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Control valves can be grouped into two major classifications: process and fluid power. Process valves control the flow of gases and liquids through a process system. Fluid-power valves control pneumatic or hydraulic systems.
Process-control valves are available in a variety of sizes, configurations, and materials of construction. Generally, this type of valve is classified by its internal configuration.
The device used to control flow through a valve varies with its intended function.
The more common types are ball, gate, butterfly, and globe valves.
Ball valves are simple shut-off devices that use a ball to stop and start the flow of fluid downstream of the valve. As the valve stem turns to the open position, the ball rotates to a point where part or the entire hole machined through the ball is in line with the valve-body inlet and outlet. This allows fluid to pass through the valve. When the ball rotates so that the hole is perpendicular to the flow path, the flow stops.
Most ball valves are quick-acting and require a 90-degree turn of the actuator lever to fully open or close the valve. This feature, coupled with the turbulent flow generated when the ball opening is only partially open, limits the use of the ball valve. Use should be limited to strictly an ''on-off '' control function (i.e., fully open or fully closed) because of the turbulent- flow condition and severe friction loss when in the partially open position. They should not be used for throttling or flow-control applications.
Ball valves used in process applications may incorporate a variety of actuators to provide direct or remote control of the valve. The more common actuators are either manual or motor-operated. Manual values have a hand wheel or lever attached directly or through a gearbox to the valve stem. The valve is opened or closed by moving the valve stem through a 90-degree arc.
Motor-controlled valves replace the hand wheel with a fractional horsepower motor that can be controlled remotely. The motor-operated valve operates in exactly the same way as the manually operated valve.
Gate valves are used when straight line, laminar fluid flow, and minimum restrictions are needed. These valves use a wedge-shaped sliding plate in the valve body to stop, throttle, or permit full flow of fluids through the valve. When the valve is wide open, the gate is completely inside the valve bonnet. This leaves the flow passage through the valve fully open with no flow restrictions, allowing little or no pressure drop through the valve.
Gate valves are not suitable for throttling the flow volume unless specifically authorized for this application by the manufacturer. They generally are not suitable because the flow of fluid through a partially open gate can cause extensive damage to the valve.
Gate valves are classified as either rising stem or non rising stem. In the non rising-stem valve, the stem is threaded into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads, while the stem remains vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate.
Valves with rising stems are used when it’s important to know by immediate inspection if the valve is open or closed, or when the threads exposed to the fluid could become damaged by fluid contamination. In this valve, the stem rises out of the valve bonnet when the valve is opened.
The butterfly valve has a disk-shaped element that rotates about a central shaft or stem. When the valve is closed, the disk face is across the pipe and blocks the flow. Depending on the type of butterfly valve, the seat may consist of a bonded resilient liner, a mechanically fastened resilient liner, an insert-type reinforced resilient liner, or an integral metal seat with an O-ring inserted around the edge of the disk.
---2 Non-rising-stem gate valve.
---3 Rising stem gate valve.
As shown, both the full open and the throttled positions permit almost unrestricted flow. Therefore, this valve does not induce turbulent flow in the partially closed position. While the design does not permit exact flow-control capabilities, a butterfly valve can be used for throttling flow through the valve. In addition, these valves have the lowest pressure drop of all the conventional types.
For these reasons, they are commonly used in process-control applications.
The globe valve gets its name from the shape of the valve body, although other types of valves also may have globular-shaped bodies. ---5 shows three configurations of this type of valve: straight- flow, angle- flow, and cross- flow.
A disk attached to the valve stem controls flow in a globe valve. Turning the valve stem until the disk is seated, which is illustrated in View A of ---6, closes the valve. The edge of the disk and the seat are very accurately machined to form a tight seal. It’s important for globe valves to be installed with the pressure against the disk face to protect the stem packing from system pressure when the valve is shut.
---4 Butterfly valves provide almost unrestricted flow.
---5 Three globe valve configurations: straight-flow, angle- flow, and cross- flow.
---6 Globe valve.
While this type of valve is commonly used in the fully open or fully closed position, it also may be used for throttling. However, since the seating surface is a relatively large area, it’s not suitable for throttling applications in which fine adjustments are required.
When the valve is open, as illustrated in View B of ---6, the fluid flows through the space between the edge of the disk and the seat. Since the fluid flow is equal on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the valve seat.
