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CONTROL VALVE
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1. Control Valves Introduction
-2-
2. Control Valve Terminology
-5-
3. FAIL-SAFE
-8-
4. Cavitation and Flashing
-9-
5. VALVE TYPES
- 13 -
6. CONTROL VALVE FLOW CHARACTERISTICS
- 28 -
7. ACTUATORS
- 33 -
8. VALVE ACCESSORIES
- 39 -
9. VALVE MATERIAL SELECTION
- 41 -
10. Valve Sizing
- 44 -
11. Self-operated pressure regulators
- 45 -
12. Industry Codes and Standards
- 47 -
13. REFERENCE
- 49 -
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1. CONTROL VALVES INTRODUCTION What Is A Control Valve? Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which can include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches. Other chapters of this handbook supply more detail about each of these control valve assembly components. Whether it is called a valve, control valve or a control valve assembly is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most important part of the loop. It is useful to think of a control loop as an instrumentation chain. Like any other chain, the whole chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link. There are many different types of final control element that are used to manipulate or control the flow of fluids. These elements can take the form of metering pumps, dampers or louvers, variable pitch fan blades or even speed regulators for engines, but, the most often used final control element is the control valve. A control valve acts as a variable resistance in a pipeline and provides a pressure drop, often referred to as throttling.
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Throughout their functional life, most control valves will be a certain percentage open for the purpose of throttling. However, there are occasions when flow has to stop; therefore a control valve must also be able to achieve a tight shut off condition. It should be noted now that subject of control valves and flowing fluid behavior has become a science within itself. There have been many books compiled by valve manufacturers and independent publishers alike, that cover all aspects of valve construction, valve characteristics, fluid behavior, valve sizing, valve noise and so on. Therefore it should be fully understood that this module is purely an introduction to the subject. Despite its wide use, there is probably no other element in a system which receives more abuse and less attention than a control valve. In most control systems, a control valve is subjected to more severe conditions of temperature, pressure, corrosion and contamination than other components, yet it still must perform satisfactorily with a minimum amount of maintenance, as it manipulates the flow of process fluids. The diagram below shows a typical pattern of pneumatically operated valve. Note that the control valve consists of two major components; the valve body assembly and the actuator. This style of valve falls into the category of sliding-stem valves. (The other group of valves being rotary shaft valves)
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Fig. 1.1- Typical control valve The operation of control valve is simple. Air pressure is applied to the top of the flexible diaphragm which exerts a downward force. The force moves the valve stem downwards against the restraining action of the spring. This downward movement will continue until either, the plug is fully mated with the seat, or, until the upward force of the spring (and
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flowing fluid pressure if valve is in service) equals the downward force of the actuator. At this point the valve stem stops moving.
2. CONTROL VALVE TERMINOLOGY Control valve terminology is quite extensive and can be best learned from manufacturers' handbooks and catalogues. However, the following paragraphs should be sufficient to give the reader some of the basic terms. 2.1. Valve Body The part of the valve that connects to the process piping and through which passes the flowing fluid, is called the valve body. The valve body must be able to withstand the same pressures and temperatures as the process piping. Small valves are connected to the piping by means of screwed threads, whereas larger valves are connected to the piping by means of flanges and welded end connections. Whatever the method of connection, these joints should be leak-free and the valve positioned so that it is easily accessible for maintenance purposes. 2.2. Bonnet The bonnet assembly is a metal casing that is threaded or bolted to the top of the valve body. It serves as a guide for the plug stem, houses the stem-seal and supports the actuator assembly. For extreme temperature service, extension or longer bonnets are used to prevent extremes of temperature damaging the sealing materials. 2.2.1. Bellows Seal Bonnet
A bonnet that uses a bellows for sealing against leakage around the closure member stem. 2.2.2. Extension Bonnet
A bonnet with greater dimension between the packing box and bonnet flange for hot or cold service. 2.3. Packing Box Assembly This is part of the bonnet assembly and is used to help prevent leakage around the valve plug stem. It allows the valve stem to move up and down with minimal leakage of the process fluid. All other valve connections, valve body to process pipework, valve body to bonnet for a little bit unclear example, are fixed or non-moving and are therefore easily gasketed. The valve stem however must move up and down. Packing rings within the packing box seal in the process fluid whilst allowing the valve stem to move with the minimum of friction. The packing rings themselves are usually made from a relatively soft and compressible material such as asbestos, Teflon, etc..
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The function of the packing or pressure gland or flange is to compress the packing rings tightly around the valve stem forming a leak proof seal. However, in normal operation of the control valve, it is not unusual to see some fluid leakage around the valve stem. With regular maintenance, the packing gland nuts can be adjusted to the correct pressure. Just tight enough to minimize leakage, but not so tight so as to cause stem binding.
Fig. 2.1 - Typical packing box
Fig. 2.2 - Typical non lubricated packing box
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2.4. Valve Trim In general terms, valve trim refers to the plug seat arrangement, but in a broader sense, trim refers to all those internal parts of the valve that come into contact with the process fluid. This would include the valve stem, valve plug (a device connected to the valve stem that controls fluid flow), seat ring (a ring that forms the valve body port), valve guides and bushings, (metal inserts that fit into the valve body and guide the moving parts), but excluding the valve body or bonnet assembly. Valve manufacturers often provide different sets of 'Trim' which can be fitted within a specific valve body. This enables a valve's characteristics to be changed without replacing the whole valve. 2.4.1. Soft-Seated Trim
The use of an elastomeric, plastic or other readily deformable material in the valve plug or seat ring to provide tight shutoff with reduced actuator forces. 2.4.2. Reduced Area Trim
Based on port area, full area and reduced area trims are available to reduce the capacity of a valve. Reduced trim may be achieved with a seat ring or cage change alone. In other designs, it may be necessary to change the plug and seat ring. 2.5. Yoke The Yoke is the upper structure of the valve installation which supports the actuator case assembly. It is rigidly fixed to the valve bonnet, usually by means of a large lock-nut. It's open frame provides access to the spring tension adjuster, valve stem and stem connector. It should also be noted that the Yoke must be strong enough to withstand any forces created by the actuator when it attempts to reposition the valve stem. Salt-water corrosion in offshore locations has been known to weaken the Yoke sufficiently to cause the Yoke to snap or break under normal working stress. 2.6. Actuator There are many different kinds of actuators (sometimes referred to as motor elements) used in industrial facilities. The type of actuator used in a specific plant application depends on many factors, including the process to be controlled, the action that is to be performed and the speed with which the action must occur. There are electrical and hydraulic actuators, but the spring and diaphragm actuator is by far the most common type of actuator used in automatic process control systems. Pneumatic actuators use air or gas pressure to produce mechanical motion. The motion produced is then used to position the controlling element anywhere within the actuator's limits of travel.
