Cottam power stations

Construction

Work was begun in April 1964 on the site of Mickleholme Farm by the Central Electricity Generating Board Midlands Region from Bournville. The modernist architects for the buildings on site were the Nottingham practice of Yorke Rosenberg Mardall. The main contractor for the construction of the 2,000 MW power station was Balfour Beatty. The coal plant was supplied by the New Conveyor Company of Smethwick. John Thompson boilers supply steam to English Electric 500 MW steam turbines. The maximum continuous rating of each boiler is 2,400 lb/sq.in and 568 °C at the superheater. The power station opened in 1969 when owned by the Central Electricity Generating Board.

The ground level before construction varied between 3.35m and 5.18m (11 and 17 ft) ordnance datum (O.D.) In order to provide adequate protection against flooding, the area in which the main building is constructed was raised to 7.92m (26 ft) O.D. by filling from borrow pits on site, but the coal store and cooling tower area remain at the original level of 4.87m (16 ft) O.D. The nature of the sub-soil was investigated by trial bores and found to be good load bearing marl at depths between 4.26m and 12.19m (14 and 40 ft) below the existing ground level and overlain by sand and gravel strata on top of which lay clay or silt and top soil.

The main building is 209.39m (687 ft) long by 124.35m (408 ft) wide and houses four 500 MW boiler-turbine units. The height of the boiler house is 65.22m (214 ft) and turbine house 34.44m (113 ft). The building is of steel construction with blockwork up to 10.66m (35 ft), above which there is a light-weight corrugated sheet cladding and windows.

An interesting feature of the construction of this building was the way in which the civil and steel erection work was phased. By completing the pile caps on units 1 and 2, the steel erection work was able to progress, while pile caps and flooring were completed on units 3 and 4. This enabled the steel work to be erected on units 3 and 4 from finished floor level while the flooring was being completed on units 1 and 2. This reduced the time taken to construct the main building.

Within the boiler house and between boilers 2 and 3, an escalator was erected which in several stages extended to the boiler drum level to assist with the movement of men and materials.

To the north of the main building is the precipitator bay and chimney. The four flues from the boiler are contained within a single chimney which stands 190.5m (625 ft) above ground level. In order to improve plume dispersal, the outer chimney casing is terminated 7.62m (25 ft) short of the four flues which stand at a height of 198.12m (650 ft)

To the east of the main building are eight cooling towers which are 114.3m (375 ft) in height and have a base diameter of 94.48m (310 ft) overall. Beyond this are the coal plant and the coal stocking area. The 400kV switching station is to the south of the site from which feeders join the National Grid system by way of other substations in the area. To the west are the station workshops and administration block of offices, which are connected to the turbine house by an enclosed overhead walkway. Other associated buildings such as the control block, water treatment plant house, oil storage compound etc., are disposed around and adjacent to the main building.

Considerable attention has been paid to landscaping the site in order to improve the visual appearance of the large plant. The cladding of the main building is painted in “Cottam Amber” colour, which blends perfectly with the brickwork of houses and farms in the vicinity. Extensive grassing and tree planting was carried out on land around the site when all the construction work was finished.

Boilers

The four 500 MW boilers at Cottam Power Station are manufactured by John Thompson Water Tube Boilers Limited, in conjunction with Clarke Chapman & Co. Ltd. Each has an evaporation rate at M.C.R. of 1.542 tonne/hour (3,400 lb/hr). The boiler plant has been designed for short time overload operation. By by-passing two of the H.P. feed heaters and increasing the firing rate by 8% an increase of 5% electrical output can be obtained.

Coal is fed from the bunker to the fuel pulverising mills by variable speed drag-link feeders. The four barrel type mills rotated at 15 rev/min (94.25 Rad/min), and are swept by hot air which carries the P.F. mixture to eight classifiers that reject any unground pieces of coal back to the mill. Eight exhausters pass the P.F. to 32 turbulent P.F. burners arranged in four rows of eight burners on the boiler front. Each P.F. burner has an integral oil burner which is used for lighting up purposes and low load operation when instability with P.F. firing would be experienced. Combustion air is fed to the burners by two forced draught fans which pass warm air ducted from the top of the boiler house through two rotary regenerative air heaters to the secondary, or combustion air registers around each burner. The air heaters are so designed that they can be by-passed on the gas and air side to facilitate optimum operating conditions.