The globe valve should never be jammed in the open position. After a valve is fully opened, the hand wheel or actuating handle should be closed approximately one-half turn. If this is not done, the valve may seize in the open position, making it difficult, if not impossible, to close the valve. Many valves are damaged in this manner. Another reason to partially close a globe valve is that it can be difficult to tell if the valve is open or closed. If jammed in the open position, the stem can be damaged or broken by someone who thinks the valve is closed.
Process-control valves have few measurable criteria that can be used to deter mine their performance. Obviously, the valve must provide a positive seal when closed. In addition, it must provide a relatively laminar flow with minimum pressure drop in the fully open position. When evaluating valves, the following criteria should be considered: capacity rating, flow characteristics, pressure drop, and response characteristics.
The primary selection criterion of a control valve is its capacity rating. Each type of valve is available in a variety of sizes to handle most typical process- flow rates.
However, proper size selection is critical to the performance characteristics of the valve and the system in which it’s installed. A valve's capacity must accommodate variations in viscosity, temperature, flow rates, and upstream pressure.
The internal design of process-control valves has a direct impact on the flow characteristics of the gas or liquid flowing through the valve. A fully open butterfly or gate valve provides a relatively straight, obstruction-free flow path. As a result, the product should not be affected.
Control-valve configuration impacts the resistance to flow through the valve.
The amount of resistance, or pressure drop, will vary greatly, depending on type, size, and position of the valve's flow-control device (i.e., ball, gate, disk).
Pressure-drop formulas can be obtained for all common valve types from several sources (e.g., Crane, Technical Paper).
With the exception of simple, manually controlled shut off valves, process control valves are generally used to control the volume and pressure of gases or liquids within a process system. In most applications, valves are controlled from a remote location through the use of pneumatic, hydraulic, or electronic actuators. Actuators are used to position the gate, ball, or disk that starts, stops, directs, or proportions the flow of gas or liquid through the valve. Therefore the response characteristics of a valve are determined, in part, by the actuator. Three factors critical to proper valve operation are response time, length of travel, and repeatability.
Response time is the total time required for a valve to open or close to a specific set-point position. These positions are fully open, fully closed, and any position in between. The selection and maintenance of the actuator used to control process-control valves have a major impact on response time.
Length of Travel
The valve's flow-control device (i.e., gate, ball, or disk) must travel some distance when going from one setpoint to another. With a manually operated valve, this is a relatively simple operation. The operator moves the stem lever or hand wheel until the desired position is reached. The only reasons a manually controlled valve won’t position properly are mechanical wear or looseness between the lever or hand wheel and the disk, ball, or gate. For remotely controlled valves, however, there are other variables that directly affect valve travel. These variables depend on the type of actuator that is used. There are three major types of actuators: pneumatic, hydraulic, and electronic.
Pneumatic actuators-including diaphragms, air motors, and cylinders-are suitable for simple on-off valve applications. As long as there is enough air volume and pressure to activate the actuator, the valve can be repositioned over its full length of travel. However, when the air supply required to power the actuator is inadequate or the process-system pressure is too great, the actuator's ability to operate the valve properly is severely reduced.
A pneumatic (i.e., compressed air-driven) actuator is shown. This type is not suited for precision flow-control applications, because the compressibility of air prevents it from providing smooth, accurate valve positioning.
Hydraulic Actuators Hydraulic (i.e., fluid-driven) actuators, can provide a positive means of controlling process valves in most applications. Properly installed and maintained, this type of actuator can provide accurate, repeatable positioning of the control valve over its full range of travel.
Some control valves use high-torque electric motors as their actuator. If the motors are properly sized and their control circuits are maintained, this type of actuator can provide reliable, positive control over the full range of travel.
---7 Pneumatic or hydraulic cylinders are used as actuators.
---8 High-torque electric motors can be used as actuators.
Repeatability is perhaps the most important performance criterion of a process-control valve. This is especially true in applications in which precise flow or pressure control is needed for optimum performance of the process system.
New process-control valves generally provide the repeatability required. How ever, proper maintenance and periodic calibration of the valves and their actuators are required to ensure long-term performance. This is especially true for valves that use mechanical linkages as part of the actuator assembly.
Process-control valves cannot tolerate solids, especially abrasives, in the gas or liquid stream. In applications in which high concentrations of particulates are present, valves tend to experience chronic leakage or seal problems because the particulate matter prevents the ball, disk, or gate from completely closing against the stationary surface.
