Valve types: gate, globe, ball, butterfly, check, control and relief valves
Deskely Completions team Singapore
Valves start, stop, throttle, divert and protect the flow in a piping system. A process plant has thousands of them, and choosing the wrong type for a duty means leaks, erosion or a valve that cannot shut off when it has to.
This guide covers the main valve types by what they do, how each one works, where it is used, the P&ID symbol for each, the design and test standards, and how valves are tagged and checked on a project.
- Isolation valves (gate, ball, plug, butterfly) are fully open or fully shut. Do not throttle with a gate valve.
- Regulating valves (globe, needle, diaphragm, control valves) are built to throttle flow.
- Check valves stop reverse flow on their own; relief and safety valves protect against overpressure.
- Every valve on a P&ID is a tag with its own installation, test and function records.
Valve types at a glance
| Valve | Motion | Main duty | Throttling | Typical use |
|---|---|---|---|---|
| Gate valve | Linear, multi-turn | Isolation | No | Isolation on larger lines with low pressure drop |
| Globe valve | Linear, multi-turn | Regulation | Yes | Flow control, bypasses, drains |
| Ball valve | Quarter-turn | Isolation | Limited | Fast shut-off, hydrocarbons, gas |
| Butterfly valve | Quarter-turn | Isolation, some regulation | Yes (some designs) | Large water and cooling lines, low pressure |
| Plug valve | Quarter-turn | Isolation, diverting | Limited | Slurries, multi-port diverting |
| Check valve | Automatic | Prevent reverse flow | No | Pump discharges, compressor outlets |
| Diaphragm valve | Linear | Isolation, regulation | Yes | Corrosive, abrasive or clean services |
| Needle valve | Linear, multi-turn | Fine regulation | Yes | Instrument lines, sampling |
| Control valve | Linear or rotary, actuated | Automatic regulation | Yes | Flow, pressure, level and temperature loops |
| Pressure relief / safety valve | Automatic | Overpressure protection | No | Vessels, pumps, heat exchangers |
Isolation valves
Gate valves lift a wedge or gate clear of the bore, so a fully open gate valve has almost no pressure drop. They are slow to operate and are not for throttling: a part-open gate vibrates and the seats erode. Common for isolation on larger lines.
Ball valves turn a ball with a bore through it a quarter turn between open and shut. They seal tightly, operate quickly and are the usual isolation valve in hydrocarbon and gas service. Full-bore designs allow pigging. Floating and trunnion-mounted balls cover small and large sizes.
Butterfly valves turn a disc in the flow a quarter turn. They are light, short and cheap in large sizes, which makes them common on water, seawater and cooling lines. High-performance and triple-offset designs extend them to higher pressures and tight shut-off.
Plug valves turn a tapered or cylindrical plug with a port through it. Lubricated and sleeved designs handle dirty services, and multi-port plugs divert flow between lines.
Regulating valves
Globe valves move a disc onto a seat in line with the flow, so the opening changes smoothly with stem travel. That makes them good for throttling, at the cost of a higher pressure drop than a gate valve. Angle valves are globe valves with the outlet at 90 degrees.
Needle valves use a fine tapered needle for precise control of small flows, typically on instrument impulse lines and sample points.
Diaphragm valves close a flexible diaphragm onto a weir or the body. The fluid never touches the moving parts, which suits corrosive, abrasive and hygienic services.
Check valves
Check valves (non-return valves) open with forward flow and close by themselves when flow reverses. They protect pumps and compressors from backflow and stop lines draining back.
- Swing check: a hinged disc, low pressure drop, horizontal or vertical upward flow.
- Lift check: a disc that lifts off the seat, like a globe valve, for small sizes and high pressure.
- Dual plate (wafer) check: two spring-loaded half discs, short and light, quick to close.
- Ball and piston checks: small sizes, instrument and utility lines.
Control valves and actuated valves
A control valve is a regulating valve (globe, rotary ball or butterfly) with an actuator and positioner that moves it in response to a signal from the control system, usually 4 to 20 mA or a digital fieldbus. It is the final element of a flow, pressure, level or temperature loop.
