Valve actuators match valve motion

Initially, valves were simply on-off devices to control flow and were actuated by hand (Fig. 1). They evolved into modulating flow, which also was done manually and controlled by eye and feel. As plants grew larger and some process fluids became hazardous, valves began to be actuated remotely. Basically, manual actuators were provided with power sources other than a strong arm to enable them to...

09/15/2002


Key Concepts

 

  • Actuators must provide sufficient force to overcome valve friction

  • Valve design determines actuator motion

  • Valve application determines if an actuator must modulate or isolate

    Sections:
    Valves
    Actuator types
    Selection
    Accessories

    Sidebars:
    Actuator type and application


    Initially, valves were simply on-off devices to control flow and were actuated by hand (Fig. 1). They evolved into modulating flow, which also was done manually and controlled by eye and feel. As plants grew larger and some process fluids became hazardous, valves began to be actuated remotely.



    Fig. 1. The type of manual actuator used depends on valve design.

    Basically, manual actuators were provided with power sources other than a strong arm to enable them to operate remotely. Power sources included electric motors, pneumatic and hydraulic cylinders, and solenoids. The choice of power source depended on location, energy available, and the motion the valve required.

    Valves

    Since actuators are used to power valves it is useful to know what forces must be overcome in the process. Valve operating force elements can be broken down into seven main components:

    • Valve seal or packing friction

    • Valve shaft bearing friction

    • Closure element on seat friction

    • Closure element in travel friction

    • Hydrodynamic force on closure element in travel

    • Stem piston effect

    • Valve stem thread friction

      • Most of these elements are present in all types of valves, but in varying degrees of magnitude. For example, the closure element in travel friction for a butterfly valve is negligible, while for a lubricated plug valve it is significant.

        It is necessary to determine the magnitude of the forces required in operating a valve to be able to size an actuator properly. Data may be available from the manufacturer or determined by test.

        Another factor to consider when selecting an actuator is the type of motion it must supply. Valve motion is usually multiturn, quarter-turn, or linear.

        Actuator types

        Valve actuators are classified by the power used to move the actuator and the type of motion provided.

        Fig. 2. Electric motor actuators usually drive gears.

        Powered multiturn actuators are one of the most common configurations. In the electric version, an electric motor drives a combination of spur or worm gears, which drive a stem nut (Fig. 2). The nut engages the valve stem to open or close the valve. These actuators can quickly operate very large valves.

        The main advantage of this type actuator is that all accessories can be incorporated in one package that is physically and environmentally protected. The primary disadvantage is when power fails the valve remains in its last position.

        Pneumatic and hydraulic motor-powered, multiturn actuators are frequently used when electric power is not readily available.

        Powered quarter-turn actuators are similar to multi-turn actuators. The main difference is the final drive element is a worm gear quadrant that produces a 90-deg output.

        The electric version is very compact and can be used on small valves. Because of their reduced size, it is possible to have fail-safe operation by using a battery backup system.

        Pneumatic and hydraulic versions can also use other mechanisms to provide a quarter-turn. These include rack and pinion (Fig. 3), scotch yoke (Fig. 4), and lever (Fig. 5). A direct quarter-turn drive uses a vane to provide motion (Fig. 6).


        Fig. 3. Rack and pinion actuators usually drive gears.

        Both the rack and pinion and vane provide constant torque output, which is desirable for smaller valves. The scotch yoke and lever are used for larger valves where high torque is required at the beginning of the stroke.


        Fig. 4. Scotch yoke actuators have high starting torque.


        Fig. 5. Lever-powered actuators require two points.

        A primary advantage of pneumatic quarter-turn actuators is that a positive failure mode can be easily accomplished by using a spring return.


        Fig.

        Powered linear actuators can be electric solenoid or fluid powered. Electric solenoid actuators are usually used on small control valves and are common on pneumatic and hydraulic machinery.

        Pneumatic or hydraulic power is used with cylinders and diaphragms to provide linear motion (Fig. 7). Diaphragms used with springs can provide relatively linear response with respect to supply pressure. In some cases this can eliminate the need for a position indicator on the valve mechanism.


        Fig. 7. Diaphragm actuators can provide a high force from compressed air.

        Cylinders can operate at higher pressures than diaphragms, making them more powerful and compact. Cylinders are nonlinear in their response and require a valve position indicator.

        Selection

        Proper actuator selection is usually a direct function of the application. Applications fall into two main categories: modulating control and isolating.

        Modulating control valves are used to continuously regulate the flow of liquid to control a process. Typically, control valves are small, frequently globe-type, and have a diaphragm-type actuator.

        Modulating valves may operate up to 1200 times per hour. Isolating valves usually operate infrequently. The actuator should be selected to provide the operating frequency required.

        Another important consideration is the environment in which the valve and actuator will be operating. The environment has a direct affect on the type of enclosure used, not only for electric motor operators, but also for pneumatic and hydraulic actuators that have electrical components associated with them.

        Another consideration is whether or not emergency shutdown operation of the valve is required. Pneumatic linear actuators lend themselves to this requirement due to their simpler design. It is easy to store energy in a spring or fluid accumulator and use this energy to drive an automated valve into the open or closed position.

        Accessories

        Accessories can be added to valve actuators, which can protect the valve, provide fail-safe operation, permit positioning, and enhance automation.

        To protect the valve, limit switches turn off the motor or power source at the ends of travel. A torque sensing mechanism in the actuator switches off power when a safe torque level is exceeded.

        Position indicating switches can be used to indicate the open and closed position of the valve. Position sensors detect how far the actuator has moved and are used to control valve modulation. Frequently, a declutching mechanism and handwheel or lever is included so the valve can be operated manually if a power failure occurs.

        Plant Engineering magazine extends its appreciation to Rotork Controls, Inc. for its assistance in the preparation of this article and in providing the cover photo. We would also like to thank Emerson Process Management, LimitorqueCorp., Pepperl & Fuchs, Inc., and Velan Valve Corp. for their assistance in the preparation of this article.



        Actuator type and application

        Multiturn (globe, gate)

        Motor-powered (electric or fluid) gearbox

        Cylinder-powered rack and pinion

        Quarter-turn (ball, butterfly, plug)

        Motor-powered (electric or fluid) screw

        Vane

        Cylinder or diaphragm-powered:

        Rack and pinion

        Scotch yoke

        Lever

        Linear (globe, gate, pinch, diaphragm)

        Cylinder

        Diaphragm

        Solenoid

        Motor-powered (electric or fluid) screw



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