Valves

VALVES [Lesson 1]

Process Valve Types And Applications
Valves are used in both domestic and industrial situations to control the flow of liquids, solids and gases.

The most common and familiar valves are the taps used in the home to control the hot and cold water. In the oil industry, valves are a major element in the control of operations. In general, valves are used for one or more of three main purposes:
1. To control the rate of flow (throttle);
2. To shut off/permit flow (ON/OFF function);
3. To isolate systems and protect products.

There are a wide variety of valve types and designs available from many suppliers in a wide range of materials; the main types and their uses are
- Gate Valve: Used for shut off - ON/OFF function.
- Ball/Plug Valves: Used for shut off - ON/OFF function
- Globe Valves: Used for control of flow and shut off.
- Butterfly Valves: Used for control of flow and shut off.
- Relief Valves: Spring loaded to open at a given pressure, and used to protect systems from over-pressure.
- Check Valves: To allow flow in one direction only.
- Fusible Link Valves/Piston Operated Valves: Quick acting and used for emergency shut off.
- Twin Sea valves: Used when tight shut off required.
- Semi-Needle Valves: Used in conjunction instruments to bleed off part of the flow.
- Ball Check Valves: Used with gauge glasses as safety precaution.

There are other less commonly used types of valves.
The actual construction/design of gate valves, for example, may vary widely depending on its application,the materials used, or the manufacturer’s own special features. The basic principle, however, will be the same.

Valves can be specially made to work at high or low temperatures (cryogenic), or to very high standards for use in explosive atmospheres, or when no leakage is permissible.

Design Features

1. Internal Sealing Systems And Materials
All valves are prone to leakage as it is difficult to obtain a perfect seal, although the use of special seal materials and designs can have very good result If high security is required, use can be made of two valves in series, one to act as the main valve and the second as a back-up should the first fail.

Some valves have bleed holes installed to detect leakage across the seal. If two valves are used together, a bleed hole may be fitted in the pipe between them, which can be opened when the valves are closed to drain any leakage. (Block and Bleed valves)

2. Body/Housing Materials
A wide range of materials is used in valve manufacture, the particular material depending largely on the fluids to be handled. Iron and steel are mainly used for oil/petroleum applications with most valves being made of mild or alloy steel. Brass valves are used for water (as well as cast iron, steel and other alloys).

Stainless steel is used for acids and other corrosive liquids. Bronze is also a commonly used material which can cope with most liquids.

3. External Sealing Systems And Materials
As well as the main seal between valve and disc, wedge, etc. there are other seals required to prevent external leaks. Gaskets or ‘0’ rings are used between surfaces such as flanges, where no relative movement takes place. The main problems occur around the valve stem, which both rotates and, in some cases, moves vertically as well.

Special glands or packings are used which can be compressed by gland nuts to increase sealing. Special materials have to be used in corrosive applications, but an asbestos based fibre is a commonly used packing material with PTFE/Teflon being increasingly common. ‘O’ rings can also be used as shaft seals and are generally made of rubber.

4. Actuation of Valves
Many smaller valves are hand operated if they are accessible. Larger valves require power actuators and inaccessible valves of all types require some form of mechanical or electrical actuator. Pneumatic (compressed air) and hydraulic cylinders and mechanisms are widely used in larger applications. Smaller valves can be operated with solenoids, but larger valves require more complex motors and mechanisms for electrical power operation.

5. Standards of Manufacture
There are many standards to which valves can be made:
• Metric/Imperial dimensions;
• British Standards BS;
• German Standards DIN;
• US Standards ANSI (previously ASA)
• American Petroleum Institution API.

Care must be taken that valves, flanges, etc. and other equipment are compatible, or leakage may occur.
API flanges and other equipment are commonly used in the oil industry. The standards lay down performance requirements as well as dimensions and material. Valves are rated according to the maximum pressure and temperature at which they can safely be used.

6. Quick Closing Valves
Quick closing valves can be installed in pipelines and systems to isolate sections in case of fire, leakage or other emergencies.

A spring is usually used to operate the valve and can be released by a number of methods:
• Fire melts fusible link;
• Remote manual cable;
• Air operated actuating cylinder;
• Electrical solenoid, etc.

If the valve can be installed so that the line pressure will help to close it, this will increase the sealing capability. Swinging check valves are often used as the basis of a quick closing valve, although ball valves, plug valves and butterfly valves are also suitable.