How to select check valves?
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Check valve selection
The check valve can be selected and sized for the application by:
Check valve cracking pressure
Check valves with a fixed cracking pressure range have a fixed spring to open the check valve at a certain upstream pressure. Adjustable check valves have an internal adjustable spring bolt that can impress the spring and locked at the required cracking pressure. This cracking pressure has to be adjusted in a test bench.
Maximum working pressure
Check valve maximum working pressure is the pressure the check valve body and seals can hold maximum. Graphs or tables with pressure information will give you the different check valve pressure ratings.
Maximum temperature, Minimum temperature
Environment and process temperatures have be taken in account. Select the check valve in its limits of temperature range. Graphs or tables with pressure and temperature information will give you the different check valve pressure ratings at different temperatures.
Check valves are flow sensitive and rely on the line fluid to open and close. The internal disc allows flow to pass forward, which opens the valve. The disc begins closing the valve as forward flow decreases or is reversed, depending on the design. The reseal pressure of a check valve is lower than the cracking pressure. Graphs or tables with reseal pressure information will give you the different check valve pressure ratings.
Body and seal material compatibility with the medium
Materials in a checkvalve that can be selected are: body material, ball material, elastomers and spring material
Don't select check valves in the same way ball valves are selected: based on line size and the desire for the largest flow rate or Cv available. These flow conditions determine the internal performance of the check valve since its disc is always in the flow stream.Typically a check valve pressure drop calculation is based on the check valve being fully open, as with ball valves / shutoff valves.
End connection type
Different designs of check valves can have similar process connections. Common connection for check valves are: flange, nominal pipe thread and tube fitting.
Check valve installation
Check valve orientation
In most cases a spring loaded check valve can be installed in any angle, lift check valves without a spring to close the disc should only be installed in a horizontal position so that the disc can fall down and closed by gravity.
Install checkvalves with clearance
Not all check valves will work in a vertical line with flow down, nor will conventional or 90-degree piston check valves perform in a vertical line without a spring to push the disc back into the flow path. The disc in some check valves extends into the pipeline when the valves are fully open. This could interfere with the performance of another valve bolted directly to the check valve.
If possible, install the check valve downstream 3 or 5 diameters away from any fitting that could cause vibration.
Follow the check valve symbol flow direction indicated by a “flow arrow” in the direction of the flow to allow the valve to perform its function. The check valve flow symbol arrow can be found on the check valve body or tag.
Preventing backflow is important because it can not only diminish process heating efficiency, but can also damage instruments, valves or steam traps. In contrast, when there is a single downward sloping pipe that is not submerged at any point, there is almost no possibility of backflow, so a check valve is not necessary.