Isolation Valves are considered on/off because they typically operate in two positions; the fully open and fully closed position (thus on/off). Valves made specifically for on/off service are designed with tight reliable shutoff in the closed position and little restriction in the open position. Ball valves, gate valves, butterfly valves, diaphragm valves and plug valves are the most commonly used isolation (shut-off) valves.
3-Way Switching Valves converge and divert fluid flow in a piping system. 3-way valves are typically used because they can do the job of two, 2-way valves. 3-way valves are typically of ball, plug and globe design, however two butterfly valves mounted on a pipe tee and connected via a lever can be more cost effective in large pipe sizes. The port configuration is of prime importance when selecting a 3-way valve. Ports must be specified to meet piping installation and flow switching requirements. 3-way valves are quarter-turn operation and easily automated. 180º turn is also available, but requires special actuation. Solenoid valves are typically used in sizes up to 1/2".
Control Valves modulate the fluid flow in a piping system. All the valves listed above can be used for manual flow control with certain limitations. Reciprocating and rotary globe control valves with pneumatic diaphragm actuators are recommended for automatic process control where precise (<1% inaccuracy) automatic flow control is required. V-Notch Ball Valves and High Performance Butterfly Valves are available, which under acceptable process conditions perform very well (typically 2-3% inaccuracy) in automatic control loops. Diaphragm valves, surprisingly enough, have excellent throttling characteristics and should be considered for sanitary type applications. Proportional Solenoid Valves are an economical way to modulate flow in small sizes up to 1". In order to automate a valve, it must be able to accept an actuator and control signal.
- Ball Valves were a welcomed relief to the process industry. They provide tight shutoff and high capacity with just a quarter-turn to operate. Ball valves are now more common in 1/4"-6" sizes. Ball valves can be easily actuated with pneumatic and electric actuators.
- Butterfly Valves have come a long way from the old damper valve days. Today's butterfly valves are designed for general as well as severe service applications. Resilient liners provide tight shutoff in general service applications. Triple offset metal seated butterfly valves are designed for severe service applications. Butterfly valves are the most economical valves per comparable capacity and are easily automated with pneumatic and electric actuators.
- Check Valves are unidirectional flow control devices used to prevent potential damage and contamination caused by backflow. Check valves use a disc, ball, or plates that open when forward flow starts in the pipeline. When pressure drops, either gravity or back pressure forces the disc back against its seat to prevent reversal of flow (backflow). A mechanical spring can be used to assist closure of the disc in low pressure/flow applications. However, closure of the disc is dependent on actual back pressure.
- Diaphragm Valves are by far the simplest valves. A resilient diaphragm provides tight shutoff and isolates the body from its operator. The operator consists of a plunger and handwheel assembly. Diaphragm valves are ideal for corrosive, slurry and sanitary services. They are easily and inexpensively actuated with pneumatic and electric actuators.
- Float Valves automatically control liquid level and prevent overfilling tanks. The valve is operated mechanically by a float which rests on the top of the liquid. As the liquid level rises, it pushes the float up and closes the valve. As the level falls, the valve opens. The amount of liquid pressure the valve can shutoff against is determined by the length of the rod and size of the float for a given valve size.
- Gate Valves have a sliding disc (gate) which reciprocates into and out of the valve port. Gate valves are an ideal isolation valve for high pressure drop and high temperature applications where operation is infrequent. Manual operation is accomplished through a multi turn handwheel gear shaft assembly. Multi-turn electric actuators are typically required to automate gate valves, however long stroke pneumatic and electro-hydraulic actuators are also available.
- Globe Valves have a conical plug which reciprocates into and out of the valve port. Globe valves are ideal for shutoff as well as throttling service in high pressure drop and high temperature applications. Available in globe, angle, and y-pattern designs. Manual operation is accomplished through a multi-turn handwheel assembly. Multi-turn electric actuators are typically required to automate globe valves, however linear stroke pneumatic and electro-hydraulic actuators are also available.
