Valve is assembled product. Valve external parts and Valve trim parts such as Body, Bonnet, Disk/wedge, Seat, Steam, Gland Packing / Stud & Bolt / Gasket / Handwheel are all manufactured independently and assembled in valve factory.
In the image below you can see the main parts of a valve.
Image – Velan
You can see the cutout of the gate valve. Main Components of the Valve are
- Seat, Packings
Let’s learn about each of these parts of a valve.
Valve Body or Shell
The body is Main Pressure retaining part and accommodate valve trim. It provides the passage for fluid flow. The body may be cast, forged, or fabricated. Sometimes valve bodies are manufactured by a combination of cast, forged, or fabricated parts. A variety of metals, alloys, and non-metals are used to manufactured valve body. The valve body is also known as a shell.
Ends of the valve are designed to connect the valve with pipe or equipment. Ends connections can be a butt, socket, threaded, flanged type and sometimes it simply sandwiches between two pipe flanges that are known as a wafer ends.
A valve body has different types of passages through which fluid pass. Design of this passages depends on the function of a valve
- First body type is Reduced bore, in this type, passage diameter of the valve is smaller than the connecting This is most common design as it will reduce overall valve cost at the same time it narrows the fluid flow.
- The second type is a Full bore, in this type inside passage diameter of the valve is same as connecting pipe. This type of body is used when pigging is required. Pigging is used for a various purpose such as cleaning and inspection of the pipeline.
- The third type is Crossflow or Split section body, this kind of body is used mainly in globe valve, piston or plug type check valve.
You can see the images of all three types of body.
From the above image, you can now easily work out the difference between full-bore valve and reduce bore valve.
Bonnet or Cover
The cover for the valve body is known as a bonnet. Like valve bodies, bonnets are also available in many designs.
Some bonnets function simply as a valve cover. For example, swing check valve as shown in the photo. While others support valve internals and accessories such as the stem, disk, and actuator. In the case of the gate, globe, stop check, and diaphragm valves, bonnet contains an opening for the valve stem to pass through. Usually, stuffing box is also a part of the bonnet.
Some valves have a bonnetless design in which valve body and bonnet are combined into one. You can see the bonnetless valve photo. In split body ball valve, there is no bonnet at all, because a body itself is split into two sections. There are many ways to connect bonnet with a body such as bolting, threading, and welding. The body-bonnet joint is one of the primary sources of the leak, that is why it should be a pressure tight. The bonnet is cast or forged of the same material as the body.
What is Valve Trim?
The removable and replaceable internal parts of the valve that come in contact with the flow medium are collectively known as valve trim. Disc, valve seat, and stem are common for all the valve. Valve Trim components will change with types of valve. Valve specific trim includes back seat, glands, spacers, guides, bushings, retaining pins and internal springs. Here in the image, you can see the gate valve trim parts. Because of the trim parts, disk movement and flow control are possible.
The disc is the part that allows, throttles or stops fluid flow depending on its position. Types of disk define the name of the valve such as gate, ball, plug and needle valve’s disk are also of the same shape as the name.
A valve disc could be cast, forged, or fabricated. Valve disk is sometimes required hard facing to improve wear resistance. Disk needed smooth machine surface to reduce the friction with a seat. Valve disk is a pressure retaining part. That means disk hold the pressure. When the valve is open, the disc does not perform pressure-retaining or -containing functions. However, when the valve is closed, the disc performs pressure-retaining functions.
A disc rested against the stationary valve seat when the valve is in the closed position. It can be moved away from the seat by the movement of the stem. However, in check and safety-relief valves, disc is moved away from the seat, by fluid flow and pressure.
The seat provides the seating surface for the disk. Here, you can see the gate valve seat in the above image. A valve may have multiple seats. In the case of globe valve and swing-check valve, there is one seat. Whereas, a gate valve and ball valve has two seats; one on the upstream side and the other on the downstream side.
The valve leakage rate is directly proportional to the effectiveness of the seal between the valve disc and seat(s). Valve seats may be integral or replaceable rings. Valves are generally provided with screwed, welded, or integrally cast or forged seat and hardened by heat treatment or by hard facing of Stellite weld overlay. A fine surface finish of the seating area is necessary for proper sealing. Some ball valve & plug valve used the non-metallic seat for non-critical services. Valve manufacturers have developed several designs of combination valve seats involving elastomer and metal seats that are effective in achieving the desired leak tightness, which cannot be achieved only by metal seats.
The back seat is comprised of a shoulder on the stem and a mating surface on the underside of the bonnet. You can see in the image. It forms a seal when the stem is in the fully open position. It prevents leakage of flow medium to the packing chamber and consequently to the environment. Back seat enables the replacing of the gland packing when the valve is in service.
The stem connects the actuator and disk. It moves and positions the valve disk. The valve stem transports the required motion to the disc, plug, or the ball for opening, closing or positioning the valve. The stem connects actuator, handwheel or the lever of the valve at one end and the disc on the other end. In gate and globe valves, linear motion of the disc open or close the valve, while in the plug, ball, and butterfly valves, the disc rotates to open or shut the valve. Stems are typically forged from stainless steel and connected to the disk by threaded or welded joints.
Bonnet Bolt & Gland eyebolt
Bonnet bolt or stud, hold the bonnet and body to create presser tight seal between them. Gland eyebolt serves two functions. First, it connects gland flange and bonnet. Second, when you tighten the bolt, it pushes the gland bush to retain gland packing in the stuffing box.
York, Yoke Bushing, Yoke Nut
The yoke is also called arms. It connects the valve body or bonnet with the actuating mechanism. The yoke and bonnet are designed as a one-piece construction in many valve designs. A yoke must be sturdy enough to withstand forces, moments, and torque developed by the actuator.
The top of the yoke holds a yoke nut. The valve stem passes through the York. It converts the rotary motion of the actuator into the linear motion and moves the valve stem.
Yoke Bushings which is also known as stem nut is an internally threaded nut held at the top of a yoke through which the valve stem pass. Usually, the yoke nut and bush are made of softer material than the stem to reduce the effort of valve opening. 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.
Non-pressure Retaining Parts of a Valve
Gland Flange is used to provide support to gland bush to keep the gland packing under tension in the stuffing box.
Gland sleeve or bush is used to keep gland packing inside the stuffing box.
Gland packing or steam packing contained in the stuffing box. Gland packings are made from graphite or PTFE as required by services. Proper compression of gland packing is required to prevent the leak from the stem. With the help of gland flange and sleeve, you can compress the gland packing. Gland packing is one of the primary sources of fugitive emission in a process plant. Regular maintenance is required to ensure proper function of packing.
Valve Trim Chart
Trim material such as Disk, seat, stem, back sheet and sleeves are grouped together and assigned one number called Trim No. or Combination number. This will element the requirement of defining material grade for each component.
- API 600 & 602 gives the list of Trim material that can be used in the valve.
- Most common trim grades are ASTM A410(13Cr), ASTM A316, Alloy 20 (19Cr-29Ni), and Monel (CuNi Alloy).
Here in the image, you can see the simplified chart of the trim material. Against trim number, material for seat, disc, backseat and stem is specified. This makes easier to order the valve as you just have to specified trim no based on the requirements and need not specify the material for each of the parts. This list is included in the resource section.