API 674 pump flange rating

[prq123]

Our API 674 positive displacement reciprocating pump has a suction pressure of ~ 2 barg and discharge pressure of ~140 barg. The vendor is proposing a 150# flange for the suction and 1500# for the discharge. Is this acceptable/normal? The concern is that if the pump is isolated (suction isolation valve closed), any leakage/reverse flow may overpressure the suction.

 

[nickelkid]

This is acceptable/normal, see Cylinder Connection section in API 674. A leaky suction valve will cause an increase in suction pressure. Do you have a psv to protect your inlet system?

 

[prq123]

There is no PSV to protect the inlet (suction) system. The arrangement is: suction block valve (150#)--> pump ---> check valve (1500#)---> discharge block valve (1500#). I should also add that we have two pumps in parallel that discharge to the same header. One pump is running and one is on standby.

Is the suction still adequately protected from overpressure if the suction block valve is closed?

I came across this article on positive displacement pump which makes me think we are not adequately protected on the suction:

http://www.driedger.ca/ce2_pdp/CE2_PDP.html

Here it states: "The suction side of the pump may also require protection. A relief valve is required unless all suction piping is rated for the full discharge pressure. Liquids, especially water, are quite incompressible. Even the smallest reverse leakage through a check valve can raise the pressure of a blocked suction sufficiently to rupture the line. This can happen even after the pump has been shut down! The discharge dampener will contain liquid at full pressure unless it has been relieved. The line rupture may occur minutes or even days after the pump has been shut down and isolated, depending on the relative sizes of the discharge and suction dampeners and the leakage rate."

 

[nickelkid]

The article answers your question, no psv - you need discharge rating on the suction side.

 

[nickelkid]

Oh I forgot, good work.

 

[prq123]

OK. I agree. Unfortunately, API 674 is not very clear on the pressure rating for the pump suction (based on my reading). I can not find anything in API 674 which states that if you don't have a PSV in your suction piping then the suction piping (and pump suction flange) needs to be same as discharge rating.

API 674 does state "Pressure containing parts" (which includes the suction manifold) are to be tested 1.5 x "maximum allowable working pressure". However, the definition of the "maximum allowable working pressure" is somewhat ambiguous. API 674 defines the maximum allowable working pressure as the maximum continuous pressure the manufacturer has the designed the equipment (or any part to which the term is applied) when handling the specified fluid.

 

[nickelkid]

Agreed. API674 does state that cylinder connection shall be suitable for the working pressure to which it is normally subjected, with a bullet, which makes you through this exercise. A little massaging of API 674 would help avoid this pitfall.