WET News

14 WET NEWS NOVEMBER 2018 | wwtonline.co.uk Correct installation wrings all the benefits from borehole drives W hen it comes to water sources, groundwater in the form of aquifers holds several attractions for wa- ter supply companies compared to surface sources such as rivers. Where aquifers exist, they are usually accessible over large ar- eas simply by sinking a bore- hole where needed. Groundwa- ter also oen requires less treatment than river extraction, for example, as it is filtered nat- urally. Contractors installing bore- hole pumps can offer their water company clients a number of advantages by specifying and installing variable speed drives (VSDs) to run the pumps. One of these is the reduction in turbid- ity, a quality measure of how much particulate matter is sus- pended in the water. Starting pumps with direct-on-line (DOL) or star delta starters tends to stir up silt from the borehole, caus- ing turbidity, whereas using a VSD achieves a gradual ramp up of speed, avoiding turbidity from occurring. Benefits stack up The benefits of using VSDs on • INSIGHT BOREHOLE PUMPING Variable speed drives are helping to make borehole extraction more efficient and reliable – but there are technical challenges that must be considered in order to wring out the greatest benefits, says ABB's Martin Richardson. borehole pumps don't stop at turbidity reduction. By running the motor to match actual demand, energy use can be sig- nificantly reduced – by as much as 50 per cent – compared to DOL driven pumps, which would always run at full speed. Periods where the system is 'pumping to waste' can also be dramatically reduced, saving even more energy. Another advantage is the ability to vary output to comply with the terms of an extraction licence. Consider, for example, a licence that specifies an extrac- tion rate of seven mega litres of water a day. A water company might have one pump that could extract five mega litres a day. Adding another pump of the same capacity would exceed the limit. By using a VSD, the speed of the second pump could be varied to ensure the pumps' combined output did not exceed the licence limit. Identifying challenges Depending on the extraction site, the pump motor may be located within the borehole pump itself, or at the head of the borehole connected to the pump by a sha. Whichever method is used, there are technical chal- lenges that must be addressed to ensure efficient and reliable extraction, as well as to reduce equipment wear and tear thereby prolonging the life of both the pump and motor. Submersible and borehole pump motors present specific challenges because of their gen- erally weaker insulation and typically long cable lengths. However, selection of the cor- rect VSD, as well as possibly an output choke or sine filter, care- ful EMC planning and good installation practices can avoid these problems. Variable speed operation puts a higher voltage stress on the motor insulation system than a DOL supply and it is rec- ommended to have either a rein- forced insulation system or a fil- ter between the motor and the drive. For instance, motors in bore- hole pumps may have a cable longer than 100 metres making it necessary to fit du/dt or sine filters. These are 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, leading to poor electromagnetic compatibility (EMC). Common installation mis- takes made by contractors can compromise EMC through fail- ure 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 application, if there is enough motor power available. It is therefore neces- sary to check the limits for both the pump and the motor. Contractors should also be aware that reverse rotation can cause tripping. Unless a non-re- turn valve is installed in the dis- charge, reverse rotation may occur on shut-down. Another priority is to avoid critical speeds and vibration – 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 critical speed is small. However, when the speed varies across a wide range, this likelihood increases dramatically. This can be reme- died by programming the drive to lock out certain speeds or speed ranges from the continu- ous operating speed range. Software protects hardware Many borehole pumps are used in rural areas and are fed by generators either permanently or as a back-up. Keep an eye on

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