This article originally appeared in Water Quality Products Commercial Water Spring issue as "The Power of Parallel Pumping"
During the 1960s, the Green Bay Packers’ power sweep dominated the game of football, helping carry the team to five NFL titles and two Super Bowl victories. When rival teams began making adjustments to stop the sweep, Packers quarterback Bart Starr read the defense at the line of scrimmage and modified the play to ensure the best results.
In a parallel pumping system, the controller is like the quarterback, dynamically making modifications based on the changing defense—or fluctuating flow and head requirements. Variable speed control in a parallel pumping system is a proven method of increasing efficiency and reducing costs, but designers can achieve even greater system efficiency and energy when using advanced logic controllers and variable speed drives together. Think of it as employing a bit of West Coast Offense to the pump staging process to create a powerful solution that comes with high efficiency, backup capacity and potential savings on space and initial investment.
In an environment where partial loading is typical, optimizing system performance is not as straightforward as optimizing pump performance at a single duty point. The entire profile of the pump’s efficiency at varying flows and speeds must be considered. In a parallel pumping environment, this makes both selection and staging strategies more complex. Technology comes into play here to help arrive at these solutions more easily, and parallel pump controllers with best efficiency staging capability can yield dividends.
Selecting a pump for parallel operation using the same methods as choosing a single pump involves cutting the flow in half or a third and picking the most efficient option. When flow gets past 120% of best efficiency point (BEP), it is time to stage the next pump. Yet, while the system will provide the required head and flow, without leveraging technology, the system likely will not be at optimal efficiency.
Achieving Optimal Parallel Pump Staging
So how can optimal parallel pump staging be achieved? The challenges and solutions of designing parallel pumping systems becomes evident when examining different staging scenarios and comparing and contrasting the weighted part load efficiency values (PLEV).
In the first scenario, the full load requirement is 4000 gpm at 65 feet of head with a control head requirement of 19.5 feet. Design parameters need to be considered carefully, as they dictate the system curve driving the selection decisions.
With such a large pump, the selection would likely be a double suction pump. In Figure A, the curve depicts an efficient solution, especially at full load. There is some tapering at 50% and 25% of load, yielding a weighted part load efficiency value (PLEV) of 81.6%. This is a 10-by 12-by-15.5 pump with a 100 hp 6-pole motor, a large investment and footprint, but one that provides zero backup capacity.
The next option provides backup capacity and even greater PLEV. Figure B shows two end suction pumps in parallel 8-by-10-by-13.5-inch pumps with 50 hp motors. This constant speed graph demonstrates efficiencies of more than 86% for portions of this curve, meaning there will only be improvements over the curves when variable speed is introduced into the equation. Overall, the system curve crosses all of the test speed curves and demonstrates acceptable efficiencies.
Figure C reflects individual pump efficiency improvements created by variable speed. Dividing the flow between both pumps at full load, efficiency is 86.3%—even higher than the double suction pump in Figure A. If both pumps continue to run at partial loads, the weighted efficiency is 77% below the double suction performance. As demand decreases, the second pump must be destaged. As system requirements drop below 2400 gpm, the two-pump efficiency is actually dropping below 80%, while the single-pump efficiency climbs to 88% as demand drops to 1,400 gpm. Based on the system curve for this pump, it appears the best staging/destaging occurs around 10% past BEP. With two parallel pumps, one pump can drop off and the system will still operate at 75% capacity.
Figure D demonstrates coverage up to 50% of load with one pump if system losses have not been underestimated. With two pumps, nearly 90% of load is covered. In fact, if the requirement was to meet full duty with two pumps, motors and drives could be upsized to 40 hp and the pumps oversped to almost the same efficiency as the pumps in Figure D, just short of 60 feet at 2,000 gpm per pump. Based on this pump’s efficiency profile, optimal system efficiency will be achieved by running two pumps from 3,600 gpm all the way down to just over 100 gpm or the 25% partial load point.
The initial investment savings on three 30 hp 4-pole motors versus a 100 hp 6-pole motor is 40%. The initial investment savings on three 6-inch pumps versus the single 10-inch pump is roughly 25%.
Figure E shows efficiency benefits that can be achieved by determining the optimal staging point for each parallel pumping solution. It is important to evaluate the system curve and pump efficiency curves to optimize staging. In these scenarios, optimal staging in a parallel pumping solution can save 3% on energy costs versus the double suction solution while increasing backup capacity and potentially eliminating system downtime required for maintenance. If pumps are not staged properly, the system could operate 3% less efficiently than the single-pump solution, resulting in more than $1,000 in increased energy costs per year depending on operating conditions and utility rates.
Determining Efficiency Benefits
The staging points are based on system design and anticipated system losses. In these examples, the system is modeled using a system curve. In actual application, this will need to be reviewed following commission. In a diverse system, there is a control area rather than a simple control curve. The more diversity in the system, the larger this area and the greater the benefit from dynamic, real-time staging performed by a capable pump controller versus a fixed staging strategy based on predetermined staging points.
As evidenced by the scenarios above, best efficiency staging for multiple pumps can be challenging for any system designer. But with the aid of a capable quarterback–a pump controller equipped with built-in best efficiency staging–these calculations happen dynamically, ensuring the system can actually deliver these theoretical efficiencies.