For years, premature wear of key components due to electrochemical corrosion has been creating challenges for power station operators. The use of EagleBurgmann diamond-faced high performance seals in boiler feed water pumps for fossil and nuclear power stations has successfully solved these problems, offering long service life.
A Need for Change
At the end of the 1970s, the conversion of the water chemistry from “alkaline” to “combination” or “neutral operation” and the use of increasingly larger and more rapidly rotating feed water pumps considerably reduced the service life of mechanical seals at many power stations.
This damage phenomenon, which caused the destruction of the face materials, had been often observed in large boiler feed pumps with high circumferential velocities. For operating periods that required at least 24,000 operating hours, the mechanical seals that were damaged by corrosion often needed to be replaced after only a few thousand hours. The premature seal replacement continually caused production losses, unnecessary expenditure, and maintenance costs.
Mechanical seals and galvanic corrosion were consequently an issue for a black coal power station operator in Germany who had already been working with EagleBurgmann since the establishment of his plant in 1971. Four units at the North Rhine Westphalia location produce an output of more than 2,200 MW. Around 7.5 billion kilowatt hours of electricity, which can cover 1.9 million single family households, are produced there each year.
In addition to the known issue of galvanic corrosion, these machines and their components are faced with further technical challenges: The changes in the power market due to seasonal causes or the addition of renewable energy sources often result in the coal-fired power stations becoming used to covering medium-load power and operating less at “max,” meaning less at full load. For the previously described power station, this meant that the more than 250 starts and stops due to the alternating load operation put the utilized seals under additional high mechanical and hydraulic loads.
In two 695 MW power station units, which have been operating for more than 30 years, two mechanical seals of the type DF-SAFI were installed in one of the two large, high-speed and turbine powered 100% full load Weller pumps. The drive side of the second pump had already been equipped with the type DF-SAFI.
The material combination for these special seals consists of a stationary-arranged seal face of carbon composite material against a rotating-arranged diamond-faced stationary seat of silicon carbide. Because of EagleBurgmann’s comprehensive field experience, the operator has been relying on this seal type since 2010. During an inspection of the Weller boiler feed water pump in 2012, one of the installed seals was inspected and then reinstalled without issue.
"Collaboration with EagleBurgmann has always worked out well for us. The problems were known and we had been working together in close dialog from the beginning. During the further development of the mechanical seals, such as the conversion of the water chemistry from alkaline to the present combination operation, we have been able to draw on EagleBurgmann’s great development and material knowledge. And we have indeed found a very good solution,” said Christoph Bolte, unit manager for mechanical engineering at the black coal power station.
Using “Vaccinations” to Extend the Operating Period
The two experts Christoph Bolte and Andreas Kretschmer (Head of application engineering Power, Pulp & Paper, EagleBurgmann Germany) know what they are talking about with regard to the technical further development in their long-standing business relationship. In the 1990s, the mini cooling circuit of the mechanical seals was reconditioned in order to circumvent the low conductivity of the feed water, which had been the trigger for electrochemical corrosion on the sliding surfaces. The circulating feed or cooling water was specifically “vaccinated” to make it softer and thereby more conductive. “In the process we added ammonia, for example, in “homeopathic” amounts so that we virtually provided the mechanical seal with an environment that corresponded to the alkaline feed water,“ explained Andreas Kretschmer, who has been supporting his customers in the Ruhr area for years.
"This let us eliminate the disruptive ‘electrochemical corrosion’ factor – and we achieved better operating periods. But of course that was not a satisfactory permanent solution for us yet,“ added Christoph Bolte. “Vaccinating” the cooling circuit not only requires constant monitoring and maintenance, but also specific HSE (Health, Safety & Environment) measures.
Innovative Solution with Diamond
The breakthrough occurred with the diamond thin-layer technology, originally developed in 2007 by EagleBurgmann together with the Fraunhofer Institute for Surface Engineering and Thin Films (IST), and has been further optimized by the Bavarian seal manufacturer over the years. It is offered by EagleBurgmann under the brand name of DiamondFace. The diamond layer up to 10 µm, which is applied to the seal faces under vacuum and at temperatures of 3,632°F (2,000°C) by way of chemical vapor deposition (CVD), is characterized by extreme hardness, high resistance to wear, excellent thermal conductivity, chemical resistance and minimal friction.
The maximum adhesion to the base material is convincing here. Another advantage is the considerably reduced heating during friction, which is slowed down by a factor of 15 in comparison to uncoated silicon carbide sliding surfaces.
“Now that DiamondFace coatings have been proven for years in the sealing of pumps, agitators and compressors in the areas of oil & gas, chemistry and pharmacy, we have also ‘discovered’ this coating for use in power station technology – and solved the difficult issue of ‘electrochemical corrosion’ in the process,” explained Andreas Kretschmer.
This uses the good electrical conductivity of the special EagleBurgmann diamond facing, which is damage resistant towards electrochemical corrosion without feed water conditioning - even for sliding surfaces with rapidly rotating contact. Furthermore, the DiamondFace layer is being tested in a facility under scientific conditions using original medium in a joint project with the Technical University of Graz, Austria. The current operating periods achieved here are more than 26,000 hours (DF/DF) and more than 18,000 hours (carbon composite material/DF). After this long operating period without signs of electrochemical corrosion, the condition of the sliding surfaces of both of the utilized material combinations can only be described as excellent and as good as new.
The innovative “diamond solution” has resulted in requests and orders from all over the world. “At present, our positive experiences in conventional power stations as well as in nuclear power stations in Europe, the U.S., India, and Asia have gotten around in the sealing industry. We are being perceived as successful problem solvers,” Andreas Kretschmer concluded.
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