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2.7. Diaphragm Actuator A fluid powered device in which the fluid acts upon a flexible component. 2.7.1. Diaphragm
A flexible, pressure responsive element that transmits force to the diaphragm plate and actuator stem. 2.7.2. Diaphragm Case
A housing, consisting of top and bottom section, used for supporting a diaphragm and establishing one or two pressure chambers. 2.7.3. Diaphragm Plate
A plate concentric with the diaphragm for transmitting force to the actuator stem.
3. FAIL-SAFE Safety at any production facility is of prime importance, so the word FAIL-SAFE should be firmly embedded in the minds of anyone who is involved with the plant or production. Most pneumatic control valves and automatic shut-down valves have spring-loaded actuators. When the loading pressure is relieved, the spring will move the final control element (valve) to one of its extreme positions, either, fully open or fully closed, the position being determined by the control valve construction. A 'Normally Open' control valve is one which opens when the diaphragm pressure is reduced to atmospheric. (Fig. 3.1) A 'Normally Closed' control valve is one which closes when the diaphragm pressure is reduced to atmospheric. (Fig. 3.2)
So, a Normally Closed valve is a Fail Close Valve, or, Air to Open. A Normally Open Valve is a Fail Open Valve, or, Air to Close. The criteria for the selection of either one depends on which action will make the process safe should there be a loss of pneumatic supply or damage to the actuator, such as a torn or split diaphragm. If such a condition occurred the valve would return to its 'Normal' or Fail-safe position of either full open or full closed. Any process operator new to a facility that uses pneumatically operated control valves, should try to familiarize himself with the Fail-safe action of those valves. "Will a particular valve fail open",or "will it fail close?" If the operator is familiar with these points, then, in the event of an emergency, he will have a much clearer idea of how the plant and process will respond and therefore will himself be able to react or respond more readily with corrective action if necessary.
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Total Electrical & Instrumentation Services
Fig. 3.1 – Fail open valve
Fig. 3.2 - Fail close valve
4. CAVITATION AND FLASHING Choked Flow Causes Flashing and Cavitation
Liquid sizing standards calculate an allowable sizing pressure drop, ΔP max. If the actual pressure drop across the valve, as defined by the system conditions of P1 and P2, is greater than ΔP max then either flashing or cavitation may occur. Structural damage to the valve and adjacent piping may also result. Knowledge of what is actually happening within the valve will permit selection of a valve that can eliminate or reduce the effects of cavitation and flashing. The physical phenomena label is used to describe flashing and cavitation because these conditions represent actual changes in the form of the fluid media. The change is from the liquid state to the vapor state and results from the increase in fluid velocity at or just downstream of the greatest flow restriction, normally the valve port. As liquid flow passes through the restriction, there is a necking down, or contraction, of the flow stream. The minimum cross-sectional area of the flow stream occurs just downstream of the actual physical restriction at a point called the vena contracta, as shown in figure below.
To maintain a steady flow of liquid through the valve, the velocity must be greatest at the vena contracta, where cross sectional area is the least. The increase in velocity (or kinetic energy) is accompanied by a substantial decrease in pressure (or potential energy) at the vena contracta. Further downstream, as the fluid stream expands into a larger area, velocity decreases and pressure increases. But, of course, downstream pressure never recovers completely to
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equal the pressure that existed upstream of the valve. The pressure differential (ΔP) that exists across the valve is a measure of the amount of energy that was dissipated in the valve. Figure below provides a pressure profile explaining the differing performance of a
streamlined high recovery valve, such as a ball valve, and a valve with lower recovery capabilities due to greater internal turbulence and dissipation of energy. Regardless of the recovery characteristics of the valve, the pressure differential of interest pertaining to flashing and cavitation is the differential between the valve inlet and the vena contracta. If pressure at the vena contracta should drop below the vapor pressure of the fluid (due to increased fluid velocity at this point) bubbles will form in the flow stream. Formation of bubbles will increase greatly as vena contracta pressure drops further below the vapor pressure of the liquid. At this stage, there is no difference between flashing and cavitation, but the potential for structural damage to the valve definitely exists. If pressure at the valve outlet remains below the vapor pressure of the liquid, the bubbles will remain in the downstream system and the process is said to have flashed. Flashing can produce serious erosion damage to the valve trim parts and is characterized by a smooth, polished appearance of the eroded surface, as shown in figure below.
Flashing damage is normally greatest at the point of highest velocity, which is usually at or near the seat line of the valve plug and seat ring. On the other hand, if downstream pressure recovery is sufficient to raise the outlet pressure above the vapor pressure of the liquid, the bubbles will collapse, or implode, producing cavitation. Collapsing of the vapor bubbles releases energy and produces a noise similar to what one would expect if gravel were flowing through the valve. If the bubbles collapse in close proximity to solid surfaces in the valve, the energy released will gradually tear away the material leaving a rough, cinderlike surface as shown in figure below.
