Why DBB before pump have same class that discharge line? 2

[VladimirWH]

Hello.
I got some comment about project - injection pump station, with diaphragm pump.
So, station consist of tank-dbb- y-strainer -dbb-pump-membrane accumulator-*PSV*-check valve-rotameter-dbb (drain system I doesn't write). Class suction line - 150, discharge - 900.
Client's specialist commented that dbb before pump should class 900. Why?
And one more question. In Russia (I work in Russia) in goverment standard I see, that pressure of PSV=1.4*Pressure Discharge, when we use a diaphragm pump. Have any recomendation om ASME abou it.

 

[mk3223]

Client's specialist commented that dbb before pump should class 900. Why?

It doesn't make a sense. Is there a pump discharge bypass return to the suction pipe, instead of the storage tank?

In Russia (I work in Russia) in goverment standard I see, that pressure of PSV=1.4*Pressure Discharge, when we use a diaphragm pump.

To ensure the PSV set pressure is not exceed the MAWP of the protected piping or equipment as refer to API-520 and 521.

 

[VladimirWH]

Is there a pump discharge bypass return to the suction pipe, instead of the storage tank?

There is no bypass. We have connection between PSV and tank.

 

[georgeverghese]

This is a recommendation found in the API standard for plunger pumps. The reason for the suction valve to be 900lb is as follows which occurs on pump stop:
a)When pump is stopped, the discharge remains at 900lb
b)Operator closes the DBB on pump suction
c)The internal discharge valve and suction valve inside the pump leaks slightly, allowing reverse flow of 900lb pressure fluid right up to the DBB on pump suction.

If your govt standard says to set the PSV at 1.4x of max normal operating discharge pressure, I think that is a good idea. Then the PSV will not lift when the membrane pulsation suppression device doesnt work well and you get severe pressure pulsations on the discharge side.

 

[LittleInch]

The only thing to add is to check what the rating of the inlet flange is on the pump. If its class 900 then your inlet valve should be the same for reasons george identifies - basically you could be holding the discharge pressure on two NRVs which are notorious for not holding 100%.

If it's only a class 150 then you really need to add in a PRV in this section to avoid over pressure of that flange.

ASME standards tend not to be so prescriptive as the GOSTS and SniPs are / were. You are more likely to find guidance on this in company engineering standards and guides, e.g. those issued by the likes of Shell, BP, Total, Exxon Mobil etc or in pump vendors recommendations.

In reality the pressure pulse might be a lot less or indeed a lot more, hence the ASME recommendation is to find out what the pressure pulse is and then design and protect the pipeline accordingly or fit pulsation dampers.

Pressure pulse analysis is now a complex business looking at pipe layout, orientation and natural frequencies.

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