dual pressure ratings of API pump 1

[rashaa]

can anyone help me with following clause of API-610:
"Unless otherwise specified, vertically suspended, double-casing, integral gear-driven (type OH6) and
horizontal multistage pumps (pumps with three or more stages) may be designed for dual pressure ratings. If
specified, suction regions shall be designed for the same MAWP as the discharge section."

when should we consider this clause? and what it means?

thanks

 

[LittleInch]

First one sounds like the inlet pressure can vary significantly and hence the outlet pressure, or inlet pressure could cover two pressures.

The second point is where there is a chance that the outlet pressure can be seen at the inlet if e.g. a valve fails to close or if the outlet pressure is such that as soon as the pump winds down the pressure can be felt at the inlet.

You could also get this on booster pumps on a pipeline where the initial inlet pressure could be high compared to the steady state pressure.

Why the question?

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[rashaa]

thank you for your response
as you mentioned in second point,in worst case, every centrifugal pump could see the shutoff pressure(even if there is minimum flow line in the discharge). so may i conclude that all pumps should be designed for dual pressure?

 

[LittleInch]

No not really. Many pumps have NRVs directly on the outlet and have somewhere for any small reverse flow to go in the inlet system such atat the inlet side never sees discharge pressure. Dual pressure on the inlet side can add considerably to the cost of the pump and its seals so if you don't need it don't fit it.

Most liquid pumps will be ( or should be) fitted with thermal reliefs on the inlet side to protect against blocked in thermal expansion exceeding the inlet pressure designation

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