Keeping storm weather flows under control

When Thames Water was overhauling its Hallingbury Road pumping station, 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

Pump Supplies has engineered an over-pumping solution to help with overhaul of Hallingbury Road pumping station, as part of Thames Water’s £16M refurbishment and extension of Bishop’s Stortford sewage treatment works (STW) in Hertfordshire.

A planned and timed project for main contractor Black & Veatch (B&V), 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.

With a population equivalent of 52,000, and the town itself predicted to increase in future, Bishop’s Stortford STW is now at full capacity. This led to Thames Water opting to extend and refurbish the current works to a 2016 design horizon of 72,000, 483l/s total works flow.

The sewage treatment works, a traditional plant operating a bio-diversity treatment process, is fed by two main pumping stations – Hallingbury and Stansted Airport, as well as a number of minor pump stations.

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 1,000m3 capacity storm tanks at Hallingbury pumping station.

Large units

The pumping station is unusually situated in a low level area requiring the pumps to lift the matter some 28m of static head and 500m to the STW. Therefore, large pumping units are required to lift the high heads of water.

The existing C-type impeller pumps were blocking, failing and cavitating weekly, mainly due to the large amount of rags, fibrous waste and debris the pumps had to deal with. This meant the site had to be attended regularly for maintenance by the Thames Water team.

A temporary over-pumping scheme was needed to allow the £1M refurbishment work at Hallingbury Road to take place, whilst ensuring undisrupted operation of the pumping station and minimising downtime. 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.

Pump Supplies’ contract manager, Rob Bessant, says: “The criterion was to cover the 400l/s storm weather flow to the storm tanks and between 80l/s and 381l/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, re-programme 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.”

He continues: “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.”

Self-cleaning design

Flygt N-Pumps are among the market’s most efficient submersibles, says Pump Supplies, 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.

Simon Blackman, site agent for Black & Veatch, says: “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 over-pumping 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.”

Blackman says: “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 assisted B&V further 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, 105kW submersible pump, while maintaining forward flows to the works.

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