Often, choosing a pump for an application amounts to referencing some curves, grabbing one that the design point fits, and calling it a day. This approach can get you by for an intermittent application or short term use, but to select a pump to produce efficiently through a long lifespan, a number of things need to be considered.
Of course, you want to select a pump that fulfills the volume requested. But does that flow rate come at a good point on the curve? Ideally, when you design closer to the Best Efficiency Point (BEP) or within 70% to 120% of BEP on average, the pump can produce more for less energy. While this is important, you will find that it also has other benefits. When a pump is working to the far left of the curve or the right end of the curve, forces become unbalanced around the rim of the impeller and cause deflection of the shaft. This may cause premature failure of seals and bearings.
Liquid Being Pumped
The product being pumped is a critical factor. Most curves are a plot of flow vs. head using cold water. The liquid may be corrosive or cause a scaling build-up. Checking materials for compatibility is critical in selecting the pump. Additionally, the Specific Gravity (SG) of the liquid is a factor in the TDH required. Be sure that TDH was calculated for any SG other than 1 which is for cold water.
Suction Lift and Suction Pipe
Be sure that any lift from below the center of the impeller is added to TDH. You will also need to be sure your suction pipe is large enough and has proper submergence. A suction pipe, just as a pump, can cause a vortex and pull in air which reduces flow as well as damage to the pump.
Make sure the materials of construction are suitable for the fluid temperature. Also, keep in mind that fluids do not all flash or boil at the same temperatures. When a fluid starts to boil, a vapor or gas is now present in the pump which again is damaging and reduces production.
NPSH and Cavitation
In considering NPSHa, all of the above points come back into play. NPSHa is a calculation of the “Positive Head” to the inlet of the pump. It includes atmospheric pressure, positive or negative lift, vapor pressure based on the temperature, piping losses and a factor of how fast water is moving in the suction pipe. Be sure the NPSHa calculated is greater than on the published curve.
If the NPSHa that is calculated is less than NPSHr, which is on the pump curves, cavitation will occur as deterioration of the impeller and surrounding parts. The NPSH should always be above the curve data at the highest flow rate of the pump system. Another type of cavitation can occur on the discharge side of the impeller. This can be caused by pumping too far to the left on the curve. If the flow rate gets too far away from the BEP that we discussed in “Flow Rate” earlier, fluids will try to recirculate around the impeller. This can cause a vaporization as well which can damage the impeller.
We have listed a number of items to “Consider” in choosing a pump, but spending a few minutes to grab and check the data can prevent early failure of the pump.
Here are a few formulas and acronyms that may come in handy:
NPSHa = Pa ± H – F – Vp + Vh
Pa = atmospheric pressure in ft.
H = fluid level above pump or minus level below
F = friction losses in suction piping
Vp = vapor pressure for temperature found on charts
Vh = velocity head often left out as a safety factor = velocity squared/2 x gravity in ft.