internal pressure times safetv factor approach, consider­ able reductions are made in the section sizes required. This results in economic savings due to reduced weld volumes and transported weight and in many cases the avoidance of the need for post-weld heat treatment.

The more traditional material for this application would be plain carbon manganese steel plate manufac­ tured by conventional steelmaking routes and fabri­ cated using established welding processes. The major disadvantages with this choice is the transport problem involved with moving the thicker sectioned penstocks and the volume of in-situ welding required.

At Dinorwig, after considering the operational and safety requirements, the decision was made to abandon the usual approach used in penstock design of applying a safety factor to the maximum internal pressure. Instead, a fitness for purpose philosophy was adopted based upon state-of-the-art fracture mechanics. This philosophy recognised the potential for crack-like defects appearing in the penstock material and evalu­ ated the contending materials against their ability to tolerate such defects under the extremes of the loading conditions expected. This appeared to be the first time such a philosophy had been used in penstock design and it required a considerable amount ot experimental and analytical work.

The basic philosophy is now commonly used and consists of th’e following steps:

• Assume the presence of a pre-existing crack in a weld with dimensions immediately below that which can be sized by ultrasonic non-destructive testing (NTD) techniques.

•Predict the amount of sub-critical crack growth that would be expected by the fatigue loading over the

,full design life of the station.

•Demonstrate that at the end of life, the crack will still be tolerated in the structure even under the most onerous loading condition imaginable.

For such a set of analyses, a pessimistic approach is adopted to ensure that failure is avoided by adopting upper-bound crack sizes with lower-bound material properties. This also requires the worst case crack locations to be considered and includes consideration of stress concentration effects and such like.

25Gas turbine plant

25.1Introduction

In a gas turbine, the continuous combustion process of the fuel is harnessed to drive a turbine shaft, which is in turn coupled to an electrical generator. The overall simplicity of the fuel system and its associated thermo­ dynamic cycle has several advantages for power genera­ tion. There is no requirement for a steam cycle, thus keeping down the capital cost of the plant. Correspond­ ingly no condenser cooling is required, enabling a

Gas turbine plant

greater freedom of plant siting to be exercised. In addition, gas turbines can be quickly started and brought on load, automatically or under remote con­ trol. However, against such advantages, comparatively high fuel and running costs have traditionally restricted gas turbines to low load factor operation on CEGB stations.

25.2 Operational requirements

Gas turbines are currently operated on the CEGB system to satisfy both auxiliary power generation and peak load generation requirements. At several of the major conventional and nuclear power stations emergency or auxiliary gas turbines based on aero­

stations they are able to contribute to the overall output of the main station during periods of high demand.

of their frequency-conscious auxiliaries. Ultimately if this situation is allowed to continue, cascade tripping of the stations can occur.

The gas turbine’s capability of starting and quickly running up to full load permits it to be synchronised to the 11 kV unit board, even when the system frequency is falling. If the system frequency continues to fall, the unit board can be isolated from the rest of the system, the gas turbine can then supply the auxiliary plant at normal frequency and so keep the auxiliaries at full speed and maintain full output from the main unit.

The major role of the CEGB’s six gas turbine stations is to satisfy the peak load requirements placed upon the system. Their output is important in control­ ling and stabilising system frequency and in meeting

the turbines coupled to the generator via a clutch in accordance with a CEGB need for power factor com­ pensation under certain conditions of system operation. The clutch automatically engages the turbine when generation is required and disengages it from the generator when reactive compensation operation is selected, i.e., the generator may be used for syn­ chronous compensation when required.

25.3 Aero-engine-derivative gas turbines

In the emergency gas turbines installed at major CEGB power stations, the supply of hot gas under pressure is provided by an aircraft jet engine. At the time that they

Station design and layout

were installed there were two British engines of suit­ able power available:

•The Avon, capable of producing sufficient gas for

14.5MW of power.

•The Olympus, capable of producing sufficient gas for

17.5MW of power.

