Machinery's Handbook, 31st Edition
2808 SYSTEM HEAD CURVES System Head Curves: System performance curves display the influence of suction and discharge line configurations. On a system head curve, head requirements of the system are plotted versus capacity. The curve represents a total static head plus friction losses. When system head is variable, as with changing reservoir levels or when there is switch- ing between lines through valving, multiple system head curves will be represented on the graph. Operating Point: The operating point is the intersection of the pump curve and the sys tem curve on a graph that sets head against flow rate. From this graph, the flow rate and head of the pump/system combination can be easily determined. An example of the oper ating point on a graph can be seen in Fig. 9. A pump/system combination will have mul- tiple viable operating points if there are multiple pump curves given for different speeds and/or geometries. The designer should choose the operating point with the highest pump efficiency. Another case where multiple operating points are seen is in systems where the system head varies and multiple system head curves are present. The designer should ensure that the pump can meet the system requirements at all operating points. When dealing with centrifugal pumps, the pump curve chosen for the operating point will in turn determine the speed and/or impeller. It is good practice to select a centrifugal pump with the largest impeller diameter that will meet the application needs while operating at or near its maximum rated speed. H H actual System resistance Operating point
H loss
H static H dynamic
Pump performance curve
Q actual
Q
Fig. 9. Operating Point of a Pump The operating point of a pump in a system can be changed by throttling the discharge line. The suction line should never be throttled because it could lead to cavitation. When a pump is throttled, the operating point moves along the pump curve as the flow rate is reduced. Operating point can also be changed through the use of a variable speed drive. Changing the viscosity of the fluid being pumped also moves the operating point in the direction of less flow for a given head. Combining Pumps in Series or Parallel: When one pump discharges into the suction line of another pump, this is a series system. Pumps in series are capable of producing higher discharge heads than single pumps. A slight increase in flow rate will also be seen when pumps are combined in series. To generate a pump curve for pumps in series, the head of each pump is added at each flow rate to create a new curve. This curve is then graphed together with the system curve to find a new operating point. Multistage pumps contain multiple impellers in one pump body to achieve a similar effect as a series of pumps. A system of pumps in parallel experiences an increase primarily in flow capacity, with only a slight increase in head. Parallel pump systems have the advantage of increased reli ability, and the ability to efficiently adjust to variations in demand. When demand varies, flow rate moves away from the operating point, resulting in decreased efficiency. When two or more parallel pumps are used, they can be engaged and disengaged to keep the system flow rate near demand. Affinity and Similarity: Comparing centrifugal pumps can be done if one assumes the efficiencies of the two pumps are the same. This becomes less true as the difference in
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online