Control Valves and their Types
Control Valves and their Types
A technological process has a large numbers of control loops which are networked together for the process to function effectively and efficiently. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, and temperature etc. within a required operating range for ensuring the efficient working of the process. Each of these loops receives and internally creates disturbances which detrimentally affect the process variable, and interaction from other loops in the network provides disturbances which influence the process variable.
For reducing 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 is to be done to get the process variable back to where it is to be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element is to implement the strategy selected by the controller. The most common final control element used for the process control is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compound, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set-point. Fig 1 shows a typical feedback control loop showing a control valve.
Fig 1 Typical feedback control loop showing a control valve
The term control valve normally refers to the control valve assembly. The control valve consists of a valve connected to an actuator mechanism which is capable of changing the position of a flow controlling element in the valve in response to a signal from the controlling system. 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, and limit switches etc. The control valve, often referred to as ‘the final control element’, is a critical part of any control loop, as it performs the physical work and is the element which directly affects the process. It is a critical part of the control loop. The control valve is to be capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal from an external control device.
Control valves predominately regulate flow by varying an orifice size. As the fluid moves from the piping into the smaller diameter orifice of the valve, the velocity of the fluid increases in order to move a given mass flow through the valve. The energy needed to increase the velocity of the fluid comes at the expense of the pressure, so the point of highest velocity is also the point of lowest pressure (smallest cross section). This occurs some distance after leaving the smallest cross section of the valve itself, in a localized area known as the ‘vena contracta’. Beyond the vena contracta, the fluid’s velocity decreases as the diameter of piping increases. This allows for some pressure recovery as the energy which has been imparted as velocity is now partially converted back into pressure. There is a net loss of pressure due to viscous losses and turbulence in the fluid.
Factors determining selection of control valves
The operation parameters of control valves vary widely in terms of pressure, pressure drops, flow rates and temperature. In addition, other crucial factors like noise, cavitation, wire drawing, leakage class, and flashing also play an important part in the selection of control valves. Selection of a control valve body assembly requires particular consideration to provide the best available combination of valve body style, material, and trim construction design for the intended service.
Since there are frequently several possible correct choices of the control valve for an application, it is important to consider various features of the control valves before their selection. These features include (i) type of fluid to be controlled, (ii) temperature of fluid, (iii) viscosity of fluid, (iv) specific gravity of fluid, (v) flow capacity required (maximum and minimum), (vi) inlet pressure at valve (maximum and minimum), (vii) outlet pressure (maximum and minimum), (viii) pressure drop during normal flowing conditions, (ix) pressure drop at shut-off, (x) maximum permissible noise level, if pertinent, and the measurement reference point, (xi) flow action (flow tends to open valve or flow tends to close valve), (xii) inlet and outlet pipeline size and schedule, (xiii) special tagging information needed, (xiv) body material, (xv) trim and packing material, (xv) end connections and valve rating, (xvi) action desired on air failure (valve to open, close, or retain last controlled position), (xvii) instrument air supply availability, (xviii) instrument signal, (xix) valve body construction (angle, double port, and butterfly etc.), (xx) valve plug action (push-down-to-close or push-down-to-open), and (xxi) valve plug guiding (cage-style, port-guided etc.).
Types and classification of control valves
There are many types of valves available, each having their advantages and limitations. The basic requirements and selection depend on their ability to perform specific functions such as (i) the ability to throttle or control the rate of flow, (ii) lack of turbulence or resistance to flow when fully open since the turbulence reduces head pressure, (iii) quick opening and closing mechanism since a rapid response is many times needed in an emergency or for safety, (iv) tight shut off since it prevents leaks against high pressure, (v) the ability to allow flow in one direction only as it prevents return, (vi) opening at a pre-set pressure which is the procedure control to prevent equipment damage, and (vii) ability to handle abrasive fluids means it is to be made of hardened material to prevent rapid wear. Fig 2 shows a cross-section of sliding stem control valve and typical flow characteristics curves.
Fig 2 Sliding stem control valve and typical flow characteristics curves
The valve stroke for control valves means the amount of flow through a pipe, and valve travel means the amount in which the valve is turned to achieve the valve stroke. Based on valve strokes valves are of three types.
Equal percentage valve – Valve which produces equal valve stroke for equal increments in valve travel. This is the most common type of valve.
Linear valves – In this type of valve, the stroke is directly proportional to valve travel.
Quick opening valves – In this type of valves, a small amount of valve travel produces a large valve stroke.
