continued: (a) lobe type; (b) internal gear type; (c) vane type; (d) impeller with stationary guiding diffuser vanes; (e) impeller with volute discharge; and (f) external gear type impeller
(Arcadio P. Sincero Sr. D.Sc. P.E., Gregoria A. Sincero M. Eng. P.E., 2003)
The figures in b and f are gear pumps. They basically operate on the same principle as the lobe pumps, except that the “lobes� are many, which, actually, are now called gears. Adjacent gear teeth traps the water which, then, along with the side of the casing, moves the water to the outlet. The gear pump in b is an internal gear pump, so called because a smaller gear rolls around the inside of a larger gear. (The smaller gear is internal and inside the larger gear.) As the smaller gear rolls, the larger gear also rolls dragging with it the water trapped between its teeth. The smaller gear also traps water between its teeth and carries it over to the crescent. The smaller and the larger gears eventually throw their trapped waters into the discharge outlet. The gear pump in f is an external gear pump, because the two gears are contacting each other at their peripheries (external). The pump in c is called a vane pump, so called because a vane pushes the water forward as it is being trapped between the vane and the side of the casing. The vane pushes firmly
NIREAS VOLUME 5 [5.2] 30
against the casing side, preventing leakage back into the inlet. A rotor, as indicated in the figure, turns the vane. Fluid machines that turn or tend to turn about an axis are called turbomachines. Thus, centrifugal pumps are turbomachines. Other examples of turbomachines are turbines, lawn sprinklers, ceiling fans, lawn mower blades, and turbine engines. The blower used to exhaust contaminated air in waste-air works is a turbomachine.
NIREAS VOLUME 5 [5.2] 31 5.2.1.4 Pumping station heads
(John M. Stubbart, 2006 )
NIREAS VOLUME 5 [5.2] 32
NOTE:This figure illustrates a pump with a suction lift. Pumps should have a suction head which means the wet well water level should be higher than the pump impeller. This pump will have difficulty starting unless it is a self-priming pump because the water level in the wet well is below the pump. Also, if air gets into the suction line, the only way it can get out is through the pump. Controls may be modified to allow the pump to operate only when a suction head exists if flooding of the service area will not result.
The discharge head on a pump is a sum of the following contributing factors:
STATIC HEAD (hd) — The vertical distance through which the liquid must be lifted
FRICTION HEAD (hf) — The resistance to flow caused by friction in the pipes. Entrance and transition losses can also be included. Because the nature of the fluid (density, viscosity, and temperature) and the nature of the pipe (roughness or straightness) affect friction losses, a careful analysis is needed for most pumping systems although tables can be used for smaller systems.
VELOCITY HEAD (hv) — The head required to impart energy into a fluid to induce velocity. Normally this head is quite small and can be ignored unless the total head is low.
SUCTION HEAD (hs ) — Reduces the pressure differential that the pump must develop when a positive head is on the suction side (a submerged impeller). If the water level is below the pump, the suction lift plus friction in the suction pipe must be added to the total pressure differential required. The amount of suction lift that can be handled must be carefully computed. Suction lift is limited by the barometric pressure (which depends on elevation and temperature), the vapor pressure (which also depends on temperature), friction and entrance losses on the suction side, and the net positive suction head required (NPSHR)—a factor that depends on the shape of the impeller and is obtained from the pump manufacturer
TOTAL STATIC HEAD (H)—Expressed by the following equation:
H’ = hd + hs
TOTAL DYNAMIC HEAD (H’)—Expressed by the following equation: H’ = hd + hf + hv + hs 5.2.1.5 Power
NIREAS VOLUME 5 [5.2] 33
Output power The power output of a pump is the useful energy delivered by the pump to the fluid. In SI units, the power output is defined as : P=γxQxH
Where •
P is the water power in kilowatts
•
γ is the specific weight of the fluid in kiloNewtons per cubic meter (kN/m3)
•
Q is the flow rate in m3/s
•
H is the total dynamic head in m
Input power Pump performance is measured in terms of the flowrate that a pump can discharge against a given head at a given efficiency. The pump capacity depends on the design, and design information is furnished by the pump manufacturer in a series of curves for a given pump. Pump efficiency, Ep, is the ratio of the useful power output (water kilowatts [wkW] or water horsepower [whp]) to the power input to the pump shaft. Hence, the brake power Bp, (bkW) that must be supplied by the drive is, in SI units :
Bp =
NIREAS VOLUME 5 [5.2] 34
Reading and understanding centrifugal pump curves is the key to proper pump selection. There are four important curves shown on the standard performance curve from the manufacturer. They are listed below and shown on the next figure. 1. Head 2. Efficiency 3. Power 4. Net positive suction head required (NPSHR)
Single line curve w/ Iso-Efficiency curves
Note that of the efficiency curve;it is broken into several iso-efficiency lines with each line representing a constant efficiency. It is read much like a topographic map, with the iso-efficiency lines corresponding to elevation lines on the map. The head that a pump can produce at various flow rates and rotational speeds is established in pump tests conducted by the pump manufacturer. During testing, the capacity of the pump is varied by throttling a valve in the discharge pipe, and the corresponding head is measured. The
NIREAS VOLUME 5 [5.2] 35
results of these tests and other tests with different impeller diameters are plotted to obtain a series of head-capacity (H-Q) curves for the pump at some given speed. Simultaneously, the power input to the pump is measured. The efficiency at various operating points is computed, and these values are also plotted in the same diagram. Together, these curves are known as "pump characteristic curves." However, pumps are not typically capable of operating continuously or for protracted periods at all positions along their characteristic curves. Severe damage can result from continuous operation too close to shut-off or too far to the right of the best efficiency point (BEP).
MORE Typical pump characteristic curve. In this example, best efficiency point (BEP) equals 83% for a fixed value for capacity equal to 450 m3/hr, and value for head equal to 38 m. Given input power for previous values equals to 75 hp.
(Garr M. Jones, PE, 2006)
Specific speed The capacity of flow rate of a centrifugal pump is governed by the impeller thickness. For a given impeller diameter, the deeper the vanes, the greater the capacity of the pump. Each desired flow rate or desired discharge head will have one optimum impeller design. The impeller that is best for
NIREAS VOLUME 5 [5.2] 36
developing a high discharge pressure will have different proportions from an impeller designed to produce a high flow rate. The quantitative index of this optimization is the specific speed. The higher the specific speed of a pump, the higher its efficiency.
Net positive suction head (NPHA) While Net Positive Suction Head (NPHA) analysis is not a concern with submersible pump design, when designing a dry pit, a NPSH analysis is critical. The following discussion demonstrates why NPHA analysis is not necessary in submersible pump design. There are two forms of NPSH. Net Positive Suction Head Required (NPSHR) is provided by the manufacturer, and net positive suction head available (NPSHA) is the amount of energy available at the inlet of the pump in relation to the system layout. NPSHA is calculated using the formula below:
NPSHA = Hatm ± hs - hvp - hf
Where : •
Hatm = Atmospheric pressure at the surface of the liquid (m)
•
hs = Suction Static Head (m) (+, if suction is above impeller eye/-, if suction is bellow impeller eye)
•
hvp = The liquids vapor pressure at the pumped temperature (m)
•
hf = The friction losses in the pipe and fittings from the suction tank to the pump inlet (m)
NPSHR is provided on the manufacturers curve. The most important thing to know about NPSH is that the NPSHA must be greater than the NPSHR. Typically, a factor of safety of 1.3 is used. Thus : NPSHA ≥ 1,3 NPSHR
The purpose of a net positive suction head analysis is to ensure that the impeller of the pump is submerged with liquid. For example, in a dry pit design the water is stored in a wet well, and the pump is stored in a separate structure and is not submerged. If the layout was such that, at some point, the water level in the wet well dropped low enough that it was not being forced into the pump impeller, the pump would begin to cavitate. In a submersible pump station with proper design of the control elevations, the pump is always submerged and forcing the fluid into the impeller thus eliminating this concern.
