Climate change, the water cycle and variable speed drives- how using variable speed drives can contribute to future water security
Climate change, increasing demands on water due to global urbanisation and the legal obligations under the EU's Water Framework Directive are making the water sector an attractive place for technologies that can help reduce costs and contribute to reducing the effects of climate change. Steve Ruddell of ABB discusses the influential role variable speed drives can play in meeting these challenges.
Yet water is becoming scarcer globally and every indication is that it will become even more so in the future. Decreasing availability, declining quality, and growing demand for water are creating significant challenges to businesses and investors who have traditionally taken clean, reliable and inexpensive water for granted.
Climate change and water management
It is only during the last five years that the likely effect of weather patterns on the water cycle has become apparent. And already it is affecting the way water is managed. The more that the water cycle is managed, the more energy intensive it becomes. The more that water is pumped, the more energy is used. The more sewage removed, the more methane is generated.
As such, some 4 to 6% of the world’s greenhouse gases come from the water industry. These are figures that urgently need to be addressed through energy efficient pumping; low energy desalination and treatment; and sludge to energy and methane capture.
Water Framework Directive
The potential driver to make things happen is the European Union’s Water Framework Directive (WFD). It is the most ambitious and far reaching European legislation, covering all aspects of water, whether surface, groundworks or marine. Although it was developed long before the impact of climate change on the water cycle was fully understood, such is its far reaching demands that it is likely to be the stimulus for additional capital spending on new technologies on a broad scale.
So armed with a serious need to meet the WFD, contribute to combating the effects of climate change and reduce the energy consumption throughout the water cycle, utilities are turning their attention to ways of controlling their pump demands, which is leading them to discover the benefits of variable speed drives (VSDs). According to recent European Union estimates, pumps use more energy than any other type of industrial equipment, consuming some 160 TWh of electric power and accounting for 79 million metric tons of CO2 emissions a year.
The power of the variable speed drive
Do not underestimate the power of VSDs. Today they are more than just devices used to vary motor speed. VSDs allow more precise control of processes such as water distribution, aeration and chemical feed. Pressure in water distribution systems can be maintained to closer tolerances. Wastewater treatment plants can consistently maintain desired dissolved oxygen concentrations over a wide range of flow and biological loading conditions by using automated controls to link dissolved oxygen sensors to VSDs on the aeration blowers.
Not to mention the benefits of using VSDs on the many different pump applications used throughout the water and wastewater industry: dosing, mains distribution, pressure boosting, raw water, inflow and sludge as well as mixers, compressors, centrifuges, screw conveyors and fans. In fact, VSDs are used throughout the entire water and wastewater treatment process – from the moment the raw water enters the inflow pumping station to the time the effluent is discharged after final purification in the wastewater plant.
Replacing mechanical devices
Traditionally for applications where flow requirements vary, mechanical devices such as flow-restricting valves or moveable air vanes are often used to control flow. This is akin to driving a car at full throttle while using the brake to control speed. This process uses excessive energy and may create punishing conditions for the mechanical equipment involved. Rarely do the pumps and motors need to operate at constant speed or all the time. But it is surprising how many of them run at unnecessarily high speeds or with over-dimensioned motors, wasting vast amounts of electric power and consuming valuable carbon credits.
Lowering energy consumption
VSDs enable pumps to accommodate fluctuating demand, running pumps at lower speeds and drawing less energy while still meeting pumping needs. By equipping each pump motor with a VSD, the speed and torque of the pump system are controlled with precision and energy consumption is kept to the minimum.
Energy savings from VSDs can be significant. Affinity laws for centrifugal pumps suggest that even a small reduction in motor speed will provide as much as 50 per cent in energy savings. A VSD controlling a pump motor that usually runs less than full speed can substantially reduce energy consumption over a motor running at constant speed for the same period.
For an 18 kW motor running 23 hours per day (2 hours at 100% speed; 8 hours at 75%; 8 hours at 67%; and 5 hours at 50%) a VSD can reduce energy use by 45%. At £0.09 per kilowatt hour, this saves over £4,560 annually. Because this benefit varies depending on system variables such as pump size, load profile, amount of static head and friction, it is important to calculate benefits for each application before specifying a VSD.
Many water utilities are saving energy using VSDs. At least £80,000 in electricity costs over a 20-year life span is being saved using ABB drives at West of Scotland Water’s waste water pumping station. The wet well is monitored with ultrasonic level instrumentation, generating a speed reference signal, which is fed to an ABB industrial drive. This increases or decreases the pump speed to give optimum energy efficiency, replacing simple on/off control of the motors. Indications are that the electricity consumption has been reduced by 48 per cent compared to the same period in the previous year. West of Scotland Water estimates that 44 per cent of the savings can be put down to maintenance and 56 per cent to the drives.
