Innovative material pairing enables low-friction operation and extends the service life five times
In countries like Canada, South Korea and Argentina, CANDU reactors (“Canada deuterium uranium reactors”) use nuclear fuel to generate electric power. Unlike the pressurized water reactors and boiling water reactors mainly used in Europe, CANDU reactors can be refueled during operation to increase availability. However, sealing the hydraulic machines used in this process represents a technical challenge.
The Canadian CANDU Owners Group (COG) regularly struggled with failing seals which led to costly shutdowns. To develop a reliable solution, material scientists from EagleBurgmann set out on a research mission which led them to observe material behavior on an atomic scale.
Low-friction mechanical seals for nuclear power plants with low leakage, high reliability and long service life.
The RAM machines are critical for troublefree reactor operation. An essential part of these highly complex hydraulic systems is a mechanical seal which seals the pressurized tube of the RAM machine against its drive gears. As the seals in place did not reliably withstand the operating stresses and failed frequently, the COG* tendered new mechanical seals for the RAM machines.
*The CANDU Owners Group is a private, not-for-profit corporation that is a leader in the field of nuclear technology. Together with its members, it drives innovation in power plant equipment and processes to achieve the highest standards of safety, efficiency and environmental performance. For more information, visit: www.candu.org
The new seal had to meet high requirements: torque and leakage had to be kept at a minimum even at pressures of 120 bar and rotational speeds of up to 261 rpm. It also had to be designed to work independent of the direction of rotation, since the RAM machine rotates in opposite directions to open and close the reactor tube. Finally, the new seal had to work reliably as this was the weak point of the previous products. When a mechanical seal fails, it costs the operator between 500,000 up to one million dollars per day in maintenance and lost revenue. In addition, service technicians are exposed to increased radiation levels when working on the RAM machine.
Unexpected problems call for a scientific approach
During the development of the mechanical seal, EagleBurgmann’s engineers encountered an unexpected problem: under certain conditions, the torque of the seal increased significantly after a few turning cycles, a behavior that no one had anticipated. To determine the cause of the rapid increase in friction, the material specialists at EagleBurgmann had to take a closer look at the atomic bonding forces between the sliding faces. Working with their colleagues at their parent company, the Freudenberg group, they conducted an atomic simulation that proved to be the key to understanding the effect.
In leak-free operation without media, it was found that cold welding occurred between individual carbon atoms on the sliding faces. In effect, the sliding faces were interlocked on a molecular level. Based on these findings, the specialists used further simulations to gradually approach the perfect material pairing.
In the end, a combination of crystalline diamond coating and silicon carbide sliding surfaces turned out to be the ideal solution.
This combination reduces cold welding to a minimum and keeps frictional resistance permanently low. Wear on the faces is also reduced. This increases the service life of the seal to four years – almost five times longer than the previous one.
As the mechanical seals will be used in proximity to the reactor, the client had particularly high demands for quality, diligence and transparency. Prior to delivery, each mechanical seal was extensively tested on a special test rig that simulates the operating conditions in the RAM machine. In addition, all materials and components used have been selected for their long shelf life to enable operators to manage their own spare parts. For example, only freshly manufactured O-rings are used in the RAM seals. Such measures ensure a long service life; the customer only needs to check seals that have been stored for years.
Finally, industry-specific packaging requirements had to be met. The seals were shrink-wrapped in a special film that is free of chlorides and other corrosive components and approved for use in the nuclear industry.
For this project, EagleBurgmann was certified to the Canadian Nuclear Power Standard „N299 - Quality Assurance Program Requirements for the Supply of Items and Services for Nuclear Power Plants“. From an audit of the entire company and its manufacturing processes to the selection of appropriate materials and the approval of suppliers, EagleBurgmann set the stage in every part of the process to ensure that the COG receives the best possible manufactured and documented product.
The new mechanical seal developed for the CANDU reactor not only met all the customer’s specifications but also performed impressively in the field. The expected service life of all installed seals was significantly exceeded. EagleBurgmann’s DF-SA DiamondFace mechanical seal has now established itself as a standard component for power plant operators. To date, more than 300 units have been successfully installed worldwide.
Things get hot in rotary kilns, very hot in fact. In the production of cement, for example, the thermometer measures more than 1,000 degrees Celsius. Sealing systems from EagleBurgmann, a company of the Freudenberg Group and EKK, make sustainable contributions to environmental protection, energy efficiency and process reliability under these "fiery" high-temperature conditions.
In countries like Canada, South Korea and Argentina, CANDU reactors (“Canada deuterium uranium reactors”) use nuclear fuel to generate electric power. Unlike the ressurized water reactors and boiling water reactors mainly used in Europe, CANDU reactors can be refueled during operation to increase availability. However, sealing the hydraulic machines used in this process represents a technical challenge.
A boiler circulation pump in a power plant conveys very hot and pressurized water. The pump has no cooling jacket, which increases the risk of a thermosiphon effect.
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