- Needle Valves have a relatively small conical seat with matching needle-like tapered plug on the end of a finely threaded screw. The fine multi-turn action provides high resolution positioning of the plug relative to the seat making it ideal for regulating fluid flow. Needle valves are ideal for precise flow metering, process dampening, and shutoff applications. Also known as gauge valves, needle valves are commonly used to protect pressure gauges and instruments from pressure spikes by dampening the flow/pressure. Needle valves can be used with pneumatic actuators to control the rate of valve open and close times (speed control).
- Plug Valves are similar to ball valves except instead of a spherical element, a cylindrical element is used as the internal restriction. Plug valves are typically more expensive than ball valves, but they are inherently more rugged as well. The plug is guided by a sleeve which acts as the sealing member. Plug valves require more torque to operate than ball valves, but are easily automated with quarter turn actuators. Plug valves are also available in 3-way and 180º configurations.
- Solenoid Valves are simple electrically operated devices. The valve plug is held in place by a spring. When electric power is applied to the solenoid (Energized), the current draw through the coil generates an electromagnetic force which opposes the spring, causing the plug to change position. When power is taken away (De-energized), the spring returns the plug to the normal position. Solenoid valves are ideal for fluid shutoff and switching in general service applications. Proportional solenoid control valves are available for modulating service.
Pressure/Temperature Rating: The process fluid's combined pressure and temperature must be within the manufacturers published rating for a given valve. The given rating will be unique to a given body shell, body and trim material combination, seal material, and end connections. Select a rating that insures these combinations are sufficient to handle the maximum possible process conditions.
- Proportional solenoid control valves are very economical solutions for general purpose applications. Variable flow is achieved by positioning the armature proportionally to a control signal. Internally, high frequency pulse-width modulation of the coil provides excellent near linear control characteristics. These miniature control valves are typically available in sizes from 1/4" up to 1".
- Reciprocating globe valves are the most rugged, and usually the most expensive, particularly in the larger sizes. The torturous path through this type of valve, provides excellent energy conversion of the process fluid resulting in accurate, repeatable control. Severe service noise and cavitation control trims are available in this valve style.
- Rotary Globe Valves consist of cammed plug and segmented "V" ball designs. They have similar control characteristics to reciprocating globe valves with the benefit of low friction from rotary motion. Rotary valves inherently have significantly more capacity and turndown (rangeability) than reciprocating globe valves. Their low cost and comparable installed accuracy performance make rotary globe valves preferred in general service applications.
- Vee-Port Ball Valves consist of a Vee Ported ball design and are available with 30, 60, and 90 degree characteristics. V-Port Ball Valves are ideal in sizes up to 4" for general flow control applications where economy, simplicity, and the sealing features of a quarter-turn ball valve are of value. Pre packaged systems are available complete with pneumatic and electric modulating actuators and positioners.
- High-Performance Butterfly Valves (HPBV) consist of Double and Triple Offset Discs which provide a modified parabolic characteristic. HPBV's are ideal in sizes greater than 3" for general flow control applications where economy, simplicity, and the sealing features of a quarter-turn butterfly valve are of value. Pre packaged systems are available complete with pneumatic and electric modulating actuators and positioners.
- Pressure regulators provide constantly controlled liquid, gas, and steam pressures in process piping systems. They are typically used to reduce inlet supply pressures to an efficient operating pressure and can also be used to maintain constant back pressures. Select regulator size based on flow requirement and operating pressure range. Typically, the lightest spring range will provide the best performance. However, a heavier spring can help dampen unstable systems.
- Pressure relief valves protect vessels and piping systems from over pressure. Relief valves begin to open as the pressure increases past the set pressure, but require about 10% over pressure to completely open. As the pressure drops, the valves begin to close, shutting fully after dropping sufficiently below the set pressure. Safety relief valves and pop-off valves open completely when the set pressure is reached. Care should be taken in selecting a relief valve that will pass enough flow to relieve the system from over pressure. Refer to ASME and API codes when selecting safety relief valves, particularly in steam, air, hot water, and chemical applications.
