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How Pressure, Temperature and Media Affect Valve Choice


September 7, 2015  


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What is a valve?

This is a simple question with different answers. If you ask an automobile mechanic, what a valve is, they may describe it as a disc at the end of a steel stem that moves up and down in the engine head, allowing a gas/air mixture in, and letting an exhaust gas out. This would be a reciprocating valve, similar to a globe valve.

A valve is a device in a flow path with the principal function of shutting off or regulating the flow of a liquid, solid or gas. Liquids can be anything from water to something as viscous as molasses. Solids can be in the form of powder to plastic pellets. Gas can be something as harmless as air, to the very lethal phosgene gas.   

The description of a valve and its function is very basic and easy to understand. Selecting the best valve for an application can be easy or complex. Regardless of the application, to select the correct valve, you need to at the very least know three things, pressure, temperature and media.

Valve types
There are many valve types to choose from, with varying functions. No one valve is suitable for all applications. A butterfly valve is very good for flow control/throttling application, however, it is not suitable for use in a raw sewage line. Knife gate valves are ideal for raw sewage, but not the best for flow control/throttling. Ball valves, plug valves, pinch valves, diaphragm valves, needle valves, check valves and globe valves are just some other types of valves, each with an application where it performs better than the others.

Not only does the application determine the correct valve to use, it also affects the valve material. Some commercial type applications like plumbing and HVAC are fairly innocuous and can be handled with basic valve types, in bronze, brass, cast iron or stainless steel. However, pressure, temperature and media are vital pieces of information that must be available to ensure correct valve selection.

Industrial applications can be more complex, requiring exotic materials to handle extreme applications involving corrosive or erosive media at elevated pressures and temperatures. Materials such as duplex and super duplex stainless steel, Alloy 20, nickel, Hastelloy, Monel, titanium and others are needed to ensure production downtime due to valve failure is reduced or eliminated.  As stated earlier, information is crucial to selecting the correct valve. More information is always better, the more critical the application is in terms of the valves purpose or the financial impact in the event of a failure.

Pressure
Pressure can fluctuate causing severe damage due to pressure surges, cavitation and water hammer. Both working and maximum pressures should be supplied. Systems are designed to a specific pressure rating, and include a safety factor, to allow for system upsets. Ratings such as ASME 150#, 300#, 600# etc. have pressure / temperature curves that help determine the correct valve for the application. Additionally, 400, 600, 1000 CWP (Cold Working Pressure) is the maximum allowable pressure under nonshock conditions at ambient temperature (-20° F to 100° F). based on 100º F. Valves themselves can be the cause of pressure surges and water hammer. This is usually the result of a valve opening or closing quickly. Easy solution, slow down the opening and or closing speed of the offending valve.

Temperature
As with pressure, both working and maximum temperatures should be provided. Impacting primarily “soft” components such as elastomeric and polymer seats, seals and packing, temperature extremes will influence material selected. Teflon, a common seat / seal material in ball valves, can work well in temperature below 180º F to about 400º F. Whereas EPDM, a common seat material in butterfly valves, can only handle -30 to 250º F.

Body/bonnet materials including bronze, ductile iron, carbon steel and stainless steel perform well when operating in the range of -20 to 400º F. However, temperatures above and below those typical operating temperatures, may have a detrimental impact in performance, as these components are subject to pipeline stresses.

For extreme low temperatures in the -250 º F range, 300 series stainless steel is required. Extreme high temperatures, above 900º F, require Molybdenum grade alloy steels. Temperature can also impact the corrosive effect an acid can have on materials.

Media
Media can alter the valve and material selection. Variables such as concentration, temperature, pressure, viscosity, percentage of suspended solids can make the difference between selecting a 316 stainless steel valve or Alloy 20. There is no such thing as too much information when it comes to gathering data to select the correct valve.

Cheap valves are rarely good, and good valves are rarely cheap. It is ultimately up to the end user to decide what valve is right for their application. The $10. 1/2" bronze valve that is fine for a city waterline would not be suitable for a sulfuric acid line used in the production of fertilizers.

It is up to the supplier not so much “sell” a valve, but rather help the user make an informed decision.
 

http://www.johnbrooks.ca

2015-09-07