Increase MSIV and MFIV Maintenance Intervals with System Medium Actuated Technology

While in some applications the traditional isolation valve technology may be acceptable, given the competitive nature of the power market today, it’s clear that system medium actuated technology’s reliability with maintenance intervals of four outages make it the right choice for both steam and water line isolation applications critical to a plant’s safe and reliable operation. This reliability has been shown with the successful installations at Northeast Utilities’ Millstone 3 Plant, KGD’s Gösgen and Oskarshamn 1 Plant, and many more where system-medium steam isolation valves have been installed as early as 1974.

The Need for Reliability

In a nuclear power plant, the need for reliability has two aspects — first is safe operation of the plant and second is the economical and competitive production of electric power. One safety-related application where a system medium operated valve may be used is main steam isolation. One or more main steam isolation valves are installed on each of up to four main steam lines connecting the reactor or steam generator to the turbine. The main steam isolation valves remain fully open during normal plant operation. However, in the event of a main steam line break or loss of coolant accident, the main steam isolation valves close to isolate the containment building and prevent the release of large amounts of radiation into the atmosphere. Therefore, when called upon, the MSIVs must perform their function without fail. This is why Germany ’s GKN-1 Nuclear Power Plant awarded CCI Switzerland a million dollar contract to provide replacement MSIV actuators. The items provided in this replacement program are serving the unit’s main steam isolation valves which in the past have been a high maintenance item at GKN-1 and are designed with larger pistons to provide additional closing force in these safety-critical gate valves.

From the economical aspect of electric power production, the equipment used in a nuclear power plant must be so reliable that unplanned outages due to equipment failures are eliminated completely and planned outages are completed in the shortest time possible. The simplicity of design afforded by the CCI system medium actuated technology makes this possible.

Reliability and Low Maintenance through Simplicity of Design

When it comes to reliability and low maintenance, the saying “Simpler Is Better” is especially true. CCI’s patented system medium actuated valve designs are the result of years of testing, operating experience, and design evolution culminating in isolation valve reliability through simplicity of design.

The key to its success lies in recognizing problems with typical isolation valves, then designing around those problems. Take for example the problem of having sufficient actuation load to operate the typical MSIV valve … there are half a dozen or so “typical” modes of actuation without a clear winner. Why? None of them are trouble-free. Most actuators (hydraulic or pneumatic) are designed with a fixed amount of thrust calculated based on the worst-case scenario. This leads to very large and very heavy actuators with complicated controls. CCI system medium operated actuators use the system medium itself as the energy source. Therefore, as the system pressure and required actuation force increase, so does the available actuator thrust.

The controls mounted on the CCI system medium actuated valves are also simple and reliable. One or more pairs of solenoid valves (based on redundancy requirements) are used to direct the system medium into the upper and lower chambers of the actuator — that’s all. When an existing valve is being replaced or the actuation changed to the system medium design, the interface of the existing valve and controls can be duplicated so there will not be a need for new operating procedures or training. From the control room’s point of view, there has been no change.

Another major benefit of a simple design, and a key economic factor, is low maintenance. Since there are no pressure boundary penetrations (no valve stem or stem packing), no elastomers in the actuator, and no dynamic graph oil seals inside the valve or actuator, nuclear plants using the CCI system medium actuated valves have standardized on maintenance intervals for the valve and actuator of not less than eight years. During that time, the valve and actuator are not disassembled. Instead, a simple operability test is conducted during the outage. Compressed air at approximately 15 psig (103.5 kPa) is used to open the valve and then the valve is allowed to drift closed under its own weight, thereby proving operability. The solenoid valves are serviced while the plant is on-line. The recommended service interval for the solenoid valves is every four years. During each outage, a number of the solenoid valves are replaced with refurbished units cutting down the maintenance time and manpower required during an outage. Once the outage is over, the solenoid valves can be refurbished, tested, and made ready for the next outage.

The needs of boiling and pressurized water reactor applications in the nuclear power industry often require a custom-made solution. CCI’s patented system-medium-actuated valve technology is ideal for applications where reliability and long maintenance intervals are essential.

Published in SOLUTIONS Spring 2001