A valve is a piping accessory used for opening and closing pipelines, controlling flow direction, and regulating and controlling parameters of the transported medium (such as temperature, pressure, and flow).
Basic Terms
Strength Performance
The strength performance of a valve refers to its ability to withstand the pressure of the medium. Valves are mechanical products designed to handle internal pressure, requiring sufficient strength and rigidity to ensure long-term operation without rupture or deformation.
Sealing Performance
The sealing performance of a valve refers to its ability to prevent leakage at various sealing points. It is the most critical technical performance indicator of a valve.
There are three sealing points in a valve:
- The contact area between the closure member and the valve seat.
- The compatibility between packing and the valve stem and packing gland.
- The connection between the valve body and the bonnet.
Leakage at the first point is known as internal leakage, or failure to seal properly, which affects the valve’s ability to shut off the medium flow. For shut-off valves, internal leakage is not permissible.
Leakage at the latter two points is known as external leakage, where the medium leaks from inside the valve to the outside. External leakage results in material loss, environmental pollution, and in severe cases, accidents.
For flammable, explosive, toxic, or radioactive media, external leakage is especially unacceptable, necessitating valves to possess reliable sealing performance.
Flowing Medium
When a medium flows through a valve, it experiences pressure loss (the pressure difference before and after the valve). This indicates that the valve imposes resistance on the flow of the medium, requiring energy to overcome this resistance.
From an energy-saving perspective, when designing and manufacturing valves, efforts should be made to minimize the resistance that valves impose on flowing media.
Opening and Closing Force and Torque
Opening and closing force and torque refer to the force or torque required to open or close a valve. When closing a valve, it is necessary to create a certain sealing surface pressure ratio between the closing parts and the valve seat, while also overcoming frictional forces between the valve stem and packing, the threads between the valve stem and nut, friction at the valve stem end support, and other frictional areas. Thus, a certain closing force and torque must be applied. The opening and closing forces and torques required during the valve’s operation vary, with their maximum values occurring at the final moment of closing or the initial moment of opening. When designing and manufacturing valves, efforts should be made to minimize these closing forces and torques.
Opening and Closing Speed
Opening and closing speed refers to the time required for a valve to complete one opening or closing action. Generally, there are no strict requirements on the opening and closing speed of valves. However, some operating conditions have specific requirements, such as rapid opening or closing to prevent accidents, or slow closing to prevent water hammer effects. These factors should be considered when selecting valve types.
Responsiveness and Reliability
This refers to the sensitivity of a valve to changes in medium parameters and its ability to respond accordingly. For valves used for adjusting medium parameters like throttle valves, pressure reducing valves, and control valves, as well as valves with specific functions like safety valves and steam traps, responsiveness and reliability are crucial technical performance indicators.
Service Life
Service life indicates the durability of a valve and is an important performance indicator with significant economic implications. It is usually expressed in terms of the number of open-close cycles that can ensure sealing requirements, or alternatively, in terms of duration of use.
Type
Classification of valves based on purpose or main structural characteristics.
Model
A numbering system for valves based on type, actuation method, connection type, structural characteristics, valve seat sealing surface material, and nominal pressure, among other factors.
Connection Dimensions
Dimensions of the valve and pipe connection parts.
General Dimensions
Overall dimensions of the valve including its open and closed heights, handwheel diameter, and connection dimensions.
Type of Connection
Various methods (such as flanged connection, threaded connection, welding connection, etc.) used for connecting valves to pipes or machinery.
Seal Test
Test to evaluate the sealing performance of the closing parts and valve body.
Back Seal Test
Test to evaluate the sealing performance of the valve stem and bonnet seal.
Seal Test Pressure
The specified pressure during a seal test of the valve.
Suitable Medium
The types of media that the valve can accommodate.
Suitable Temperature
The temperature range of the media that the valve can accommodate.
Sealing Face
The two contacting surfaces where the closing parts and valve seat (or valve body) fit tightly together to create a seal.
Closing Parts
A general term for components used to cut off or regulate the flow of media, such as the disc in gate valves or the valve plug in throttle valves.
Packing
Material inserted into a stuffing box or packing box to prevent leakage of the medium from around the valve stem.
Packing Seat
The component that supports the packing and maintains its sealing.
Gland
A component used to compress packing to achieve a seal.
Yoke
A component on the bonnet or valve body that supports the valve stem nut and actuating mechanism.
Dimension of Connecting Channel
Structural dimensions of the assembly connection between the closing part and valve stem.
Flow Area
The minimum cross-sectional area between the inlet of the valve and the sealing face of the valve seat (but not the “curtain” area), used to calculate theoretical flow rate without any resistance effects.
Flow Diameter
The diameter corresponding to the flow area.
Flow Characteristics
The functional relationship between outlet pressure and flow rate of a pressure reducing valve under stable flow conditions, with inlet pressure and other parameters unchanged.
Flow Characteristics Derivation
The variation in outlet pressure caused by changes in flow rate of a pressure reducing valve under stable flow conditions, with inlet pressure and other parameters unchanged.
General Valve
Valves commonly used in various industrial pipelines.
Self-Acting Valve
Valves that operate based on the capabilities of the medium (liquid, air, steam, etc.) itself.
Actuated Valve
Valves operated by manual, electric, hydraulic, or pneumatic methods.
Hammer Blow Handwheel
A handwheel structure that uses impact force to reduce valve operating force.
Wormgear Actuator
A device using a worm gear mechanism to open, close, or adjust a valve.
Pneumatic Actuator
A driving device that uses air pressure to open, close, or adjust a valve.
