Investigations on the Performances of Centrifugal Pumps with Various Alterations of the Impeller

Abstract

Several years ago the writer conferred in settling the problem of changing the operation of a pump delivering 5500 gal. per minute at 168 feet head to that of delivering 3000 gal. per minute at 146 feet head. The driving motor was a 2200 volt A.C. synchronous motor. The driving speed could not be altered, and the diameter of the impeller could be cut down only a certain amount before the delivery head would be below that required. At the lowest permissible head of operation the delivery was still excessive unless throttling was resorted to. The head delivery curve was so flat at that operating range, throttling of considerable extent would have to be resorted to in order to have sufficient delivery head leaway to meet the slightly fluctuating operating conditions. The writer made the suggestion that the impeller be altered by blocking off part of the ports, having in mind that such an alteration would permit the pump to continue delivering at sufficient head to meet the requirements, but the capacity would be reduced to the desired amount without the necessity of throttling. It was hoped to gain a substantial savings in operating power by such a procedure. Inquiry was made of the pump manufacturers to get all possible information concerning any such alterations as might he made to so change the operating characteristics of the pump. They had no information on that phase of the subject they said, and suggested a different impeller. Inasmuch as the change was deemed to be but temporary, it was thought inadvisable to purchase a different impeller, and further, inasmuch as the suggested alteration of the impeller was apparently in a strange field with no information available on the subject, and continuity of service of the pump was of paramount importance, it was thought inadvisable to alter the impeller other than turning down the diameter as far as the delivery head would permit, a good margin of safety being necessary on account of the almost horizontal characteristic of the head-delivery curve at that operating condition. Due to the experience recounted the following research was conducted to obtain the information desired concerning the alteration of the impeller of a centrifugal pump, such that in the field the impeller may be readily and cheaply altered so the pump will deliver more economically, with the desired factor of safety, at operating conditions of lesser capacity than those for which the pump was originally designed.