Dealing with the rigours of cyclic CCGT operation: an operator’s perspective19 October 2001
To be successful in the electricity market place will increasingly require combined cycle turbine plants to operate cyclically. This poses challenges for technology that was originally intended to fulfil a baseload role. Innogy has undertaken a major programme of research into the issues involved. This has led to the carrying out of modifications and a programme of inspections designed to alert plant staff to impending problems. Dave Bogert, Innogy PLC, Swindon, UK
The pace of evolution in the UK's energy markets continues to pose significant engineering challenges for electricity generators striving to ensure power plants can maintain the levels of reliability and flexibility required to match present-day market conditions.
While much of the attention during the past decade has focused on optimising the performance of coal-fired plants, Innogy has also gained significant experience of the challenges facing operators through the requirement for flexible operation of its portfolio of combined cycle gas turbine (CCGT) plants.
Brought on stream to move the UK into a new era of highly efficient and more environmentally friendly electricity production, combined cycle gas turbine plants were primarily designed for continuous operation. And for much of their lives to date, that has indeed been the case.
The move to deregulation in the UK, however, has introduced market conditions where, at current gas prices, there is now surplus CCGT capacity. Coupled with the new electricity trading arrangements (NETA), it means that increasingly gas-fired plants will be exposed to the rigours of cyclic operation during the next five to ten years.
Against that emerging scenario, Innogy has carried out extensive research into the challenges this will pose and has already identified potential solutions. This has resulted in a number of modifications being designed and implemented to minimise some of the issues and a regime of inspections determined that will allow early warning of any developing problems.
Innogy research has raised a number of issues associated with heat recovery steam generator (HRSG) technology. This is based on an in-depth assessment of all aspects of the design and operation of both the horizontal and vertical duct style of heat recovery steam generator. The assessment has considered areas such as start-up and shut-down characteristics, risks involving plant chemistry and damage management issues.
Ranking the risks
In most heat recovery steam generator designs little consideration is given to access for repair and maintenance. Look at a typical horizontal duct HRSG. Plant damage and tube leaks can lead to prolonged forced outage times and refurbishment can also take extended periods, both commercially unacceptable for two-shifting operation. This makes it imperative to assess the risks, introduce modifications and develop effective condition monitoring and maintenance programmes. It is important to have a clear understanding of the impact of cyclic operational sequences on plant, so that they can be optimised to balance the commercial benefits of plant flexibility against the risks to plant integrity.
Many of the challenges involving the move from the baseload operation of HRSGs to the stop–start nature of cyclic operation are centred on component fatigue. These can be broken down into three specific types:
• thermal; and
Innogy has carried out a series of studies using thermocouples in HRSGs to assess and understand the nature of each particular fatigue risk in each of the plant components.
Facing up to fatigue
Thermal fatigue is, of course, no stranger to power plant operators, since it occurs in conventional boilers and manifests itself in cracking caused by the different rates of thermal expansion present during plant start-up and shutdown.
The risk of thermal fatigue in HRSG headers is comparatively small since the operating temperatures themselves are relatively low compared with large fossil-fired boilers and the headers are therefore smaller and thinner. However, the heat recovery steam generator steam conditions for advanced gas turbines now under development will soon match those of conventional plant.
One particular risk of thermal fatigue has been identified. This can arise when large volumes of condensate form in superheater elements while the plant is being gas purged or is shutdown.
Certain headers are more at risk than others. It is useful to rank this risk by taking account of position, thickness, geometry and operating temperatures. Particular attention is given to examining the detailed tube layout and support arrangements to identify potential loading mechanisms from differential tube and header expansion.
With a ranking of these risks, a targeted inspection programme can be implemented to identify problems as they develop and allow preventative maintenance.
When considering preheaters and economisers the likelihood of external dew-point corrosion and steaming when off-load is important. In some low temperature circuits it is often possible for temperatures to be higher off-load than when the plant is on-load because heat is released from the HP circuits off-load.
Additional risks for economisers are header stratification and stub cracking caused by economiser shock as cold water is introduced at start up. This can be a particular problem in vertical tube economisers which are generally less flexible to contraction of the inlet tubes compared with other rows as cold water is introduced. The solution to this problem may involve recirculating the water to reduce the temperature differentials. Introducing additional flexibility into the routing of tubing or support modification can also be effective.
The tube elements in vertical duct HRSGs are likely to be more "mechanically" flexible and less at risk of failures than in horizontal duct designs. This is mainly because in vertical duct designs the tube modules are arranged in horizontal serpentines between inlet and outlet headers and can readily accommodate tube differential expansions within the tube supports. This compares with horizontal duct designs with vertical tubes where the need for intermediate headers at the ends of tube passes causes a constraining effect. Horizontal tubing also eliminates the possibility of steam locking that is a characteristic of the vertical tube designs with horizontal ducts.
In both superheaters and reheaters, the risk of fatigue through flexible operation comes at the stub to header weld and other stress concentrating features. The fatigue is caused by the high rates of temperature changes at plant start-up and the resulting differential thermal expansion, often associated with condensate formation and drainage. Another consideration is the added effect of creep that can exacerbate the damage caused by fatigue as the plant becomes older.
