Generating power in the water treatment process16 June 2000
Steve Cryer from the UK’s South West Hydro Group explains how hydro power can be harnessed successfully within the infrastructure of the water industry
The Crownhill water treatment works in Devon, UK, has been supplying water for the city of Plymouth since the last century. The main source of this water is an impounding reservoir at Burrator, situated in the Dartmoor National Park. In 1957
a Francis turbine and generator was installed at the treatment works, working under gravity off the 73m head available from the reservoir, via a 69cm diameter raw water main. A reserve raw water reservoir, sited adjacent to the water treatment works, was used to dump
water from the turbine to avoid over pressurisation of the system.
However, the reservoir was abandoned when the old cast iron raw water main (circa 1890) was renewed and the turbine became redundant. An intermediate underground storage facility had been constructed at Roborough, a site midway between the impounding reservoir at Burrator and the Crownhill treatment works. This facility was able to accept water from a pumping station on the river Tamar to augment supplies from Burrator during dry periods. Water for the treatment works is fed from this facility to the works’ inlet tank, via a new 900mm ductile iron raw water main, which also includes Larner Jonson pressure reducing valves to break the head pressure down to 55m at the works.
The Crownhill treatment works is sited in an urban area, with adjacent private domestic dwellings. Shortly after commissioning the new raw water main, the owner of the treatment works, South West Water, realised that the operation of the Larner Jonson valves was becoming a nuisance for the neighbouring properties through vibration and noise transmitted through the substructure of the surrounding land.
At about this time, South West Water acquired the 3.2MW South West Hydro Group of power stations in West Devon from National Power. Following this acquisition a hydro management group was formed. The group looked at the Crownhill works and suggested that design and installation of a water turbine to break the head of the works inlet, working in conjunction with the pressure reducing valves, could overcome the noise and vibration problems. In addition the output from a generator coupled to the turbine could be used to offset the costs of running the treatment process.
The hydro management group carried out a feasibility study. A proposal was subsequently put forward to renovate the original turbine and generator and relocate them, along with one of the Larner Jonson pressure reducing valves, into a nearby redundant pump house that had been made available following decommissioning of the reserve storage reservoir. The valve was installed in parallel with the turbine to act as a back-up and to balance flows to the treatment process for continual operation of the water treatment plant.
Using the original turbine and generator presented a problem as well as a major cost-saving. The turbine had worked previously on a higher head and, with no technical data or even drawings available, the hydro management group was not sure if it would work on the reduced available head of 55m (as opposed to 73m). The group worked with Hydroplan consultants and produced a set of operating scenarios for the turbine under these conditions. From this, two major problems were identified:
•How to ensure that any load changes did not rapidly fluctuate flows and adversely affect the set parameters in the works treatment process.
•How to control flow fluctuations in the event of loss of mains power and subsequent pressure surges in the raw water main.
Modelling of the pressure transients was carried out by Halcrow. Information was provided that enabled precise planning for handling the expected maximum pressure fluctuations within the system, without major disruption to the works process. During the testing and commissioning of the turbine and Larner Jonson valve it was found that a 1.5bar back pressure existed, due to the difference between the turbine draft tube and the water treatment works’ inlet tank. This restricted the surge pressure and flow increase, so that it had a negligible effect on control of the treatment process.
A logic control system, backed up by an uninterruptible power supply, was designed by Tecker of Falmouth to match the phased opening and closing of both the turbine guide vanes and pressure reducing valve. This was necessary to ensure smooth flow regulation at a
rate of no more than 1% of maximum flow rate per minute, which was needed
to safeguard the integrity of the
The refurbishment of the turbine, which included pressure testing of the spiral casing, was carried out by Gilkes of Kendal. The induction generator was overhauled by Lewis BERL of Bath, and Tecker built the switchgear and control systems. The civil contractors were Hannon and Young, supported by
the Engineering Department of South West Water. The South West Hydro management group was responsible for the design of the turbine and generator foundations, overhauling the outboard bearings and assisting Gilkes in the installation work. All aspects of future maintenance are the responsibility of the hydro management group.
The project started in May 1999 and was commissioned in mid-November. Since that time the turbine has performed well and the noise and vibration problems have been alleviated to the satisfaction of the company and local residents. This was achieved within the set time and financial constraints. Invaluable experience has been gained from the work at Crownhill and South West Hydro is now confident that this system can be used elsewhere and is planning further projects. Being able to successfully use hydro generation within the water industry’s infrastructure opens up new opportunities. With support from the UK Government through its new Renewable Energy Trading Arrangements (see IWP&DC April 2000, pp15-17), such opportunities should come to fruition.