6 replies to this topic

[Psingh]

Dear all in case of reverse flow through diverse check valves API 521 section 4.3.4.4 says that:

 

"Where no specific experience or company guidelines exist, one may estimate the reverse flow through series

check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal flow diameter"

 

I have a water injection pump (2 working 1 standby) with following details: 

 

Suct pr                   : 7 barG

suct design pr        : 17.5 barG 

discharge pressure: 275 barG.

normal flow            : 700 m3/hr.

discharge line        : 16 inch,

 

The discharge lines connect to a header connecting all these pumps....discharge line of each pump has a set of diverse check valves to avoid incidence of high pressure to the suction side.

 

Now in case of that these check valves fail what do you recommend:

1. 1% of the normal volumetric flow( 1/10 of dia will give 1%  area only) through the discharge line as per API as the PSV design Load.
2. If not then what are the equations which I shall use to calculate the reverse flow through the assumed RO of 1.6" (16"/10) in these high differential pressure conditions, I want to develop an excel tool for this case.

I have read several forums where people are suggesting for 10% vol flow to be taken for PSV load calc but I did not find any basis for that so thought of writing to cheresource....

 

Please give your opinion.

 

B.R.

Pramukh

Edited by pramukh, 12 April 2015 - 05:56 AM.

[fallah]

pramukh,

 

I think you should follow the second method means the maximum expected differential across the check valve should be used for restriction orifice calculation with diameter equal to one tenth of the largest check valve diameter...

[Psingh]

Fallah,

 

Thanks for your quick response,

 

Can you please refer me some standard / equations to calculate the flow in second case, since the Delta P is very high (257.5 bar) in this case.

 

B.R.

 

Pramukh

[fallah]

Pramukh,

 

I guess the pump is positive displacement type and the scenario of reverse flow is pump trip; then if so, i don't think there would be such high differential pressure across every single check valve among a series check valves in the case of pump trip...as the overall differential pressure of 257.5 bar will be shared among all valves in the series...

Edited by fallah, 11 April 2015 - 02:42 PM.

[Psingh]

Oh I forgot to mention the pump is centrifugal and the PSV is at the suction side.
Can anyone refer me some equation/standard to calculate the flowrate through the assumed RO as guided by API 521.

My problem now is to estimate the flowrate.

[fallah]

pramukh,

 

As a quick estimation with an online calculator the mass flow rate, as expected, was very high and around 200 kg/sec.

 

It's strange considering a centrifugal pump for water injection with such huge discharge pressure and moderate flow rate. Obviously, a positive displacement pump, appears to be best choice...also location of PSV at pump discharge will allow very fast reverse rotation of pump leading to....

[Psingh]

Oh I misquoted the flowrate was 700 M3/hr

 

@ fallah : btw what ws the online tool.