NIREAS VOLUME 5 [5.2] 37
5.2.1.7 Electrical motors Electric motors are the most frequently used drivers in pumping stations, primarily because of their versatility, compactness, and low maintenance. The most common machine is the polyphase (threephase) squirrel-cage induction motor; these motors range in size from less than one to several thousand horsepower. Induction motors up to about 600 kW (800 hp) are usually used for adjustable-speed drives, but larger drives tend to be more economical with a wound-rotor or a synchronous motor. Large squirrelcage motors, however, have high efficiencies and power factors that make the operating costs approach those of synchronous motors without the high capital cost. Furthermore, the controls are more complex for synchronous and wound rotor motors. Singlephase motors are used to drive small loads and are not considered as drivers in pumping stations.
The following minimum amount of information shall be given on all nameplates of singlephase and polyphase induction motors :
1. Manufacturer's type and frame designation. "Type" is often used by motor manufacturers to define motor as single- or polyphase, single- or multi-speed. Motors of a given horsepower rating are built in a certain size of frame or housing. For standardization, a frame size has been assigned for each integral horsepower motor so that shaft heights and dimensions will be the same to allow motors to be interchanged. 2. Horsepower. The rated shaft output of the motor. 3. Time rating. The time rating or "Duty" defines the length of time during which the motor can carry its nameplate rating without exceeding design limits. Pump motors are rated for continuous duty or "Cont." 4. Maximum ambient temperature for which the motor is designed (i.e., usually 40 or 500C). 5. Insulation system designation. Class A, B, F, or H. Insulation classes are directly related to motor life. Class A insulation has a recommended temperature limit of 105 Degrees oC Class B goes to 130 Degrees oC Class F to 155 Degrees oC Class H to 180 Degrees oC
NIREAS VOLUME 5 [5.2] 38
MORE Class A insulation was the standard insulation used on older U Frame motors between 1952 and 1964. Since 1964, T Frame motors use class B insulation as the standard insulation. Most common fractional horsepower motors use either insulation class A or B. Class B is used on most integral horsepower motors. Classes F and H are generally used for motor designs which are special applications. Insulation ratings assume the motor is operating within its rated ambient temperature. Ambient temperature is the air temperature surrounding the motor and is also indicated on the nameplate. Motors should be replaced by motors with the same or higher insulation class to avoid reductions in motor life and nuisance tripping of the motor overload device. Each 10 Degree oC rise above the motor's rating can reduce motor life by onehalf.
From this chart, you can compare Class A, B, F, and H insulation systems, all with ambient temperatures of 40 Degrees C. You can see how they differ in the total temperature they can
withstand.
Based
on
the
ambient
temperature of the application and the hours of operation you can select an insulation class that will provide dependable motor life.
For example: A motor operating at 180 Degrees C will have an estimated life of only 300 hours with a Class A insulation system. If Class B insulation is used, estimated life is increased
to
1,800
hours.
If
Class
F
insulation is used, 8,500 hours of life can be expected from the motor and with Class H insulation motor life will increase to tens of thousands of hours.
NIREAS VOLUME 5 [5.2] 39
6. Ingress protection (IP) of electric motor . Classification is a measure of the capacity of the motor to resist ingress of dust and of water. Objects, dust, or water may enter the motor providing they cannot have any detrimental effect upon its operation.
MORE Two numbers follow the letters IP. The first number defines resistance to dust and the second to water. This classification system utilizes the letters "IP" ("Ingress Protection") followed by two or three digits. (A third digit is sometimes used. An "x" is used for one of the digits if there is only one class of protection; i.e. IPX4 which addresses moisture resistance only.)
The first digit of the IP code indicates the degree that persons are protected against contact with moving parts (other than smooth rotating shafts, etc.) and the degree that equipment is protected
0 1 2 3 4 5 6
against
solid
foreign
bodies
intruding
into
an
enclosure.
No special protection Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter. Protection against fingers or other object not greater than 80mm in length and 12mm in diameter. Protection from entry by tools, wires, etc., with a diameter of thickness greater than 1.0mm. Protection from entry by solid objects with a diameter or thickness greater than 1.0mm Protection from the amount of dust that would interfere with the operation of the equipment. Dust tight.
The second digit indicates the degree of protection of the equipment inside the enclosure against the harmful entry of various forms of moisture (e.g. dripping, spraying, submersion, etc.)
0 1 2 3 4 5 6 7 8
No special protection Protection from dripping water. Protection from vertically dripping water. Protection from sprayed water. Protection from splashed water. Protection from water projected from a nozzle Protection against heavy seas, or powerful jets of water. Protection against immersion. Protection against complete, continuous submersion in water.
NIREAS VOLUME 5 [5.2] 40
The following examples are fairly typical classifications of motors used to drive generalpurpose fans. The descriptions are abbreviated.
Protection Class
Object Protection
Water Protection
IP44
Solid objects over 1mm Splashed water
IP54
Dust resistant
Splashed water
IP55
Dust resistant
Hosed water
IPW55
Dust resistant
Rain water
IP56
Dust resistant
Powerful water jets
IP65
Dust exclusion
Hosed water
7. RPM at rated full load 8. Frequency. 60 hertz in North America, 50 hertz in Europe. 9. Number of phases. Usually three-phase for motors 0,5 hp and larger, one-phase for less than 0,5 hp. 10. Rated load current 11. Voltage
Oil – filled Vs Air – filled motors Oil-filled motors offer several benefits. Due to a much higher thermal transfer capacity of oil as compared to air (approximately 7x) oil-filled motors tend to run cooler. The oil also provides continuous lubricant for the bearings and the windings. Some manufacturers claim that the vibration, or start up torque pulses, of the windings causes the insulation to wear subsequently leading to shorts within the motors. Oil-filled motors are designed to lubricate the windings and prevent degradation from chaffing during start-up. There are also studies in support of the claim that oil-filled motors prevent moisture from getting into the hydroscopic insulation on the windings, a benefit since the insulation tends to breakdown more quickly in moist environments. Air-filled motors have a lower amount of drag loss as compared to an oil-filled motor. Typical estimates range from 1% to 2% less loss. Air-filled motors work best in applications where the liquids are always cool and provide plenty of heat dissipation. If heat dissipation might be an issue, oil-filled motors have the advantage over air-filled.
Rated power of the motor (Bm) is greater than the power input of the pump (Bp) by a percentage called margin of safety which takes into account the transmission losses (if any) from the motor to
NIREAS VOLUME 5 [5.2] 41
the pump. The safety margin varies between 40% and 10% and is higher for small pumps and smaller for large pumps. If excess flow variations are expected, the nominal motor power must be selected for maximum pump flow from operating curves.
Where : •
Bm = Rated power of the motor (kW)
•
Bp = Pump’s maximum power input (kW)
•
Em = motor efficiency at selected operation point (%)
Power Loss due to motor and pump inefficiency
(John M. Stubbart, 2006 ) 5.2.1.8 Calculations Water to electricity relation
Typically, a motor efficiency of no less than 85% is used. Thus :
Bm=
NIREAS VOLUME 5 [5.2] 42
A simple explanation of electrical measurements can be made by comparing the behavior of electricity to the behavior of water.
Volts (potential) can be compared to the pressure in a water pipe (psi).
Amperage (current) can be compared to quantity of flow or Resistance (ohms) can be likened to the friction loss in a pipe.
Velocity of a Fluid through a Pipeline The speed or velocity of a fluid flowing through a channel or pipeline is related to the crosssectional area of the pipeline and the quantity of water moving through the line; for example, if the diameter of a pipeline is reduced, then the velocity of the water in the line must increase to allow the same amount of water to pass through the line.
Velocity (m/sec) =
NIREAS VOLUME 5 [5.2] 43
Friction Head = Roughness factor x
NIREAS VOLUME 5 [5.2] 44 5.2.1.9 Operation & Maintenance Refer to the manufacturer’s operations and maintenance recommendations for specific guidance. These suggestions are general in nature. The type of equipment that is in operation determines how and when maintenance takes place. Water quality and equipment history play a predominant role in scheduling maintenance. Above all, safety is the main concern when performing any duty on equipment. Electrical, mechanical, and confined-space safety practices must be a part of any preventive maintenance checklist.
Daily (or during routine visits when pump is in operation) •
Visually observe pump and motor operation.
•
Read the amperage, voltage, flows, run hours, and other information from motor control center.
