The Features and Performance of Butterfly Valve
Jan 18,2023Read More
It is usually not enough to simply select the type of valve suited to given process parameters. Selecting compatible materials of construction helps ensure the lifespan of the valve, as well as the safety of the workforce, the environment and the public.
Selecting the most appropriate materials of construction of control valves is guided primarily by the service of the valve, then secondarily by cost; the least expensive material that is compatible with the service will be chosen to be used. Proper material selection promotes safety by avoiding materials of construction that may react with or be corroded by the process fluid.
The principle materials that need to be selected carefully are the wetted materials, that is, the materials that come into contact with the process fluid. For example, the material in the types of valves used onboard ship. These generally include the ball (for ball valves), the disk (for butterfly valves), and the plug (for plug and globe valves). Also included are the seats, which is the area where the plug or disk "sits" when closed to provide the actual shut off. The seals and the valve body are usually wetted as well.
There are many resources that contain what resources are compatible with a wide variety of process fluids, such as the Cole Parmer Chemical Resistance Database and the Cat Pumps’ Chemical Compatibility Guide.
There are design parameters inherent in the valve designs themselves that increase safety. For high service pressures (or in case of fire) some valves are designed with initial flexible seal rings that function as the primary seals. Behind these primary seals would be a backup seal of a more durable material such as 316 stainless, inconel or hastelloy. These backup seals assist in handling the additional pressure and heat.
In the highly specialized case where the process fluid is so dangerous or unsafe that any release of process fluid is unacceptable, the valve’s packing can be slightly pressurized with a barrier fluid. As long as the pressure of the barrier fluid is higher than the process fluid, any leakage between the valve and the process will leak barrier fluid into the process, and not process fluid into the environment. Though as a side note, these applications usually require double containment piping and a whole host of other precautions beyond simply the safety of the valve. The most common barrier fluid is water or a water/antifreeze mix for freeze protection.
Some other considerations when selecting a material for valve are longevity/reliability of the valve and the temperature range of usage. If the valve is a control valve that gets constant use, it is important to select durable materials or to plan for replacement of the valve frequently. Service temperature is also important; materials need to be selected so the mechanical integrity of the valve is maintained throughout the entire service temperature.
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