What are the essential parts of a valve? It is a complicated mechanical device assembled with several different components: The body (the pressure-containing shell built of cast or forged steel with flanged, threaded, or socket weld pipe connections), the bonnet, the trim (the disc, seat, stem, and other wet components), the packing, and the actuator (manual, gear, or automated).
Parts of Valve
The body is an essential component because it keeps the moving components in contact with the fluid and must withstand the pressure in the pipe network. The body can be cast or forged from steel in a variety of shapes, designs, and material grades. ASTM A216 is the most commonly used material in the oil and gas industries for cast bodies, and ASTM A105 is the most commonly used material for forged bodies (high-temperature service). For low-temperature usage, ASTM A352 LCB/LCB and ASTM A350 LF2/LF3 are utilized for cast and forged bodies, respectively. As the temperature, pressure, or corrosion increases, stainless steel bodies are required: ASTM A351 CF8 (SS304) and CF8M (SS316) for cast devices; and ASTM A182 Fxx (F304, F316, F321, F347) for forged types.Special material grades with even higher corrosion resistance, such as super austenitic stainless steels (SMO 254), duplex, and super duplex steels (F44, F51, F53, F55), and nickel alloys, are utilized for specialized purposes (Inconel, Incoloy, Hastelloy). Nonferrous materials or alloys, such as Monel, Cupronickel, Aluminum-bronze alloys, and other alloys combining Nickel, Copper, and Aluminum, are the best options for marine applications. Cast iron bodies, the most affordable type, are used for water distribution (low-pressure applications).
Body End Connections
Valves can connect to other mechanical devices and pipes in a number of ways. For larger diameter devices, the major end types are flanged and buttweld; and for smaller diameter devices, socket weld or threaded/screwed (NPT or BSP). More articulated end types on the butterfly include lugs, wafers, and double-flanged ends.
The device includes two flanged ends that can be joined to a pipe with the use of a mating (companion) flange. The ASME B16.5 specification specifies the number of stud bolts and nuts required for a flanged connection, as well as an appropriate gasket. Because they provide long-lasting, strong connections, flanged joints are employed for large diameter valves. The flange face can be raised, flat, ring joint, tongue and groove, male and female, and finished in any of the varying types (the most frequent finish is the RF type, i.e. raised face) (stock, serrated or smooth).
Socket Weld And Buttweld Ends
The connecting pipe is welded to the valves using ASME B16.11 socket welds or ASME B16.25 buttweld ends. Welded connections are more expensive to execute than flanged joints because they involve more effort, however, they are more durable and less prone to leaks in the long run. High-pressure pipes have socket weld and buttweld ends (socket weld for smaller sizes, below 2 inches, and buttweld for larger diameters). Buttweld connections require complete welding of the beveled ends of the two components to be connected, whereas socket weld connections use fillet welds.
Threaded End Connection
In this situation, the device is connected to the pipe through a thread (tapered thread), which might be BSP or NPT (more common in the petrochemical industry). This form of connection is used for low-pressure pipes with bore sizes of less than 2 inches that are not affected by mechanical forces like vibration and elongation. Threaded connections are simpler and less expensive to install since the pipe is simply screwed into the valve without the use of flanges, stud bolts, or welding processes. However, in the case of a leak, threaded connections must be replaced and cannot be fixed (which is instead possible for the two previous types of pipe-to-end connections).
The bonnets are additional significant parts for the gate and globe types. By removing the bonnet, maintenance workers have access to the internal systems and may replace components such as the seat, stem, and so on (trim). Valve bonnets are made of cast steel or forged steel and come in a variety of types (the most common being bolted, round bolted, welded, and pressure seal) (generally using the same material grade as the body). The bonnet is connected to the body by threaded, bolted, or welded joint connections, and several types of gaskets are used to provide leak-free connections between the body and the bonnet.
Trim refers to all of the interior valve parts that may be removed and changed (these parts are also referred to as “wet” since they come into full contact with the conveyed fluid). The trim typically includes components such as the disc, the seat, the stem, the glands, the bushings, and the sleeves needed to guide the stem (the actual list of parts that make up the trim depends, actually, on the type of device). Trim is an important component, and materials should be selected with consideration. Things to consider include pressure, temperature, and the type of fluid. The API trim chart has standardized typical trim combinations for gate, globe, and check valves.
The disc opens, shuts, or regulates the fluid flow. In the case of a gate valve, the gate (wedge) is the valve disc (whereas the disc is called a ball for ball valves). After the body and the bonnet, the disc is the third most crucial component for a valve’s successful functioning. Discs are typically made of forged steel and are frequently reinforced (hard-faced) to improve the mechanical qualities of the base material.
The movement of the disc is enabled by the seats. A globe or a swing-check valve has one seat that, if combined with the motion of the disc, may open and close the fluid route, shutting off and/or throttling the fluid.
Gate valves typically have two seats, one on the upper and one on the lower side. Seats are frequently hard-faced. When the device is closed, the correct finish on the seats provides a strong seal.
The stem of a valve is used to close and open the valve as well as to move the disc. The stem is connected to the valve actuator or the manual hand wheel (or lever) at one end and the valve disc at the other.
The stem regulates a linear movement on the disc in gate and globe valves, while in ball, butterfly, and plug valves, the disc rotates to close or open the device (“quarter-turn valves”).
Stems are composed of forged steel and are threaded or otherwise fixed to the disc. To avoid leaks, the surface of the stem must be properly coated.
The packing is the gasket that seals the stem with the bonnet and consists of the following components:
- Gland follower, a sleeve that is compressed into the stuffing box by a gland.
- Gland, a bushing that compresses the packing into the stuffing box.
- Stuffing box, a chamber that compresses the packing
- Packing comes in various materials including PTFE, elastomers, fibrous material, and so on.
- A backseat is located within the bonnet. The back seat acts as a seal between the stem and the bonnet, preventing system pressure from building up on the valve packing once the valve is fully open. In gate and globe valves, back seats are often used.
- The valve packing must be correctly designed and constructed to reduce the risk of stem damage and fluid leakage. On the other hand, keep in mind that too tight packing may cause the stem to crack.
A hand-operated or manual valve often has a hand wheel that can be turned clockwise or counter-clockwise to open and/or close the valve (typical for gate and globe valves). A lever is used to move a ball, plug, or butterfly (manual quarter-turn valves). The use of manual valves is not feasible or recommended in the following situations:
- Large-diameter valves that operate at high pressures
- Valves that should be remotely controlled
- Valves that must open or close very quickly due to the nature of the process
A valve actuator is required in all of these situations. The actuator generates linear and rotational movement capable of opening or closing a valve (the actual motion depends on use on the type of the valve, linear or quarter turn).
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