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include the gas and steam turbines and to investigate the implications for control and instrumentation. Trials of the optimised two-shift procedure would then be carried out and the extent of improvement assessed. Many of the design features that determine how flexible a HRSG is are described in Section 3.2. However, the quality of manufacture and construction and the operational procedures adopted also play a key role in ensuring that the effects of flexible operation (predominantly thermal fatigue damage) are minimised. Manufacturers are aware of the threats posed by cyclic operation, and have already started to address the issues raised by the flexible operation of power plant in response to customer demand. Both vertical and horizontal gas flow designs can be equally suited to flexible operation providing sufficient measures are taken during the design stage. Horizontal gas flow designs tend to be more susceptible to flexible operation damage due to lack of flexibility between the header systems (particularly if bottom-supported), though this is being addressed on more modern designs. The use of serpentine tube banks on vertical gas flow HRSGs inherently makes them more mechanically flexible, although there can be difficulties with drainage of the horizontal tube banks leading to the risk of off-load corrosion. Relatively simple ways of improving the ability of an HRSG to withstand the rigours of flexible operation include the correct sizing of drains and vents and the use of bypass valves and recirculation systems. GT and control and instrumentation reliability are important in avoiding trips, as hot re-starts are particularly damaging to upstream HRSG components. More substantial features such as the inclusion of a stack damper or the use of higher-grade alloys to reduce component thickness should be made at the design stage, as these are much more expensive to retrofit later in the plant life. 3.8 HRSG Costs, Reliability and Maintenance 3.8.1 Capital Cost Capital costs for new build CCGT plant are difficult to predict accurately without going out to tender, and even then a wide spread of costs can be possible at any given time. However the approximate total project cost for a new build CCGT in the UK [52] is estimated to be in the region of £425/kW. This includes not only the EPC (engineer/procure/construct) contract, but also other items such as project management, connection to gas/electricity networks etc. The HRSG it likely to account for around 10-15% of this total. This is still significantly lower than for other forms of fossil fuel generation e.g. the equivalent capital cost for a new build advanced pulverised fuel plant is around £800/kW [53] . CHP plant capital expenditure is generally more expensive at around £750/kW [52] with the HRSG likely to account for 10-15% of this total. (73)
3.8.2 Operating Costs Fixed operating costs, excluding fuel, are estimated to be around £12/kWyear for an existing CCGT plant [54] , although this could be higher for the most advanced class of GT plant due to GT maintenance costs. Generally, though, this represents the lowest value across the range of fossil fuel plant, the next lowest value being that of £14/kW for an existing coal fired plant without FGD. These CCGT fixed operating costs will again be dominated by the gas turbine, perhaps even more so than with the capital costs. Taking into account all costs (fuel cost at 23p/therm, fixed operating cost and cost of capital), the estimated delivered energy cost for a CCGT in the UK is around 2.2p/kWh [53] . 3.8.3 Reliability Reliability/availability will vary greatly depending upon the original build quality and design of the HRSG, the operating regime and the maintenance performed. EPRI [55] predicted theoretical total availability of a drum type HRSG to be 98.52% e.g. 1.48% availability loss. Powergen data from 1997- 1999 [56] suggests a figure of around 0.2% average HRSG availability loss, which may be due to the relative youth of the plant and a fairly tight functional specification. The losses [56] appear to be mainly due to one of three causes; tube leaks, leaks from flange connections or trips due to incorrect (high or low) drum level on start up. A survey of the causes of tube leaks on Powergen CCGT and CHP plant indicates that around 50% are due to ‘wear out’ mechanisms such as flow accelerated corrosion, fretting, long term overheating, on load corrosion, stub weld cracking etc. The remaining 50% can roughly be categorised as arising either from original manufacturing (usually weld) defects / previous site repairs or of being of a miscellaneous nature [57] . 3.8.4 Maintenance As well as the scheduled routine maintenance (e.g. valve/pump maintenance, safety valve maintenance and testing, instrumentation checks, etc), typical preventative actions would include annual HRSG visual inspections of: - • HRSG/duct supports, expansion and alignment. • HRSG/duct external framing & internal stiffeners. • HRSG/duct internal insulation (if fitted). • Bypass damper and stack. • Main HRSG stack. • Tube modules and headers. • Duct fabric expansion joints (including thermal imaging whilst on-load to identify areas operating at above-design temperatures and areas of gas leakage). • The condition and tightness of pipe penetration seals • The condition and movement of main feed and main steam pipe supports. (74)
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