By Ann Chambers, Associate Editor
Multistage valve trim retrofits lessen vibration
Ever since the 1973 commissioning of Commonwealth Edison`s two 828 MW boiling water reactors at the Quad Cities Nuclear Power Plant, the 14-inch residual heat removal (RHR) valves and related system piping and components have suffered severe vibration and damage whenever the system was in required periodic test operation. In addition, the high vibration raised fears of potential fatigue failures in the RHR system piping.
Several “fixes” were tried with little improvement before the original single-stage RHR valve trim was replaced with multistage, tortuous path trim. The new trim assemblies were installed into the existing valve bodies, and the damaging vibration problems ceased.
The original RHR valves at Quad Cities Units 1 and 2 were conventional, single-seat, globe valves of flow-to-open design. Table 1 shows the operating conditions specified with the two pumps running. Under these low differential pressure conditions, most control valve manufacturers would design a single-stage trim similar to the one originally installed.
The industry change to perform periodic RHR pump testing required the use of a pump bypass loop. The loop was necessary so that the safety-related pumps could be tested under a dynamic simulation. Control valves were installed to achieve the proper fluid conditions. Excessive vibration of these valves and piping systems occurred during system tests, and rated flow had to be reduced. During test operation, the RHR valves and associated piping vibrated excessively due primarily to a high level of cavitation which extended several diameters downstream from the valves themselves. Vibration measurements were taken in terms of velocity on the RHR valve bonnets and actuators, as well as on a nearby spring can and elsewhere in the system (Table 2).
Experience at other Commonwealth Edison power plants showed the feasibility of installing new, multistage, pressure-reducing trim into the existing valve bodes, as this could be done without valve body modification or the need to cut the valves out of the piping. The retrofitting process using the existing valve body and actuator produced considerable cost reduction and permitted significant time savings over total valve replacement.
Table 3 shows post-retrofit vibration levels and percent reduction from pre-retrofitting values at the same locations. The peak velocity has been reduced by 91 percent. However, the peak vibration now is due to the fluid turbulence acting on the piping system as demonstrated by the peak frequency of about 20 Hz. The vibration attributed to the flow control valve now shows up at peaks at 2,460 Hz and 1,530 Hz. The peak velocity of 0.010 inches per second at 2,460 Hz is an even more significant vibration component reduction, and less than 1 percent is attributable to the presence of the valve.