•
Inspect mechanical seals.
•
Check operating temperature.
•
Check warning indicator lights.
•
Check oil levels.
•
Note any unusual vibration.
Weekly •
Test per-square-centimeter levels of the relief valve system; these should be set just above the normal operating pressure of the system.
•
Inspect stuffing box and note the amount of leakage; adjust or lubricate packing gland as necessary. A leakage rate of 20 to 60 drops of seal water per minute is normal for a properly adjusted gland; inadequate or excessive leakage are signs of trouble. Do not overtighten packing gland bolts. Clean drain line if necessary.
•
Check valve lubricant levels.
•
Test the priming system and perform preventive maintenance as necessary.
•
Inspect motor for indications of overload or electrical failure. Check for burnt insulation, melted solder, or discoloration around terminals and wires.
•
Check for and remove any obstructions in or around the impeller, screens, or intake, as appropriate. (Be sure to shut off the pump.)
•
Test transfer valve, if applicable.
NIREAS VOLUME 5 [5.2] 45
Monthly •
Check bearing temperatures with a thermometer.
•
Clean strainers on system piping including strainers-on automatic control valves.
•
Perform dry vacuum test.
•
Check oil level in pump gearbox; add oil as necessary.
•
Inspect gaskets.
•
Check motor ventilation screens and clean or replace as necessary.
•
Check pressure gauge reliability.
•
Check foundation bolts.
•
Clean pump control sensors (may be required weekly, depending on water quality).
•
Check drive flange bolts, if applicable, and tighten as necessary.
Methods to remove air from a centrifugal pump •
You can fill the pump and suction piping with liquid and start all over again.
•
You can attach a priming pump to the discharge side of the pump to remove any air in the pump and suction piping. Be sure this pump has a mechanical seal. You never want to use packing in a priming pump because air will leak into the stuffing box through the packing.
•
Some people install a foot valve at the end of the suction piping to insure that the fluid will not drain from the pump and suction piping. These valves seldom work out because, like all check valves, they leak.
NIREAS VOLUME 5 [5.2] 46
General guidance of maintenance in submersible pumps
Before any maintenance you need: •
Emptying, washing and aeration of the wet pumping chamber
•
The pump must be completely disconnected from the sewer and electrical network
•
Equipement with appropriate protective equipment such as gloves, mask, tools etc.
•
Ensure immediate escape route from the well chamber of the pumping station
•
You should conduct periodic audits in accordance with the manufacturer's instructions and according to the hours of operation.
•
During regular inspections you should check: The good condition of the cable Condition and Volume of the oil in the oil pan The vertical position of guide rods Proper function of level controls The gap between the impeller and suction cap The cable used must have very good properties for underwater applications. Cables must exhibit high resistance to corrosion by sewage wastewater. In a submersible pumping system, particularly important is the maintenance of tightness, so monitoring of the mechanical cable glands is required. By monitoring the situation, longevity of the pump is ensured. Monitoring is accomplished by checking and replacing the oil at regular intervals depending on usage, but in general changing once a year for equipment operating at 50-70% of its capacity is mandatory . A small sewage entering the oil chamber (about 10-20%) over one year or 4,000 hours is normal. Replacing the oil and the sealing ring (O-ring) results in the operation of the pump, without problem for a further period. It is important that each time a service of
the pump is programmed, even for
replacing oil, the seal rings must be replaced Basic principle: if disassemble of the mechanical seal take place, mechanical seal must be replaced, since absolute tightness cannot be achieved
General guidance of maintenance in dry – pit pumps
NIREAS VOLUME 5 [5.2] 47
•
During the first start-up, or after a maintenance work, suction pipe and pump impeller chamber should be filled with water, in order to remove the air from the vent plugs or vents.
•
Common maintenance tasks: Periodic inspection of the condition of the tires connections (couplings) Leak Checking in sealing points. Permanent leak is caused by poor contact between the surfaces of the mobile and fixed bearing gland Temperature control of the pump bearings, which should be constant throughout the operation. The bearing temperature may reach up to 50 degrees oC above the ambient temperature, but in no case is can exceeds up to 80 ºC. Condition and Volume of the oil in the oil pan
During operation, the pump must work quietly. If any vibration detected, possible causes should seek out and normal functioning must be restored
5.2.2 Pump Control systems Pump operations usually control only one variable: flow, pressure, or level. All pump control systems have a measuring device that compares a measured value with a desired one. This information relays to a control element that makes the changes. The user may obtain control with manually operated valves or sophisticated microprocessors. Economics dictate the accuracy and complication of a control system.
Most centrifugal pumps require some form of pump control system. A typical pump control system includes a sensor to determine when the pump should be turned on or off and the electrical/electronic controls to actually start and stop the pump. The control systems currently available for the centrifugal pump range from a very simple on/off float control to an extremely complex system capable of controlling several pumps in sequence. The following sections briefly describe the operation of various types of control devices/systems used with centrifugal pumps
5.2.2.1 Float control system Currently, the float control system is the simplest of the centrifugal pump controls. In the float control system, the float rides on the surface of the water in the well, storage tank, or clear well and is attached to the pump controls by a rod with two collars. One collar activates the pump when the liquid level in the well or tank reaches a preset level, and a second collar shuts the pump off when the level in the well reaches a minimum level. This type of control system is simple to operate and relatively inexpensive to install and maintain. The system has several disadvantages;
NIREAS VOLUME 5 [5.2] 48
for example, it operates at one discharge rate, which can result in: (1) extreme variations in the hydraulic loading on succeeding units, and (2) long periods of not operating due to low flow periods or maintenance activities.
Float level control system in a pump station
Float level control system installation in a pump station
NIREAS VOLUME 5 [5.2] 49
5.2.2.2 Electrode control system The electrode control system uses a probe or electrode to control the pump on and off cycle. A relatively simple control system, it consists of two electrodes extending into the clear well, storage tank, or basin. One electrode activates the pump starter when it is submerged in the water; the second electrode extends deeper into the well or tank and is designed to open the pump circuit when the water drops below the electrode. The major maintenance requirement on of this system is keeping the electrodes clean.
Electrode control system for pump control
NIREAS VOLUME 5 [5.2] 50 5.2.2.3 Sonar control system A sonar or low-level radiation system can be used to control centrifugal pumps. This type of system uses a transmitter and receiver to locate the level of the water in a tank, clear well, or basin. When the level reaches a predetermined set point, the pump is activated; when the level is reduced to a predetermined set point, the pump is shut off. Basically, the system is very similar to a radar unit. The transmitter sends out a beam that travels to the liquid, bounces off the surface, and returns to the receiver. The time required for this is directly proportional to the distance from the liquid to the instrument. The electronic components of the system can be adjusted to activate the pump when the time interval corresponds to a specific depth in the well or tank. The electronic system can also be set to shut off the pump when the time interval corresponds to a preset minimum depth.
Sonar control system for pump control
NIREAS VOLUME 5 [5.2] 51 5.2.2.4 Motor controllers Several types of controllers are available that protect the motor not only from overloads but also from short-circuit conditions. Many motor controllers also function to adjust motor speed to increase or decrease the discharge rate for a centrifugal pump. This type of control may use one of the previously described controls to start and stop the pump and, in some cases, adjust the speed of the unit. As the depth of the water in a well or tank increases, the sensor automatically increases the speed of the motor in predetermined steps to the maximum design speed. If the level continues to increase, the sensor may be designed to activate an additional pump.
5.2.2.5 Protective instrumentation Protective instrumentation of some type is normally employed in pump or motor installation. (Note that the information provided in this section applies to the centrifugal pump as well as to many other types of pumps.) Protective instrumentation for centrifugal pumps (or most other types of pumps) is dependent on pump size, application, and the amount of operator supervision; that is, pumps under 500 hp often only come with pressure gauges and temperature indicators. These gauges or transducers may be mounted locally (on the pump itself) or remotely (in suction and discharge lines immediately upstream and downstream of the suction and discharge nozzles). If transducers are employed, readings are typically displayed and taken (or automatically recorded) at a remote operating panel or control center.
