Reason for specifying suction pressure 1

[extremechanical]

While data sheet for centrifugal pump is prepared suction pressure and npsh available are provided . I understand npsh available takes into consideration of atmospheric pressure, ( if applicable) minus the vapor pressure and minus the friction losses . In that case , please let me know is it not this npsh available matters to the pump to work with ? Then why we need another parameter suction pressure which is simply the static head converted to pressure or pressure available near suction flange , depending on case to case?

 

[LittleInch]

You forgot about head difference from top of liquid to c/l of inlet flange in your NPSHA calculation....

Maximum suction or inlet pressure is more about what is the max pressure the pump seals see if the inlet valve is open but you have no flow.

For something like a booster pump in a pipeline or many other systems, the static no flow inlet pressure can be significant even though inlet pressure in operation might be much lower and the pump seals need to be good enough not to leak at that pressure.


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

 

[extremechanical]

Little inch , Thanks a lot for correcting me on npsh availability calculation. I also understood the suction pressure is important to see how much seal would see. But is the reason for specifying suction pressure is only for that reason or any other reason? Because there is another term rated pressure ! So in specific I want to know please - sorry for repeating the same question

 

[bimr]

NPSHR is a significant pump characteristic; it is often taken as a measure of minimum suction head needed to operate a pump satisfactorily.

 

[1gibson]

Do you want the pump to have the correct MAWP? Then specify the max suction pressure.

It's also used to calculate the pump thrust, also required for the mechanical seal (and system, if applicable.)

 

[BigInch]

As LittleInch said!!!
Pump 1 is discharging 2000 psi. Next pump in the line is pump #2 and it is seeing 1750 psig from pump 1 after subtracting 250 psi line loss through the 50km of pipeline between them. Better be sure that pump #2 can take at least 1750 psig, and maybe the full 2000 pisg when flow from pump #1 is slow, or stopped completely, as pump #2 might see full discharge pressure from pump #1 in these types of installations.

 

[LittleInch]

extreme mechanical

Basically yes - it is the seal issue which is important for max inlet pressure. The other issue is also at pump start, the pump might then have a momentary very high discharge pressure until the inlet pressure reduces to its "normal" pressure. Also if the outlet valve is closed (no flow), the discharge pressure could easily exceed a class rating if your "normal~" discharge was close to it (say 90 barg for a class 600 system). The pump could then need to be designed for a much higher pressure than its rated or normal discharge pressure.

Rated pressure affects the pump casing design.

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

 

[extremechanical]

Little inch and big inch ( are you guys engineering twins ?!) thanks so much ! I am also thankful to all who replied .

 

[BigInch]

Anything but.

 

[Pumpsonly]

Suction pressure also decide type of pump to use. End suction or over hung pump has lower suction pressure capability than between bearing pump. A high suction pressure system can have low NPSHa due to the high vapour pressure of the liquid.