I have been writing “Common Pumping Mistakes” for Pumps & Systems for more than three years. Typically the hardest part of the job is topic selection so it will be fresh, educational and interesting. This month, I am writing on a collection of shorter subjects and baking them up into one article. Instead of a meal, we will have hors d’oeuvres. Hopefully it will satisfy your appetite. If you have been reading my column, many of these tidbits will be a review. These comments are based on single-stage overhung centrifugal pumps moving ambient temperature clear water, except when otherwise noted.
Pumps are really designed to operate at only one point. That hydraulic condition of one point of head and flow is the best efficiency point (BEP), also known as the best operating point. Anywhere else on the published set of curves is simply a commercial compromise. It would be too expensive for most end users to have a pump designed and built for their unique set of hydraulic conditions.
Pay attention to the published pump curves. Manufacturers’ pump performance curves are based on clear water at approximately 65 F, unless stated otherwise. They will not be corrected for fluid viscosity. The horsepower stated may or may not be corrected for specific gravity or viscosity.
When the manufacturers’ published pump curve stops at some point of flow and head, it is for a good reason. Do not operate the pump at the end of the curve; if there was more performance to be generated from the curve beyond that point, the manufacturer would have extended the curve. Operating at or near the end of the curve will be fraught with performance issues.
Pumps are stupid. A centrifugal pump is simply a machine, where for a given set of fluid properties, impeller geometry and operating speed it will react to the system in which it is installed. The pump will operate (flow and head) where its performance curve intersects the system curve. The system curve dictates where the pump will operate.
Understand the system curve. The system curve represents all of the friction, static and pressure head baked into the system. Velocity head is also present, but typically too small of a component to be concerned about.
Pumps do not suck fluids. This is a common misunderstanding, but realize that some energy source other than the pump must supply the energy required for the fluid to get to the pump. Normally these are gravity and/or atmospheric pressure. Lastly, fluids do not have tensile strength. Consequently the pump cannot reach out and pull fluid into the suction.
The maximum realistic suction lift is about 26 feet. See the previous section where pumps do not suck. If you are at sea level the atmospheric pressure will be 14.7 pound per square inch absolute (psia), which translates (multiply by 2.31) into about 33.9 feet of absolute head. So, in a perfect world, if there was no fluid friction or vapor pressure working against the system you might be able to lift cold water 33 feet.
In reality, fluid friction and the negative consequences of vapor pressure will work against you and preclude fluid lifts of much more than 26 feet. Always calculate the net positive suction head available (NPSHa) and compare to the pump’s net positive suction head required (NPSHr) value. The higher the margin, the better.
A pump running backwards does not reverse the flow direction. The flow will still go in the suction and exit from the discharge nozzle. Depending on the specific speed (Ns) of the pump (think impeller geometry), the flow and head will be reduced by some significant amount because the pump is much less efficient. For lower specific speed pumps the flow will be approximately 50 percent of rated and the head will be 60 percent of rated. An American National Standards Institute (ANSI) pump running backwards will cause the impeller to unscrew from the shaft and lodge itself in the casing.
You cannot vent air from the impeller eye of an operating pump. A pump is in many ways like a centrifuge, and so the heavier water is expelled to the outside diameter and the lighter air remains in the middle or center. The pump should be at rest to be properly vented. Pumps with centerline discharges are essentially self-venting.
Industrial pumps do not come from the factory ready to “plug and play.” There are exceptions to this comment, but never assume. The pump will require oil to be added to the bearing housings. The impeller clearance must be ascertained and set for the fluid (temperature) to be pumped. The driver will need to be aligned to the pump. Yes, the alignment may have been performed in the factory, but the second the unit was moved for transport the alignment was lost.
You will need to check alignment again after the piping is installed, and again when the base is grouted in. The direction of rotation should be ascertained and matched to the phase rotation on the motor driver.