Making the most of refurbishment15 January 2001
Hydro power projects across the UK proved to be the focal point of a recent conference which looked at the latest developments in uprating and refurbishment. Robert Freer reports from the Institution of Mechanical Engineers in London
HYDRO power in the western world is now at a mature stage in its development. Many of the best sites have been developed and the electricity supply companies are now concentrating on improving the reliability and efficiency of existing power stations and supply systems rather than developing new sites. Power station owners also recognise that machinery ages with time, requires more regular maintenance and eventually needs to be replaced.
Fortunately machinery designs and the materials and techniques used in construction have steadily improved with time, so that the decision to replace worn out equipment and machinery with new equipment is usually also an opportunity to increase, not only the performance and the efficiency of the machines, but also the availability and output of the hydro power station.
With deregulation and privatisation of the power industry becoming more widespread in the developed world there is greater pressure on power station owners to operate their commercial assets efficiently. Privatisation has also meant that government funds are usually no longer available to finance new works or renovations, and the owner will have to use their own funds or form financial partnerships with others.
The main alternative contractual procedures are either the traditional multi-contract packaging or turnkey contracts. The multi-contract method is well established in western countries but has a greater risk of delay than turnkey contracts and leaves the owner more exposed to the additional costs arising from technical problems and delays.
Turnkey contracts pass the responsibility for contract co-ordination to one contractor and give the bidder flexibility in planning the work to be done. There is less room for dispute and the owner is better placed to recover reasonable damages if the contract completion date is not met. There is no one answer to the most appropriate choice of contract and the merits of different strategies have to be carefully evaluated at the outset.
The problems associated with ageing in hydro power stations and some examples of the solutions adopted to deal with them were discussed at ‘Hydropower Developments – New Projects and Rehabilitation’, a meeting held at the Institution of Mechanical Engineers (IMechE) in London, UK on 30 November 2000. The event was organised by the Fluid Machinery Committee of the Power Industries Division of IMechE.
The conference attracted a large audience and was a good opportunity for those concerned with the growing interest and activity in the renovation of power systems to discuss a range of technical, commercial and contractual possibilities when undertaking this type of work.
Refurbishing Scottish hydro
Jim Smith, the engineering manager of Scottish and Southern Energy (SSE), described the policy considerations which led to the decision to carry out an extensive replacement programme for 30 of SSE’s main hydro power stations. Most of these hydro plants were built in the 1950s and 1960s and the established programme of plant maintenance and overhauls tackled the known problems and halted deterioration of the main equipment. But for a number of reasons the company was not getting the full benefit or the required availability from this piecemeal and incremental approach. For example, integrating new plants with old often fails to deliver the full benefits in improved performance and reliability expected from the new plant, and the multiple outages involved in a programme of discrete refurbishment reduces the operational availability of the plant.
Another important factor was that such an incremental programme is expensive in terms of the time and resources of the management and engineering staff. SSE therefore decided to undertake a full refurbishment of 30 power stations to a standard which would ensure that the stations would have a reliable service life of 30 years without the need for further significant capital expenditure. It was decided to award the refurbishment contracts on a turnkey basis. The economics of the refurbishment programme under the present circumstances were described by Smith as fragile, but all the refurbishment projects have achieved the required investment return criteria.
The UK government decision to exclude hydro plants with a capacity greater than 10MW from its Renewables Obligation will threaten the long term viability of some of these stations once refurbishment becomes essential (see IWP&DC April 2000, pp15-17). Suppliers of hydro power believe this exclusion may seriously affect the government’s targets to promote the use of renewable energy, and it is particularly unfortunate that this exclusion affects a reliable and useful renewable energy source which is technically well established.
The refurbishment of the first project at Glenmoriston (commissioned in 1957) was described in detail. It was found that restoring the plant to an ‘as new’ condition improved the turbine generator efficiency at the plant by 10%. Four other refurbishment contracts have also been let for power stations at Rannoch, Errochty, Pitlochry and Clachan.
Rannoch power station was originally commissioned in the early 1930s. An historic problem of penstock resonance had resulted in load restriction and limited pressure in the penstock. Tests were carried out on the penstock and draft tube to locate the source of the vibrations. The designers were then able to recommend to the owners that it was desirable to replace not only the turbine runner but also the spiral casing, draft tube and relief valve. Together these improvements would increase the efficiency of the power station by 5.03%.
At the other end of the scale, two papers were presented which described the current work on improving the performance of seals for valves and gates. Large butterfly valves used as turbine inlet valves, or as pipeline protection valves on high head power stations, will be subjected to considerable loads during emergency closure. The high flow rates and the large dynamic heads during emergency closure can cause large variations in the static pressure around the valve disc.
Dr PT Ireland of Oxford University, UK, described the research work which is being carried out for GE Energy (UK) to establish the fundamental design parameters during emergency closure of a lattice valve, and to evaluate the seal loads and deflections during high flow closing. The experimental model, which is approximately 1/8th scale, was tested using air as the working fluid. This was an acceptable alternative because cavitation effects are not significant, the air flow is turbulent and the model flow Mach number is sufficiently low that the air flow is incompressible. The conclusion of this work showed that dimensional analysis applied to real seals will lead to better seal designs.
