Understanding pump pressure that’s available vs. the required pressure

Photo credit: ©istockphoto.com/Craig Cozart

I was lucky to have had as one of my teachers the late, great Gil Carlson. Gil was Bell & Gossett’s director of technical services and when he would come to New York City to speak at an ASHRAE event, my old boss would ask me to sit with Gil and keep him company during the day.

This was difficult because Gil was a good listener and by this I mean that he didn’t talk that much, or at least not to me. Now that I’m older than he was then, I realize he probably didn’t have much to say to a dopey kid.

So I would ask him question after question. I might say, “Mr. Carlson, how did you come to realize that circulators in closed systems should always pump away from the compression tank?” And he would answer, “It’s just something I’ve always known.”

And then I was left to come up with the next dopey question. This would go on all day long. It’s the main reason why I can now stand in front of big groups and speak nonstop for hours and hours. Dead air is inspirational.

When the time came to get Gil to the ASHRAE meeting, we’d gather his overhead slides and cigarettes and head out. He smoked like it was his job and walked slowly, but when he got in front of a group of engineers it was magical. He’d sit next to his overhead projector and talk to the slides. But every now and then he’d stand and beam this somewhat maniacal grin from one side of the room to the other. Then he’d take a drag on his cigarette, sit and continue talking to his slides.

I later learned that the managers at B&G had sent Gil to a class on public speaking, where he was told that it is important to smile and make eye contact with your audience. This is why Gil turned into a human lighthouse every 15 minutes or so. It was quite unsettling but somehow it worked for him.

Now I’m not an engineer and what I know about engineering came to me by way of books, listening and paying attention, and mostly years after the days I was trying to get Gil to those ASHRAE meetings on time. When I’d sit and listen to his lectures, I was mostly in a state of utter confusion because Gil would launch into this stratospheric ballet of math and charts and graphs.

It was only his visual stuff that stuck with me — the word pictures. And fortunately, there were enough of those to make me want to learn more. I don’t think in numbers; I think in pictures.

Positive pressure

At one point during one of these talks, Gil put up an overhead slide that read NPSH and he asked, “Does anyone know what this acronym stands for?”

An engineer in the front row answered, “Net Positive Suction Head.”

“Well, there’s that,” Gil said. “But it has another meaning. Anyone?” And at this point he once again became a lighthouse. After he sat, he repeated, “Anyone?”

No one answered. Gil lit yet another cigarette, smiled and said, “It stands for Not (he underlined the N with his crayon) Pumping (he underlined the P) So (and so on) Hot!” And then he beamed out another of those wild smiles and the place went nuts.

That’s how I came to understand NPSH. I learned from Gil that there are two kinds: Net Positive Suction Head Available and Net Positive Suction Heat Required. The first is what you find on the job; the second is what the pump manufacturer determines the pump needs.

But now I have to back up a step and ask you to consider the shape of a centrifugal-pump impeller. It has a hole in its end and we call that the “eye” because it acts like the eye of a hurricane but not as nasty. Branching out from the eye we have a bunch of curved vanes and these end at the outer edge of the impeller.

When the impeller spins, it takes the water that’s within the eye and flings it out along those vanes to the edge of the impeller and from there to the outlet of the pump. The instant this happens, a pressure lower than the pressure that’s at the edge of the impeller will appear at the impeller’s eye.

Water in the pump’s suction line will rush in to take the place of the water that’s no longer within the eye of the impeller. Since you can’t compress water, and because the system is completely filled with water, the whole big wheel of water turns.

Now imagine what happens if we clog the pump’s suction line with debris so that when the impeller throws the water from its eye, not enough new water can enter to take the place of the missing water.

Think of a pump serving a cooling tower. Imagine a lot of crap is stuck in the pump’s inlet strainer. Or think of a boiler-feed pump, where the condensate is red hot and close to the boiling point. What if the impeller tosses out that hot water and there’s not enough new water to take its place?

In both of those cases, there often isn’t much positive pressure available to enter the suction side of the pump. The level of water in the cooling tower, or the boiler-feed pump receiver, probably isn’t that high. And since both those systems are open to the atmosphere, the only positive pressure you have going for you is the weight of the water above the inlet to the pump.

Subtract from that positive pressure the pressure drop through a strainer and the approach piping — you wind up with NPSHA. It’s what’s available (and why we put strainers after the pump on cooling towers).

And that brings us to the pump manufacturer. It will test each pump to see how much Net Positive Suction Head the pump will require, and then publish this on its pump curves. If what the pump requires isn’t available, then you run into the Not Pumping So Hot situation.

Destructive vapor

Most engineers (other than Mr. Carlson) call this “cavitation.” Water boils at 212° F, but only at sea level. If you go to Aspen, Colo., you’ll be able to boil water at 197°. When pressure drops, so does the boiling point of water.

When an impeller spins, the pressure at its eye drops and if there’s not enough NPSH Available, the water will reach the point where it flashes to vapor. This isn’t steam because we’re not adding any heat; it’s just vapor, but it’s still going to ruin your day.

The vapor bubbles form at the eye and get flung along the impeller vanes to the outer rim of the impeller, where the pressure is high. The bubbles then collapse and the surrounding liquid water screams in to fill up the spaces formed as the bubbles cave it. The water hits the edges of the impeller with incredible force. It’s enough to quickly chew up an impeller and wreck the pump.

Standing on the outside, it sounds like the pump is moving gravel. What you’re hearing is the sound of the water wrecking the impeller.

Oh, and note how the cavitation happens only when there’s not enough NPSH Available. To make this happen in a closed, hot-water heating system, you’d need a pump that can produce a differential pressure greater than the system’s static fill pressure. You’d also need to be pumping right at the point of no pressure change (the location of the compression tank).

This is rare, and the reason why we don’t see much cavitation in hydronic heating systems. You really have to screw up to make it happen, but that doesn’t mean you won’t be successful.

For instance, set up a two-temperature system (say radiant along with high-temperature radiators) and use a three-way mixing valve to get the lower temperature. It’s possible to pipe this in a way so that when the three-way valve goes to bypass, it isolates the secondary (radiant) pump from the system’s compression tank. The moment that happens, the radiant pump will cavitate and if no one notices, it will die.

It is now time for me to stand and beam a crazy smile across the room.