Trimming the Pump Impeller Can Cut Costs

Determining Flow and Head
The pump is installed and running, but how do you know if it is operating at its design point? There is a simple way to check. Knowing that a pump will provide a certain flow at a given head, we can determine the point at which the pump is operating. To determine the head, a few gage readings will be necessary. Take one reading from the suction of the pump and one from the discharge after the system is balanced and with all the control valves wide open. The difference between the two gage readings will give you the head that the pump is providing. Remember to convert your gage readings to feet of head. Knowing the head and the impeller size, you can determine the flow of the pump.

 

Now that we have the flow and head of the pump, let’s see how close we are to the design point. Most often, the head will be less than what we expected, and the flow will be more. Why does this happen? There are many reasons, but it does no good to blame anyone. Let’s just fix the problem.

Solutions
Trimming the impeller is one of best solutions. Before we can trim the impeller, we need to determine where the pump is operating. In the pump curve above, let’s call point “D” the design point, and draw the system curve that corresponds with that design point. Point “A” is where we actually are, which we determined from our gage readings. Along with that is our actual system curve. Remember that we are concerned with the actual system curve. This shows us how our system operates, not how it was designed. Operational and design points are often completely different. We would like to be on the unmodified actual system curve, but where on that curve? If our load has not changed and our heat transfer is the same, we want to be at our design flow. That is“I,” the ideal point.

Trimming the Impeller
But how do we get there? Although it’s off our impeller curve, we can trim our impeller down to the right size. In this particular case, our ideal impeller size falls between 10-1/2” and 11-1/2 (actually about 11”). Fortunately, trimming an impeller is not too difficult or expensive, and in fact it pays for itself very quickly. Notice from the figure that when we trim our impeller we lose some pump efficiency, but we’re more concerned about the cost of operating our pump and that cost has dropped tremendously. In this case we have dropped from 85Hp to 40 Hp-that’s a lot. Even if your electric rates are low and you don’t operate all year long, there is still the potential for great energy savings.

Consider the Savings
Looking at the pump curve you can determine the horsepower savings between the different operating points, and if you know kilowatthour costs you can figure your savings. Once you get the cost savings per year for trimming the impeller, a simple payback period can be determined to see if trimming is economically feasible. Payback periods of less than a year are not uncommon.

This example shows how adding a small safety factor can lead to a large energy cost penalty. It also shows why it’s not a good idea to install a larger impeller for some higher demand in the future, and in the interim close down on the discharge valve. It is better to wait and buy a new impeller when the increased need arrives in the future.
An impeller cannot be trimmed indefinitely. The further you trim the impeller, the lower its efficiency becomes. Plus, you are now getting near the hub of the impeller eye. It is best not to trim the impeller any smaller than the minimum size impeller that the manufacturer shows on his pump curve.

There may be times when waiting for the impeller to be trimmed is not possiblefor example, when shutting down the system that the pump is supplying would be impossible or too costly. In that case you may want to order an additional impeller and run the process until the new impeller can be installed. Lastly, remember to rebalance the impeller after trimming.