Innovative approach taken in Belfast deep-shaft project

Rethinking the site of the drive motor, saved costs in one of Northern Ireland's biggest infrastructure projects, says Frank Cooper, sales director at Bedford Pumps

The installation of six 15-tonne stormwater pumps, some of the largest pumps in Northern Ireland, has recently been undertaken by Bedford Pumps, part of the Hidrostal group, in collaboration with Morgan Farrans JV. The system was part of one of the biggest infrastructure projects in Northern Ireland, the £160M Belfast Sewers project, commissioned by Northern Ireland Water to overhaul the mainly-Victorian sewer system.

The extreme weather conditions Belfast has been experiencing in conjunction with the city's effluent on overburdened sewers has resulted in major issues with flooding and pollution. Pump manufacturer Bedford Pumps made its installation 40m below ground level at the project's Terminal Pumping Station located within the Belfast Duncrue Street Wastewater Treatment Works (WwTW).

The station lies at the end of a 10km network of new storm tunnels that have been constructed over the past three years. The main stormwater tunnel provides the city with protection against a one-in-30-year storm event.

The pumps are fundamental to the whole project to alleviate storm conditions and reduce the risk of flooding in the city. Each has the capacity to deliver 3,500l/s, which means that when all six pumps are running they could empty an Olympic sized swimming pool in less than two minutes.

They are driven by close coupled, 950 kW, 3.3kV, immersible, flameproof motors.

In addition to the six large storm pumps, the station also includes two smaller, 450kW, forward flow to treatment (FFT) pumps, which will be used to discharge the lower, base load flows, to the WwTW. These pumps are also driven by immersible, flameproof motors.

As FFT pumps always had to pass flows to the WwWT, irrespective of the sump water levels the following design criteria had to be met:
  • The ability of the pump to maintain a constant flow of 750l/s over a static head range of 10m to 36m, whilst remaining hydraulically stable and operating efficiently
  • Ensure a minimum self-cleansing velocity greater than 1.8m/s
  • Minimise the possibility of valve-slam by strategic positioning of the non-return valve (NRV)
  • Construction materials to be capable of resisting severe abrasion due to the high levels of grit present in the pumpage
Bedford Pumps used an innovative technique when designing their pumping solution, moving away from the traditional arrangement adopted for deep well stations. The norm for this type of application would be to install the drive motor at ground level thereby ensuring that it is kept dry in the event of a flood.

This type of installation would add cost to both the mechanical and civil installation in addition to the ongoing maintenance requirements on long drive shafting needed to couple the motor to the basement mounted pumps.

Instead, Bedford eliminated the need for drive shafts, bearing and supporting structures by incorporating a motor fitted directly above the pump.

The direct drive motor, manufactured by Bedford Pumps, is floodproof (IP68), flameproof and fully immersible. This was close-coupled to the pump, minimising the overall cost of the installation whilst ensuring optimum reliability. Both the FFT and the storm pump shaft sealing arrangement is of the conventional packed-gland type, which affords the client low capital and maintenance cost.

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