Friday, August 29, 2008

How to select the right valve for your application

Valves have long been more than just a simple device for turning on and shutting off flow. Valve design has kept in step with industrial progress - the development of piping techniques, and the ever-growing list of fluids for processing, power, and finished product. Industry loses millions of dollars each year through the consequences of improper valve selection. Improper valve selection can promote valve failures, which can result in loss of system fluids, out of "spec" product, downtime expenses, unsafe workplace conditions, and environmental damage.

So, how can you confidently select a valve that will install easily, perform safely and reliably, and offer the lowest maintenance and overall cost in your system? You must ask yourself three questions below:

1. What type of fluid will the system carry?
Before selecting a valve, consider the type of fluid the system will carry. Is the fluid viscous or thin? Gas or liquid? Corrosive or inert? Such variables can affect system components and operation. For example, fluid viscosity affects system flow and valve requirements. Fluids that are more viscous reduce system flow and leakage. On the other hand, a high-pressure, light gas will move freely along its flow path, but can be more difficult to seal.
Some gases, such as hydrogen and methane, present significant ignition hazards, and even the smallest leak to the atmosphere can be catastrophic. If the system fluid is a toxic gas, such as arsine or phosphine, leakage to the atmosphere can be even worse. Corrosive gases or liquids such as hydrogen chloride, hydrogen sulfide, or even steam can damage components and actually remove material by chemical or physical attack.

2. What are the system operating conditions?
System operating conditions, such as temperature and pressure, are also important factors in choosing a valve. For example, consider material selection in high- or low-temperature applications; component materials with varying expansion rates can allow fluid leaks. Plastic components can shrink and leak, or they can absorb water and other system media and become brittle at low temperatures. Elastomers, too, can harden and crack in cryogenic service, and they have high thermal coefficients of expansion. In addition, differential pressure can affect sealing capability. For example, a system operating at 1000 psig (68.9 bar) can leak 10 times the amount of the same system operating at 100 psig (6.89 bar).

3. Do you know the different characteristics of different valves?
Progress in valve design puts at the piping engineer's elbow a great variety of valve types, each with some special qualification for service. From these you may choose the right one to provide dependable and economical performance in each particular need.

Gate Valve

Commonly used in industrial piping, this type of valve, as a rule, should be used as a stop valve...to turn on and shut off the flow, as opposed to regulating flow. It gets its name from the gate-like disc which operates at a right angle to the path of flow.

Globe and Angle Valves
The flow through globe valves follow a changing course, thereby causing increased resistance to flow and considerable pressure drop. Because of the seating arrangements, globe valves are the most suitable for throttling flow. The valve is named after its globular body.
Angle valves, similar in principle and a companion line to the globe, are designed to permit a 90 degree turn in piping and are less resistant to flow.

Check Valve


Sometimes referred to as the non-return valve, the check valve stops backflow in the piping. Unlike the gate and globe valves, this simplest of types operates automatically.


Ball Valve

Unique in design, this valve controls the flow of a wide variety of fluids. It can be opened or closed in a quarter-turn of the operating handle. The name "ball" is derived from the ball-shaped disc located within the body. A hole through the center of this disc provides the straight-through flow which is characteristic of ball valves. Light and durable, these are the valves that are playing increasingly important roles in our nation's missile projects, as well as in industry and commercial buildings.

Butterfly Valve
Here's a valve that is extremely durable, efficient and reliable. The butterfly valve derives its name from the wing-like action of the disc which operates at right angles to the flow. Its chief advantage is a seating surface which is not critical. The reason for this being the disc impinges against a resilient liner to provide bubble tightness with low operating torque.

In summary, choosing the correct valve is not always easy. It's a matter of knowing every detail of the job to be done - working pressure, temperature, fluid, volume of flow, corrosive elements, valve operating cycle, etc. other equally. When you choose the right valve, you help maintain a safe environment, eliminate costly downtime, and benefit from increased reliability and leak-tight operation and performance.

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