Control valves are available with a wide variety of valve bodies in various styles, materials, connections and sizes. There are four general types of valves as given below.
Electronic or electrical valves – The movement of the ball or flap which controls the flow is controlled electronically through circuits or digitally. These types of valves have very precise control but can also be very expensive.
Non‐return valves – These valves allow flow in only one direction. When pressure in the opposite direction is applied, the valve closes.
Electro- mechanical valves – These valves have electro magnets controlling whether the valve is open or closed. These valves can only be fully open or fully closed.
Mechanical valves – These valves use mechanical energy in the process of opening and closing the actual valve. Larger valves can be opened and closed using mechanical processes such as levers and pulleys, whereas smaller mechanical valves can be opened or closed via a turning wheel or pulling a level by hand.
There are two main types of control valve designs, depending on the action of the closure member. These are namely (i) sliding-stem, or (ii) rotary. Sliding-stem valves use linear motion to move a closure member into and out of a seating surface. Rotary valves use rotational motion to turn a closure member into and out of a seating surface. In addition, control valves are also classified according to their guiding methods, characterization methods, and the nature of services they are applied within. Classification of control valves is shown in Fig 3.
Fig 3 Classification of control valves
Examples of linear control valves and rotary control valves are given in Fig 4 and Fig 5.
Fig 4 Linear control valves
Fig 5 Rotary control valves
Common type of control valves
There are a large number of valve types available for implementation into technological process systems. The four major types of the valves normally used in technological processes are (i) ball valves, (ii) butterfly valves, (iii) globe valves, and (iv) plug valves. There is also an range of many other types of valves specific to certain processes. Ball valves are good for on/off situations. They are easy to clean. A common use for a ball valve is the emergency shut off for a sink. Butterfly valve has good flow control at high capacities and is economical. Globe valve has good flow control but it is difficult to clean. Plug valve is suitable for extreme on/off situations. It is more rugged and costly than ball valve. The four main valve types are described below.
Ball valve – A ball valve is a valve with a spherical disc which is the part of the valve controlling the flow through it. The sphere has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow occurs. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked. There are four types of ball valves.
A full port ball valve has an over sized ball so that the hole in the ball is the same size as the pipeline resulting in lower friction loss. Flow is unrestricted, but the valve is larger. This is not required for general applications since all types of valves like gate valves, plug valves, and butterfly valves etc. which are normally used have restrictions across the flow and does not permit full flow. This leads to excessive costs for full bore ball valves which is generally an unnecessary cost.
In reduced port ball valves, flow through the valve is one pipe size smaller than the valve’s pipe size resulting in flow area becoming lesser than pipe. But the flow discharge remains constant as it is a multiplier factor of flow discharge (Q) is equal to area of flow (A) into velocity (V). A1V1 = A2V2. This means that the velocity increases with reduced area of flow and decreases with increased area of flow.
A V-port ball valve has either a ‘v’ shaped ball or a ‘v’ shaped seat. This allows the orifice to be opened and closed in a more controlled manner with a closer to linear flow characteristic. When the valve is in the closed position and opening is commenced, the small end of the ‘v’ is opened first allowing stable flow control during this stage. This type of design needs normally a more robust construction due to the higher velocities of the fluids, which quickly damages a standard valve.
A trunnion ball valve has a mechanical means of anchoring the ball at the top and the bottom, this design is generally applied on larger and higher pressure valves (say, above 100 mm and 40 kg/sq cm pressure).
Butterfly valves – Butterfly valves consist of a disc attached to a shaft with bearings used to facilitate rotation. The characteristics of the flow can be controlled by changing the design of the disk being used. For example, there are designs which can be used in order to reduce the noise caused by a fluid as it flows through. Butterfly valves are good for situations with straight flow and where a small pressure drop is needed. There are also high performance butterfly valves. They have the added benefit of reduced torque issues, tight shutoff, and very good throttling. It is necessary to consider the torque which acts on the valve. It has the fluid moving on both sides and when being used to throttle the flow through the valve it becomes a big factor. These valves are good in situations with high desired pressure drops. They are normally used because of their small size, which makes them a low cost control instrument. Some kind of seal is necessary in order for the valve to provide a leak free seal.
Globe valves – A globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. The valve can have a stem or a cage, similar to ball valves, which moves the plug into and out of the globe. The fluid’s flow characteristics can be controlled by the design of the plug being used in the valve. A seal is used to stop leakage through the valve. Globe valves are designed for easy maintenance. They normally have a top which can be easily removed, exposing the plug and seal. Globe valves are good for on, off, and accurate throttling purposes but especially for situations when noise and cavitation are factors.