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Cavitation damage may extend to the adjacent downstream pipeline, if that is where pressure recovery occurs and the bubbles collapse. Obviously, high recovery valves tend to be more subject to cavitation, since the downstream pressure is more likely to rise above the liquid’s vapor pressure. 4.1. Valve Selection for Flashing Service In general erosion is minimized by: _ preventing or reducing the particle (liquid droplets in this case) impact with the valve surfaces. _ making those surfaces as hard as possible _lowering the velocity of the erosive flow Selecting a valve with as few fluid directional changes as possible provides the least number of particle impacts. Sliding-stem angle valves are traditional solutions which provide such a flow path. Some rotary valves, such as eccentric rotary plug, and V− ball valves, also offer straight through flow paths. Valves with expanded flow areas downstream of the throttling point are beneficial because the erosive velocity is reduced. For those areas where the fluid must impact the valve surfaces, at the seating surfaces for example, choose materials that are as hard as possible. Generally the harder the material the longer it will resist erosion. Fluids that are both flashing and corrosive can be especially troublesome. Flashing water in a steel valve is an example of the synergistic result of both corrosion and erosion. The water causes corrosion of steel and the flashing causes erosion of the resultant, soft, oxide layer; these combine to create damage worse than either individual mechanism would. The solution in this case is to prevent the corrosion by selecting, as a minimum, a low-alloy steel. 4.2. Valve Selection for Cavitation Service The first is to eliminate the cavitation and thus the damage by managing the pressure drop. If the pressure drop across the valve can be controlled such that the local pressure never drops below the vapor pressure, then no vapor bubbles will form. Without vapor bubbles to collapse, there is no cavitation. To eliminate cavitation the total pressure drop across the valve is split, using multiple-stage trims, into smaller portions. Each of these small drops keeps its vena contracta pressure above the vapor pressure so no vapor bubbles are formed. The second method does not eliminate the cavitation but rather minimizes or isolates the damage much the same as with flashing solutions. This method aims to isolate the cavitation from valve surfaces and to harden those surfaces that the cavitation does impact. A third method is to
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change the system in a manner to prevent the causes of cavitation. If the P2 can be raised enough so that the vena contracta pressure does not fall below the vapor pressure, that is the valve is no longer choked, then cavitation will be avoided. P2 can be raised by moving the valve to a location that has more static head on the downstream side. Applying an orifice plate or similar backpressure device can also raise P2 at the valve; the downside is the potential for the cavitation to transfer from the valve to the orifice plate. 4.3. Noise Control In closed systems (not vented to atmosphere), any noise produced in the process becomes airborne only by transmission through the valves and adjacent piping that contain the flowstream. The sound field in the flowstream forces these solid boundaries to vibrate. The vibrations cause disturbances in the ambient atmosphere that are propagated as sound waves. Noise control employs either source treatment, path treatment, or both. Source treatment, preventing or attenuating noise at its source, is the most desirable approach, if economically and physically feasible. Recommended cage-style source treatment approaches are depicted in figure below.
The upper view shows a cage with many narrow parallel slots designed to minimize turbulence and provide a favorable velocity distribution in the expansion area. This economical approach to quiet valve design can provide 15 to 20 dBA noise reduction with little or no decrease in flow capacity. _ Complementary body designs prevent flow impingement on the body wall and secondary noise sources.
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5. VALVE TYPES 5.1. Single Ported Valves The term 'Single-ported' refers to the fact that a single path exists for passage of fluid through the valve. The diagrams on the following pages show two forms of single ported valves which are quite clearly two similar units (Fig. 5.1 & 5.2). However, one obvious difference is that the plugs are reversed with respect to one another. Notice that with one valve, the downward movement of plug causes the valve to close (Fig. 5.1), whilst with the other; a downward movement of the plug causes the valve to open (Fig. 5.2). The valve that has a downward motion to close, is a direct acting valve and the valve that has a downward motion to open is a reverse acting valve. Compared to Double Ported Valves, Single Ported Valves are generally cheaper to purchase and due to the single plug and seat area, are more resistant to leakage when a tight shut-off is required. There is a major disadvantage to single ported valves however, and that is when dealing with high fluid pressures that create unbalanced forces across the plug when it is in the closed position. Larger actuators are used to overcome these forces. Single ported valves are usually installed so that the fluid pressure tends to force the plug away from the seat. This results in smoother operation and reduces the tendency of the plug to slam shut against the seat. 5.2. Double-Ported Valves The double ported valve was developed to provide a valve that would require less force to position its plug to any position between full open and full closed than was required by the single ported design (Fig. 5.3). With the double ported design, dynamic forces tend to be more in balance due to the fact that flow tries to open one port and close the other. These reduced dynamic forces allow better control of the valve and permit choosing a smaller actuator than would be used on a single ported valve of the same capacity. Many double ported valves are also reversible. (i.e. a downward movement of the plug to close or a downward movement of the plug to open.) The major disadvantage of this valve is that it cannot provide such good 'shut-off' as the single ported valve. This is due to the problems of uniform alignment and of course wear on 'two' sets of plugs and seats.
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Fig. 5.1 - Single port unbalanced reversible plug valve
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Fig. 5.2 - Single port valve with plug reversed
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Fig. 5.3 - Double port semi balanced valve
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5.3. Globe Style The most common control valve body style is in the form of a “globe”, globe valves are often Particularly used in sizes from 1” to 4”, and can be either single or double seated as described below: Port guided valve plugs (single port) are often used on on-off or low pressure throttling services and top and bottom guided valve plugs (double port) furnish stable operation for severe service conditions 5.3.1. Single Port Globe
Advantages a) High rangeabilitiy b) Generally specified for application with stringent shutoff requirement metal to metal seating surfaces or "Soft- seating" with nitrile or other elastomeric materials forming the seal, can handle most service equipments. c) Reversible plugs available d) Most common body style, simple in construction e) Since high- pressure fluid is normally loading the entire area of the port, the unbalance force created must be considered in selecting actuator for single- port control valve bodies. f) Many modern single-seated valve bodies use cage-style construction to retain the seat ring, provide valve plug guiding, and provide a means for establishing a particular flow characteristic. Cage- style trim offers advantages in ease of maintenance and interchangeability of cages to alter valve flow characteristic. g) Cage-style single-seated valve bodies can also be easily modified by change of trim parts to provide reduced-capacity flow, noise attenuation, reduction or elimination of cavitation. Disadvantages a) Unbalanced design requires relatively large actuator. b) Low pressure recovery characteristic. 5.3.2. Double Port Globe
Advantages a) High flow capacity comparing to single port valves of the same size. b) High rangeability
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c) Balanced design requiring smaller actuator in comparison to single port design. d) Reversible port available ( valve plug can be installed as either "push- down-to-open" or" push-down-to-close") e) Usually furnished in the sizes of 6 inch or larger. Disadvantages a) Relatively high leakage rated at shut off, for (0.5% of rated capacity), when valve is new or not worn.
