Chapter 9 Actuators Control Automático (3º Ing. Ind.) Dpto. Ing. de Sistemas y Automática Universidad de Sevilla
Actuators Electric motors DC motors AC motors Valves Butterfly valves Globe valves Ball valves Valve actuators Membrane Piston Electric
Electric motors DC Motors Driven by DC electric power Easy to change rotation sign Wide speed operation range Widely used in servomechanisms AC Motors Driven by an alternating current Need a variable-frequency drive A system for controlling the frequency of the electrical power supplied to the motor
Valves A valve is a device that regulates the flow of substances (either gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valve parts: Body and internal parts to regulate flow Actuator or servomotor to modify the state of the valve Pneumatic controlled valve
Butterfly valves A flat circular plate is positioned in the center of the pipe. The plate has a rod through it connected to an actuator on the outside of the valve. Rotating the actuator turns the plate either parallel or perpendicular to the flow. Advantages High flow Low pressure drag Wide temperature operating range Small size Low price Disadvantages: Needs high power actuators
Ball valves Opens by turning a handle attached to a ball inside the valve. The ball has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. Advantages Excellent control for high viscosity, erosive fluids High flow Disadvantages: High price Not appropriate for all fluids Generates signal noise
Globe valves Globe Valves are named for their spherical body shape with the two halves of the body being separated by an internal baffle. This has an opening that forms a seat onto which a movable plug can be screwed in to close (or shut) the valve. Advantages: Wide family of valves for different applications (Flow limits, fluid properties,...) Disadvantages High price Large pressure drops
Globe valves
Globe valves Different glove valves: Cage: Holed body Membrane: High corrosive fluids Three way valve: Used to separate a flow in two, or to merge two flows in one
Actuators Pneumatic diaphragm actuator Most used in industry Air pressure moves spring to control opening Closed at no pressure Open at no pressure Security issues Advantages High reliability Low price Disadvantages Small force Cannot be applied to valves with a large range Diaphragm actuator: fail closed, air-to-open. Diaphragm actuator: fail open, air-to-close.
Actuators Piston valve Control the motion of a fluid along a tube or pipe by means of the linear motion of a piston within a chamber or cylinder. Advantages: Ability to deliver high force High response speed Able to control large range valves Disadvantages: Need of a servomechanism
Actuators Electric (Electro-mechanical, electro-hydraulical) Advantages: No need of an pneumatic system Low energy consumption Low installation and maintenance cost Operated by electric signals (no need of transducer) Disadvantages: High price Need of electromagnetic protection Risk of explosion Need a servomechanism Low force Slow response
Valve opening regulator Closed-loop control of the opening position of a valve to counter disturbances and errors in the system Position sensor Controller Electro pneumatic transducer
Control valves properties Data needed to chose the appropriate valve: Fluid properties Pressure before the valve Pressure drop in the valve Flow Leakage properties Flow/pressure ratio with respect to the opening of the valve (%) Iso-percentage Linear Open-Closed FLow % max. Opening % max