Pertinent Characteristic of Pumps

Pertinent Positive Displacement Pump Characteristics

1. The discharge pressure produced is a function of the system requirement only and is independent of pump capacity.
2. The flow rate pulses between maximum and minimum values for each revolution of the crankshaft.
3. The pulsating flow imposes an acceleration used that adds to the net positive inlet pressure required.
4. Being more energy efficient, the positive displacement pump is normally the lead pump and the centrifugal pump is the supplement when operated in parallel.

Pertinent Centrifugal Pump Characteristics

1. The dynamic head of (or pressure rise) produced is a function of pump capacity as well as of system requirements.
2. For satisfactory operation, the flow must be kept within a limited range of the best efficiency capacity.
3. When used as a suction booster, the centrifugal pump must be designed so it cannot introduced air into the gas intolerant reciprocating pump. (Gas in positive displacement pumps, like liquids in positive displacement compressors, may cause severe hydraulic and mechanical shock).
4. Centrifugal pumps do not generate acceleration heads that impose on not positive suction head required. However, if a centrifugal pump is connected in parallel to a common suction line with a reciprocating pump, some of the system acceleration head loss from the positive displacement pump may affect the centrifugal pump.

Only two cases need to be considered: centrifugal pumps feeding:

1. In series into reciprocating pump to increase suction NPSHA to the reciprocating pump
2. In parallel to augment the delivered capacity. It would be most unusual to encounter a positive displacement pump feeding into the suction of a centrifugal pump because of the high pressure that could be imposed on the centrifugal pump suction and because of the amplification of flow pulsations resulting from interaction of the characteristic of the two pumps, which could be deleterious to both pumps.