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Understanding NPSH Part 3
Tue, March 03, 2019
Last time, we looked at how NPSH can be impacted by a poor pump selection, such as using a 3500 RPM pump selected near the end of its published curve. Sometimes this can be remediated by reducing the pump speed, but not always. If the NPSHr curve is relatively flat, the speed reduction may help. If the NPSHr curve is more vertical however, the reduction in speed won’t provide much help. It’s also important to know what the NPSHr curve actually represents: in order to eliminate ALL cavitation, the pump suction may require as much as 20 psi or more! It’s hard to get rid of all the cavitation everywhere within the flow channels. Velocities are high and turbulence is an issue. So, at what point does the pump manufacturer indicate what the NPSHr is? The answer comes from the Hydraulic Institute (HI), an association to which most pump manufacturers belong. HI allows pumps to have a 3% reduction in pump head for the given flow before having to post the NPSHr. In other words, the pump is tested at a specific flow and temperature, then the suction pressure is reduced until there is a 3% reduction in pump head for the same flow. Notice, this means the pump is already cavitating at the stated NPSHr. For most HVAC applications, this isn’t much of an issue. For larger pumps however, the 3% reduction needs to be understood. You may not hear the cavitation, but you may notice the drop in pump head. One last thing: if you have a situation where the pump cavitation is too severe and capacities are falling off, you may be able to help the situation by adding a small amount of air to the pump suction. It seems counter-intuitive, but that small amount of air will increase the pump suction pressure. Be careful, too much air can cause air entrainment issues. If you have an NPSH problem, give us a call at Bornquist. We will be happy to take a look and offer potential solutions. After all, we have been doing this since 1937! Next- Efficiency Islands and Part Load Efficiencies.
Tags: Bornquist , Education , engineering , HVAC , NPSH , pumps , systems design
Last time, we looked at how NPSH can be impacted by a poor pump selection, such as using a 3500 RPM pump selected near the end of its published curve. Sometimes this can be remediated by reducing the pump speed, but not always. If the NPSHr curve is relatively flat, the speed reduction may help. If the NPSHr curve is more vertical however, the reduction in speed won’t provide much help. It’s also important to know what the NPSHr curve actually represents: in order to eliminate ALL cavitation, the pump suction may require as much as 20 psi or more! It’s hard to get rid of all the cavitation everywhere within the flow channels. Velocities are high and turbulence is an issue. So, at what point does the pump manufacturer indicate what the NPSHr is? The answer comes from the Hydraulic Institute (HI), an association to which most pump manufacturers belong. HI allows pumps to have a 3% reduction in pump head for the given flow before having to post the NPSHr. In other words, the pump is tested at a specific flow and temperature, then the suction pressure is reduced until there is a 3% reduction in pump head for the same flow. Notice, this means the pump is already cavitating at the stated NPSHr. For most HVAC applications, this isn’t much of an issue. For larger pumps however, the 3% reduction needs to be understood. You may not hear the cavitation, but you may notice the drop in pump head. One last thing: if you have a situation where the pump cavitation is too severe and capacities are falling off, you may be able to help the situation by adding a small amount of air to the pump suction. It seems counter-intuitive, but that small amount of air will increase the pump suction pressure. Be careful, too much air can cause air entrainment issues. If you have an NPSH problem, give us a call at Bornquist. We will be happy to take a look and offer potential solutions. After all, we have been doing this since 1937! Next- Efficiency Islands and Part Load Efficiencies.
Tags: Bornquist , Education , engineering , HVAC , NPSH , pumps , systems design
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