The boiler feed water which is passed through the economiser before entering the drum is circulated around the combustion chamber water wall tubes by any three of the four boiler water circulating pumps. Steam from the drum is passed to the H.P. turbine through a horizontal primary super heater bank, pendant super heater platens, and pendant final super heater bank. The super heater platens which are situated directly above the combustion chamber differ from other heaters in that the heat is imparted to the steam by radiation as well as convection. The other haters being later in the gas duct rely mainly on convection for heat transfer. Steam temperature control is achieved by two stages of attemperation, one of which s between the primary super heater and the super heater platens and the other, which controls the final steam temperature, is immediately before the final super heater inlet. Exhaust steam from the H.P. turbine is returned to the boiler for re-heating at a constant pressure before being returned to the I.P. turbine. This achieved by a horizontal primary and a pendant final re-heater situated in the gas duct. Steam temperature control is achieved by non-contact type attemperators.

The boiler is kept clean by 42 gun type soot blowers in the combustion chamber and 42 long retractable blowers which clean the pendant and convection surfaces, all operating under automatic control from the control room.

The hot gasses from the combustion chamber are drawn through the super heater and re-heater banks, air heater and precipitators by induced draught fans which expel the gasses through the F.G.D. to the chimney. The fans maintain a slight vacuum within the combustion chamber in order to prevent combustion gasses leaking into the boiler house. The precipitators which are entirely of steel construction collect the dust by electrostatic means and there are no mechanical collectors.

Turbo-alternators

Each of the four turbines is an English Electric Company multi-cylinder impulse reaction machine operating on a single re-heat cycle with terminal steam connections of 158.6 bar (2,300 lbf/in2), 566 degrees Celsius (1051 degrees Fahrenheit) and exhausting at a back pressure of 1.5 in Hg. Steam from the boiler passes through four strainers and two pairs of combines stop and emergency valves, each pair being associated with two throttle valves which regulate admission of steam to the H.P. cylinder inlet belt. The H.P. cylinder comprises eight stages in total. The steam expands through the first five stages towards the governor end and then re-versus its direction and flows between the inner and outer case to the last three stages. Reversing the steam flow within the H.P. cylinder helps to balance the thrust thus relieving the load on the single thrust bearing. From the H.P. cylinder the steam is fed to the boiler re-heater and returned to the I.P. cylinder through two strainers and two pairs of I.P. emergency valves, each pair of which is associated with a pair of interceptor valves which are attached to the cylinder. The I.P. section of the turbine is double flow with seven stages to each flow.

The exhaust steam from the I.P. cylinder is passed to the three L.P. cylinders by cross-over pipes reducing cross sections, which distribute the flow of steam equally to each L.P. cylinder through which it expands to the condenser. Each cylinder is double flow with five stages to each flow. A special feature of the Cottam machines is the radial condenser in which the tube nests are disposed right round the turbine L.P. shafts in a common casing. This reduces the steam velocity in the exhaust space, which together with the reduced losses in the exhaust duct improves the efficiency of the condenser. The weight of the whole structure is some hundreds of tons less than earlier structures, and the foundation block is also greatly simplified, since only a rudimentary structure above basement floor level is necessary under the L.P. turbine.

Feed Water System

Following condensation of the low pressure exhaust steam in the condenser, the feed water is passed to a condensate sump mounted under the condenser. From this receiver the condensate is drawn by one or two extraction pumps and passed through a discharge strainer. The low pressure feed heating system consists of five Direct Contact heaters with one high level deaerator to provide a high feed pump suction head. The Direct Contact L.P. heaters are arranged in two banks, comprising three and two heaters respectively. Within each bank the heaters are stacked above each other so that the condensate drains from the lower pressure heater below by gravity. The heights of the heaters are approximately inversely proportional to the bled steam pressures. The advantages of Direct Contact heaters are essentially that thermal efficiency is improved, capital cost is reduced due to fewer numbers of extraction pumps being required, and the system is entirely ferrous.