5.2.2.6 Temperature detectors Resistance temperature devices (RTDs) and thermocouples are commonly used as temperature detectors on the pump prime movers (motors) to indicate temperature problems. In some cases, dial thermometers, armored glass-stem thermometers, or bimetallic-actuated temperature indicators are used. Whichever device is employed, it typically monitors temperature variances that may indicate a possible source of trouble. On electric motors greater than 250 hp, RTD elements are used to monitor temperatures in stator winding coils. Two RTDs per phase are standard. One RTD element is usually installed in the shoe of the loaded area employed on journal bearings in pumps and motors. Normally, tilted-pad thrust bearings have an RTD element in the active, as well as the inactive, side. RTDs are used when remote indication, recording, or automatic logging of temperature readings is required. Because of their smaller size, RTDs provide more flexibility in
NIREAS VOLUME 5 [5.2] 52
locating the measuring device near the measuring point. When dial thermometers are installed, they monitor oil thrown from bearings. Sometimes temperature detectors also monitor bearings with water-cooled jackets to warn against water supply failure. Pumps with heavy wall casings may also have casing temperature monitors.
5.2.2.7 Vibration sensors Vibration sensors are available to measure either bearing vibration or shaft vibration direction directly. Direct measurement of shaft vibration is desirable for machines with stiff bearing supports where bearing-cap measurements will be only a fraction of the shaft vibration.
NIREAS VOLUME 5 [5.2] 53
5.2.3 Valves Any wastewater operation will have many valves that require attention. A maintenance operator must be able to identify and locate different valves to inspect them, adjust them, and repair or replace them. For this reason, the operator should be familiar with all valves, especially those that are vital parts of a piping system. A valve is defined as any device by which the flow of fluid may be started, stopped, or regulated by a movable part that opens or obstructs passage. As applied in fluid power systems, valves are used for controlling the flow, the pressure, and the direction of the fluid flow through a piping system. The fluid may be a liquid, a gas, or some loose material in bulk (such as a biosolids slurry).
Designs of valves may vary, but all valves have two features in common: a passageway through which fluid can flow and some kind of movable
(usually machined) part that opens and closes
the passageway.
Valves may be controlled manually, electrically, pneumatically, mechanically, or hydraulically, or by combinations of two or more of these methods.
Valves are made from bronze, cast iron, steel, stainless steel, and other metals or alloys. They are also made from plastic and glass.
Valves are made in a full range of sizes that match pipe and tubing sizes. Actual valve size is based on the internationally agreed-upon definition of nominal size (DN), which is a numerical designation of size that is common to all components in a piping system other than components designated by outside diameters. It is a convenient number for reference purposes and is only loosely related to manufacturing dimensions. Valves are made for service at the same or higher pressures and temperatures that piping and tubing is subject to.
NIREAS VOLUME 5 [5.2] 54 Basic valve operation
(Frank R. Spellman, 2011)
Main types of valves include :
• Ball valves • Gate valves • Globe valves • Needle valves • Butterfly valves • Check valves • Quick-opening valves • Diaphragm valves • Relief valves • Pressure-Reducing valves • Air Relief valves
NIREAS VOLUME 5 [5.2] 55
5.2.3.1 Ball valves Ball valves, as the name implies, are stop valves that use a ball to stop or start fluid flow. The ball performs the same function as the disk in other valves. As the valve handle is turned to open the valve, the ball rotates to a point where part or all of the hole through the ball is in line with the valve body inlet and outlet, allowing fluid to flow through the valve. When the ball is rotated so the hole is perpendicular to the flow openings of the valve body, the flow of fluid stops. Most ball valves are the quick-acting type and require only a 90째 turn to either completely open or close the valve; however, many are operated by planetary gears. This type of gearing allows the use of a relatively small hand wheel and operating force to operate a large valve; however, it increases the operating time for the valve. Some ball valves also contain a swing check located within the ball to give the valve a check valve feature. The two main advantages of using ball valves are that: (1) the fluid can flow through it in either direction, as desired; and (2) when closed, pressure in the line helps to keep it closed. Ball valve
NIREAS VOLUME 5 [5.2] 56 Ball valve – 3D representation
5.2.3.2 Gate valves Gate valves are used when a straight-line flow of fluid and minimum flow restriction are necessary; they are the most common type of valve found in a water distribution system. Gate valves are so named because the part that either stops or allows flow through the valve acts somewhat like a gate. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve bonnet. This leaves an opening for flow through the valve the same size as the pipe in which the valve is installed. For these reasons, the pressure loss (pressure drop) through these types of valves is about equal to the loss in a piece of pipe of the same length. Gate valves are not suitable for throttling purposes. Generally, gate valves are not installed where they will have to be operated trequently because they require too much time to operate from fully open to closed.
NIREAS VOLUME 5 [5.2] 57
Double disc NRS (nonrising stem) gate valve
(Garr M. Jones, PE, 2006)
Knife gate valve
(Garr M. Jones, PE, 2006)
NIREAS VOLUME 5 [5.2] 58
5.2.3.3 Globe valves Probably the most common valve type in existence, the globe valve is commonly used for water faucets and other household plumbing. The globe valves have a circular disk (the globe) that presses against the valve seat to close the valve .Globe valves seat very tightly and can be adjusted with fewer turns of the wheel than gate valves; thus, they are preferred for applications that call for frequent opening and closing. On the other hand, globe valves create high head loss when fully open; thus, they are not suited in systems where head loss is critical.
Vee-ported low headloss control valve in wye pattern fitted with vee-port throttling plug. Flow is from right to left
(Garr M. Jones, PE, 2006)
NIREAS VOLUME 5 [5.2] 59 5.2.3.4 Needle valves Although similar in design and operation to the globe valve (a variation of globe valves), the needle valve has closing element in the shape of a long tapered point, which is at the end of the valve stem. Next figure shows a cross-sectional view of a needle valve. As you can see in the figure, the long taper of the valve closing element permits a much smaller seating surface area than that of the globe valve; accordingly, the needle valve is more suitable as a throttle valve. In fact, needle valves are used for very accurate throttling.
Common needle valve
NIREAS VOLUME 5 [5.2] 60
5.2.3.5 Butterfly valves This valve itself consists of a body in which a disk ("butterfly") rotates on a shaft to open or close the valve. Butterfly valves may be flanged or wafer design, the latter intended for fitting directly between pipeline flanges. In the full open position, the disk is parallel to the axis of the pipe and the flow of fluid. In the closed position, the disk seals against a rubber gasket-type material bonded either on the valve seat of the body or on the edge of the disk. Because the disk of a butterfly valve stays in the fluid path in the open position, the valve creates more turbulence (higher resistance to flow and thus higher pressure loss) -than a gate valve. On the other hand, butterfly valves are compact. They can also be used to control flow in either direction. This feature is useful in water treatment plants that periodically backwash to clean filter systems.
Butterfly valve
NIREAS VOLUME 5 [5.2] 61
5.2.3.6 Check valves Check valves are usually self-acting and designed to allow the flow of fluid in one direction only. They are commonly used at the discharge of a pump to prevent backflow when the power is turned off. When the direction of flow is moving in the proper direction, the valve remains open. When the direction of flow reverses, the valve closes automatically from the fluid pressure against it. Several types of check valves are used in wastewater operations, including: • Slanting disk check valves • Cushioned swing check valves • Rubber-flapper swing check valves • Double-door check valves • Ball check valves • Foot valves • Backflow-prevention devices
In each case, pressure from the flow in the proper direction pushes the valve element to an open position. Flow in the reverse direction pushes the valve element to a closed position.
Note: Check valves are also commonly referred to as nonreturn or reflux valves.
Nonreturn valve with rubber blockage
Open valve
Closed valve
NIREAS VOLUME 5 [5.2] 62 Nonreturn valve with spring blockage
5.2.3.7 Quick opening valves Quick-opening valves are nothing more than adaptations of some of the valves already described. Modified to provide a quick on/off action, they use a lever device in place of the usual threaded stem and control handle to operate the valve. This type of valve is commonly used in wastewater operations where deluge showers and emergency eyewash stations are installed in work areas where chemicals are loaded or transferred or where chemical systems are maintained. They also control the air supply for some emergency alarm horns around chlorine storage areas, for example. Moreover, they are usually used to cut off the flow of gas to a main or to individual outlets.
