Evaluation of Main Steam Isolation Valve and Main Steam Check Valve
A nuclear power plant underwent an Extended Power Uprate (EPU) that included an increase in the mass flow rate through the steam generator. The power plant selected Kalsi Engineering to evaluate the design of the Main Steam Isolation Valve (MSIV) and the Main Steam Check Valve (MSCV) to determine if they could perform their safely functions under high energy line break in the main steam line.
The MSIV and MSCV are directly connected (see Figure 1). The MSCV prevents back flow through the main steam line if a pipe break occurs upstream of the MSIV. The MSIV isolates the main steam line if a pipe break occurs downstream of the MSCV. These valves are required to survive extreme flow-induced forces as they close and isolate against the high energy line break conditions.
Kalsi Engineering developed a dynamic model to predict the disk closing speed of the MSIV and MSCV under pipe break and spurious closure conditions. The dynamic model was integrated into RELAP (a thermal-hydraulic code). The integrated model included transient predictions for the following key model components:
- System information including flow rate, pressure throughout main steam line, and choking conditions at the subject valve based on valve position and closing velocity.
- Flow-induced forces based on pressure conditions and disc angle.
- Net actuator force based on actuator pressure, spring force, and position.
The model was developed to predict the flow-induced forces utilizing flow coefficients that were obtained by performing a matrix of CFD analyses. The matrix included incompressible flow (water) and compressible flow (steam) conditions for multiple disc positions.