Simply installing a valve with the same inlet and discharge size as the piping used in the process is not acceptable. In most cases, the valve must be larger than the piping to compensate for flow restrictions within the valve.
Operating methods for control valves, which are designed to control or direct gas and liquid flow through process systems or fluid-power circuits, range from manual to remote, automatic operation. The key parameters that govern the operation of valves are the speed of the control movement and the impact of speed on the system. This is especially important in process systems.
Hydraulic hammer, or the shock wave generated by the rapid change in the flow rate of liquids within a pipe or vessel, has a serious and negative impact on all components of the process system. E.g., instantaneously closing a large flow-control valve may generate in excess of 3 million foot-pounds of force on the entire system upstream of the valve. This shock wave can cause catastrophic failure of upstream valves, pumps, welds, and other system components.
Changes in flow rate, pressure, direction, and other controllable variables must be gradual enough to permit a smooth transition. Abrupt changes in valve position should be avoided. Neither the valve installation nor the control mechanism should permit complete shut off, referred to as deadheading, of any circuit in a process system.
Restricted flow forces system components, such as pumps, to operate outside of their performance envelope. This reduces equipment reliability and sets the stage for catastrophic failure or abnormal system performance. In applications in which radical changes in flow are required for normal system operation, control valves should be configured to provide an adequate bypass for surplus flow to protect the system.
E.g., systems that must have close control of flow should use two proportioning valves that act in tandem to maintain a balanced hydraulic or aerodynamic system. The primary or master valve should control flow to the downstream process. The second valve, slaved to the master, should divert excess flow to a bypass loop. This master-slave approach ensures that the pumps and other up stream system components are permitted to operate well within their operating envelopes.
---9 One-way fluid-power valve.
Fluid power control valves are used on pneumatic and hydraulic systems or circuits.
The configuration of fluid power control valves varies with their intended application. The more common configurations include one-way, two-way, three-way, and four-way.
One-way valves are typically used for flow and pressure control in fluid-power circuits. Flow-control valves regulate the flow of hydraulic fluid or gases in these systems. Pressure-control valves, in the form of regulators or relief valves, control the amount of pressure transmitted downstream from the valve.
In most cases, the types of valves used for flow control are smaller versions of the types of valves used in process control. These include ball, gate, globe, and butterfly valves.
Pressure-control valves have a third port to vent excess pressure and prevent it from affecting the downstream piping. The bypass or exhaust port has an internal flow-control device, such as a diaphragm or piston, that opens at predetermined setpoints to permit the excess pressure to bypass the valve's primary discharge. In pneumatic circuits, the bypass port vents to the atmosphere. In hydraulic circuits, it must be connected to a piping system that returns to the hydraulic reservoir.
A two-way valve has two functional flow-control ports. A two-way, sliding-spool directional control valve is shown in ---10. As the spool moves back and forth, it either allows fluid to flow through the valve or prevents it from flowing.
In the open position, the fluid enters the inlet port, flows around the shaft of the spool, and flows through the outlet port. Because the forces in the cylinder are equal when the valve is open, the spool cannot move back and forth. In the closed position, one of the spool's pistons simply blocks the inlet port, which prevents flow through the valve.
A number of features common to most sliding-spool valves are shown. The small ports at either end of the valve housing provide a path for fluid that leaks past the spool to flow to the reservoir. This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When these valves become worn, they may lose balance because of greater leakage on one side of the spool than on the other. This can cause the spool to stick as it attempts to move back and forth. Therefore, small grooves are machined around the sliding surface of the piston. In hydraulic valves, leaking liquid encircles the piston, keeping the contacting surfaces lubricated and centered.
Three-way valves contain a pressure port, cylinder port, and return or exhaust port. The three-way directional control valve is designed to operate an actuating unit in one direction. It’s returned to its original position either by a spring or the load on the actuating unit.
---10 Two-way, fluid-power valve. IN; OPEN; CLOSED
---11 Three-way, fluid-power valve.
---12 Four-way, fluid-power valves.
Most actuating devices require system pressure to operate in two directions. The four-way directional control valve, which contains four ports, is used to control the operation of such devices. The four-way valve also is used in some systems to control the operation of other valves. It’s one of the most widely used directional-control valves in fluid-power systems.
The typical four-way directional control valve has four ports: pressure port, return port, and two cylinder or work (output) ports. The pressure port is connected to the main system-pressure line, and the return port is connected to the reservoir return line. The two outputs are connected to the actuating unit.
