What is a Valve?
A Valve is a device comprising of an obturator or closure member that is used to control flow or pressure in a piping system.
Valves are one the most important component of a piping system and specifying the correct type, design and material of valve is crucial for plant operation. The function of valves in a piping system is one of the following:
- Start and stop flow (On-off valves)
- Control or regulate flow (Regulating valves)
- Prevent back-flow (Check valves)
- Divert flow (Multi-port valves)
- Relieve pressure from a piping system (Pressure relief valves or Pressure-vacuum relief valves)
Types of Valves
The various types of valves commonly used in any plant facilities are:
- Gate valves
- Globe valves
- Check valves
- Ball valves
- Plug valves
- Butterfly valves
- Pinch valves
- Diaphragm valves
Valves which relieve pressure are excluded from this discussion. They are discussed in a separate article.
Classification of Valves based on Closure Element
In addition to the function of the valve, valves can also be classified based on the movement of the closure element.
Based on movement of Obturator or Closure Element, valves can be classified into the following broad categories.
Closing Obturator: In this type of valve the closure member i.e. disc or plug moves along the seat axis, towards or away from the direction of valve seat or port. Globe valve, Needle valve and Piston valve fall in this category of valves.
Sliding Obturator: In this type of valve, the closure member i.e. wedge gate or parallel gate moves perpendicular to the direction of flow at the valve port. Gate valves fall in this category of valves. Ball valves, Plug valves and Butterfly valves fall in this category of valves.
Rotary Obturator: In this type of valve, the closure member has a port which is turned through 90 degrees such that the port aligns with the direction of flow allowing full flow across the valve or the port is at right angles to the direction of flow shutting off the flow.
Flexible Obturator: In this type of valve, the closure member is a flexible passage which is flattened or pinched to restrict the flow and vice versa to let flow pass through the valve. Diaphragm valves and Pinch valves fall in this category of valves.
Components of a Valve
Pressure Containing Components: Valve body, bonnet or cover, disc, and body-bonnet bolting are specified as pressure-containing components of a valve as they are subject to the full design pressure of the specified valve. The pressure containing components are designed to withstand the design pressure and temperature conditions specified in the valve data sheet. Since the pressure containing components come in contact with the process fluid their material is required to be compatible with the service. In many cases for a cost effective design the pressure containing components are specified in carbon steel material with internal lining or overlay to withstand the corrosive fluid.
Non-Pressure Containing Components: Components of a valve such as valve seats, stem, yoke, packing, gland bolting, bushings, hand wheel, lever, gear operator or other valve actuators are not required to be designed for pressure containment and are considered as non-pressure containing components.
Valve Body
The valve body is the main pressure containing part of the valve and its thickness is determined by the governing standard that defines the pressure rating of the valve. For example ASME B16.34 defines the thickness of valve bodies for various types of valves and pressure ratings. Valve body gets subject to shell test pressure as per the applicable testing standard of the valve.
The body of the valve can be in cast or forged construction. Very small sized valves are in some cases manufactured out of bar stock. In most cases, valves up to 1½" are purchased in forged construction and valves 2" and larger in cast bodies. The shape of the valve body depends on the type of valve.
The ends of a valve can be flanged, socket-welded, threaded or butt-welded connections depending on the piping material class requirements. Most company specifications require the flanges to be integral with the body of the valve. Valves with flanged connections welded to the body are seldom accepted by most companies.
If the valve is specified for an application that requires a corrosion allowance larger than 3 mm, the requirement to compensate the valve body thickness with higher corrosion allowance must be clearly highlighted in the valve specifications and data sheets. It may be noted that internal lining or cladding on the valve body does not contribute to the pressure design thickness of the valve body.