5.2.3.8 Diaphragm valves Diaphragm valves are glandless valves that use a flexible elasto-meric diaphragm (a flexible disk) as the closing member and in addition effect an external seal. They are well suited to service in applications where tight, accurate closure is important. The tight seal is effective whether the fluid is a gas or a liquid. This tight closure feature makes these valves useful in vacuum applications. Diaphragm valves operate similar to globe valves and are usually multi-turn in operation; they are available as weir type and full bore. A common application of diaphragm valves in wastewater operations is to control fluid to an elevated tank.
NIREAS VOLUME 5 [5.2] 63
Diaphragm valves
NIREAS VOLUME 5 [5.2] 64 5.2.3.9 Relief valves Some fluid power systems, even when operating normally, may temporarily develop excessive pressure; for example, whenever an unusually strong work resistance is encountered, dangerously high pressure may develop. Relief valves are used to control this excess pressure. Such valves, are automatic valves; they begin to open at a preset pressure but require a 20% overpressure to open wide. As the pressure increases, the valve continues to open farther until it has reached its maximum travel. As the pressure drops, it starts to close and finally shuts off at about the set pressure. Main system relief valves are generally installed between the pump or pressure source and the first system isolation valve. The valve must be large enough to allow the full output of the hydraulic pump to be delivered back to the reservoir.
Note: Relief valves do not maintain flow or pressure at a given amount but prevent pressure from rising above a specific level when the system is temporarily overloaded. Pressure Relief valve
NIREAS VOLUME 5 [5.2] http://www.youtube.com/watch?v=DAqnpaHf2Qs
65 5.2.3.10
Pressure-reducing valves
Pressure-reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system. In practice, they are very much like pressure-regulating valves. A pressure-reducing valve reduces pressure by throttling the fluid flow. Ά reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the valve is set, the reduced pressure will be maintained regardless of changes in supply pressure (as long as the supply pressure is at least as high as the reduced pressure desired) and regardless of the system load, providing the load does not exceed the design capacity of the reducer. Pressure-reducing valve
NIREAS VOLUME 5 [5.2] 66
5.2.3.11
Air Relief valves
A common Air Relief Valve is a combination of a kinetic and an automatic air relief valve. During the filling of the system, the air of the piping is released through the chamber and the exit orifice (St.1). When the water level starts rising and the pressure is 0.5 atm, the floater A' rises and closes the exit orifice. It remains to this position while there is no pressure in the system, (St.2) The automatic air relief valve starts functioning when air bubbles appear. These bubbles are going out from the holes of the floater A' (St.3). During the evacuation of the system and when the pressure decreases to the minimum, the floaters fall down, due to their weight (St.1) and the orifice of the air relief valve is being released.The system is filled with wellcoming air which protects the piping from damages caused from the depressure.
Air Relief Valve function
During the filling of the system, the air, existed inside the pipes, is being released through the chamber and the exit orifice
The water rises the floaters and the orifice is closed from the floater A'.
The air bubbles appeared to the upper part of the chamber, displace floater B' and escape through the holes of floater A'
NIREAS VOLUME 5 [5.2] Air Relief Valve’s typical connection
67
5.2.3.12
Valve operators
In many modern wastewater operations, devices called operators or actuators mechanically operate many valves. These devices may be operated by air, electricity, or fluid—that is, by pneumatic, magnetic, and hydraulic operators.
Pneumatic, ElectroMagnetic, and Hydraulic valve operators
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5.2.4 Mixers Mixing is an important unit operation in many phases of wastewater treatment including (1) mixing of one substance completely with another, (2) blending of miscible liq uids, (3) flocculation of wastewater particles, (4) continuous mixing of liquid suspensions, and (5) heat transfer. Most mixing operations in wastewater can be classified as continuous-rapid (less than 30 s) or continuous (i.e., ongoing). •
Continuous rapid mixing is used, most often, where one substance is to be mixed with another. The principal applications of continuous rapid mixing are in (1) the blending of chemicals with wastewater.
•
Continuous mixing is used where the contents of a reactor or holding tank or basin must be kept in suspension such as in equalization basins, flocculation basins, suspendedgrowth biological treatment processes, aerated lagoons, and aerobic digesters
Following equation is widely used in the design and operation of systems with mechanical mixing devices. (Camp, T. R., and P. C. Stein, 1943)
Where : •
G = average velocity gradient, T-1, 1/s
•
P = power requirement, W
•
μ = dynamic viscosity, N*s/m2
•
V = flocculator volume, m3
•
t = detention time, s
•
Q = flowrate, m3/s
NIREAS VOLUME 5 [5.2] 69
Typical values that have been used for G for various mixing operations are reported in next table : Typical detention time and velocity gradient G values for mixing and flocculation in wastewater
(Eddy, 1999)
Following equations are widely used for estimating the power input in a mixer, as well as the pumping capacity of the mixer. (turbulent flow is mandatory for the application of following equations, Rn>10.000) P = NP* p * n3 * D5 Qi = NQ * n * D3
Where, P = power input, W NP = power number for impeller, unitless p = density, kg/m3 n = revolutions per second, r/s D = diameter of impeller, m Qi = pump discharge, m3/s NQ = flow number for impeller, unitless Typical power and Flow numbers for various impellers
NIREAS VOLUME 5 [5.2] 70
(Eddy, 1999)
5.2.4.1 Continuous rapid mixing Many types of mixing devices are available, depending on the application and the timescale required for mixing. Most common types of mixers for continuous rapid mixing are : •
Static mixers
•
In-line mixers
•
High-Speed Induction Mixer
•
Pressurized Water Jets
•
Turbine and Propeller Mixers
Static Mixers Static in-line mixers contain internal vanes or orifice plates that bring about sudden changes in the velocity patterns as well as momentum reversals. Static mixers are principally identified by their lack of moving parts. Typical examples include in-line static mixers that contain elements that bring about sudden changes in the velocity patterns as well as momentum reversals and mixers that contain orifice plates and nozzles.
NIREAS VOLUME 5 [5.2] 71 Typical in-line static mixer
Inline Mixers In-line mixers are similar to static mixers but contain a rotating mixing element to enhance the mixing process.
NIREAS VOLUME 5 [5.2] 72 Typical in-line mixer
NIREAS VOLUME 5 [5.2] 73 High-Speed Induction Mixers The high-speed induction mixer is an efficient mixing device for a variety of chemicals. The system consists of a motor-driven open propeller that creates a vacuum in the chamber directly above the propeller. The vacuum created by the impeller induces the chemical to be mixed directly from the storage container without the need for dilution water.
Typical induction mixer
NIREAS VOLUME 5 [5.2] 74 Pressurized Water Jet mixers Pressurized water jet mixers, can also be used to mix chemicals. An important design feature of pressurized water jet mixers is that the velocity of the jet containing the chemical to be mixed must be sufficient to achieve mixing in all parts of the pipeline.
Typical jet mixer
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Turbine and Propeller Mixers Turbine and propeller mixers are used commonly in wastewater-treatment processes for mixing and blending of chemicals, for keeping material in suspension, and for aeration. Turbine or propeller mixers are usually constructed with a vertical shaft driven by a speed reducer and electric motor. Two types of impellers are used for mixing: (1) radial-flow impellers and (2) axial-flow impellers. Various types of propeller mixers
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Typical mixers used in wastewater treatment for rapid mixing: (a) in-line static mixer with internal vanes, (b) in-line static mixer with orifice for mixing dilute chemicals, (c) in-line mixer, (d) inline mixer with internal mixer, (e) high-speed induction mixer, (f) pressurized water jet mixer with reactor tube
(Eddy, 1999)
5.2.4.2 Continuous mixing
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Continuous mixing operations are used in biological treatment processes such as the activatedsludge process to maintain the mixed liquor suspended solids uniformly mixed state. In biological treatment systems the mixing device is also used to provide the oxygen needed for the process. Thus, the aeration equipment must be able to provide the oxygen needed for the process and the energy needed to maintain mixed conditions within the reactor. Both mechanical aerators and dissolved aeration devices are used. Diffused air is often used to fulfill both the mixing and oxygen requirements. Alternatively, mechanical turbine-aerator mixers may be used. Horizontal, submersible propeller mixers are often used to maintain channel velocities in oxidation ditches, mix the contents of anoxic reactors and aid in the destratification of reclaimed water storage reservoirs.
