Introduction to Valves - Only the Basics

What are Valves?

Valves are mechanical devices that controls the flow and pressure within a system or process. They are essential components of a piping system that conveys liquids, gases, vapors, slurries etc..

Different types of valves are available: gate, globe, plug, ball, butterfly, check, diaphragm, pinch, pressure relief, control valves etc. Each of these types has a number of models, each with different features and functional capabilities. Some valves are self-operated while others manually or with an actuator or pneumatic or hydraulic is operated.

Functions from Valves are:

  • Stopping and starting flow
  • Reduce or increase a flow
  • Controlling the direction of flow
  • Regulating a flow or process pressure
  • Relieve a pipe system of a certain pressure

There are many valve designs, types and models, with a wide range of industrial applications. All satisfy one or more of the functions identified above. Valves are expensive items, and it is important that a correct valve is specified for the function, and must be constructed of the correct material for the process liquid.

Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing. The basic parts of a valve are illustrated in the image on the right.

Valve Body

The valve body, sometimes called the shell, is the primary boundary of a pressure valve. He serves as the main element of a valve assembly because it is the framework that holds all the parts together.

The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints.

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 or flanged.

Valve bodies are cast or forged in a variety of forms and each component have a specific function and constructed in a material suitable for that function.

Valve Bonnet

The cover for the opening in the body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are in many designs and models available.

A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is commonly connected to the body by a threaded, bolted, or welded joint. During manufacture of the valve, the internal components, such as stem, disk etc., are put into the body and then the bonnet is attached to hold all parts together inside.

In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts. Valve bonnets, although a necessity for most valves, represent a cause for concern. Bonnets can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost, and are a source for potential leakage.

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.

A Valve's trim performance is determined by the disk and seat interface and the relation of the disk position to the seat. Because of the trim, basic motions and flow control are possible. In rotational motion trim designs, the disk slides closely past the seat to produce a change in flow opening. In linear motion trim designs, the disk lifts perpendicularly away from the seat so that an annular orifice appears.

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 600 Valve's Trim No

Valve Disk and Seat(s)


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. The disk is the third most important primary pressure boundary. With the valve closed, full system pressure is applied across the disk, and for this reason, the disk is a pressure related component.

Disks are usually forged, and in some designs, hard surfaced to provide good wear properties. Most valves are named, the design of their disks.


The seat or seal rings provide the seating surface for the disk. 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. A gate valve disc has two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow.

To improve the wear-resistance of the seal rings, the surface is often hard-faced by welding and then machining the contact surface of the seal ring. A fine surface finish of the seating area is necessary for good sealing when the valve is closed. Seal rings are not usually considered pressure boundary parts because the body has sufficient wall thickness to withstand design pressure without relying upon the thickness of the seal rings.

Valve Stem

The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. 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 disc is rotated to open or close the valve.

Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.

There are five types of valve stems:

In the main Menu "Valves" you will find some links to detailed (large) images of Rising and NON Rising Stem valves.

Valve Stem Packing

For a reliable seal between the stem and the bonnet, a gasket is needed. This is called a Packing, and it is fitted with e.g. the following components:

An important aspect of the life time of a valve is the sealing assembly. Almost all valves, like standard Ball, Globe, Gate, Plug and Butterfly valves have their sealing assembly based upon shear force, friction and tearing.

Therefore valve packaging must be properly happen, to prevent damage to the stem and fluid or gas loss. When a packing is too loose, the valve will leak. If the packing is too tight, it will affect the movement and possible damage to the stem.

Typical sealing assembly

1. Gland Follover  2. Gland  3. Stuffing Box with Packing  4. Back Seat

Compression packing for valve fitting guide

Valve Yoke and Yoke Nut


A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the Yoke holding a Yoke nut, stem nut, or Yoke bushing and the valve stem passes through it. A Yoke usually has openings to allow access to the stuffing box, actuator links, etc.. Structurally, a Yoke must be strong enough to withstand forces, moments, and torque developed by the actuator.

Yoke Nut

A Yoke nut is an internally threaded nut and is placed in the top of a Yoke by which the stem passes. In a Gate valve e.g., the Yoke nut is turned and the stem travels up or down. In the case of Globe valves, the nut is fixed and the stem is rotated through it.

Valve Actuator

Hand-operated valves are usually equipped with a handwheel attached to the valve's stem or Yoke nut which is rotated clockwise or counter clockwise to close or open a valve. Globe and gate valves are opened and closed in this way.

Hand-operated, quarter turn valves, such as Ball, Plug or Butterfly, has a lever for actuate the valve.

There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include:

These valves are usually equipped with an actuator.
An actuator in the broadest definition is a device that produces linear and rotary motion of a source of power under the action of a source of control.

Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There a many different types of actuators, but the following are some of the commonly used valve actuators:

For more information about Actuators see main Menu "Valves" -Valve Actuators-

Classification of Valves

The following are some of the commonly used valve classifications, based on mechanical motion:

Classification of Valves based on Motion

Valve Types Linear Motion Rotary Motion Quarter Turn
Butterfly NO YES YES
Swing Check NO YES NO
Diaphragm YES NO NO
Safety YES NO NO
Relief YES NO NO
Valve Types Linear Motion Rotary Motion Quarter Turn

Class Ratings

Pressure-temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes: standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves.

Valve types and operation


On this page are defined a number of basic information from valves.

As you may have seen in the main Menu "Valves", you can find also information about several and often applied valves in Petro and chemical industry.
It can give you an impression, and good understanding of the differences between the various types of valves, and how these differences affect the valve function. It will help to a proper application of each type of valve during the design and the proper use of each type of valve during operation.