Pressure Retaining Parts
Valve body, bonnet or cover, disc, and body-bonnet bolting are classified as pressure- retaining parts of a valve. The following provides a brief description of these parts:
Body. The valve body houses the internal valve parts and provides the passage for fluid flow. The valve body may be cast, forged, fabricated, or made by a combination of cast, forged, or fabricated portions. Valve bodies can be and are made from a variety of metals and alloys. Also, valve bodies can be and are made of nonmetals; however, these must be within certain size and pressure-rating limits. The valve-body ends are designed to connect the valve to the piping or equipment nozzle by different types of end connections, such as butt or socket-welded, threaded, flanged or bolted, soldered, brazed, solvent cement joint, mechanical joint, or coupling. Refer to Table A10.6.
Bonnet or Cover. The bonnet or cover is fastened to the valve body to complete the pressure-retaining shell. In case of gate, globe, stop check, and diaphragm valves, it contains an opening for the valve stem to pass through. Usually, it contains a stuffing box. It provides access to valve internals, especially when the valve is installed. The top works of valves include bonnet, yoke, and operating mechanism. The bonnet is the base that supports the valve top works. The valve bonnet is attached to the valve body by many different types of joints: bolted, pressure-seal joints—breach lock and standard pressure seal joints— threaded, welded, union joint, and clamp seal. Some valves have a bonnetless design in which valve body and bonnet are combined into one. This is also known as an integral bonnet. The bolted bonnet design is commonly used with cast or ductile iron, cast or forged steel, and alloy valves, which are rated NPS 2¹⁄₂ (DN 65) and larger, and Class 600 and below. Valves with a pressure seal–bonnet design are usually manufactured in Class 600 and higher rating classifications and are considered superior in regard to the leaktightness of the body bonnet joint. Bolted bonnet designs are also produced in valves rated Class 900 and higher. The user needs to consider the potential consequences of flow-medium leakage through body bonnet joints and take the necessary measures to prevent or contain the leakage.
Bonnet or Cover Bolting. Bolting includes bolts, nuts, and washers. The bolting to be used must be made from materials acceptable for the application in accordance with the applicable code, standard, specification, or the governing regulation. Refer to the applicable valve standard for acceptable bolting materials.
Disc. The disc is the part which allows, throttles, or stops flow, depending on its position. In the case of a plug or a ball valve, the disc is called plug or a ball. A valve disc could be cast, forged, or fabricated. A disc is seated against the stationary valve seat or seats when the valve is in the closed position. It can be moved away from the valve seat(s) by motion of the valve stem, with the exception of check and safety-relief valves, in which the disc is moved away from its seat(s) by fluid flow and pressure. At times some users do not consider the valve disc to be a pressure-retaining or -containing part. The reasoning advanced is that when the valve is in an open position, the disc does not perform a pressure-retaining or -containing functions. However, when the same valve is closed, the disc performs pressure-retaining functions. Refer to Table A10.7 for disc materials.
Valve Trim. The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as valve trim. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim. Valve trim parts may be constructed of assorted materials because of the different properties needed to withstand different forces and conditions. Bushings and packing glands do not experience the same forces and conditions as do the valve disc and seat(s). Flow-medium properties, chemical composition, pressure, temperature, flow rate, velocity and viscosity are some of the important considerations in selecting suitable trim materials. Trim materials may or may not be the same material as the valve body or bonnet. API has standardized trim materials by assigning a unique number to each set of trim materials. Refer to Table A10.8 for API trim materials.
Nonpressure Retaining Parts. Valve seat(s), stem, yoke, packing, gland bolting, bushings, handwheel, and valve actuators are some of the major nonpressure retaining parts of a valve.
Valve Seat(s). A valve may have one or more seats. In the case of a globe or a swing-check valve, there is usually one seat, which forms a seal with the disc to stop the flow. In the case of a gate valve, there are two seats; one on the upstream side and the other on the downstream side. The gate-valve disc or wedge has two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow. Multiport plug and ball valves may have several seats, depending upon the number of ports in the plug or ball. The valve leakage rate is directly proportional to the effectiveness of the seal between the valve disc and its seat(s). The valve standards MSS SP 61, API 598, and ASME B16.34 specify acceptable leak rates. A user may specify more or less restrictive leak rates to satisfy the application requirements. Valve manufacturers have developed several designs of combination valve seats involving elastomer and metal seats that are effective in achieving the desired leaktightness, which is not readily accomplished with metal seats. Valve seats may be integral, replaceable, or renewable seat rings. Small valves generally are provided with screwed-in, swaged-in, welded, or brazed-in valve seats. Large valves may have any of the seat designs listed for small valves, or have seats integrally cast or forged with the valve body and hardened by heat treatment or surfaced with hard material such as Stellite. Stellite is a trade name of the Deloro Company. There are other metals that can be used for hardening the seating surfaces.
Galling Prevention. In order to prevent or minimize galling of the valve disc and valve seats, it is common industry practice to maintain a hardness differential between the stationary seating surfaces of valve seats and the moving seating surfaces of the valve disc. The stationary valve seats are hardened slightly more than the disc-seating surfaces. When both the valve seats and the disc are hardened by use of Stellite, the valve is termed fully stellited. When only the seats are hardened by use of Stellite, the valve is called half stellited. Heat treatment is another method of hardening.