Reducing maintenance costs
But it is not just in energy saving where VSDs offer benefits. Manufacturers have been working closely with water utilities and original equipment manufacturers (OEMs) to explore ways in which drives can help improve every stage of the water and wastewater cycle. While energy saving remains the number one reason for installing a drive, it is in the operation and maintenance of pumps where drives can also make a big impression.
Eliminating water hammer and preventing leakage
VSDs reduce mechanical stress on pipes, pumps, valves and other key equipment, and diminish the likelihood of water hammer by enabling the flow rate to be increased gradually and safely. Water hammer occurs when the flow of water in a pipe is stopped suddenly, causing a shock wave to ripple through the water and impact on the structure of the pipes, leading to damage. Over time, this can result in burst pipes. Residual water in the pipeline can also be propelled at high pressure causing water hammer and subsequent risk of damage to pipelines.
Using a VSD allows demand to be smoothed out, reducing the sudden stops and starts that lead to water hammer. Single-speed drives start motors abruptly, subjecting the motor to high torque and current surges up to 10 times the full-load current. In contrast, VSDs offer a "soft start" capability, gradually ramping up a motor to operating speed. This lessens mechanical and electrical stress on the motor system and can reduce maintenance and repair costs and extend motor life.
Stress on the electrical system is mitigated by starting the pump motor softly and with considerably lower peak current than direct-on-line starting. And drives lower the risk of cavitation by monitoring the pressure of the incoming pipeline.
The benefit of soft-start was experienced at Howden Water Treatment Works in Scotland. Consisting of a borehole pumping station as well as the treatment works itself, the site was facing water hammer problems. VSDs were installed to control it.
The soft start capabilities of the drives are also critical to the application, helping the facility meet the electricity utility’s regulations on motor starts. To reduce the current at start-up, these must not exceed 68 A at 11,000 V and each motor must be started separately, with a two second minimum time between starts.
Avoiding pump impellor ragging
Another issue affecting maintenance teams is the jamming or ragging of pump impellors. This occurs when debris within the waste water being taken into the treatment works clogs the pumps. Normally this is an expensive repair, whereby the pump is lifted from its position using a crane, allowing a maintenance team to manually clean the rags from the impellors. The pump is reassembled and lowered back in-situ.
To overcome this, ABB developed an anti-jam routine, now built within one of its drive types. This enables the drive to perform preventive maintenance on the pump. When triggered, the pump runs at high speed through a number of user-defined cleaning cycles to prevent the build-up of particles.
Severn Trent Water (STW) in the UK is one of the early adopters of this technology, saving up to £2,400 a week on pump maintenance. The company installed four submersible foul pumps at its Worcester Water Treatment Works, pumping raw sewage from a new foul well.
STW experienced a number of blockages in the foul pumps, caused by rags sticking to the impeller. ABB suggested the installation of its anti-jam software. Taking minutes to complete, the cleaning cycle removes the debris from around the pump volute, preventing it from entering the pump and blocking it when the pump ramps up from zero to its nominal operating speed.
Following installation, STW has not experienced a single blockage of the foul pumps.
Reducing harmonic distortion
Because of the switching technology used by drives, they can sometimes produce harmonic distortion, adversely affecting power quality, and subsequently, other electrical machinery. However, manufacturers have developed many solutions to correct this problem.
ABB, for example, has developed a range of low harmonic drives that produce exceptionally low harmonic content in the drive input. It achieves this by way of an active rectifier, along with the motor control method, Direct Torque Control (DTC) and the use of a low pass filter, giving a total current distortion of less than 5.0 per cent. The electrical system is also more reliable, due to avoiding external harmonic filters. Expensive multi-winding transformers are also unnecessary.
Intelligent pump control
Because of the many and varied challenges the water industry faces within pump control, VSD manufacturers are developing drives that are packed with all the functions that can single handily meet these challenges. The idea of application-specific drives is something which research organisation, IMS, has identified as a growing trend. With such drives the user can reduce the total cost through shorter start-up times, lower integration costs and improve machine productivity.
ABB, for instance, has developed a version of its ABB industrial drive that is loaded with intelligent functions that meet the demanding needs of squared torque pump control, for single and multi-pump systems. All these pumps have specific requirements from level control through to multi-pump and priority pump control, and all of which can be solved using the latest software now available. The drives’ pump-specific functions decrease the lifecycle cost of the pumping systems, helping to save time and money, boost energy efficiency and reduce carbon dioxide emissions.
Nearly half of all readily available renewable supplies of water have already been abstracted for human use. A report by the Ceres/Pacific Institute concludes that climate change will exacerbate these water risks, especially as the world population grows by 50 million a year. The more that the water cycle is managed; the more energy intensive pumping is utilised. So the role for energy-saving technologies like VSDs appears secure.