Suction and discharge pressure on a positive displacement pump

[abbt489]

@ LittleInch
At what point you change the design pressure accross a pump is dependant on your system, but commonly if you have an inlet isolation valve, the rating of this is the same as the discharge side because if the outlet valve is opened before the inlet valve and pump is in operation, then the inlet side can see the outlet pressure even with a non return valve.

@ 130684
The suction design pressure is sometime considered equal to the discharge design pressure because while one pump is running, the spare one can have the discharge isolation valve open (operator error)...but is considered equal, only up to the suction isolation valve (the section between the suction isolation valve and pump).


Hi, Noted the following from one of your previous threads, can you please advice if these are in any design requirements or standard codes? I am having problems justifying it to one of my contractors

 

[racookpe1978]

Safety and common sense design to prevent catastrophic failure in the event of an operator error or failure of a pump seal/valve seal are difficult to get across to some clients.

Ask the person opposing your design to accept in writing ALL responsibility for short-circuiting common sense safety precautions and ALL responsibility for future operator error and mechanical damage for the life of the plant just to save 150.00 dollars in thicker pipe material.

 

[abbt489]

@ racookpe

The issue is slightly complicated. The suction line are rated the same as the discharge line. Due to a manufacturing error the suction chamber of the positive displacement pump (Parallel operation: 2 pumps in use and 1 standby) may not now be fit for the intended design pressure. There is a challenge now that the suction chamber and suction lines should not have been rated to the discharge line in the first place.

 

[LittleInch]

As the first time I've ever been asked in person to respond I suppose I had better answer this.

Can't recall which tread that was in, but probably referred to a centrifugal pump.

Of course "positive displacement" is a wide term also and the previous advise may or may not hold. The key factor is whether in any circumstances which includes a single failure, the inlet piping and valves can see the discharge pressure.

The codes are not designed to be specific in these instances nor should they be so the answer isn't there.

Some key passages from 31.3 include (my emphasis)

The Code sets forth engineering requirements deemed necessary for safe design and construction of pressure piping. While safety is the basic consideration, this factor alone will not necessarily govern the final specifications for any piping installation. The designer is cautioned that the Code is not a design handbook; it does not eliminate the need for the designer or for competent
engineering judgment.

In determination of the design pressure section the following applies

(a) The design pressure of each component in a piping
system shall be not less than the pressure at the most
severe condition of coincident internal or external pressure
and temperature (minimum or maximum) expected
during service, except as provided in para. 302.2.4.

302.2.4 refers to allowances for short duration overpressure only (a maximum of 33%)

So, in your case, you need to determine and be satisfied yourself, that in no circumstances can the inlet side see the discharge side pressure. If there is sufficient protection and installed equipment, there might be, but I doubt it.

Usually these sort of things should get picked up in a design review or HAZOP where you need to be able to justify and defend your design when questioned about the relevant ratings. Without any more details it is difficult to comment further.

I agree with what racoope says also. Not sure why you are having to justify to a contractor, but you don't say which party you work for. However you should be able to defend your design.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

OK.

Add a pressure relief valve on the inlet side, set to relieve at the max pressure the inlet side has been found to be rated for. End of problem. Discharge the relief valve pipe to a safe place, add a flow alarm so the control room knows when it relieves.