What a valve is for
Valves do a small number of jobs. They start and stop flow (isolation), regulate it (throttling), keep it moving one way only (non-return), or protect a system from excess pressure (relief). Most problems with valves come from using a type designed for one of those jobs to do a different one. A valve that isolates well may wear quickly if it is used half-open for control, for example.
Pump lines make this clear. A pump discharge normally needs an isolation valve so the pump can be serviced, a way to prevent backflow when the pump stops, and sometimes a means of regulating flow. Each of those jobs points to a different valve design.
Gate valves
A gate valve raises and lowers a flat or wedge-shaped gate across the flow path. When fully open, the flow passes almost straight through, so pressure loss is low. That makes gate valves a traditional choice for isolation in larger lines.
They are not intended for throttling. A partly open gate can vibrate, and the flow can erode the gate and seat. Gate valves also open and close slowly, because the stem must turn many times, which can be an advantage when you want to avoid sudden changes in flow.
Globe valves
A globe valve closes by pressing a plug against a seat positioned in the flow path. The flow has to change direction inside the body, so pressure loss is higher than in a gate valve, but throttling is much better behaved. Globe valves are widely used where flow needs adjusting or where tight shut-off with regular use is wanted.
Their higher resistance means they are not the best choice where pressure drop is critical, such as on the suction side of many pumps.
Ball valves
A ball valve uses a ball with a hole through it. A quarter turn of the handle moves from fully open to fully closed. They shut quickly, seal tightly in many services, and take up relatively little space, so they are very common for on/off service in both small and mid-sized lines.
Full-port types leave the flow path almost unrestricted, while reduced-port types are smaller and cheaper but add resistance. Ball valves are generally not ideal for fine throttling, since partly open positions can wear the seats. Because they close fast, shutting one quickly on a long line can cause a pressure surge, so in some systems slow-acting actuators or procedures are used.
Check valves
Check valves are self-acting. They let flow pass one way and close when it tries to reverse. No handle or stem is involved. Their role is to stop backflow, protect pumps from spinning backward and keep lines primed. Common check valve types include swing, lift, piston, wafer and nozzle (silent) designs, which behave differently when closing. A slow-closing swing check on a fast-stopping pump line can slam, while quicker-closing types can reduce slamming.
Because a check valve depends on flow to open, it sets a minimum flow below which it may chatter. Orientation also matters. Some designs are made for horizontal lines only.
Other designs worth knowing
- Butterfly valves use a disc turned a quarter turn in the pipe. They are compact and economical for large diameters and are used for isolation and, in some versions, moderate regulation.
- Plug valves use a tapered or cylindrical plug and are common for on/off service with difficult fluids.
- Diaphragm valves isolate the fluid from the mechanism and are used for corrosive, sanitary or slurry services.
- Relief and safety valves open at a set pressure to protect equipment and are a safety-critical category with their own rules.
- Control valves adjust flow automatically in response to a controller, and the way they regulate flow is a subject in itself.
How to choose
Work through a short list of questions.
- What job must the valve do? Isolate, throttle, prevent backflow or protect? Pick a design built for that job.
- What is the fluid? Corrosive, abrasive or sticky liquids affect body and trim materials, seat type and whether the fluid can be allowed to contact the moving parts.
- What are the pressure and temperature? Valves are rated for combinations of pressure and temperature, and the rating can fall as temperature rises. The rating must cover worst-case conditions.
- How often will it operate? Rarely used isolation valves have different needs from valves that cycle many times a day.
- How fast should it act? Quick closing can cause pressure surges, while very slow operation may be inconvenient in emergencies.
- How will it be operated? A hand wheel may be enough, but some valves need automation, and valve actuators or solenoid valves in the control circuit may be part of the specification.
- What are the end connections and space limits? Threaded, flanged, welded and wafer styles all fit differently, and installation access matters.
- How will it be maintained? Check availability of spare parts and whether it can be serviced in place.
Leakage and sealing
Not every valve that is “closed” is bubble-tight. Shut-off ratings vary by design and seat material, and the degree of leakage that is acceptable depends on the service. For hazardous fluids, the stem seal also deserves attention because it is a common leak path to atmosphere.
Safety
Valves operate on pressurized lines, often with hazardous contents. Work on them calls for depressurizing and isolating the line, following lockout and tagout style procedures, and relying on trained and qualified personnel. Use the manufacturer’s manuals and ratings, and check local regulations and codes. This article is general information, not engineering advice.