^ The valves are controlled by an electric actuator and a specific declutchable manual emergency control located behind the secondary containment (i.e., outside the reactor building)
Article by Kévin Fort, Quotation Engineer, KSB SAS
Following the Fukushima accident, regulatory agencies have requested that nuclear power plants should be fitted with filtered vent systems, to enable containment systems to be safely depressurized and filtered in the event of a severe nuclear accident. This is because should an accident occur, the containment system may be subject to overpressure from steam formation or non-condensable gas generation.
FCVS systems can depressurize the containment area while capturing radioactive aerosols and iodine emissions and thereby mitigating the consequences of a severe accident to the environment. From its facility in France, KSB has been involved for the design and manufacture of triple offset butterfly valves with a nominal diameter of NPS 16” and a remote actuation system specifically developed for such FCVS units.
All the supports, anchor bolts and mechanical wall penetration devices (see Figure 1) are designed and supplied by KSB taking account of irradiation levels (2000 kGy) and mechanical loads. In normal operating conditions, the containment isolation valves are normally closed and operated only during a severe accident (or during periodical test). In the event of an accident, the valves would be exposed to high levels of radiation, high temperatures and high pressures.
Thermal binding due to a temperature rise during an accident has been taken into consideration by performing theoretical calculations and by running tests in the laboratory as per ASME QME-1. The valves are located in the annular space using a spindle extension system, which passes through a passage in the secondary containment wall; permitting the system to be controlled from outside the containment area by an electric actuator and/or a manually declutchable handwheel in the event of an emergency (see Figure 2). The complete arrangement has been designed to allow for a certain flexibility in length and also the need to be able to absorb movements during seismic activity or thermal expansion.
Furthermore, this system is designed, manufactured and tested as per ASME III, Subsection NC (Class 2) and conform to code case N62-7 requirements.
Thanks to the ASME N’STAMP certification (N and NPT), KSB has a strong quality management system organization in accordance for this kind of project, which has been processed according to Quality Management ASME NQA-1. KSB has established an internal nuclear safety process for all nuclear projects.
The TRIODIS range
The TRIODIS range is a high performance triple offset butterfly valve. This kind of design minimizes seat wear, allows for a long valve lifetime and permits excellent, long-term tightness performance.
This type of equipment can be applied in numerous applications involving liquid or gas processes. KSB experts will select the optimum valve materials in accordance with the service conditions.
The body design can be a wafer, full-lug, flanged or butt-weld side entry as required by the customer. The body can be forged or cast depending on the requirements. The packing shaft is designed in accordance with nuclear requirements. Graphite packing can be replaced without special tools.
The tightness performance required by engineering company TRACTEBEL was for a leakage rate lower than 3cc/h per 25mm of seat diameter. The measured performance of KSB’s TRIODIS valve with a lamellar seat is no leakage.
The design temperature required for the project is 250 °C; well within the allowable temperature range of TRIODIS valve which, by using suitable materials, extends from -250°C to +500°C. The valve pressure rating Class 300 is used for this installation. The design pressure of the TRIODIS valve goes up to class 900.
An irradiation design 200 MRad was specified for the valve and the complete actuation system. The TRIODIS butterfly valve is able to support several thousands of cycles.
The lifetime of the whole equipment is forty years for the project. Forged stainless steel was therefore selected for the valve body and disc. The seat gasket (mounted on the disc) is made of steel and graphite (other possibilities are available on request as solid seat). The body seat is made with a Stellite 21 hard welding deposition (other materials are available on request if Stellite 21 is prohibited).
Further, the limit switches and electric actuator are qualified for nuclear application and the valve is equipped with a double sealing system on the flange faces and a leak-off recovery system between the two joints. The leak-off recovery system on valve packing is realized with a lantern.
A specific mock-up was designed and manufactured in the workshop to test and demonstrate the proper functioning of the whole system before delivery. Qualification tests were successfully completed in laboratories under TRACTEBEL/ELECTRABEL and third party supervision.
This project is the result of close cooperation among all involved departments inside KSB from our commercial office in Belgium up to the operations and service through the design and quality assurance in France.
KSB is proud to be an historic partner of TRACTEBEL/ELECTRABEL from the beginning of nuclear program in Belgium.
About the Author
Kévin Fort is nuclear sales department collaborator (Quotation Engineer) based in KSB amri butterfly valves, actuators and systems factory in France near Bordeaux. He has twelve years of experience at KSB and nine years in the nuclear market.
He spent the first three years of his career as a quality control engineer for nuclear components before moving to the nuclear sales department.
Mr. Fort is currently in charge of nuclear projects quotations for the worldwide market.