Pumps, positive-displacement plunger pumps

Positive-displacement pump—A pump that conveys fluid by directly moving it using a suitable mechanism such as a piston, plunger, or screw. 

Plunger or diaphragm pumps do not handle large flow rates in excess of 100 m /hr (4400 US gpm), but they are suitable for a wide range of applications at higher volumetric concentrations than centrifugal pumps. Positive displacement pumps can pump slurries with a weight concentration of 70%. 

Reciprocating or positive-displacement pumps with valve action piston pumps, diaphragm pumps, plunger pumps... 

In the first major class of pumps a definite volume of liquid is trapped in a chamber, which is alternately filled from the inlet and emptied at a higher pressure through the discharge. There are two subclasses of positive-displacement pumps. In reciprocating pumps the chamber is a stationary cylinder that contains a piston or plunger in rotary pumps the chamber moves from inlet to discharge and back to the inlet. 

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. 

The piston plunger pump is the simplest form of a positive displacement pump. These pumps can be powered by a variety of prime movers, internal combustion engines, and electric motors (and in some cases, powered by a gas turbine motor). In such applications, the separate pump unit is connected to the prime mover by a power transmission. 

Fig. 6.37 Schematic representation of four geometrical configurations utilizing external mechanical pressurization giving rise to positive displacement flow, (a) Axially moving plunger in a cylinder, (b) Squeezing disks, (c) Intermeshing gear pump, (d) Counterrotating intermeshing twin screws.

Positive-displacement reciprocating pumps (a) plunger pump (b) diaphragm pump-... 

Figure 3-12 classifies pumps as positive-displacement or kinetic. Positive-displacement pumps move liquid at a relatively constant rate and are used when discharge pressures are high, when the pump must be self-priming, and when liquid must be metered at an accurately prescribed rate. Flow through a reciprocating pump is produced by the movement of a piston or a plunger... 

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