Valve Bonnet
The bonnet covers the valve body and acts as a support and guide for the stem and gland packing. Not all valves have bonnet. Check valves have only cover. Similarly there is no defined bonnet for a conventional two piece or three piece design ball valve. Bonnet design will vary with the design of the valve. The bonnet is in most cases bolted or screwed to the valve body. For a full welded design the bonnet is welded to the body of the valve. The connection between the valve body and bonnet is subjected to the full valve design pressure and is designed accordingly. In case of bolted bonnet joints the gaskets used shall be compatible with the fluid service and suitable for the design conditions of the valve. The material of bonnet is normally same as valve body.
Valve Bonnet Design can be one of the following types:
Threaded Bonnet: In a threaded bonnet design the bonnet is attached to the body by screwing it into or onto the body neck. Threaded Bonnet can be Screw-In type or Screw-On type. In a screw-in bonnet the bonnet has external threads and the neck of the body has internal threads. In a screw-on bonnet, the bonnet has internal threads and the neck of the body has external threads.
Union Bonnet: Union Bonnet design offers a quick and easy method of assembling and disassembling the bonnet from the valve body. The union bonnet provides a tight closure with least effort
Bolted Bonnet: In a bolted bonnet design, the bonnet and the body flanges are bolted together. This type of bonnet design results in an Outside Screw and Yoke (O.S&Y) design with rising stem. The stem has threads on the outside of the valve body and hence does not come in contact with the fluid service. This eliminates the possibility of corrosion or erosion of the stem threads. The stem on the outside of the body can be easily lubricated for extending the life of the valve.
Valve Stem
The valves stem transfers the rotary motion or torque applied on the hand-wheel, lever or actuator to the obturator i.e. disk, ball or plug causing it to open, close or throttle the flow. In case of ball, plug and butterfly valves the rotary motion of stem is transferred to the disk as is. However, in case of gate and globe valves the rotary motion of stem is translated into linear motion of the disk. Since torque is applied on the stem, it should have enough mechanical strength to withstand the shear forces on the stem. The breakaway torque of a valve is usually larger than its runaway torque. Breakaway torque is the torque required for moving a valve from a fully closed position and under full differential pressure. Stem design shall be based on the maximum torque the stem will be subjected to during its operation. In addition to shear force a stem may be subject to a thrust force due to pressure acting along the stem axis as in case of a globe valve. In such cases the stem design shall also taken into consideration the maximum thrust force acting on the stem. Typically, the stem is not specified as a pressure containment part while preparing valve data sheets.
In valves where the stem is normally in contact with the fluid service, the stem material must be compatible with the piping service. In such cases the stem design shall ensure no leakage of fluid between the stem and the gland packing. The stem is required to have a fine surface finish to prevent any leakage across the gland packing. Stems are normally of forged construction and connected to the obturator by means of the following types of stem designs:
Rising Stem with Outside Screw and Yoke: The exposed part of the stem is threaded, while the part of stem inside the valve is smooth. The exposed stem threads, therefore do not come in contact with the process fluid. There are two styles of stems, one with the handwheel fixed to the top of the stem which causes the handwheel to move at tandem with the stem, and the other with a threaded sleeve that causes the stem to rise through the center of handwheel which is at a fixed level. Rising stem with outside screw and yoke (OS&Y) is commonly specified for valves 2 inches and larger.
Rising Stern with Inside Screw: The threaded part of the stem is inside the valve body, and the stem packing is encases the smooth section of exposed stem. In this design, the stem threads come in contact with the flow medium. When rotated, the stem and the handwheel rise together to open the valve. This design is commonly used in the smaller-sized low pressure gate and globe valves.
Non-Rising Stem with Inside Screw: The threaded section of the stem is inside the valve and does not rise. The valve disc travels linearly with respect to the stem. In this design, the stem threads come in contact with the flow medium. Therefore, this design is used where linear movement of stem is restricted due to space constraints and where process fluids are not corrosive, erosive and do no cause wear and tear of stem material.
Sliding Stem: The stem does not rotate, and it is without a thread. It slides in and out of the valve packing to close, open, or position the valve closure member. This design is used in hand-lever-operated, quick-opening valves. It is also used in control valves operated by hydraulic or pneumatic cylinders.
