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Understanding NPSH, Part 1
Mon, February 02, 2019
NPSH is probably the least understood part of a manufacturers pump curve. It’s easy to see why: we use atmospheric pressures as reference, the effects aren’t always audible, and it’s hard to tell pump cavitational noises from air entrapment. NPSH stands for Net Positive Suction Head. Roughly interpreted, it’s the amount of absolute pressure at the suction flange of a pump. There are two numbers that have meaning when it comes to NPSH - the first is NPSHr, which represents the minimum suction pressure that the manufacturer wants to see before cavitation becomes a problem. The second is NPSHa, which is the amount of pressure available to the pump suction. The NPSHa must exceed the NPSHr, or bad things can happen. Liquids will boil if pressures are too low. For example, if we are at sea level, water will boil at 212F. But if we climb to 30,000 feet, the boiling point is only 160F, which is why it takes longer to boil food at high altitudes – the temperature is lower. If we reduce the pressure to only 0.1 psia, water will boil at 36F! In other words, we need pressure to keep water from flashing into a vapor. But what happens in a centrifugal pump? Water enters the suction flange and sees a rapid acceleration, which reduces its pressure. If the pressure is reduced enough, the water will flash to a vapor. That vapor will then see a sudden increase in pressure in the impeller vanes or the volute and will implode to become a liquid again. It’s this implosion that we hear as cavitation. It is also the cause of damage to the impeller and volute. As each vapor bubble collapses, it chips away at the impeller. Over time, this can make the impeller look like swiss cheese. These photos show what an impeller can look like after being subjected to cavitation. Cavitation can also cause damage to pump seals and pump shafts.
But take heart: NPSH is stated in absolute pressure, not gauge pressure. Gauge pressure takes atmospheric pressure out of the equation. The absolute pressure at the Earth’s surface is 14.7 psi. Since this is where we live, we prefer to call that pressure “zero”. But it’s really 14.7 psia, or 34 feet of head absolute. NPHS is stated in feet of head absolute, where zero is the pressure as seen in the void of outer space.
Next time – How to fix an NPSH problem.
Tags: bell & gossett , Bornquist , centrifugal pumps , Education , HVAC , NPSH , optimization , pumps , systems design
NPSH is probably the least understood part of a manufacturers pump curve. It’s easy to see why: we use atmospheric pressures as reference, the effects aren’t always audible, and it’s hard to tell pump cavitational noises from air entrapment. NPSH stands for Net Positive Suction Head. Roughly interpreted, it’s the amount of absolute pressure at the suction flange of a pump. There are two numbers that have meaning when it comes to NPSH - the first is NPSHr, which represents the minimum suction pressure that the manufacturer wants to see before cavitation becomes a problem. The second is NPSHa, which is the amount of pressure available to the pump suction. The NPSHa must exceed the NPSHr, or bad things can happen. Liquids will boil if pressures are too low. For example, if we are at sea level, water will boil at 212F. But if we climb to 30,000 feet, the boiling point is only 160F, which is why it takes longer to boil food at high altitudes – the temperature is lower. If we reduce the pressure to only 0.1 psia, water will boil at 36F! In other words, we need pressure to keep water from flashing into a vapor. But what happens in a centrifugal pump? Water enters the suction flange and sees a rapid acceleration, which reduces its pressure. If the pressure is reduced enough, the water will flash to a vapor. That vapor will then see a sudden increase in pressure in the impeller vanes or the volute and will implode to become a liquid again. It’s this implosion that we hear as cavitation. It is also the cause of damage to the impeller and volute. As each vapor bubble collapses, it chips away at the impeller. Over time, this can make the impeller look like swiss cheese. These photos show what an impeller can look like after being subjected to cavitation. Cavitation can also cause damage to pump seals and pump shafts.
But take heart: NPSH is stated in absolute pressure, not gauge pressure. Gauge pressure takes atmospheric pressure out of the equation. The absolute pressure at the Earth’s surface is 14.7 psi. Since this is where we live, we prefer to call that pressure “zero”. But it’s really 14.7 psia, or 34 feet of head absolute. NPHS is stated in feet of head absolute, where zero is the pressure as seen in the void of outer space.
Next time – How to fix an NPSH problem.
Tags: bell & gossett , Bornquist , centrifugal pumps , Education , HVAC , NPSH , optimization , pumps , systems design
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