Drain Valve. Image Credit: Pexuniverse Regulator. Image Credit: Beswick Engineering. Solenoid air control valve. Image Credit: Grainger
Valves are mechanical devices that control the flow and pressure of liquids, gases, and slurries within a system. They are also known as regulators and are used in a wide variety of applications. For all types of media that are conveyed through pipes, there must be valves to control their flow. Valves vary greatly in size, design, function, and operation. It is vitally important to select a valve that will provide the most satisfactory performance, valve life, and least amount of maintenance.
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Although there are many different types of valves and many are designed for specific applications, there are several valve components that are uniform throughout all valves. When selecting a valve it is also important to consider the fitting and material that will needed for the system.
Basic parts of a valve.
Image Credit: RoyMech
Valves can vary greatly in size and design but there are several basic components to valve functionality.
The body of the valve holds the parts together. The ends are designed to connect into the pipe or equipment in the system and generally are butt or socket welded, threaded or flanged. The body is the first pressure boundary to come into contact with the surrounding environment and system media. The environment is an important consideration when selecting the body material.
The bonnet is the cover for the opening in the body. This is the second most important boundary of a pressure valve and is made from the same material as the body. The bonnet can also support internal valve parts, such as the stem, disk, and actuator.
Trim is a term used for the replaceable internal parts such as the disk, seat, stem, and sleeves used to guide the stem. The trim is responsible for the basic motions and flow control features of the valve.
The disk and seat provide the capability for permitting and prohibiting fluid flow. The system is under full pressure when the disk is closed. The seat provides a surface for the disk to seal to in order to stop the flow. The valves may have one or more seats depending on the type. For example, a gate valve has two seats; one on the upstream side and the other on the downstream side. The design of the disk is generally where valves get their name.
The stem is responsible for the movement of the disk, plug or the ball for opening or closing the valve. It is usually forged and connected to the valve hand-wheel, actuator, or the lever by threading. The stem moves the disc in a linear or rotary movement to open or close the valve.
- There are five types of valve systems depending on the application.
- Rising stem with outside screw and yoke - The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are separate from the flow medium by the stem packing. This type of valve is common for larger valves.
- Rising stem with inside screw - The threaded part of the stem is inside the valve body, and is in contact with the flow medium. When rotated, the stem and the hand-wheel rise together to open the valve.
- Non-rising stem with inside screw - The valve disc travels along the stem, like a nut as the stem is rotated. Stem threads are exposed to the flow medium so this model is appropriate when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem material.
- Sliding stem - The valve stem slides in and out of the valve to open or close the valve. This design is for hand-operated, rapid opening valves, and control valves that operate by hydraulic or pneumatic cylinders.
- Rotary stem - This is a commonly used model in ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.
Stem packing is used to form a tight seal between the stem and the bonnet. The packing is fitted with one of several components: a gland follower, a gland, stuffing box, packing material, or a backseat. Packing is important in preventing damage to the stem and fluid or gas loss. It is commonly a fibrous material or compound (such as Teflon®) that forms a seal between the internal and the outside parts of a valve.
The yoke and yoke nut are used to connect the body with the actuating mechanism. The yoke must be strong enough to withstand the forces, movements, and torque developed by the actuator. The nut is used to control the movement of the stem.
The valve actuator operates the stem and disk to open and close the valve. There are several types of actuators to consider depending on the needs of the system such as the torque necessary to operate the valve, speed and the need for automatic actuation.
To select the correct valve actuator, visit GlobalSpec’s How to Select Valve Actuators.
Material of Construction
Valves are made of a wide variety of materials including metallic and nonmetallic options. When selecting a material, the operating environment (i.e. ambient heat), lifespan (i.e. maintenance), and media (i.e. gas or corrosive liquid) should be considered. The most common material is carbon steel because it does very well in high heat, is easily available, and is inexpensive, but it is not suited for corrosive materials. Stainless steel is strong and exhibits resistance to both corrosion and high temperatures, but costs more than carbon steel. Special alloys are used for severe applications such as high pressure or extremely corrosive materials.