Reciprocating pumps flow-rate

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

Figure 32.21 Reciprocating pump variation of flow rate with numbers of cylinders...

Positive displacement, reciprocating, pumps are normally used where a high head is required at a low flow-rate. Holland and Chapman (1966) review the various types of positive displacement pumps available and discuss their applications. 

Reciprocating and rotary positive displacement pumps are commonly used where moderately low vacuum is required, about 10 mmHg (0.013 bar), at moderate to high flow rates such as in vacuum filtration. 

The piston is driven in and out of a solvent chamber by an eccentric cam or gear. On the forward stroke, the inlet check valve closes, the outlet valve opens, and mobile phase is pumped to the column on the return stroke the outlet valve closes and the chamber is refilled. Unlike syringe pumps, reciprocating pumps have an unlimited capacity, and their internal volume can be made very small, from 10-100 p. The flow rate can be varied by changing the length of stroke of the piston or the speed of the motor. Access to the valves and seals is usually fairly straightforward. 

In the single-headed reciprocating pump shown in the figure, the mobile phase is being delivered to the column for only half of the pumping cycle. During the drive stroke of the piston, the flow rate is not constant (because the speed of the piston changes with time). The output of the pump is shown in Fig. 2.2e (z). Use of a twin-headed pump with the two heads operated 180° out of phase... 

Compared to syringe type or reciprocating pumps, pneumatic amplifier pumps are very cheap. They tend to be rather difficult to dismantle for repairs, and some types are very noisy in operation. Because they do not provide a constant flow of mobile phase, they are not used much in analytical hplc. They can, however, operate at high pressures and flow rates and so are used mainly for packing columns, where high pressures are needed and variations in the flow rate through the column do not matter. 

Normally, a positive displacement reciprocating pump is used for pressurizing the pipeline during testing. The flow capacity of the pump should be adequate to provide a reasonable pressurizing rate. The pressure rating of the pump must be higher than the anticipated maximum test pressure. 

In Figure 30.3, the flow rate of a twin-head reciprocal pump has been plotted against time. The stage-A depicts the drive while the refill zone is vacant while the stage-B evidently shows the two-heads functioning simultaneously thereby the drive and the refill both zones could be visualized. 

The Micromeritics 7000 liquid chromatograph (Fig.3.30) also uses a reciprocating pumping system. The pump has several unique features. It delivers pulseless flow without a pulse damper, it can operate from an unlimited reservoir, it requires only a single pump for gradient elution and it is capable of operating at constant pressure or constant flow-rate. 

The Perkin-Elmer Model 604 liquid chromatograph (Fig.3.31) has a reciprocating pump with positive displacement. The delivery of a constant volume is digitally controlled from flow-rates of 0.05 ml/min to 6.0 ml/min at pressures of up to 7000 p.s.i. Solvent pulsations are removed by means of a damper system. 

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