Reciprocating positive pumps

For reciprocating positive pumps and compressors the fluid displacement per stroke depends on the geometry, the fluid compressibility, the polytropic coefficient, and the internal leakages and volumetric losses, but basically not on the speed of machine. 

Positive Pumps. Positive pumps employed by the food industry have a rotating cavity between two lobes, two gears that rotate in opposite directions, or a crescent or stationary cavity and a rotor. Rotary positive pumps operate at relatively low speed. Fluid enters the cavity by gravity flow or from a centrifugal pump. The positive pump also may use a reciprocating cavity, and may be a plunger or piston pump. These pumps are not truly positive with respect to displacement, but are used for metering product flow. 

Reciprocating FW pumps may have either one or two steam cylinders. They are positive-displacement pumps and usually operate at only slow speed. They are efficient but can rapidly lose efficiency when either the steam or water valves are worn. 

Constant-flow systems are generally of two basic types reciprocating piston and positive displacement (syringe) pumps. Reciprocating piston pump can maintain a liquid flow for indefinitely long time. 

Reciprocating positive displacement pumps may have adjustment of the length or frequency of the stroke as another control feature. These may be solenoid or pneumatic devices that can be operated off a flow controller, as shown on Figure 3.21(d). 

Figure 7.4. Performance of several kinds of pumps, (a) Comparison of small centrifugal and turbine pumps (Krista and Annett, 1940). (b) An axial flow pump operating at 880 rpm (Chem. Eng. Handbook, 1973). (c) An external gear pump like that of Figure 7.12(e) (Viking Pump Co.), (d) A screw-type positive displacement pump, (e) NPSH of reciprocating positive displacement pumps.

Because of the undesirability of using gland packing, reciprocating pumps are not suitable for pumping solvents and, if a positive pump is needed, double-diaphragm air-operated pumps are worth considering. 

Figure 21.12 Reciprocating positive displacement pump operation...

As the viscosity of a liquid increases, the maximum speed at which a reciprocating positive displacement pump can properly operate decreases. Therefore, as viscosity increases, the maximum flow rate through the pump decreases. 

Table 21.3 Common failure modes of reciprocating positive-displacement pumps...

Vetter, G. and Schweinfurter, F. (1989) Computation of Pressure Pulsations in Pumping Systems with Reciprocating Positive Displacement Pumps, in Proc. 3rd Joint ASCEIASME Mech. Conf. San Diego, Pumping Machinery, pp. 83-89. 

Reciprocating positive displacement machines exist in versions such as piston pumps, membrane pumps, piston compressors, and membrane compressors. These machines achieve the highest pressures and the pumps are additionally able to reach a high conveying precision. The main reason for these features is the leakproof discharge chambers (operating areas of the positive displacement machines). 

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