Just pick the pump with the highest efficiency! Right?” That’s the line of reasoning most novice pump professionals will follow. Efficiency is king! The most efficient pump will use the least amount of power, and run the best! All of that is true. Except when it isn’t.
Sometimes the Design Condition Should NOT Fall at BEP
Sometimes the design condition we’re provided isn’t actually the condition where the pump is expected to operate the majority of the time. This can be frustrating at times, but it happens because most pump systems are designed around a maximum capacity requirement. Typically the design condition is a reflection of the maximum capacity. However, for the most part, the maximum capacity will not be very close to the average or most typical operating conditions.
In most applications the pump will be expected to operate against a range of heads and not just at a single head condition. A well-intentioned pump professional selecting a pump on the basis of a single operating condition might pick the most efficient selection only to find that it is a terrible fit for the customer’s system. Hopefully this will be discovered before the pump is purchased, and not after a poorly-selected pump has already been installed.
Consider the Entire Pump System
Limiting your pump selection criteria to a single operating condition means taking a huge risk. It’s important to take a “system approach” to pump selection. What this means is that when making a pump selection realize that every pump operates within a system that is constantly changing. As a result, no pump operates at a single condition all of the time (except in some rare closed-loop applications – but we’re going to focus on the other 99.9% of applications).
In the vast majority of applications the head that the pump will be operating against will change. As a result, the operating point will change, and the pump will be pumping at a different head and flow than the one for which it was selected. So what can you do? How do you pick the right pump?
The first thing to do when provided a single design condition and no additional information is to demand more information. Insist on knowing the full range of heads and flows that the pump will be expected to produce. In cases where you get pushback to this request, insist on getting as much information as possible. You’re protecting yourself and your customer by requiring this information.
Base a Pump Selection on the Most Frequently Encountered Pumping Conditions
Next you need to know the average or most typical operating condition. Where will the pump operate the vast majority of the time? This is the point that needs to fall at the highest efficiency, and as close to the pump best efficiency point as possible.
Finally, you need to know the range of operating heads that the pump is expected to operate against continuously. What is the complete range of heads that the pump will be required to pump against? Take the maximum and minimum values and make sure they fall within the allowable operating range as defined by the pump manufacturer.
Once you locate a selection that puts the average condition near pumps best efficiency point and keeps the maximum and minimum head values within the allowable operating range you’ve just made the RIGHT pump selection.
Disaster Averted
Once you’ve made the right pump selection you will still need to verify that you have adequate suction pressure (NPSHa), that you’ve selected the right pump design for the application, and that the pump is a good fit for other application specifics. Once you’ve covered all of these bases you are well on your way to avoiding the mistake of picking the wrong pump.