Floyd Bensinger, Flowserve, USA
Main steam isolation valves (MSIVs) are clearly critical safety equipment in nuclear plants. Their sole function is to close the steam supply system in the unlikely event of a steam pipe rupture downstream of the MSIV. This action supports the heat removal from the nuclear-fueled reactor.
Today, these valves must close quickly, typically within three to five seconds, and must operate successfully under all plant conditions including emergency situations, even if all normal power supplies fail.
This critical valve application has evolved since the beginning of commercial nuclear power generation. Initially, MSIVs were Y-pattern globe valves with electric motor actuators. As plant technical design criteria became more demanding and required operational times to be reduced, electric motor operators could no longer close the valves fast enough and pneumatic actuators with mechanical springs were used in their place.
Eventually, newer plants took advantage of the more efficient flow characteristics of gate valves in place of Y-pattern globe valves. This created the need to use gas/hydraulic actuators to actuate the gate valves.
Quickly and reliably closing MSIVs, which can stand close to 5 metres tall and weigh more than 18 metric tonnes, is no easy task. It requires several hundred-thousand pounds of force to be delivered instantaneously, without fail. Stored energy actuators remain the most cost-effective and reliable solution for actuation to handle this application.
MSIVs rely on low-pressure sensors on the steam line to trigger the MSIV closure signal (control signal to the gas/hydraulic actuator solenoid valves). This electrical signal causes the release of hydraulic fluid in the rod end of the actuator, allowing the high-pressure stored energy of springs or compressed gas to drive the MSIV closed.
For more than 30 years, gate valves equipped with gas/hydraulic actuators have been the safest, most reliable and cost-effective way to close MSIVs without depending on an external energy source. Their small size – compared to other available designs – translates into lower installation, operation and energy costs.
In addition, industry tests have shown gas/hydraulic actuators are capable of performing safety-related functions under harsh conditions, such as high-radiation exposure, earthquakes and potential plant accident environmental conditions, resulting from a loss of coolant or a main steam line break.
The Gas/Hydraulic Concept
Gas/hydraulic actuators rely on high pressure to open and close the valve. The actuator cylinder rod end is pressurized to open the MSIV and depressurized to close it. The depressurization of the rod end is controlled by the hydraulic circuit to regulate the rate or speed of MSIV closure.
The cap end of the cylinder contains a high-pressure gas accumulator. On the most recent designs, this accumulator is integral with the cylinder and, when filled with gaseous nitrogen, provides the stored energy to drive the MSIV shut, when required.
To close the valve, a quick-release hydraulic circuit discharges the fluid from the cylinder rod end to a reservoir, permitting the stored energy in the pressurized gas of the cylinder cap end to extend the actuator. To open the valve, a hydraulic power unit pumps fluid to the rod end of the cylinder to drive the MSIV open and fully pressurize the nitrogen in the cap end accumulator. While the concept is simple, the principle has to be delivered in a way that satisfies both the reliability and redundancy requirements of a nuclear power plant.
The first requirement is that stored energy must always be available. Gas stored in remotely mounted accumulators could be unavailable when needed, especially if a leakage occurs in the flexible connections between the accumulators and the valve. Even if accumulators are mounted on the valve, the connections between accumulators and the actuator cylinder could be vulnerable to leaks or failures.
As a result, the most efficient actuators have a built-in stored gas volume. In this instance, gas is stored in an accumulator coupled with the cap end of the actuator cylinder. Besides eliminating potential leaks, this design stops pressure losses during the stroking of the valve, which ensures quick closure. Whenever the MSIV is open, the stored energy to close the MSIV is always available.
Small Size Reduces Cost, Increases Safety
In the past, some nuclear plants used large, low-pressure actuators to close the equally large valves (typically size 16 to 32). These large actuators were expensive to purchase, install and maintain. In contrast, using smaller, high-pressure stored energy systems to actuate valves minimizes costs. They have a lower purchase price and are more energy efficient, while being more reliable than earlier MSIV actuators.
Actuator performance during an earthquake is paramount to ensuring plant safety. Previous actuator designs were challenged to meet stringent seismic requirements due to their large size and piping connections. The small size and integral design of modern gas/hydraulic stored energy actuators allow for reliable performance, even during a seismic event, without sacrificing the power needed to reliably close the MSIV.
Redundancy Increases Safety
Redundancy of safety-related equipment is necessary in actuator design to ensure reliability. No credible, single component failure can prevent the closure of the MSIV. Redundancy is applied to the hydraulic control system and components of the MSIV actuator to ensure the actuator will open and close the valve when the control circuitry gives the appropriate signal.
Redundancy is provided through two separate hydraulic manifolds, mounted on opposite sides of the actuators, containing identical sets of electrical and hydraulic equipment to ensure release of hydraulic fluid from the actuator cylinder rod end to the reservoir.
Flexibility Solves Customer Problems
Because the specific requirements of power plant valve applications vary, the control logic and instrumentation for actuators must be flexible. Many users require a fail-safe arrangement such as a solenoid valve that will open automatically during a power loss.
Other users want to avoid potential, inadvertent MSIV closure, so these nuclear plant operators request systems requiring a positive power signal or additional controls redundancy to initiate main valve closure. In such cases, the customer must assume part of the safety and redundancy requirements and furnish duplicate external power busses and control circuitry.
Instrumentation could include pressure switches, transducers or transmitters to monitor actuator pressures, and position switches or proximity switches to monitor the MSIV position.
Continuous Design Improvements Ensure Safety
Because the operating conditions of each nuclear plant are unique actuator/valve suppliers can help nuclear plant operators identify any custom specifications that might be necessary to ensure best total reliability at their facility.
Gas/hydraulic stored energy actuator design is being constantly improved to reduce maintenance, increase performance and functionality, support new nuclear plant designs, and comply with changing nuclear environmental, safety, and functional standards.
Gas/hydraulic stored energy actuators help modern power plants meet their goals of increasing plant safety, plant availability and decreasing operational costs.
The actuators can close MSIVs during an emergency even when an external power source is unavailable. Redundancy applied to the hydraulic controls and components and improved performance during seismic events give plant operators confidence that the MSIV and its actuator will perform when required. The relatively small size of the device helps trim costs because of its reduced energy consumption and lower initial investment.