Number of Pumping Units

A single pump is the cheapest first-cost installation. However, if downtime has any value such as in lost production, in hazards created in rest of process, etc., then a stand-by duplicate unit should be considered. A spare or stand-by can be installed adjacent to the operating unit, and switched into service on very short notice, provided it is properly maintained. Spare pumps which do not oper- 

If solids are carried in the fluid, this can present a difficult problem if they are not properly flushed from the pump on shutdown. Some spare or second pumps are selected for 100 percent spare others are selected so that each of two pumps operate in parallel on 50 percent of the flow, wiih each being capable of handling 67 to 75 percent of total load if one pump should fall off the line. This then only reduces production by about 25 percent for a short period, and is acceptable in many situations. These pumps are usually somewhat smaller than the full size spares. 

Wien it is necessary to plan several pumps in parallel, the pump manufacturer must be advised, and care must be taken in arranging suction piping for the pumps, otherwise each may not carry its share of the flow. 

There are many flow conditions, and pumps should be selected to operate as efficiently as possible over the widest range of capacity. 

If the flow is expected to vary during the system operation, the high and low GPM (and corresponding heads) should be given to allow proper evaluation. 

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