I just want to know what is the action of a non-slam swing check valve. My understanding is that it is non-slam in one direction (i.e. when the disc slams against the body during opening) and that the non-slam action arises due to the opening of the disc in a direction of compression of the spring.

During closing action, however, the spring assists, in addition to gravity and reverse pressure force, and the closure of the valve is quicker.

Various mfrs implement this "non-slam" action by different designs. Some call it "cushion" check valves too.

I am looking forward to comments from members here on the purported action of this "non-slam" or "cushioning" device, as I am trying to sort out some confusions over the matter with my colleagues.

My limited recall of cushioned slam swing check valves is that the cushioning occurs on valve disk closing against the mainseat rather than on disk opening against the valve body. The latter action is generally "soft" with the disk approching the backseat body-stop asymtotically over a fairly wide flow range. This can be damaging to the disk pivot pin and bushings if the disk flutters and repeatedly impacts the valve body backseat. However, it is the the mainseat slamming that is more critical because of the high closing velocities that increase the fluid pressure surge that propagates both upstream and downstream from the closed disk position such that the disk experiences twice the pressure surge magnitude plus the impulsive closing mechanical force when it hits the mainseat. One cushioned slam valve design we tested in the 1960's had a spring-loaded circular plate closing a circular hole in the middle of the disk which throttled flow through the nearly closed disk and bled-off some of the pressure surge buildup. Another approach is to have an orifice in the valve body that bleeds a limited flow around the closed disk. This is most easily done with a positive valve seat angle where the protruding shell making up the positive seat is relatively thin-walled for drilling and leakage flow orifice insertion. The orifice is usually sized to limit leakage flow to prevent breakaway and reverse rotation of a centrifugal pump being protected by the check valve.

Your enlightening post shows that my 1960s brush with cushioned swing check valve technology is way out-of-date by now. My longtime interest in swing check valve dynamics relates to interaction with centrifugal pumps in parallel pressurized water flow systems. These pumps have to withstand closing slam pressure surges for many thousands of cycles at a number of different pressure levels and the discharge flow from the pumps must be stable and non-rotational to not cause repetitive  backseat tapping which is both damaging to the valve pivot hardware and excessively noisy. Cushioned valves are not used but seat bypass orifices provide a slam cushioning bleedoff of pressure buildup. Considerable attention is paid to disk hydrodynamic design profile so that disks "fly" smoothly in the flowstream and don't flutter near the backseat position.

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