A valve is a mechanical device that regulates the flow of fluids (either gases, fluidised solids, slurries or liquids) by opening, closing, or partially obstructing various passageways.
Valves are used in a myriad of industrial, military, commercial, and residential applications.
There are many different types of valves:
- A ball valve (often called a quarter turn valve) is a valve that opens by turning a handle attached to a ball inside the valve.
- The ball has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked.
- The handle position lets you "see" the valve's position.
- The body of ball valves may be made of metal, ceramic, and/or plastic.
- The ball may be chrome plated to make it more durable.
- There are three general types of ball valves: full port, standard port, and reduced port.
- A full port ball valve has an oversized ball so that the hole in the ball is the same size as the pipeline resulting in lower friction loss.
- Flow is unrestricted.
- A standard port ball valve is usually less expensive, but has a smaller ball and a correspondingly smaller port.
- Flow through this valve is one pipe size smaller than the valve's pipe size resulting in slightly restricted flow.
- In reduced port ball valves, flow through the valve is two pipe sizes smaller than the valve's pipe size resulting in restricted flow.
- Manually operated ball valves can often be closed quickly and thus there is a danger of water hammer. Some ball valves are equipped with an actuator that may be pneumatically or motor operated.
- These valves can be used either for on/off or flow control.
- A pneumatic flow control valve is also equipped with a positioner which transforms the control signal into actuator position and valve opening accordingly.
There are also three-way ball valves, with a T-shaped hole through the middle.
With such a valve the flow can be directed to either one or the other or both sides or be closed off completely.
- A Butterfly valve is a type of flow control device, used to make a fluid start or stop flowing through a section of pipe.
- The valve is similar in operation to a ball valve.
- A flat circular plate is positioned in the center of the pipe.
- The plate has a rod through it connected to a handle on the outside of the valve.
- Rotating the handle turns the plate either parallel or perpendicular to the flow of water, shutting of the flow.
- It is a very robust and reliable design.
- However, unlike the ball valve, the plate does not rotate out of the flow of water, so that a pressure drop is induced in the flow.
- A gate valve is a valve that opens by lifting a round or rectangular gate out of the path of the fluid.
- Gate valves are sometimes used for regulating flow, but many are not suited for that purpose, having been designed to be fully opened or closed.
- When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.
- Most gate valves have a rising or a nonrising stem.
Rising stems give a visual indication of valve position.
Nonrising stems are used where vertical space is limited.
- Gate valves may have a screw-in, union, or bolted bonnet.
Screw-in bonnet is the simplest, offering a durable, pressure-tight seal.
Union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength.
Bolted bonnet is used for larger valves and higher pressure applications.
Bonnets provide leakproof closure for the valve body.
- Globe valves are named for their spherical body shape.
- The two halves of the valve body are separated by a baffle with a disc in the center.
- Globe valves operate by screw action of the handwheel.
- They are used for applications requiring throttling and frequent operation.
- Since the baffle restricts flow, they're not recommended where full, unobstructed flow is required.
- A bonnet provides leakproof closure for the valve body. Globe valves may have a screw-in, union, or bolted bonnet.
Screw-in bonnet is the simplest bonnet, offering a durable, pressure-tight seal.
Union bonnet is suitable for applications requiring frequent inspection or cleaning. It also gives the body added strength.
Bolted bonnet is used for larger or higher pressure applications.
- Many globe valves have a class rating that corresponds to the pressure specifications of ANSI 16.34.
- A check valve is a mechanical device, a valve, that normally only allows fluid to flow through it in one direction.
- A double check valve is often used as a backflow prevention device to keep potentially contaminated water from siphoning back into municipal water supply lines.
- A clapper valve is a type of check valve used in or with firefighting, and has a hinged gate (often with a spring urging it shut) that will only remain open in the outflowing direction.
- Some types of irrigation sprinklers and drip irrigation emitters have small check valves built into them to keep the lines from draining when the system is shut off.
Pressure Relief Valves
- A pressure relief valve opens to release excess pressure when the pressure is too high to protect the vessel or other equipment from overpressurization.
- A relief valve is like a safety valve in that it is for liquids only.
- A safety valve is for gases only
About the Author
Anton Dooley is a Piper with over 25 years experience covering process plant engineering, design & training. He is the founder of pipingdesigners.com