Pumping Savings Realized

When applied correctly, variable-speed pumps can enhance pump station reliability and performance while delivering on the promise of lower energy usage.
Pumping Savings Realized
When flushing the discharge line regularly, it is possible to reduce the minimum fluid velocity below 2.5 feet per second without having sedimentation problems.

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Pumping with variable-speed pumps can improve process control, save energy, smooth out operation and reduce maintenance for wastewater pump station operators.

However, some users of variable-speed pumps report little or no energy savings, or even increased energy usage. More frequent pump clogging is also common. The solution lies in intelligent control algorithms and a holistic pump system approach. By correctly applying variable-speed pumps, it is possible to maximize the benefits of variable-speed technology while avoiding pitfalls.

Pump and pump system aspects

Performance optimization depends in part on proper care of the pump and related systems.

Pump sumps. It is important to keep the pump sump clean to prevent sedimentation of organic and inorganic material. Variable-speed controls can be programmed to perform regular sump cleaning cycles to reduce buildup of sediment and accumulation of floating debris.

Pump suction and discharge piping. With variable-speed pumping, it is possible to reduce fluid velocity below the normally recommended 2.5 feet per second for extended periods because variable-speed technology enables flushing of the discharge line by temporarily increasing the fluid velocity. A variable-speed-controlled pump station increases flexibility to clean the force main by flushing.

Check valves. In applications with variable-speed pumps, the liquid velocity will be lower than normal in the pipe system. Swing check valves have lower friction losses at low velocities than ball check valves and therefore increase energy savings.

Water hammer. Variable-speed pumps have the capability to soft-start and soft-stop by gradually increasing and decreasing pump speed. By changing the water velocity slowly in the discharge line, water hammer and slamming of check valves can be minimized or eliminated.

Minimum speed. Certain pump designs may have limitations on the lowest allowable pump speed. The minimum speed depends on proper operation of the cooling system, shaft resonance frequencies and other issues.

Pump controls and variable-speed drive aspects

An efficient system must exert control over each pump and over the pump station as a whole.

Controlling pumps. Variable-speed drives must be controlled by software to control individual pumps, while the pump station must have a master controller to properly sequence and control all pumps. The most modern controllers for wastewater pumps are hosted inside variable-speed drives.

Drive starting current and starting torque. A wastewater pump must be equipped with a variable-speed drive that can deliver nominal torque at startup and sustain twice the nominal torque at full operating speed for at least one second. Using the feedback data from the motor, the starting current can be reduced while the starting torque can be increased.

Process control aspects

Daily inflow to a wastewater pump station varies considerably – it is typically low during the night and peaks in the morning and evening. To minimize energy consumption, operators should focus on two areas.

Reducing the total pumping head. Total head is the sum of static head and losses. As a wastewater pump is normally sized to handle maximum inflow to the pump station, it is possible to reduce the pumped flow during normal operation and thereby reduce the total head.

Maximizing pump efficiency. To achieve maximum pump efficiency, it is important to select pumps that deliver sustained efficiency (self-cleaning pumps) and that operate as close to the best efficiency point as possible. To achieve maximum efficiency, a pump operated at reduced speeds should be selected well to the right of the best efficiency point at full speed.

Today’s intelligent pump controls have algorithms that automatically optimize speed for minimum energy usage. The algorithms determine the optimal speed and adapt for system changes such as reduced pump performance or increased sedimentation in the force main.

Pump sump level control

Two-speed pumps and variable-speed-operated pumps allow the station to operate at flows closer to actual inflows as opposed to design flows. Correctly implemented variable-speed operation can lead to lower energy consumption. There are four basic methods of level control.

Traditional constant-level control. In this scenario, pumps often operate outside their preferred operating range, reducing life expectancy. This happens when the pump is run at lower than the energy-optimal speed (the speed at which the pump’s specific energy consumption is at a minimum). Partial pump clogging is also more likely with non-self-cleaning pumps, such as traditional closed-impeller and vortex pumps.

Optimal constant-level control. Using a combination of reduced speed and intermittent draw-fill operation is the most energy efficient method of controlling variable-speed pumps and is recommended for wastewater pump stations.

Variable-level control. The advantage of this common control method is that the inflow can be buffered in the sump, resulting in a smoother pumped flow. Compared to constant-level control, variable-level control is a softer control strategy that helps smooth out shorter inflow peaks.

Minimum flow control. To be energy efficient in this scenario, the pump speed must not fall below the optimal speed, even if the liquid level decreases at a slower rate.

The hydraulic end

For pumps with self-cleaning hydraulics, which have been on the market for more than 10 years, the risk of debris accumulation is low. This is due to a hydraulic impeller design with extremely “swept back” leading edges, a relief groove and sometimes other hydraulic augmentations. The self-cleaning mechanism remains constant regardless of flow and speed. Therefore, as long as the duty point remains within the pump’s allowable operating range, a self-cleaning pump can handle reduced speeds, to as low as 50 percent of full speed, without increased risk of clogging.

Conclusion

To pump reliably and efficiently at variable speed, it is necessary to consider the pumped media, the pump type, the control method and the process requirements. Use of a standard drive for variable-speed wastewater pumping requires substantial engineering and investigations and implementation of the right control algorithm. Some drives are preprogrammed for controlling pumps with algorithms for various pumping applications. These drives must still be configured and engineered for the specific application.

Intelligent wastewater controls that are preprogrammed with advanced algorithms and preconfigured to ensure reliable wastewater pumping and ease of commissioning are just now entering the market. These devices will increase the pump station reliability and ensure that calculated energy savings are realized. In addition, engineering hours will not be wasted finding a custom control algorithm to suit a common application challenge.

About The Author

Stefan M. Abelin is director of project execution for the Flygt branded wastewater pumping business of Xylem. He can be reached at stefan.abelin@xyleminc.com. An extended version of this paper was first presented at the Singapore International Water Week 2014.



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