Valve Stem. The valve stem imparts the required motion to the disc, plug, or the ball for opening or closing the valve. It is connected to the valve handwheel, actuator, or the lever at one end and the valve disc on the other. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball, and butterfly valves, the valve disc is rotated to open or close the valve. With the exception of stop-check valves, check valves do not have valve stems.
Rising Stem with Outside Screw and Yoke. The outermost part of the stem is threaded, while the portion of stem inside the valve is smooth. The stem threads are isolated from the flow medium by the stem packing. Two different styles of this design are available: one having the handwheel fixed to the stem so that they rise together, and the other having a threaded sleeve that causes the stem to rise through the handwheel. The rising stem with outside screw and yoke (O. S. & Y.) is a common design for NPS 2 (DN 50) and larger valves. Some codes, such as ASME B31.1, Power Piping, require that an outside screw-and-yoke design be used for NPS 3 (DN 80) and larger valves in pressures above 600 psi (4140 kPa). See Fig. A10.1.
Rising Stem with Inside Screw. The threaded part of the stem is inside the valve body, and the stem packing is along the smooth part that is exposed to the atmosphere outside. In this case the stem threads are 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-to-moderate pressure gate, globe, and angle valves.
Nonrising Stem with Inside Screw. The threaded part of the stem is inside the valve and does not rise. The valve disc travels along the stem like a nut when the stem is rotated. Stem threads are exposed to the flow medium and, as such, are subjected to its impact. Therefore, this design is used where space is limited to allow linear stem movement, and the flow medium does not cause erosion, corrosion, or wear and tear of stem material. See Fig. A10.2.
Sliding Stem. This stem does not rotate or turn. It slides in and out of the valve to close or open the valve. This design is used in hand-lever–operated quick opening valves. It is also used in control valves that are operated by hydraulic or pneumatic cylinders.
Rotary Stem. This is a commonly used design in ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.
Stem Packing. Stem packing performs one or both of the following two functions, depending on the application:
Prevent leakage of flow medium to the environment
Prevent outside air from entering the valve in vacuum applications
Stem packing is contained in a part called the stuffing box. Packing rings are packed and compressed by tightening a packing nut or packing gland bolts. Compression must be adequate to achieve a good seal. Sometimes it requires regular inspection and tightening of packing rings, if required, to stop leakage. If this does not stop the leakage, the packing may need to be replaced. Belleville washers are used to maintain live loading, or the required compression of packing to achieve an effective seal against leakage.
A stuffing box may be provided with some or all of the following features as dictated by the valve application:
Two sets of packing rings separated by an intermediate lantern ring
A bottom junk ring
A leak-off connection which detects leakage past the lower set of packing rings and is piped off to a leakage collection tank
A blow-off connection for removal of packing rings using compressed air
Belleville washer, live loading
A steam-seal connection, where an external steam supply is used to prevent leakage from the packing chamber
As an alternate to steam-seal connection, a grease or sealant-seal connection used to prevent loss of vacuum within the valve
Inverted-V packing for vacuum service
Figure A10.3a shows a standard graphite packing arrangement; Figure A10.3b depicts the inverted Teflon packing arrangement for vacuum service; Figure A10.3c reflects a lantern-ring packing arrangement; and Figure A10.3d shows a live-loading packing system.
Stem Protector. In the case of outside-screw-and yoke rising-stem gate and globe valves, a portion of the threaded valve stem is exposed to the outside environment when the valve is in the open position. Airborne dirt and other substances may be deposited on the exposed portion of the threaded stem and impair its smooth operation or shorten its stem-bushing life. A stem protector in the form of a clear plastic sleeve, tubing, or a pipe with a cap at the end is installed to protect the stem. The length of the stem protector must be adequate to allow full stem travel.
Back Seat. Back seat is comprised of a shoulder on the stem and a mating surface on the underside of the bonnet. It forms a seal when the stem is in the fully open position. It prevents leakage of flow medium from the valve shell into the packing chamber and consequently to the environment. Back seat enables dismantling of the valve beyond the bonnet, without disrupting the fluid flow through the valve. In addition, it allows the replacing of the stuffing box while the valve is in service.
Yoke. Yoke is also called yoke arms. It connects the valve body or bonnet with the actuating mechanism. In some cases, it provides support for the gland-pull- down bolts. On many valves, the yoke and bonnet are designed as one-piece construction. The top of the yoke holds a yoke nut, stem nut, or yoke bushing and the valve stem passes through it. See Fig. A10.1. For power-actuated valves, the yoke arms are of a heavier construction to provide adequate support to the actuator. The yoke usually has openings or windows to allow access to the stuffing box, position-switch dogs, actuator couplings, et cetera. Structurally, a yoke must be sturdy enough to withstand forces, moments, and torque developed by the actuator.
Yoke Bushings. An internally threaded nut held in the top of a yoke through which the valve stem passes. In gate and diaphragm valves, the yoke nut is turned and the stem travels up or down depending upon the direction of rotation of the nut. In the case of globe valves, the nut is held fixed and the stem is rotated through it. Usually, the yoke nut or yoke bushing is made of softer material than the stem for valves requiring medium effort to actuate. Valves which require greater effort to open or close are provided with anti-freeze yoke-sleeve bearings that minimize the friction between the hardened stem and the yoke bushing.