A further important point that affects the level of fatigue risk in headers is the design of the tube stub to header weld. These are frequently of a partial penetration design similar to socket welds, but with up to 19 000 separate welds present in a large heat recovery steam generator, this type of weld represents a significant risk, especially in thin section tubing.
Structural components such as ducts, seals, casings and insulation also have the potential to suffer from fatigue and tube plate movement and tube fin wear may also need to be addressed in vertical duct designs. Often, these issues could have been overcome simply at the plant concept and design stages but, once installed, present considerable maintenance difficulties.
The right chemistry
Operational chemistry concerns must also be assessed as these also potentially affect the integrity of HRSGs during flexible operation. The wall thickness of tubing in many HRSGs is much less than on conventional boiler plant, making it less tolerant of loss-of-wall section through pitting attack or other corrosive loss.
One consideration that must be taken into account with the horizontal design is that, by their nature, they are not self-draining. Consequently, chemical control must be considered for off-load periods to prevent off-load corrosion taking place.
Flexible operation makes it increasingly difficult to regulate and control boiler water chemistry. Problems such as phosphate hideout and the hideout return can be difficult to manage. This has led to phosphate dosing being dropped for HP circuits on some Innogy plant in favour of all volatile treatments.
Attention also needs to be paid to the control of dissolved oxygen. In a two-shifting regime control of oxygen is more problematic and frequently the original plant deaeration systems are deficient under this mode of operation.
Far greater care needs to be taken with regular monitoring of boiler water chemistry in two-shifting regimes. Extra emphasis needs to be placed on sampling techniques and batch sampling may only be of limited use. The choice of sampling locations and techniques for flexible operating needs to be reconsidered.
Studies by Innogy have uncovered a number of issues relating to HRSG pipework. When major new-build projects such as CCGTs are executed, there can be interface problems between the design of the pipework and the needs of particular items of plant such as the HRSG. These issues are normally identified early in a plant's life to avoid it affecting its ability to operate flexibly without problems. Particular attention needs to be paid if two or more HRSGs are interconnected as the likelihood of temperature differentials in common pipework is greater.
Experience has shown that pipework support systems may not be capable of the operational duties imposed by two-shifting. A programme of both hot and cold surveys is needed to ensure that during a prolonged period of flexible operation, the movement of pipework and stress levels are within acceptable limits, and that support and pipework defects are identified.
Valve-related issues have been identified at a number of Innogy plants. Leaking or passing valves together with inadequate actuation and instrumentation for two shifting are issues to be addressed. Frequently, the original quality of the OEM supplied valves is low or there are no master and martyr arrangements. In some instances this has caused significant problems, for example, a passing main reheat isolation valve on one particular unit caused back heating and resulted in pressurisation of the sister heat recovery steam generator.
Returning to the critical issue of the formation of condensate and its subsequent removal, the source of the problem is the loss of heat from the HRSG while it is shutdown in between periods of activity. One of the most successful ways of helping prevent this is by using a stack damper. Reducing the formation of condensation, and also loss of pressure, will see the plant return to normal operating conditions more quickly on restart.
The normal purge operation of the gas turbine after tripping or planned shutdown involves quantities of relatively cold air being passed through the HRSG. This is a particular problem in HRSG leading superheater circuits where large quantities of condensate form. If the condensate is still present when the turbine bypass valves are open, it is likely to impact on the outlet headers and outlet pipework system, causing both fatigue and distortion. There is also the possibility of fatigue being caused to the header stubs.
Frequently original drains are only designed to cope with cold start conditions, which are not the most onerous. HRSG drainage systems at several Innogy locations have been modified.
To give a greater insight into the scale of the problem, Innogy has modelled how condensate forms during plant shut-down or start-up operational sequences. This allows drains to be resized and located at the optimum points to facilitate the removal of condensate from the HRSG. It also enables the operator to determine whether modifications are needed to the controls for plant start-up and shutdown during two-shifting.
Particular problems have also been encountered with blowdown vessels that were again originally designed only for cold start conditions and baseload operation. These issues can be surmounted by increasing the blowdown capacity and by modifying the operation during a start to prevent overloading though operations such as drum lowering.
Sequence control limitations
Innogy recognises that in many CCGT plants automated start-up sequence controls can be a limitation when two-shifting. Sequence logic can be upset by minor faults in the plant instrumentation and control system causing considerable delays that, due to the complexity of the system, can be difficult to understand and address. There is frequently a requirement for considerable control & instrumentation engineering input to ensure safe and yet commercially viable flexible operation.
These are some of the particular component and design issues in the flexible operation of HRSGs that have been identified and addressed by research and studies carried out by the team at Innogy.
As CCGT plants are called on to operate to a more flexible regime in future years, there will almost certainly be other issues that will come to light. Innogy is continuing with its research programme to ensure it is able to meet these challenges with tried and tested solutions both for its own plant fleet and those of its growing portfolio of external customers around the world.