Atomic Energy of Canada, Ltd., a subcontractor of Societatea Nationala Nucleoelectrica S.A. (SNN) of Romania, contracted Badger Meter to model, test, and produce a set of precision valves for Cernavoda Unit 2, the second nuclear power plant in Cernavoda, Romania. The main concern for the construction and operation of the valves was their survivability and continued functioning after enduring an earthquake. In nuclear power plants, such valves control the cooling of the nuclear reactors where continued flow of water around the nuclear core is essential for safety. After the earthquake that precipitated the eruption of Mt. Saint Helens in 1980, testing criteria for valves routinely has included their capability to ensure the safe functioning of the reactor after seismic events, at least in terms of cooling capacity.
The original process control valve assembly, consisting of valve and switching units, was modeled in a 3D solid modeling program, and was designed to be part of a temperature control system. In all, the complete system consists of 16 separate units, including 11 major assemblies and four to five minor variations of one part. The assembled 3D model interfaces with the piping structures (lower horizontal pipes), also modeled in the CAD system's piping program.
Materials were defined for the various parts using the CAD tool's built-in library. The parts consist of 300 Series stainless steel, brass, higher-end nickel alloys, and high-pressure alloys of brass and stainless steel, depending on the stresses and temperatures to which the various parts would be exposed.
The CAD model was reduced from its original configuration to a simplified model of five components. The removed components were simplified into a series of lumped nodal masses and later reattached to the finite element analysis (FEA) model with massless, virtual beams at appropriate locations. The mass, centroid, and moment of inertia data for the nodal masses was formulated in the CAD program and then applied in ALGOR FEA software. Simplifying the model, only the necessary properties for analysis were used, thus reducing the solving time.
The important material properties were defined using 316 stainless steel alloys from the software's material libraries. A mesh convergence analysis was performed to increase mesh density as needed, while eliminating unnecessary detail. In some parts, the mesh refinement was as great as 1/1000", while in others, it was as coarse as 1/8". In all, the mesh yielded 29,448 elements. Constraints were then defined at the rigid connection of the bonnet to the valve body.
The goal of the analysis was to simulate the valve assembly's condition and functionality after an earthquake. That is, failure was defined not in terms of status at the moment of seismic impact, but rather after the event's impact initially was felt. As part of an effort to improve the accuracy of the experiment, engineers were not given access to the maximum allowable stress target for the entire assembly — the company was in a blind data scenario with regards to the overall target stresses.
The impact event — an earthquake — was defined as inputs of 1 to 4 Gs applied over a period of 1/10 second. The event was simulated by using a natural frequency analysis, incorporating the results into the model, and then conducting a response spectrum analysis.
The results of the analysis showed that the maximum stress of 1,811 psi was found in the yoke — the section connecting the valve's mount to the piping. This is less than the maximum allowable stress of 23,100 psi for material of this kind (ASTM A-27 Cast Steel), according to the ASTM standards.
In its use of FEA, the company realized savings in terms of the time, materials, testing funds, and parts that would have been required to prototype the valve assembly and perform physical tests. The valve assembly met the criteria for use in the Cernavoda nuclear facility, whose construction is now nearly 40% complete. The valve assembly is part of the new, functioning nuclear facility.
This work was done by Wayne Hall, mechanical designer, and Wayne Hays, senior valve engineer, of Badger Meter, Tulsa, OK, using software from ALGOR, Inc., Pittsburgh, PA. For more information, Click Here