4 Valve Automation Considerations

4 Valve Automation Considerations

Aside from the control system itself, the most widely discussed component of any process system is the valve. Whether the application is a simple open / close manual valve, heavy duty flow control valve, or critical isolation valve, engineers and suppliers spend hours discussing, specifying and designing valve packages that can withstand to a multitude of factors for functioning. One component of that package that is equally critical, but sometimes overlooked, is the actuator.

The decision to automate a valve requires several considerations. Here are some, but certainly not all, factors that an engineer must consider when choosing an actuator:

  • application
  • control system diagram
  • reliability and criticality of assets
  • cost

1. Application

What are we trying to achieve in the system with this valve? Is it a flow control circuit, isolation valve, pump protection? What are the process factors, such as pressure, temperature and flow, required for the application? While these are generally taken into consideration when specifying a valve, they are still critical to understanding how they can affect the performance of an actuator.

For example, if an actuator is undersized or not specified correctly for the valve and process variables, it may not have the proper force to fully close the valve, resulting in unreliable process control. A typical multiple of the valve operating torques to ensure correct actuator operation is 25%. This difference between the torque requirement of the valve and the output capacity of the actuator ensures that the unit continues to operate correctly and provides a measure of protection against process changes.

Image 1: Type 14 diaphragm valve with A202DN PST pneumatic positioner.
Image 2: Diaphragm valve type 14 with PST 101 pneumatic positioner.

2. Control system diagram

Users should consider how the control system will interact with the actuator. They should also determine whether the actuator will use air or electricity to operate. Will there be a control signal in the form of a relay and solenoid output or a 4-20 milliamp (mA) signal to a controller? What feedback requirements are required by the actuator? For example, a requirement might be a feedback signal in the form of 0-10 volts to verify the valve position to the programmable logic controller (PLC).

3. Reliability and criticality of resources

Another negative result of choosing the wrong actuator is the loss of repeatability in the control. The wrong actuator can operate in an application for a short time but, if this valve malfunctions, there could be a process impact that could affect users or possibly the environment. Users should consider what action the unit should take in the event of a loss of control or power. Understanding how the valve, actuator, and all controls work together is critical to success. The various actuation technologies and the benefits they provide are critical to success. Also, review any legal requirements, codes, or standards that require a specific level of performance. Regulations and certifications may vary by industry and application.

4. Cost

While this is not the only deciding factor in making the most technical and economical choice, every engineer must consider the overall project budget when making the choice of actuator. While there is consideration of the initial investment cost, there is also a need to operate and maintain an asset throughout its life cycle. Depending on air or electrical consumption or maintenance and repair costs, you may lose a lower upfront cost advantage over time when trying to troubleshoot a problematic drive.

Each actuator must perform, at a minimum, the following functional purposes:

  1. The actuators process the control input to move the closing element of the valve, be it ball, disc or shutter, to the desired stroke position; they must be able to hold the latch in that position reliably until the control signal changes. In a pneumatic actuator, this is achieved by applying air to a diaphragm or piston to move the valve. Pneumatic positioners and electro-pneumatic positioners are common methods for converting small pneumatic or electronic signals for modulating service. An electric actuator receives a control signal of a change in voltage, current or resistance and responds by using an electric motor and gear train to convert the rotational force into torque or thrust. When a shut-off valve is fully closed or opened, the actuators must have the torque or thrust available to provide the desired shut-off and ratchet torque available to move the latch out of the seat. In pneumatic actuators, the amount of torque depends on the size of the surface receiving the supply air, such as a piston, and the pressure applied, typically measured in pounds per square inch (psi). Electric actuators use motor size and gear ratio to achieve output torque and speed.
  2. Actuators must operate the valve at the speed necessary to achieve optimal results for the process by reducing mechanical wear and ensuring the longest possible life cycle for the package. The application of a large amount of force quickly and repeatedly can cause damage to the valve and actuator if not specified correctly. In a pneumatic application, it may be necessary to increase the size of the actuator to ensure more power is available, or to use specialized control accessories such as positioners, solenoids, boosters or quick exhaust to increase cycle times. The speed of an electric actuator can be manipulated in several ways. Some actuators have electronic boards that manipulate the output speed while maintaining a constant torque. Other actuators must be assembled with the appropriate gear ratio and motor configurations, limiting this adjustment to purely mechanical.
  3. Actuators should have a certain level of safety in the event of a failure, whether in closing, opening or in position, and be able to hold that position until normal operation is restored. In a pneumatic system, this requirement has various levels of complexity depending on the safety requirement. The most common option is a spring return unit which forces the actuator to open or close in the event of a loss of supply air or control signal. Sometimes, additional accessories such as a storage tank, specialized solenoids, and pneumatic relays may need to be added to supply air to springless units or meet fault location requirements. In an electric actuator, battery or capacitor, failsafees are fairly common solutions. A battery, bank, or capacitor stores energy and supplies it when needed when power or control signal is lost. The mechanical tension of the motor and gear train then holds the valve in position until power is restored.

Engineers are looking for vendors to provide technologies and equipment that not only serve the basic purpose of process control in their facilities, but also more features and benefits that maximize productivity, increase results, and solve problems contributing to loss. of uptime and profitability.

.

Leave a Comment

Your email address will not be published.