Part I — Performance Terminology

1. Strength Performance The strength performance of a valve refers to its capacity to withstand the pressure exerted by the working medium. As pressure-bearing mechanical components, valves must possess sufficient structural strength and rigidity to ensure long-term, reliable operation without cracking or permanent deformation.

2. Sealing Performance Sealing performance refers to a valve’s ability to prevent medium leakage at all sealing interfaces, and is the most critical technical performance indicator. Three primary sealing points exist:

• The contact interface between the closing element and the valve seat sealing surfaces;
• The interface between the packing, valve stem, and stuffing box;
• The joint between the valve body and the valve bonnet.

Leakage at the first point is termed internal leakage, which compromises the valve’s shut-off capability and is impermissible in isolation valves. Leakage at the latter two points is termed external leakage, where medium escapes to the surrounding environment. External leakage causes material loss, environmental contamination, and potential safety hazards. For flammable, explosive, toxic, or radioactive media, external leakage is strictly unacceptable; therefore, all valves must maintain reliable sealing integrity.

3. Flow Resistance As medium flows through a valve, a pressure drop occurs across the inlet and outlet, indicating that the valve imposes resistance on the flow. The medium must expend energy to overcome this resistance. From an energy-efficiency standpoint, valve design and manufacture should aim to minimize flow resistance to the greatest extent practicable.

4. Opening and Closing Force and Torque These refer to the force or torque required to actuate the valve between open and closed positions. During closing, a defined seating load must be established between the closing element and the valve seat, while simultaneously overcoming friction at the valve stem–packing interface, the stem–nut thread engagement, the stem-end support, and other frictional components. The required actuating force and torque vary throughout the stroke, reaching maximum values at the final instant of closing or the initial instant of opening. Valve design should seek to minimize both closing force and closing torque.

5. Opening and Closing Speed Opening and closing speed is expressed as the time required to complete a full stroke. While no strict requirements generally apply, specific process conditions may impose special demands — for instance, rapid actuation to contain an emergency, or slow closure to prevent water hammer. These factors should be considered during valve selection.

6. Sensitivity and Operational Reliability This refers to the responsiveness of the valve to changes in medium parameters. For regulating devices — such as throttle valves, pressure reducing valves, and control valves — as well as functionally specific devices such as safety valves and steam traps, sensitivity and operational reliability are critical performance indicators.

7. Service Life Service life indicates the durability of a valve and is an economically significant performance indicator. It is typically expressed either as the number of open/close cycles achievable while maintaining sealing integrity, or as total service duration.

Part II — Classification and General Terminology

8. Valve Type Classification of valves according to their intended service or primary structural characteristics.

9. Valve Model A designation that encodes valve type, drive method, end connection style, structural characteristics, seat sealing surface material, and nominal pressure rating.

10. End Connection Dimensions The dimensional specifications at the interface between the valve and the connecting pipeline.

11. Overall Dimensions Key dimensional data including valve opening/closing height, handwheel diameter, and end connection dimensions.

12. Type of Connection The method used to connect a valve to a pipeline or piece of equipment, including flanged, threaded, and welded connections.

13. Seat Leakage Test (Seal Test) A test performed to verify the sealing integrity of the closing element against the valve seat.

14. Stem Seal Test (Back Seat Test) A test performed to verify the sealing integrity between the valve stem and valve bonnet.

15. Test Pressure The pressure specified for use during valve leakage testing.

16. Applicable Medium The fluid or gas for which the valve is rated and designed.

17. Applicable Temperature Range The permissible temperature range of the medium for a given valve.

18. Sealing Faces The two mating contact surfaces between the closing element and the valve seat that together form the primary seal.

19. Closing Element (Obturator) A collective term for components that interrupt or regulate medium flow, such as the gate in a gate valve or the disc in a throttle valve.

20. Packing Deformable sealing material installed in the stuffing box to prevent medium leakage along the valve stem.

21. Packing Seat (Packing Washer) A support component that retains the packing in position to maintain the stem seal.

22. Gland The component used to compress the packing axially, thereby effecting a stem seal.

23. Yoke The structural element on the valve bonnet or body that supports the stem nut and actuating mechanism.

24. Stem-to-Disc Connection Dimensions The dimensional specifications of the mechanical interface between the valve stem and the closing element.

25. Flow Area The minimum cross-sectional area between the valve inlet and the valve seat sealing surface (exclusive of the “curtain” area), used to calculate theoretical flow capacity without accounting for resistance losses.

