Managers know that valves are important to plant operation. That’s why keeping them in top condition is a priority. Upgrading to advanced valve designs and reducing stress on valve materials are just a couple of steps that can improve outcomes.
It’s not an exaggeration to say that there are hundreds, if not thousands, of valves installed in each and every utility-scale power plant in the world. The number in each plant varies, of course, based on the size of the plant and the complexity of its systems, but every valve has a purpose and its proper operation is important in ensuring the most efficient and safest plant operation possible.
Yet, there are many valves at nearly every plant that don’t perform as intended. Often, valves don’t seal as designed. In fact, valve seats and discs can be damaged during the initial hours of operation and never work properly again. It’s akin to having a car accident as you drive a new vehicle off the show floor—how disheartening.
Wet Steam
Wet steam presents a particular challenge for power and process plant managers. Wet steam forms when saturated steam and condensate water molecules combine, which can result from factors such as incorrect plant operation logic or poor drainage systems. Failure to control wet steam can lead to the erosion of critical valve trim components, causing safety hazards and substantial operational inefficiencies, which can ultimately lead to costly unplanned downtime.
Examples of critical steam valve applications that are prone to condensate erosion are high-, intermediate-, and low-pressure bypass valves. Other applications that face condensate erosion include vent, steam letdown, and auxiliary steam valves. Erosion can be minimized or even prevented by admitting only dry saturated steam into control valves, but often, the presence of wet steam cannot be avoided. This leads to condensate erosion of sealing surfaces (Figure 1).
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| 1. A valve component damaged due to wet steam. Courtesy: IMI Process Automation |
Sealing surface erosion leads to leakage and downstream temperature rise. In some designs, this results in frequent opening and closing of spray water valves, leading to spray water valve trim damage, loss of controllability, damage to pressure reducing stages, and potential cracking in downstream pipes. It may also lead to water hammer and even more consequential system damage.
The EroSolve Solution
To counter the wet steam problem, IMI Process Automation launched a solution called EroSolve Wet Steam in 2019. The company has been installing the enhanced components at power and process plants throughout the world since then, and on July 10, 2024, it announced it had installed its 200th EroSolve Wet Steam solution.
EroSolve eliminates erosion problems through the combination of an innovative design that keeps flashing fluids away from sealing surfaces and a special material that resists water droplet erosion. The solution is offered as a trim upgrade and can be implemented in both IMI Process Automation and other manufacturers’ valves.
Several customers have raved about the results. At one plant in India, for example, annual cost savings of £275,000 and a reduction in CO2 emissions equivalent to 10,000 tonnes per valve, per year have been attributed to the EroSolve Wet Steam solution (Figure 2). Meanwhile, at a major U.S. power facility, the use of EroSolve Wet Steam reportedly enabled substantially improved valve longevity. Prior to the upgrade, the valve in question would leak approximately three to six months after installation. However, after implementing EroSolve Wet Steam, the plant has experienced no leakages in more than three years, according to IMI Process Automation.
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| 2. EroSolve Wet Steam trim example. Courtesy: IMI Process Automation |
In other customer testimonials, managers at a 1,200-MW combined cycle plant in the U.S. said they were happy with the solution’s performance and intended to upgrade two additional valves. At a 420-MW coal plant in India, managers said IMI Process Automation was a good partner capable of analyzing the complete system and providing a suitable solution to eliminate their erosion problem.
“Wet steam is a significant environmental and economic problem for power and process plants worldwide, which is why it was identified as a key challenge to solve as part of the first round of our Growth Hub initiative back in 2019. Since then, we have been able to deliver substantial savings to customers across industries, both in terms of costs and emissions, as a result of reduced leakage and downtime,” said Kevin McKown, president for the Americas at IMI Process Automation. “This is a great example of our mission to deliver ‘breakthrough engineering for a better world’ in practice. EroSolve Wet Steam has made a very significant contribution to supporting the industry both in its commercial and environmental sustainability.”
Metal Fatigue and Cracking
Beyond issues caused by wet steam, critical bypass valves are also susceptible to metal fatigue and cracking. The combination of high vibration, frequent thermal cycling, and extreme pressure drops can lead to metal fatigue in the valve body, water connections, and surrounding piping, resulting in cracks and potentially catastrophic failures, if not addressed.
Regular inspections and non-destructive testing are important on these valves. Routinely inspect the valve body, welds, water connections, and surrounding piping, using non-destructive examination techniques to detect developing metal fatigue issues before they become critical.
Consider upgrading to flow-up valve designs, which can reduce wear and extend service life compared to traditional flow-down designs. If flow-down valves are the only practical option, vertical mounting of the actuator can result in reduced and more even wear.
Using advanced materials can also help. Employ more erosion-resistant, hard-facing materials for valve components like the plug-stem assembly, cage, and seat ring. Consider high-temperature alloys for applications in supercritical and ultrasupercritical plants.
DMWs
Dissimilar metal welds (DMWs) can also be a source of trouble. DMWs involve joining two different alloy systems or metals together, rather than welding identical materials. Common power plant examples of DMWs include welding carbon steel to stainless steel or joining different grades of stainless steel. DMWs are more complex than welding similar metals due to differences in physical, chemical, and mechanical properties between the materials being joined. Key challenges are caused by differences in melting points, thermal expansion rates, and formation of brittle intermetallic compounds at the weld interface.
When valves are installed using DMWs, it’s important to locate the connection in lower-stress areas, if possible. It’s often best to use spool pieces for material transitions rather than welding directly to the valve body. The key to successful dissimilar metal welding is understanding the metallurgical compatibility of the materials, and selecting appropriate welding parameters and filler metals to create a sound joint.
Other Stress Reduction Tips
To reduce stress on valve components during operation, implement advanced control algorithms and precision actuation technology. Valve bodies and trim should be designed to withstand severe thermal shock, such as can be caused by temperature changes of 200C or more, while still ensuring reliable operation. Vibration should also be limited as much as possible, as this also contributes to metal fatigue. Incorporating noise abatement technology can help.
The importance of proper maintenance cannot be overemphasized. Replace soft goods and spray nozzles regularly to prevent damage to more critical components. It can be worthwhile to install manual block valves upstream and downstream of bypass valves to allow for online repairs when necessary.
For a thorough understanding of how a valve might perform in a system, finite element analysis (FEA) models can be useful. FEA is a computerized method for predicting how a product or structure will react to real-world forces, vibration, heat, fluid flow, and other physical effects. FEA breaks down a complex object into thousands, or even millions, of smaller elements (like cubes or tetrahedrons). Mathematical equations are used to predict the behavior of each element. Then, the computer combines all individual behaviors to predict the overall behavior of the object. Advanced FEA models can be used to analyze complex geometries in control valve designs, locate stress concentrations, and incorporate design modifications to minimize stresses.
