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Reducing leaks with AC drives
12
Applications
WORLD PUMPS
June 2012
Water & wastewater
Reducing leaks with AC drives Leakage is a common problem among many water utilities. It is often caused, and made worse, by the pressure in the water mains being kept at a higher level than demand necessitates. One solution to this problem is to use AC drives to regulate and maintain a more appropriate system pressure, bring savings in cost, water and energy.
M
any water supply networks maintain a constant pressure over time, regardless of whether this pressure is needed or not. Typically, the pressure is provided by one or more pumps and is controlled by a relief valve. Over time, this
practice is wasteful and causes leaks in the supply network. Pump capacity is normally selected according to the maximum flow and head, perhaps even with a view to future needs, plus a
certain safety margin. However, the average pumping capacity may be only a fraction of the maximum capacity, and during periods of low demand, the required capacity will be even less. If the pressure corresponds to demand, leakage across the water industry could be significantly reduced. Reducing pressure by a little cuts leakage by a lot – a mere 20% reduction can result in 50% less leakage. When the pressure is high, water is forced out through existing leaks. Further leaks are generated through excessive strain on the joints as pumps start and stop. Meanwhile, energy is wasted through unnecessary friction in the system and by pumping water through leaks. The water lost through leaks has to be replaced, resulting in more pump running time and further energy wastage. The energy waste in turn harms the company’s environmental performance. At the same time, everyday life in the company's service areas is disrupted by maintenance work more often than necessary. Ultimately, this affects the company’s financial performance. By using AC drives, water mains pressure can easily be controlled and the impact of the problems above can be significantly reduced or eliminated.
The right speed Different pipeline systems have different requirements. With an AC drive, it is easy to change the output of the pump system to suit conditions.
www.worldpumps.com
The objective of a water company is to move water from A to B as cheaply and effectively
14
Applications
WORLD PUMPS
installation and then fine-tuned over time. Modern drives are easy to operate and adjust. A system with five bar during daytime could, for instance, be automatically reduced to three bar at night using the inbuilt variable-speed drive (VSD) control. This could cut leakage by as much as 80% during the night.
Similarly deceleration towards ‘sleep’ can be controlled via the flow calculation feature to slowly decelerate at near zero flow to close a check valve at a rate that does not initiate ‘water hammer’. This can reduce the need for pipe replacement or help postpone renewal to a later date.
Some AC drives, such as the ACQ810 and ACS800 from ABB, have a flow calculation function that enables flow measurement without the installation of a separate flow meter. The default flow calculation function is sensor-less using information derived from the pump datasheet, as well as providing the option to measure the flow rate using pressure transmitters.
Network cleaning
Automatic functions
If the pressure corresponds to demand, leakage across the water industry could be significantly reduced.
as possible. If the system uses AC drives, flows can be adjusted to ensure that pressure levels are appropriate, and that the right reservoirs are filled with the right volume at the right time. Providing correct pressure corresponding to demand makes common sense. Who in their right mind would drive a car on the motorway in low gear and with full throttle unless they were overtaking? Nevertheless a great number of pump systems run at what amounts to 'overtaking-speed' at all times, because many pump stations are built by contractors whose priority is cost-effective design and construction, not cost-effective operation. This is false economy. Most analyses show that energy consumption is the dominating element of a pump station’s life cycle costs, especially if the pumps run more than 2,000 hours annually. If the object is to reduce the life cycle cost of a pump station, using AC drives makes sense as their cost is relatively small compared with the total life cycle cost savings. This would also address many of the industry’s problems with maintenance and leak management.
