Electric valve actuators are ubiquitous in today’s industrial space. They can be found in various industries and applications, including water treatment and wastewater treatment plants, hydroelectric power plants, refineries, shipyards, and many processing industries such as chemicals, food and beverage, pulp and paper, and pharmaceuticals. It is crucial for anyone responsible for selecting and operating these products to understand the valve force requirements for the application, available power sources, control, feedback, commissioning, safety, backup power, and fail-safe mechanisms.
Power Supply
Electric actuators must have a power supply available. It is essential to determine the type of power available on-site before selecting the actuator.
- Single-phase AC power
- Three-phase AC power
- DC power
This will determine the type of electric actuator chosen for the application. The power will operate the motor through a set of reversing contactors (located inside the actuator or in a remote cabinet). The power must be capable of providing the appropriate voltage and current required by the electric actuator to output the force needed to operate the valve. An electric actuator may be required to operate the valve as an on/off or modulating actuator; since the operating frequency is critical to the selection of electric actuators, ensure that the actuator’s manufacturer can operate in any of the above applications.
Control
The control device is the mechanism for operating the electric actuator, so the next step is to determine how to control the actuator.
There are two main types of controls:
-
Local Control
is directly mounted on the actuator.
-
Remote Control
is a system for operating the actuator remotely.
Most commonly used for on/off valves, it can also be used for modulating service. This is accomplished by sending open/stop/close signals to the actuator from the PLC or remote control station via buttons or switches (pilot devices). The control power is typically a 24VDC or 110VAC signal.
Remote control power can be provided on-site, or in some cases, the actuator is equipped with a control power transformer (CPT) that provides this voltage for the customer’s control system.
Analog control signals are shared with modulating valves. Analog control signals may also be referred to as positioning signals or set points. Most analog controls use a 4-20mA signal, where 4mA is fully closed and 20mA is fully open. A 0-20mA signal is typically not used because 0mA can be misinterpreted as a loss of signal. By setting the mA control signal to positions between 4mA and 20mA, the valve can be positioned between closed and open. This signal can be sent from the control room, or in some cases, it may be the output from a flow meter upstream of the valve used for flow modulation.
Digital bus communication can be used for on/off valves or modulating valves. Digital bus communication is also known as fieldbus or two-wire control, as the control signal communicates along wires connecting a set of actuators and a digital bus controller.
To use fieldbus communication, first determine the protocol to be used.
Some available protocols include:
-
Foundation Fieldbus
-
Profibus
- Modbus RTU (Remote Terminal Unit)
- Modbus TCP/IP (Transmission Control Protocol/Internet Protocol)
-
Hart
- Ethernet IP (Internet Protocol)
-
Device Net
-
Back Net
If communication is done via fieldbus on-site, ensure that the actuators can use the same protocol.
Some actuators can choose to operate using discrete inputs and fieldbus communication.
Feedback
Feedback is the signal from the actuator to the control system, also known as the output signal.
Analog Feedback The actuator can provide a 4-20mA signal, indicating the existing valve position to the control system, similar to the set point analog control signal. 4mA or 20mA can be set based on the application to indicate open or closed positions. If required by the application, some actuators can also provide torque feedback in the form of a 4-20mA signal, indicating the output torque of the actuator.
Feedback Signal Many actuators come with status relays available for customer use. The signal actuator has status relays available for the customer, indicating actuator position, torque failure, general failure, operational indication, or intermediate position. If it is a non-intrusive actuator (defined below), they will have relays that can be programmed to meet the customer’s feedback signal requirements.
Bus Communication Feedback Can be monitored through fieldbus signals, containing more signals than using discrete feedback capabilities. For example, this can include the actuator’s position or status, as well as the fieldbus system status, in addition to many other statuses depending on the fieldbus protocol.
Limit/Torque Switches These contacts can serve as voltage-free dry contacts to provide position/torque feedback status.
Visual Position Feedback Most actuators are equipped with physical position indicators that show the position of the valve and actuator; this can be in the form of a mechanical dial or screen indication. If the valve actuator is mounted on an additional gearbox, a pointer cover is typically provided to give visual position indication.
Commissioning
Commissioning of the actuator is needed to ensure proper operation of the valve and integration with the control system. Commissioning requirements include but are not limited to setting end positions, setting torque, confirming stroke counts, and feedback signals. Commissioning will depend on the type of actuator provided.
Intrusive Actuators
These actuators must have their covers opened and use tools to set limit switches and torque switches within the actuator.
Non-Intrusive Actuators
These smart actuators can be programmed using local controls. Other options, such as Bluetooth and laptop or mobile applications, may also be used for commissioning.
Safety
When it comes to electric actuators, different levels of safety protection can be provided to prevent accidental operation of the valve.
Physical Safety: The actuator can be equipped with lockable guiding devices, such as selector switches. Additionally, lockable covers can be provided to cover local control devices. It is also possible to install padlocks on handwheels/separation devices for manual operation of the valve.
System Safety
- Bluetooth: Can enable/disable Bluetooth, password protection, or completely eliminate Bluetooth.
- Allow Signal: When setting up the control system, an allow signal can be sent to the actuator before electrical operation occurs. This can be used to ensure that the valve does not operate accidentally. Interlock Signals: Provide allow signals to the actuator from separate actuators or other devices within the facility. This interlock is used if opening or closing the valve requires a specified state from elsewhere in the facility.
Backup Power
Backup or emergency power may be crucial in the event of a main power supply interruption on-site. It must be decided what type of actuator operation is required in case of main power loss. Does the actuator still need to operate the valve normally or place the valve in a safe position?
These options can operate the actuator and/or place the actuator in a safe position during a main power outage:
Uninterruptible Power Supply (UPS): If a solution is needed for the actuator to operate normally during a power outage, a UPS backup power system may be a good choice for the actuator. This will allow customers to open or close valves during an emergency power outage.
DC (Direct Current Power): The actuator also has versions with DC voltage. If there is backup DC voltage available on-site, these power sources can be used to operate the actuator during an emergency power outage.
Fail-Safe
Actuators can be equipped with a device to place them in a safe position. This device typically uses springs or other technologies to position the actuator in a specified safe position, and the selection of these devices should consider the force required by the valve.
Usage Considerations
Power Source
- Ensure that there is available power and current on-site to operate the provided actuator.
- Keep power cables separate from signal cables.
Control Type
Ensure that the provided actuator matches the control devices available on-site.
Feedback
- Ensure that appropriate application feedback is provided for the actuator.
- Ensure you know who is providing the analog feedback power.
Commissioning
- Always hire trained professionals familiar with the provided equipment.
Safety
- Keep track of changes to actuator passwords. OEM may be able to reset the password to factory defaults.
Backup Power
- Ensure that the actuator and backup power are compatible (voltage, current, etc.)
Fail-Safe
Ensure the actuator is equipped with the required fail-safe position. The fail-safe position may not be changeable after delivery.
Electric actuators play a crucial role in the safe and efficient operation of valves. Understanding the various elements (as discussed here) allows users to successfully define, set up, and operate basic control systems.
