Parallel And Series Operation

Two or more pumps may be arranged for parallel or series operation to meet a wide range of requirements in the most economical manner. If the pumps are close together, that is, in the same station, the analysis given below should be adequate to secure satisfactory operation. If the pumps are widely separated, as in the case of two or more pumps at widely spaced intervals along a pipeline, serious pressure transients may be generated by improper starting or stopping procedures. The analysis of such cases may be quite complicated, and References 46 to 48 should be consulted for methods of solution.

Parallel Operation Parallel operation of two or more pumps is a common method of meeting variable-flow-rate requirements. By starting only those pumps needed to meet the demand, operation near maximum efficiency can usually be obtained. The head-flow characteristics of the pumps need not be identical, but pumps with unstable characteristics may give trouble unless operation only on the steep portion of the characteristic can be assured. Care should be taken to see that no one pump, when combined with pumps of different characteristics, is forced to operate at flows less than the minimum required to prevent recirculation. See the discussion that follows on operation at other than normal flow rate. Multiple pumps in a station provide spares for emergency service and for the downtime needed for maintenance and repair.

FLOW RATE Q, GPM

FIGURE 40 Head-flow curves of pumps operating in parallel

FLOW RATE Q, GPM

FIGURE 40 Head-flow curves of pumps operating in parallel

The possibility of driving two pumps from a single motor should always be considered, as it usually is possible to drive the smaller pumps at about 40% higher speed than a single pump of twice the capacity. The saving in cost of the higher-speed motor may largely offset the increased cost of two pumps and give additional flexibility of operation.

One of the first steps in planning for multiple-pump operation is to draw the system-head curve, as shown in Figure 40. The system head consists of the static head Hs and the sum Hf of the pipe-friction head and the head lost in the valves and fittings (see Sections 8.1 and 8.2). The head curves of the various pumps are plotted on the same diagram, and their intersections with the system-head curve show possible operating points. Combined pump head curves are drawn by adding the flow rates of the various combinations of pumps for as many values of the head as necessary. The intersection of any combined HQ curve with the system-head curve is an operating point. Figure 40 shows two pump head curves and the combined curve. Points 1, 2, and 3 are possible operating conditions. Additional operating points may be obtained by changing the speed of the pumps or by increasing the system-head loss by throttling. Any number of pumps in parallel may be included on a single diagram, although separate diagrams for different combinations of pumps may be preferable.

The overall efficiency h of pumps in parallel is given by

sp. gr. = specific gravity of the liquid k = 3960 USCS (0.1021 SI) XQ = sum of the pump flow rates, gpm (l/s) SP = total power supplied to all pumps, hp (W)

Series Operation Pumps are frequently operated in series to supply heads greater than those of the individual pumps. The planning procedure is similar to the case of pumps in parallel. The system-head curve and the individual head-flow curves for the pumps are plotted as shown in Figure 41. The pump heads are added as shown to obtain the combined pump head curve. In this example, Pump 2 operating alone will deliver no liquid because its shutoff head is less than the system static head.

There are two possible operating points, 1 and 2, as shown by the appropriate intersections with the system-head curve. As with parallel operation, other operating points

FLOW RATE, Q

FIGURE 41 Head-flow curves of pumps operating in series

FLOW RATE, Q

FIGURE 41 Head-flow curves of pumps operating in series could be obtained by throttling or by changing the pump speeds. The overall efficiency of pumps in series is given by

wherein the symbols are the same as for parallel operation. It is important to note that the stuffing box pressure of the second pump is increased by the discharge pressure of the first pump. This may require a special packing box for the second pump with leakoff to the suction of the first pump. The higher suction pressure may increase both the first cost and the maintenance costs of the second pump.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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