eye diameter, eye area, and vane inlet angle. Cutting the impeller diameter has no

effect on the suction geometry, and should have no effect on NPSHR.

of impeller diameter but there are also some that show two NPSHR curves: one

for maximum impeller diameter and another for minimum diameter. I noticed that

the NPSHR for minimum diameter is higher than for maximum diameter. You said

that NPSHR does not change with impeller cut diameter. Please explain this

apparent inconsistency.

with how NPSHR is determined rather than with actual change in NPSHR. These

explanations were given:

This is an anomaly that is caused by internal flow recirculation and how flow rate

is measured. In some instances cutting the impeller diameter will result in higher

internal flow recirculation. Radial flow impellers whose ratio of eye diameter to

impeller diameter is greater than 0.50 are more sensitive to this phenomenon.

For clarity, assume a hypothetical situation where a pump has:

Qd, flow at discharge nozzle = 100 GPM

Qi, internal flow recirculation = 10 GPM

Qs, flow at impeller suction, Qd+Qi = 110 GPM

During test the NPSHR is measured at 100 GPM discharge flow rate. That

NPSHR is considered the NPSHR at 100 GPM but in reality it applies to 110 GPM,

the actual flow rate at the impeller suction, of which 10 GPM is due to internal

recirculation.

Now assume that the impeller diameter is cut and, as a result, the internal

recirculation flow increases to 15 GPM. The NPSHR at 100 GPM discharge flow

will now appear to be higher because in reality it is the NPSHR for 115 GPM

suction flow.

The problem is that there is no practical way to measure internal flow recirculation

and hence there is no practical way to correct the NPSHR.

Another explanation:

It is common practice in the pump industry to measure NPSHR based on a 3%

head loss. But 3% of what head? If a pump is tested at maximum impeller

diameter then it is 3% of the head at maximum impeller diameter; if tested at

minimum diameter, then it becomes 3% of the head at minimum diameter. In my

opinion this practice is inconsistent and needs to be corrected.

Here's why: Say that a pump, at 1000 GPM, has a head of 500 FT at maximum

impeller diameter. Under the present industry-wide practice a head drop of 15 FT

will determine its NPSHR. But if that same impeller is cut 20% of its diameter and

the head at 1000 GPM is reduced to 300 FT then its NPSHR will be based on a

head drop of only 9 FT.

See the inconsistency? The same pump, with same impeller and same suction

geometry, ends up with different NPSHR at same capacity because the NPSHR

are based on different absolute values of head drop. This seems to explain why

the NPSHR at impeller cut diameter may at times appear to be higher.

Specific speed (NS) and suction specific speed (NSS) are always calculated

based on data at maximum impeller diameter for data consistency. It makes

sense that NPSHR should also be based on 3% head drop at maximum impeller

diameter regardless of actual impeller cut diameter.

R: 0210-IMNP

C: basics, operation

F: NPSHR

"Make it simple"

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