Sawsan311
Chemical
- Jun 21, 2019
- 303
Dear All,
I have been looking into the impact of pump curve on NPSHR requirements and how larger impeller pumps (for the same rated point) although having less NPSHR requirement at the rated capacity as per the definition of NPSHR API 610 principle of being the point corresponding to cavitation at every 3% reduction in the head YET there will be higher NPSHA/NPSHR ratio towards the low flow rate when suction flow recirculation becomes more prominent with larger pumps due to flow separation at the eye of the impeller.
Also for a multispeed VSD electric motor driven pump, I have seen vendors quoting one NPSHR curve only instead of multiple for every speed.. do you agree that not always there will be the fact of having lower NPSHR at lower speeds since it depends on the shape of the NPSHR curve being cubical or parabolic?
Additionally, I have always known that vendor intentionally tend not to extend NPSHR curve backwards towards low flow rate due to two key main reasons : a) not capturing the thermal effect of recirculation by the NPSHR test b) not capturing the effect of recirculation at very low rates and which the same can't be predicted by the NPSHR test. However, I was surprised to see some vendors reporting inverted NPSHR curves in a parabolic shape i.e. NPSHR shown as rising on both side from one point at the bottom of the curve. Is that even possible or acceptable to show?? why NPSHR be reported to have a rising trend towards both high and low flow rate and is this envisaged in drooping curves?
I got the attached snap from one of the curves in a published paper showing an example of the shape of the NPSHR which I am questioning.
Appreciate your views
I have been looking into the impact of pump curve on NPSHR requirements and how larger impeller pumps (for the same rated point) although having less NPSHR requirement at the rated capacity as per the definition of NPSHR API 610 principle of being the point corresponding to cavitation at every 3% reduction in the head YET there will be higher NPSHA/NPSHR ratio towards the low flow rate when suction flow recirculation becomes more prominent with larger pumps due to flow separation at the eye of the impeller.
Also for a multispeed VSD electric motor driven pump, I have seen vendors quoting one NPSHR curve only instead of multiple for every speed.. do you agree that not always there will be the fact of having lower NPSHR at lower speeds since it depends on the shape of the NPSHR curve being cubical or parabolic?
Additionally, I have always known that vendor intentionally tend not to extend NPSHR curve backwards towards low flow rate due to two key main reasons : a) not capturing the thermal effect of recirculation by the NPSHR test b) not capturing the effect of recirculation at very low rates and which the same can't be predicted by the NPSHR test. However, I was surprised to see some vendors reporting inverted NPSHR curves in a parabolic shape i.e. NPSHR shown as rising on both side from one point at the bottom of the curve. Is that even possible or acceptable to show?? why NPSHR be reported to have a rising trend towards both high and low flow rate and is this envisaged in drooping curves?
I got the attached snap from one of the curves in a published paper showing an example of the shape of the NPSHR which I am questioning.
Appreciate your views
