Multiple Pump Arrangements
Advantages
There are many advantages to using combinations of smaller pumps in place of a single large one.
Operating Flexibility
As shown in Figure 1, the use of several pumps in parallel broadens the range of flow that can be delivered to the system. Additionally, by energizing and de-energizing pumps, the operating point of each pump can be kept more closely near its BEP. Caution should be used in operating parallel pumps to ensure that the minimum flow requirement is not violated for any pump.
Redundancy
Failure of one unit does not force a system shutdown. With a multiple pump arrangement, one pump can be repaired while the others serve the system.
Maintenance
Multiple pump configurations allow each pump to be operated close to its BEP, which lowers bearing wear and permits the pumps to run more smoothly. Other benefits include less reliance on energy-dissipating flow control options such as bypass lines and throttle valves. The use of a single, large pump during low flow demand conditions forces the excess flow to be throttled or bypassed. Throttling the flow wears the throttle valves and creates energy losses. Similarly, bypassing the flow is highly inefficient, since all the energy used to push the excess flow through the bypass lines is wasted.
Excess flow creates maintenance problems in terms of piping vibrations which fatigue welds in the piping and piping supports and loosen flanged joints.
Efficiency
A potential advantage from multiple pumps is a higher overall efficiency level since each pump can operate close to its BEP. By energizing or de-energizing pumps as necessary to meet changes in system demand, each pump can operate over a smaller
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Multiple Pump Arrangements
Additionally, at a given head and flow, high speed pumps tend to be more efficient than slow speed pumps. (Exception: pumps with specific speed values greater than 3,000 tend to be less efficient at higher speeds; however, this is not typical of most pumps). Since smaller pumps require smaller motors, the use of multiple high speed pumps can provide an efficiency advantage over a single, slow speed pump. However, this efficiency advantage should be balanced against the tendency for higher speed machines to require more maintenance.
Other Options
Other system designs that can be used to handle widely varying operating conditions include pony pumps, multiple speed pumps, and variable frequency drives. For more information on pony pumps see the Fact Sheet titled Pony Pumps. Information on variable frequency drives can be found in the Fact Sheet titled Variable Frequency Drives.
Multi-speed pumps are used in similar ways as multiple pump configurations in that the
fluid power generated can be matched to the demands of the system. Shifting a pump to higher or lower speeds moves the entire performance curve up or down respectively as shown in Figure 2.
Although at any given operating point, multi-speed pumps tend to perform less efficiently than single speed pumps, their ability to operate over a wide range of conditions offers a key advantage. Multiple speed pumps also offer space savings. Their compact operating package avoids the additional piping and valves required for parallel pumps.
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Flow (GPM) Head
(Feet)
High speed
Medium speed
Low speed Pump Curves
System Curve
Figure 2 Multiple Speed Pump Performance Curves
Pumping System Fact Sheet #9 Table of Contents
Basic Principles... 53 When to Consider Pony Pumps... 53 Costs of Intermittent Pump Operation... 54 Costs of High Flow Velocity... 54 Recovering the Costs of Installing a Smaller Pump... 55 Basic Principles
Many pump applications have a diverse range of flow needs. Often, there is a large difference between the flow required during normal system operation and that required during peak load conditions. For example, rainwater collection and some cooling system applications typically require a relatively low flow rate. Occasionally, however, a heavy storm or exceptionally large heat load due to a sudden increase in production demand creates a need for high pumping capacity.
If the pumps are sized to handle peak flow, or worst-case conditions, they may operate at substantially inefficient levels for long periods before operating briefly at their design points during periods of high demand. These oversized pumps tend to wastefully consume energy and require frequent maintenance because they operate far away from their best efficiency points.
Similarly, in sewage treatment plants the normal operating demands on pumps may be relatively low. During storms, however, the amount of fluid which must be drained from the various holding ponds or tanks increases dramatically. Obviously, the pump(s) that maintain holding pond levels must be capable of handling storm conditions.
To avoid the high friction losses and maintenance problems that accompany continuous operation or frequent starts of oversized pumps, a plant can install smaller (pony) pumps to handle normal operating conditions. The larger pumps would then only be used to handle the occasionally severe load conditions, providing considerable cost savings.
When to Consider Pony Pumps
Indicators of a need for a smaller pump to handle normal operating conditions include: