Advantages of parallel pumping

It is generally true that the larger the fluid flow requirement, the more viable parallel pumping can be. However, parallel pumping can be successfully applied to both large and small systems (Fig. 1).

It is necessary to ensure that pumps are selected to satisfy worst case conditions on heating and cooling systems. Consider that these full-load conditions occur only in 10%-15% of the pump’s operating life. The remainder of the time the system requires much less flow.

Systems with two-way valves vary flow by modulating closed during part-load conditions, based on their control sequence. Since parallel pumps are applied with smaller motors to better match part load, the savings potential is enhanced.

Using software analysis, a comparison can be made that is useful and interesting. For example, consider a heating system with a capacity of 400 gpm at 90-ft TDH and compare using a standalone pump with parallel pumps. Although the comparison is typical, note that load profiles are easily modified to match any system, and the analysis can be job specific.

For this heating system, assume 4500 hr/yr of operation and a typical load profile. Most heating system designs include a standby pump in the secondary or distribution loop. The table summarizes the criteria used and conclusions reached.

Note that the duty point brake horsepower for 200 gpm at 90-ft TDH is 6.26 hp. Further analysis shows the system requires a 10-hp motor to run the parallel pumps. Brake horsepower prior to turning on the second pump peaks at over 8 hp. It would be a costly mistake to use a 7 1/2-hp motor for the parallel pumps. Not all pump suppliers would go to a larger motor.

In the specifications, bidders should perform this analysis to confirm a nonoverloading condition before turning on the second pump. A performance-based specification is well worth the extra ink in this instance.

The strategy for closed loop heating and cooling can work very well when applied properly and is a flexible solution, especially in pumping systems with high flow rates. An analysis is shown for two pumps, but larger systems with up to six pumps in parallel have proven effective when staged and controlled accurately.

Another application where multiple pumps can be used is in pressure boosting applications. Although similar in configuration, open pumping systems require special considerations. Pressure fluctuations, pump curve type, unequal capacity splits, and minimum pressure requirements must be evaluated and considered to select the most effective combination.

Final considerations

It is common to require a dedicated standby pump for a main heating or process loop. It is usually a duplicate of the main pump. This approach doubles the cost of the single pump option and reduces the payback period for the use of parallel pumps.

The staging point for the parallel option is 360 gpm, providing 90% design flow redundancy. The requirement for full flow is rare and the 360 gpm can be seen as an overall redundancy closer to 95%. There are several measurements that can initiate pump staging controls (kilowatts, amperes, pressure, or flow) and add a higher level of performance.

There can be electrical installation cost savings for breakers/electrical panels for smaller horsepower motors, particularly when a large, dedicated standby pump is required.

Careful analysis with specialized software is an important tool to use in properly selecting and applying centrifugal pumps when operating in parallel.

Parallel pumping with variable speed can save energy and introduce significant advantages in operation and control.

A pump curve (Fig. 2) illustrates some of the finer points on redundancy and duty-point brake horsepower.

— Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, j.foszcz@cahners.com

Key concepts

Determine the full flow and most common rate of a process to see if parallel pumping can be used.

Two small pumps in parallel provide redundancy and save on installation and operating costs.

Use software to analyze a piping system, select the proper pumps and motors, and determine operating costs.

Two important checks

1. The pump that turns on first must have an impeller curve that crosses the system curve at all flows. Operation past the right end of the pump curve can cause erratic and unpredictable flow as well as significantly increase the chances of destructive cavitation.

2. Brake horsepower must be considered at the point where the first pump curve crosses the system curve, just before turning on the second pump. The bhp peaks in this location, and usually makes a nonoverloading motor selection mandatory.

Benefits of parallel pumping

– Provides up to 90% redundancy of the design flow with a single pump, which equates to significant standby protection that supports the system when one pump is down. Since heat transfer varies as a square function of flow, a single pump operating to supply a process is very close to design heat transfer rates, which further increases system redundancy.

– Reduces the motor size required for a standalone pump, making this redundancy cost effective. In some cases, electrical wiring savings can be significant.

– Matches flow to load better, which means a single large pump is not required to continually pump full flow against a control valve at high turndown.

– Saves amps on motor starts for plants that are restricted by demand charges or electrical entrance limitations.

Heating system, standalone vs parallel pumps

Pumping system Standalone Parallel

design criteria pumping system pumping system

Pump capacity 400 gpm @ 90 ft 200 gpm @ 90-ft each

Pump selected Base mounted Base mounted

(3-in. discharge, 4-in. suction) (2 1/2-in. discharge, 3-in suction)

Duty point bhp 12.02 hp @ 400 gpm 6.26 hp @ 200 gpm

BHP prior to staging N/A 8.32 hp @ 360 gpm

Pump motor required 15 hp (nonoverloading) 10 hp (nonoverloading)

Pump-only price $3260 $4650 (2 pumps)

Annual operating cost $3800 operating in a $3200 staging with

variable volume system standard control panel

Simple payback (based on operating costs and constant speed operation) is 2.3 yr

More info

The author is available to answer questions on parallel pumping. He can be reached at 506-849-0334.

See the fluid handling channel on our web site, Plant Engineering Online,(www.plantengineering.com) for more articles related to this topic.

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