Pump Suction Press Vs. Vap. Press.

[me3]

In the data sheets of some new pumps I noticed that the pump Suction Press. is equal to or a bit less than the pumped liquid Vapour Press. at the pumping temp.

Wouldn't that cause a cavitaion problem?

In addition, the data sheets shows that there is NPSHa (few meters). But how come we have NPSHa, while the pump suction press. is already less than the pumped liquid vap. press?!!

Thanks,

 

[25362]

Please check "where" is the suction absolute pressure measured. If it is on the liquid suction vessel, then the NPSHA could be a few meters, as you say, o/a of the elevation of the liquid level over the pump's eye.

Then, of course, check whether the suction pressures, as given, are absolute or gage. Kindly report back.

 

[upendrapatel]

I think the simplest way to varify the available energy at eye of the impeller is to apply basic Bernoulli equation taking impeller eye elevation as datum. This will chow clearly the available NPSHA. In this case the liquid is at boiling point. Refer book "Centrifugal Pump, Design and Application" by Labanoff & Ross, Figure 9.26, Page 116, of second edition.

 

[me3]

Thanks for your reponces. For "25362", the data sheet is not metioning clearly where the suction press. is measured. But, it is written in the "Operating Conditions" section which talks mainly about the pump itself.

In addition, tow values are given for the Suction Press. Max. and Rated. So, I would assume they mean the suction at pump inlet.

And both suction and vap. press. are given in Abs.

 

[Scipio]

If you have a suction pressure in absolute less than the vapour pressure in absolute, I agree with your confusion as to where a few meters of NPSHA is coming from. You might want to double check those NPSH calcs, I've seen once or twice where someone relatively new at running those calcs used the maximum suction pressure for calculating the NPSHA instead of the rated suction pressure.

 

[25362]

To me3:

1. Since NPSH is defined as the difference between total head (static and dynamic) and the vapor pressure, both at the pump intake, the only "contributor" left for the NPSHA is the velocity head u²/2g, which is usually negligible, except when using inducers, which are, in effect, small "booster" pumps that are capable to handle a bit of vaporization without suffering cavitation problems.

2. However, if the suction pressure P_s is taken at the reservoirs, you still have the elevation difference between the reservoir and the pump's eye less the friction loss in the suction pipe, as a contributor to the NPSHA, a difference that may be larger than the small positive or negative (P_s-P_v) difference you mentioned.

Kindly comment.