Start with the duty, not the type
The single most common specification error is choosing a valve designed for isolation and then using it to throttle. An isolation valve is built to be fully open or fully shut; held partly open, flow accelerates across a partially exposed seat and erodes it. A regulating valve is built for exactly that condition and is wasteful and expensive used as a simple shut-off.
| Type | Primary duty | Notes |
|---|---|---|
| Gate | Isolation | Full bore, very low pressure drop when open. Slow to operate. Not for throttling. |
| Globe | Throttling and regulation | Good control characteristic; significant permanent pressure drop across the seat. |
| Ball | Isolation, fast operation | Quarter-turn, tight shut-off, low pressure drop. Full bore or reduced bore. |
| Butterfly | Isolation and coarse throttling | Compact and economical in large sizes; disc remains in the flow path. |
| Check | Prevent reverse flow | Swing, lift, dual-plate and nozzle types differ sharply in closing behaviour. |
| Safety relief | Overpressure protection | Set pressure and relieving capacity are the governing specification. |
| Diaphragm / pinch | Corrosive, slurry and hygienic duty | Isolates the mechanism from the fluid entirely. |
Pressure class and temperature rating
Valve pressure classes — 150, 300, 600 and upwards — are not fixed pressure limits. Each class defines a pressure–temperature curve, and the allowable pressure falls as temperature rises. A body that holds a given pressure at ambient will be rated well below that at elevated temperature, and the material affects the curve, so a carbon steel and a stainless steel body of the same class do not carry the same rating at temperature.
Specify both the design pressure and the design temperature. A class chosen from pressure alone will be wrong for any hot service.
Body and trim materials
Body material is selected against the process fluid, temperature and external environment. Trim — the seat, disc or ball and stem — takes the wear and often needs to be a different, harder or more corrosion-resistant material than the body.
- Carbon steel bodies suit general water, oil, steam and non-corrosive service
- Stainless steel suits corrosive process fluids, hygienic duty and higher temperatures
- Cast iron and ductile iron remain common in water, HVAC and low-pressure service
- PVC, CPVC and lined valves handle aggressive chemicals such as hypochlorite and acids that attack metals
- Soft seats give tight shut-off but limit temperature; metal seats survive heat and abrasives with less perfect sealing
End connections
The valve has to join the pipe it sits in. Confirm the connection type, size and standard before ordering — a flanged valve to the wrong flange standard will not bolt up regardless of how correct the rest of the specification is.
- Flanged — the standard industrial connection; confirm the flange standard, class and facing
- Threaded — common in small bore, typically up to about 2 inches
- Butt weld and socket weld — used where a permanent, leak-free joint is required
- Wafer and lug — compact mounting for butterfly and dual-plate check valves between flanges
Manual or actuated
Manual operation is the default where the valve is accessible and operated infrequently. Actuation is specified where the valve is remote, has to respond to a control signal, has to fail to a defined position, or is simply too large to turn by hand.
- Pneumatic actuators are fast and are the usual choice where instrument air is available
- Electric actuators suit slow, precise positioning and sites without an air supply
- Fail-safe action — fail open, fail closed or fail in place — is a safety decision, not a preference
- Large gate and butterfly valves often need gearboxes even for manual operation
What to include in a valve enquiry
- Valve type and duty — isolation, throttling or non-return
- Nominal size and pressure class
- Design pressure and design temperature
- Process fluid, its concentration and whether it carries solids
- Body and trim material requirements
- End connection type and standard
- Manual or actuated, and the fail position if actuated
- Quantity, delivery city and required date