Pump Control Strategies for Energy Savings
In water and wastewater treatment plants, the greatest energy user is the pump. In a typical plant, the energy consumed by pumps may account for as much as 50% of the total energy bill. It is easy to see that the key to energy management is effective pump control.
While energy costs continue to rise, tighter environmental regulations increase the need to control and treat flows, and these processes use more energy. There has never been a more pressing time to improve process efficiency to keep energy bills under control.
For most level measurement applications in the water and wastewater industry, ultrasonic equipment has long been the technology of choice. This non-contacting technology is low maintenance, easy to install, reliable and cost-effective.
Recent technical advances in ultrasonic systems provide the key to wise energy management because they can provide the information necessary to monitor and control pumps effectively. The capabilities of these advanced systems are powerful tools for the plant operator. The more sophisticated systems will effectively automate plants without the high cost of programmable logic controllers (PLCs).
Plants can reduce energy costs by implementing the following pump control strategies:
- Economy pumping to avoid peak rate periods;
- Monitoring flow trends for effective management; and
- Monitoring pump performance to remove inefficient pumps from service.
Economy pumping is a valuable strategy in areas where energy costs vary at different times of the day. The ultrasonic system allows users to configure independent pump setpoints based on zones or time periods. The well is pumped down to its lowest operational level just before the high cost period. During the peak cost period, the well is allowed to fill to a higher level. If this level is reached during the high cost period, pumping occurs to a point that is higher than the lowest operational level, thus skimming the collection and reducing the energy used to start the pump. In many cases, it is possible to use the network capacity to avoid pumping completely during the high cost period.
Of course, it is important that if pumps are needed, the system responds. It is essential that regulations are adhered to and failsafe systems are in place. It is also important to ensure that the system will detect weather flow or storms, and switch back automatically to the normal pumping routines in these conditions. Some systems will also monitor overflow discharge, logging the date, time, duration and volume. This data is valuable for regulatory reporting.
Economy pumping also can be applied on standing water tanks (reservoirs) in a water distribution control system. Users can program the pumps to work during the night, priming the system to meet demand while using lower cost energy.
Depending on energy rates in specific areas, investment in an advanced controller that can perform these economy-pumping functions can pay for itself quickly.
The benefits are very real, as demonstrated at South West Water in the United Kingdom. South West Water holds the water and sewerage appointments for southwest England, serving 1.5 million people. Since the U.K. introduced a climate change levy in April 2001, energy unit costs have differential rates based on the time of day and season, which can increase overall costs by up to 15%. During high cost periods, additional charges based on transmission charges (triads) can occur, during which time users are notified to reduce energy consumption. This information can come at short notice, even within a few hours of the period, requiring rapid adjustments to avoid added costs.
As part of an initiative to reduce overall operating costs, South West Water installed Siemens ultrasonic controllers in seven pumping stations. Through economy pumping strategies, the plant cut its energy costs by almost 10% at certain sites, and successfully avoided high tariff charges. Previously, each station required a level measurement system, PLC and remote terminal unit for telemetry. The advanced ultrasonic controller eliminates the need for multiple systems because it provides all the functionality required to control pumps, collect data and communicate over the networks. South West Water experienced a very rapid return on its investment.
The benefits of economy pumping are not restricted to jurisdictions that have high tariff periods. The ability to modify pump control according to the time of day also helps with load balancing throughout a network. This can be used where power supplies suffer brown-outs due to high loads, for example, by staggering pump operations throughout the network at peak demand times.
Monitoring flow trends is a basis for effective management. Advanced systems can accurately determine and totalize pumped volume without the need for a primary device. Users can log the data or transmit it by telemetry to a central SCADA system to obtain cost-effective flow-trending data across the entire network. Studying uncharacteristic flows pinpoints current and future problems.
One example is infiltration monitoring. In a closed sewer system, it is inefficient to pump rainwater that has entered the network through degraded pipes. If flow increases during a storm, it may indicate degraded pipes that should be repaired to protect the integrity of the system, reduce pumping and save energy.
