Isolation valves are special types of valves that completely obstruct the path of flow of a fluid, thereby isolating a portion of the system from fluid flow. Under normal operating conditions, isolation valves remain open. It is typically only under certain special circumstances, such as for safety reasons or for system maintenance or repair, that they are closed.
Factors involved in the selection of isolation valves include whether the fluid flowing is a liquid or a gas, system pressure, fluid temperature, and rate of fluid flow. Such factors are generally taken into account when choosing not only the type of valve to be used, but also the material and valve size. Expected function of the valve is also generally considered in valve selection.
Isolation valves generally fall into two categories: rotary movement valves and linear movement valves. In a rotary movement valve, the part that obstructs fluid flow rotates about an axis so it is perpendicular to the fluid when the valve is closed. Ball valves and butterfly valves are examples of rotary movement valves. Linear movement valves involve the obstructing portion of the valve being moved into or out of the fluid flow in a straight line. Gate valves, globe valves, and diaphragm valves are all linear movement valves that may be used for isolation in a system.
A ball valve consists of a hollow sphere with holes on two sides directly opposite each other. When the holes are aligned with the path of flow of fluid, they present no obstruction, so the valve is open. Rotating the ball a quarter of a turn closes the valve because the solid part of the ball is now in the path of the fluid flow. This simple quarter turn makes it quick and easy to shut off fluid flow, and ball valves form a tight seal. These features make ball valves good choices for isolation valves in a variety of applications.
In a butterfly valve, a disc rotates around a shaft such that the plane of the disc is parallel to the fluid flow when open and perpendicular to it when closed. Ball valves, rather than butterfly valves, are usually used as steam isolation valves because the latter typically do not achieve as tight a seal as former. Butterfly valves are typically used in low-pressure situations and when space is at a premium due to their compact size. They are also often chosen when an expensive valve material is called for in a design because their compactness means less material is required.
Gate valves involve a disc that retracts completely out of the path of fluid flow when open. They are typically used in systems requiring minimal pressure drop and uninterrupted fluid flow. Globe valves consist of a tapered plug that is pushed into the path of the fluid to reduce or shut off its flow. Due to their lower likelihood of leakage as opposed to gate valves, they are more likely to be used in high-pressure or high-volume systems.
Pressure is applied to a flexible diaphragm in a diaphragm valve. This helps move it into the path of the fluid and forming a seal that obstructs its flow. These types of valves are commonly used in applications with corrosive fluids or fluids containing suspended solids because the fluid never contacts the moving parts of the valve.
Many industries depend on isolation valves for a variety of functions. They are used, for example, in chemical manufacturing, oil and gas processing and transport. Power generation, sewage and mining are other areas in which isolation valves are used. Isolation valves may be used for routine purposes such as isolating a portion of a system for maintenance or repair. They may, however, be critical in emergencies such as in serving as mechanical barriers in the event of fire or explosion.