Actuator Options for Process Automation
The process industries have control valves with actuators designed to open, close and control them. Many people prefer pneumatic actuators given their long history, reliability and high performance. Where an air supply isn’t available or realistic for pneumatic actuators, an electromechanical actuator is an option. However, those tend to not perform as well as the pneumatic options because they are known for either operating much slower than pneumatics, or over-shooting. Many of them also degrade in their performance over time as their gearing wears. They typically use little motors and a significant amount of gearing to reduce cost and package size, unless one specifies the top tier models that are usually several times more expensive. Then there are electro-hydraulic options, which tend to be bulkier, require higher maintenance efforts and are much more expensive, but offer high torques/thrusts and excellent performance.
Interestingly enough, in the discreet automation world, a precision motion control world of robotics and pick and place gantries, the technological evolution and expectations are much different. Pneumatics are the least expensive option with the lowest expected performance. Hydraulics have their place because of their extremely high-force to package-size ratio. However, both are fraught with their various leak, maintenance, and performance limitations, causing people to often want to go “all electric” if they can. In this world, if you want high performance and low maintenance, electromechanical is high on the list of options.
Back to the process industries, if the typical available solutions don’t meet a given application’s performance requirements, it is important not to let experience bias prevent using solutions from a different world. If that valve’s pneumatic actuator’s ¼ percent - ½ percent, accuracy isn’t good enough, then an electromechanical solution from the world of discreet automation may be the answer. A ¼ percent accuracy on a valve actuator with a 200mm stroke works out to an accuracy of +/- 1mm. This is a very easy target for precision motion control products where +/-0.05mm is a typical performance specification.
Process systems usually use 4-20mA analog or hybrid HART/4-20mA control signals. 0-10vdc is more typical of either HVAC or commercial systems, or manufacturing floor controllers. Some newer systems use fieldbuses like Foundation Fieldbus or Profibus. If the control system uses an Industrial DCS, there is usually a gateway to typical PLC protocols like Modbus or DeviceNet available. Serial interfaces are rare.
Existing electromechanical valve actuators are typically controlled by analog signals, 0-10Vdc or 4-20mA, from a control system. These same signals can be used to control electromechanical servo-motor actuators. However, 4-20mA is a much less common signal interface than the 0-10Vdc option. Analog signals also tend to drive up the cost of these controllers because of the Analog-to-Digital chips and the more sophisticated controllers required to handle them. There are options available that are more cost effective, easier-to-use, and probably higher performing. These can vary from bus-based interfaces like Profibus, DeviceNet and EtherCAT to serial-based ones with RS232 or RS485 to typical motion control signals like Step & Direction. The motion can even be pre-programmed for move profiles or positions and simply triggered with discreet inputs. This could be something like Input #1 sends it to 10 percent closed, Input #2 goes to 20 percent closed, etc. However, it is very important to understand that these options are more difficult to retrofit into an existing system controlling typical valve actuators.
There aren’t just linear electromechanical actuators that are controlled by servo motors. They can be rotary as well. Even simpler though is just using a rotary servo motor as a rotary actuator for something like a ball valve. The rotary servo motor wouldn’t need a mechanical actuator and would be coupled directly to the stem of the rotary valve. To some people, the motor would then be the actuator. In the discreet automation world, a “motor” and an “actuator” denote two different components of the system. As mentioned above, the words and technologies may be the same, but the meanings and connotations may differ.
Typical ApplicationsWhat applications would benefit from electromechanical servo-motor controlled actuators? Any application where the customer would receive financial benefits from higher performance. These benefits could be higher accuracy, better repeatability, more controlled motion, or lower maintenance that result in an improved bottom-line performance. These are probably going to be critical applications needing tight control with expensive ingredients like the chemical industry with their liquid pressure and pH control applications. Flow control applications may experience huge benefits. There are even refiners that found the higher performing solutions so critical to their applications that they purchased full sets of spares so they never would be without a working system.
Specifying the Right SolutionTrial-and-error is a common method for product selection in some industries. Try an actuator and if it doesn’t work, try a larger one. In the discreet automation industry, trial-and-error isn’t an effective method due to the cost and lead times of these high-precision products. Proper sizing and selection is the preferred method, but this can mean digging up or guessing at information that isn’t readily available. An alternative, of course, is grossly over-sizing the solution, but then this makes it more expensive than otherwise needed.
Most valves have seating or unseating loads that are higher than the load of actually moving the valve. The discreet automation world may refer to this as “stiction”, a non-technical term referring to sticky friction which is the force required to break the mechanics free of a static location with no motion at all. This unseating load could be covered under a servo motor’s “peak” torque range while the torque to otherwise move the valve could be covered under the same servo motor’s “continuous” torque range. Since globe valves typically require a ‘seat load’ to meet the shutoff standards required, the servo motor would need to be properly sized to hold that specific thrust level indefinitely. A brake can also be used very easily on the motor to hold that position which could potentially reduce the needed size of the motor. It certainly would reduce the power consumption of the motor and its controls.
Environmental ConsiderationsValve actuators are often protected in housings sealing them against the outdoor conditions. Servo motors can be and are designed for harsh conditions ranging from washdown IP65 to cryogenic and submersive conditions. But these are special conditions and need to be specified so don’t assume anything. The right housing with the right connectors with the right feedback device on the servo motor will meet most of these requirements.
Manual Over-RidesAnother easy assumption to make would be that electromechanical servo-motor controlled actuators will include manual over-rides. This is certainly possible but isn’t the default design of these actuators. Manual over-rides are so commonplace on valve actuators that people assume they will be included with any actuator. The solution is simple with an extended shaft on the back of the motor with a hand-wheel mounted to it. It just isn’t commonly used in the Discreet automation world so make sure you specify it if you need it.
One can always shotgun and approach and try a solution. Perhaps this will save time, and therefore money, if guessed correctly. Alternatively, one can take the time to do it correctly and be sure to take all the requirements into consideration. After all, there are many options and points one must consider in order to improve his or her company’s bottom line.
Article featured in Processing Magazine
You were tireless in your support and it will not be forgotten!
Latest from Valin's Blog
The NIST Chemistry WebBook contains a great deal of information regarding the properties of a broad range of chemicals and is helpful for those who deal with chemical processes.In this article, Jon Monsen has outlined the procedure for finding the actual density of a gas using the WebBook.