metal
to
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seating
b) Low pressure recovery characteristic. c) Erosion may occur on high pressure drop applications due to leakage charactristic. d) Not good for high flow with low pressure drop application 5.3.3. Three-Way Valve Body
Three-way valves are a design extension of a typical double-ported globe valves. These valves may be used for diverting services or mixing combining services (Fig. 5.5). Advantages a) Three pipeline connections to provide general converging (flow-mixing) or diverging (flow-splitting) services. b) Variations include trim material selected for high temperature service. Standard connections (flanged, screwed, butt weld, etc.) can be specified to mate with almost any piping scheme. Disadvantages b) Actuator selection demands careful consideration, particularly for constructions with unbalanced valve plug. c) Cannot control total flow
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Fig. 5.4- Three Way Valve with Balanced Valve Plug
5.3.4. Angle Valve
A valve design in which one part is co-linear with the valve stem or actuator, and the other part is at a right angle to the valve stem. Angle valves are nearly always single ported (Fig. 5.6). Advantages a) They are commonly used in boiler feed water and heater drain services and in piping schemes where space is at a premium and the valve can also serve as an elbow. b) A typical angle (y-type) valve with streamlined body interior passage tends to prevent an accumulation of solid on the body wall. c) Some angle valves have a self draining design which is suitable for handling of certain dangerous fluids such as radioactive liquids is required. d) May be used in cases where the piping layout doesn't allow the installation of globe valve. e) May be used for handling certain erosive fluids such as abrasive catalyst material. f) Can handle sludges and slurries
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Disadvantages Energy conversion takes place in downstream pipe and occasionally leading to severe pipe vibration and noise problem.
Fig. 5.5- Angle-Style Control Valve Body
5.4. ROTARY SHAFT VALVES Butterfly valves, ball valves and …, fall into this category. The trim element is 'rotated' by the actuator. 5.4.1. Butterfly Valve
The butterfly valve is basically a circular metal disc which is fitted in a short flangeless pipe-spool. The disc is rotated by a shaft which is coupled to an actuator. Butterfly valves require minimum space for installation and provide high capacity with low pressure loss through the valve. However, they often require high-output piston
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actuators or larger diaphragm actuators since operating torques can be quite high. The use of soft seating materials such as T.F.E. or Nitrile provide for good shut off service. Conventional discs provide throttling control for up to 60° rotation and show equal percentage flow characteristics. Flangeless Valve Valve style common to rotary-shaft control valves. Flangeless valves are held between ANSIclass flanges by long through-bolts (sometimes also called wafer-style valve bodies). The diagram overleaf shows a typical butterfly valve installation showing the valve mounted between two flanges (Fig. 5.7). Advantages a) Require minimum space for installation. b) Provide high capacity with low pressure loss through the valve. c) Very economical, particularly in larger sizes and in terms of flow capacity per investment. d) The flangeless types are mate with standard raised-face pipeline flanges e) Readily available in large sizes. f) Standard liner can provide good shut off and corrosion protection with nitrile or TFE liner. Disadvantages a) May require high output or large actuators if the valve is big or the pressure drop is high, since operating torques may be quite large. b) Tight shutoff depends on use of resilient seats which are temperature limited. c) Throttling control on some designs is limited to 60° travel.
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Fig. 5.6 - Butterfly valve in flangeless spool 5.4.2. Ball Valve
A typical ball valve houses a sphere or ball that contains a circular port which is normally equal in size to the bore of the process pipe. The ball can be rotated through 90° from its "full-open" position to its 'fully closed" position by a drive shaft coupled to an actuator. The ball is in continuous contact with a sealing ring which provides a tight shut off. Ball valves are commonly used as "shut down" valves, being held either fully open or fully
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closed. But, a modified ball valve with a "Vee" shape cut into the circular port will provide equal percentage flow characteristics and is ideal for controlling the flow of viscous fluids that contain entrained solids or fibers. Flow has a tendency to rotate the ball to the closed position; therefore the actuator must counteract this effect. Full bore ball valve The flow controlling member of rotary shaft control valves use a complete sphere with a flow passage through it. The flow passage equals or matches the pipe diameter (Fig. 5.8). Advantages a) High capacity b) Good rangeability c) Reasonable cost d) Good for slurry service e) Control and shutoff capability Disadvantages a) Limited operating pressure b) Not good for high pressure drop c) Not good for various characteristics (quick opening only). V-Notch Ball Valve Body Modified ball valve with a “V” shape cut into the circular port will provide equal percentage flow and is Ideal for controlling the flow (Fig. 5.9). Advantages a) Suited to control of erosive or viscous fluid, paper stock or other slurries containing entrained solids or fiber. b) Uses standard diaphragm or piston rotary actuators, or conventional linear travel actuators with minor modifications. c) Ball remains in contact with seal during rotation, which produces a shearing effect as the ball closes and minimizes clogging. d) Available with either metal or heavy-duty TFE filled composition ball seal ring to provide excellent rangeability in excess of 300:1
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e) Bolts between ANSI class flanges with long through-bolts. f) The V-notch produces an equal percentage flow characteristic. g) Available in flangeless or flanged body end connections. Both flanged and flangeless valve mate with class 150, 300 or 600 ANSI or DIN flanges. Disadvantages a) Straight-through flow design produces little pressure-drop b) Limited operating pressure
Fig. 5.7 – Full bore ball valve plan view
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Fig. 5.8 – V-Notch Ball valve
5.4.3. Rotary Eccentric Plug Valves
The operation of this valve is based on an eccentrically rotating spherical plug contained within the valve body. The plug is attached to a drive shaft which is rotated through 50° by a lever Iinked to an actuator. When moving from open to closed position, the leading edge of the plug passes the seat by a very close margin. As it continues to rotate, the trailing edge comes into contact with the seat. At this point the leading edge is almost in contact with the seat, but not quite. Further rotation and application of stem torque by the actuator cause the plug arms to flex and forces the leading edge of the disc into contact with the seat. Flow through the valve can be in either direction, but, it must be noted that depending on the direction of flow, the dynamic forces will tend to open or close the valve. Therefore the flow direction, through the valve is determined by its fail-safe requirements, i.e. if it is desired to fail-open then the fluid flow is in the direction which will tend to open the valve.