26. Flow Diameter The diameter corresponding to the flow area.

27. Flow Characteristics Under steady-state flow conditions with constant inlet pressure, the functional relationship between the outlet pressure of a pressure reducing valve and the flow rate.

28. Flow Characteristic Deviation Under steady-state flow conditions with constant inlet pressure, the change in outlet pressure of a pressure reducing valve resulting from a variation in flow rate.

29. General-Purpose Valve Valves of standard design widely used across various industrial pipeline systems.

30. Self-Acting Valve A valve that operates automatically in response to the energy of the medium itself (liquid, gas, or steam), without external actuation.

31. Power-Actuated Valve A valve operated by an external power source, including manual, electric, hydraulic, or pneumatic actuation.

32. Impact Handwheel A handwheel design that utilises impact force to reduce the manual effort required to operate the valve.

33. Worm Gear Actuator A device that opens, closes, or modulates a valve via a worm-and-gear mechanism.

34. Pneumatic Actuator A drive device that opens, closes, or modulates a valve using compressed air.

35. Hydraulic Actuator A drive device that opens, closes, or modulates a valve using pressurised liquid.

36. Hot Condensate Capacity The maximum rate at which a steam trap can discharge condensate under a specified pressure differential and temperature condition.

37. Steam Loss The mass flow rate of live steam that escapes through a steam trap per unit time.

Part III — Valve Definition Terminology

1. Valve A mechanical device incorporating a movable element used to control the flow of media within a pipeline system.

2. Gate Valve A valve in which the closing element (gate) is driven by the valve stem and travels perpendicular to the flow path along the seating surfaces.

3. Globe Valve (Stop Valve) A valve in which the closing element (disc) is driven by the valve stem and travels along the axis of the valve seat.

4. Throttle Valve A valve that regulates flow rate and pressure by varying the cross-sectional area of the flow passage through movement of the closing element.

5. Ball Valve A valve that opens and closes by rotating a spherical closing element about an axis perpendicular to the flow direction.

6. Butterfly Valve A valve that opens and closes by rotating a disc about a fixed axis within the flow passage.

7. Diaphragm Valve A valve in which a flexible diaphragm, actuated by the valve stem, opens and closes the flow passage while isolating the actuating mechanism from the process medium.

8. Plug Valve (Cock) A valve that opens and closes by rotating a cylindrical or tapered plug about its own axis.

9. Check Valve (Non-Return Valve) A self-acting valve that opens and closes automatically under the force of the flowing medium to prevent reverse flow.

10. Safety Valve (Relief Valve) A self-acting valve that automatically opens to discharge medium when system pressure exceeds a predetermined set point, and automatically reseats when pressure falls below that value, thereby protecting pipelines and equipment from overpressure.

11. Pressure Reducing Valve A valve that reduces medium pressure through throttling of the closing element and automatically maintains downstream pressure within a defined range, responding directly to downstream pressure.

12. Steam Trap A self-acting valve that automatically discharges condensate while preventing the escape of live steam.

13. Drain Valve (Blowdown Valve) A valve used to discharge sludge, sediment, or condensate from boilers, pressure vessels, and similar equipment.

14. Low Pressure Valve Valves with a nominal pressure rating of PN ≤ 1.6 MPa.

15. Medium Pressure Valve Valves with a nominal pressure rating of PN 2.0 MPa to PN < 10.0 MPa.

16. High Pressure Valve Valves with a nominal pressure rating of PN ≥ 10.0 MPa.

17. Ultra-High Pressure Valve Valves with a nominal pressure rating of PN ≥ 100.0 MPa.

18. High Temperature Valve Valves designed for media at temperatures exceeding 450 °C.

19. Low Temperature Valve Valves designed for media at temperatures between −40 °C and −100 °C.

20. Cryogenic Valve Valves designed for media at temperatures below −100 °C.

Part IV — Valve Structure Terminology

1. Face-to-Face / Centre-to-Face Dimension (Structural Length) The distance between the end faces of the valve’s inlet and outlet connections; or, for angle-pattern valves, the distance from the inlet end face to the outlet centreline.

2. Through-Way Valve — Face-to-Face Dimension The distance between two planes perpendicular to the valve axis, located at each end of the valve body flow passage.

3. Angle Valve — Face-to-Face / Centre-to-Face Dimension The distance between a plane perpendicular to the axis at one end of the valve body passage and the centreline axis at the opposite end.

