Flow measurment (water and wastewater)/ Mjer. protoka vode i otpadnih voda by AG-Metal Zenica

Flow Measurement

Water is the best of all things. Pindar (C. 522–C. 438 B.C.E) Olympian Odes Flow is measured…one cannot afford not to measure…because the competitive environment dictates that one does “not” have the luxury of producing “wastefully.” Someone is always watching; if not the plant manager, then the vice-president of finance; if not the plant neighbors, then the EPA; and, of course, there are laws of physics to keep everyone honest.1

9.1 INTRODUCTION Flow is one of the most difficult variables to measure accurately. If we wanted to use an approximate (but very simple) method to determine open channel discharge, we would measure the velocity of a floating object moving in a straight uniform reach of the channel. If we know the cross-sectional geometry of the channel and the depth of flow is determined, we can compute the flow area. From the relationship Q = A ¥ V, we can estimate the discharge Q. The average velocity of flow in a reach is approximated by timing the passage of the floating object along a measured length of channel. While it is a useful way to obtain a ballpark estimate for the flow rate as part of a preliminary field study, this technique is not suitable for routine measurements required in water and wastewater treatment plant operations. Another simple (but impractical) method of determining flow is the weight per unit time method. This method assumes a basic premise of fluid mechanics: mass is a conserved quantity. Simply, the mass entering a system is equal to the mass leaving the system when both are measured over the same time interval. Using the weight per unit time method to measure flow requires catching the flow in a container and weighing it over a given interval of time. The impracticality of using this method in water and wastewater operations can be seen in closed-loop processes commonly associated with chemical applications. Consequently, other methods must be used to obtain flow measurements. Any seasoned water and wastewater operator knows that flow measurement is an essential part of water and wastewater treatment. Unit processes are designed for specific flow levels, and process adjustments (e.g., adjustments made on pumping rates, chlorination rates, filter rates, aeration rates, etc.) are based upon current levels of flow; in many cases, they are controlled by flow rate

adjustments. Accurate flow measurement is a key element in any attempt to identify, correct, and prevent operational problems. Therefore, it is important to operators tasked with operating the plant at optimal efficiency. In this chapter, we briefly discuss methods of measuring flow, many of the calculations used to determine flow, and various flow measurement problems.

9.2 METHODS OF MEASURING FLOW Measuring the flow in water and wastewater operations requires a thorough, detailed understanding of the process and the substance being measured. Two factors that determine the method of flow measurement and the flowmeter most suited to an application are the quantity of the flow and the type of substance being measured. We already mentioned one of the methods of measuring flow, using the Q = A ¥ V formula method. When using this method, we measure the velocity of the flow and the channel width/water depth first, then the formula is used to find flow rate. This procedure can be used in any location where you can measure the water crosssectional area and velocity. Note: The flow rate of a substance can be described using a number of terms including feet per second, gallons per minute, cubic feet per minute, and tons per hour. The unit chosen to indicate flow rate is an important factor in flow measurement applications, and varies according to the indicating requirements specific to the process. Another simple method that can be used to measure flow is known as the fill and draw method. Accomplished by measuring the amount of time required to transfer or pump a given volume of water from one point to another, use the fill and draw method at any location where changes in liquid volume (or depth) can be measured. From the description of the two rudimentary measurement techniques just described, we can see that even in those cases where flow measurement is provided, some method can be found to measure — or at least estimate — flow rates. However, the majority of water and wastewater operators do not measure flow by the two methods. Instead, modern treatment plants and current practices normally include the use of other methods. For example, the bucket and stopwatch technique has been replaced with other methods, up to and including complex electronic systems.

