Valve Glossary: 88 Terms Every Piping and Welding Engineer Should Know
This valve glossary explains the words on every valve data sheet, from gate and globe to Cv, pressure class and shell test, so that purchase orders, welding procedures and inspection plans mean the same thing to everyone.
Quick Answer: A valve glossary defines the terms used to specify, buy, weld and test valves. The core groups are valve types (gate, globe, ball, check), parts (body, bonnet, stem, trim), end connections (flanged, butt-weld, socket-weld), pressure class and Cv, standards such as ASME B16.34 and API 600, and tests such as shell and seat tests.
Each term is defined in plain English with the standard that governs it, linked to the relevant ASME B31.3 process piping guide and welding topics. Use the welding terminology A-Z glossary for general welding terms.
Definitions are summaries; the valve standard, data sheet and manufacturer instructions govern.
Figure 1: Common valve types – gate, globe, ball and check – arranged for comparison.
Key Takeaways
- Valve terms fall into seven groups: types, parts, ends and dimensions, ratings and flow, standards, testing, and materials and service.
- Pressure class (150 to 2500) is a rating index, not a psi value; allowable pressure also depends on material group and temperature in ASME B16.34.
- Gate and ball valves isolate, globe and control valves regulate, and check valves stop backflow; never throttle with a gate valve.
- Cv equals Q times the square root of SG over pressure drop, so 100 gpm of water at 4 psi drop needs a Cv of 50.
- Welders meet valve terms in butt-weld and socket-weld ends, body and trim materials, and hardfacing such as Stellite 6.
What Is a Valve Glossary and How Should You Use It?
A valve glossary is a grouped list of the technical terms used to describe, specify, order, weld and test industrial valves. Terms are grouped in the order a valve is chosen and used: type, parts, ends, rating, standard, test and material. Reading in that order turns 88 separate words into one story, and the wider welding vocabulary is in the welding terminology A-Z glossary.
What is a valve?
A valve is a mechanical device that starts, stops, regulates or directs fluid flow by opening, closing or partially obstructing a passage. The main types are gate, globe, ball, plug, butterfly, check and relief valves, and each is specified by type, size, pressure class, end connection, body material and trim.
| Group | Question it answers |
|---|---|
| Types | What does the valve do? |
| Parts | What is it made of? |
| Ends and dimensions | How does it connect? |
| Ratings and flow | How much pressure and flow? |
| Standards | Which specification applies? |
| Testing | How is it proved? |
| Materials and service | What fluid and duty? |
What Are the Main Types of Valves?
Valve types differ by motion and by duty: linear-motion valves (gate, globe) are multi-turn, while ball, plug and butterfly valves are quarter-turn. The glossary below is followed by a comparison table and a selection chart.
- Gate valve
- A linear-motion valve in which a wedge or parallel gate slides across the bore to start or stop flow. Used for on-off isolation, not throttling, because partial opening erodes the gate and seats.
- Globe valve
- A linear-motion valve in which a plug moves against a seat in the flow path. Suited to throttling and regulation, with a higher pressure drop than gate or ball valves.
- Ball valve
- A quarter-turn valve whose bored ball rotates 90 degrees between open and closed. Gives fast shutoff and, in full-bore designs, very low pressure drop.
- Plug valve
- A quarter-turn valve with a tapered or cylindrical plug containing a port. Used for isolation, including dirty or abrasive services in lubricated or sleeved designs.
- Butterfly valve
- A quarter-turn valve with a disc on a central shaft. Compact and light for large diameters, supplied with wafer, lug or flanged bodies.
- Check valve
- A non-return valve that opens with forward flow and closes automatically when flow reverses. Also called a non-return valve (NRV).
- Swing check valve
- A check valve with a hinged disc that swings off the seat under forward flow. Common in horizontal lines and can slam if flow reverses rapidly.
- Piston (lift) check valve
- A check valve whose piston or disc lifts straight off the seat. Typical of small forged steel valves and suited to high-pressure lines.
- Dual-plate check valve
- A wafer-style check valve with two spring-loaded half-plates on a central hinge pin. Short and light, and it closes faster than a swing check.
- Needle valve
- A small globe-style valve with a tapered needle plug for fine flow control, common on instrument and sampling lines.