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Fig. 5.9 - Rotary eccentric plug valve
Fig. 5.10 - Rotary eccentric plug valve and actuator
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This control valve makes exaggerated use of the offset center, as used in butterfly valve designs, to obtain contact at closure without rubbing. The seat portion of the plug has the form of a spherical segment which is rotated 50 degrees for maximum opening. The support arms for the plug flex upon closure to cause tighter contact with the seat upon increase in actuator force. Advantages a) Valve assembly combats erosion. The rugged body and trim design handle temparatures to 430°c and shutoff pressure drops to 1500 PSI (103 bar) . b) Path of eccentric plug minimizes contact with the seat ring when opening, reducing seat wear and friction, prolonging seat life, and improving throttling performance. c) Self – Centering seat ring and rugged plug allow forward or reverse flow with tight shutoff in either direction. Plug, seat ring and retainer are available in hardened materials, including ceramics, for selection of erosion resistance. 5.4.4. Eccentric- Disc Control Valve Body
The operation of this valve is based on an eccentrically rotating disk contained within the valve body. They are frequently applied in applications requiring large size and high temperature, (540°C) due to their lower cost relative to other styles of control valves. (See fig. 5.12) Advantages a) Effective throttling control for pressure drops up to 740(psig) or 51 (bar). b) Provides linear flow characteristic through 90 degrees of disc rotation. c) Eccentric mounting of disc pulls it away from seal after it begins to open, minimizing seal wear. d) Available in sizes through 24-inch compatible with standard ANSI flanges. e) Uses standard pneumatic diaphragm or piston rotary actuators. f) Standard flow direction is into the concave side of the disc : Reverse flow results in reduced capacity.
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Fig. 5.11 - Rotary Eccentric- Disc Control Valve Body
6. CONTROL VALVE FLOW CHARACTERISTICS The flow characteristic of a valve refers to the relationship between the fluid flow through the valve and the percentage travel or movement of the valve plug stem. (valve lift). Valve plugs are produced in a variety of shapes and forms, each with a particular flow characteristic in mind. They can be designed to produce all forms of flow characteristics from "on - off" service to any desired form of throttling action, the choice of which depends upon the process to be controlled. To generalize, there are three main types of flow characteristics two of which have relevant modifications. (See fig. 6.1 overleaf). A - Quick opening. B - Linear. C - Equal percentage.
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Fig. 6.1 - Flow characteristics for control valves 6.1. Quick Opening The curve shows that a maximum flow rate change occurs with a relatively small stem travel, up to 35% and thereafter, there is little increase in flow as the stem approaches its wide open position. This characteristic is required in on-off or two position control valves, self-actuated valves, regulators and relief valves. Some examples of quick opening plugs are shown below.
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Fig. 6.2 - Quick opening plugs 6.2. Linear The linear curve shows that the flow rate is directly proportional to the valve travel throughout the travel range, for example. at 70% of travel, the flow is nearly 70% of maximum. This characteristic is used in liquid level control valves and in control systems requiring a constant gain, that is, the gain in the proportional increase or decrease in flow for a given controller output. The figure below shows some examples of linear plugs in single or double ported valves.
Fig. 6.3 - Linear plugs
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6.3. Equal Percentage An equal percentage characteristic is one in which equal increments of stem travel produce equal percentage changes in flow rate. The actual curve (Fig. 6.1 flow characteristic curves for control valves), shows that when the valve is nearly closed, the change in flow rate with respect to travel is small, but is relatively high when the valve is nearly fully open. In practical terms this means that the valve will give accurate throttling control over its lower travel range and rapidly increasing capacity as it approaches the wide open limit. The shape of a typical equal percentage curve is due to the fact that for every % of valve stem travel, the rate of flow through the valve increases by a given percentage increment. A typical example is that for every 10% change in valve stem position, the rate of flow through the valve changes by 50%. Relating to Fig. 6.1 it can be seen that if the valve stem is at 30% of its travel there will be approximately 9% of maximum flow through the valve. If the stem travel is increased by a further 10%, the flow rate will increase by approximately 50%, in this example, to 13% of maximum flow. A further 10% change of valve stem travel will result in another 50% change in the maximum rate of flow. This time the flow rate is approximately 18% of maximum flow. A further 10% change of valve stem position will result in the flow rate changing by a further 50% and so on. The diagram below shows three forms of equal percentage plugs which are normally used for pressure control in process systems where only a small proportion of the total pressure drop is available for control purposes.
Fig. 6.4 - Equal percentage plugs
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6.4. Cage Trim Valves The flow characteristics so far studied have been determined by shaping the plugs so that the unobstructed flow area changes in size and shape as the plug moves through its travel range. In cage trim valves the characteristic is determined by shaping the cage ports or 'windows' to produce quick opening, linear or equal percentage curves. The characteristic for any selected valve is changed by installing a different cage making it unnecessary to change the plug to alter the flow characteristic. The following diagram shows the shapes of cage windows used to give quick opening, linear and equal percentage flow characteristics.