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Rectangle weir

Head Weir crest Crest length

Triangular weir V-notch angle

Head Weir crest FIGURE 9.1 (A) Rectangular weir; (B) triangular V-notch weir. (From Spellman, F.R. and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

9.2.1 WEIRS The simplest, least expensive, and probably the most common type of primary measuring device used to measure flow in open channels is the weir. A weir is simply a dam, rectangular obstruction, or V-notch crest (over which water flows), placed in the channel so that the water backs up behind it and then flows over it (see Figure 9.1-A and Figure 9.1-B). For open channel measurements, weirs can be used for rectangular and circular clarifiers (see Figure 9.2). The crest or edge allows the water to spring clear of the weir plate and to fall freely into the air. Each type of weir has an associated equation for determining the flow rate through the weir. The equation is based on the depth of the liquid in the pool formed upstream from the weir. Simply, in measuring, measure the head a specified distance behind the weir (constriction) in the channel, and then use the head to calculate the flow or to determine the flow using a table or graph. The edge or surface over which the water passes is called the crest of the weir, as shown in Figure 9.1-A. Generally, the top edge of the weir is beveled with a sharp upstream corner (aptly called sharp-crested weirs) so that the liquid does not contact any part of the weir structure downstream, but rather springs past it. This stream of water leaving the weir crest is called the nappe. A disadvantage of a weir is the relatively dead water space that occurs just upstream of the weir where organic solids may settle out, causing odors. Note: The operation of the weir is sensitive to any foreign material or debris that may be present ÂŠ 2003 by CRC Press LLC

Flow Rate (gpd)

Weir

Flow Rate (gpd)

Weir FIGURE 9.2 (A) Weir overflow for rectangular clarifier; (B) weir overflow for circular clarifier. (From Spellman, F.R. and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

upstream of the flowmeter or on the weir plate itself. Therefore, the weir should be periodically inspected and any accumulated debris removed. This action will also reduce organic settling, and thus reduce odors.

9.2.2 THE OSCILLATING DISK WATER METER Many individual household and apartment dwellers are familiar with the oscillating disk water meter, even if they do not think they are. They at least they know that there is a meter that is read by the utility routinely and that their water bill is based on that meter reading.

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This standard household water meter is known as the oscillating disk water meter, which is a common positivedisplacement meter. The meter has a measuring chamber of known volume containing a disk that goes through a cyclic motion as water passes through. A recording register, which can be mounted exterior to the house, records the rotation resulting from filling and emptying of the chamber. This type of meter is very reliable, simple in construction, highly sensitive, accurate, and has low maintenance costs.

Again, these characteristics make it particularly suitable for flow measurement in wastewater operations. The flume is also used in water flow measurement applications.

9.2.4 VENTURI METER

Note: For customers requiring high flow rates, another type of water meter is used: the compound meter. The general-service compound meter consists of a positive-displacement, current meter with an automatic valve arrangement that directs water to the current meter during high rates of flow and to the displacement meter at low rates.

A venturi is a restriction with a relatively long passage with smooth entry and exit. The venturi meter is another flow measuring device found in pipe systems in wastewater collection systems and treatment plants. It utilizes the principle of differential pressure — the flow must pass through a section with a smaller diameter in a device. The change in pressure that occurs while passing through the smaller diameter section is related to the rate of flow through the pipe. It is often used in wastewater streams since the smooth entry allows solids to be swept through instead of building up as it would in front of an orifice.

9.2.3 FLUMES

9.2.5 MAGNETIC FLOWMETER

Besides the weir, another device commonly used to measure flow in open channels is the flume. Figure 9.3 shows a Parshall flume — the most commonly used measuring device. The Parshall flume is named after Dr. Ralph L. Parshall formerly of the U.S. Soil Conservation Service. In 1922, Dr. Parshall modified the existing venturi flume design. This perfected device measures the head a specified distance behind the narrow point (throat of the flume), then the head measurement is used to calculate the flow or the flow is determined by using a table or graph. Because wastewater contains suspended and floating solids, it prohibits the use of enclosed meters. This is where the flume comes in. The principal advantages of the Parshall flume are its:

The magnetic meter (magmeter) is another flow measuring device commonly used to measure flow of the wastestream through pipes. In operation, wastewater is passed between the poles of a magnet. The flow creates an electrical current that the meter measures. The amount of current produced is related to the amount of flow. With obstructionless design, there are no moving parts to wear and no pressure drop other than that offered by a section of pipe with equal length and inside diameter. Moreover, the magmeter has the advantages of a linear output, corrosion-resistant wetted parts, and highly accurate output. “The greatest disadvantage of this type of meter is its initial cost and the need for trained personnel to handle routine operation and maintenance.”2 Table 9.1 provides a list of many different types of methods and devices applicable to fluid flow measurement.