- Diaphragm valve
- A valve in which a flexible diaphragm presses onto a weir or seat. Suited to clean, corrosive or sanitary service because the stem stays out of the fluid.
- Pinch valve
- A valve that closes by squeezing a flexible sleeve. Used for slurries and abrasive media.
- Pressure relief valve (PRV)
- A spring-loaded or pilot-operated valve that opens at a set pressure to protect equipment from overpressure and recloses as pressure falls. Vessel requirements are summarised in the ASME Section VIII Division 1 overview.
- Safety valve
- A pressure relief valve with rapid pop-open action, used mainly on gas and steam duty.
- Control valve
- A valve with an actuator and positioner that modulates flow in response to a control signal to hold pressure, flow, level or temperature.
- Solenoid valve
- An electrically operated on-off valve in which a solenoid coil moves the plug. Used on small lines and for pilot or air service.
| Valve | Motion | Best use | Avoid for |
|---|---|---|---|
| Gate | Linear, multi-turn | On-off isolation, low pressure drop | Throttling |
| Globe | Linear, multi-turn | Throttling and regulation | Lines needing minimal pressure drop |
| Ball | Quarter-turn | Fast shutoff, full-bore isolation | Prolonged throttling |
| Plug | Quarter-turn | Isolation, dirty fluids | Fine control |
| Butterfly | Quarter-turn | Large lines, light weight | Tight shutoff at high pressure (unless high-performance type) |
| Check | Self-acting | Preventing backflow | Isolation |
| Diaphragm | Linear | Clean, corrosive, sanitary | High pressure and temperature |
What Are the Parts of a Valve Called?
Valve parts are best learned from the outside in: the pressure boundary (body, bonnet, packing), the operating gear (stem, yoke, actuator) and the internal trim. Figure 2 labels the main parts of an OS&Y gate valve.
- Body
- The main pressure-containing shell with the flow passage and end connections. Forged or cast, and the part that carries the pressure rating.
- Bonnet
- The cover over the body opening through which internal parts are fitted. Types include bolted, welded and pressure-seal bonnets.
- Pressure-seal bonnet
- A bonnet in which internal pressure forces a seal ring tighter against the body, so higher pressure improves sealing. Commonly used in high-pressure, high-temperature valves.
- Stem
- The shaft that transmits motion from the handwheel or actuator to the gate, disc or ball. May be rising or non-rising.
- OS&Y (outside screw and yoke)
- A design where the stem threads sit outside the pressure boundary and the stem rises as the valve opens, giving visible position indication. Standard on API 600 gate valves.
- Disc, gate and wedge
- The moving element that contacts the seat to block flow. Called a gate or wedge in gate valves and a disc in globe and check valves.
- Seat
- The stationary sealing surface that the disc, gate or ball closes against. May be metal or soft (PTFE, nylon, elastomer).
- Trim
- The internal parts in contact with the process fluid, chiefly the seat surfaces, disc or gate, stem and backseat bushing. Trim material is chosen for corrosion and wear.
- Backseat
- A sealing surface that seats the stem against the bonnet when the valve is fully open, limiting stem leakage while packing is serviced as the manufacturer permits.
- Packing
- Compressible sealing material, such as graphite or PTFE, around the stem in the stuffing box that stops leakage along the stem.
- Stuffing box
- The chamber around the stem that holds the packing.
- Gland
- The follower that compresses the packing, tightened by gland nuts or bolts.
- Yoke
- The frame on the bonnet that supports the stem nut, handwheel or actuator.
- Actuator
- A manual, electric, pneumatic or hydraulic device that operates the valve, including gear operators.
- Bellows seal
- A flexible metal bellows welded to the stem that seals it without packing. Used for low-leakage stem sealing on hazardous fluids.
What Do Valve End Connections and Dimensions Mean?
End connection terms tell you how the valve joins the line, and dimension terms tell you whether it fits. Weld-end valves need a bevel and procedure, while flanged valves depend on flange class and face.
- Flanged end
- A valve end with a flange bolted to the mating pipe flange. Types and faces are covered in the flange types and faces guide.
