I thought that I would ask this. Everyone says or knows NPSHA should be greater then NPSHR by at least, bit I have not come across anyone who knows exactly what will happen if it does, apart from cavitation? Will the pump stop, will it work at a different dp? will it grow wings and fly? .....what will happen? apart from cavitation that is.
I'll put in my 2c after hearing what people have to say...?

cavitation alone is enough reason not to operate out of the NPSHR range, unless of course you are using cavitation as a means of breaking up filamentous bacteria

No Terry, suppose accidentally your calculations were erroneous and NPSHR became slightly lower then NPSHA. What do you do then? Re-supply of the pump will take time so you cannot stop the pump. Moreover, your client suffers a loss if u stop the pump.
This is a subject where I have not been able to find any information upon. The situation happened to a M+P pump once which I had sold to a refinery owned by IOCL(Indian Oil Corp). Luckily, I was given wrong data.
But I wanted to know any experience of other group members in this regard.

Hi Arindom,
Even at the published NPSHr rating by the pump manufacturer, the pump starts to cavitate, that's why we use a safety factor when calculating NPSHa. What will happen if you are off the mark? Depending on the application, you will have premature failure of the bearings, the seal, etc. What do you do then? You can look at changing the pump speed via vfd, if you are pumping out of a tank, can the level be adjusted higher? You can trottle the pump discharge to work on a better place on the curve.

Arindom, i am at a loss here. anytime NPSHR is lower than Available that is a good thing. Maybe i am missing something

Hi Terry,
Sorry for the typo.It should be ' NPSHA becomes lower then NPSHR.

Hi Francoise,
Yes-i am comfortable with a margin of 1m though 0.5 m also works. You can fit an inducer, you can lower the discharge etc..these are preventive measures. I want to know that what is the worst possible scenario. Cavitation will be there in the suction side, the suction flange may become uneven and leak a little etc etc. But is that all? Damage to the pump in the long run. Yes, pump will fail earlier but is there any known empirical data proving that some extensive unwanted phenomenon will take place apart from cavitation and pitting, because of the NPSH. On bearings-yes i presume that cavitation will lead to dynamic UN-balancing because of which, bearings may be affected, but
I have failed to find any written info anywhere or anyone who says" at x, y will happen" etc".
Even Karrasik is silent on that. I have personally opened a cavitating HSC pump and the damage was quite minor after 8 months.That pump will easily run for 3 years more after which it won't have much value anyway-due to depreciation Isn't this an important part of Pumping Technology? But information available is very scanty, to my knowledge.
I just want know something on this as its very interesting (to me-at least)

Hi Arindom, your original question "what will happen apart from cavitation" is the pump will eventually be destroyed. How long depends on the severity of cavitation. At the 3% head drop NPSHR curves from the manufacturer, the pump is cavitating sufficiently to lose 3% of its head.

In addition it will be noisy and vibration will increase over the normal running condition. That vibration could cause the shaft to break due to fatigue on between bearing impeller pumps, holes to appear through the impeller vane, bearings to have a shorter life and erosion of the impeller vanes and pump casing.

You will not find in a vehicle owners manual the statement " do not drive faster than 220 km per hour because the engine will drop out " and so I do not expect a statement in a pump users manual to the effect of " do not run the pump at the NPSHR on the curve because it will explode". They will warn you of the ill effects if you try it and see.

The margin to apply is not simply 0.5 metre or 1 metre and it goes away. With these values you may still have a noisy pump. It looks like some manufacturers are now quoting minimum NPSHR needed for satisfactory running as well as the NPSHR at 3% head drop.

This should reduce the number of pumps in the field that are cavitating.

What engineers and system designers should be looking at is providing the pump with adequate NPSHA to allow it to run reliably within its expected lifetime. There can be nothing more alarming to have an unexpected pump failure on a critical service late at night. This everyday irritation for some then becomes a crisis.

A better question may be what if I provide plenty of NPSHA for the pump. It will be a simple answer, the pump will run well!

One possible way to avoid cavitation in this case (where at the duty point the NPSHA is less than NPSHR) close the valve on the discharge side of the pump until the NPSHR reduces and becomes below the NPSHA. You will reduce the Flow Rate of the Pump but the pump will operate without cavitation and you do not have to wait for the replacement pump.

Trimming or replacing the impeller is another inexpensive option. Check the curve for the next smallest size and see if the NPSHA is under the NPSHR.

Anis, A replacement pump should be ordered as it is likely that throttling back on the discharge valve will tear up the pump's shaft, seals and internals as you may be operating too far off the curve. This could be a bad idea. Rob,s advise is better.