Refining and enhancing the steam cycle power generation process is essential to provide more efficient and cleaner power for the future. But keeping advanced technology operational in a cost-effective way sets the pace of change, writes Philippe Claeys.

Supercritical (SC) power stations are becoming more commonplace and the technology behind them is quickly developing into the norm for new projects in industrialised countries across the world.

In the past few years, new SC plants in China, India, Russia and South Africa have also come on line. Technological developments are expected to continue, with more and more new plants employing ultra-supercritical (USC) technology. Currently, there are USC plants in operation in Germany, Denmark and Japan, with a number of units under construction in China and India.

SC and USC technology improves thermal efficiency by converting water directly to steam, without the boiling phase. In this way, higher pressures and temperatures can improve the efficiency of the thermal cycle. Supercritical heating occurs around 221 bar, in comparison to conventional subcritical stations, which usually operate between 150-180 bar.

USC plants increase these pressures even further, resulting in improved efficiency in steam generation and a reduction in fuel consumption at the start of the combustion process.

Up to now, limitations in metallurgy have curbed the development of SC and USC power plants. However, companies such as Tyco Valves & Controls are investing in the research of nickel and chrome super alloys to engineer more robust components that can withstand higher operating pressures and temperatures.

Matching the pace of development with resilient, innovative technology is essential in keeping a plant viable. The International Energy Agency, in its Power Generation from Coal, 2011 report, estimates that switching a plant from subcritical to current USC operation would improve efficiency by between 4 and 6 per cent, reducing fuel consumption and emissions by 25-30 per cent.

These figures underline the importance of development in this area. With ‘efficiency’ becoming the watchword for power generation across the globe, all aspects of a plant’s lifecycle come under scrutiny, including cost-in-use, maintenance and repair operations (MRO) and safety implications, to name but a few.

Efficiency in MRO

Maximising preventative maintenance to minimise downtime is key in improving efficiency. The boiler trim needs regular inspection and maintenance because steam and sediment can have destructive effects.

The consequences from faulty or low-quality equipment can be serious, where damage to machinery, injury and periods of downtime are all possible. When working with high temperature steam, valves must be welded in to avoid steam leakage, which limits MRO options.

Effective maintenance to prevent this damage and ensure repair of any wear and corrosion is one of the most important facets of maximising uptime and maintaining productivity. Selecting components that enable MRO to be completed quickly and effectively is therefore a good first step in improving a plant’s day-to-day efficiency. Taking into account that the typical lifetime of a plant is several decades, it makes sense to take a long-term view.

For example, choosing generic components not designed for a particular purpose may reduce costs in the short term. But if their performance is compromised, this can result in increased cost-in-use within just a few years. High pressures and high temperatures can damage pipeline components, requiring regular maintenance and replacement, and increasing running costs.

Inside the pipeline

For small bore, high-pressure vent, drain and blow off applications of heat recovery steam generators, Yarway designed the Welbond high-pressure globe valve. With a focus on long-term cost benefits, the Welbond globe valve can be repaired in line quickly and easily. Stem, disk and packing can be removed through the yoke, and the solid Stellite seat can be fully exposed for re-seating.

Welbond high-pressure globe valve
The Welbond high-pressure globe valve offers long-term cost benefits

The alternative option to a globe valve in many applications can be a metal-seated ball valve. Low-cost and versatile, ball valves are often a first choice for shut off and other applications. However, their limitations mean that their cost-in-use can spiral, and any maintenance usually requires the entire valve to be cut out of the pipeline, dismantled and replaced. This can lead to the pipeline being out of service for several days.

The valve body design and construction plays an important role in the potential for leakage. General wear aside, internal leakage is a common, yet preventable, cause of damage to a valve. Metal seated ball valves are often cast in a two-piece or three-piece body design. Where the body parts are joined there is a potential leak path. Gaskets and body welds can give rise to internal leaking in a similar way. The Welbond globe valve was therefore designed in a one-piece forged body with no gaskets, pressure welds or seal welds, eliminating these leak paths and potentially extending its lifespan.

Lifecycle cost

Calculating the estimated lifecycle cost of a valve is an important factor in valve selection. Factoring in downtime and maintenance costs over the working life of a valve is essential, and can massively impact specification decisions. In-line repairable globe valves are suitable for pressure classes 1500, 2500 and 4500, making them suitable for high-pressure, high-temperature steam line environments. Table 1 gives estimates for cost savings with a valve with in-line repair capabilities.

table 1

As Table 1 illustrates, the initial cost for a ball valve is significantly lower. However, within a few years, the valve seat of both valves will begin to wear and it will be necessary to replace them. An in-line repairable globe valve can be re-set quickly and easily by on-site MRO personnel, using a specially designed re-seating tool. A metal seated ball valve, however, needs the whole valve replacing, which involves shutting down the pipeline while the valve is cut out and replaced.

Downtime makes an impact after just one replacement and, as Table 2 shows, within one repair the in-line repairable option could be more financially sustainable. In-line repairable metal seated ball valves are available, but are typically suitable for low-pressure applications only, up to pressure class 600. Table 3 illustrates how maintenance and repair costs add up over a decade or more: selecting a durable, long-lasting valve that allows for less frequent repair can save time and cost. With reserve seat material already in place, a Yarway Welbond globe valve can be re-seated up to five times. A metal seated ball valve may need to be removed and replaced five times in the same period.

table 2
table 3

Durability in design

Each time the Yarway Welbond globe valve is re-seated, it provides an as-new solid Stellite seat, designed for resistance and durability. The one-piece disc and seat ring provide the robustness required in this environment and are resistant to thermal cracking. Metal seated ball valves often feature a layered Stellite seat, which can be weaker and more susceptible to cracking. The errosiveness of steam, coupled with the associated high temperatures found around the boiler, mean that a durable design is vital for any boiler trim valve.

A high-flow capacity can also reduce flow turbulence and minimise erosion. Through generous port sizes and disc retraction beyond requirements, flow velocities in the Yarway Welbond globe valve are minimised, protecting the Stellite seat and disc from damage, and potentially extending service life.

With hundreds of valves required throughout a plant, making cost and time savings on each will quickly add up. Cutting time for maintenance in this one key area contributes to the overall MRO and materials costs of the plant. With a portfolio of brands and technologies, Tyco can provide advice and support for specifying valves and actuators that will provide savings in the long term. Plant requirements, pipeline temperatures and pressures and on-site capabilities for MRO all play a part in the decision-making process. However, the most important point to bear in mind is to think beyond the next 12 or 18 months, and instead to focus on the benefits across the entire working life of the plant.

Philippe Claeys is a marketing manager at Tyco Valves & Controls. For more information, visit www.tycoflowcontrol.co.uk/valves.