Modern Pump Selection Part 4: Why You Should NEVER Over Head a Variable Speed Pump

By Chad Edmondson

Variable speed drives are an excellent way to enhance the efficiency of a well-designed system. They are not and should never be considered an insurance policy for pumps. 

Variable speed drives do not compensate for an imperfect pump selection. In fact, they can make the situation worse. It is okay to be slightly conservative in your pump selection if you are not 100% sure how the pumps are going to be piped, but common sense must be applied to the pump head calculation.  Also, don’t forget ASHRAE 90.1’s requirement that a detailed pump head calculation be performed and documented.

Cavitation at Full Flow

The problems of an over headed pump in a variable speed system will always rear their head at full flow conditions.  Granted, full flow only occurs about 1% of the time, but it does occur.

The variable speed pump curve (Figure 1) with the control curve shown in purple illustrates what happens. 

 FIGURE 1

FIGURE 1

In this system we have a pump capable of generating 110 feet of head at a design flow of 800 GPM.  At 100% speed we’re fine, but what if we don’t need 100% speed to achieve full flow?  The variable speed drives will begin to slow the pump down.  In this example, the drives will slow the pump down to 80% speed, at which point the pump crops off its curve. 

What happens when a pump falls off its curve? It cavitates.  Suddenly you are experiencing an explosive amount of noise and vibration and your pump impeller is taking a huge beating. The situation has to be resolved and the only way to do that without buying a new pump it is to throttle the existing pump at the discharge valve – essentially adding resistance to the system. So much for the advantages of variable speed drives. So much also for meeting ASHRAE 90.1 (and the building code) which says you must minimize pump throttling losses.

Another Case for Using Efficiency Islands

Clearly, it’s better to avoid this situation in the first place by improving your pump selection.  First, start with a more accurate calculation of your total system resistances and design flow.  Don’t select a pump for 110 feet of head when 70 feet of head will do the trick.  Furthermore, don’t select a pump based on BEP at full flow conditions, which only occurs about 1% of the time.  It’s better to select the pump based on “efficiency islands” as discussed in Part 1 of this series.  This means selecting a pump that will operate to the left of its BEP most of the time.

All this begins to make much more sense as we start to understand how varying load conditions constantly alter the position of the system curve.  That’s a topic we will review (again) in our next blog so stick with us!