Rotary Stem: This is the most commonly used design in stem of ball, plug, and butterfly valves. A quarter-turn rotary motion of stem causes the valve closure member to fully open or shut the valve.
Valve Disk and Seats
The valve disk, plug or ball is considered as a pressure containment component since it gets exposed to the full differential pressure when the valve is in closed position. In partially open position the disk, plug or ball is not subject to full differential pressure and does not act as a pressure containment component. When the valve is cracked open the valve port can see high fluid velocities that can cause erosion in the seat and disk. Not all valve types are designed for throttling and hence due care must be exercised as ball valve and gate valve disks are not designed for throttling or regulating flow.
The disk plug or ball is held by the seats to form a seal for stopping the flow. Gate and ball valves have two seating surfaces whereas globe, butterfly and check valves have one seating surface. The disk provides the capability for permitting and prohibiting fluid flow. A highly smooth surface finish and a differential hardness is maintained between the disk and seat to achieve good sealing characteristics. In most cases seat rings are threaded, welded or press fit to the body of the valve and can be replaced if they get damaged. There are some designs where the body surface serves as the seating surface, but this is not a preferred design. Disks are usually manufactured in forged construction and specified with hard overlays such as tungsten carbide or stellite overlay to improve their wear resistance.
Valve Trim
As per API 600, trim of a valve includes the stem, the gate seat surfaces, the body (or seat ring) seat surfaces and the backseat stem contact surface. Table 13 of API 600 also specifies the basic trim materials.
Valve Stem Packing
The stem packing acts a seal between the valve internals which contains the pressurized process fluid and the external environment. Manufacturers have their patented design for stem packing to ensure that the packing is compatible with the process fluid while maintaining the leak tight conditions. For example graphite seals are not used as primary containment seals but only as back-up seals for fire resistance properties.
There is a worldwide focus now on limiting fugitive emissions. A good stem seal design is important in controlling fugitive emissions. Two commonly used designs in achieving effective sealing is chevron packing or O-rings. The packing in the stuffing box is held in compressed state by screw type connection or bolted connection. The flange type seal easy maintenance and replacement of the stem packing. As the valves are cycled into open and closed positions the stem seals are subject to wear and need replacement.
Valve stem packing is inspected periodically. If there are any signs of leakage the packing is usually compressed by using gland nuts.
Valve Backseat and Stem Protector
Backseat: Stems in gate and glove valves are provided with a shoulder inside the bonnet which serves as a backseat. When the valve is in the fully open position the stem backseat forms a seal with the stem and shuts off pressure to the stuffing box thereby preventing the leakage of process fluid through the stem packing. The backseat essentially serves as a backup to the primary stem packing seal. The backseat permits the replacement of stem packing on-line without having to shutdown the operations. Backseat test is carried out as part of the valve inspection and testing process.
Stem Protector: In gate and globe valves of OS&Y rising stem design, a stem protector is used to protect the stem from damage
Valve Actuator
The valve actuator operates the stem and disk gate plug or ball assembly. An actuator may be a manually operated handwheel operated, lever operated, gear operated, motor actuated, pneumatically actuated or hydraulically actuated. When the actuator is not provided by the manufacturer, close interface is required between the valve manufacturer and actuator supplier to ensure overall integrity of the actuated valve. In such situations it is important to have a single point responsibility for the entire assembly.
Valve Codes and Standards
The material that follows includes the most commonly used standards for valves used in ASME B31 process projects from a variety of recognized bodies. These codes and standards contain the rules and requirements for design, pressure-temperature ratings, dimensions, tolerances, materials, nondestructive examinations, testing, and inspection and quality assurance. Compliance to these and other standards is invoked by reference to codes of construction, specifications, contracts, or regulations.
