Water & Wastewater Treatment

14 WWT PUMP AND VALVE SUPPLEMENT 2019 Opinion: pump reliability down and replacing damaged compo- nents, re-assembling, pressure testing, transportation back to site and, nally, the re-installation and commissioning of the repaired unit. Suddenly the cost of a mechanical seal becomes just the tip of the iceberg. It's surely obvious that the most critical component within a submersible pump is therefore the device that prevents water ingress into the electric motor - the mechanical seal. Increase the life expectancy and performance of this component and the life expectancy of the entire machine will be improved. Improving seal performance A number of factors work against e• cient sealing. In a submersible pump, two seals are used in tandem, with the chamber between the two seals lled with an oil 'bu• er • uid', which forms a liquid barrier between the process • uid and the electric motor. Progressively, but inevitably with traditional seals, the dirty process • uid migrates across both seal faces, eventually working its way into the electric motor and bearing chamber, causing failure. Another familiar feature of pump breakdown, particularly in sewage applications, is seal 'wragging' on single coil spring seals. Put simply, as the seal spins, solids get caught up in the spring, wrap around it and gradually build up. Where rubber is used as the sealing material, the abrasive/corrosive solids cause them to perish, resulting in leakage and eventual seal, and pump, failure. In worst case scenarios the build-up pushes the seal o• its drive mechanism, or lug, causing complete loss of seal drive. Submersible seals designed to work in reduced • uid lm conditions help to reduce leakage, however they signi - cantly increase wear rates. While it is recognised best practice to liƒ pumps and replenish the oil chamber with uncontaminated oil every 12 months, much as you would replace the oil in your car, the pressures of sta• ng and cost mean this is widely ignored. So what options are open to the wastewater company wanting to extend the life of the mechanical seals and ensure pump reliability and longevity? Firstly, select seals made from the right materials. We have carried out tests over 400+ hours comparing OEM seals typically used in submersible pumps (rubber/plastic housing and drives and ceramic-on-ceramic seal faces) with advanced component seals (all-metallic materials employing solid tungsten carbide against silicon carbide faces). Contin- ued leakage began within as little as two days on the OEM seal and seal faces showed extreme wear averaging 0.7mm and loss of surface nish, while the advanced component seal remained leak-free throughout testing and exhibited minimum wear. Multiple-spring sealing devices are also readily available which o• er a signi - cantly more controlled spring rate, improving seal face • uid lm conditions in marginal lubrication regimes and preventing wragging. Preventing contamination Everyone knows a regular oil-change keeps their car engine running more e• ciently and extends its lifespan. The same is true of submersible pumps. The logistical problems presented by submersible pumps can be overcome by connecting the oil chamber through • exible hosing to a surface mounted reservoir with a self-contained circulat- ing pump. This involves signi cantly increasing the oil bu• er • uid volume from a couple of litres, to 25+ litres. However, with an oil lter and water separator tted in the circuit, the circulated oil in the seal chamber is constantly recycled, remaining cooled and uncontaminated – making any migration a moot issue and creating the ideal conditions for optimum seal performance and extended pump life. Although oil changes would be possible on the surface, they are highly unlikely to be needed. Cost evaluation The costs of investment in modern technologies to achieve improved pump life are not prohibitive. Take the example of one UK water company, which had experienced repeated water ingress problems with a submersible pump, requiring removal for rebuild at least every six months. Following the installation of an oil circulating reservoir system and upgraded mechanical seals the pump performed faultlessly for 18 months and the company achieved payback on its investment within a year. To calculate this at a macro level, there are around 1,200 submersible pumps per one million human popula- tion served. With an average installed life of three years, 400 pumps would require replacement or repair at a total average cost of £1,500 per unit. If pump installed life was only doubled, this would reduce the repair bill by 50 per cent - meaning a large water company with several thousand submersible pumps could be looking at savings in excess of £1,000,000. The reality is that even a doubling of pump life would not bring the wastewa- ter industry in line with modern industry standards. But the huge cost impact of investment in modern technology should be incentive enough to make companies consider making a foray into the 21st century. Opinion: pump reliability advanced component seals (all-metallic materials employing solid tungsten carbide against silicon carbide faces). Contin- ued leakage began within as little as two days on the OEM seal and seal faces showed extreme wear averaging 0.7mm and loss of surface nish, while the advanced component seal remained leak-free throughout testing and exhibited minimum wear. Multiple-spring sealing devices are also readily available which o• er a signi - cantly more controlled spring rate, improving seal face • uid lm conditions in marginal lubrication regimes and preventing wragging. optimum seal performance and extended

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