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myv65 · 12-02-2002, 08:48 PM

OK, I didn't read the other thread, yet. What I would say is that many people don't seem to understand cavitation and what pump manufacturers call NPSH. My apologies in advance for covering "old territory". Consider this an explanation of BillA's reference to NPSH above.

Cavitation is simply a condition where the pressure on a liquid is insufficient to maintain it in the liquid state. Some of the liquid converts to vapor. When the vapor hits the impeller and the pressure rises, the vapor bubbles collapse. Both the collisions and the collapsing are the sounds of cavitation.

NPSH is net positive suction head. For every pump a curve may be generated for NPSH vs flow rate. NPSH is the minimum permissible pressure that the pump requires to avoid cavitation. The curve for NPSH tends to look much like the curve for P/Q, but going in the other direction. At zero flow, you essentially require zero net suction head. As flow grows, so does NPSH, slowly at first and rapidly as you approach the max flow of the pump.

So how do you calculate NPSH? It is nothing more than the static pressure that exists at the pump suction minus velocity head and the fluid's vapor pressure. Static pressure is basically one atmosphere +/- depending on your elevation and your particular system. System configuration doesn't matter much as one atmosphere is roughly 33' (10 m) of head. Velocity head is V^2/2g, where "V" is the average velocity (flow rate / area) and "g" is the gravitational constant (in whatever units you use).

So what does this mean? It means that as flow rate goes up, you're more likely to have cavitation. If you have cavitation and throttle the outlet, the cavitation will generally diminish or cease. Here's the kicker. For a given flow rate, a higher volume, higher pressure pump will not cavitate as easily as a smaller volume, lower pressure pump.

Many times when people think they have cavitation it is nothing more than air getting into the pump inlet (not truly cavitation) or the rattling of a cheap impeller under load (also not cavitation).

For anyone seeking a technical explanation from a pump supplier, feel free to look here.

12-02-2002, 08:48 PM	#21
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	OK, I didn't read the other thread, yet. What I would say is that many people don't seem to understand cavitation and what pump manufacturers call NPSH. My apologies in advance for covering "old territory". Consider this an explanation of BillA's reference to NPSH above. Cavitation is simply a condition where the pressure on a liquid is insufficient to maintain it in the liquid state. Some of the liquid converts to vapor. When the vapor hits the impeller and the pressure rises, the vapor bubbles collapse. Both the collisions and the collapsing are the sounds of cavitation. NPSH is net positive suction head. For every pump a curve may be generated for NPSH vs flow rate. NPSH is the minimum permissible pressure that the pump requires to avoid cavitation. The curve for NPSH tends to look much like the curve for P/Q, but going in the other direction. At zero flow, you essentially require zero net suction head. As flow grows, so does NPSH, slowly at first and rapidly as you approach the max flow of the pump. So how do you calculate NPSH? It is nothing more than the static pressure that exists at the pump suction minus velocity head and the fluid's vapor pressure. Static pressure is basically one atmosphere +/- depending on your elevation and your particular system. System configuration doesn't matter much as one atmosphere is roughly 33' (10 m) of head. Velocity head is V^2/2g, where "V" is the average velocity (flow rate / area) and "g" is the gravitational constant (in whatever units you use). So what does this mean? It means that as flow rate goes up, you're more likely to have cavitation. If you have cavitation and throttle the outlet, the cavitation will generally diminish or cease. Here's the kicker. For a given flow rate, a higher volume, higher pressure pump will not cavitate as easily as a smaller volume, lower pressure pump. Many times when people think they have cavitation it is nothing more than air getting into the pump inlet (not truly cavitation) or the rattling of a cheap impeller under load (also not cavitation). For anyone seeking a technical explanation from a pump supplier, feel free to look here.