Other actuated valves open and shut only: on/off valves for sequencing, shutdown valves that close on a trip from the safety system, motor-operated valves for large or remote valves and solenoid valves for small lines and for venting actuators. Each has a fail position (fail open, fail closed or fail last) shown on the P&ID.
Control valve loops are proved by loop checks and stroke tests before commissioning.
Pressure relief and safety valves
A pressure safety valve (PSV) or pressure relief valve (PRV) opens automatically when the pressure reaches its set pressure and closes again when it falls. It is the last line of protection against overpressure for vessels, exchangers, pumps and piping.
Relief valves open in proportion to the overpressure and suit liquids; safety valves pop fully open and suit gases and steam; safety relief valves do both. Bursting (rupture) discs are a non-reclosing alternative, sometimes fitted upstream of a PSV.
Each PSV is set and tested, tagged with its set pressure and certificate, and installed with its inlet and outlet isolation valves locked in position. On a project, PSVs are usually removed or blinded for the pressure test and reinstated afterwards.
Valve standards
| Standard | Covers |
|---|---|
| ASME B16.34 | Flanged, threaded and welding-end valves: pressure-temperature ratings, wall thickness |
| API 6D | Pipeline and piping valves: gate, plug, ball and check |
| API 600 / API 602 | Steel gate valves (bolted bonnet) and compact steel gate, globe and check valves |
| API 608 / API 609 | Metal ball valves and butterfly valves |
| API 594 | Check valves: flanged, lug, wafer and butt-welding |
| API 526 | Flanged steel pressure relief valves |
| API 598 / ISO 5208 | Valve inspection and testing: shell and seat tests |
| IEC 60534 / ISA-75 | Industrial process control valves |
Valve tags and symbols on P&IDs
Manual valves are often shown by symbol and line number only, or with a short valve tag. Actuated and instrumented valves carry ISA-style tags: the letters say what the valve does, and the loop number ties it to its instruments.
| Tag | Usual meaning |
|---|---|
| FV, PV, LV, TV | Flow, pressure, level and temperature control valves |
| XV | On/off (automated) valve |
| SDV, ESDV | Shutdown valve, emergency shutdown valve |
| BDV | Blowdown valve |
| PSV, PRV | Pressure safety valve, pressure relief valve |
| MOV | Motor-operated valve |
| HV | Hand-operated valve (some projects) |
Conventions vary by project and company, so check the P&ID legend. The P&ID symbols library has every valve symbol in ISA and ISO form, and how to read a P&ID explains the rest of the sheet.
Valves in completions and commissioning
Each valve is a tag with its own records. Typically: the factory shell and seat tests and certificate (API 598), receipt inspection, installation check (orientation, flow direction, access), inclusion in the line's pressure test, and for actuated valves a function test with stroke time, limit switch and fail-safe checks. PSVs add a set pressure certificate and the lock position check.
Building that list starts with reading every valve off the P&IDs into the tag register. That is the slow part on a new project, and the part Deskely Extract does: it reads the P&IDs and hands the engineers a register to check.
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Frequently asked questions
What are the main types of valves?
Gate, globe, ball, butterfly, plug, check, diaphragm, needle, control and pressure relief or safety valves. They are grouped by duty: isolation, regulation, non-return and overpressure protection.
What is the difference between a gate valve and a globe valve?
A gate valve lifts a gate clear of the bore for isolation with little pressure drop and should not be used to throttle. A globe valve moves a disc onto a seat and is built for throttling, with a higher pressure drop.
Which valves can be used for throttling?
Globe, needle, diaphragm and control valves are designed for it. Butterfly and some ball valves can throttle within limits. Gate valves should not be used to throttle.
What is a non-return valve?
Another name for a check valve: it lets flow pass in one direction and closes on its own when the flow reverses.
What is the difference between a relief valve and a safety valve?
A relief valve opens gradually in proportion to the overpressure and is used for liquids. A safety valve pops fully open and is used for gases and steam. Safety relief valves can do both.
What does XV mean on a P&ID?
Usually an automated on/off valve. SDV is a shutdown valve, BDV a blowdown valve, and FV, PV, LV and TV are control valves. Check the project legend, as conventions vary.
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