- Raised face (RF)
- A flange face with a raised ring around the bore that concentrates gasket load. The most common face on process valves.
- Ring-type joint (RTJ)
- A flange face with a machined groove for a metal ring gasket, used for high-pressure and severe service.
- Butt-weld (BW) end
- A valve end bevelled for butt welding to the pipe, with dimensions per ASME B16.25. Joint geometry is explained in welding joint types.
- Socket-weld (SW) end
- A socket into which the pipe is inserted and fillet welded. Used on small-bore forged valves.
- Threaded end (NPT)
- Tapered pipe threads, usually per ASME B1.20.1, for small-bore, non-critical service.
- Wafer
- A body clamped between two pipe flanges by long through-bolts, with no flanges of its own.
- Lug
- A body with threaded lugs so each pipe flange is bolted to the valve separately, which can allow downstream piping removal within the manufacturer’s limits.
- Face-to-face dimension
- The distance between the end faces of the valve, standardised in ASME B16.10 so that valves interchange. Called end-to-end for weld-end valves.
- NPS and DN
- NPS is nominal pipe size in inches and DN is the metric equivalent, so NPS 4 equals DN 100.
- Full bore (full port)
- A valve bore equal to the pipe bore, giving minimal pressure drop and allowing pigging.
- Reduced bore (reduced port)
- A valve bore smaller than the pipe bore, usually one size smaller, giving a lower cost and a higher pressure drop.
What Do Valve Pressure Class, Rating and Cv Mean?
Pressure class and Cv answer two different questions: how much pressure the valve can hold, and how much fluid it can pass. The glossary entries are followed by approximate class ratings and a worked Cv calculation.
- Pressure class
- A rating index (150, 300, 600, 900, 1500, 2500) that sets allowable pressure for a given material group and temperature. It is not a direct psi value; see the B16.5 pressure-temperature guide.
- Pressure-temperature (P-T) rating
- The table of maximum allowable pressure against temperature for each class and material group in ASME B16.34. Allowable pressure falls as temperature rises.
- Design pressure and design temperature
- The conditions a piping system is designed for under the piping code, such as the ASME B31.3 process piping guide. Valve ratings must equal or exceed them. Wall thickness checks use the pipe wall thickness calculator.
- MAWP
- Maximum allowable working pressure, the highest permitted pressure at the coincident temperature, used mainly for pressure vessels.
- CWP and WOG
- Cold working pressure is the non-shock pressure rating at ambient temperature. WOG on a valve body means the rating applies to water, oil and gas.
- Flow coefficient (Cv)
- The flow in US gpm of 60 degrees F water that passes a valve with a 1 psi pressure drop. A higher Cv means more capacity.
- Kv
- The metric flow coefficient: flow in cubic metres per hour of water with a 1 bar drop. Cv is about 1.156 times Kv.
- Cracking pressure
- The minimum upstream pressure at which a check valve begins to open.
- Set pressure
- The pressure at which a pressure relief valve starts to lift.
| Pressure class | Rating (psig) | Rating (bar) |
|---|---|---|
| 150 | 285 | 19.6 |
| 300 | 740 | 51.1 |
| 600 | 1,480 | 102.1 |
| 900 | 2,220 | 153.2 |
| 1500 | 3,705 | 255.3 |
These are typical group 1.1 values at ambient temperature; the allowable pressure drops as temperature rises, so always read the current ASME B16.34 table for the material and temperature.
Flow coefficientCv = Q x sqrt(SG / dP)
Q = flow (US gpm), SG = specific gravity (water = 1), dP = pressure drop (psi) Worked example: water, Q = 100 gpm, dP = 4 psiCv = 100 x sqrt(1 / 4)
Cv = 100 x 0.5 = 50 Metric equivalentKv = Cv / 1.156 = 50 / 1.156 = 43.3 m3/h per sqrt(bar)
Q = flow (US gpm), SG = specific gravity (water = 1), dP = pressure drop (psi) Worked example: water, Q = 100 gpm, dP = 4 psiCv = 100 x sqrt(1 / 4)
Cv = 100 x 0.5 = 50 Metric equivalentKv = Cv / 1.156 = 50 / 1.156 = 43.3 m3/h per sqrt(bar)
Which Standards Apply to Valves?
