NPSH safety margin criteria
NPSH safety margin criteria
(OP)
L.S.,
I looked in several pump books for a good safety margin for the NPSHA compared to the NPSHR-3%. Different sources tell me to keep a fixed safety margin of 0.5 meter, others of 1 to 1.5 meter. I heard about criteria from the Hydraulic Institute, where even a factor is being applied to the NPSHR to calculate the NPSHA, but I haven't got them at my disposal. So far I applied 1 meter, but as I am confronted with a case, where I think I should have more reliable ground I am looking for a sound set of rules concerning this issue.
Could anyone of you shed a light on this subject or refer me to the right literature?
I looked in several pump books for a good safety margin for the NPSHA compared to the NPSHR-3%. Different sources tell me to keep a fixed safety margin of 0.5 meter, others of 1 to 1.5 meter. I heard about criteria from the Hydraulic Institute, where even a factor is being applied to the NPSHR to calculate the NPSHA, but I haven't got them at my disposal. So far I applied 1 meter, but as I am confronted with a case, where I think I should have more reliable ground I am looking for a sound set of rules concerning this issue.
Could anyone of you shed a light on this subject or refer me to the right literature?
Karel Postulart, The Netherlands
Nuon Power Generation





RE: NPSH safety margin criteria
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: NPSH safety margin criteria
In our plant, we require a fixed margin of 3 feet for hydrocarbon and 5 feet for water. But, we also require that the NPSHa is calculated using a conservative method. The level is defined at the bottom nozzle or bottom tangent (depending on the vessel arrangement) of the suction vessel. We have had very good results with our method.
Johnny Pellin
RE: NPSH safety margin criteria
Also, if you have a situation where you can test your pump to verify the NPSHr curves, then once verified, you can margin accordingly. I've done that in manufacturing situations where we had to use the same pump(s) repeatedly. We tested and knew whose NPSHr curves were accurate and whose were not so good.
rmw
RE: NPSH safety margin criteria
RE: NPSH safety margin criteria
RE: NPSH safety margin criteria
http:/
RE: NPSH safety margin criteria
RE: NPSH safety margin criteria
http://www.EngineeringGuideBook.com
RE: NPSH safety margin criteria
Thank you for all of your reactions. What I will do is try to get a hold of Grit's book about cavitation. Alas there are no general rules that can be applied. It does surprise me though that so many different books present different rules. I think the best when opting for a certain ( type of ) pump is to let the manufacturer, who designed it, give instructions how the high the NPSHA should be compared to the NPSHR over the whole region of operation.
A last additional question. What inaccuracy can be expected while calculating the NPSHA? I heard that an inaccuracy of about 20% is not exaggerated.
With best regards,
Karel
Karel Postulart, The Netherlands
Nuon Power Generation
RE: NPSH safety margin criteria
RE: NPSH safety margin criteria
1. The vapour pressure which enters the NPSHa is strictly related to the fluid temperature and so the closer to reality is the temperature used in the design phase the more accurate the value of NPSHa computed.
2. The minor losses due to fittings in the suction line can be calculated with different methods (Crane, 2-K method, 3-K method) and this will also effect the value of NPSHa computed.
3. Head loss due to friction are strictly related to the pipe's roughness, and the correspondence between roughness value assumed and real roughness will affect the value of NPSHa computed accordingly.
I consider the accuracy applied when calculating NPSH available reflects to the safety margin required.
RE: NPSH safety margin criteria
RE: NPSH safety margin criteria
I just was trying to calculate NPSHa for a diesel pump and had some problems to select the correct value of the vapor pressure. Is ther any data I can use always for ambient temperature? I didn't find a standard but only some "no believable" data. Most of them said 0,3 kPa, is that correct?
Anyway, I saw in most of the booked I read, a security margin of 0.5 m, but normally they talked about water, not hydrocarbons.
Thank you
RE: NPSH safety margin criteria
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