Working on behalf of the Ouse & Derwent Internal Drainage Board, pump supplier AxFlow has installed two new submersible Bedford pumps at the Stillingfleet Surface Water Pumping Station. The two pumps removed from service are also being overhauled.
The purpose of the pumping station is to lift surface water which can no longer drain by gravity into the nearby Beck because mining subsidence has lowered the surface ground level. The pumping station was constructed by the British Coal Corporation in the early 1990s as part of its mining remediation plans.
The need for new pumps was identified when high moisture readings were recorded within the pump linings containing the electrical equipment. “This was causing an operational concern,” reported Mark Redgrove, AxFlow’s Technical Support Manager. “The pumping station is equipped with two large 800mm rising main storm and two 600mm rising main dry weather pumps. To fix the problem the pumps needed removing. However the pumping station required its pumping capacity to be sustained. The solution was to purchase two new submersible Bedford pumps which could replace the existing ones when they were removed. We were called to remove the old Bedford SB pumps and install the new pumps.”
Scaffolding plan scuppered
“Logistically, at first it appeared to be a straightforward job to remove and replace the pumps using scaffolding. The scaffolding is usually erected inside the wet well with a crane being used for removing and replacing the pumps to be changed. However, installing scaffolding in the pump wet well was not practical. This was because of the limited space along with the [fact that the] pumping station wet well was not totally sealed from the flows. The rising water levels required one pump running on and off to keep the level down,” says Redgrove.
In view of this, the potential practical options and health and safety implications of a confined space were considered as to how safely the work could be carried out. The option to be progressed required the utilisation of a man-riding basket suspended in the wet well from a crane because of the constrained area and conditions. This made it necessary to have two cranes on site: one for lifting the pumps and a second crane for operating the man-riding basket. According to AxFlow, very careful planning and control was needed to get both the man-riding basket in place to position the pump lifting tackle and to carry out the pump removal and installation works with the two crane jibs working in such a small opening.
Pump fall risk
“Our plan was to remove the old flange bolts and pumps and reconnect the new pumps to the existing pipework with new bolts,” said Redgrove. “The larger pump was standing on three feet on a concrete platform in the wet well and with the removal of some of the bolts the weight of the pump (4 tonnes) tended to pull it away from the flange on which it was fitted, putting the pump at risk of falling into the wet well. This was addressed by leaving some bolts in place until it was actually ready to lift.”
The job of removing the old pumps and installing the new pumps took AxFlow three days to complete. Further to this work AxFlow were also asked to take the old pumps to their Huddersfield Service Base for inspection. Redgrove said: “We now have the pumps at our base and having stripped them down we have found that they were in pretty good condition, even after 25 years of service. However, some oil and water has got into the motor windings, this appearing to be caused by perishing cable glands which can be relatively easily rectified. We further looked at replacing the usual components that can be subjected to wear, including bearings and mechanical seals and washed out the stator windings and reassembled the pumps.
The impellers were in good condition as they clearly had a relatively easy life, even though they are in a high profile pumping station and over the years had been subjected to high levels of continuous operation.”
The two original pumps still operating at the Stillingfleet Pumping Station will be removed in the future to be replaced with the two overhauled pumps at AxFlow’s Huddersfield workshops to finalise the current work on the pumping station.
