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J.H. WRIGHT & ASSOCIATES

NPSH MADE SIMPLE ?

The writer has been challenged to write a simple explanation of Net Positive Suction Head without using any equations, graphs or words with more than three syllables. Since I'm probably the last graduate engineer still using a slide rule, here goes.

We begin with an Italian gentleman by the strange name of Bernoulli, who for no good reason I can think of, put pressure gauges on a venturi. What's a venturi? Just a necked down section of pipe.

Actually, he didn't use pressure gauges because they hadn't been invented, but he did arrange to measure the pressure at the three points shown above. He discovered that the pressure is reduced in the narrow pipe section where the liquid has to speed up. If it isn't obvious to you that the velocity of the liquid is higher in the narrow opening, we've both got a problem. Anyway, Mr. Bernoulli has been immortalized in the history of physics with "Bernoulli's Theorem".

The second part of my explanation involves a pot of water heating up on your stove. As you heat the water, the little molecules get more and more active - you'd jump around too if someone was setting fire to the soles of your shoes.

Finally, at 212ºF, they can't stand the heat anymore and they jump out of the water and form a gas; steam. This effectively keeps us from getting the water temperature over 212ºF.

As anyone who has tried to do some cooking at higher altitudes such as Denver can tell you - out there you can't even get the water up to 212ºF before the little molecules start jumping out and forming into gas. The reason is that at 5,000 ft. of altitude, the air pressure is less than the 14.7 psi we have at sea level and it's easier for the molecules to jump out, so they do it at lower temperatures. Therefore, it should be obvious that either higher temperatures or lower surface pressures or a combination of the two will cause a liquid to change into a gas, O.K.?

Now, what's all this got to do with NPSH? Well, let's look at the suction of a centrifugal pump.

If you can't see the similarity to a venturi right at the impeller suction, I need to take more drawing lessons.

When the pump is pumping, there is therefore a pressure reduction on the liquid right at the impeller eye. If the pressure reduction is enough and/or the temperature of the liquid is high enough, the liquid turns to gas right at the impeller eye. So what, the liquid behind should push it right through the pump shouldn't it? Well, not always, sometimes a big bubble is formed and the pump will stop pumping completely. After all, it's a pump, not a blower.

Sometimes the following liquid does push the gas deeper into the pump impeller where the pressure is higher. The gas turns back into liquid and continues right on through the pump. Great. No, not great because as each molecule implodes (not explodes), it reaches out and grabs a little piece of impeller metal to take with it as a souvenir of the journey. Before long, your impeller looks like a smorgasbord for termites.

So - what is NPSH? Well, there are two kinds - NPSH (Required) which is really a measurement of how much "necking down" or pressure reduction the designer had to design into the pump. The other is NPSH (Available) which is how much pressure you've got to arrange to have on the liquid before it goes into the pump so the conversion to gas does not occur.

No curves, equations or long words!!

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