6 Workshop Exercise 2: Selection and Verification of Combined Pumps in Parallel

Question

6 Workshop Exercise 2: Selection and Verification of Combined Pumps in Parallel Arrangement The local community center has received a donation of two Bell and Gossett Series 60 in-line mounted centrifugal pumps with 3 in. x 3 m. x 514 n casing and 1750 rm mpel er speed. The pumps are slated for installation and use in a water system. The system shoukd provide 180 gpm of water over a 20 ft. w.g·head. An engineer has proposed that the pumps be istalled n parallel to meet the required low. However, given the additional costs that are associated with this proposal, suitable justifications mist be presented to the community center. If it is decided that the pumps should be installed in parallel, specify the flow rate and pump head for one pump in "stand-by" operation. Will full "stand-by" operation be achieved? That is, what percent of the maxinmm design flow rate will be achieved with one pump in stand-by operation? Will this be reasonable to a client? If the pump were a base-mounted pump, what motor size would be required to avoid overloading of the pump along the entire system curve? Note: Additional pages are provided for caleulations and justifications.

Explanation / Answer

Over a head of 20 ft. only pumps with impeller diameter of 4.75’’ and 5.25’’ are suitable. For the first of them the flow achieved is around 88 gpm (88/180=0.49=49%) and for the second one is around 145 gpm (145/180=0.81=81%). As the flow is added up at the same head when two pumps are connected in parallel, the demand can be barley supplied with two 4.75’’ impeller diameter pumps. Two 5.25’’ impeller diameter pumps seem to be an oversized design.

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