Increasing plant pressure

The efficiency of conventional fossil fuel power plants is strongly dependent on steam temperature and pressure. If plant efficiency can be increased, a corresponding reduction in fuel usage and carbon dioxide emissions per megawatt hour (MWh) can be achieved.

Present conventional plant technology operates at temperatures up to 565°Celcius and pressures up to around 24 megapascals (MPa). New generation supercritical plants operate at temperatures up to 650°C and ultra-supercritical plant will operate at temperatures around 760°C, with pressures approaching 38 MPa, placing a new demand on high-strength, high temperature creep-resistant materials.

The immerging alloys that will achieve temperatures of around 650°C are
T/P911, T/P92, T/P23, T/P24, 347HFG and 304H while the alloys for the ultra-supercritical plant and higher temperatures will most likely be nickel-based alloys such as Hayes 230, Inconel 740 and modified 617.

With the introduction of high efficiency plant using advanced materials and operating at high pressures and temperatures, issues of plant integrity and reliability will be even more important as the consequences of component failure could be catastrophic. This will place stringent requirements on plant owners and operators to ensure that the most appropriate inspection techniques are applied to their plant in a planned and timely fashion. It will also require that all risk analyses, inspection and maintenance plans, reports and documentation are filed systematically to comply with all legal obligations.

Corrosion and chemical attack

With the introduction of new advanced power plant materials operating at these high temperatures, oxidation in air and steam will become a key design issue.

Excessive steam-side oxide growth cannot be tolerated, as the inevitable exfoliation of this scale will block superheater and reheater tube bends and erode turbines. Furthermore, the development of a thick steam-side and/or fire-side layer could thermally insulate the tubing, significantly decreasing its heat transfer capability thereby increasing fuel requirements, carbon dioxide emissions and decreasing efficiency. It will also increase tube metal temperature raising concern for tube integrity and life.

Managing potential failures and leaks

The deterioration of plant leading to component failures is managed through routine non-destructive testing.

Traditionally, radiography testing, hardness testing, dye penetrant, ultrasonic and magnetic particle inspections have been used to inspect for defects and cracks. With the need for more accurate crack detection and sizing, techniques such as time of flight diffraction (TOFD), Cracksizer (ACFM), phased array, acoustic emission (AE), microstructural replication (R) and others have been developed and are now often used routinely during maintenance outages.

Assisting these techniques are the evaluation codes, such as:

  • BS 7910, “˜Guide on methods for assessing the acceptability of flaws in metallic structures’
  • British Electric R5 “˜Assessment procedures for the integrity of high temperature structures’ and R6 “˜Assessment for the integrity of structures containing defects’
  • API 579 “˜Fitness for service’
  • AS/NZ3788 “˜Pressure vessels’.

Clearly the advent of stress analysis using finite element modelling and piping analysis programs have made these calculations possible and available within reasonable time and cost frameworks.

Managing any potential failures is imperative as an unscheduled plant outage can cost a significant amount of money. For example, a conventional 500 MW unit producing electricity at $50 per MWh can lose $600,000 per day; 500 MW x $50/MWh x 24h = $600,000. A simple boiler tube failure can take between two and three days to repair, resulting in a loss in revenue of between $1.2 and $1.8 million.

Generally with an open-cycle gas turbine (OCGT) or a combined-cycle gas turbine (CCGT), where a gas turbine/heat recovery steam generator (HRSG) unit combination is relied on, production losses incurred due to plant issues are lower than that which would be incurred through singularly operating gas turbines or HRSGs. While the lower production loss is more favourable and highlights the flexibility of CCGT plants, such losses remain an unacceptable cost to business. For older HRSGs, poor access can extend outage duration and add to repair costs and production loss.

The impact of ageing plant life

Many conventional Australian power stations are now between 20 and 50 years old and as this plant ages, the risk of failure increases, the costs of inspection and maintenance escalates, and the need to accurately assess the remaining life of strategic components to enable timely replacement becomes critical.

As for conventional plant, OCGT and CCGT will also become less reliable with age. However, strict maintenance regimes and key component replacement strategies, as recommended by gas turbine equipment manufacturers, can prolong turbine lives.

HRSGs, however, were originally plagued by poor design such that no allowance was made to inspect and maintain these plants. The result was plant that could only be inspected/repaired by “˜cutting in’ and “˜welding out’. More recent designs have allowed for inspection doors, space for access and improved management of condensate related issues, flow accelerated corrosion and thermal fatigue.

In recent times, the introduction of risk-based inspection to ASNZ3788 has enabled limited inspection and maintenance budgets to be strategically focused on the most vulnerable areas of plant. This has improved reliability while containing costs, but has increased the vigilance required by station owners and operators to ensure they have correctly understood how their plant is ageing and realise that this can be a moving target.

Originally designed for 30 years, the expectation of owners and financiers is that power plant lives can be extended to 50 and 60 years, which can place a strain on safety margins inherent in the original design. Add to this the changing environment from baseload operation to cyclical operation (two shifting), and the assumed safety factors can be substantially less than originally believed. Consequently, the owner/operator must understand all aspects of their plant’s deterioration with time or have proven specialists who can advise in these matters to ensure preventable failures are avoided.

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