ASME Standards
- ASME B16.10 - Face-to-Face and End-to-End Dimensions of Valves
- ASME B16.20 - Metallic Gaskets for Pipe Flanges: Ring Joint, Spiral Wound, Jacketed
- ASME B16.21 - Non Metallic Flat Gaskets for Pipe Flanges. 2.4 Valves 83
- ASME B16.38 - Large Metallic Valves for Gas Distribution (Manually Operated, NPS 2%" to 12, 125 psig Maximum)
- ASME B16.40 - Manually Operated Thermoplastic Gas Shutoffs and Valves in Gas Distribution Systems
- ASME B18.2.1 - Square and Hex Bolts and Screws
- ASME B18.2.2 - Square and Hex Nuts
AWWA Standards and Specifications
- AWWA C500 - Metal-Seated Gate Valves for Water Supply Service
- AWWA C501 - Cast-Iron Sluice Gates. C504, Rubber-Seated Butterfly Valves
- AWWA C507 - Ball Valves, 6 in. through 48 in. (150 mm through 1200 mm)
- AWWA C508 - Swing Check Valves for Waterworks Service
- AWWA C509 - Resilient-Seated Gate Valves for Water Supply Service
- AWWA C510 - Double Check Valve Backflow Prevention Assembly
- AWWA C511 - Reduced Pressure Principle Backflow Prevention Assembly
- AWWA C512 - Air-Release, Air-Vacuum, and Combination Air Valves for Waterworks Service
- AWWA C540 - Power-Actuating Devices for Valves and Sluice Gates
- AWWA C550 - Protective Epoxy Interior Coatings for Valves and Hydrants
British Standards and European Norms
- BS 1868 - Steel check valves (flanged and butt-welding ends) for petroleum, petrochemical and allied industries.
- BS 6755 - Testing of valves - Part 2. Specification for fire type testing requirements
- BE EN 1984 - Industrial Valves - Steel Gate valves
- BS EN ISO 17292 - Metal ball valves for petroleum, petrochemical and allied industries
- BS EN 15761 - Steel gate, globe and check valves for sizes DN100 and smaller, for the petroleum and natural gas industries
- BS EN 12266 Part 1 and 2 - Industrial valves Testing of valves
- EN 10204 - Metallic products - Type of inspection documents
- DIN EN ISO 5210 - Industrial valves - Multi-turn valve actuator with attachments
American Petroleum Institute Specifications
- API 6D - Specification for Pipeline Valves (Gate, Plug, Ball, and Check Valves)
- API 6FA - Specification for Fire Test for Valves
- API 6FB - Specification for Fire Test for End Connections
- API 6FC - Specification for Fire Test for Valves with Automatic Backseats.
- API 6FD - Specification for Fire Test for Check Valves
- API 14A - Specification for Subsurface Safety Valve Equipment.
