NPSH REQUIRED Relationship to Impeller Diameter

[SAeng]

Hi,

We have replaced an existing centrifugal pump impeller. the new impeller Head Curve is already plotted on the original manufacturers pump curve.

However only one NPSH required Curve has been drawn on the Pump Curve.

If the pump impeller is changed would the NPSHreq not change as well. from the curve it implies that a single curve is applicable to a number of impellers(with different diameters).

Can the affinity laws be used to determine the NPSHreq curve? (h1/h2) = (d1/d2)^2 - for constant speed.

or does the manufacturer have to retest the pump to provide a new NPSH req curve?

The question is how do i get an NPSHreq curve for the other impeller diameter?

can someone point me in the right direction.
Thanks

 

[BigInch]

No you can't use the affinity laws for NPSH. NPSHR correlations are to suction specific speed.

 

[pumpkind]

The curve given by the Manufacturer is valid for all the listed impellers. Use the relevant data at the flow provided by the pump with the new impeller.

 

[Yorkman]

Not that I'm recommending this but just thinking out loud, experimentally could you momentarily shut down/valve off the suction of the pump and with a compound gage record the "pressure/vacuum" at the suction connection and convert to Ft/Hd? I more curiuos than advocating this, I've used this method to get an idea of the Ft/Hd availabe at the discharge side why would'nt it work on the suction side? I would think the manufacturer has the data your looking for, with the impellar change. And I'm sure one of the other members could comment on the use of the proper formula provided you can supply the needed data.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[jet1749]

What Yorkman suggests is basically how you perform an NPSHr test
Pick a couple of points on the performance curve. Run pump at constant speed, vary the suction pressure whilst trying to maintain constant head and flow at the selected point. Where head reduces by 3% that is your NPSHr at that operating point. Do a couple more points to generate your curve and thats it.

 

[BigInch]

I think I've found a "backdoor affinity law" for NPSHR using suction specific speed.

Calculate your suction specific speed Nss with the current impeller, Nss = RPM * Q^0.5 / NPSHR

where all Qs are gpm
NPSHR is ft

Calculate Flowrates with the new impeller diameter
Q2 = Q1 * D2^3/D1^3


NPSHR2 = RPM * Q2^0.5 / Nss

 

[Artisi]

SAeng (Chemical)

What are you asking, are you changing speed or diameter?

If you are replacing an impeller of the same diameter, of the same configuration and operating at the same speed nothing will change.
If the impeller diameter is to be less than the original but speed remains the same, the NPSHr at your new duty will be as shown on the pump data sheet NPSHr curve.
The NPSHr curve is probably drawn from testing at full diameter at a constant speed at a number of different flow rates. Reducing the impeller diameter will not change NPSHr at a particular flow rate as NPSHr is a function of flow into the impeller and not a function of diameter.



Naresuan University
Phitsanulok
Thailand

 

[25362]

In Sulzer's Centrifugal Pump Handbook, I've seen the NPSH requirements for a 2950 rpm pump Type ZA 80-250 for 259 mm and 200 mm impeller diameters.

For a constant flow rate of 100 m³/h, beside a reduction in head and power consumption, there was an increased demand in NPSH from 2.5 m to
~ 4.5 m when using the smaller impeller, with an attached drop in efficiency.

 

[BigInch]

NPSHR relationship to Suction Specific Speed. See above, or see it here,

http://www.lawrencepumps.com/newsletter/news_v04_i5_oct.html

Going the Big Inch!

http://virtualpipeline.spaces.msn.com