Pneumatic mixing
In pneumatic mixing, a gas (usually air or oxygen) is injected into the bottom of mixing or activatedsludge tanks, and the turbulence caused by the rising gas bubbles serves to mix the fluid contents of the tank. In aeration, soft bubbles are formed with an average diameter of 5 mm while the air flow is about 10 percent of the liquid flow.
When air is injected in mixing or flocculation tanks or channels, the power dissipated by the rising air bubbles can be estimated with the following equation
Where,
P = power dissipated, kW pa = atmospheric pressure, kN/m2 Va = volume of air at atmospheric pressure, m3/s pc = air pressure at the point of discharge, kN/m2
Mechanical Aerators
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The principal types of mechanical aerators used for continuous mixing are high-speed surface aerators and slow-speed surface aerators. Typical power requirements for mixing with mechanical aerators range from 20 to 40 kW/103 m3, depending on the type of mixer and the geometry of the tank, lagoon, or basin. Mechanical Aerators main use is for enrichment of waste with oxygen, therefore they are more thoroughly examined in the next chapter with title “Aeration Systems�.
Typical Mechanical Aerators installations
5.2.4.3 Calculations Problem:
NIREAS VOLUME 5 [5.2] 79 A vertical flat-blade turbine is installed centrally in a baffled vessel. The vessel is 2.0 m in diameter. The turbine, 61 cm in diameter, is positioned 60 cm from the bottom of the vessel. The tank is filled to a depth of 2.0 m and is mixing alum with raw water. The water is at a temperature of 25 oC and the turbine is running at 10 rpm. What horsepower will be required to operate the mixer? What is the maximum pump discharge from the mixer?
Solution: Flow characterization
Rn=
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•
Mixers: Regular inspection and preventive maintenance by cleaning at regular intervals, is mandatory
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Inspection interval: Depending on the stress of the mixing group, but it cannot exceeds one year in any case
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Maintenance and monitoring should be performed in accordance with the manufacturer’s manual
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Strong vibrations or abnormal function: Possible causes are: -
Very small overlap of the helix by the mixing liquid
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Air inlet in the propeller
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Incorrect rotation of the propeller
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Parts of the mixing asembly as supporting elements or parts of the link are faulty or have been dismantled.
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Electrical power cables: Cleaning and checking for damage to the insulation, once a month.
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Power consumption: Check with amperemeter.
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Lifting mechanism: Test for proper function every six months.
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Propeller: Visual inspection for the presence of cracks or damage from rough or abrasive rags
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Insulation engine: once a year or every 4,000 hours of operation it is necessary to test the resistance of the motor insulation and proper functioning of the monitoring.
5.2.5 Aeration systems There are several types of aeration systems used for wastewater treatment.
The two basic
methods of aerating wastewater are (1) to introduce air or pure oxygen into the wastewater with submerged diffusers or other aeration devices or (2) to agitate the wastewater mechanically so as to promote solution of air from the atmosphere.
5.2.5.1 Diffused-air aeration systems A diffused-air system consists of diffusers that are submerged in the wastewater, header pipes, air mains, and the blowers and appurtenances through which the air passes.
Diffusers
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Three categories of diffusers are defined: (1) porous or fine-pore diffusers, (2) nonporous diffusers, and (3) other diffusion devices such as jet aerators, and aspirating aerators.
Description of commonly used air diffusion devices
(Eddy, 1999) 1. Porous diffusers
Numerous materials have been used in the manufacture of porous diffusers. These materials generally fall into the categories of rigid ceramic and plastic materials and flexible plastic, rubber, or cloth sheaths.
Typical porous air diffusers: (a) aluminum oxide disk, (b) ceramic dome, (c) polyethylene disk, and (d) perforated membrane.
NIREAS VOLUME 5 [5.2] 82 (Eddy, 1999)
With all porous diffusers, it is essential that the air supplied be clean and free of dust particles that might clog the diffusers. Air filters, often consisting of viscousimpingement and dry-barrier types, are commonly used. Precoated bag filters and electrostatic filters have also been used. The filters should be installed on the blower inlet.
2. Nonporous diffusers
Nonporous diffusers produce larger bubbles than porous diffusers and consequently have lower aeration efficiency; but the advantages of lower cost, less maintenance, and the absence of stringent air-purity requirements may offset the lower oxygen transfer efficiency and energy cost.
Nonporous diffusers used for the transfer of oxygen: (a) orifice and (b) tube.
(Eddy, 1999)
3. Other diffusion devices
Jet aeration combines liquid pumping with air diffusion. The pumping system recirculates liquid in the aeration basin, ejecting it with compressed air through a nozzle assembly. This system is particularly suited for deep (>8 m) tanks.
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Aspirating aeration consists of a motor-driven aspirator pump. The pump draws air in through a hollow tube and injects it underwater where both high velocity and propeller action create turbulence and diffuse the air bubbles .
Other devices used for the transfer of oxygen: (a) static tube mixer where air is introduced at the base of the aerator that contains mixing elements, (b) jet reactor in which pressurized air and liquid are combined in a mixing chamber (As the jet is emitted, the surrounding liquid is entrained to enhance oxygen transfer.), (c) jet aerator in a manifold arrangement, and (d) aspirating aerator
(Eddy, 1999)
The efficiency of oxygen transfer depends on many factors, including the type, size, and shape of the diffuser; the air flowrate; the depth of submersion; tank geometry including the header and diffuser location; and wastewater characteristics.
Oxygen transfer efficiency (OTE) of porous diffusers may also decrease with use due to internal clogging or exterior fouling. Internal clogging may be due to impurities in the compressed air that
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have not been removed by the air filters. External fouling may be due to the formation of biological slimes or inorganic precipitants.
Typical membrane diffusers installation in an aeration tank
NIREAS VOLUME 5 [5.2] 85 Operation and maintenance of diffusers and aeration system appurtenances •
Preventative maintenance of air disc: Protects the diffuser of particles that can cause clogging of the pores Aeration system is maintained at the desired levels of performance, and the entry of solids in the air distribution network is not allowed Includes: inspection, cleaning and replacement of air filter blower, whenever required by the manufacturer.
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Cleaning: Cleaning methods used to restore the efficiency of diffusers are: either intermittent (the aeration tank is off) either continuous operation (no access to the tank).
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Some purification methods used are washing with acidic or alkaline cleaning, gas injection, cleaning with high pressure water (jetting) and purification by high pressure air (air bumping).
NIREAS VOLUME 5 [5.2] 86 Blowers and compressors
There are three types of blowers commonly used for aeration: centrifugal, rotary lobe positive displacement, and inlet guide vane-variable diffuser. Centrifugal blowers can deliver air flows greater than 425 m3/min of free air. Rated discharge pressures range normally from 48 to 62 kN/m2 .Centrifugal blowers have operating characteristics similar to a low-specific-speed centrifugal pump. For higher discharge pressure applications (55 kN/m2) and for capacities smaller than 425 m3/min of free air per unit, rotary-lobe positive displacement blowers are prefered. The positivedisplacement blower is a machine of constant capacity with variable pressure. The units cannot be throttled, but capacity control can be obtained by the use of multiple units or a variablespeed drive. Rugged inlet and discharge silencers are essential.
On the other hand, inlet guide vane-variable diffuser mitigates some of the problems and considerations associated with standard centrifugal and positive-displacement aeration blowers. The design is based on a single-stage centrifugal operation that incorporates actuators to position the inlet guide vane and variable diffusers to vary blower flowrate and optimize efficiency. Blower capacities range from 85 to 1700 m3/min at pressures up to 170 kN/m2 . Principal disadvantages are high initial cost and a sophisticated computer control system to ensure efficient operation.
Rotary-lobe positive displacement blower
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Typical Centifugal blower installation in a WWTP
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Operation and maintenance of blowers •
Maintenance and precautions: When checking the operation and maintenance of a blower, it is necessary to take into account all issues about safety of the staff.
