Internal Sealing Systems And Materials
Body/Housing MaterialsA 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.
External Sealing Systems And MaterialsAs 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.
Actuation of ValvesMany 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.
Standards of ManufactureThere 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.
Quick Closing Valves
One common type is the rising stem type. In other designs, the wheel is fixed to the stem and rises with it.
The position of the stem/stem and hand wheel indicates whether the valve is open or shut. If the stem is raised, the valve is open.
Ball/Plug ValvesPurpose: Ball and plug valves are used to provide a quick, simple shut-off. They are operated by turning the ball or plug through 90º. Ball and plug valves should not be used for throttling, as a restriction in the flow will lead to erosion of the valve.
Some ball and plug valves are lubricated to provide a seal and prevent wear, and should be regularly lubricated with the proper lubricant.
The high pressure is usually on the bottom of the plug, so that the stem, seal, etc. are not under continuous pressure. Applications are widespread, including domestic water taps.
Some butterfly valves have a direct acting lever, others are operated through gearboxes when finer control is required. In most cases, a clockwise movement will close the valve.
pressure. The spring force will then release the valve.
Relief valves operate automatically and are usually pre-set to a specified relief setting by the manufacturers or adjusted when in use, if required. Re-calibration is then required.
Purpose: Check valves allow flow in one direction only. One common application is in the discharge line of a centrifugal pump to prevent reverse suction.
The two designs operate on the same principle: flow through the valve holds the plug or disc in an open position. If flow ceases or falls to below the backpressure ahead of the valve, then gravity or the back pressure will tend to return the plug to its seat.
The opening action consists of two stages:
1. Unsealing the sealing segments.
2. Rotating the plug.
Operation: Refer to Figure
rate of flow can be adjusted as required by raising the needle.
Ball-Check Gauge Glass Valves
|Ball-Check Gauge Glass Valves|
Operation: Refer to Figure
Direction of Flow: Direction of flow is usually marked with an arrow in the case of check valves and globe valves.
Direction of Opening/Closing: Hand wheels and levers - clockwise to close, anti-clockwise to open.
Levers - usually lever in line with pipe - open, lever at 90º to pipe - closed.
Open and close valves slowly.
Valves should always be opened and closed slowly, except in emergencies. Too rapid closing can cause pressure waves to build up and travel back through the system, possibly causing severe damage, burst or injuries. This phenomenon is known as “water hammer” in domestic water systems and a loud knocking noise can be heard in the pipe.
Gate and Ball/Plug Valves: Gate and ball valves must only be used in the fully open or closed positions.
Intermediate setting can cause turbulence, which can wear away the valve very quickly and cause internal leakage. Gauge Glass Valves: Gauge or sight glass valves must be fully opened as soon as the fluid has reached its level or there will be no protection if the glass breaks.
Interlock/Keys: Some valves are not fitted with hand wheels or levers and can only be operated by special keys or spanners. This is because the setting of the valve is critical and must not be altered except by an authorised person.
Similarly, some valves are sealed with wire; locks or other means and must not be tampered with or altered as serious damage could result.
Do not Open/Close too far or use unnecessary force.
If gate and globe valves are jammed too far open, they may seize or be damaged. This is called “backseating” and puts unnecessary strain on the disc, which may break off. It is best to re-close gate and globe valves by 1/2 to 1 full turn after they are fully opened.
Similarly, over-tightening the valve when closing it may damage the disc and seat leading to seizure or leakage.
If valves prove stubborn to open, mechanical assistance such as a valve wrench or spanner can be used. The minimum amount of force should be used and before applying the “persuader”, check that the valve is not already open. Do not use persuaders on twin seal valves or on very small valves, which may break off.