4. Construction Type The primary structural and geometric characteristics that define a valve’s design.

5. Through-Way Pattern A valve body configuration in which the inlet and outlet axes are collinear or parallel.

6. Angle Pattern A valve body configuration in which the inlet and outlet axes are perpendicular to each other.

7. Y-Pattern (Oblique-Body Type) A valve body configuration in which the flow passage is straight and the valve stem is positioned at an acute angle to the flow passage axis.

8. Three-Way Pattern A valve body configuration incorporating three flow port directions.

9. T-Pattern Three-Way A three-way configuration in which the plug or ball bore is T-shaped, allowing connection between all three ports.

10. L-Pattern Three-Way A three-way configuration in which the plug or ball bore is L-shaped, allowing connection between two adjacent ports.

11. Balanced Type A design configuration that employs medium pressure to counterbalance axial forces acting on the valve stem.

12. Lever Type A design configuration in which a lever mechanism drives the closing element.

13. Normally Open Type A design configuration in which the closing element defaults to the open position in the absence of external actuation.

14. Normally Closed Type A design configuration in which the closing element defaults to the closed position in the absence of external actuation.

15. Steam-Jacketed Type Valves incorporating a steam-heated jacket for temperature maintenance of the process medium.

16. Bellows Seal Type Valves incorporating a metal bellows assembly to provide a leak-free stem seal.

17. Full-Bore Valve A valve in which the internal flow passage diameter is equal to the nominal pipe bore throughout all sections of the valve.

18. Reduced-Bore Valve A valve in which the internal flow passage is reduced below the nominal pipe bore.

19. Reduced-Port Valve A valve with a reduced internal flow passage in which the port opening of the closing element is non-circular.

20. Uni-Directional Valve A valve designed to provide sealing in one direction of medium flow only.

21. Bi-Directional Valve A valve designed to provide sealing in both directions of medium flow.

22. Double-Block-and-Bleed (DBB) Valve — Dual Bi-Directional Seats A valve with two seating elements, each capable of sealing in both directions of flow.

23. Double-Block-and-Bleed (DBB) Valve — One Uni-Directional, One Bi-Directional Seat A valve with two sealing pairs that simultaneously maintain a sealed state in the closed position, with a port provided between the two seals to relieve trapped body cavity pressure. Designated DBB.

24. Back Seat A secondary sealing feature that prevents medium leakage past the stuffing box when the valve is in the fully open position.

25. Pressure-Seal Bonnet A bonnet design in which medium pressure acting within the valve body automatically energises the body-to-bonnet joint, providing a self-sealing effect.

26. Stem-Head Dimensions The dimensional specifications at the connection between the valve stem and the handwheel, handle, or other operating device.

27. Stem-End Dimensions The dimensional specifications at the connection between the valve stem and the closing element.

Part V — Valve Component Terminology

1. Body (Valve Body) The principal pressure-containing element that connects directly to the pipeline or equipment and forms the primary flow passage.

2. Bonnet (Valve Cover) The main closure element that connects to the valve body, forming the pressure-containing chamber — either directly or through an intermediate component such as a diaphragm.

3. Closing Element (Obturator) A collective term for the component(s) responsible for interrupting or regulating medium flow (e.g., the gate in a gate valve, the disc in a globe valve).

4. Disc The closing element of globe valves, throttle valves, check valves, and similar valve types.

5. Valve Seat (Body Seat Ring) A component installed within the valve body that forms a sealing pair with the closing element.

6. Sealing Faces The two mating contact surfaces between the closing element and the valve seat.

7. Valve Stem (Spindle) The principal load-transmitting element that conveys operating force or torque to the closing element.

8. Stem Nut (Yoke Bushing) The component that forms a motion pair with the stem thread, converting rotational to linear motion.

9. Stuffing Box (Packing Box) A cavity in the valve bonnet or body filled with packing material to prevent medium leakage along the valve stem.

10. Packing Compressible sealing material housed in the stuffing box to effect a dynamic stem seal.

11. Gland The component that applies compressive load to the packing to create and maintain the stem seal.

12. Packing Rings Individual rings of packing material stacked within the stuffing box.

13. Packing Seat (Packing Washer) A support ring that retains the packing stack and maintains the integrity of the stem seal.

14. Yoke The structural bracket mounted on the bonnet or body that supports the stem nut and actuating mechanism.

15. Impact Handwheel A handwheel incorporating a percussive mechanism to multiply the operator’s input force, used to break a valve free from a stuck position.