Suction and Discharge Piping

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It is absolutely imperative that the suction piping be sized to assure that the NPSH available exceeds the NPSH required by the pump. Often, this can be arranged by elevating the suction tank or by providing a lowhead centrifugal charge pump to feed the reciprocating pump. If the NPSH available is too low, valve breakage and pump maintenance costs will be excessive.

If two or more pumps are installed in parallel it is best to install separation suction lines between the tank and the individual pumps. However, in most cases this is not practical and the suction lines are manifolded together. If this is done, the lines should be sized so that the velocity in the common feed line is approximately equal to the velocities in the lateral lines feeding the individual pumps. This avoids abrupt velocity changes and minimizes acceleration head effects.

The suction and discharge piping should be short with the minimum number of elbows and fittings. Where possible, pipe should be laid out using 45° ells for elevation and plan changes rather than 90° ells. Pipe diameter changes should be made with eccentric reducers with the flat side up to eliminate gas pockets.

Table 12-1 lists some suggested maximum flow velocities for sizing suction and discharge piping for reciprocating pumps. A low flow velocity for the suction piping is particularly important. Some operators tend to use maximum velocities of one foot per second no matter what the pump speed.

table-12-1


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