How to select electric actuators?
Electric rotary actuators are basically geared motors with a variety of electrical accessories (limit switches, heater, circuit boards) that may be specified to meet the exact valve automation requirements of a process control system. As an electrical device it is housed in an enclosure designed for specific area classifications (NEMA Rating).
The following guide is for electric actuators and controls only. It is assumed that the valve to be automated has already been selected based on its own unique ability to handle the service for which it is intended.
A complete valve specification for which an automated package may be specified should include:
· Valve Size
· Valve Type
· Valve Manufacturer
· Model #
· Valve Torque (In-Lb)
· Line pressure (Psi)
· Media Temperature (°F)
· Media (Slurry, Gas, etc.)
The most important step in developing an automated valve specification is to determine a sizing criteria. If a valve is to operate in a process handling clean liquids at moderate pressures and temperatures, the manufacturer’s published operating torque is usually adequate for actuator sizing. Under certain conditions, however, the torque required to operate a valve may increase. In this case a sizing safety factor may need to be applied based on the guidelines
A. Specification Checklist
· NEMA Rating (NEMA 4, NEMA 7 or NEMA 4X)
· Motor Voltage: 12, 24, 115, 220 VAC or VDC
· 3-phase specify voltage
· Compartment heater
B. Torque Switches
Torque switches are recommended on electric actuators with an output torque of 800 inch pounds and higher to protect the automated valve and motor. They are also necessary for tight closure of torque seated valves.
· Torque switch: Open or Close
C. Manual Override
A manual override is designed to provide manual valve operation in the event of electrical failure or for start-up and setup procedures. The handwheel should not rotate during electrical operation.
D. Separate Controls
· Local/Remote Switch
· 3 Position Rotary Switch (open/stop/close)
· 3 Pushbuttons (open/stop/close)
E. Electronic Speed Control
As a geared motor an electric actuator has a fixed speed of output rotation that is a function of the layout of the gear train. Some manufacturers offer their products with a rotary output speed of 2 seconds but most are in the range of 5 to 25 seconds for 90° depending upon the model. If a cycle time greater than what is available is necessary an electronic speed control should be specified.
· Electronic speed control
· Speed of operation sec/90°
F. Position Transmitter
A position transmitter provides continuous feedback of valve position.
· Slide wire (potentiometer)
· 2-wire 4-20 maDC output (integral power supply)
· 4-wire 4-20 maDC output (requires a power supply)
G. Limit Switches
Limit switches are used to provide the necessary number of electrical contacts in the open, close or intermediate position. Standard electric actuators have two SPDT limit switches, one for the open position and one for the closed position to control rotary travel as desired.
Specify the number of additional contacts required in each position:
· Open (make & break)
· Close (make & break)
· Intermediate position: rotary position in degrees (make & break)
Positioners are used with electric actuators to position the valve in response to a control signal.
· Input signal (maDC)
· Position transmitter required?
I. Control Options for
One of the most appealing characteristics of electric rotary actuators is its inherent versatility.
Electronic control cards, switches and wiring are easily re-configured to meet the control system needs.
· 3-position control package can provide 0, 90, 180 rotation for multi-ported, 3-position valves.
· mid position stop from both / either extreme. If either, then from: CW & CCW
Integral Local Control Station
· mounts directly to actuator housing and includes Local / Remote switch plus open/close switch for local operation.
· Options: Key lock, Open/Close lights
· Interposing Relay
· Plug & Socket Connector
· DC Motor
· Battery Backup for Fail-Safe Operation
· Special Coatings