Check valve type for VFD controlled pumps

[DGStavlas]

Dear all,
We are looking for the suitable type of a check valve to be installed at the discharge line of a VFD controlled pump. Our first thought was to use typical swing check valves but as I have read in some posts herein a lot of colleagues suggest the use of non slam check valves (e.g noreva or Mokveld). Our process fluid is mine water (probably solids presence in low concentration). Are non slam check valves suitable for this application?
The following info might also be useful:
-Two pumps / parallel operation discharge in one main header/ VFD controlled
-Regarding water hammer protection a pressure vessel and air valves at strategic locations (combination) are also included in the design.
-8" CS sch.80 discharge pipeline / 12" CS sch.80 main header

Thank you in advance

 

[LittleInch]

Given that for VFD operation you can ramp down flow over a much longer period than a simple on/off pump I can't really see the issue here.

Any system which either has a high pressure from a parallel unit which can still be in use or a system with a high static head has potential for check valve slamming.

I would either close the isolation valve whilst the pump is still running then turn the pump off when closed or gently ramp the pump down to prevent reverse flow.

If this is pumping water up a mine shaft then you have a high static pressure and this sort of operation would be subject to check valve slamming almost regardless of how you run the pumps down.

a bit more data wouldn't go amiss...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Swing check valves are a bad choice when you need good flow turndown performance - at low flows, swing checks tend to chatter. I'd go a for a low dp spring loaded dual plate check valve at the least ( mounted in a horizontal section of pipe obviously).

 

[DGStavlas]

Dear LittleInch,
Yes the application is pumping water up a mine shaft (lucky guess or succesful searching I presume ). The issue is the selection of the proper type of check valve.
Additional info: max surge pressure around 60 bars, multistage centrifugal pumps, isolation valves would be gate.

Dear georgevergese,
Do Dual plate check valves have issues with the presence of solids even in low concentration;

 

[LittleInch]

You said your fluid was "mine water" so I guessed.

A slow acting / non slam check valve looks to be a good idea here unless you can normally isolate the pump prior to run down / shut off to prevent the reverse flow from occurring.

At those pressures though even one accidental pump trip could destroy your check valve.

True non slam valves are not cheap but not fitting them could lead to some damage to pump or valve very easily.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGStavlas]

Dear littleInch,
First of all thanks for the quick response.
Yes normmally we will be able to isolate the pump prior to run down / shut off and in the case of an accidental pump trip air valves and a pressure vessel would act as surge protection.
Would you still prefer non slam over swing check? Also are you aware if non slam valves have any issues with fluids containing solids?

 

[LittleInch]

Then you might be able to get away with not installing non slam valves. Experience of similar pump outs might help you make an informed decision.

Non slam are always better, but price difference can be significant. all depends on mode of operation, number of stop starts, reliability of equipment and impact (literally) of a check valve slamming shut.

No valve likes solids, but all that happens with an NRV is it might leak a bit more, but very few are reliable sealing anyway.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

My guess ( in the absence of a description of how this occurs) is that you have this water hammer surge pressure of 60bars when both pumps trip simultaneously and their discharge check valves close AND another check valve much further downstream ( at the top of the mineshaft?) also closes suddenly to prevent reverse flow??

 

[DGStavlas]

Dear georgeverghese,
Check valves are only installed at the pumps discharge. At the top of the mineshaft a combination air valve will be installed. Yes the surge pressure happens when both pumps trip simultaneously.

 

[georgeverghese]

So have you explored the possibility of keeping surge pressures within limits by slowing down the closing speed of this air operated valve at the top of the mineshaft? There is a simple formula in the 6th edition of Perry Chem Engg Handbook for liquid hammer surge pressure estimates - the Joukowski (??) equation.