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Especially during the visual inspection phase when the blower is in operation, personnel must wear noise protection equipment, safety glasses and protective gloves.
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Particular care is needed in the moving parts of the blower and the exhaust air which is at a very high temperature.
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Attention needs also to the blower’s suction point which should not be approached with clothing or other items that may be caught in the suction.
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Necessary is to follow the manufacturer's instructions.
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All maintenance must be performed by qualified personnel.
Lubrication •
The lubrication of the gears and the bearings located on the opposite side of the movement is made by oil.
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The lubrication of bearings on the movement side is made through grease. The outer points requiring lubrication are:
•
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Oil level plug
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Oil filling with oil
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Cap oil removal
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Global grease nipples
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Connection of the manometer
The oil level in the sump of the gear can be controlled by removing the corresponding plug. When the machine is leveled, there will be a slight overflow of oil from this opening.
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Avoid the use of high doses of oil in order to avoid overheating. The same applies to bearings.
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The first greasing can be made after 3000 hours of operation with the help of special grease.
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Correct tactic is the use of mineral oil without additives EP (eg generic oil or hydraulic oil) or synthetic base oil ofelinis.
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5.2.5.2 Mechanical aerators Mechanical aerators are commonly divided into two groups based on major design and operating features: aerators with vertical axis and aerators with horizontal axis. Both groups are further subdivided into surface and submerged aerators. •
Surface Mechanical Aerators with Vertical Axis
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Submerged Mechanical Aerators with Vertical Axis
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Surface Mechanical Aerators with Horizontal Axis
•
Submerged Mechanical Aerators with Horizontal Axis
1. Surface Mechanical Aerators with Vertical Axis
Surface aerators consist of submerged or partially submerged impellers that are attached to motors mounted on floats or on fixed structures. The impellers are fabricated from steel, cast iron, or noncorrosive alloys, and are used to agitate the wastewater, entraining air in it. Surface aerators may be classified according to their rotation speed to low-speed and high-speed, depending on the application.
Surface Mechanical Aerator with Vertical Axis
2. Submerged Mechanical Aerators with Vertical Axis
NIREAS VOLUME 5 [5.2] 90 Air or pure oxygen in submerged aerators may also be introduced by diffusion into the wastewater beneath the impeller or downflow of radial aerators. The impeller is used to disperse the air bubbles and mix the tank contents.
Submerged Mechanical Aerator, Vertical Axis type with supplementary air introduced below the turbine
(Eddy, 1999)
NIREAS VOLUME 5 [5.2] 91 3. Surface Mechanical Aerators with Horizontal Axis
The surface aerator is patterned after the original Kessener brush aerator, a device used to provide both aeration and circulation in oxidation ditches. The brush-type aerator had a horizontal cylinder with bristles mounted just above the water surface. The bristles were submerged in the water and the cylinder was rotated rapidly by an electric motor drive, spraying wastewater across the tank, promoting circulation, and entraining air in the wastewater.
Surface Mechanical Aerator with Horizontal Axis
Surface Mechanical Aerator Installation in Oxidation Ditch : Plan view
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4. Submerged Mechanical Aerators with Horizontal Axis Submerged horizontal-axis aerators are similar in principle to surface aerators except disks or paddles attached to rotating shafts are used to agitate the water. The disk aerator has been used in numerous applications for channel and oxidation ditch aeration.
5.2.6 Other equipment – operation & maintenance 5.2.6.1 Dismantling joints The dismantling joint in a hydraulic installation is the necessary joining element between different fittings so that they are easily adjusted to the net or removed from it. Rubber sealing ring is of special material and thickens, in order to assure a tight sealing and prevent the dismantling joint from being damaged. Typical dismantling joint
NIREAS VOLUME 5 [5.2] 93 5.2.6.2 Settling tank’s skimmer •
Reciprocating bridges should be checked daily both visual and auditory. Regular inspection should be done on the wheels of the shuttle bridge in order to identify possible damage or possible deviation in alignment.
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The drive must be cleaned daily from dust and dirt in order to ensure cooling.
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Change the gear oil every 4000 hours or at the latest after 1 year of operation.
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Settling tanks should be emptied periodically so that it is possible to control the wetted parts (support bars scrapers, struts, bottom scrapers).
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In case of damage of the bottom scraper, this must be replaced.
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Periodic lubrication of bearings in accordance with the manufacturer's instructions is necessary.
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GLOSSARY 5.2.7 Glossary Axial-flow pumps—Pumps that transmit the fluid pumped in the axial direction. Base plate—The foundation under a pump. It usually extends far enough to support the drive unit. The base plate is often referred to as the pump frame. Bearings—Devices used to reduce friction and to allow the shaft to rotate easily. Bearings may be sleeve, roller, or ball. • Radial (line) bearing—In a single-suction pump, it is the one closest to the pump. It rides free in its own section and takes up and down stresses. • Thrust bearing—In a single-suction pump, it is the bearing located nearest the motor, farthest from the impeller. It takes up the major thrust of the shaft, which is opposite from the discharge direction. Note: In most cases, where pump and motor are constructed on a common shaft (no coupling), the bearings will be part of the motor assembly.
Best operating efficiency—Value of the efficiency that corresponds to the best operating performance of the pump. Blade—The impeller element in a paddle wheel. Brake or shaft power—The power of the motor or prime mover driving the pump. Brake efficiency—Ratio of the power given to the fluid to the brake input power (brake power) to the pump. Cavitation—A state of flow where the pressure in the liquid becomes equal to its vapor pressure. Casing—The housing surrounding the rotating element of the pump. In the majority of centrifugal pumps, this casing can also be called the volute. • Split casing - A pump casing that is manufactured in two pieces fastened together by means of bolts. Split casing pumps may be vertically split (perpendicular to the shaft direction) or horizontally split (parallel to the shaft direction).
Centrifugal pump—A pump that conveys fluid through the momentum created by a rotating impeller. Coupling—Device to join the pump shaft to the motor shaft. If the pump and motor are constructed on a common shaft, the assembly is referred to as a close-coupled arrangement. Discharge—In a pumping system, the arrangement of elements after the pumping station. Discharge velocity head—The velocity head at the discharge of a pumping system. Displacement pumps—Pumps that literally pushes the fluid in order to move it. Dynamically similar pumps—Pumps with head coefficients that are equal. Extended shaft—For a pump constructed on one shaft that must be connected to the motor by a coupling.
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Fittings losses—Head losses in valves and fittings. Frame—The housing that supports the pump bearing assemblies. In an end-suction pump, it may also be the support for the pump casing and the rotating element. Friction head loss—A head loss due to loss of internal energy. Gear pump—A pump that basically operate like a lobe pump, except that instead of lobes, gear teeth are used to move the fluid. Geometrically similar pumps—Pumps with corresponding parts that are proportional. Hydraulic mixers—Mixers that utilize, for the mixing process, the agitation that results in the flowing of the water. Homologous pumps—Pumps that are similar. Similarities are established dynamically, kinematically, or geometrically. Impeller—The rotating element in the pump that actually transfers the energy from the drive unit to the liquid. Depending on the pump application, the impeller may be open, semi-open, or closed. It may also be single or double suction. Impeller eye—The center of the impeller, the area that is subject to lower pressures due to the rapid movement of-the liquid to the outer edge of the casing. Inlet dynamic head—The sum of the inlet velocity head and inlet manometric head of a pump. Inlet manometric head—The manometric level at the inlet to a pump. Kinematically similar pumps—Pumps whose flow coefficients are equal. Lobe pump—A positive-displacement pump whose impellers are shaped like lobes. Manifold pipe—A pipe with two or more pipes connected to it. Manometric level—The height of liquid corresponding to the gage pressure. Mixed-flow pump—Pump with an impeller that is designed to provide a combination of forward and radial flow. Mixing—A unit operation that distributes the components of two or more materials among the materials producing in the end a single blend of the components. Net positive suction head (NSPH)—The amount of energy possessed by a fluid mat the inlet to a pump. Non-pivot parameter—The counterpart of pivot parameter. Outlet dynamic head—The sum of the outlet velocity head and outlet manometric head of a pump. Outlet manometric head—The manometric level at the outlet of a pump. Paddles—Impellers in which the lengths are equal to 50 to 80% of the inside diameter of the vessel in which the mixing is taking place.