The main extraction pump discharges the condensate to No.1 D.C. heater from which it cascades down to No.2 and then to No.3 D.C. heater by gravity. Two lift pumps deliver the condensate to No.4 D.C. heater, from which it cascades through No.5 D.C. heater to two deaerator lift pumps. The deaerator lift pumps discharge the condensate to the deaerator and from there it passes to the boiler feed pump suction main through microwire and magnetic filters. The main boiler feed pump is driven by a steam turbine which receives its steam from the main H.P. turbine exhaust. It comprises a single cylinder eleven stage turbine turning at 5,000 rev/min (31.42 Rad/min) and driving a multi-stage pump to give a delivery pressure of 2,067 tonne/m2 (2,940 lbf/in2). Two electrically driven Starting and Standby feed pumps are provided.

The high pressure heating system consists of two parallel banks of two heaters being numbered 7 and 8 stages in the system. Each of the H.P. heaters is vertical and of the non-contact type. From the H.P. heaters the condensate, at a temperature of 253 degrees Celsius (455 degrees Fahrenheit), is passed into the boiler economiser. During unit shutdowns the whole of the feed system can be ‘blanketed’ with nitrogen gas. This is an attempt to reduce the rate at which copper and ferrous oxides form, hence reduce the ‘carrying over’ of these oxides to the boiler drum.

Boiler feed water make-up is supplied by the Water Treatment Plant. This has a continuous rating of 3,672,000 Litres (970,000 gallons) in 24 hours at 153,300 Litres (40,500 gallons) per hour. The plant comprises three groups of cation, anion and mixed bed ion exchange resin units, together with vacuum degassing and pressure filtering plant. Each group processes 76,650 Litres (20,250 gallons) per hour and normally two groups are in service at one time, with the third on standby or being regenerated.

Gas Turbines

Four 25MW gas turbine generating sets were installed in the Station, in a separate building adjacent to the 400kV switching Station. Their provision basically covered the following functions, and provided for:-

(i) An independent supply to the Station auxiliary system in time of heavy overload and low frequency on the system. Operation to meet this condition was automatic including run up, synchronising of the gas turbine and load transfer.

(ii) Peak lopping by export through the unit or Station Transformers.

(iii) Standby to the unit transformers.

(iv) Start up supplies when isolated from the grid system.

Each unit consisted of two Rolls-Royce Avon RA29 Stage 6A (1533) gas generators in parallel feeding hot gas into an English Electric Co. ltd., two stage turbine which drove a 25MW alternator. The exhaust gasses were passed into independent metal stacks, each 106.68m (350 feet) in height. The plant which was normally left un-attended could be run up to full load in a little under two minutes. The gas turbines were later decommissioned on the arrival of the new CCGT installation at Cottam Power Station.

Cottam Development Centre

The Cottam Development Centre is a 400 MW combined cycle gas turbine (CCGT) power station, fuelled by natural gas. It was built as a joint venture between Powergen and Siemens, as a testbed for Siemens to develop CCGT technology.

Construction of the station commenced in July 1997 on a football and cricket pitch adjacent to the coal-fired power station. During construction, heavy components weighing up to 400 tonnes were brought onto site using inland waterways, to avoid damaging local roads. The station opened in September 1999. In May 2002 the station was bought out by Powergen for £52 million.

Specification

The power station generates electricity using a single Siemens V94.3A (now called a SGT5-4000F) gas turbine, one BENSON heat recovery steam generator and one steam turbine. Electricity from the station has a terminal voltage of 21 kilovolts (kV), and enters the National Grid via a transformer at 400 kV. The plant has a thermal efficiency of 58%.

2004 Wildcat strike

Wildcat strike action was taken in December 2004, when scaffolding collapsed and injured one worker.

2006 Wildcat strike

In February 2006, 51 British workers at the station walked out on Wildcat strike action due to the underpayment of Hungarian workers during construction of Flue Gas Desulphurisation equipment. The Hungarians had worse working conditions than the British workers and were underpaid £1 million by employer SFL. 15 British workers who walked out in solidarity were made redundant.