Valve components, such as wedge, disc, ball, and seat, are hardfaced to reduce the effects of friction, wear and corrosion and thus enhance their in-service life. Cobalt-based alloys are commonly used due to their unique galling performance, low coefficient of friction, and superior corrosion resistance at temperatures up to 650°C (1,200°F). These features help to prevent the valve components from seizing, suffering surface deterioration, and/or being affected by gross material transfer between two counter bodies.
However, the increasing demand of severe-service applications with extreme operating conditions of temperature, pressure, high-cycle, corrosive, and erosive mediums has pushed the limits of these commonly used materials, states Dr. Khelfaoui. “Considering this, colleagues and I at Velan are developing alternative materials and processes that are needed to sustain the extreme operating conditions of the valves. In fact, Rejean Rene, Luc Vernhes and I have just prepared a paper on this very topic which I will have the pleasure of presenting at the upcoming Valve World Conference in Dusseldorf, Germany, in late November.”
Talking more about her plans for the Conference, Dr. Khelfaoui indicates her presentation will discuss material challenges associated with valves used in three distinct areas. These are enhanced ferritic steel valves for supercritical power plants, nickel-based superalloy valves for advanced ultrasupercritical power plants, and finally titanium and duplex valves for slurry and mining. Asked if she could go into more details about the supercritical power plans, she notes that in the past twenty years, combustible costs, environmental policies, and a growing demand for electric power have motivated engineering companies to improve the efficiency of coal-fired power generation technologies.
“These state-of-the-art facilities, called supercritical (SC) combined cycle power plants, work at temperatures above 538°C (1,000°F), use modified 9% chromium steel alloy valves with superior high temperature properties, and are equipped with heat recovery steam generators (HRSG),” indicates Dr. Khelfaoui. “Several cases have been reported where the Co-based hardfacing failed prematurely, leading to plant shutdowns. One of the typical failure modes is delamination. Developmental initiatives are ongoing to either enhance the performance of Cobased hardfacing or to find alternative solutions to improve valve reliability.”
To address such hardfacing challenges, Velan was a founding member of the EPRI P87 program investigation into cracking and disbonding of hardfacing alloys in CCPP valves. “Being a very active company, Velan teamed up with researchers and universities to thoroughly investigate and validate alternative hardfacing materials,” says Dr. Khelfaoui.
A metallurgist’s paradise
Disbonding of valve components such as discs, seat rings and wedges is an issue that needs to be taken very seriously, continues Dr. Khelfaoui. “The consequences of such failure may compromise the valve functionality. Depending on the severity of the failure, it can also result in the liberation of segments of the hardfacing that may damage downstream components such as the turbine.” Velan therefore offers several alternatives to conventional Co-based hardfacing. “Our complete hardfacing and coating portfolio has more than thirty different materials. Our latest coating, VEL-8, was launched last spring for severe-service ball valves,” states Dr. Khelfaoui.
And of course, Velan continues to support internal and external R&D initiatives. Dr. Khelfaoui: “in terms of development we continue to run extensive tests in-house. At the same time, we are also collaborating intensively with strategic partners, including Electrical Power Research Institute (EPRI), National Research Council of Canada (NRC), universities, suppliers, etc. In 2012, for example, Velan became one of the founding members of the largest industrial research chairs in Canada focused on new coating development.” Another recent development is a unique, hightemperature tribometer, recently constructed for Velan to measure static and dynamic friction, and sliding wear up to 800°C (1,460°F). This piece of equipment was built to simulate a valve.
Dr. Khelfaoui concludes: “so as you can see our work involves dealing with a wide range of materials covering a wide product portfolio for a broad spectrum of key industry segments. It is therefore not without good reason that I say that Velan is an absolute paradise for a metallurgist!”
Meet Dr. Fadila Khelfaoui
Asked about the origins of her interest in metallurgy, Dr. Khelfaoui recalls a motivating incident as a very young child. “I was reading a book about atoms and was fascinated how various atoms could be combined to form alloys. Perhaps that’s why as a child I played with and broke things to see how they worked!” Dr. Khelfaoui went on to earn M.Sc. and Ph.D. degrees in materials science and engineering from the National Institute of Applied Science in Lyon (France) and an engineering degree in metallurgy from the University of Science and Technology in Algeria.
“By coincidence, around the time I was approaching the decision point in my undergraduate engineering education, the university created a brand-new Department of Metallurgy. The timing was perfect to pursue a degree in metallurgy.”
On completing her studies, Dr. Khelfaoui worked for companies in Europe, the USA and Canada holding positions such as engineer and director, joining Velan in Canada in 2016 where she now serves as a Corporate Engineer, Metallurgy.
She explains: “My role at Velan is to act as a subject matter expert for materials working on both shortand long-term R&D projects. I conduct failure analysis, lead materials and coatings development projects, as well as manage our technological roadmaps in collaboration with third parties in line with Velan’s strategy and customer needs. Our department is based in Montreal, Canada, but we have a global reach. Projects often start following customer feedback or based on marketing initiatives from strategic areas.” With over eighteen years of experience in materials, surface treatments and related process technologies, Dr. Khelfaoui is a member of the Professional Engineer Order in Quebec.