Plug valves – Plug valves are valves with cylindrical or conically-tapered ‘plugs’ which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passage ways going sideways through the plug, so that fluid can flow through the plug when the valve is open. Plug valves are simple and often economical. There are two types of plug valves. One has a port through a cylindrical plug which is perpendicular to the pipe and rotates to allow the fluid to proceed through the valve if in an open configuration. In the closed configuration, the cylinder rotates about its axis so that its port is no longer open to the flow of fluid. An advantage of these types of valves is that they are excellent for quick shut-off. The high friction resulting from the design, however, limits their use for accurate modulating / throttling.
The other type of plug valve is the eccentric plug valve. In this design, the plug rotates about a shaft in a fashion similar to a ball valve. To permit fluid flow, the plug can rotate so that it is out of the way of the seat. To block fluid flow, it rotates to the closed position where it impedes fluid flow and rests in the seat.
In addition to the four main types of control valves, there are several other valves which can be necessary to manipulate fluid flow in a technological process system. Some important types of valves important to certain processes are given below with a brief description of their design and application.
Angle valves – These valves include inlet and outlet ports which are oriented at a 90 degree angle. The fluid leaves at right angles to the direction in which it enters the valve.
Bleed valves – These valves vent signal line pressure to atmosphere before removal of an instrument or to assist in calibration of control devices. Common bleed valves include ball and plug bleed valves.
Check valves – These valves are one way valves. Check valves only allow fluid in one way and out the other. They are often placed on individual fluid streams when mixing fluids so as to prevent the mixture from flowing back into the original streams. Also, the speed with which the valve closes is significant to prevent reverse‐flow velocity. There are different types of check valves which include lift check, swing check, tilting disk and diaphragm valves.
Lift check valves are quick closing. This valve is to be used only for low viscosity fluids since they can be slowed by viscous fluids. Swing check valve has a disc like closing method from a hinge which can or cannot be spring loaded. The tilting disk check valve is spring loaded for quick response. These are often more expensive and harder to fix when broken. Diaphragm valves have excellent shut‐off characteristics and are used when there is particulate matter in the fluids. Diaphragm valves are not a good choice for controlling flow. A diaphragm valve has both a flexible and a rigid section. One advantage is that there are no crevices which affect the flow of the fluid when open.
Piston valves – These valves have a closure member shaped like a piston. When the valve opens, no flow is observed until the piston is fully moved from the seat bore, and when the valve closes, the piston removes any solids which might be deposited on the seat. Hence, piston valves are used with fluids which have solid particles in suspension.
Gate valves – These valves work by raising a gate or barrier out of the way of the flowing fluid. The gate valve has the positive quality that, when fully open, the fluid flow is totally unobstructed. Two major types of gate valves are generally used. These are (i) parallel, and (ii) wedge gate valves. The wedge gate valve, in which the closure member has a wedge shape, provides the advantage of sealing against low pressure flow, as well as high pressure flow. Gate valves have the ability to open very quickly.
Advantages of gate valves are that they have a high capacity, have good seals, relatively inexpensive, and do not have very much resistance to flow. Some disadvantages of gate valves are that (i) they sometimes can have poor control, (ii) they can cavitate at lower pressures, and (iii) they cannot be used for throttling.
Needle valves – These valves are similar to gate valves. However, they are normally applied to a much smaller orifice. Needle valves are excellent for precise control of fluid flow, typically at low flow rates.
Flush bottom valves – These valves are generally at the lowest point of a tank or reactor and used to drain out contents. They are unique since it leaves no dead space in the valve when it is closed. This eliminates the problem of product build-up within the valve.
Pinch valves – These valves are mainly used in order to regulate the flow of slurries in certain processes and systems. Pinch valves have flexible bodies which can be shut by pinching them. These valves are often used when it is necessary for the slurry to pass straight through when the valve is not pinched. Pinch valves can be controlled mechanically or with fluid pressure.
Knife valves – These valves are used in systems which deal with slurries or powders. They are mainly used for on and off purposes; whether or not the slurry or powder flows or not. A knife gate valve can be used for fibrous material because it can cut through to close the valve.
Ball-cock valves – These valves are used in controlling levels in tanks. The valve is connected to a float in the tank using a lever. When the level in the tank rises, the float rises and forces the valve to be shut at the maximum level of the tank allowed. Ball-cock valves are used mostly in water tanks and other low‐risk areas within a certain process.