1. Capabilities for self-cleaning (i.e., its design and smooth construction does not offer any place where solids may collect behind the metering device) 2. Relatively low head loss 3. Ability to function over a wide operating range while requiring only a single head measurement. Converging inlet

Diverging outlet Throat

Flow

Top view Stilling well for measuring hear FIGURE 9.3 Parshall flume. (From Spellman, F.R. and Drinan, J., Water Hydraulics, Technomic Publ., Lancaster, PA, 2001.)

9.3 FLOW MEASUREMENT CALCULATIONS While it is true that flow can be measured electronically or by using various tables or charts, water and wastewater operators should be skilled in making flow computations. Even with the use of a chart or graph, appropriate conversions and calculations are required. In this section, we discuss the calculations required to determine flow rates using the fill and draw, V-notch weir, and the Parshall flume. We also provide a few simple flow calculation problems.

9.3.1 CALCULATION METHOD USED AND DRAW TECHNIQUE

FOR

FILL

The mathematical procedure for determining flow in gallons/minute is:

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TABLE 9.1 Types of Flow Measurement Devices Typical Usea

Principle of Flow Measurement

Elbow meter Rotameter Pitot tube venturi meter Acoustic meter Turbine meter Electromagnetic meter Orifice meter Flow nozzle meter

w w w ww ww ww ww ww ww

For Pressure Pipes The differential pressure is measured around a bend The rise of float in a tapered tube is measured The differential pressure is measured The differential pressure is measured The sound waves are used to measure the velocity A velocity driven rotational element (turbine, vane, wheel) is used Magnetic field is induced and voltage is measured The differential pressure is measured The differential pressure is measured

Flumes Weirs Pitot tube Current meter Acoustic meter Depth measurement

ww ww w w ww ww

For Open Channels Critical depth is measured at the flume Head is measured over a barrier (weir) The differential pressure is measured Rotational element is used to measure velocity The sound waves are used to measure velocity and depth Float is used to obtain the depth of flow

Devices

w = water application; ww = wastestream application

Source: Adapted from Qasim, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic Publ., Lancaster, PA, 1994, p. 221. With permission.

Flow (Q [gal min]) =

(

( )

Tank Volume ft 3 ¥ 7.48 gal ft 3

9.3.2 CALCULATION METHOD USED VELOCITY/AREA TECHNIQUE

where H = Head in feet K = Constant related to the weir angle

FOR

For determining flow in cubic feet per second or gallons per minute using the velocity/area technique, the following equations can be used:

)

Q ft 3 sec = Channel Width (ft ) ¥ Water Depth (ft ) ¥ V (ft sec)

(9.2)

Q (gal min) =

(

)

(9.3)

1.55 ft sec MGD ¥ 1440 min d

Weir

Constant

22.5∞ 30∞ 45∞ 60∞ 90∞ 120∞

0.497 0.676 1.035 1.443 2.500 4.330

9.3.4 WEIR OVERFLOW (WEIR LOADING RATE)

Q ft 3 sec ¥ 1, 000, 000 3

(9.4)

(9.1)

Time Re quired (min)

(

)

Q ft 3 sec = K ¥ H 2.5

Weir overflow rate is the effluent flow rate expressed in gallons per linear foot of weir per day (see Figure 9.2). It can be used to evaluate actual operating conditions by comparing current values with design specifications.

Weir Overflow =

Q (gal d ) Weir Length (ft )

(9.5)

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EXAMPLE 9.1

EXAMPLE 9.2

Problem:

The circular settling tank is 90 ft in diameter and has a weir along its circumference. The effluent flow rate is 2.50 MGD. What is the weir overflow rate in gallons per day in feet?

A grit channel 3 ft wide has water flowing to a depth of 16 in. If the velocity through the channel is 0.8 ft/sec, what is the ft3/sec flow rate through the channel?

