- Pipework length – 175 metres
- Distance to STW – 500 metres
- Over-pumps – Flygt NT 3312’s 180kw Ex. Proof
- DWF pumps – 4 no. Flygt NT3231, 105kw explosion proof submersible pump
- SWF pumps – 3no. Flygt NT3312, 140kW explosion proof submersible pump
- Combined total operating head on temporary line – 38 metres
- Static head – 28 metres
- Hours run approx. - 2500
A planned and timed project for main contractor Black & Veatch, Pump Supplies lent its skills and expertise to design a one-off flexible system within a confined space, thus reducing the overall capital expenditure of the scheme.
The market town of Bishop’s Stortford has a population of about 35,000 however the busy M11 motorway, expected enlargement of the nearby Stansted Airport, and the train links to London and Cambridge, show that the population is likely to increase in future to 45,000.
As a result, Bishop’s Stortford STW with a population equivalent of 52,000 is now at full capacity, leading to the requirement from Thames Water to extend and refurbish the current works to a 2016 design horizon of 72,000, 483l/s total works flow.
The sewage treatment works is fed by two main pumping stations – Hallingbury and Stansted Airport, as well as a number of minor pump stations. It is a traditional works operating a bio-diversity treatment process.
Within the scope of Black & Veatch’s (B&V) remit was the need to overhaul Hallingbury Road pumping station. It comprises four submersible pumps for dry weather flows (DWF) acting on a duty, assist and standby basis handing flows forward to Bishop’s Stortford STW for treatment, and two shaft driven pumps for the storm weather flows (SWF), pumping to three 1000m3 capacity storm tanks at Hallingbury PS.
The pumping station is unusually situated in a low level area requiring the pumps to lift the matter some 28m of static head and 500 metres to the STW. It therefore requires large pumping units to lift the high heads of water.
The existing C type impeller pumps were blocking, failing and cavitating on a weekly basis, mainly due to the large amount of rags, fibrous waste and debris the pumps had to deal with, requiring the site to be attended regularly for maintenance by the Thames Water team.
A temporary over-pumping scheme was needed to allow the £1 million refurbishment work at Hallingbury Road to take place, whilst ensuring undisrupted operation of the pumping station and minimising down time. This involved a clean out of the DWF and SWF wet wells, installation of new Flygt N-Pumps and new MCC.
In assessing the job, and due to the compact nature of the site and enclosed working space, Pump Supplies proposed in its design to use the temporary pumps firstly as temporary replacements for the storm pumps, whilst the existing units were replaced and new ones tested and commissioned, and then to use the same pumps with some control adjustments for the second phase of the job in the dry weather flow section where the work was of a more critical nature.
Commenting, Pump Supplies’ contract manager, Rob Bessant, said: “The criterion was to cover the 400 l/s storm weather flow to the storm tanks and between 80 l/s and 381 l/s for the DWF. By installing two Flygt NT 3312 180kw explosion proof pumps controlled by variable frequency drives, we could set a maximum speed for the storm pumps, and then once that element was complete, reprogramme the drives for the DWF aspect. In this way we reduced the amount of pumps which were required and the labour cost of the job as a whole.
“Because space was at a premium we simply would not have had enough room for four temporary pumps and allow room for B&V’s contractors to get the existing pumps out.
“The configuration of 175m of pipework connected to the temporary pumps, together with digital flow meters, ultrasonic level controls, gate valves and 2 no. 200kw variable frequency drives, allowed us to send the correct flow to the correct destination, depending upon what part of the site was being worked on. The flow meters also enabled us to control the amount of flow through the works so it was continual throughout the day rather than a series of purges, which helps the system work better altogether.”
Flygt N-Pumps are the market’s most efficient submersibles and offer sustained efficiency because fibrous material cannot build up on the impeller through its self-cleaning design. The pumps were installed vertically within the dry well design of the pumping station itself.
The SWF over-pumps were in place for an eight-week period whilst the in-situ pumps were replaced, tested and commissioned. The valves in the pipework were then swapped over to cover the DWF sent to the works. The second phase was critical and Pump Supplies’ duty pump ran 24 hours a day for 105 days trouble-free - some 2,500 operating hours - with no failures, blockages or problems.
Commenting Simon Blackman, site agent for Black & Veatch, said: “The final temporary works scheme took several months to plan; we were liaising with Thames Water operations closely to ensure that the process was not put at risk of failing. The pumping station drywell is 13m deep so the working at height issues posed a high risk for the installation activities.
“Pump Supplies worked with us to develop the final overpumping scheme and designed the temporary works. Due to space limitations we were restricted to 1no duty and 1no standby pump only which formed part of its proposal.
“The existing installation was blocking on a weekly basis regularly requiring night time call outs. Once the temporary pumps were in place we had no night time call outs and forward flows to both the storm tanks and main works were maintained at all times during the construction period.
“The temporary pumps also provided us with an early indication of how the permanent installation was going to perform as they were from the same range. Six months on from final completion and nearly a year from initial commissioning the new pumps have not yet blocked or failed.”
Pump Supplies further assisted B&V with the supply of four Flygt FS 3152 chopper pumps to handle the heavy build up of rags and fibrous waste in the DWF wet well. This was to allow new valves to be installed on the existing suction pipes to feed each new Flygt NT3231,105 kw submersible pump – whilst still maintaining forward flows to the works.
As the leading distributor of Flygt drainage, sewage and sludge pumps, Flygt and Pump Supplies worked in tandem to provide a robust and reliable solution for the project.