By Maty Yeminy, Constellation Nuclear and Dave A. Stemler, CCI
Constellation Nuclear’s Nine Mile Point Unit 2, a 1,124 MW Boiling Water Reactor, was commissioned in 1987. In the mid-90s the plant started to experience cavitation problems with the unit’s 2 inch x 2 inch cooling water temperature control valve. This valve controls the temperature of the turbine lubricating oil.
More recently a 4-inch x 4-inch flow reject valve (reactor-spill valve to the reactor water clean up system) also started developing control problems. The original equipment manufacturer (OEM) had modified the 4-inch x 4-inch valve flow reject valve to improve its control capability. Unfortunately, this reduced the valve’s capacity to the point where it could no longer handle the reactor fluid volume during startup.
The OEM replacement trim provided a maximum flow of less than 100 gpm while the control-room operators required 180 gpm. The reduced flow prolonged the startup process by about three hours at an added cost of more than $100,000 (based on $30 per MWhr).
Because of the environment, and the fluid being handled by the flow reject valve, both valves are “hot” (radioactive). Whenever work is performed on the valves, therefore, the objective is to minimize personnel radiation exposure and minimize the amount of scrap material (solid radwaste) requiring decontamination.
Since the valves are welded into the piping systems, Constellation Nuclear preferred a retrofit solution rather than total replacement. Retrofitting is less expensive than replacing a valve because no new body is required. It also reduces radiation exposure to plant personnel.
Control Valve Retrofit
The retrofit trim, Figure 1a, for the 2 inch x 2 inch 900 ANSI valve body was designed for a maximum flow of 16 gpm, a maximum inlet pressure of 1,390 psia and maximum ΔP of 1,300 psia at 170 F.
However, the ability to design the retrofit trim without intruding into the “hot” area was a challenge. To do this, the project team took advantage of miscellaneous spare parts on hand at Nine Mile Point. These were measured to create a dimensional “model” of a retrofit trim design that could be fitted into the existing valve body.
The new bonnet design used the only non-reactive valve related item available, a spare bonnet-to-body ring gasket. However, because the existing bonnet was not the size required for the retrofit, excess material was added to the design. The “fat” was built into the bonnet design to allow for rapid in-plant machining. It also provided additional material to accommodate the bolt ring diameter.
Existing documentation and other available data provided valuable design information to the design engineers. Because the original hydraulic actuator had failed, a new pneumatic actuator was installed.
The retrofit trim, Figure 1b, for the 4-inch x 4-inch 600 ANSI globe reactor spill valve was designed for a maximum flow of 250 gpm, a maximum inlet pressure of 1,215 psia and a maximum ΔP of 1,200 psi at 130 F. It was also designed for tight shutoff.
Control of the valve between 250 gpm and 13 gpm had previously caused problems for the control-room operators. Prior to the upgrade the valve had severe leakage problems due to erosion. However, the new extreme trim characterization has been designed to permit rapid plug lift off under low-flow conditions.
The valve’s characterization provides for a 30 percent stroke travel at only 10 percent flow, Figure 2, reducing the possibility of plug seat erosion. Whereas the OEM modified valve had three stages of pressure reduction, the new upgraded valve has 20 stages of pressure reduction, reducing the exit velocity to less than 100 ft/sec. ANSI Class V leakage requirements were designed into the valve. A new pneumatic actuator was also installed.
The retrofitted trim design was made simpler by the purchase of a spare trim set for the previously OEM modified valve. This trim was measured and used for determining the available envelope within the existing body for the new trim. Constellation Energy was also able to obtain other critical details from the OEM for use in designing a new bonnet for the valve. As a result radiation exposure to the mechanics was reduced.
Benefits to the Plant
The control-room operators have reported better than expected results during startup of Unit 2 after the 2000 refueling outage. This included an increase in the reactor “reject flow” rate to 225 gpm, good controllability and tight shutoff. By increasing the flow rate, the plant has reduced startup by more than three hours.
Maty Yeminy has more than 23 years experience in the nuclear power industry and for the last 17 years has been a design engineer for the Nine Mile Point nuclear station. He has BS and MS degrees in mechanical engineering from the Polytechnic Institute of New York.
Dave Stemler has more than 27 years of experience in engineering, construction and procurement for large scale fossil and nuclear power plants. He has a degree in Business Management from California State University at Fullerton. Currently he is Director, Power Projects, CCI, California.
Multistage Pressure Reduction
In severe-service control valve applications, advanced multistage, pressure-reduction technology solves many operating problems. High trim velocity is eliminated in the control valves by dividing the fluid stream into multiple, but discrete, flow passages.
A stack of flat metal disks forms the trim assembly, Figure 3. Each disk has a flow pattern of successive right-angle turns machined into the flat surface of the disc. When stacked, the disks can be matched for linear flow/stroke operation or mismatched in groups to produce a specific flow/stroke characterization.
The individual disks create a flow pattern that enables the trim to be infinitely tuned to control flow. This not only maintains positive operating characteristics, it also prevents cavitation throughout the operating flow range of the valve.
Figure 4a illustrates how this multistage, pressure-reduction technology eliminates cavitation. Uncontrolled trim velocity can cause the fluid pressure to drop below the vapor pressure and can cause damage to the valve from cavitation. However, with a multistage pressure reduction design, the pressure is gradually reduced to prevent the fluid pressure from dropping below the vapor pressure, Figure 4b.
Each disk in the retrofitted stack trim incorporates a pressure equalizing ring on its inside diameter, thus ensuring that equal pressure acts radially around the circumference of the plug at any position in its stroke. It also keeps the plug centered at all loads to prevent plug vibration.
Figure 3:Typical tortuous path disks and disk stack.