Valve standards define design, dimensions, ratings, testing and marking, and a valve specification usually names several at once. The list below groups the standards that appear most often on data sheets.
- ASME B16.34
- Valves – Flanged, Threaded and Welding End. Covers pressure-temperature ratings, dimensions, materials, examination and testing.
- ASME B16.10
- Face-to-face and end-to-end dimensions of valves.
- ASME B16.5
- Pipe flanges and flanged fittings, NPS 1/2 to 24, which set the flange ends of most valves.
- ASME B16.25
- Butt-welding ends, giving the weld bevel dimensions for BW valves and fittings.
- ASME B31.3
- The process piping code that governs the system in which the valve is installed.
- API 600
- Steel gate valves with flanged and butt-welding ends and bolted bonnets, for refinery and similar service.
- API 602
- Compact steel gate, globe and check valves, mainly forged and NPS 4 and smaller.
- API 608
- Metal ball valves with flanged, threaded and welding ends.
- API 609
- Butterfly valves: double-flanged, lug-type and wafer-type.
- API 594
- Check valves: flanged, lug, wafer and butt-welding types.
- API 6D
- Pipeline and piping valves covering gate, ball, check and plug valves in Classes 150 to 2500.
- API 598
- Valve inspection and testing, including shell, backseat and seat tests.
- API 607
- Fire test for quarter-turn valves and valves with non-metallic seats.
- ISO 15848
- Industrial valves: measurement, test and qualification procedures for fugitive emissions.
- NACE MR0175 / ISO 15156
- Material requirements for equipment used in H2S-containing (sour) environments.
How Are Valves Tested and What Do the Terms Mean?
Valve testing terms describe how strength and tightness are proved before shipment. The numbers below are from the standards’ typical requirements; the purchase order and edition in force govern.
- Shell test
- A hydrostatic test of the body and bonnet to prove strength, with the valve partly open. B16.34 and API 598 set it at 1.5 times the 38 degrees C rating.
- Backseat test
- A test that the stem backseat seals when the valve is fully open.
- Seat (closure) test
- A test of leakage past the closed seat, typically a liquid test at 1.1 times the rating plus a low-pressure gas test.
- Seat leakage class
- The permitted leakage at the seat, for example ISO 5208 rates A to G or ANSI/FCI 70-2 classes for control valves.
- Bubble-tight
- Seat leakage so low that no bubbles appear in a standard test; zero visible leakage.
- Fire-safe test
- A test, such as API 607, showing that a valve with soft seats still seals acceptably after fire exposure.
- Fugitive emissions test
- A test of stem and body leakage of volatile gases under ISO 15848 or similar procedures.
- Special Class (B16.34)
- A quality class that adds volumetric examination of castings to the Standard Class requirements. Methods are outlined in the ASME Section V overview.
- Material test certificate (MTC)
- The mill or foundry document giving chemistry and mechanical properties, commonly an EN 10204 type 3.1 certificate. Where it fits is shown in the ITP preparation guide.
Figure 4: A valve hydrostatic shell and seat test set-up with pressure gauge and hand pump.
Worked example: Class 300 carbon steel valve, 38 degrees C rating 740 psigShell test = 1.5 x 740 = 1,110 psig (rounded up per B16.34 rules)
Seat closure test = 1.1 x 740 = 814 psig
Which Valve Materials and Service Terms Should You Know?
Material grade codes and service terms tell you what the valve is made of and where it can safely work. Grouping by P-Number for procedure qualification is explained in the P-Number, F-Number and A-Number guide.
- A105
- ASTM A105, forged carbon steel for piping components, used for forged valves and flanges.
- WCB
- ASTM A216 Grade WCB, cast carbon steel for valve bodies in general service.
- LCC
- ASTM A352 Grade LCC, cast carbon steel for low-temperature service with impact testing.
- CF8M
- ASTM A351 Grade CF8M, the cast equivalent of 316 stainless steel. Duplex options are covered in the duplex stainless steel guide.
- WC6, WC9 and C12A
- ASTM A217 cast Cr-Mo grades: WC6 is 1.25Cr-0.5Mo, WC9 is 2.25Cr-1Mo, and C12A is the cast 9Cr-1Mo-V grade matching Grade 91, covered in the P91 requirements guide. Effects of chromium and molybdenum are in alloying elements in steel.
