Reductions in centrifugal pump NPSHR (net positive suction head required 12-1)
The curve that came with your pump shows the NPSH required for any given impeller size and capacity. This number was determined by pumping cold water through the pump while reducing the suction head until the pump showed a reduction in discharge head of three percent (3%), due to the low suction head and any formation of bubbles within the pump. This point is called “the point of incipient cavitation”.
Please take a look at the pump curve shown in the next drawing. It demonstrates that if you had a 13 inch (330 mm) impeller and you wanted to pump 300 gpm (68m3/hr.) you would need at least 10 (3 meters) feet of NPSH. If you are pumping hot water or some hydrocarbons you can, in some cases, operate with a lower NPSH required than shown on the pump curve.
If you reference Technical paper Volume #9, Number #12 , you will learn that we used a similar reduction when we were calculating the suction specific speed number (SSS) of the impeller.
The NPSH reduction chart, will show you how to calculate this reduction. As you use this chart please keep the following in mind:
- This chart were created using pure liquids of the type you find in tank farms. Many liquids are mixtures and have entrained gases, or air that could require additional NPSH.
- Your product may be a combination of several hydrocarbons with different vapor pressures.
- Sometimes the temperature at the suction side of the pump can vary with outside temperature.
- Pump discharge recirculation lines can have a major affect on the pump suction temperature. These recirculation lines frequently raise the temperature of the liquid at the pump inlet.
- If a cleaner or solvent is going to be pumped through the lines at the end of a batch, depending upon the fluid, you could get into a cavitation situation.
- This reduction is possible because the expansion rate of hot water and some hydrocarbons is not as great as that of cold water.
Using the chart is not very complicated:
Find the temperature of your product and proceed either up or down to the vapor pressure of your product in either psia. or kPa. (100 kPa = 1 atmosphere)
From this point follow along or parallel to the sloping lines to the right side of the chart where you can read the NPSH reduction in feet or meters.
- If this value is greater than one half of the cold water NPSH required by the pump manufacturer, deduct one half of the value from the pump manufacturer’s cold water NPSH to obtain the corrected NPSH required.
- If this value is less than one half of the cold water NPSH required by the pump manufacturer, deduct the chart value from the pump manufacturer’s cold water NPSH to obtain the corrected NPSH required
- The chart is restricted to a maximum reduction of ten feet (3 meters) It is recommended that you do not extrapolate beyond this number or in any case use a reduction of more than 50% of the NPSH required by the pump for cold water.
- Your pump curve shows you need a 16 foot (5 meters) NPSHR (net positive suction head required) for the capacity you are pumping.
- The product you are pumping is liquid Propane at 55° F (13°C), which has a vapor pressure of 100 psia (700 kPa).
- The chart says you need a reduction of 9.5 feet (2.9 meters) which is greater than one half of the NPSH required
- The corrected value of NPSH required is therefore one half the cold water requirement given to you by the pump manufacturer or 8 feet (16 – 8 = 8), or (5.0 meters – 2.9 meters = 2.1 meters.)
Example #2 :
- Lets assume the same pump is now going to handle propane at 14°F (3.3°C) where it has a vapor pressure of 50 psia (345 kPa).
- The chart now shows a reduction of 6 feet (1.8 meters), which is less than one half of the cold water requirement. The corrected value of NPSH is therefore 16 feet minus 6 feet or a new value of 10 feet (16 – 6 = 10 feet) ( 5 meters – 1.8 meters = 3.2 meters)
If your calculations show that you have a potential cavitation problem you have several choices:
- Reduce the NPSH required.
- Increase the head on the suction side of the pump.
- Lower the temperature of the incoming liquid.
If you will refer to my Technical paper Volume #1 Paper number #3 you will see that I have covered the above subjects in good detail.
Here are a couple of more thoughts on the subject:
- If your pump is a multi-stage design this same discussion applies to the first stage head. The following stages are not considered because they should have plenty of head available to them.
- Where dissolved air or other non condensable gases are present in the liquid, pump performance may be adversely affected even though you have the NPSH required as shown on your pump curve. You are going to have to increase the NPSH available to prevent the release of these gases.
- Absolute pressure can vary with weather conditions. If you are playing it close, and we often do, this could lead to a cavitation problem.
- Low flow usually means a lower NPSH required, but low flow can also mean a temperature build up inside the pump.
- Some operators have been known to throttle the suction of a pump to reduce capacity. This may be valid for some unique dangerous products that would become a hazard if the discharge were throttled and the heat increased inside the pump, but except for these special cases, suction throttling is a bad idea.