Fig. 6.5 - Cage trim
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7. ACTUATORS Spring loaded pneumatically operated actuators are the most commonly used motor elements for the movement of control valve plugs. This is basically due to the relatively simple construction. It is totally non electrical so is therefore intrinsically safe. It is proven to be operationally reliable and is easy to maintain. 7.1. Single Acting Diaphragm Actuators Actuator designs do vary from one manufacturer to another, but the principle of operation is similar in each case. A pneumatic pressure signal is applied to one side of a flexible diaphragm. (Remember that Force = Pressure × Area). The resultant force created pushes the diaphragm, diaphragm plate, and actuator stem in one direction. As it does so, it is being opposed by an actuator spring. The greater the pressure applied, the greater the stem movement and the more the spring opposes by being compressed. When the force of the diaphragm in one direction, equals the force of the spring in the opposite direction, the actuator stops moving. If the pneumatic pressure is relieved, the force of the spring will be greater and therefore push the diaphragm, diaphragm plate and hence the actuator-stem back to its "normal" position. Actuators can also be direct or reverse acting. Loading pressure forcing the diaphragm down, is regarded as direct-acting, whereas loading pressure forcing the actuator up, is regarded as reverse-acting. This may now lead to some confusion. Direct or reverse acting Valves? Now direct or reverse acting actuators? Remember, that a Control valve consists of both valve and actuator. Increasing pressure to a control valve causing it to close, is direct acting. Increasing pressure to a control valve causing it to open, is reverse acting. Therefore a direct acting valve with a direct acting actuator is a direct acting control valve. However, a reverse acting valve with a reverse acting actuator will also be "air to close", therefore is regarded as a direct acting control valve. Any other combination will produce a reverse acting control valve. The most important factor to remember is the valve's "Normal" position or it's Fail-Safe action. e.g. When there is a loss of pneumatic signal, will the valve fail to the open or closed position? Operating pressures for diaphragm actuators are normally between 0.2-1 bar, (3-15 psi) or 0.4-2 bar (6-30 psi)
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Fig. 7.1 - Direct acting actuator
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Fig. 7.2 - Reverse acting actuator
7.2. Piston Actuators Piston actuators are more ruggedly constructed than diaphragm actuators. This enables them to be operated at much higher pressures so therefore capable of delivering a much greater force that may be required to overcome large pressure drops across control valves. Operating pressures of around 7-10 bar (100-150 psi ) are quite typical. Another advantage to piston actuators is that they can be manufactured to produce a substantially longer stroke than could be achieved from a diaphragm actuator. Shutdown valves with piston actuators are generally spring opposed. Air pressure moves the piston against spring opposition. When the pressure is relieved, the spring returns the actuator to its "normal" position. It should be noted that some shutdown systems use a double acting piston, that is, one that is moved in either direction by pneumatic pressure. An accumulator or air reservoir is then used to move the actuator in the event of air supply failure. Control valves that make use of piston actuators are not normally spring loaded. As a rule they will also be double acting and will include a valve positioner. They make ideal OnOff valves and when a positioner is used, provide accurate positioning.
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Beware! Without an opposing spring, there can be no FAIL-SAFE position unless there is some form of pneumatic accumulator incorporated into the air system.
Fig. 7.3 - Pneumatic piston actuator
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7.3. Manual Actuators With some control valve installations hand wheels are fitted onto the actuators for the purpose of manually positioning the final control element. There are several circumstances where a hand wheel would be used. For example: • • • •
During plant start-up. During an emergency. On failure of pneumatic supply to the actuator. Where the control valve is not provided with a by-pass valve.
Hand wheels can either be top mounted or side mounted. Top mounted hand wheels on direct acting diaphragm actuators can be used as adjustable stops to limit the upward direction, or to manually close "push-down" to close valves. On reverse acting diaphragm actuators the unit can be used as an adjustable travel stop in the downward direction or to manually close "push-down" to open valves. Side mounted actuators can be used with either direct or reverse acting actuators. This unit can be used to limit the travel of the actuator in either direction or to position the valve manually. But, it should be noted that for normal service in automatic control, any hand wheel should be positioned at its "NEUTRAL" position to enable the controller to effectively control the system.
Fig. 7.4 - Top mounted handwheels
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Fig. 7.5 - Side mounted handwheel
7.4. Electrical Actuators Traditional electric actuator designs use an electric motor and some form of gear reduction to move the valve. Through adaptation, these mechanisms have been used for continuous control with varying degrees of success. To date, electric actuators have been much more expensive than pneumatic for the same performance levels. This is an area of rapid technological change, and future designs may cause a shift towards greater use of electric actuators.
Fig. 7.6 – Electric Actuator
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8. VALVE ACCESSORIES 8.1. VALVE POSITIONERS A control valve must be capable of responding quickly and smoothly to control signal changes. In many instances a properly sized valve with a properly sized actuator will do an adequate job without the use of a valve positioner. However there are certain instances where a valve positioner should be considered.
These are: •
Where the required diaphragm pressure is greater than the controller signal pressure.
•
To "Split-Range" the controller output to more than one valve.
• Where distances between controller output and valve actuator are long, thereby creating a delay between the time of controller output change and subsequent reaction of the actuator. When a valve positioner is not used, the controller output is connected directly to the actuator. Any change in controller output pressure must be effective over the diaphragm surface area. If the volume of the actuator is large, then it may take some while for the actuator to react. When a positioner is used, the controller output goes to a bellows within the positioner. The volume of the bellows is small compared to the actuator. Hence, a small change in controller output is more readily detected by the bellows. The motion produced by the bellows is used to move a pilot valve or flapper (if using a flapper-nozzle system,) which in turn causes a large volume of air to rapidly enter the actuator thereby ensuring that it responds rapidly to control signal changes. From the foregoing, it can be seen that a valve positioner is capable of converting small signal changes into a very powerful force for positioning the final control element. But, it should be noted and clearly understood, that in some loops, the use of a valve positioner would most surely lead to control loop instability. Such systems are those which exhibit fast response times such as flow control loops, liquid pressure control loops or some gas pressure control loops. In these situations a 'booster' may be considered as an alternative. If the system is relatively slow, such as a liquid-level control loop or a temperature control loop, then a valve positioner will perform quite satisfactorily.
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Fig. 8.1 - Control valve and positioner 8.2. Limit Switches Limit switches operate discrete inputs to a distributed control system, signal lights, small solenoid valves, electric relays, or alarms. An assembly that mounts on the side of the actuator houses the switches. Each switch adjusts individually and can be supplied for either alternating current or direct current systems. Other styles of valve-mounted limit switches are also available.
