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initiated and presented to the operator. In this situation, there is a danger that important alarms could be overlooked, leading to a higher risk of plant trips. Some of these problems are related to the presence of poorly specified instrumentation, particularly actuators and valves, and this means that upgrading of such equipment is an ongoing process on new plant. It is known that the incidence of plant trips that are control and instrumentation related drops considerably in the first months and years of operation from levels that can be as high as 50% during commissioning. It is vital to eliminate spurious trips due to faulty instrumentation early in a plant’s lifetime, as these are very damaging. For example, calculations performed on a P91 superheater header with full penetration welds under an optimised hot-start/shutdown procedure demonstrated that a hot restart following a unit trip is 41 times more damaging in terms of thermal fatigue damage than a hot start following a normal shutdown [51] . The level of detail provided on some sequence displays is often insufficient to identify the plant condition preventing a sequence from progressing. The sequence logic is often so complex that even minor faults can be difficult to pinpoint and rectify, with sequences having to be overridden manually and stepped through to try and identify the sequence hold. It is usually essential to have a member of staff proficient in control and instrumentation issues available to deal with any automation related problems that may arise. The start-up of a unit can be potentially influenced by a wide range of active operational constraints generated from within the GT itself and also from the HRSG and the ST. These constraints adversely affect the run-up process by inhibiting firing on the GT until the current active constraint has been relieved. This gives rise to the risk of variable run-up times for each start-up on each unit and clearly becomes even more relevant in the event of moving the operational regime away from base-load. Faults with field devices can exacerbate this, making it difficult to predict overall run-up and loading times with absolute accuracy. The above factors have become highly significant under the New Electricity Trading Arrangements (NETA), where significant financial penalties can exist for not getting up to load on time. 3.7 Flexible Operation of HRSG Plant To take economic advantage of fluctuations in the wholesale price of electricity, it has become advantageous within certain markets (particularly the UK) for generating plant to operate flexibly. This may involve regular ‘twoshifting’ where plant is taken off load for several hours overnight, shut down at weekends and/or fluctuations between full load and part load or minimum stable generation. In the UK, the volatility in gas price and increased dominance of gas generation also means that companies with a portfolio comprising generation reliant on more than one fuel source can make up their contracted output as they see fit. These factors have resulted in many CCGTs (and hence HRSGs) designed for largely base-load operation being subjected to a flexible operating regime, with many plants conducting daily starts for at least part of the year. Many of the effects of flexible operation on HRSG (71)
components in terms of thermal fatigue damage, cycle chemistry and control/instrumentation are described in Sections 3.3, 3.4, 3.5 and 3.6 above. As far as the HRSG is concerned, the effects of load fluctuation are small compared to those arising from plant shutdown/start-up where the temperature differentials and ramp rates involved are much more damaging in terms of fatigue life. Power Technology’s approach to assessing the level of risk to HRSG components under a flexible operating regime is to carry out a flexible operation study, which is conducted in three phases: Phase 1 involves establishing the existing level of instrumentation on the HRSG and highlighting those that would be required for a series of monitored flexible operation trials. The critical HRSG components at risk of early failure/increased degradation or that could create operational problems under a flexible operation regime are also identified based upon station-specific HRSG component materials and geometry, inspection results, previous flexible operation experience and known problem areas on the plant being studied. The number and location of additional instruments (typically thermocouples on boiler headers and stubs) required for flexible operation trials are then specified, with their installation justified against the potential risks identified. Thermocouples can also be fitted to structural components such as expansion joints, duct supports, casings and so on to quantify the temperature differentials across them. A desk study of the effects of flexible operation on water/steam chemistry would also be completed and would typically review phosphate hideout and its control, flow accelerated corrosion risk in low temperature / pressure circuits, water treatment plant capacity, steam quality at start-up, de-aeration capability on start-up and condenser integrity. After the additional instrumentation required had been installed, a programme of shutdown/start trials would be agreed with the station for Phase 2 and all plant data received would be processed and analysed. The extent of any damaging transients (specifically ramp rates, through-wall temperature differentials and peak temperatures) would be quantified and a detailed thermal fatigue stress analysis carried out on the worst affected components. The damage would be quantified with the results expressed in terms of impact on component life and the likelihood of any failure mechanisms. In addition, one or two starts would be observed on site to fully understand any operational problems being experienced at first-hand. Appropriate recommendations to manage any issues identified would then be made. These might typically include enhanced, targeted non-destructive testing/visual inspections, proposed modifications to operating procedures to minimise impact on HRSG components and/or changes to the cycle chemistry. Phase 3 (if required) would explore in more detail with the station the feasibility of any proposed modifications to operating procedures in order to reduce component damage and / or reduce start time. Where the required changes are significant, it may be necessary to widen the scope of the study to (72)
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