Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/1055150
28 | DECEMBER 2018 | WWT | www.wwtonline.co.uk quite effective at reducing the rate of voltage change (du/dt) and the peak motor voltage to within pump manufacturers specifications. Guard against interfer- ence and distortion Other potential problems can include undesirable network distortion, which is possible if the VSD uses a simple diode bridge, especially at higher power ratings. Using low har- monic drives can avoid this. High rates of voltage change can also lead to radio frequency interference, lead- ing to poor electromagnetic compatibility (EMC). Common installation mistakes made by contractors can compromise EMC through failure to provide correct earthing especially where junction boxes are concerned. Motor earth cables should be connected directly to the VSD earth, with no intermediate connections or cross bonding to junction box steelwork, for example. Getting up to speed Although VSDs are generally used to reduce speed, they may also be used to increase speed if required by the ap- plication, if there is enough motor power available. It is therefore necessary to check the limits for both the pump and the motor. It should also be noted that reverse rotation can cause tripping. Unless a non-return valve is installed in the dis- charge, reverse rotation may occur on shut-down. Another priority is to avoid critical speeds and vibra- tion - the risk of the pump or motor reaching a critical speed increases when using a VSD. When running at fixed speed, the chance of the fixed speed coinciding with a criti- cal speed is small. However, when the speed varies across a wide range, this likelihood increases dramatically. This can be remedied by program- ming the drive to lock out certain speeds or speed ranges from the continuous operating speed range. The Knowledge Pumps and drives So ware that can protect hardware Many borehole pumps are used in rural areas and are fed by generators either perma- nently or as a backup. Keep an eye on the harmonic content, as generators might be an additional source of, or be susceptible to, harmonics. Pump bearings are another factor. Pump manufacturers usually state that the pump should achieve a speed of around 30 Hz in less than a second to build up a flow of water to lubricate and cool the bearings. Some VSDs offer so'ware features such as a quick-ramp feature that allows a pump to reach its minimum speed as quickly as possible, extending pump bearing life and reducing unplanned out- ages. When the quick ramp stage ends, a slower ramp can be used ensuring minimal turbid- ity and smooth operation once the pump starts creating flow. Retrofits also a major option VSDs for borehole pumps are best introduced as part of the overall design for the borehole pump system to get the great- est benefit. However, retrofit- ting a VSD later can still bring many benefits and is a major option when considering optimising the performance of boreholes. In fact, the majority of VSDs installed on borehole pumps are retrofit projects. Anglian Water is saving £2,000 a year on a borehole pump's electricity costs follow- ing the installation of an ABB VSD on a submersible pump motor. The borehole is one of two operated by Anglian Water, at separate locations, that feed the reservoir serving customers in Sudbury, Suf- folk. Prior to fitting the VSD at Blackhouse Lane, both pumps were run at fixed speed all of the time but had to be turned off frequently as the reservoir had reached the desired capacity. Anglian Water suspected that this sys- tem was inefficient as it was pumping more water than was required. The frequent stopping and starting of the pumps was also causing ex- cess turbidity in the extracted water. The decision was taken to install a VSD on the pump motor at Blackhouse Lane. The drive alters the speed of the pump's motor to top up the reservoir as required, while the borehole pump at Woodhall Road, without a VSD, runs at full speed to provide most of the demand. This has reduced the amount of energy used by the Blackhouse Lane pump motor by £2,000 per year. It has also reduced the turbidity in the extracted water to an acceptable level. VSD retrofit at pumping station pays for itself in just 12 days In the most extreme cases, turbidity can make water extraction financially unviable. This was the situation facing one water supply company which was considering closing a borehole as a result. The site in question uses two 90 kW extraction pumps which were switched on and off as required. As the pump motors where connected DOL, each start created an excessive amount of turbidity that mean the utility had to wait 14 days for it settle out before it could start supplying water. ABB installed a 90 kW on one of the two pump motors for a trial period. The drive was programmed to have stepped ramp periods to allow for lubrication of pump but reduce the stirring up effect of the silt at the bottom of the aquifer. The result was an immediate 80 percent reduction in turbidity, bringing levels within legal limits, and a 40 percent reduction in energy use. The site now produces 2.7 M/L of potable water each day – providing a payback period of just 12 days from installation of the drive.