8.3. Solenoid valves A solenoid valve is a combination of two basic functional units: a valves and a solenoid. (Electro magnetic device) there are several types: direct action, internal pilot operated, external pilot operated, two way, three way and four way, Figures 8.5 show several types of solenoid valves. They come in different enclosures to suit various electrical classifications, and there are many variations relative to electrical characteristics as well as mechanical and operating characteristics. The function of solenoid valve is to provide an on-off switching option in the
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system. They accomplish in a pneumatic or liquid system what an electrical relay accomplishes in an electrical system. They are frequently used in conjunction with control valves to open or close the valve at predetermined conditions or limits. A solenoid valve utilizes an electromagnetic coil to actuate an armature or valve stem in a magnetic field to control fluid flow. Solenoid valves are either fully opened or fully closed and are actuated by electric signals from remote locations. When electrical power is supplied to the electromagnet, a magnetic field is built in the valve coil, which causes the solenoid to be positioned in the coil. The plunger is connected to a valve disc which opens or closes the orifice dependent on valve actuation-whether the valve is energized to open or energized to close . 8.4. Pressure supply regulator with filter and moisture trap Supply pressure regulators, commonly called airsets, reduce plant air supply to valve positioners and other control equipment. Common reducedair-supply pressures are 20, 35 and 60 psig. The regulator mounts integrally to the positioner, or nipple-mounts or bolts to the actuator. Pressure in the event of supply pressure failure. These devices can be used with volume tanks to move the valve to the fully open or closed position on loss of pneumatic air supply. Normal operation resumes automatically with restored supply pressure. Functionally similar arrangements are available for control valves using diaphragm actuators.
9. VALVE MATERIAL SELECTION 9.1. General - Material selection should be guided by the piping specification and by the process conditions. - If uncertainty about material selections exists, the final material selection should be made in consultation with those specializing in material science or with the control valve supplier. - For valves in a flammable hydrocarbon service, body materials such as plastic, cast iron, bronze, and aluminum should be avoided so that process integrity can be maintained in an emergency situation involving an external fire.
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9.2. Valve Body - The valve body material should be dictated by the piping specification but will commonly be WCB or WCC carbon steel or ASTM type 300 series stainless steel (SS). - Valves should be designed to meet the design pressure and temperature. - All materials used in the valve should be compatible with the process for normal and abnormal conditions. - Table 1 provides metallurgy guidance at different design temperatures. - NACE MR-01-75 should be used when selecting material for processes containing hydrogen sulfide (H2S). This NACE standard provides metallic material requirements for resistance to sulfide stress cracking. - Physical effects of turbulence, fluid impingement, flashing, erosion, cavitation, changes in flow, pressure, temperature occasioned by maintenance (e.g., when line is steamed out), and other abnormal operating conditions may necessitate a different material for the valve body than for the piping in which it is installed. Corrosion, which may have minimal (or acceptable) effects on the pipe, may be exacerbated in the valve body. Passivation films that form under relatively quiescent conditions may be worn away by high-velocity fluids
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9.3. Valve Trim General - The effects of wear, galling, erosion, and corrosion are more pronounced on valve trim than on the valve body. To minimize these effects and to simultaneously control cost, a valve trim of a different metallurgy than that of the valve body may be utilized. - Standard valve trim, generally 316 SS, should be given first consideration. Higher alloy grades (e.g., 416 SS, 17-4 PH SS) have a proven history in severe service applications. These alloys are offered as standard on some valves. - Valves in cavitating or flashing services, those containing erosive or solid-bearing fluids, or those in high-pressure applications should be specified with trim having a hardness of at least 38 Rc (hardness Rockwell C). - The following guidelines are not exhaustive but are generally accepted. These guidelines should be compared with actual experience because the issues that impact trim material selection are complex.
9.3.1. Series 300 and 400 Series SS (for further information)
- Series 300 and 400 series SS are widely used and are frequently available as standard offerings. - The 300 series, e.g., Types 304 and 316 are relatively ductile and resistant to many types of corrosion. The 300 series metals cannot be hardened by heat treatment. The relative softness of 300 series metals, compared with Types 410 and 416, makes them less desirable in erosive, wear-producing applications. - The 400 series SS are generally less corrosion resistant than are the 300 series SS. - For general service, the 300 and 400 series SS can be applied in processes from -20°F to 650°F. Refer to valve manufacturer’s literature for limitations. As an example, cavitation trim typically has a more limited range. - Assuming compatibility with the chemistry of the process, types 300 and 400 components can be plated or hard-faced with materials to increase their wear resistance. 9.3.2. Material Overlays (for further information)
- Hard-facing material includes stellite No. 6 (CoCr) and tungsten carbide. - Hard-facing is effective in erosive applications, in steam and water applications in which pressure drops exceed 50 psi, and in general applications in which pressure differentials are greater than 500 psi. - The facing material can wear or corrode away over time, leaving the base, softer material unprotected.
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9.3.3. Other Materials (for information)
- Type 17-4 PH SS metallurgy may be specified for components such as valve plugs, cages, and guide bushings requiring greater strength, hardness, and galling resistance. - Valve trim parts may be specified with more exotic metallurgy while using a less expensive material for the body. For example, trim parts of Monel™ or Hastelloy®, Moneil™, stellite, and 17-4 PH SS can be utilized with carbon steel bodies if the process contains trace quantities of hydrofluoric acid, sulfuric acid, or dry chlorine gas or where the process is a maximum of 20% caustic material. - Valve trim selection may also depend on the valve body geometry. For example, streamlined bodies having high-pressure recovery coefficients are more likely to exhibit cavitation. Valves having high-pressure recovery coefficients, such as rotary valves, could require special or hardened trim. 9.3.4. Cast Carbon Steel, ASTM A216 Grade WCC (for information)
WCC is the most popular steel material used for valve bodies in moderate services such as air, saturated or superheated steam, non-corrosive liquids and gases. WCC is not used above 800_F (427_C) as the carbon rich phase might be converted to graphite. It can be welded without heat treatment unless nominal thickness exceeds 1 1/4 inches (32 mm). 9.3.5. Cast Type 304L Stainless Steel, ASTM A351 Grade CF3 (for information)
This is a good material offering for chemical service valves. 304L is the best material for nitric acid and certain other chemical service applications. Optimum corrosion resistance is retained even in the as-welded condition. 9.3.6. Cast Type 316 Stainless Steel, ASTM A351 Grade CF8M (for information)
This is the industry standard stainless steel body material. The addition of molybdenum gives Type 316 greater resistance to corrosion, pitting, creep and oxidizing fluids compared to 304. It has the widest temperature range of any standard material: −425_F (−254_C) to 1500_F (816_C). The rough castings are heat treated to provide maximum corrosion resistance. 9.3.7. Cast Iron (ASTM A126)
Cast iron is an inexpensive, non-ductile material used for valve bodies controlling steam, water, gas and non-corrosive fluids.