Hydraulic Actuator
A driving device that uses hydraulic pressure to open, close, or adjust a valve.
Hot Condensate Capacity
The maximum amount of condensate water discharged by a steam trap under given pressure differential and temperature.
Valve Definition Terms
Valve
A general term for a mechanical product with a movable mechanism used to control the flow of media within a pipeline.
Gate Valve (Slide Valve)
A valve where the closing part (gate) is driven by a valve stem and moves up and down along the valve seat (sealing surface).
Globe Valve (Stop Valve)
A valve where the closing part (valve plug) is driven by a valve stem and moves up and down along the axis of the valve seat (sealing surface).
Throttle Valve
A valve that adjusts the flow rate or pressure by changing the cross-sectional area of the passage through the closing part (valve plug).
Ball Valve
A valve where the closing part (ball) rotates around a line perpendicular to the passage.
Butterfly Valve
A valve where the closing part (butterfly plate) rotates around a fixed axis.
Diaphragm Valve
A valve where the closing part (diaphragm) is driven by a valve stem and moves up and down along the stem axis, isolating the actuating mechanism from the medium.
Cock (Plug Valve)
A valve where the closing part (plug) rotates around its axis.
Check Valve (Non-Return Valve)
A valve where the closing part (valve disc) automatically prevents the reverse flow of media by the action of the medium itself.
Safety Valve (Relief Valve)
A valve that automatically opens to discharge excess pressure when the pressure inside the pipeline or equipment exceeds a specified value, and closes automatically when the pressure falls below the set value, thereby protecting the pipeline or equipment.
Pressure Reducing Valve
A valve that reduces the pressure of the medium through throttling by the closing part (valve disc) and maintains the downstream pressure within a specified range through direct action of the downstream pressure.
Steam Trap
A valve that automatically discharges condensate while preventing steam leakage.
Draining Valve
A valve used for discharging sludge from equipment such as boilers and pressure vessels.
Low Pressure Valve
Valves with a nominal pressure PN ≤ 1.6 MPa.
Middle Pressure Valve
Valves with a nominal pressure in the range of PN ≥ 2.0 to PN < 10.0 MPa.
High Pressure Valve
Valves with a nominal pressure PN ≥ 10.0 MPa.
Super High Pressure Valve
Valves with a nominal pressure PN ≥ 100.0 MPa.
High Temperature Valve
Valves used for media with temperatures above 450°C.
Low Temperature Valve
Valves used for media with temperatures between -40°C and -100°C.
Cryogenic Valve
Valves used for media with temperatures below -100°C.
Valve Structure Terms
Structural Length (Face-to-Face Dimension, Face-to-Center Dimension)
The distance between the face surfaces of the valve’s inlet and outlet, or the distance from the inlet face to the outlet axis.
Through Way Type Valve Structural Length
The distance between two planes perpendicular to the valve axis at the ends of the valve body’s passage.
Angle Type Valve Structural Length
The distance between a plane perpendicular to the axis at one end of the valve body’s passage and the axis at the other end.
Type of Construction
The main characteristics of various valves regarding their structure and geometric shape.
Through Way Type
A valve type where the inlet and outlet axes are either coincident or parallel.
Angle Type
A valve type where the inlet and outlet axes are perpendicular to each other.
Y-Type (Y-Globe Type, Y-Type, Diaphragm Type)
A valve type where the passage is in a straight line, and the valve stem is positioned at an acute angle to the valve body’s passage axis.
Three-Way Type
A valve type with three passage directions.
T-Pattern Three-Way
A three-way valve where the passage through the plug (or ball) is in the shape of a “T”.
L-Pattern Three-Way
A three-way valve where the passage through the plug (or ball) is in the shape of an “L”.
Balance Type
A valve type that uses medium pressure to balance the axial force on the valve stem.
Lever Type
A valve type where the closing part is operated by a lever mechanism.
Normally Open Type
A valve type where the closing part is automatically in the open position without external force.
Normally Closed Type
A valve type where the closing part is automatically in the closed position without external force.
Steam Jacket Type
Valves with a steam heating jacket structure.
Bellows Seal Type
Valves equipped with a bellows seal structure.
Full-Opening Valve
A valve where the internal passage diameter is the same as the nominal pipeline diameter throughout the entire valve.
Reduced-Opening Valve
A valve with a reduced diameter of the internal passage.
Reduced-Bore Valve
A valve with a reduced diameter of the internal passage, where the passageway in the valve’s closing part is non-circular.
Unidirectional Valve
A valve designed to seal against medium flow in only one direction.
Bidirectional Valve
A valve designed to seal against medium flow in both directions.
Twin-Seat, Both Seats Bidirectional Valve
A valve with two sealing seats where each seat can seal both directions of medium flow.
Twin-Seat, One Unidirectional and One Bidirectional Seat Valve
A valve with two sealing pairs, where both pairs can maintain a sealed state in the closed position, and the valve body between the two pairs has an interface to relieve medium pressure. Denoted by DBB.
Back Seal
A sealing structure that prevents medium leakage at the packing box when the valve is fully open.
Pressure Seal
A structure that uses medium pressure to achieve automatic sealing at the connection between the valve body and bonnet.
Dimension of Valve Stem Head
The structural dimensions where the valve stem connects with the handwheel, handle, or other actuating mechanisms.
Dimension of Valve Stem End
The structural dimensions where the valve stem connects with the closing part.
Dimension of Connecting Channel
The structural dimensions of the connection between the closing part and the valve stem.
Type of Connection
Various methods used for connecting the valve to pipes or machinery, such as flanged, threaded, or welded connections.
