Valve Magazine - Fall 2011

Much testing and precise quality control goes into manufacturing nuclear actuators. Here, a highly trained quality control inspector is performing a quality test on two control packages for a nuclear pneumatic linear actuator.

be used for an application.

When a qualified actuator with the appropriate pedigree is mated to an “N” stamp valve, the integral unit must pass one more important test. This test is to assure the “N” stamp valve combined with the approved attached actuator has sufficient integrity to withstand difficult seismic requirements. The test is governed by another standard known as QME- 1. Although it is essentially a valve test, the actuator is an important component so it is part of the testing.

technology can only be used in applications that do not require active testing.

ENHANCEMENTS UNDER
STRINGENT TESTING

Because motor operators are very simple with just a motor, gear set, and limit and torque switches within robust cast iron and steel housing, some advanced options are available that can assist the nuclear industry in certain applications.

For example, several major actuator manufacturers offer a thrust monitoring component so the nuclear plant can

WHEN AND WHERE
EQUIPMENT CAN BE USED

As explained, only the most rugged actuators manufactured today are suitable for use inside a nuclear reactor. When equipment is not part of the safety system and it is outside of the reactor, standard industrial equipment can be used that takes advantage of some of today’s newer technologies.

An example is smart actuators. The modern electric actuator used in many process plants today consists of an integral motor control center with a digital control module. That configuration allows remote monitoring of valve and actuator data useful for maintenance and diagnostics. Unfortunately, the technology of today does not allow these sophisticated electronics to reside within an actuator or to pass the environmental and seismic testing required for inside the reactor. Actuators with advanced

monitor valve thrust requirements and actuator thrust outputs. While that capability is a standard item with a smart actuator, it’s not as simple with the nuclear actuator. The typical way this capability is achieved in a nuclear actuator is by measuring the deflection of the motor shaft via the spring pack on the end of the motor.

Another example of a nuclear option becoming very popular is the torque limiter. When sizing an actuator for a valve in a nuclear application, the criteria are different than for most other applications. The valve factors used to calculate the thrust required are very high so sufficient safety margins are ensured. The safety margins are set by the industry and monitored very closely.

Another factor that comes into play is power voltage. The actuators must be sized to work on reduced voltages that might be available during accident conditions. When a motor is running on reduced voltage, it produces reduced torque. The combination of these challenges means an actuator might be much larger for a valve than normally used.

Motor operators are also very powerful and have a reserve torque and thrust capability. The industry calls them stall torques, and they can more than double the rated output torque of an actuator. Because of the demands, the associated valve must have a stronger superstructure as well as a larger stem than what might be expected. The larger stem then might require a larger thrust, and the exact balance becomes a challenge for specifying engineers.

The torque limiter is a brake that will reduce the force put on the valve during abnormal operation so that the valve does not need as large a structure to hold the actuator. Another option used for high-speed valve applications is thrust compensators, which are essentially disc spring packs that allow for a gradual loading of the actuator as the valve seats.

This self-contained, linear hydraulic actuator is designed for a nuclear plant’s main steam isolation valve or main feedwater isolation valve. It uses an electric motor to turn the hydraulic pump, which generates the fluid power pressure and flow to extend and retract the main hydraulic cylinder. It is coupled to the isolation valve to safely shut down the plant.