Isolation valves block fluid flow for system maintenance or security. Selection factors include fluid type, pressure, temperature, and flow rate. Types include rotary and linear motion valves such as ball, butterfly, gate, globe, and diaphragm valves. They are used in various industries and can be vital in emergencies.
Isolation valves are special types of valves that completely block the flow path of a fluid, thereby isolating a portion of the system from the fluid flow. Under normal operating conditions, the isolation valves remain open. It is usually only under certain special circumstances, such as for security reasons or for system maintenance or repairs, that they are closed.
Factors involved in isolating valve selection include whether the flowing fluid is a liquid or gas, system pressure, fluid temperature, and fluid flow rate. These factors are generally taken into consideration when selecting not only the type of valve to be used, but also the material and size of the valve. When selecting the valve, the intended function of the valve is generally also considered.
Isolation valves are generally divided into two categories: rotary motion valves and linear motion valves. In a rotary valve, the part that obstructs the flow of fluid rotates around an axis so that it is perpendicular to the fluid when the valve is closed. Ball valves and butterfly valves are examples of rotary motion valves. Linear motion valves involve the obstructing portion of the valve being moved in or out of the fluid flow in a straight line. Gate valves, globe valves and diaphragm valves are all linear motion valves that can be used for isolation in a system.
A ball valve consists of a hollow ball with holes on two sides directly opposite each other. When the holes are aligned with the fluid flow path, they are clear of any obstructions, so the valve is open. Turning the ball a quarter turn closes the valve because the solid part of the ball is now in the path of fluid flow. This simple quarter turn makes shutting off fluid flow quick and easy, and the ball valves form an airtight seal. These features make ball valves a good choice 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 the former. Butterfly valves are typically used in low pressure situations and when space is limited due to their compact size. They are also often chosen when expensive valve material is required in a project because their compactness means less material is required.
Gate valves involve a disc that retracts completely out of the fluid flow path when it is opened. They are typically used in systems that require minimal pressure drop and uninterrupted fluid flow. Globe valves consist of a conical plug that is pushed into the fluid path to reduce or stop its flow. Because of their lower likelihood of leaking than 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 fluid path and form a seal that obstructs its flow. These types of valves are commonly used in applications with corrosive fluids or fluids containing suspended solids as 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 production, processing and transportation of oil and gas. Power generation, wastewater and mining are other areas where isolation valves are used. Isolation valves can be used for routine purposes such as isolating part of a system for maintenance or repair. They can, however, be vital in an emergency, for example by acting as mechanical barriers in the event of a fire or explosion.
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