Bristol pumps it up

A critical pumping station for the Bristol area required new storm pumps and a screening chamber. Trant Construction¡¦s M&E project manager, Simon Whaley, explains the installation

Ashton Avenue Sewage Pumping Station (SPS) is one of Wessex Water's critical pumping stations.
It is contained within a steel palisade fence with its own access road off Clift House Road, Cumberland Basin, with the highway to the south of the station, and, to the west, Bristol City Central Park.

The east side is bounded by a warehouse and garden centre, and on the north side is the River Avon. Ashton Avenue SPS was originally commissioned in July 1970. The incoming gravity sewer to the SPS is a 3.6m by 3.6m box culvert and takes the majority of the flow from southern Bristol - the Malago Drainage Area Plan catchment.

Existing works
Prior to project commencement the existing works consisted of the following:
  • Free dry weather flow (DWF) pumps, which lifted the foul sewage up into the Southern Foul Water Interceptor (SFWI) were replaced in 1994 and the fourth pump was refurbished. These were three dry-well submersible pumps, and a centrifugal-flow pump with the motor located at ground level. The fifth pump was abandoned.
  • The theoretical pumped forward flow to treatment (FFT), delivered by the DWF pumps, was 2,150l/s including two 65kW pumps at 650l/s plus one 70kW pump at 850l/s; duty/ assist/assist, with another 850l/s pump on standby. This flow was pumped up in to the SFWI box culvert (2.51m wide x 2.13m high) for onward flow to Avonmouth Sewage Treatment Works for treatment.
  • There were three storm pumps, wet-well axial flow pumps, shaft-driven with 200kW motors located at ground level. These pumps were each rated at 1,700l/s and normally operated duty/assist/standby. These pumps were original from 1970.
  • There were three abandoned comminutors upstream of the DWF pumps, thus there was no screening whatsoever of any flows at this station.
A revised discharge consent was issued by the Environment Agency (EA) for compliance in December 2011. One of the conditions stated: "Storm sewage, and sewage discharged in an emergency, for all flows up to a one-in-five year return period shall pass through 6mm, 2D, autocleaning screens. Screenings are to be removed from this storm discharge."

Refurbishment
The pumping station (PS) refurbishment required each of the existing (1994) DWF pumps to be removed, cleaned, motors checked, rewound if necessary and new impellers installed.

This work was phased, with one pump being removed at a time, involving close interface with Trant, Wessex Engineering & Construction Services (WECS) and Wessex Water's commissioning and operations team.

Two new submersible volute pumps from Bedford Pumps, with associated pipework and valves, were also installed by Trant, to provide greater security and capacity to the pumping station. The existing storm pumps were isolated and removed. Removal of the old storm pumps, together with installation of the new, required careful programming to ensure that the pumping station remained fully operational during a storm event. The new installation consists of three new Bedford storm pumps,operating as duty/assist/assist.

These are mixed-flow ¡¥canister¡¦ pumps with submerged motors and are supplied with pump condition monitors and a ¡¥latch lift¡¦ lifting system to ensure speed, efficiency and cost savings when removing or installing the pumps.

Each pump is rated at 2,500l/s at 8m static head (approximately 10.6m total design head), complete with variable speed drives.

The storm sump has been hydraulically modelled to ensure optimum flow paths are achieved to the pumps. Modifications, including the introduction of baffle walls, benching and stainless steel deflectors to the storm sump have been completed.

Screenings station
The screening chamber was constructed from reinforced concrete with dimensions of 13.2m x 13.2m and wall thicknesses of 500mm.

Interlocking PU20 Larsson piles were used as part of the temporary works and then part of the permanent works.

During the construction phase, using interlocking piles mitigated any issue with ingress of groundwater. The interlocking piles were driven to a depth of 18m to give a toe into the mud rock strata of 2m. CFA piles 350mm diameter were bored into the ground to a depth of 20m to support the new screening chamber.

The chamber was ultimately constructed in two sections, a wet well and a dry well. The wet well contains four new Huber RoK 1/700/6 screens capable of operating at flows of 2000l/s each, together with all associated control and isolation facilities.

The new chamber comes into action during a storm event, all flows are pushed through the automatic mechanically cleaned screens. The flows then pass down the newly constructed pre-cast box culverts which discharges into the River Avon adjacent to the works.

Wet well
The dry well area consists of screenings return pumps and associated pipework and valves for returning all screenings back to the SFWI flow which ultimately discharges to Avonmouth STW. With the difficulties associated with maintaining a live operational works at all times, commissioning routes were designed in at an early stage.

These successfully allowed the works to remain fully operational whilst commissioning of the new storm pumps and screening chamber was completed.

Motor control
A new motor control centre (MCC) from GPS, with a new dual supply system, has been installed to provide security of supply to the works, under the management of Trant. It is fed from two new 2MVA transformers to either side of a normally open bus coupler ¡V which, in the event of a failure of either supply, will close and thus feed the whole MCC.

There are three variable speed drives for the storm pumps, each rated at 450kW, together with all the drives and controls associated with the DWF pumps, screening chamber equipment and building services. There is a dedicated ICA panel, housing a Siemens 319 CPU and touch-screen; all software programming was completed by WECS's automation and commissioning team in conjunction with Trant.

Design
The generation of a 3D model using Trant's in-house capability allowed for a simplified design for access walkways that were not initially obvious on the original layout drawings and thus generated a significant cost saving. The MCC was relocated from the initial design in a new building to a relocated position in the pump station next to the existing MCC.

This brought big cost savings to the project, but required much tighter management and interface with Wessex Water's operational team to maintain operability of the pumping station during the phased changeovers.

Trant oversaw the production of operation and maintenance manuals. These are available online and on CD to the operational and maintenance team. The project sees Ashton Avenue SPS compliant with a revised discharge consent from the EA. The project is due to be completed in August, one month ahead of programme and under original budget.

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