Flow trends also help users detect the implications of population growth and other scenarios so they can manage them effectively in the long term. One of the great challenges over time is that of specification to meet future population demand. It is not uncommon for pumps to be under or over specified. With the ability to store and trend flows throughout the network, increases in demand become apparent and can be identified for future planning and engineering.
Monitoring pump performance
A pump running below capacity wastes energy and money. As pumps wear or become blocked with material, their performance deteriorates.
An advanced ultrasonic system allows users to monitor each pump’s performance, including pump status, efficiency and total pumped volume. It will also log run time and the number of starts, and integrate pumps and components for effective monitoring and control. It automatically removes a non-performing unit from operation and starts a more efficient one. The operator will receive an exception report or an alarm when a failing or under-performing pump needs maintenance or replacement. Getting a pump quickly back into a high-performing state is a good investment in energy efficiency. At the same time, pump performance monitoring also shows when pumps are working well, reducing the need for field inspections or unnecessary maintenance.
In a station that contains pumps with varying capacities, the operator can program the controller to turn the appropriate pumps on and off depending on flow. The high capacity, high energy-consuming pumps can be engaged only when needed. Advanced controllers offer a variety of pump control routines and subroutines such as fixed duty assist and fixed duty backup; alternate duty assist and alternate duty backup; service ratio duty assist and service ratio duty backup; and first-on, first-off duty assist. These routines manage pumps efficiently to save energy. This information provides the opportunity to optimize operations with pump control by level rate of change, service ratios to balance pump run times, pump start delays to reduce power surges, and pump run-on or valve flushing to clear sediments.
The scum line reduction feature allows the operator to automatically vary the pump-on level within a specified range. The end result is a reduction in scum line buildup and a reduced need for regular cleaning and maintenance within the sump.
With all these capabilities, advanced ultrasonic controllers can help optimize operations. Automating a plant may be the best investment in productivity, efficiency and cost savings. From the comfort of the office at the main treatment plant, the operator can remotely collect data and set pump control routines. The controller liberates the technicians from daily remote inspections and leaves them free for other tasks. It improves plant operation and quality control by making more data available for decision-making. Tighter control results in lower energy costs.
Selecting a system
There are a number of systems on the market today, and they offer significant benefits. When assessing pump controllers, start by making a thorough inventory of all the pumps, and their locations and functions. Then, compare the plant’s needs against the specifications of various products to get the best match.
Ease of use is important. Look for controllers that offer broad functionality, but with the control algorithms and subroutines all preprogrammed for easy installation and configuration without complex programming or ladder logic. Users should be able to make changes quickly and easily in the field.
No matter how advanced the controller, the measurement accuracy of the system is largely dependent on the capability of the transducers, so always select the best quality sensors. Ensure the supplier has a portfolio of transducers for different distances (ranges) and needs. Look for high output efficiency for good performance. Choose a product with safety approvals appropriate for the operation environment. Check the accuracy under the environmental conditions in which it will be operating. If it is a harsh environment with dust, moisture, corrosion, vibration, flooding or temperature extremes, be sure the transducer can handle these conditions. Check the specifications to ensure a transducer is suitable for the application and that it is compatible with the intended transceiver.
The plant setup will determine the controller version: rack, panel or wall mount. Controllers should integrate easily with the existing SCADA, distribution control system or PLC system to provide remote monitoring of all process variables, instrument diagnostics, status and programming parameters. More available data means improved monitoring, maintenance scheduling, crew dispatch information and asset management capability.
For multiple or remote sites, the controller needs built-in communications or the ability to connect to fieldbuses by telephone, leased line, radio modems, satellite or direct PC connections.
Energy savings and reduced maintenance make advanced ultrasonic controllers a very cost-effective asset management tool. The power of advanced pump control can significantly help the bottom line.