Solenoid valves – These valves are used very frequently for the technological processes. The valves have a loop of wire which is wrapped around a metal core. A current is passed through the valve creating a magnetic field, which in turn opens or closes plungers in pipelines controlling flow in the pipe. There are three types of solenoid valves namely (i) electro-mechanical solenoid valves which use electrical energy, (ii) pneumatic solenoid valves which use compressed air, and (iii) hydraulic solenoid valves which use energy from pressurized fluids.
The valves which come under the category of safety valves (Fig 6) are (i) pressure relief valve, (ii) steam traps, and (iii) other safety valves consisting of ‘rupture disc’ and ‘pressure vacuum valves’.
Fig 6 Safety valves
Pressure relief valves – Pressure relief valves are used as a safety device to protect equipment from over-pressure occurrences in a technological process. Loss of heating and cooling, mechanical failure of valves, and poor draining and venting are some of the common causes of over-pressure. The relieving system depends on the process at hand. Pressure relief valves either bypass the fluid to an auxiliary passage or open a port to relieve the pressure to the atmosphere. Technological processes operating at high pressure normally have several pressure relief valves to follow the safety codes and procedures specified for these processes. Each of the pressure relief valves has different levels of pressure ratings to release different amounts of material to the atmosphere in order to minimize environmental impact. Three examples of pressure relief valves are described below.
The first example is the ‘conventional spring loaded valve’. As the pressure rises, this causes a force to be put on the valve disc. This force opposes the spring force until the set pressure when the forces are balanced and the disc starts to lift. As the pressure continues to rise, the spring compresses more, further lifting the disc and relieve the higher pressure. As the pressure decreases, the disc returns to its normal closed state. The advantages of this type of valve are (i) versatility, and (ii) higher reliability. The disadvantages are (i) pressure relief is affected by the back pressure, and (ii) susceptible to chatter.
The second example is the ‘bellows spring loaded safety relief valve’. This valve has the same principle as the conventional spring valve, with the exception of a vent located on the side of the valve. This vent releases the contents of the valve out to the surrounding environment. The advantages of this type of valve are (i) pressure relief is not affected by back pressure, (ii) can handle higher built-up back pressure, and (iii) spring is protected from corrosion. The disadvantages are (i) bellows can be susceptible to fatigue, (ii) not environmentally friendly (can release of toxics into atmosphere), and (iii) needs a venting system.
The third example is the ‘pilot operated safety relief valve’. This valve is also similar to the conventional safety relief valve except a pneumatic diaphragm or piston is attached to the top. This piston can apply forces on the valve when the valve is closed to balance the spring force and applies additional sealing pressure to prevent leakage. The advantages of this type of valve are (i) pressure relief is not affected by back pressure, (ii) can operate at 98 % of set pressure, and (iii) less susceptible to chatter. The disadvantages are (i) pilot is susceptible to plugging, (ii) has limited chemical use, (iii) valve can have problems with the condensation, and (iv) potential for back flow.
Steam traps – Steam traps are devices which exist in low lying places within a pressurized steam line to release condensate and non-condensable gases from the system. Steam lines are normally used to open / close control valves, and heat trace pipelines to prevent cooling etc. These steam traps are generally used to save money on the prevention of corrosion and loss of steam. When these traps fail, it means loss of steam and hence the money. There can be several numbers of steam traps for a technological process, and hence it is important to maintain and check the condition of each trap at a planned interval. The checks can be done by visual, thermal, or acoustic techniques. There are many types of steam traps which differ in the properties they operate on including mechanical (density), temperature, and thermo-dynamic (pressure).
Other safety valves – These are (i) rupture disc, and (ii) pressure vacuum valves. A rupture disc (also called a ‘bursting disc’ or ‘safety disc’) is a thin membrane of material (usually metal) which acts as a one‐time use pressure relief device. At a critical pressure, the disc fails and ruptures allowing flow and the release of pressure. Often, rupture discs are used as a back‐up to a conventional spring‐controlled safety valve. Given primary safety valve failure (either no or inadequate pressure relief) the rupture disc bursts open and relieve pressure. The pressure vacuum (PV) valves protect against both the over‐pressure and the under‐pressure conditions. They are generally used in atmospheric storage tanks, to prevent the build-up of excessive pressure or vacuum which can dangerously unbalance the system or damage the storage vessel.