Solution: Solution: Weir Overflow =

3 ft ¥ 1.3 ft ¥ 0.8 ft sec = 3.12 ft 3 sec

2.50 ¥ 1, 000, 000 gal MG 3.14 ¥ 90 ft

EXAMPLE 9.3

= 8846.4 gal d ft

9.3.5 CALCULATION METHOD

FOR

Problem:

PARSHALL FLUME

As with the V-notch weir, calculating flow through a Parshall Flume requires the use of charts or graphs and appropriate conversions. Then, the following equations may be used.

(

60 sec min ¥ 1440 min d = 3, 150, 576 gal d

)

Q ft sec = K ¥ H

(9.6)

A grit channel 35 in. wide has water flowing to a depth of 9 in. If the velocity of the water is 0.80 fps, what is the ft3/sec flow in the channel?

0.9920 2.060 3.070

Solution:

n = exponent constant related to throat width 3 in. 6 in. 9 in.

1.547 1.580 1.530

(

)

(9.7)

where W = Throat width L = W0.026

9.3.6 TYPICAL FLOW MEASUREMENT PRACTICE CALCULATIONS The answers to the examples provided in this section are derived using equation standard Q = AV, including conversions where appropriate. © 2003 by CRC Press LLC

2.9 ft ¥ 0.75 ft ¥ 0.80 ft sec = 1.74 ft 3 sec

9.4 FLOW MEASUREMENT OPERATIONAL PROBLEMS

2. For flume throats 1 to 8 ft wide Q ft 3 sec = 4 ¥ W ¥ H1.522 L

EXAMPLE 9.4 Problem:

where K = Constant related to the throat width 3 in. 6 in. 9 in.

Solution: 3 ft ¥ 1.25 ft ¥ 1.3 ft sec ¥ 7.48 gal ft 3 ¥

1. For flume throats less than 12 in. wide 3

A grit channel 3 ft wide has water flowing at a velocity of 1.3 ft/sec. If the depth of water is 15 in., what is the gal/d flow rate through the channel?

Operators are often responsible for troubleshooting flow measurement problems. Our experience indicates that flow measurement problems (indicative of problems with the flow measuring method or device used) typically fall into two categories: (1) a sharp drop or increase in recorded flow, or (2) inconsistent or inaccurate flow measurement using a weir. A number of causes could be responsible for a sharp drop or increase in recorded flow. For example, the problem could be caused by an obstruction to the float (if used). Removing the obstruction and/or keeping the float clean and free of grease correct this problem. Another type of flow measuring device may malfunction because of improper airflow or a damaged bubbler tube. In correcting this problem, the bubbler tube should be cleaned, the airflow adjusted, and grease removed from the assembly.

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In another type of measuring device, the problem might be indicative of grease buildup on magnetic meter coils. Solving this problem is a simple matter of removing grease buildup. A weir plate clogged with debris could also cause a sharp drop in recorded flow. To correct this problem, debris should be removed and the frequency of weir cleaning should be increased. Inconsistent or frequent inaccurate flow measurement using a weir usually indicates that the weir is not level and needs to be adjusted.

9.5 CHAPTER REVIEW QUESTIONS AND PROBLEMS 9.1. Why are flow measurements important? 9.2. A grit channel 2.5 ft wide has water flowing to a depth of 18 in. If the velocity of the water is 0.8 ft/sec, what is the ft3/sec flow in the channel? 9.3. A grit channel is 2.5 ft wide with water flowing to a depth of 15 in. If the flow velocity

ÂŠ 2003 by CRC Press LLC

through the channel is 1.6 ft/sec, what is the gal/min flow through the channel? 9.4. A grit channel 3 ft wide has water flowing to a depth of 10 in. If the velocity through the channel is 1 ft/sec, what is the ft3/sec flow rate through the channel? 9.5. If you had the choice of installing a weir system or a Parshall flume flow measurement device in your plant, which one would you choose or prefer? Why?

REFERENCES 1. Spitzer, D.W., in Flow Measurement: Practical Guides for Measurement and Control, Spitzer, D.W., Ed., Instrument Society of America, Research Triangle Park, NC, 1991, p. 3. 2. Qasim, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic Publ., Lancaster, PA, 1994, p. 226.