- Stellite (hardfacing)
- A cobalt-chromium alloy deposited by weld overlay on seats and discs for wear and galling resistance. Stellite 6 is the common grade; see the weld overlay and cladding guide.
- 13Cr trim
- Martensitic Type 410 stainless trim giving corrosion and erosion resistance in hydrocarbon service.
- Sour service
- Service containing hydrogen sulfide, where materials and hardness must meet NACE MR0175 / ISO 15156.
- Cavitation
- Formation and collapse of vapour bubbles downstream of a restriction, causing noise, vibration and damage to valve trim.
- Water hammer
- A pressure surge caused by sudden flow stoppage, for example rapid valve closure or check valve slam.
- Fail-safe (fail open, fail closed)
- The position an actuated valve moves to on loss of air, power or signal.
- Double block and bleed (DBB)
- An arrangement of two isolation valves with a vent between them, giving positive isolation for maintenance.
How Do Valve Terms Affect Welding and Fabrication?
Valve terms matter to welders in three places: end preparation, protection of internals during welding, and repair or overlay. Always follow the valve manufacturer’s welding instructions.
- End preparation: butt-weld ends follow ASME B16.25 and socket-weld ends need the correct gap before fillet welding.
- Protecting internals: many manufacturers specify the valve position and a limit on body temperature during welding to protect soft seats and seals.
- Hardfacing and repair: seat overlays use cobalt alloys such as Stellite 6, and casting repairs follow the material specification and B16.34.
- Documentation: certificates and PMI records link the valve to its specified material.
Regional note. On Indian and Gulf EPC projects, valve data sheets typically quote API, ASME and ISO standards together, and clients commonly require MTCs and PMI for alloy valves. Check which edition the project specification names.
Common Mistakes and Limits When Reading Valve Terms
Valve terms are easy to misread, and requirements depend on the code edition and contract specification applicable to your project.
- Treating pressure class as a psi value.
- Using a gate valve for throttling.
- Confusing face-to-face with end-to-end dimensions on weld-end valves.
- Assuming a reduced-bore valve has the same capacity as a full-bore valve.
- Ignoring the temperature when reading a P-T rating.
Limit. This glossary explains terms; it does not replace the valve standard, data sheet or manufacturer’s instructions.
Quick Reference: Valve Abbreviations Cheat Sheet
| Abbreviation | Meaning |
|---|---|
| BW | Butt-weld end |
| SW | Socket-weld end |
| RF | Raised face |
| RTJ | Ring-type joint |
| FF | Flat face |
| OS&Y | Outside screw and yoke |
| NRV | Non-return valve (check valve) |
| PRV | Pressure relief valve |
| DBB | Double block and bleed |
| CWP | Cold working pressure |
| NPS | Nominal pipe size |
| WOG | Water, oil, gas rating |
| Cv | Flow coefficient (US gpm per psi drop) |
Frequently Asked Questions
What is the difference between a gate valve and a globe valve?
A gate valve slides a wedge or gate across the bore and is used for on-off isolation with low pressure drop. A globe valve moves a plug against a seat in an S-shaped flow path and is used for throttling. Using a gate valve partly open erodes the gate and seat.
What does pressure class 150 or 300 mean on a valve?
Pressure class is a rating index, not a pressure in psi. The allowable pressure depends on the class, the material group and the temperature, taken from the ASME B16.34 tables. For carbon steel at ambient temperature, Class 150 is about 285 psig and Class 300 about 740 psig.
What is the difference between API 600 and API 6D valves?
API 600 covers bolted-bonnet steel gate valves, mainly for refinery process piping. API 6D covers pipeline and piping valves, including gate, ball, check and plug valves, in Classes 150 to 2500. Choose by service and by what the project specification names.
What is Cv and how is it used?
Cv is the flow coefficient: the US gpm of 60 degrees F water that passes with a 1 psi drop. It is used to size control valves and estimate pressure drop. The worked example in the ratings section shows the calculation.
What is the difference between a shell test and a seat test?