For all control valves, flow rates of fluid through the valve depend on the percentage of a full valve opening. In ball and butterfly valves, the valve opening is based on rotation. In the case for butterfly valves, an open valve is a result of a 90 degree rotation. When the valve is at the open position, the flow is parallel to the valve. The flow is uninterrupted, and hence, no pressure is on the valve. When the valve is throttling, as is common for globe valves, the flow rate is reduced or increased depending on the opening of the valve, and there is unequal pressure on the ends of the valve.
The ball valve flow stream involves at least two orifices which are one for inlet and one for outlet flow. While traveling through an open ball valve, the fluid continues in its flow straight through, with little pressure loss. When the ball valve is throttling, however, the fluid is subject to shearing and a change in flow rate in accordance with the percentage of the valve that is open. With high velocity liquids, the valve is susceptible to cavitation and erosion, and can produce noise. Also, at sonic velocities, the vena contracta stops expanding, and create choking.
Globe valves are dependent on shape of the plug of the valve, in addition to the opening size, for flow variations. Lifting the globe valve causes it to open. A flat plug is used for a quick opening while a conically shaped plug creates linear flow as the valve is raised, and a rectangular shaped plug, where the bottom converges to one point directly in the middle of the plug, creates an equal percentage of flow through the valve.
Actuator is the mechanical equipment which supplies the force necessary to open or close a control valve. Actuators can be pneumatic, motion conversion, hydraulic, or electric. Actuators are, essentially, the alternative to manual operation of a valve. The method of applying the opening / closing force to the valve is what differentiates the various types of actuators. When selecting the actuator, the most important feature to specify is whether the valve is to be fail-safe open or closed. This is determined entirely by a careful analysis of the process to decide which is safer. If all the power goes out or some other emergency occurs, the decision on the fail-safe mode of the valve is a huge factor in preventing accidents.
Pneumatic actuators – Pneumatic actuators are the most popular type of actuators. They have a delayed response which makes them ideal for being resilient against small upsets in pressure changes of the source. The standard design of a pneumatic actuator consists of a pre-compressed spring which applies force against a disk on a sealed flexible chamber. The disk is normally attached to the stem of the valve it is intended to control. As the chamber is compressed with air, the chamber expands and compresses the spring further, allowing axial motion of the valve stem. Knowing the relationship between the air pressure in the chamber and the distance the stem moves allows one to accurately control flow through the valve.
The biggest advantage of the pneumatic actuators is their fail-safe action. By design of the compressed spring, the engineer can determine if the valve is going to fail close or open, depending on the safety of the process. Other advantages include reliability, ease of maintenance, and widespread use of such devices.
Motion conversion actuators – These actuators are generally used to adapt a common translational motion from the actuator’s output to a rotary valve. The rod which moves axially from the translational motion actuator is connected to a disk and the connection is pivoted. The disk itself is also pivoted about its centre. This system of pivots allows the translational motion to be converted into the rotation of the disk, which opens or close the rotary valve.
The main advantage of this set-up is that an inexpensive translational motion actuator can be used with rotary valves. The key draw-back is that the applications in which this can be used is very limited. Specifically, this set-up is useless in the common case where the rotary motion required is greater than 90 degrees.
Hydraulic actuators – These actuators work using essentially the same principal as pneumatic actuators, but the design is usually altered. Instead of a flexible chamber, there is a sealed sliding piston. Also, instead of using a spring as the opposing force, hydraulic fluid is contained on both sides of the piston. The differential pressure across the area of the piston head determines the net force. Hydraulic actuators on the other hand use an incompressible fluid, so the response time is essentially instantaneous.
Hydraulic actuators offer the advantages of being small and yet still providing immense force. Draw-backs of hydraulic actuators are primarily the high capital cost and difficulty in maintaining them.
Electric actuators – These actuators typically use standard motors, powered by either AC induction, DC, or capacitor-start split-phase induction. The motor is connected to a gear or thread which creates thrust to move the valve. As a fail-safe, some motors are equipped with a lock in last position on its gear. This means that the gear cannot move from forces outside of the electric motor. This helps prevent overshoot on the motor as well as helps create better positioning for the gear.
Another type of motor which can be used is called a stepper motor. It uses increments on gear reduction to alleviate problems with positioning and over-shoot. The increments are in a range of 5,000 to 10,000 increments in a 90 degree rotation. A problem with using electric actuators is that a battery operated back-up system is needed or else the system is useless during power failure. Also, the actuator needs to be in an environment which is rendered safe, meaning a non-explosive environment.
Manual actuators – These actuators are normally used for overrides of power actuators described above. This is an important safety measure in case the power actuator fails. Manual actuators typically consist of either a lever or a wheel (used for larger valves) connected to a screw or thread which turns the valve.