Significant development has taken place in aero-engine technology since the initial machines were installed. Rolls Royce has expanded its range of aero-engine­ based gas turbine units and machines are currently available with a peak output of up to 70 MW for future consideration. The current range uses the Spey, Avon, Olympus C and RB2I1 engines.

These gas generators supply the hot gas under pressure to a mechanically separate power turbine running at synchronous speed. Figure 2.98 shows the layout of a typical aero-engine-based, peak-load set.

Acro-engine-derivativc gas turbines have very quick start times and can be brought up to lull load in two to three minutes. Another advantage of such machines is repair by replacement of the gas generator units. The relatively light weight of the gas generator (approxi­ mately 2 tonnes) is also an advantage in this context and the set can be returned to operation with refurbish­ ment of the gas generator at central stores or works.

Chapter 2

25.4 Industrial gas turbines

During the 1970s it was decided to install industrial gas turbines. The incentive for doing so was a practical determination of the comparative economics of these and aero-engine-based units. At the time, the available evidence favoured the industrial unit for reasons of longer design life and the absence of any weight limita­ tions. Also the design of industrial gas turbines pro­ duced a machine that was more robust than aero­ engine-derivative plant and suitable for extended periods of generation. There was also the possibility that greater cost savings with scale would be shown by the industrial over the acro-engine-bascd machine, where improved output is obtained through a multipli­ city of gas generators. Industrial units also have a greater tolerance of lower grade fuel, thus giving the possibility of reduced operating costs.

Two prototype GEC sets have been installed using EM610 machines at Leicester gas turbine power station. I Tie'll unit has an output of 52 MW. Industrial gas turbine units of up to about 130 MW are currently available from the major manufacturers for future

order of 20 to 30 minutes from first rotation to full load.

25.5 Gas turbine power station layout

25.5.1Introduction

Gas turbines require relatively few items of auxiliary plant for their operation. They occupy relatively small areas of land and require few site resources, thus enabling great flexibility to be exercised in their siting. The following sections briefly outline the .major plant items that have an influence on the overall layout of a gas turbine power station.

25.5.2Station plant

The major plant requirements for a gas turbine station may be summarised as follows:

Gas turbines

Two optional formats of gas turbines are currently available for both the aero-engine-derivative and indus­ trial type machines. The units may be located in a small power station building represehting a traditional CEGB approach of housing generating plant. Alternaliveiy the gas turbine units tire available from the manufacturers as works-assembled packaged units. This format requires no building around the units and

offers advantages of lower cost and a shorter lead time from order to commissioning.

Fuel storage and handling

The basic concept envisaged for future gas turbine stations is for natural gas fuel with a fuel oil capacity as back-up. For industrial machines this may include both residual fuel oil (termed ORF) and distilate fuel oil (termed ODF) capacity. To enable start-up and shut­ down when firing ORF, an ODF system must also be included to facilitate initial combustion and to prevent problems of residues depositing in fuel oil pipework whilst firing on natural gas. Thus, in addition to a natural gas receiving valve station, both ORF and ODF storage tanks and delivery systems may be required.

Pumping plant for unloading supplies of residual and distillate fuel oil from rail tankers into storage vessels and feeding the gas turbines is essential. Treatment plant for both types of fuel oil is also required. Residual fuel oil heaters are also required to provide fuel oil to the turbines of the correct viscosity if this fuel type is chosen.

Site services

The remaining site area is occupied by:

• Fire protection water storage tanks and pumphouse.

Station design and layout

•Workshop and stores buildings.

•Lagoon for contaminated drainage and to receive discharge from the fire protection system and fuel oil tank contents in the event of a fire.

