Reciprocating pumps flow profile

With the drive mechanism used in a simple reciprocating pump, the profile of the piston position is sinusoidal. Hence the flow-rate profile, which is the first derivative of the piston position profile, provided that the back-pressure is nearly atmospheric, will be half sinusoidal, as shown in Fig, 9a, It can be seen that the flow delivered by a single head is basically pulsed, as the delivery occurs only during the forward stroke. The maximum flow-rate is 3.14 times the average flow-rate. 

Figure 9.24. (a) Schematic and (b) flow profiles of a dual-head reciprocating pump. Reprinted from Yost, Ettre, and Conlon, Practical Liquid Chromatography, 1980 by courtesy of Perkin-Elmer. 

As seen above, the flow of sinusoidal drive reciprocating pumps is greatly pulsed. Even when a triplex pump is used, a residual pulsation remains (Fig. 9b). Most detectors in general use, especially differential refractoineters, are flow sensitive, the pulses increasing the baseline noise, resulting in a poorer detection limit. Under certain conditions, the pulses can even affect the profile of the eluted peaks and thus the resolution, as the HETP is not a linear function of the actual flow-rate. For these reasons, pulse damping is often necessary. 

Fig, 9. Piston position and flow in reciprocating pomps. In each figure the upper curve gives the position of the piston and the lower curve the flow-rate profile as a function of time, (a) Single sinusoidal piston movement head A = backward stroke, B = forward stroke, (b) Three sinusoidal piston movement heads. In the lower diagram the thick solid line indicates the total flow-rate, (c) Dual-head, special-piston movement pump. 1 piston A accelerates 2 piston A moves at constant speed 3 piston A decelerates 4 piston A retracts 1 to 4 similar movements of piston B. The broken line indicates the total flow-rate. 

Figure 4. (a) Design details of the drive cam and (b) resultant liquid flow-rate profile in a modem single reciprocating piston pump. 

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