- API 14D - Specification for Wellhead Surface Safety Valves and Underwater Safety Valve for Offshore Service
API Standards
- API 526 - Flanged Steel Pressure Relief Valves
- API 527 - Seat Tightness of Pressure Relief Valves
- API 589 - Fire Test for Evaluation of Valve Stem Packing
- API 594 - Wafer and Wafer-Lug Check Valves
- API 598 - Valve Inspection and Testing
- API 599 - Metal Plug Valves-Flanged and Welding Ends
- API 600 - Steel Gate Valves-Flanged and Butt-Welding Ends
- API 602 - Compact Steel Gate Valves-Flanged, Threaded, Welding, and Extended-Body Ends
- API 603 - Class 150, Cast, Corrosion-Resistant, Flanged-End Gate Valves
- API 607 - Fire Test for Soft-Seated Quarter-Turn Valves
- API 608 - Metal Ball Valves-Flanged, Threaded, and Welding Ends
- API 609 - Butterfly valves, double flanged, lug and wafer type
MSS Standards
- MSS-SP-6 - Standard Finishes for Contact Faces of Pipe Flanges and Connecting-End Flanges of Valves and Fittings
- MSS-SP-9 - Spot Facing for Bronze, Iron and Steel Flanges
- MSS-SP-25 - Standard Marking System for Valves, Fittings, Flanges, and Unions
- MSS-SP-42 - Class 150 Corrosion Resistant Gate, Globe, Angle, and Check Valves with Flanged and Butt-Weld Ends
- MSS-SP-45 - Bypass and Drain Connection Standard
- MSS-SP-53 - Quality Standard for Steel Castings and Forgings for Valves, Flanges, and Fittings and Other Piping Components-Magnetic Particle Examination Method
- MSS-SP-54 - Quality Standard for Steel Castings and Forgings for Valves, Flanges, and Fittings and Other Piping Components-Radiographic Examination Method
- MSS-SP-55 - Quality Standard for Steel Castings and Forgings for Valves, Flanges, and Fittings and Other Piping Components-Visual Method
- MSS-SP-60 - Connecting Flange Joint between Tapping Sleeves and Tapping Valves
- MSS-SP-61 - Pressure Testing of Steel Valves. MSS-SP-67, Butterfly Valves
- MSS-SP-68 - High Pressure-Offset Seat Butterfly Valves. MSS-SP-70, Cast Iron Gate Valves, Flanged and Threaded Ends
- MSS-SP-71 - Cast Iron Swing Check Valves, Flanged and Threaded Ends
- MSS-SP-72 - Ball Valves with Flanged or Butt-Welding Ends for General Service
- MSS-SP-78 - Cast Iron Plug Valves, Flanged and Threaded Ends
- MSS-SP-80 - Bronze Gate, Globe, Angle and Check Valves
- MSS-SP-81 - Stainless Steel, Bonnetless, Flanged Knife Gate Valves
- MSS-SP-82 - Valve Pressure Testing Methods
- MSS-SP-84 - Valves-Socket-Welding and Threaded Ends
- MSS-SP-85 - Cast Iron Globe and Angle Valves, Flanged and Threaded Ends
- MSS-SP-86 - Guidelines for Metric Data in Standards for Valves, Flanges, Fittings, and Actuators.
- MSS-SP-88 - Diaphragm Type Valves
- MSS-SP-91 - Guidelines for Manual Operation of Valves. MSS-SP-92, MSS Valve User Guide
- MSS-SP-93 - Quality Standard for Steel Castings and Forgings for Valves, Flanges, and Fittings and Other Piping Components-Liquid Penetrant Examination Method
- MSS-SP-94 - Quality Standard for Steel Castings and Forgings for Valves, Flanges, and Fittings and Other Piping Components-Ultrasonic Examination Method
- MSS-SP-96 - Guidelines on Terminology for Valves and Fittings
- MSS-SP-98 - Protective Epoxy Coatings for the Interior of Valves and Hydrants
- MSS-SP-99 - Instrument Valves
- MSS-SP-100 - Qualification Requirements for Elastomer Diaphragms for Nuclear Service Diaphragm Type Valves
- MSS-SP-101 - Part-Turn Valve Actuator Attachment-Flange and Driving Components Dimensions and Performance Characteristics
- MSS-SP-102 - Multi-Turn Valve Actuator Attachment-Flange and Driving Component Dimensions and Performance Characteristics
- MSS-SP-105 - Instrument Valves for Code Applications
- MSS-SP-108 - Resilient Seated-Eccentric Cast Iron Plug Valves
Valve Data Sheets
Valve data sheets are prepared and issued by Engineering Companies for procurement purposes. The valve data sheets are prepared for each type of valve specifying the main features as defined in the relevant piping material class and the applicable specification. The valve data sheets should specify the service fluid to ensure compatibility of the valve seals and gland packing material for the intended service. For soft seated valves it is customary to specify the actual maximum and minimum operating temperatures instead of the design temperatures which may be much higher. For pipeline valves, the data sheets must also specify the pipeline internal diameter and tolerances.