Valve Selection
Valves isolate, switch and
control fluid flow in a piping system. These valves can be operated manually
with levers and gear operators or remotely with electric, pneumatic,
electro-pneumatic, and electro-hydraulic powered actuators. Manually operated
valves are typically used where operation is infrequent and/or a power source
is not available. Powered actuators allow valves to be operated automatically
by a control system and remotely with push button stations. Valve automation
brings significant advantages to a plant in the areas of process quality,
efficiency, safety, and productivity.
Valves must be properly
selected to operate safely, efficiently, and reliably without excessive
maintenance. First select the most cost effective valve type suitable for the
given application. Then select the appropriate pressure/temperature rating,
materials of construction, end connections, and actuation based on the service
conditions and system requirements.
- 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 valves.
- 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.
- 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. Multiturn 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. Availablle in globe, angle, and y-pattern designs. Manual operation is accomplished through a multi-turn handwheel assembly. Multiturn electric actuators are typically required to automate globe valves, however linear stroke pneumatic and electro-hydraulic actuators are also available.
- 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.
- 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.
- Control Valves modulate the fluid flow in a piping system. All the valves listed above can be used for manual and automated flow control with certain limitations. Reciprocating and rotary globe control valves are recommended for automatic process control where precise (<2 a="" able="" accept="" acceptable="" accuracy="" actuator="" an="" and="" are="" automate="" available="" ball="" be="" butterfly="" characteristics.="" conditions="" control="" diaphragm="" enough="" excellent="" flow="" have="" high="" in="" is="" it="" must="" of="" order="" perform="" performance="" process="" required.="" signal.="" span="" surprisingly="" throttling="" to="" under="" valve="" valves="" versions="" very="" well.="" which="">
- 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 than reciprocating globe valves. Their low cost and comparable installed accuracy performance make rotary globe valves preferred in general service applications.
- 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.
- 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.