- Body Materials: Select body and trim materials based on their strength (pressure/temperature rating) and resistance to corrosion and erosion of a given process fluid. Plastic is used on very low pressure systems where corrosion is of primary concern. Brass/bronze is very economical and fairly corrosion resistant. Iron is very cost effective and can be economically coated or lined for compatibility with corrosive fluids. Select carbon steel where strength is needed. Stainless steel has very good strength and corrosion resistance. Other exotic alloys and molys can be supplied where needed.
- Seal Materials: Further select elastomeric and plastic seals, liners and diaphragms based on their chemical compatibility to the process fluid. Elastomeric elements (natural and synthetic rubbers) have better sealing characteristics, however plastics (PTFE, PFA, etc.) are typically chosen for harsh chemicals. Refer to chemical resistance guides for proper selection.
- End Connections: Valve body end connections are typically chosen based on initial cost, plant standard, and/or maintenance preference. Maintenance consideration is the preferred method of selection. Threaded ends (NPT/screwed) have a low initial cost, but are subject to leak paths and stripping. Use threaded ends where maintenance is not a concern. Welded ends provide for rigid, leak tight connections. They have low initial hardware cost, but high maintenance cost should they need to be cut out of the line for repair or replacement. Flanged ends have the highest initial cost, but are preferred from an installation and removal standpoint. Wafer bodies give the benefits of a flanged installation with very low initial cost. Use wafer bodies only where the pipe is rigid or fully supported. 3-Piece ball valve designs give the benefit of threaded or welded joints with integrally flanged wafer bodies.
Actuator Selection: As mentioned previously, actuators are used to operate valves automatically and/or remotely. Actuators typically use pneumatic, electric and hydraulic power to actuate a valve shaft. The force output of the actuator must be sufficient to overcome valve static friction and dynamic torque. Static friction is developed in the metal-to-metal surfaces, seats, and seals. Dynamic torque is that unbalanced force of the process acting on the plug, disc, or ball. Valve torque requirements are supplied by the manufacturer and based on pressure drop across the valve. A minimum of 10-20% safety factor should be added to insure reliable operation. The on/off actuator positions the valve in the open or closed position. Modulating actuators use controllers/positioners to maintain a valve position based on an input signal.
Accessories are components within a valve automation system. The components are required to operate, override, and support the actuation assembly. Select accessories based on the valve, actuator, and control system requirements.
- Pneumatic double acting actuators use gas, typically air or nitrogen to pressurize a double acting cylinder. An unbalance in the opposing cylinder areas drive the connected shaft in the required direction. A 4-way solenoid valve is used to switch pressure between the cylinders. The normal position is the position of the valve with the solenoid de-energized. Available output force is equal to the pressure times the area of the piston (F=PxA). Piston actuators are typically sized for 80 psi minimum. Double acting actuators do not have an inherent fail safe action. If only electrical power is lost, then the valve will go to or remain in its normal position. If pneumatic power is lost, and the friction in the valve and actuator is sufficient to overcome the dynamic unbalance in the valve, then the valve will remain in its last position. If the dynamic unbalance force is greater, then the valve will drift in the dominating direction. Accumulator systems with trip valves are available to provide pneumatic fail safe operation.
- Pneumatic spring return actuators are spring opposed pneumatic cylinders. A 3-way solenoid valve is used to operate a spring return actuator. The normal position is the position of the valve with the spring extended and the solenoid de-energized. The available force is the pressure times the area of the piston less the spring force [F=(PxA)-Fs] in one direction and the available spring compression force in the opposite. Spring force is determined by the spring rate (K) times the compression, in inches (F=KxIn) at any given position of stroke. Spring and piston actuators are typically sized for 80 psi minimum. Spring and diaphragm actuators (for Control Valves) should be sized for 35 psi (6-30 psi signal). Spring return actuators are inherently fail safe. Trip valves are available for lock-in-last position fail action.
