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When a European pumping station supplying fresh water to a capital city required a refurbishment, one pump needed reverse engineering to bring it back to the original specification. The decision was taken in the knowledge that AxFlow had the necessary engineering resources to return the pumps to full long term operating condition and that this also provided the best return on investment.
UK’s South East Water’s Bray Gravels pumping station has an essential role to play in supplying fresh water to around 300,000 homes immediately to the west of London. Built more than 30 years ago, the pumping station draws water from boreholes close to the River Thames at Maidenhead and pumps it to the Surrey Hills and Crowthorne Reservoirs some 10 miles away. However, although continuing to supply water to these reservoirs, by 2011 the three Weir two stage high head split volute water transfer pumps, which by now were obsolete, had become less than efficient and unreliable.
South East Water’s programme of investment for capital maintenance identified the need to reduce possible future failure either by replacement or refurbishment. To assist in making the choice resources of AxFlow Windsor were requested.
Fluctuating operating regimes
In the period between the three pumps first being installed in the early 1980s and 2011, the demands on the pumping station had changed, exposing the pumps to fluctuating operating regimes. Consequently, over the years each of the pumps had at times been run off the true pump curve to detrimental effect. In late 2011, AxFlow Windsor to a look at one particular troublesome pump and once removed and opened up, examination revealed substantial wear and component fatigue largely attributable to cavitation.
Specifically, the suction impeller was eaten away and required reverse engineering to bring it back to the original specification. The reverse engineering included the making of new sand casting patterns and the casting of a new impeller from bronze. This new rough casting was then to be machined to the same dimensions as the old impeller. Faced with the option of purchasing a replacement unit or carrying out a comprehensive repair and rebuild programme, South East Water went for the refurbishment option.
The decision was taken in the knowledge that AxFlow had the necessary engineering resources to return the pumps to full long term operating condition and that this also provided the best return on investment. The initial programme of works included replacing the pump bearings, motor bearings, mechanical seal, propshaft, universal joints, gaskets, efficiency liner, together with reverse engineering the suction impeller, dismantling the motor and testing the windings, rebalancing the propshaft, shot-blasting, painting and reassembly.
Coupled with this were the logistics of dismantling the motor and pumps and removing them from site and then reversing the process for installation, testing and commissioning. Given the size and weight of the pump and motor, this was not exactly a straightforward task and further complicated by the fact the pumps had been installed before much of the current pump house had been constructed. This was not the end of the enterprise, for once the pump had been stripped down and analysed back at AxFlow’s premises, other elements were identified that required attention.
During the period when the pump was at AxFlow’s service base, the two remaining pumps and motors failed, leaving the station with a reduced capacity to transfer water. As a result of the two pumps running at high speeds and temperatures, both the motors burned out on the same day. AxFlow alleviated the problem by locating a temporary motor that enabled Bray Gravels to maintain operations.
But even this was not straightforward because the existing motors which had failed were fitted with European Standard shafts, making it impossible to get a motor-off- the shelf. However, AxFlow were able to locate a suitable motor in another part of the country and make modifications that would allow it to be used on one of the pumps; this response meant that the station was up and running within days without there being any impact on the supply to South East Water’s customers.
Following this emergency, South East Water embarked on a programme of repairing the two remaining pumps and after a due tendering process, the contract was awarded to AxFlow. Work commenced on the refurbishment of these pumps in 2012.
Matching up new to old
A major difficulty when refurbishing older pumps is matching up new to old. The existing motors were so old they were not suitable for running with inverters, because they did not have insulated bearings. Therefore, new motors were required for all three refurbished pumps. However, because non- standard motors were required AxFlow had to make a special adaption plates to accommodate the motors. Furthermore, the new motors had larger shafts making it necessary to make three new couplings to fit the new motors in order that they could be coupled up to the propshafts.
With all three refurbished pumps now installed and commissioned, Bray Gravels has the capacity to pump up to 360 megalitres per day at a head up 11.5bar, using just two pumps. By sharing the load at peak demand, the pumps can operate within their curves and can be run at lower speeds, thereby removing the potential for cavitation.
The refurbishment of all three pumps, together with the installation of the new motors and recommisioning was completed this summer. The project grew from sorting out problems on one particular pump in to a major overhaul of the water transfer pumping capability. The pumps and motors have been given a new lease of life and South East Water can look to many more years of service from Bray Gravels.
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