Selection of Pump
Selection of Pump
(OP)
For Heater charge , we have two pumps ( both are motor driven) . One will be running and another as standby which will be kept in autostart mode . The standby pump starts in autostart mode when the discharge header experiences low low pressure.We have sufficient NPSH margin ( more than 2 metres) at rated conditions and nearly 0.6 meter at 120% of the rated flow. The rated point is nearly 82% of its BEP.
But at the end of the curve the NPSH margin is negative. My query is, when the pump starts in auto, since the NPSH margin is negative , whether the pump will experience cavitation? If so should I have a relook on the selection of the pump? The pumps are driven by 500 kw motors and the motors are selected for operating at end of curve operation.
Thanks to clarify.
But at the end of the curve the NPSH margin is negative. My query is, when the pump starts in auto, since the NPSH margin is negative , whether the pump will experience cavitation? If so should I have a relook on the selection of the pump? The pumps are driven by 500 kw motors and the motors are selected for operating at end of curve operation.
Thanks to clarify.





RE: Selection of Pump
Even if you are using actual running level for the NPSH margin, I would not worry about a couple of seconds of low margin (or negative margin) operation during an unusual and rare operating condition.
Johnny Pellin
RE: Selection of Pump
Since they are now two 50% pumps in parallel, system resistance throttles them back to lower flows where less NPSH is required.
Sounds pretty much inherently safe to me, you have almost immediate correction in the event of pump 1 operation beyond rated flow, or cavitating.
I don't think pump 2 is in any danger at all, at least not compared to pump 1. Unless it could be subject to excessive start/stops within a short time frame (check with motor manufacturer.)
RE: Selection of Pump
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If this is only a transient event then a risk analysis will determine if there is any concern based on the frequency and length of the events occurrence.
RE: Selection of Pump
I found so many documents online that explains the steps that are taken in order to select an appropriatye pump.
http://www.firavia.com/mecheng.html
you have to be carefull it is hard to use a system curve for parallel branched piping network.
RE: Selection of Pump
This thread provides a link to a pump selection software (that I claim no knowledge of). More importantly, it also offers a link to the McNally Institute which has a lot of information on pump selection: thread407-280011: Pump selections.
Firavia
Seems like you've put together quite a library.
Patricia Lougheed
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RE: Selection of Pump
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com
RE: Selection of Pump
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com