"non-slam" check valves 1

[alwayslearning]

Dear All,
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.

Regards

 

[vanstoja]

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.

 

[alwayslearning]

Thanks for the post van. I just explored things a bit.

1) In our industry we have the non slam check valves by Eagle Industry Co Ltd. They call it "Free Flow Reverse current valve with Oil Cylinder"! In principle it is a normal swing check valve and the hinge pin extends beyond the body to connect to a spring loaded piston which travels in a cylinder. The bottom portion of cylnder is connected to Oil supply header (~5kg/cm2g) and the upper portion to oil return header . When the disc swings open, the action is to compress the spring and the pressured oil (@5 kg/cm2g) fills up the displaced volume.

This action, in my opinion, effects a non-slam action against the body.

Upon disc closure, the spring force (with its stored potential energy) quickly swings into action and aids in quick closure. The, trick now is that the spring closes quickly but is moving against a pressured header which offers resistance. So in effect the valve closure upto 80-90% (just a guessed number, depends on spring constant) is very quick but the rest of the travel is slow. Thus this kind of check valve perhaps effects non-slam action in either directions. Before i did this post, my understanding was that of "non slam action only in one direction, i.e. while opening".

2) I saw the air-cushioned and oil cushiooned swing check valves offered by APCO valves. The operation of their valve is well explained in their catalogue.

Essentially both use the piston and cyliner. And in addition they have a "COUNTERWEIGHT" arm; this counter weight arm can be adjusted to give varying load arm (moment).
2.1) In the air cushioned design, the disc opening action draws in air (freely) in to the cylinder through a (yet another) small check valve - which is actually a "control check valve". There is no spring to be compressed here, so the disc travel is free (and this is clearly highlighted in the APCO catalogue) and must "slam" the valve body.

In the pressure reversal case, the disc closes quickly under the influence of the counter-weight; the fastness of closure being adjustable (depending on the load-arm selected). the quick closure takes the valve to about (say) 80% during which time the piston is compressing air within the cylinder . After this 80% closure, the pressurised air in the cylinder acts as a cushion and the full closure is ensured under the controlled release of air from this control check valve.

The APCO catalog states that the user can alter the closure time by altering the load-arm; and alter the cushioning effect by playing with the control knob on the "control check valve" of the cylinder.

2.2) the oil operated case is similar to the air cushioned swing check valve as explained above. APCO states that, sometimes during rapid flow reversal, the "air cushiooning" may lose its effectiveness; and in such cases they recommend the oil controlled check valves.

I would like to thank you again for your post; it just made me look at the other side of the coin too. Please correct me if i have lapses in my understanding . Thanks

PS: Just for your information, we are using the swing check valves as explained in 1) above in Gas and steam services.


Regards

 

[vanstoja]

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.

 

[gustorf]

alwayslearning and vanstoja,
You may not be aware of a non-slam nozzle type check valve.
It combines all advantages of the other check valve styles.

1. non-slam performance without any dampening system. it has very small valve disk strokes, that gives small reverse velocities, when the disk hits the seat. (Imagine clapping your hands with a distance of 1 inch and 10 inch and you know the difference)

2. Low pressure loss due to venturi-shaped design leading to big energy cost savings.

3. Maintenance-free design due to metal-to-metal sealing without wearing parts to be replaced regulary for several years of operation.

If you need further info please look at

http://www.noreva.de

 

[tyrels]

Check out

http://www.dft-valves.com

They might be able to assist you.

 

[RobV]

The designer of the nozzle type non-slam checks can be found at

http://www.mokveld.com,

the TKZ-Y. I'd still say that they lead the market in the non-slam check valves.

Another one is from Stockham, the "noz-check"