The criteria that determine performance of fluid-power valves are similar to those for process-control valves. As with process-control valves, fluid-power valves also must be selected based on their intended application and function.
When installing fluid power control valves, piping connections are made either directly to the valve body or to a manifold attached to the valve's base. Care should be taken to ensure that piping is connected to the proper valve port. The schematic diagram that is affixed to the valve body will indicate the proper piping arrangement as well as the designed operation of the valve. In addition, the ports on most fluid-power valves are generally clearly marked to indicate their intended function.
In hydraulic circuits, the return or common ports should be connected to a return line that directly connects the valve to the reservoir tank. This return line should not need a pressure-control device but should have a check valve to prevent reverse flow of the hydraulic fluid.
Pneumatic circuits may be vented directly to atmosphere. A return line can be used to reduce noise or any adverse effect that locally vented compressed air might have on the area.
The function and proper operation of a fluid-power valve are relatively simple.
Most of these valves have a schematic diagram affixed to the body that clearly explains how to operate the valve.
---13 Schematic for a cam-operated, two-position valve.
---14 Schematic of two-position and three-position valves.
---15 Schematic for center or neutral configurations of three-position valves.
---16 Actuator-controlled valve schematics.
This is a schematic of a two-position, cam-operated valve. The primary actuator, or cam, is positioned on the left of the schematic and any secondary actuators are on the right. In this example, the secondary actuator consists of a spring return and a spring-compensated limit switch. The schematic indicates that when the valve is in the neutral position (right box), flow is directed from the inlet (P) to work port A. When the cam is depressed, the flow momentarily shifts to work port B. The secondary actuator, or spring, automatically returns the valve to its neutral position when the cam returns to its extended position. In these schematics, T indicates the return connection to the reservoir.
This illustrates a typical schematic of a two-position and three-position directional control valve. The boxes contain flow direction arrows that indicate the flow path in each of the positions. The schematics don’t include the actuators used to activate or shift the valves between positions.
In a two-position valve, the flow path is always directed to one of the work ports (A or B). In a three-position valve, a third or neutral position is added. A type 2 center position is used. In the neutral position, all ports are blocked and no flow through the valve is possible.
This is the schematic for the center or neutral position of three-position directional control valves. Special attention should be given to the type of center position that is used in a hydraulic control valve. When type 2, 3, and 6 are used, the upstream side of the valve must have a relief or bypass valve installed. Since the pressure port is blocked, the valve cannot relieve pressure on the upstream side of the valve. The type 4 center position, called a motor spool, permits the full pressure and volume on the upstream side of the valve to flow directly to the return line and storage reservoir. This is the recommended center position for most hydraulic circuits.
The schematic affixed to the valve includes the primary and secondary actuators used to control the valve. This provides the schematics for three actuator-controlled valves, as follows: (1) double-solenoid, spring-centered, three-position valve; solenoid-operated, spring-return, two-position valve; double-solenoid, detented, two-position valve.
The top schematic represents a double-solenoid, spring-centered, three-position valve. When neither of the two solenoids is energized, the double springs ensure that the valve is in its center or neutral position. In this example, a type 0 configuration is used. This neutral-position configuration equalizes the pressure through the valve. Since the pressure port is open to both work ports and the return line, pressure is equalized throughout the system. When the left or primary solenoid is energized, the valve shifts to the left-hand position and directs pressure to work port B. In this position, fluid in the A-side of the circuit returns to the reservoir. As soon as the solenoid is de-energized, the valve shifts back to the neutral or center position. When the secondary (i.e., right) solenoid is energized, the valve redirects flow to port A, and port B returns fluid to the reservoir.
The middle schematic represents a solenoid-operated, spring-return, two position valve. Unless the solenoid is energized, the pressure port (P) is connected to work port A. While the solenoid is energized, flow is redirected to work port B. The spring return ensures that the valve is in its neutral (i.e., right) position when the solenoid is de-energized.
The bottom schematic represents a double-solenoid, detented, two-position valve. The solenoids are used to shift the valve between its two positions.
A secondary device, called a detent, is used to hold the valve in its last position until the alternate solenoid is energized. Detent configuration varies with the valve type and manufacturer. However, all configurations prevent the valve's control device from moving until a strong force, such as that provided by the solenoid, overcomes its locking force.
