- 11 April 2007 -

Optimizing energy efficiency in parallel pump installations

Saving energy in systems with parallel pumps is complex. To assist with this task, ABB Oy in Finland has developed intelligent pump control software for its low voltage AC industrial drives. Jukka Tolvanen and Heikki Kervinen present the features of the new software and explain how it can be used to improve the energy efficiency of pumping systems without the need for an external programmable logic controller.

In wastewater pump stations it is quite common for the pumped volume to vary widely depending on the process, time of day, consumption and other factors. One option to handle the varying volume requirements and provide over-dimensioning is to run smaller pumps in parallel. When the flow demand is lower, a smaller pump is optimal. When the flow demand increases, additional parallel pumps are started up to meet the capacity requirements. However, saving energy in installations where parallel pumps are used for tank filling/emptying is a complex task. In a parallel installation each pump affects all the others. Moreover, in parallel installations with dissimilar pumps, selecting the optimal running order and method by conventional means presents significant challenges.

In general, AC drives are the most energy-efficient way of controlling pumps. This paper describes how intelligent pump control (IPC) software developed for ABB industrial drives can further boost energy efficiency in both single and parallel pump installations. This solution is particularly energy efficient in wastewater applications. In ABB's pump control solution, the primary controlled variable is energy efficiency and the secondary controlled variable is the tank level.

Affinity laws

The affinity laws shown below describe the relationship between speed (n), flow (Q), head (H) and power (P). Speed and flow are proportional. The power required is proportional to the cube of the speed or flow. If the speed is reduced by half, the power consumption is only one eighth of the original. At half speed the flow is naturally only half the original value, so the time required to pump the same volume is doubled. However, the energy requirement is still only one quarter of the original figure.

Note that the affinity laws do not take the static head into consideration. The calculation above is applicable only to systems with a zero static head. To adapt the system curve to the efficiency speed approach, it is necessary to define specific energy.

Specific energy

Specific energy, Es (Wh/V), is calculated as follows:

Specific energy = Energy used/pumped volume

= Pin/Q

Energy-saving potential

The lowest energy consumption can be achieved through the efficiency speed approach. Savings of 10-50% in energy consumption can be realized, especially in wastewater applications where the static head is small and system performance is not limited if the pump speed is slightly reduced.

Software solution

Intelligent pump control is an optional software package for ABB industrial drives. Incorporating all the most common functions required by pump users, it eliminates the need for an external programmable logic controller (PLC) and other additional components. A pump system with fewer electrical components is always more reliable, especially in the harsh environments typical of wastewater pump stations.

IPC can help save energy, reduce downtime and prevent pump jamming and pipeline blocking.

All of the IPC features presented here are innovations. The software package was developed on the basis of ABB's extensive experience of pump and fan control software for its drives. In part, development and testing of the new features was done in collaboration with ABB's pump customers all over the world.

Level control mode

The software feature used to optimize energy efficiency in parallel pump installations is level control mode (patent pending). This is typically used in the filling or emptying of wastewater storage tanks, and can be used with a single pump, or up to eight pumps in parallel. Energy consumption is minimized by operating the pumps at their efficiency speed as long as possible.

On the basis of the theory presented earlier, this approach makes energy savings of around 10-50% possible, while still providing a control method that has the flexibility to cope with wide variations in the liquid level and pumped volume.

Other IPC benefits

Equipping a pump station with parallel pumps means that a redundant system can be created. With the IPC's level control mode, system redundancy is 100%. If one of the pumps, motors or drives stops operating, the system will continue uninterrupted. The drives are interconnected with a fast fibre optic link, and even if the master drive in the parallel system stops operating, one of the other drives can take over as the 'new master' in only 500 ms. System redundancy of 100% guarantees high usability and risk-free operation of the pump system, even if a fault should occur.

Pipeline blockage can be a problem when pumping liquids that contain particles, especially with slow operating speeds and/or smooth control. Level control mode features fast-ramp starting to create a flush effect that helps to keep pipelines clear. In addition, when the pumps are operated they are always run close to the nominal point where the risk of pipeline problems is lower due to a higher flow. The software also seeks to prevent sediment build-up on the tank walls by randomly varying the surface level within a range preset by the user.

The IPC's anti-jam function enables drives to perform preventive maintenance on pumps. When this function is triggered, the pump is run at a high speed and then either reversed or stopped in a number of user-defined cleaning cycles. This helps to prevent congestion through the build-up of particles. The trigger parameters are set by the user, with four different options available (high current, run-on time, external input and every start).

Pump priority control balances the operating time of all the pumps in the system over the long term. This facilitates maintenance planning and can boost energy efficiency by operating pumps closer to their best efficiency point. In a system where the consumption rate is higher during the day, for example, the drive can be programmed to operate higher capacity pumps during the daytime and smaller units at night.

Summary

By making efficiency speed the main control parameter for single and parallel wastewater pumps, it is possible to achieve up to 50% savings in energy consumption. ABB's IPC software package supports this approach, and delivers a number of usability benefits. Redundancy of 100% keeps everything up and running even when one part of the system fails - no matter whether it is the master or follower in a parallel system. Other features of the software, such as the anti-jam function, flush effect and pump priority control, guarantee trouble-free operation in wastewater pumping applications. 

www.abb.com/motors&drives

 

 

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