OVERSIZED PUMP O021

Conduct a survey of any process plant and you will find that a high percentage of the centrifugal pumps are oversized. There must be a reason why this is such a common problem, so here are a few possibilities:

- Safety margins were added to the original calculations. Several people are involved in the pump buying decision and each of them is afraid of recommending a pump that proves to be to small for the job.
- It was anticipated that a larger pump would be needed in the future, so it was purchased now to save buying the larger pump later on.
- It was the only pump the dealer had in stock and you needed one badly. He might have offered you a “special deal” to take the larger size.
- You took the pump out of your spare parts inventory. Capital equipment money is scarce so the larger pump appeared to be your only choice.
- You purchased the same size pump as the one that came out of the application and that one was over-sized also.

Obviously this larger pump and motor required a higher investment in power, but since we are not using the full power, are we really paying too much for the daily operation? The easiest way to find the answer to this question is to look at a typical pump curve and make our calculations from the numbers we get.

You can use any of the following formulas to make your calculations:

300 gpm. to a 156 foot head, with an efficiency rating of 60%

required to do this

As shown in the above drawing, we should be using the smallest diameter impeller “E” to do this, but we have an oversized pump and we are using the larger impeller “A” with the pump discharge valve throttled back to 300 gpm. giving us an actual head of 250 feet and a 50% efficiency. Now our Kilowatts look like this:

required to do this.

If 28.2 KW is being used and only 14.7 KW are required, it means that we are paying for an extra 13.5 KW to pump against the throttled discharge valve.

If this pump runs 24 hours per day that would be 8760 hours this year, and at a power cost of $0.05 cents per Kilowatt-hour it would cost your company an additional:

8760 hours x .05 cents per kilowatt-hour x 13.5 kilowatts = $5913.00 per year, extra operating cost.

Now we will work the same problem in the metric system:

Assume that we need to pump 68m^{3}/hour to a 47-meter head with a pump that is 60% efficient at that point.

required to do this.

As shown in the drawing, we should be using impeller “E” to do this, but we have an oversized pump and we are using the larger impeller “A” with the pump discharge valve throttled back to 68 cubic meters per hour giving us an actual head of 76 meters. Now our kilowatts look like this:

is required to do this. So:

28.6 – 14.8 = 13.8 extra kilowatts being used to pump against the throttled discharge valve. If the pump runs twenty four hours a day that would be 8760 hours per year, times 13.8 extra kilowatts equals 120,880 additional kilowatts pr year.

Multiply this number by how much you spend for a kilowatt-hour of electricity and you will see that the over sized pump is costing you a lot of money. In this example the extra cost of the electricity could almost equal the cost of purchasing the pump. As an example if we calculate our kilowatt cost at U.S. four cents per kilowatt-hour, we would get:

120,880 KW x $0.04 = $4,835.20 to pump against the throttled valve.