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Paddle arm—The element extending from the axis of rotation in a paddle wheel. Paddle wheel—The paddle configuration in a flocculator compartment. Parallel connection—Mode of connection of more than one pump where the discharges of all the pumps are combined into one. Pitch—Assuming no slippage, the ratio of the distance traveled by the water to the diameter of the impeller. Pneumatic mixers—Mixers that uses gas or air bubbles to induce the agitation. Positive-displacement pump—A pump that conveys fluid by directly moving it using a suitable mechanism such as a piston, plunger, or screw. Power dissipation—In fluid motion, power lost due to frictional resistance and is equal to the power given by the agitator. Propeller pumps—The same as axial-flow pumps. Priming—Filling the casing and impeller with liquid. If this area is not completely full of liquid, the centrifugal pump will not pump efficiently. Propellers—Impellers in which the direction of the driven fluid is along the axis of rotation Pump assembly—The pump arrangement in a pumping station. Pump characteristics—Set of curves that depicts the performance of a given particular pump. Pump loss—Head losses incurred inside the pump casing. Pumping—A unit operation used to move fluid from one point to another. Pumping station—A location where one or more pumps are operated to convey fluids. Pumping system—The pumping station and the piping system constitute the pumping system. Radial-flow pump—Pump with an impeller that directly throws the flow radially into the side of the chamber circumscribing it. Rotational mixers—Mixers that use a rotating element to effect the agitation. Scaling laws—Mathematical equations that establish the similarity of homologous pumps. Seals—Devices used to stop the leakage of air into the inside of the casing around the shaft. • Gland—Also known as the packing gland, it is a metal assembly that is designed to apply even pressure to the packing to compress it tightly around the shaft. • Lantern ring—Also known as the seal cage, it is positioned between the rings of packing in the stuffing box to allow the introduction of a lubricant (water, oil, or grease) onto the surface of the shaft to reduce the friction between the packing and the rotating shaft. • Mechanical seal—A device consisting of a stationary element, a rotating element, and a spring to supply force to hold the two elements together; may be either single or double units. • Packing—Material that is placed around the pump shaft to seal the shaft opening in the casing and prevent air leakage into the casing.
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• Stuffing box—The assembly located around the shaft at the rear of the casing. It holds the packing and lantern ring.
Series connection—A mode of connecting more than one pump where the discharge of the pump ahead is introduced to the inlet of the pump following. Shaft—The rigid steel rod that transmits the energy from the motor to the pump impeller. Shafts may be either vertical or horizontal. Shaft sleeve—A piece of metal tubing placed over the shaft to protect the shaft as it passes through the packing or seal area. In some cases, the sleeve may also help to position the impeller on the shaft. Shroud—The metal plate that is used to either support the impeller vanes (open or semi-open impeller) or enclose the vanes of the impeller (closed impeller) Shut-off head—The head or pressure at which the centrifugal pump will stop discharging. It is also the pressure developed by the pump when it is operated against a closed discharge valve. This is also known as a cut-off head. Similar or homologous pumps—Pumps where the head, flow, and pressure coefficients are equal. Similarity, affinity, or scaling laws—The equations that state that the head, flow, and power coefficients of a series of pumps are equal. Slinger ring—A device to prevent pumped liquids from traveling along the shaft and entering the bearing assembly. A slinger ring is also called a deflector. Specific speed—A ratio obtained by manipulating the ratio of the flow coefficient to the head coefficient of a pump. Values obtained are values applying at the best operating efficiency. Static discharge head—The vertical distance from the pump centerline to the elevation of the discharge liquid level. Static suction head—The vertical distance from the elevation of the inflow liquid level above the pump centerline to the centerline of the pump. Static suction lift—The vertical distance from the elevation of the inflow liquid level below the pump centerline to the centerline of the pump. Suction—In a pumping system, the system of elements before the pumping mstation. Suction velocity head—The velocity head at the suction side of a pumping system. System characteristic—In a pumping system, the relationship of discharge and the associated head requirement that excludes the pump assembly. Total dynamic head or total developed head—The head given to the pump minus pump losses. Total developed head requirement—The equivalent head loss corresponding to a given discharge.
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Total static head—The vertical distance between the elevation of the inflow liquid level and the discharge liquid level. Transition losses—Head losses in expansions, contractions, bends, and the like. Turbines—Impellers shorter than paddles and are only about 30 to 50% of the inside diameter of the vessel in which the mixing is taking place. Turbomachine—Fluid machine that turns or tends to turn about an axis. Vane pump—A pump in which a vane pushes the water forward as it is being trapped between the vane and the side of the casing. Volute—Casing of a centrifugal pump that is shaped into a spiral. Wearing rings—Devices that are installed on stationary or moving parts within the pump casing to protect the casing and the impeller from wear due to the movement of liquid through points of small clearances. • Casing ring—A wearing ring installed in the casing of the pump. A casing ring is also known as the suction head ring. • Impeller ring—A wearing ring installed directly on the impeller. • Stuffing box cover ring—A wearing ring installed at the impeller in an end-suction pump to maintain the impeller clearances and to prevent casing wear.
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ASSIGNMENTS SECTION
QUESTIONS
1. _____________viscosity materials are thick.
2. When the_____________of a pump impeller is above the level of the pumped fluid, the condition is called suction lift.
3. When a pump is not running, conditions are referred to as _____________;when a pump is running, the conditions are _____________ 4. 5. The sum of total static head, head loss, and dynamic head is called_____________
6. What are the four basic types of curves used for centrifugal pumps?
7. With the pump shut off, the difference between the suction and discharge liquid levels is called_____________
8. The casing of a submerged pump encloses the pump impeller, the shaft, and the_____________
9. Name three types of impellers.
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10. Large or small capacity pumps have better efficiency?
11. Which type of pump is the most common in the entrance pump –lift station, for medium to large WWTP ?
12. Which type of pump is more easily repaired ? Centrifugal Dry type or centrifugal submerged type?
13. Given the fact that both pumps have the same impeller opening, which type of pump is easier to clog? Semi-open type or vortex impeller?
14. Given the fact that both pumps have the same impeller opening, which type of impeller present the better efficiency ? semi-open, open, vortex, or closed?
15. Given the fact that a new submersible pump is installed in a sewage pump station, what is the demanded ingress protection (IP) of the electrical motor?
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16. Gate valves are installed where they will have to be operated trequently. True or false?
17. Globe valves create high head loss when fully open. True or false?
18. Check valves are also commonly referred to as _____ or ______ valves
19. Relief valves are designed to control excess pressures that may harm rest of the network (ducts, pumps, etc). True or false?
20. A pressure-regulating valves keeps the ___________at a ____________ level.
21. If a piping system is subjected to depressures that may damage the network, which type of valve would you propose?
22. Static in-line mixers use moving parts. True or false?
23. What is the 2 types of aeration systems that can be used in the aeration tank of an activated-sludge process?
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24. Which are the three main categories of diffusers used?
25. Which type of diffusers generates larger bubbles ? Porous or nonporous?
26. Which type of blower can deliver higher discharge pressure applications? Centifugal or rotary-lobe positive displacement blowers?
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SUGGESTED ANSWARS:
1. High 2. Impeller eye 3. Static condtions , dynamic conditions 4. Total dynamic head 5. Head, Efficiency, Power & Net Positive suction head required (NPSHR) 6. Total static head 7. Electrical motor 8. Open, Semiopen, closed, vortex, axial flow, mixed flow 9. Large capacity pumps 10. Rotary screws pump 11. Centrifugal Dry type 12. Semi-open type 13. Closed 14. IP68 15. False 16. True 17. Nonreturn, reflux 18. True 19. Pressure ; constant/stable/fixed 20. Air relief valve 21. False 22. Mechanical aerators and dissolved aeration systems 23. (1) porous or fine-pore diffusers, (2) nonporous diffusers, and (3) other diffusion devices such as jet aerators, and aspirating aerators 24. Nonporous 25. Rotary-lobe positive displacement blowers
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