As with process-control valves, actuators used to control fluid-power valves have a fundamental influence on performance. The actuators must provide positive, real-time response to control inputs. The primary types of actuators used to control fluid-power valves are mechanical, pilot, and solenoid.
Mechanical The use of manually controlled mechanical valves is limited in both pneumatic and hydraulic circuits. Generally, this type of actuator is used only on isolation valves that are activated when the circuit or fluid-power system is shut down for repair or when direct operator input is required to operate one of the system components.
Manual control devices (e.g., levers, cams, or palm buttons) can be used as the primary actuator on most fluid-power control valves. Normally, these actuators are used in conjunction with a secondary actuator, such a spring return or detent, to ensure proper operation of the control valve and its circuit.
Spring returns are used in applications in which the valve is designed to stay open or shut only when the operator holds the manual actuator in a particular position. When the operator releases the manual control, the spring returns the valve to the neutral position.
Valves with a detented secondary actuator are designed to remain in the last position selected by the operator until manually moved to another position.
A detent actuator is simply a notched device that locks the valve in one of several pre-selected positions. When the operator applies force to the primary actuator, the valve shifts out of the detent and moves freely until the next detent is reached.
Pilot Although a variety of pilot actuators are used to control fluid-power valves, they all work on the same basic principle. A secondary source of fluid or gas pressure is applied to one side of a sealing device, such as a piston or diaphragm. As long as this secondary pressure remains within pre-selected limits, the sealing device prevents the control valve's flow-control mechanism (i.e., spool or poppet) from moving. However, if the pressure falls outside of the pre-selected window, the actuator shifts and forces the valve's primary mechanism to move to another position.
This type of actuator can be used to sequence the operation of several control valves or operations performed by the fluid-power circuit. E.g., a pilot operated valve is used to sequence the retraction of an airplane's landing gear.
The doors that conceal the landing gear when retracted cannot close until the gear is fully retracted. A pilot-operated valve senses the hydraulic pressure in the gear-retraction circuit. When the hydraulic pressure reaches a pre-selected point that indicates that the gear is fully retracted, the pilot-actuated valve triggers the closure circuit for the wheel-well doors.
Solenoid--Solenoid valves are widely used as actuators for fluid-power systems.
This type of actuator consists of a coil that generates an electrical field when energized. The magnetic forces generated by this field force a plunger that is attached to the main valve's control mechanism to move within the coil. This movement changes the position of the main valve.
In some applications, the mechanical force generated by the solenoid coil is not sufficient to move the main valve's control mechanism. When this occurs, the solenoid actuator is used in conjunction with a pilot actuator. The solenoid coil opens the pilot port, which uses system pressure to shift the main valve.
Solenoid actuators are always used with a secondary actuator to provide positive control of the main valve. Because of heat build up, solenoid actuators must be limited to short-duration activation. A brief burst of electrical energy is transmitted to the solenoid's coil and the actuation triggers a movement of the main valve's control mechanism. As soon as the main valve's position is changed, the energy to the solenoid coil is shut off.
This operating characteristic of solenoid actuators is important. E.g., a normally closed valve that uses a solenoid actuation can be open only when the solenoid is energized. As soon as the electrical energy is removed from the solenoid's coil, the valve returns to the closed position. The reverse is true of a normally open valve. The main valve remains open except when the solenoid is energized.
The combination of primary and secondary actuators varies with the specific application. Secondary actuators can be another solenoid or any of the other actuator types that have been previously discussed.
Although there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure. This lists the more common causes of these failures.
Special attention should be given to the valve actuator when conducting a Root Cause Failure Analysis. Many of the problems associated with both process and fluid-power control valves are actually actuator problems.
In particular, remotely controlled valves that use pneumatic, hydraulic, or electrical actuators are subject to actuator failure. In many cases, these failures are the reason a valve fails to properly open, close, or seal. Even with manually controlled valves, the true root cause can be traced to an actuator problem. E.g., when a manually operated process-control valve is jammed open or closed, it may cause failure of the valve mechanism. This over-torquing of the valve's sealing device may cause damage or failure of the seal, or it may freeze the valve stem. Either of these failure modes results in total valve failure.
----1 Common Failure Modes of Control Valves
The Causes The Problem Manually Actuated Valve Fails to Open Valve Fails to Close Leakage Through Valve Leakage Around Stem Excessive Pressure Drop Opens/Closes Too Fast Opens/Closes Too Slow Pilot Actuated Solenoid Actuated