Hydropower to the people!
Hydropower generation has been on Northumbrian's wishlist for Mosswood WTW but it had not been a viable option, until now, says Alan Morritt.
Investment in renewable energy production is part of Northumbrian Water’s (NWL) Environmental Strategy in addition to its over-arching drive for efficient operations. The concept of hydropower generation at Mosswood WTW was considered several times previously, but had continually failed to offer a commercially attractive investment opport-unity. Previous proposals had required new building structures to house the turbine, and could not offer as high efficiency turbine on low head high flow conditions.
NWL invited JN Bentley to offer proposals for a hydro-electric installation to the intake of the Mosswood site, using the head derived from Derwent Reservoir. A major requirement was to maintain the operational workings of the site during the development, which resulted in a phased installation process.
This was derived out of a collaborative ‘one team’ planning approach at the design stage to ensure seamless service continuity for NWL and its customers. The £1M design and construct project commenced in March 2011, and was commissioned and operational by March 2012.
A double regulated Z-axial flow turbine, derived from a detailed feasibility study, offered NWL a commercially viable solution for investment. Drivers for the scheme included the potential for savings on energy costs; and reduction in site grid-demand and the application of Feed in Tariff (FiT) payments (reducing the payback period significantly).
The existing inlet pipe work consisted of two mains; north and south, running in parallel from the Derwent Reservoir to the works. A cross connection on site allowed the flow from either main to feed into the first stage of treatment. The flows in each of the existing mains were not balanced, with the greater portion of flow in the south main. At the cross connection, the south main flow joined the north main and this fed into the flash mixer via a butterfly valve which was electrically actuated and responded to a set point from the site Scada system. A connection from the sludge supernatant return pipe work returned top water from the sludge system at that point, to be reused in the process.
The hydroturbine unit was installed into the existing north main inlet pipe work as it enters flash mixer No 1. The turbine connects to a bespoke MCC panel and provides about a third of the power demand for the site.
Modifications to the existing building’s structure were required to allow the turbine unit to be installed in the existing inlet room basement. The reinforced concrete floor slab was removed and replaced with a removable open mesh GMS deck. The control panel was also housed at ground floor level in the same building.
The second inlet main (south main) was modified and a new flow control valve installed in the inlet room basement. A manual isolation valve was installed in this main near to the existing cross connection and the new equipment was connected to the existing site Scada.
The hydroturbine scheme provides Mosswood WTW with about 150kW of its base load electrical demand, through renewable means, using the natural 11.5m head of water into the works from Derwent Reservoir.
Close collaboration between NWL and the project team during invasive works to maintain programme was critical, as system demands varied with weather conditions. Engineering and workmanship was critical in constructing the pipework modifications, incorporating time-critical tasks and minimal space in work areas. The system was designed to pass entire flow to the works through the turbine when in operation.
A number of supply chain partners deliver the hydroelectric scheme including Newmills Engineering (turbine design and construction), and Blackhall Engineering (bypass valve; and framework sub-contractors Armah Switchgear, IDEC Technical Services and Dimewest.
The project team encountered a number of challenges throughout the scheme, which were overcome due to close collaboration throughout the delivery team. These challenges included:
- Maintaining the required throughput to the works during the construction: Detailed planning with client operations and distribution teams, in addition to enabling works undertaken on site allowed the diversion of incoming flow onto clarification stage and ensured that throughput was maintained
- Further work done in the distribution network for alternative supplies: The result of which is now also available for future work and provides additional security of supply by the modification of a link main
- The physical dimensions of equipment to be installed relative to space in the building and close tolerances also provided logistical challenges: This was negated through careful sequencing of work and the skill demonstrated by the installation teams
- Weight of kit in excess of available lifting equipment: External means of uprating the facility needed to be designed and installed
- The control system had to provide automatic “bump-less” transfer of flow from turbine to bypass in the event of a shutdown while maintaining control of flows into the flash mixer: Achieved using a PLC to monitor the turbine and flow control valve conditions, along with inputs from newly installed ultrasonic level instruments and existing site flow-meters.
Health and safety was a high priority from the scheme’s inception due to the fact that confined space working, lifting operations involving large items of equipment in restricted spaces, large excavations and working at height were all present on the scheme, which was conducted in a live operational environment. Control and planning of tasks involving a multi-disciplined team in limited space was a priority, with daily briefings being a valuable tool in highlighting the risks and undertaking the processes safely.
The team demonstrated exemplary health and safety performance, including; 21,290 hours worked, 365 people working on the project on site, 0 reportable injuries, 0 dangerous occurrences, an Accident Frequency Rate (AFR) of 0 and only one minor injury.
Eighty-eight behavioural discussions were carried out by the project management team, which are observations of workplace behaviours followed by discussions intended to challenge unsafe behaviours, to reinforce safe behaviours and provide trend analysis information.
The installed hydroturbine at Mosswood provides the facility with approximately 150kW of its base load electrical demand, through entirely renewable means. Using the natural 11.5m head of water into the works from Derwent Reservoir the turbine is producing more power than stated output under most head/flow conditions.
The project at Mosswood WTW was NWL’s first retro-fit hydroturbine installation into an operational water treatment works.
The scheme was also recognised by NWL’s Water Production manager, Noel Cooper who comments: “From the outset this project was a challenge, as we always knew, we were up against the clock… The attitude, focus and passion shown by all involved has been first class.”
The project was com-missioned within the planned programme and the system produces power to offset site demand and accrue FITS payments, whilst not impacting the primary function of the site – producing high quality potable water for Sunderland, South Shields and Washington.
Alan Morritt is project leader with JN Bentley
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