Influence of the selection of valve equipment used in power generation plants
To keep pace with market trends, power providers must remain ahead of legislation, anticipate the evolution of the industry and implement the necessary adaptations. One significant illustration of this reality is the increasing trend to construct new power plants that use alternative fossil fuels.
The transition of the power industry from coal-fired power plants to combined-cycle power plants has compelled, in part, a conversion from cast-steel valves to forged equipment. Combined-cycle power plants seek higher efficiencies to maximize returns; as a result, operating temperatures and pressures have increased, necessitating superior valve solutions that can withstand the conditions without suffering the impact of the harsher environment.
Market conditions also demand faster start-up times, increasing the severity of gas turbine starts. This in turn increases the thermal transients, with higher gas turbine acceleration and higher gas flows at increased temperatures. Every time the plant is turned on, then turned off (cycling), the gas turbine, steam lines, steam turbines and auxiliary components undergo drastic thermal and pressure stress. This can quickly damage the equipment and dramatically accelerate the wear and tear on valves. As a result, non-cast valve solutions are becoming increasingly sought by both supercritical coal and combined-cycle power producers.
Because process equipment in these conditions is subject to a lifetime of thermal cycling (hot, warm and cold start), load changes and trip scenarios, valves can become susceptible to material creep and fatigue damage, which may dramatically limit service life. By being inherently free of the internal discontinuities typically found in cast products, forged valves are free from defects like blow holes and shrinkage. As a result, they are becoming increasingly popular in some power generation applications.
Despite the growing popularity of large forged valves, there are still hundreds of applications throughout a power plant where castings are also well suited, including high-pressure steam. Cast technology is proven and has been used in the majority of power projects for over a century. Without the additional expense of forging dies and machining, cast valves are generally more cost-effective than their forged counterparts. They are successfully used in continuously-run modern plants, where many of the problems originally attributed to cast valves have been eliminated. Furthermore, cast valves are often easier and more practical in terms of design modification, fabrication and upgrade.
While both types of valves have distinct advantages in the proper applications, neither is completely impervious to the potential for defects. Forged valves are susceptible to laps, seams, poor grain structure and bursts—internal tears that can result in valve cracking. Castings are vulnerable to surface discontinuity, sand inclusions, porosity, hot tears and shrinkage cavities. Despite these possibilities, however, good forging and casting processes can eliminate the risk of defects, which underscores the importance of selecting a reliable valve supplier.
Castings remain appropriate for a wide range of applications, especially in cases where a unique metal composition is required, the part is relatively large or complex, or weight and cost are prohibitive factors. However, the inherent ability of forgings to provide longer life in today’s demanding applications is attractive for the volatile environment of a frequently cycling, combined-cycle power plant. To truly understand the best valve solution for a particular application, valve users and manufacturers must work together to evaluate product design, quality and cost, and select the most appropriate product for their needs.
With coal and natural gas fueling the next generation of power production, valves and other process equipment must be designed to address specific conditions of the changing environment.
Power professionals worldwide seek to assure their operations are increasingly efficient. But productivity gains come at a cost because increases in plant cycling, temperatures and pressures place additional burdens and stress on the equipment. Consequently, valve users seek solutions that are reliable, safe and above all, efficient to meet the energy needs of an increasingly-demanding nation. Forged and cast valve solutions alike meet this need in the harsh environment, while offering users the quality and dependability commanded by the ever-dynamic industry.