Seal design was also discussed by Mr Dickson of Weir Pumps and Mr Jones of Edison Mission Energy (First Hydro Company) who were interested in the practical performance of seals used in gates which are designed to act either as water level control gates or as isolating gates. They discussed the different types of seal suitable for various operating conditions and water pressures, and they were particularly interested in the seals with a ‘piano note’ section made from extruded elastomer which will be used for the main and stop gates at the Dinorwig plant in Wales, owned by Edison Mission Energy. This seal was introduced because it is a robust section with a degree of self-adjustment and can conform well to deviations in the sealing face to ensure a tight seal.
Dickson and Jones found that seal suppliers did not have sufficient documented information about their seals to enable the contractors to confirm their performance in service to the power utilities. They decided it was necessary for them to undertake independent tests to determine the operating parameters of this type of seal and to explore its behaviour under various conditions. Their results showed that details such as the degree of tightening of the clamping bolts had an imperceptible effect on the flexibility and hysteresis of the seal in operation. Also reducing the degree of support behind the flange of the seal made it easier to deflect. Sealing the corners of gates always presents a problem and the authors showed that there are benefits in using a mitred seal.
tackling Welsh hydro
Another piece of refurbishment successfully undertaken at Dinorwig was described by DJ Carson-Mee and WO Moss of Edison Mission Energy (First Hydro Company). The existing spiral wound generator motor heat exchangers had deteriorated as a result of stress corrosion and fatigue at the tube plates. The original design consisted of a spiral wound, tin coated, copper fin coolers and failure was caused by tube fretting around the brazed areas at the tube sheet.
New plate fin heat exchangers have now been installed and their immediate effect has been to reduce the temperature in the generator motor core windings by 10ºC on load, therefore prolonging the life of the generator. Further developments in the design of fin heat exchangers are expected to reduce the temperature in the core windings by a further 80C. The authors believe that the benefits gained in increasing the life of the generator motor and reducing down time exceeds the capital cost of the new coolers.
First Hydro also owns the Ffestiniog pumped storage power station in North Wales. This innovative plant has four separate pump and turbine units, with each pump and turbine mounted on the same shaft. The turbine units are subjected to frequent pressurisations and de-pressurisations, and since the station started operating in 1965 the turbines have undergone 90,000 turbine pressurisations. First Hydro bought the station in 1995 and commissioned GE Energy (UK), to undertake a survey to ensure its life continued for another 25 years.
Part of GE Energy’s assessment included an operating target of a further 75,000 turbine pressurisations. First Hydro also wanted assurance that at the end of the 25 years the plant’s components would still be able to withstand the worst case of over pressurisation which could occur with a trip of the 90MW turbines. The continued life was dependent on the integrity of the major high pressure components, especially the embedded parts such as the turbine and pump spiral casings. The main inlet and pump discharge valves were also included in the survey.
To perform an engineering critical assessment (ECA) to determine the fitness for purpose of the components, it was first necessary to collect all the information about the critical components either from original drawings and records or from site measurements. Non destructive techniques such as ultrasonic and magnetic particle examination were used for the site measurements. This data, together with the results of the finite element analysis, enabled the owners to make an assessment of the condition of the equipment. Particular attention was paid to the turbine casings because a failure occurred in one of the casings during the initial hydraulic tests in 1960 before the station was commissioned.
The results of the ECA revealed defects originating at manholes in two of the turbine casings which were corrected by careful welding. The conclusion of the work gave the owners confidence that the station could continue operating for a further 25 years.
Three new installations were also discussed at the conference. The Lesotho Highlands Water Project for the Lesotho Highlands Development Authority was described by GC Jones and RG Taylor of GE Energy (UK). The project is a multi purpose development to improve the water supply in Lesotho and South Africa and to generate 72MW in an underground power station at the end of a 45km long headrace tunnel. The upstream reservoir has an operating range in excess of 60m and the head losses in the tunnel are significant. These factors led to the need to select a turbine suitable for a large range of heads.
GE Energy supplied the three Francis turbines, all the other mechanical equipment and the heating and ventilation system. This equipment was funded mainly from grants and loans raised in the UK.
The installation of the third unit at the Gitaru power station in Kenya was described by Mr Griffin of Mott MacDonald, consulting engineers to the Kenya Electricity Generating Company. The installation work was unusual in that the power station had already been built some twenty years previously to house the first two units, and space had been left for the third unit. Only a limited amount of civil engineering work had to be done for the third set, and most of this work was the supply and installation of mechanical and electrical equipment.
Although it is unusual to require the mechanical and electrical contractor to be responsible for the civil engineering works, the client’s engineers decided that the financial risk to the client would be minimised if the contract was let as a complete turnkey contract, including the civil engineering design and construction. On completion of the contract the authors believed that the turnkey contract was in fact the correct arrangement.
Mr Gowen described the Beeston scheme on the river Trent at Nottingham, UK, which was undertaken by Hyder Industrial. Beeston is a run-of-river scheme with two bulb turbines, each with an output of 650kW. The turbines make use of a 2m head provided by the existing Beeston weir.
This scheme represented a successful development within the limits of the local planning and environmental requirements and by making use of a Non Fossil Fuel Obligation (NFFO) grant. Similar developments in the future will need to seek financial support from the Renewables Obligation.
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