Young & Franklin develops a new torque matching valve actuator

by Weijian Ma and Gabriel Silva

As industry moves towards the use of low cost, low Btu synthetic gases for power generation, larger valves are required to operate the turbines. These larger valves, typically configured as quarter-turn ball or butterfly, have high torque requirements that can range from 3,000 Nm to more than 30,000 Nm. High torque actuators currently available to control these valves are both large and expensive. Actuator size is a product of both the high forces required to operate the valves and code requirements which require a mechanical (spring) close to ensure fail-safe closure of the fuel valves when there is a failure in the control signal or loss of hydraulic pressure.

Confronted with the challenge of designing a compact, cost effective actuator to operate 406 mm high torque valves for a new SYNGAS installation, Young & Franklin has incorporated an innovative mechanical variable torque intensifier (patent pending) to a linear actuator so as to match the actuator output to the derived torque curve requirements of the valves. As an added safety feature, it also provides a mechanical lock at the close position.

Standard Linear Actuator Design

Typically, fail-safe hydraulic actuator designs for industrial gas turbines include a servo-valve, a single acting hydraulic cylinder, mechanical spring(s), a trip valve, an LVDT or RVDT for position feedback and a hydraulic filter. The hydraulic cylinder is mounted on, and integrated within, a spring housing which attaches to the valve. The cylinder rod is connected to a linkage through a coupling and spring guide plate structure to the valve shaft, thereby converting linear motion into rotary motion.

The valve is controlled by an input signal to the servo valve. The servo valve directs the flow of hydraulic fluid to the cylinder to extend/retract, compress/decompress the spring, and open/close the metering valve. In normal operation, the emergency trip valve is energized, and the servo valve controls valve position in conjunction with the LVDT or RVDT. When the trip valve is de-energized , the cylinder port opens to return, the cylinder loses pressure and the compressed spring extends the rod and closes the valve. The servo valve is also biased to direct the cylinder rod to extend in the event of loss of the command signal which, in conjunction with the spring, closes the valve.

The typical servo actuator linkage geometry is basic, and provides easy coupling and adjustment. However, the standard quarter-turn valve arm linkage provides maximum torque at mid-stroke instead of at valve opening or closing where the valve's torque requirements are the highest.

New Design

The inefficiency inherent in sizing the actuator to meet the valve's highest torque requirement throughout its stroke is substantial for high torque valves. Finding a way to match the output of the actuator to the torque requirement of the valve throughout its stroke would make it possible to control the valve with a smaller actuator and spring than conventional designs. The solution lies in the linkage mechanism. A special torque modulating mechanism was designed by Young & Franklin that provides maximum torque at valve opening or closing. The torque modulating mechanism is composed of a primary linkage A, a triangular crank arm (cam), a secondary linkage B, and a standard valve crank arm. When the hydraulic cylinder extends, linkage A is forced downward, rotating the triangle crank arm in an anti-clockwise direction. The crank arm then forces the linkage B from the open position to the closed position. The torque produced by the constant hydraulic force of the cylinder translates through the valve crank arm from an initial full opening value to the maximum torque required by the value at its closing position.

Three additional benefits of this approach are:
(1) An inherent mechanical lock in the closed, de-energized position. At close, the triangle crank arm's rotation center is in line with the linkage B, creating a "lock point", which mechanically holds the valve closed independent of the fail-save spring force. This guarantees a tight and stable close.
(2) There is more accurate control and system stiffness in the near close position. In this position, linkage A travel is a multiple of the distance traveled by the valve crank arm, so small variations of actuating stroke produce no noticeable valve crank arm area change. This ensures the most accurate control of the valve in this critical area.
(3) The inherent deceleration of the mechanism at closing reduces inertia effects and valve shock.

Valve Actuator Sizing

A torque matching actuator was designed for, and tested using a 406 mm line size butterfly valve with the following specifications: unseating and seating torques of 3028 Nm; hydraulic system pressure of between 83-117 bars; and a fail-safe trip time of < 0.5 seconds.

The manufacturer's listed valve actuator requirements were a torque of 3028 Nm during both seating and unseating operations (0° angle) and a minimum torque of 1170 Nm at the 70 angle of the valve's rotation. Estimates were made from valve technical literature that for the 30° and 90° angles of rotation the valve torques are 40% and 15%, respectively, of the maximum valve seating torque. The dimensions for the new linkages, geometry and spring size were then derived to meet these design requirements.

Test results showed that the designed actuator satisfied the valve torque requirements with a torque margin in excess of 35% during the 0° - 90° valve rotation. The test results also showed that the torque increased exponentially at closing, as predicted by computer simulations.

Qualification

The torque matching valve actuator was tested for more than 55,000 cycles with a hydraulic loading device simulating valve load. The actuator showed no degradation in its performance throughout the test. Other tests such as frequency response, hydraulic slew rate, and trip time were also performed. The results show very low hysteresis, a trip time below 0.5 seconds, and slew rates below 1.0 second.

Application

The torque matching actuator was selected for nine stop ratio and control valves to be used in a recent alternative fuel/low Btu project by a major gas turbine manufacturer. All units were tested and performed per specification. They will be installed for the gas turbine by the end of 1999.

Applications for the torque matching actuator can be extended to torque requirements up to 30,000 Nm by simply varying the mechanism geometry and actuator sizing. Additional merits of this design include a choice of cylinder stroke orientation, ease of assembly and disassembly, and mounting of the fail-safe spring. This robust and innovative actuator is an evolutionary extension of a time proven design used for decades by major land-based gas turbine manufacturers. It incorporates the same basic components as a typical actuator, and it operates in the same manner, but is designed to be a compact and cost effective answer to the problem of high torque valve control.

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