Right pressure Different pipeline systems have different requirements. With an AC drive, it is easy to change the output of the pump system to suit conditions. Settings can be adjusted at www.worldpumps.com
June 2012
Many modern drives have very sophisticated control functions built in. It is, for instance, possible to vary automatically the pressure according to demand. Some drives also have a ‘sleep’ function that enables the pump to 'rest' if there is no demand. If the drive senses low demand during the rest period, and this remains steady, it will be identified as a leak, which means the pump can stay in sleep mode for longer. Before entering sleep mode, the drive can activate a ‘sleep boost’ function, which runs the pump to boost the pressure in the pipeline or water level in the tank prior to shutdown. This extends the pump's sleep time and saves energy. It also avoids unnecessary starting and stopping. AC drives can also prevent leaks from occurring in the first place. Switching pumps on and off causes rapid changes in flow. These flow changes are followed by rapid pressure transients, known as water hammer, which cause stress on the network and damage pipes, pipe supports and valves, resulting in leaks. And when pipework is replaced and leaks repaired, pressure can build elsewhere in the network, causing leaks in new locations. The use of AC drives enables the user to gradually ramp the acceleration at a safe rate to avoid hammering. ABB’s drives for water and wastewater have a ‘pipe fill’ function which provides a pump with a soft-start enabling a smooth build-up of flow in pipes. This avoids pressure peaks, for example in irrigation systems where pipes are momentarily empty and controlled pipe-filling is required. The lifetime of the pipe work and pumping system is thereby increased.
One of the reasons for maintaining high pressure can be to keep the network clear of sediment. This can also be arranged with the help of AC drives, but far more economically and effectively than merely using high system pressure. The sleep boost function mentioned above, for instance, helps to flush the pipelines. When changing pump speed, the change can be programmed to take place rapidly, creating a powerful flush effect in the pipelines and in the pump itself, keeping the system clean. These changes take place within safe limits, to avoid water hammer. Some AC drives also have a pump clean function, providing a pump cleaning cycle by running between maximum and minimum speed settings with aggressive ramps in between. The user defines whether forward and/or reverse operation is allowed, depending on pump design, plus the duration/number of cycles to be used per cleaning cycle. The user sets the VSD triggering condition to automatic when monitoring high levels of, for example, current and torque. The triggering condition is also set for regular cleaning cycles, for example on every pump start or after a set time duration or number of revolutions. Keeping a pump as clean as possible helps ensure the highest hydraulic efficiency for the system.
A better system As many systems only require high pressure for a fraction of the time, the pressure can often be significantly reduced, particularly at night. With AC drives it is easy to maintain an efficient system with low maintenance requirements, and they do not cost any more than a traditional system, as the drives pay for themselves through more energy-efficient operation. The result is a network that works better for all involved – staff, customers, shareholders and the regulator. www.abb.com/drives
Applications
WORLD PUMPS
June 2012
Water & wastewater
Reducing leaks with AC drives Leakage is a common problem among many water utilities. It is often caused, and made worse, by the pressure in the water mains being kept at a higher level than demand necessitates. One solution to this problem is to use AC drives to regulate and maintain a more appropriate system pressure, bring savings in cost, water and energy.
M
any water supply networks maintain a constant pressure over time, regardless of whether this pressure is needed or not. Typically, the pressure is provided by one or more pumps and is controlled by a relief valve. Over time, this
practice is wasteful and causes leaks in the supply network. Pump capacity is normally selected according to the maximum flow and head, perhaps even with a view to future needs, plus a
certain safety margin. However, the average pumping capacity may be only a fraction of the maximum capacity, and during periods of low demand, the required capacity will be even less. If the pressure corresponds to demand, leakage across the water industry could be significantly reduced. Reducing pressure by a little cuts leakage by a lot – a mere 20% reduction can result in 50% less leakage. When the pressure is high, water is forced out through existing leaks. Further leaks are generated through excessive strain on the joints as pumps start and stop. Meanwhile, energy is wasted through unnecessary friction in the system and by pumping water through leaks. The water lost through leaks has to be replaced, resulting in more pump running time and further energy wastage. The energy waste in turn harms the company’s environmental performance. At the same time, everyday life in the company's service areas is disrupted by maintenance work more often than necessary. Ultimately, this affects the company’s financial performance. By using AC drives, water mains pressure can easily be controlled and the impact of the problems above can be significantly reduced or eliminated.
The right speed Different pipeline systems have different requirements. With an AC drive, it is easy to change the output of the pump system to suit conditions.
www.worldpumps.com
The objective of a water company is to move water from A to B as cheaply and effectively
14
Applications
WORLD PUMPS
installation and then fine-tuned over time. Modern drives are easy to operate and adjust. A system with five bar during daytime could, for instance, be automatically reduced to three bar at night using the inbuilt variable-speed drive (VSD) control. This could cut leakage by as much as 80% during the night.
