Flow through check valve and orifice 1

[ssctsa1]

I am installing an orifice in a system with a check valve. A far as flow is concerned in relation to the cracking pressure of the check valve, does it matter if I put the orifice upstream or downstream of the check valve? If I put the check valve upstream of the orifice, then I think it is possible that I will not get the necessary delta P across the check valve to ensure the valve remains open (it may reseat due to the low delta P and then start chattering). If I put the check valve downstream of the orifice then I should not have a concern with check valve cracking/seating pressure. Does this sound logical?

I know there are alot of other system parameters that affect this but I was looking for a general rule of thumb or typical installation configuration?

 

[BigInch]

The introduction of an orifice in the system increases the dP across the orifice by reducing flow in the entire branch, therefore all points upstream of the orifice will tend to increase in pressure and all points downstream will tend to decrease in pressure. The differential pressure in the branch is increased with an orifice installed. Now the same flow in the branch occurs at a higher differential branch pressure, although the differential cracking pressure of the check valve remains the same. No change as far as the check valve hydraulics is concerned, but since branch inlet/outlet pressure is greater, there's a chance it might need a new pressure rating, if the net effect is to increase upstream pressure and the check is installed upstream of the orifice.

The check valve only sees its dP across itself, the check valve. It does not see the dP across the orifice, nor does it see the dP from beginning of branch to end of branch, so it doesn't really care where it is located in relation to the orifice, as far as what affects its opening or closing differential pressure. The cracking dP across the valve remains the same. When ... it sees that dP, and opens or closes, is another story.

So now the question becomes, how will the reduction of flow in the branch by the addition of an orifice affect the operation of the check valve? Starting from a lower pressure upstream of the check valve, the reduced flow in the upstream branch segment will delay the opening of the check valve, since at reduced flow it will take longer for the upstream piping to reach the cracking pressure at the check valve. Beginning with a higher pressure upstream of the check, reduced flow would tend to keep that pressure higher for a longer time and tend to keep the valve open longer.

In the same manner, pressure in the downstream segment of the branch is also at reduced flow, so pressure changes there are slower too. The check would tend to open later, since reducing pressure downstream would tend to take a longer to reach and it would tend to stay open longer, since increasing pressure downstream would take longer to reach the valve at reduced branch flow. The net effect is, branch differential pressure increased and time constant of the branch pressure change response to flow change is increased.

Chattering is a transient response and depends on how fast the piping upstream of the check valve changes pressure in relation to how fast the downstream segment changes pressure. When the whole differential pressure in the branch is relatively small, chattering occurs. A relatively large differential branch pressure (much greater than check valve cracking pressure) and a consequently more sustainable flowrate in the branch will tend to avoid chattering. An orifice, increases branch differential pressure, but decreases flowrate, hence it may or may not be effective in reducing chatter. In cases where it is not effective, consider using a preloaded check valve.


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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ssctsa1]

Now that's what I call an answer. Thanks

 

[gerhardl]

Thank You to BigInch, I understood more than before, star for you!

Additional to ssctsa1: why do you want to place an orifice here? Is this for pressure measuring or flow restriction?

If this is to restrict to avoid chattering, it will (and anyway) have far better results for all pressures and flow conditions if you put in the right type and dimension of preloaded check valve.

Also consider that if you pump the fluid, soft start and soft stop of pumps and pump speed regulating are becoming more and more common. This will give varying flow conditions, and at unfortunate flow conditions chattering can worsen or start for a check valve that else is OK.

A break in pump current can also give very high pressure peaks with a slamming type checkvalve, instead of a preloaded soft closing check valve.

A preloaded spring type operated check valve, or best soft closing nozzle check valve (with spring), dimensioned correctly for minimum pressure loss, would probably solve chattering problems.

If you have free fall, and/or relatively constant pressure,things might be a bit different, and an orifice may be used to restrict the flow to a roughly given value. In case: check valve (if needed) upstream or downstream according to local pipeline layout and actual flow conditions, but correct type and sizing!

 

[BigInch]

Admittingly a bit long-winded compared to my usual responses.

From small D, short pipes and relatively incompressible liquids to big D, long pipes and gases, the time constants could vary from milliseconds to hours or more.

I tend to think an orifice wouldn't be very effective. It doesn't seem to address the problem directly anyway.

One could also attempt to eliminate chattering by having odd lengths of upstream and downstream piping to try to keep any reflected pressure waves from reinforcing themselves. Stay away from integer length multiples, etc. as you would do for recip compressor piping. At least I'd try it before installing an orifice. Something like moving the valve to 0.3 or 0.7 x branch length.

(thanks for the * gerhardl)

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

some check valve dynamics,

http://www.valveandequipment.com/pdfs/vm8.pdf

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/