Saving Time and Energy is an Open and Shut Case at GKN

Nuclear power plant operator Gemeinschaftskernkr— aftwerk Neckarwestheim (GKN) and CCI Switzerland recently developed a new actuating concept for an existing valve. The challenges that confronted them often force operators to re-think or change very extensive, complex and expensive maintenance activities:

• Reduction of the available budget for maintenance, spare and consumable parts.
• Reduction of present resources (personnel reduction).
• Reduction of maintenance and preparation times (bench-marking).
• The gradual elimination of manufacturers’ know-how.

When the 3-loop pressurized water reactor GKN1 was built in 1976, the MSIV (Main Steam Isolating Valves) installed were operated by an oil hydraulic actuator, assisted by a small system medium actuator, working on the depressurizing principle. Due to insufficient positioning forces, the original gate valves and their oil hydraulic systems were redesigned and an identical valve was tested under disturbance conditions in several 1:1 scale tests on the Large Valve Test Bed in Karlstein (GAP).

In 1985, the oil hydraulic systems of the main steam isolating valves were modified and all components were designed for resistance to earthquakes, aircraft crashes and explosion shock waves. Because of the large number of different components that were susceptible to disturbance, the total maintenance and in-service testing and inspection costs, from 1984 to 2000 were approximately Euro 102,000 (USD 100,200) per year and valve, not including GKN’s own costs.

To combat the increasing challenges mentioned above, in 1999 GKN’s specialist department developed a concept to modify the actuation of the main steam isolation valves. Once the concept had been agreed within GKN, it was discussed with different manufacturers and then inquiries were issued. The most innovative and flexible as well as the most cost effective, was CCI.

Figure 1: Old design of the main steam isolating gate valve controls

For the new concept, the working principle of the system medium actuator was changed from the ‘depressurizing principle’ to the ‘pressurizing principle’. This meant the elimination of the steam supply to the actuator cylinder during normal plant operation. Due to its solid design, the actuating cylinder as well as the pilot control valves mounted directly on it are always hot, about 500 F (260 C), and consequently hotter than the system medium flowing into the actuator, dependent on the pressure conditions.

Once CCI had manufactured the actuators, they were taken to the CCI works test bed and assembled to a valve body and plate assembly identical to those installed in the plant. In 1:1 full scale testing, a total of 90 strokes were carried out, under operating conditions and under (partially) differential pressure conditions. The series of tests were agreed and carried out with the independent authority (TÜV) and included tests with built-in individual errors in the controls and pilot valves, at varying steam and auxiliary medium pressures. Even with this large number of stroking operations, no serious pilot valve leakages occurred. The leakage behavior confirmed the chosen seat geometry.

Figure 2: New design of the main steam isolating gate valve controls

The test results were compared with the fluid dynamic calculations carried out beforehand and with the results of the Large Valve Test Bed (GAP) tests. Since the results agreed with those from the calculations and the GAP, the calculation procedure was verified and confirmed. By reason of these tests and the calculated proof, it was not necessary to carry out new actuator tests on the GAP. This was one of the main objectives of the project because of the immense costs of using the GAP in Karlstein. By the above measures the actuator concept was tested and confirmed in advance.

After the conversion components had been delivered to the plant at the beginning of May 2001, the installation followed during the outage. The valve actuators, with a total weight of approximately 15 tons, were installed in less than three days. In addition, within 21 days approximately 10 tons of old parts were removed. In the same time period the complete power supply and control system was converted, and the cold and hot commissioning was done.

Thanks to the new actuator concept, about 400 active components of the additional oil hydraulic system were eliminated, which improved the emergency power load of the plant. Also, the simpler control reduced the number of control interlocks and the required in-service testing and inspection. The design of the new actuators has resulted in a low maintenance, with maintenance intervals that can be increased. This also contributes to a considerable cost savings. Space conditions at the valve’s location are also improved by the low installed height of the actuator. The whole modification will pay for itself within the next five years.

This example shows that with innovative new developments, plant operators and valve manufacturers with their own know-how are in a position to fulfill safety-technical requirements and lower plant operating costs considerably.