Similarly deceleration towards ‘sleep’ can be controlled via the flow calculation feature to slowly decelerate at near zero flow to close a check valve at a rate that does not initiate ‘water hammer’. This can reduce the need for pipe replacement or help postpone renewal to a later date.
Some AC drives, such as the ACQ810 and ACS800 from ABB, have a flow calculation function that enables flow measurement without the installation of a separate flow meter. The default flow calculation function is sensor-less using information derived from the pump datasheet, as well as providing the option to measure the flow rate using pressure transmitters.
Network cleaning
Automatic functions
If the pressure corresponds to demand, leakage across the water industry could be significantly reduced.
as possible. If the system uses AC drives, flows can be adjusted to ensure that pressure levels are appropriate, and that the right reservoirs are filled with the right volume at the right time. Providing correct pressure corresponding to demand makes common sense. Who in their right mind would drive a car on the motorway in low gear and with full throttle unless they were overtaking? Nevertheless a great number of pump systems run at what amounts to 'overtaking-speed' at all times, because many pump stations are built by contractors whose priority is cost-effective design and construction, not cost-effective operation. This is false economy. Most analyses show that energy consumption is the dominating element of a pump station’s life cycle costs, especially if the pumps run more than 2,000 hours annually. If the object is to reduce the life cycle cost of a pump station, using AC drives makes sense as their cost is relatively small compared with the total life cycle cost savings. This would also address many of the industry’s problems with maintenance and leak management.
Right pressure Different pipeline systems have different requirements. With an AC drive, it is easy to change the output of the pump system to suit conditions. Settings can be adjusted at www.worldpumps.com
June 2012
Many modern drives have very sophisticated control functions built in. It is, for instance, possible to vary automatically the pressure according to demand. Some drives also have a ‘sleep’ function that enables the pump to 'rest' if there is no demand. If the drive senses low demand during the rest period, and this remains steady, it will be identified as a leak, which means the pump can stay in sleep mode for longer. Before entering sleep mode, the drive can activate a ‘sleep boost’ function, which runs the pump to boost the pressure in the pipeline or water level in the tank prior to shutdown. This extends the pump's sleep time and saves energy. It also avoids unnecessary starting and stopping. AC drives can also prevent leaks from occurring in the first place. Switching pumps on and off causes rapid changes in flow. These flow changes are followed by rapid pressure transients, known as water hammer, which cause stress on the network and damage pipes, pipe supports and valves, resulting in leaks. And when pipework is replaced and leaks repaired, pressure can build elsewhere in the network, causing leaks in new locations. The use of AC drives enables the user to gradually ramp the acceleration at a safe rate to avoid hammering. ABB’s drives for water and wastewater have a ‘pipe fill’ function which provides a pump with a soft-start enabling a smooth build-up of flow in pipes. This avoids pressure peaks, for example in irrigation systems where pipes are momentarily empty and controlled pipe-filling is required. The lifetime of the pipe work and pumping system is thereby increased.
One of the reasons for maintaining high pressure can be to keep the network clear of sediment. This can also be arranged with the help of AC drives, but far more economically and effectively than merely using high system pressure. The sleep boost function mentioned above, for instance, helps to flush the pipelines. When changing pump speed, the change can be programmed to take place rapidly, creating a powerful flush effect in the pipelines and in the pump itself, keeping the system clean. These changes take place within safe limits, to avoid water hammer. Some AC drives also have a pump clean function, providing a pump cleaning cycle by running between maximum and minimum speed settings with aggressive ramps in between. The user defines whether forward and/or reverse operation is allowed, depending on pump design, plus the duration/number of cycles to be used per cleaning cycle. The user sets the VSD triggering condition to automatic when monitoring high levels of, for example, current and torque. The triggering condition is also set for regular cleaning cycles, for example on every pump start or after a set time duration or number of revolutions. Keeping a pump as clean as possible helps ensure the highest hydraulic efficiency for the system.
A better system As many systems only require high pressure for a fraction of the time, the pressure can often be significantly reduced, particularly at night. With AC drives it is easy to maintain an efficient system with low maintenance requirements, and they do not cost any more than a traditional system, as the drives pay for themselves through more energy-efficient operation. The result is a network that works better for all involved – staff, customers, shareholders and the regulator. www.abb.com/drives