A shell test pressurises the body and bonnet to prove the pressure boundary is strong enough, normally at 1.5 times the 38 degrees C rating. A seat test checks leakage past the closed seat, normally at 1.1 times the rating plus a low-pressure gas test.
What does OS&Y mean on a gate valve?
OS&Y means outside screw and yoke. The stem threads sit outside the fluid, and the stem rises as the valve opens, so the position is visible. It is the standard design for API 600 gate valves.
Why do welders need to know valve terms?
Welders and welding engineers deal with valve end connections (butt-weld and socket-weld), body and trim materials, hardfacing, and the rating that governs inspection. Misreading a term can mean the wrong end preparation, filler or heat treatment.
Key Terms to Learn First
- Pressure class
- A rating index, not a psi value.
- Cv
- Flow capacity of a valve.
- Trim
- Internal parts that touch the fluid.
- OS&Y
- Rising stem with threads outside the fluid.
- BW and SW
- Weld-end connections.
- Shell test
- Hydrostatic strength test of the body.
- PMI
- Positive material identification of the alloy.
Conclusion
This valve glossary gives 88 terms in seven groups: types, parts, ends, ratings, standards, tests and materials. Learn pressure class, Cv, trim, OS&Y, end connections, shell test and PMI first, then use the standards named on your data sheet for details. Welders should pair this vocabulary with the welding terminology A-Z glossary and the ASME B31.3 process piping guide.
About This Guide
This guide was prepared by the WeldFabWorld technical team from the standards listed in References. Verify all requirements against the code edition and specification applicable to your project.
Standards and References
- ASME B16.34 – Valves – Flanged, Threaded, and Welding End. American Society of Mechanical Engineers (ASME).
- ASME B16.10 – Face-to-Face and End-to-End Dimensions of Valves. American Society of Mechanical Engineers (ASME).
- ASME B16.5 – Pipe Flanges and Flanged Fittings: NPS 1/2 Through NPS 24 Metric/Inch Standard. American Society of Mechanical Engineers (ASME).
- ASME B16.25 – Buttwelding Ends. American Society of Mechanical Engineers (ASME).
- ASME B31.3 – Process Piping. American Society of Mechanical Engineers (ASME).
- API 600 – Steel Gate Valves – Flanged and Butt-welding Ends, Bolted Bonnets. American Petroleum Institute (API).
- API 602 – Gate, Globe, and Check Valves for Sizes DN 100 (NPS 4) and Smaller. American Petroleum Institute (API).
- API 6D – Specification for Pipeline and Piping Valves. American Petroleum Institute (API).
- API 598 – Valve Inspection and Testing. American Petroleum Institute (API).
- API 607 – Fire Test for Quarter-turn Valves and Valves Equipped with Nonmetallic Seats. American Petroleum Institute (API).
- ISO 15848 – Industrial valves – Measurement, test and qualification procedures for fugitive emissions. International Organization for Standardization (ISO).
- ISO 5208 – Industrial valves – Pressure testing of metallic valves. International Organization for Standardization (ISO).
- NACE MR0175 / ISO 15156 – Materials for use in H2S-containing environments in oil and gas production. NACE International / ISO.
- EN 10204 – Metallic products – Types of inspection documents. European Committee for Standardization (CEN).
Official sources: ASME, American Petroleum Institute and ISO.
Related Guides
Flanges — Types, Face Types, Grades, Gaskets and ASME StandardsFlange types and faces used on valve ends.ASME B31.3 Process Piping — The Complete Engineer’s GuideThe piping code governing valves in process plants.ASME B31.1 vs B31.3 — Power Piping vs Process Piping: Key Differences Every Engineer Must KnowHow the two piping codes differ.Weld Overlay / Cladding — Types, Process & ASME Requirements: The Complete Technical GuideHardfacing for valve seats and wear surfaces.Pressure Vessel Fabrication & Types: Complete Engineering GuideVessels, nozzles and relief devices.Mechanical Properties of Metals — Complete Engineering GuideStrength and hardness behind valve materials.Mechanical Testing of Materials — The Complete Engineering GuideTests behind material certificates.Welding Consumable Selection – A Smart Guide for Engineers and Quality ProfessionalsFillers for valve and piping welds.