Designed for teaching: Small two pump set to demonstrate series and parallel system

Abstract

A range of small self‐contained hydraulic machine test sets are now available which permit small groups of students to obtain the usual performance characteristics of pumps and turbines with some ease‐though at the expense of efficiency due to scale effects. As most lecture courses include the application of pumps to systems as well as the basic considerations affecting pump performance, it was felt that a layout that allowed the student to combine two pumps in series and then in parallel would introduce him to the hydraulic considerations involved as well as give a more flexible piece of equipment that would permit these tests and a performance test to be conducted (for the basic performance curves would have to be produced first). The apparatus evolved is described, and skeleton laboratory instructions are attached as an appendix. When two similar pumps are connected in series, the fluid passes through each in turn, and the total head produced is approximately double that of one pump as illustrated in figure 1. When two similar pumps are connected in parallel, the total head produced is that of one pump, but the flow is approximately doubled, as shown in figure 2. Figure 3 is a diagrammatic layout of the hydraulic system. The apparatus consists basically of two similar pumps A and B which can be run as single units, connected and operated in series, or connected and run in parallel as will be seen from figure 3, the valves 2 and 4 are closed when series operation is desired, and flow controlled by valve 6; when operation in parallel is required valve 3 is closed, all others being open. A single sump tank is provided, flow measurement is achieved by simple volumetric means and pressure is measured by bourdon‐type gauges, a, b, c and d. The pumps are supplied by Stuart Turner with a duty of 50 feet at approximately 1300 gpm at 2900 rpm, they may operate at 1450 rpm or 2900 rpm, are provided with swinging stators for torque measurement and with a revolution counter for average speed determination. As will be seen from the appendix, the sequence of tests to be performed requires the student to obtain the basic performance characteristics of each pump at the two rotational speeds possible, and to compare them using the Similarity laws; then to test the pumps connected in series and parallel and to compare the performance obtained with that predicted. Figures 4 and 6 give some typical curves and show that the actual performances in series and parallel do not quite fulfil those predicted ignoring losses‐the student is required to consider the contributory factors in his discussion. The writer is grateful to Plint and Partners Limited for their co‐operation.