- Electric actuators use a reversing electric motor and gear reduction to drive a valve shaft. A given motor size and gearbox are matched to provide the most effective force output. The output force is measured in inch or foot pounds of torque. Standard voltages are 220-480VAC/3-phase, although single phase and 120VAC options are available. Torque switches are used to position the valve open and closed. The normal position is the position of the valve with the solenoid de-energized. Electric actuators inherently fail in the last position. Battery back up is needed to drive the actuator to a specific fail safe position.
- Solenoid Valves use electromagnetic force to switch pressure ports. A spring holds the valve plug in position (usually closing a port). When electric current is applied to the coil of the solenoid valve, the spring opposed plug opens or switches ports within the valve. Pneumatic pressure is then allowed to load or exhaust the actuator cylinder. 3-way valves are used on spring-return actuators and 4-way valves are used on double acting actuators. Solenoid valves can be normally open (de-energized to load actuator) or normally closed (de-energized to vent actuator). Solenoid valves typically come in aluminum, brass and stainless steel. Select the appropriate coil voltage and area classification.
- Limit Switches are activated by cams connected to the actuator shaft. When activated, they send discrete electrical signals to indicate actual valve position, usually confirming that a valve fully opened or closed operated remotely. In addition, these signals can be used as interlocks in a control system. For example, locking out a pump until its supply valve has been opened. Mechanical switches should be housed in an enclosure (Switchpak), which will also provide visual position indication. Proximity (magnetically latching switches) are hermetically sealed which makes them less susceptible to moisture intrusion. Select the appropriate switch type and area classification.
- Positioners are used to position a valve based on a control signal. The signal can be pneumatic or electric. The positioner is mechanically attached to the valve stem. It compares the valve position to the input signal and sends the required output to the actuator to bring the valve to the correct position. The positioner can be analog or digital. Analog positioners use mechanical or electro-mechanical methods to position the valve. Digital (Smart) positioners use microprocessor technology. Smart positioners can operate on digital networks as well as traditional analog signals. They provide 2-way communication with the microprocessor which allows remote configuration and auto calibration, operating information such as actual valve position, and diagnostics information such as cycle count, deviation, friction band, and valve signature. Digital positioners should be used whenever possible.
- I/P Transducers are used to convert a current signal to a pneumatic signal. Typically a 4-20mA control signal to an existing 3-15 psi pneumatic positioner. I/P's are very economical and can be used to directly control a valve, however an I/P positioner will usually provide much better performance.
- Filter Regulators should be used whenever possible. They are used to filter and regulate the supply pressure for pneumatic instruments. Instruments have small orifices that can become restricted and plugged by dirty air, moisture can corrode and short electrical components, and pneumatic instrument performance is optimum with regulated supply pressures. Set regulators to 5 psi greater than the maximum supply pressure required by the actuator. Solenoid valves with built in timers are ideal for blowing off regulator drain cocks.
- Speed Control Valves are used to control the rate at which automated pneumatic valves operate. Metering valves reduce valve speeds by restricting the flow in and/or out of the actuator. Volume boosters increase the actuator loading time and exhaust valves increase the actuator unloading time.
- Fail Safe Accumulators are used to provide a fail safe means for double acting pneumatic actuators. This system consists of a volume tank, inline check valve and trip valve. Upon loss of supply pressure the trip valve causes the stored gas to drive the actuator to its fail safe position. Stand alone trip valves can be used to lock a double acting actuator in last position upon loss of supply pressure. Solenoid valves can be added to provide electric override or fail safe action.
- Manual Overrides can be used to manually operate a valve in an emergency or to bypass automatic operation. A handwheel gear operator is mounted in between the valve and the actuator. The handwheel operator is engaged by locking the gear. Manual overrides should be considered on critical valve applications and for emergency shutdown valves.
- Strainers are placed in process piping lines to remove large solid contaminants from the flow stream. Stainers help protect valves, pumps, and other expensive equipment against dirt, scale, chips and other sediment. Strainers should always be installed upstream of solenoid valves to prevent clogging of internal passages.