10 Pump flow
Pump flow
The term pump flow refers here to the control of fluid flow through pumps and piping systems as opposed to process control involving total industrial processes. The words control and regulation are otherwise considered to be synonymous in this and other chapters.
When dealing with fluid transportation, a pump installation is dimensioned to cope with a maximum flow which, in practice, might never come about. This principle of dimensioning is nevertheless correct, since inadequate pumping capacity on isolated occasions can lead to the most damaging consequences. It is also obvious that sensible ways of varying the flow are important, not only for the functioning of the pump installation itself but also for the process for which the pump is included as an aid.
Procurement costs for pumps generally amount to less than 10 percent of the total investment cost of a plant and yet the functional quality of a pump may be the decisive factor for the overall function and its associated running costs. Advanced methods of flow control, e.g. speed regulation of pumps, are today’s best methods of varying the flow but with this disadvantage, that the procurement costs for the pump equipment are just about doubled. Since the share of costs due to the pump is very small, the extra investment for, say, a speed converter is easily compensated for in other ways in the installation and might even bring about large reductions in total costs. The savings may be in the form of reduced pipeline dimensions, considerably reduced costs for storage, water towers and pump sumps for example, and sometimes even the size of buildings.
With regard to existing installations, the method of flow regulation affects, most of all, the energy costs. As a result of increasing electricity costs, the optimum methods of pump flow control have changed greatly from those with low outlay costs and high energy consumption to those with relatively high investment costs and good power economy. For pumps with a power requirement greater than 10 kW, it will even now pay to convert to more power-saving methods of regulation.
Pumps can be controlled in steps, dis-continuous flow regulation or smoothly continuous flow regulation. Combinations of the two methods are also used. The choice of method is determined by the overall requirement of the pump installation, evenness of flow, economy and technical possibilities.
The operating point of the pump is the intersection of the system curve, the pipeline characteristic and the pump curve as in figure 10a. At this point, the braking “force” the system curve, is equal to the driving “force”, i.e. the pump curve.
Figure 10a The operating point, also called duty point, is at the intersection of system and pump curves.
Pump flow variations
It is only in exceptional circumstances that a pump can be used without any means of control or regulating the flow. The unregulated flow will fail to agree with the required flow because of:
- Safety margins and hydraulic tolerances.
- Failure to utilize full production capacity.
- Variations in requirement caused by climatic variations, e.g. summer, winter, rain, drought.
- Variations in production processes.
Flow variation with time can be described either by a running curve, figure 10b or by a constancy curve, figure 10c.
Figure 10b Flow as a function of time – running curve.
In the constancy curve, time has been summed up for all periods of flow having the same magnitude. A read-off on the constancy curve gives the time during which the flow at least amounts to a particular value.
Figure 10c Constancy curve for flow.
Constancy curves can be made up for various periods of time ranging from a full 24 hours to the lifetime of the installation. Constancy curves are usually made up for a period of time of 1 year, however.
Unfortunately, constancy curves are not that easy to compile. but a simple analysis of the actual fluid transportation often gives adequate guidance for the determination of maximum, minimum and average flow, The mean flow, for example can be determined from the annual production capacity and maximum flow from peak consumption.
The pump size is obviously determined by the maximum flow figure, whereas pump economy is determined by the duration of flow. Economic assessments are greatly facilitated if the volume flow is known at 100%, 50% and 0% constancy (duration). It would be even better, of course, if the values at 25% and 75% are also known. The following simple equation is obtained by the use of Simpson’s integration formula:
where
Qmean = Mean flow
Q100 = Flow at 100% constancy, i.e the flow taking place 100% of operational time
Q50 = Flow at 50% constancy
Q0 = Flow at 0% constancy (Q0 = Qmax)
or if more points on the curve is known:
where Q100, Q75……Q0 are expressions fpr the flow at 100%, 75%…..0% constancy, see also figure 10d.
Figure 10c Illustration for Equation 10a and 10b