25.5.3Industrial gas turbine site layout

Figur# 2.100 shows idealised conceptual layouts for future gas turbines incorporating industrial type ma­ chines capable of running on a range of fuel options. The overall layout shows 'station based’ gas turbine units, whilst the lower inset shows the gas turbine units in ‘package’ form. The layouts chosen provide an econ­ omic arrangement for interconnections between the various site services. In reality, however, site specific features and restrictions may have a major influence on the overall layout for any future station. The design concept and orientation of plant items and buildings for these conceptual layouts is briefly explained as follows:

Fuel oil tanks

It is envisaged that four ORF tanks would be installed in total since this would allow flexibility in arrangement and operation. Long term tank outages for cleaning or repair can therefore be accommodated with the station still in service.

ODF contains fewer contaminants than ORF, and due to the absence of tank heaters, an ODF tank outage is a much more remote possibility. It is envisaged that only two ODF tanks would be required.

Fuel oil pumphouse and tanks

The fuel oil pumphouses, including the heating and fuel treatment plant, are situated in the centre of the site to minimise pipe runs to the unloading points, the fuel oil tanks and the gas turbine units. For the same reason, ORF treatment plant is situated next to the ORF tanks.

Fire protection pumphouse and tanks

The fire protection pumphouse and water tanks are again situated in the centre of the site but adjacent to the ODF tanks. This is because ODF is more volatile than ORF and is thus seen to constitute the main fire risk.

Other plant items

The natural gas receiving valve station is situated adjacent to the gas turbines and away from the ODF tanks since they constitute the main potential risk of a fire. This layout also minimises natural gas pipe runs.

The contaminated drainage lagoon is sited adjacent to the fuel oil tank farm, to receive bund drainage in the event of a fire or tank failure.

Chapter 2

25.5.4Cowes gas turbine station layout

Station and site details

Figure 2.101 shows the site layout for Cowes gas turbine station, which is representative of the latest of the CEGB’s peak-load gas turbine generating installa­ tions. The station is situated on the east bank of the River Medina on the southern outskirts of East Cowes on the Isle of Wight. The station consists of two 70 MW acro-cngine-based gas turbine units each powered by four Olympus gas generators, two at each end of the gas-turbine houses, which exhaust into power turbines located at each end of a centrally-mounted electrical generator.

The gas turbine generating station, the fuel oil reception and storage complex, and the 132 kV sub­ station have been constructed partly on land previously occupied by the Cowes coal-firpd power station and partly on land acquired from British Gas, who occupy the adjoining site. The total site area is approximately 4.75 hectares. The site is bounded to the south by farmland, with the south eastern corner of the site occupied by an 11 kV/33 kV substation. Over the northern half of the eastern boundary, the site is bounded by land scheduled for light industrial develop­ ment. Along its northern and western boundaries, the site is bounded by the Southern Gas Works and the River Medina.

It is very rare in the UK that power station design engineers are provided with an ideal greenfield site to work with due to the limited land areas that are available for such developments. It is thus often required to be able to design an effective power station layout based upon the limitations and restrictions imposed by any particular site. The gas turbine station at Cowes provides a good example of such a develop­ ment, where the shape and nature of the land available has imposed certain restrictions on the layout of individual plant items within the station.

Site development

The main elements of the installation comprise:

•Two turbine houses each with exhaust system and chimney.

•Control block linking the two turbine houses with integral station and generator transformer com­ pounds.

•Amenity and workshop building.

•132 kV substation.

•Fuel oil handling pumphouse and fire protection pumphouse.

•Bunded oil storage area.

•Combined heavy load and oil unloading berth.

•Stores (hiodified existing workshop building).

•An extended 33 kV substation.

The location of these various elements has been dictated by the shape and size of the land area available, the slopes and levels of the ground and the intention to make the route of the oil from reception to

pumphouse to storage vessel to turbine houses as simple as possible.

Xfi \de fl conibined access and service road. The

*°!. a flat S,te from east t0 west could not be

terrace, each of which would have given rise to stability

of fill, creating stability problems on the slope.

Due to difficult foundation conditions, the onlv reasonable location for the fuel oil storage tanks is at the southern end of the old power station site.