- The engineering challenge: a replacement suction impeller had to be rough cast using a sand pattern before being machined to the original dimensions
Work at Bray Gravels pumping station secures supply
Reverse engineering key to success at Bray Gravels where matching old with new meant creating a bespoke solution.
- After 30 years the transfer pumps at the pumping station were inefficient, unreliable and, by now, obsolete
- Failure of two remaining motors when one pump was removed for repair posed a very real risk to supply
- The return on investment calculation showed that the most cost-effective solution was to undergo a comprehensive refurbishment, despite the challenges that presented, rather than replace all three pumps
- The logistics challenge: Each pump and motor had to be dismantled and removed to AxFlow’s service base – a process complicated by the fact they were installed before the current pump house was built
- The main pump hall at Bray Gravels after all the pumps had been reinstalled and commissioned.
- The capacity challenge: already worn and in need of repair, the two remaining pumps and motors failed when the third was removed for refurbishment
- The obsolescence challenge: the old motors could not be used with inverters, so replacements were needed. However, the new motors had larger shafts than their non-standard predecessors necessitating the manufacture of three bespoke couplings.
After more than three decades in service, the pumps at South East Water’s Bray Gravels pumping station were reaching the end of their life. However, when an ROI calculation showed refurbishment was a better option than replacement, it led to a major overhaul of the water transfer pumping capability.
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 unreliable and inefficient.
A major contributing factor was the fact that since installation in the early 1980’s, the demands on the pumping station had changed, exposing the pumps to fluctuating operating regimes. Consequently, at different points over the years each of the pumps had been run off the true pump curve, to detrimental effect.
This became apparent when, in late 2011, AxFlow was called out to one of the three pumps, and discovered substantial wear and component fatigue, largely attributable to cavitation. Such was the damage, that the suction impeller was eaten away.
The choices were to purchase a full replacement unit or, if possible, carry out a comprehensive repair and rebuild programme. On analysis, the latter proved to have the best return on investment and so South East Water opted for refurbishment.
The initial programme of works included replacing the pump bearings, motor bearings, mechanical seal, propshaft, universal joints, gaskets, efficiency liner, dismantling the motor and testing the windings, rebalancing the propshaft, shot-blasting, painting and reassembly.
However, replacing the damaged suction impeller was more problematic than simply sourcing a replacement. It meant going back to the drawing board and using reverse engineering to create the part in line with the original specification. A new rough casting was made in bronze using new sand casting patterns. This was then machined to the same dimensions as the original impeller.
AxFlow removed the dismantled pump and motors from site for the work to be carried out, before reversing the process for installation, testing and commissioning - not a straightforward task given the size and weight of the pump and motor. And a process further complicated by the fact the pumps had been installed before much of the current pump house had been constructed.
However, the removal of one pump proved too much for an already struggling system. As a result of the two pumps running at high speeds and temperatures, both the motors burned out on the same day, leaving the station with a reduced capacity to transfer water and a very real risk of disruption to supply.
A temporary replacement had to be found, but this was far from straightforward because the existing motors were fitted with European Standard shafts, making it impossible to get an off- the shelf stop gap.
AxFlow had to quickly locate a suitable motor in another part of the country and make the necessary modifications to allow it to be used on one of the pumps, thus preventing any impact on South East Water customers. However, the emergency prompted South East Water to initiate a programme to fully repair the two remaining pumps, putting the project out to tender.
Sourcing compatible motors for the pumps then became a very real issue. A major difficulty when refurbishing older pumps is matching new to old. The existing motors were so old they were not suitable for running with inverters, because they did not have insulated bearings.
However, because non- standard motors were required AxFlow, who were awarded the contract, had to make special adaption plates to accommodate the motors. In turn, because those new non-standard motors had larger shafts, bespoke couplings had to be fabricated to allow them to be coupled up to the existing propshafts.
The refurbishment of all three pumps, together with the installation of the new motors and recommissioning was completed last summer. 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 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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