re: Acceptable NPSH margin 5

[TurbineBlade]

Hi,

I would like to ask what is an acceptable NPSH margin in the industry. I have a pump in an open system that is for accident mitigation purpose. Therefore, should I specify a higher NPSH margin like 50% above the NPSH required.

What will happen if the pump runs below the NPSH required? Will the pump still deliver the flow but at a much lower flow rate?

thanks

Turbineblade

 

[ccfowler]

TurbineBlade,

The NPSHr curve from the manufacture show when the flow rate is 15% above the BEP, the NPSHr starts to increase substantially.

This suggests to me that operating beyond 115% of BEP flow should be avoided. Commonly, the published NPSHr curve ends imply the upper and lower flow limits for other than momentary operation. Any operation beyond the upper limit can be quite risky (as in very rapid failure). Operating below the lower limit usually results in longer term cavitation and erosion damage.

Your reference to a high elevation source suggests the true flooded-suction condition that is probably necessary for reliable startup, but the suction piping can still cause NPSHa problems.

 

[JJPellin]

I probably shouldn't be weighing in on this debate this late in the thread. But, I will anyway. NPSH margin is very difficult to generalize. I have pumps that are running with an NPSHa of three times the NPSHr, running at exactly BEP that cavitate enough to destroy the impeller every 6 months. I also have pumps that run well below NPSHr for 10 years at time with no repairs needed. Any pump that still has serious, damaging cavitation running within the range of 75 to 105% of BEP with an NPSH margin of 15% probably has something else wrong. The pumps in my first example were redesigned with the suction nozzle offset from the centerline. Previous versions of the same model had the nozzle right on centerline. When they offset the nozzle, it created a pre-rotation that causes the pump to cavitate despite an incredible NPSH margin.

In my experience, the overwhelming majority of pumps will run very, very well and suffer no ill effects from cavitation if they have an NPSH margin of 5 feet or 15% as long as they are operated in the range of 75 to 105% of BEP. To find those extremely rare pumps that might have problems under these conditions, you need to consider suction specific speed, specific speed and suction energy. A number of good books tie in these parameters with recommended NPSH margin ratios. The one that happened to catch my eye was "Pump Users Handbook" by Bloch and Budris.


Johnny Pellin

 

[sheiko]

You will find relevant infos in:

1/ Vlaming, D. J., "Analysis of Cavitation Provides Advanced NPSH Estimates for Centrifugal Pumps," Oil & Gas Journal, Nov. 19, 1984.

2/ Henshaw, T., "NPSHA - How much is enough?," Hydrocarbon Processing, Oct. 2004.

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."