Valve actuators deliver either linear or rotary force. It’s important that the force they provide to the valve is sufficient to operate it effectively, but not so great the seats or stems of the valve might get damaged.
For that reason, almost all actuators have some form of force-limiting mechanism.
TYPES OF MECHANISMS
On most applications, the force output of the fluid power actuator is limited by the system pressure. In almost all cases, this system pressure is controlled by a pressure regulator.
The typical electric actuator has an internal torque-limiting mechanism. This is to ensure sufficient force for seating the valve with prevention of excessive force that could gall the seats or cause the valve to jam.
For actuators with worm and wheel gears, the axial force reaction of the worm shaft against the worm wheel is directly proportional to the output torque of the actuator. The worm shaft moves in proportion to the force against a spring. Alternatively, some devices use an electronic pressure sensor to measure the force reaction on the worm shaft.
Electric motor current measurement or motor speed is also an indication of the level of torque generated.
Once the predetermined force or torque limit is reached, a torque sensor is tripped, and power is shut off to the motor. The predetermined force can be adjusted so that one size of actuator can be used on different valves or applications.
Position sensing is important for two reasons:
- To give feedback on the position of the valve. This could be transmitted to the remote control room or displayed locally on the actuator.
- To allow the valve to be seated correctly.
Some valve designs require a specific amount of force to seat the closure element sufficiently so that the pipeline medium cannot pass. Typically, these valves are wedge gate, globe or triple offset butterfly valves.
Other valves, such as most quarter-turn valves (ball, plug and resilient- seated butterfly valves), along with some slab or knife gate valves, are designed to seal at a certain position.
To ensure proper seating on these “position-seating” valves, the actuator must move to the correct position and stop. One way to achieve this is to have a mechanical stop in the actuator. This often is provided on electric or fluid power actuators for quarter-turn valves, but position sensing is still needed as an indicator. That sensing is usually achieved by a direct drive from the part-turn actuator output shaft to switch trip mechanisms such as cams or levers. In addition, a potentiometer or a 4-20 milliamp transmitter can be driven from the same shaft to give continuous remote position indication.
For operational purposes, the actuator must always know where it is relative to the valve position. Because of this, a position-measuring system is mandatory in multi-turn actuators for position-seating valves. A mechanism driven from the output of the actuator is frequently used.
The multi-turn actuator needs a versatile counting mechanism to accommodate the variety of output turns that different valves require.
Some mechanisms count the output turns of the actuator using a geared rotating counter mechanism similar to an odometer but bi-directional. Another method uses a rotating threaded shaft on which a nut travels in proportion to the actuator output turns. Both types of mechanical drive mechanisms have a finite number of output turns they can accommodate.
The more common method is to use an electronic encoder that can measure a much greater span of output turns.
The two types of encoders in general use for valve actuators are the absolute and the incremental. The incremental encoder will count the number of turns from a set position established in its memory during set up. It can only count when powered; if an incremental encoder is powered down and the actuator moves, it loses position reference. As a result, a backup battery is usually incorporated into the design to power the encoder and the processor in case the actuator loses main power.
The absolute encoder works differently. It doesn’t need to be powered when moved because each position has a unique signature that can be read at any time. It does not rely on an incremental count to maintain position reference.
The position-sensing mechanism is used to trip the motor power at the ends of valve travel on position-seating valves.
All types of actuators need some kind of direction control to move the valve in the opening or closing direction. For electric actuators this usually takes the form of a motor starter. For fluid-powered actuators, a direction control valve is used.
In both cases, an external control signal is used to energize the coils of the motor starter or the solenoid coils of the direction control valve.
Control wires connect the control room to these directional controls and are often 110VAC or 24VDC.
For electric actuators, an important decision is how the motor is controlled. Two types of motor control layout are:
- A separate motor control center (MCC) that contains the motor starters for the actuators in one central location separate from the valves. This configuration is often used when actuators are located in hostile environments.
- Motor starters integral to the actuator located at the valve. This configuration provides a simpler and often less expensive installation.
The separate MCC most likely originated in the power industry where motor starters are typically grouped together. The control wiring runs from the control room to the MCC with feedback wires from the valve actuator position-indicating and torque-sensing switches. The power cables run from the MCC to the individual actuators.
Advantages of the MCC layout for actuator control include:
- Motor starters are all located in a single area for easy maintenance.
- The motor controls are removed from vibration, steam, dirt, water and other contaminants.
- The actuator requires a smaller space envelope and has less weight.
Advantages to the integral control method include:
- Torque and limit switches can be wired in the actuator allowing the unit to be self-contained and factory tested.
- Less site wiring is required.
- Automatic phase correction can be incorporated in the controls.
- A digital field bus link can be employed to reduce the multiple control and indication wires and also bring back diagnostic information.
The MCC layout was the global standard in 20th century power plant design and is still seen in water treatment facilities and power plants of German design. In contrast, integral controls and starters are the preference in the oil, gas, petrochemical and most other industries.
Most manufacturers of electric actuators have product designs to accommodate either configuration.
The sensors and controls described in this column are the fundamental requirements needed in a modern valve actuator, whether fluid or electric powered. There are many refinements and additional controls available to enhance performance and protect the automated valve assembly. However, without this fundamental control of force and position, the valve actuator cannot perform effectively.