10. VALVE SIZING Standardization activities for control valve sizing can be traced back to the early 1960’s when a trade association, the Fluids Control Institute, published sizing equations for use with both compressible and incompressible fluids. The range of service conditions that could be
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accommodated accurately by these equations was quite narrow, and the standard did not achieve a high degree of acceptance. In 1967, the ISA established a committee to develop and publish standard equations. The efforts of this committee culminated in a valve sizing procedure that has achieved the status of American National Standard. Later, a committee of the International Electro technical Commission (IEC) used the ISA works as a basis to formulate international standards for sizing control valves. (Some information in this introductory material has been extracted from ANSI/ISA S75.01 standard with the permission of the publisher, the ISA.) Except for some slight differences in nomenclature and procedures, the ISA and IEC standards have been harmonized. ANSI/ISA Standard S75.01 is harmonized with IEC Standards 534-2-1 and 534-2-2.(IEC Publications 534-2, Sections One and Two for incompressible and compressible fluids, respectively.)
11. SELF-OPERATED PRESSURE REGULATORS Self-operated pressure regulators are control devices which measuring units draw their energy from the process medium which creates sufficient force to move the final control element. The regulators consist of a valve and an actuator which either opens or closes the valve when the pressure increases. The regulators are proportional regulators controlled by the process medium. Each deviation from the adjusted set point is assigned a certain valve plug position. 11.1. Pressure reducing valves Pressure reducing valves or pressure reducing stations withdraw as much energy from a pressure vessel with a higher pressure level as needed to maintain a nearly constant pressure level in downstream equipment, although consumption fluctuates. The pressure p2 to be controlled (controlled variable x) produces the force Fm = P2.A, which is proportional to the controlled variable, on the diaphragm area A. This force corresponds to the actual value and is compared at the plug stem with the spring force FS = set point w. FS is adjustable at the set point adjuster. If the pressure pS changes, and in this way also the force Fm, the valve plug is being adjusted until Fm = FS. In the version shown in Fig. 10.1, the valve closes when the pressure to be maintained constant rises. The regulator, in this case a pressure reducing valve, adjusts the pressure p2 downstream of the valve to the value adjusted at the set point adjuster.
11.2. Excess pressure valve The controlled variable p1 is picked up in the valve body and applied to one side of the actuator disc. The force of the actuator Fx = p1 .A is compared via the plug stem to the force FS = set point w of the set point spring. In steady state (x = w) Fx = FS. If the pressure p1 increases, the actuator force increases and the travel of the plug increases against the force of the set point spring. This causes the outlet flow to increase and the pressure p1 to decrease until a new equilibrium is reached between actuator and spring force. In the version shown in Fig. 10.2, the valve opens when the pressure to be maintained constant rises. The regulator, in this case an excess pressure valve, adjusts the pressure p1 upstream of the valve to the value adjusted at the set point adjuster.
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Fig. 11.1 Pressure reducing valve The valve closes when the downstream pressure rises (p2 > p1)
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Fig. 11.2 Excess pressure valve The valve opens when the upstream pressure rises (p1 > p2)
1 Valve body 2 Valve seat 3 Plug 4 Plug stem 6 Set point adjuster 7 Positioning spring 8 Actuator
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12. INDUSTRY CODES AND STANDARDS American Petroleum Institute (API) – API 609 - Butterfly Valves: Double Flanged, Lug- and Wafer-Type American Society for Mechanical Engineers (ASME) – ASME B31.3 - Process Piping American Society for Testing and Materials (ASTM) – ASTM A193 - Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High-Temperature Service – ASTM A194 - Standard Specification for Carbon and Alloy Steel Nuts for High- Pressure or High-Temperature Service, or Both – ASTM A216 - Standard Specification for Steel Castings, Carbon, Suitable for Fusion Welding, for High-Temperature Service – ASTM A320 - Standard Specification for Alloy/Steel Bolting Materials for LowTemperature Service – ASTM A351 - Standard Specification for Castings, Austenitic, Austenitic-Ferric (Duplex), for Pressure-Containing Parts – ASTM A352 - Standard Specification for Steel Castings, Ferritic and Martensitic for Pressure-Containing Parts, Suitable for Low-Temperature Service – ASTM A217 - Standard Specification for Steel Castings, Martensitic Stainless and Alloy, for Pressure-Containing Parts, Suitable for High-Temperature Service The Instrumentation, Systems, and Automation Society (ISA) – ANSI/ISA 75.11 - Inherent Flow Characteristic and Rangeability of Control Valves – ISA RP75.23 - Considerations for Evaluating Control Valve Cavitation – ANSI/ISA 75.25 - Control Valve Dynamic Testing NACE International – NACE MR-01-75 - Sulfide Stress Cracking Resistant Metallic Materials for Oil Field Equipment
Government Regulations U.S. Department of Labor, Occupational Safety and Health Administration (OSHA) – OSHA 1910.95 - Occupational Noise Exposure
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U.S. Environmental Protection Agency (EPA) – Clean Air Act (CAA), Section 112, National Emission Standard for Hazardous Air Pollutants – HON Rule - Valves in Gas/Vapor Service and in Light Liquid Service
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13. REFERENCE 1. CONTROL VALVE HANDBOOK - Fourth Edition www.EmersonProcess.com/Fisher 2. WWW.MASOLENIAN.COM 3. PROCESS CONTROL HANDBOOK B.LIPTAK 4. APPLIED INSTRUMENTATION IN THE PROCESS INDUSTRIES W.G. ANDREW H.B.WILLIAMS 5. CONTROL VALVE SELECTION AND SIZING DRISKELL 6. WWW.SAMSON.DE 7. WWW.Pfeiffer-Armaturen.COM
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