A valve actuator is a mechanism that opens and closes a valve. By definition it seems simple but in reality there  is nothing simple about it. There are two basic valve operating designs: linear and rotary. Therefore, basic actuator designs are linear, rotary and sometimes rotary linear. Typically, rotary-operated valves provide the best overall value in valve automation.

Valves that need to be opened, closed or throttled frequently are popular candidates for pairing with an actuator. Actuators are attached or built into a valve body to automatically adjust the flow of fluids, gases, steam, solids or slurries through a valve. If you’re in the market for an actuated valve, there are many things to consider. First, there are several types of actuators: pneumatic, electric, water hydraulic, oil hydraulic and self-contained electro-hydraulic (oil). The two most common types are pneumatic and electric. These technologies are very different and understanding the benefits and limitations of each one is critical. In general, it’s important to consider the application, location and intended functionality.

Pneumatic

Pneumatic actuators typically provide high force and speed in a smaller footprint than electric actuators depending on available air pressure supply. The force and speed of these actuators are semi-independent from each other, the greater the force required, the larger the actuator size and therefore, slower operating speed ranges. These ranges are adjustable with air throttling mechanisms to accommodate different speed requirements. 

There are several different pneumatic actuator designs, including diaphragm and piston-cylinder types for both linear and rotary valve operating designs. Rotary actuators provide the most variety in design. Piston-cylinder designs include scotch yoke, rack and pinion, and modified scotch yoke. Secondly, there are also vane and diaphragm-type rotary designs. All of these provide air to open and close the valve. With the addition of a spring or springs, a fail position on the loss of air supply can be provided. An electric solenoid valve is usually required to allow the actuator to shift air supply from open to closed and vice versa.

An important factor when determining price is the added cost of a compressor and air tubing to each actuator. While pneumatic actuators are significantly less expensive in terms of upfront costs, they are most economical when appropriately matched with compressor size. Small compressors are economical only when they are used to power a small number of actuators. The same is true with larger compressors. Unused compressor capacity can be a waste of money; therefore it’s best to only use a larger compressor when there is a need to power many pneumatic actuators. 

Although initial pneumatic unit actuator price is considerably lower, compressor costs, tubing and solenoid wiring costs, and maintenance and operating costs can be higher overall. These costs include replacing components that commonly wear out over time, like actuator cylinders, and paying for the electricity it takes to power compressors, which eventually adds up higher than most would expect. If there is already a compressor with extra capacity at your facility, a pneumatic actuator can be the best value.

Electric

Electric actuators, also known as electric motor operators, are known for their precise control and positioning capabilities. Typically, they can be heavier but because of what it takes to achieve that precision, they can be substantial in cost. The accuracy comes from the high quality components. High precision screws and anti-backlash mechanisms can create positioning accuracy to ten-thousandths of an inch and even standard components can generate exactness to a few hundredths or thousandths of an inch. 

Electric actuators tend to not have the speed and thrust associated with pneumatic due to the nature of electric motors. Thrust must be sacrificed to achieve high speeds and vice versa. For any electric actuator, more thrust is available at low speeds. Because of this, initial sizing of electric actuators is extremely important. If a thrust or speed increase is needed after installation, it will require a larger and more powerful actuator. 

If you decide to go electric, make sure you understand its application in real conditions under load. With electric actuators, the cost between different sizes increases exponentially, so understanding specifications of your system will help in choosing the right actuator while minimizing cost. One area operators see lower costs with electric actuators is in operating expenses. Operating costs for these units come mostly from the power draw of the motors. When calculating cost versus benefit between pneumatic and electric, look past initial cost and remember to factor in operating time and costs.

Other considerations

It’s important to keep in mind that environmental regulations are getting tighter each year and strict emission laws are frequently being put in place. Electric actuators are currently gaining popularity because they do not require the use of supply gas and do not create emissions. Installing a vapor recovery unit or using compressed air can help cut down on emissions associated with pneumatic actuators.

Another case for electric actuators is automation. A significant benefit with electric actuators is they can control your valves from an offsite location. Automation is also possible with pneumatic but only through an I/P valve controller that converts an electric signal to pneumatic, ultimately adding another component to buy and maintain.

If it’s time to replace actuators or if you’re purchasing them for the first time, take some time to consider your system and the exact role the actuators will serve. Consider future expansion and your overall budget and always ask reputable experts for recommendations.

Laura Jensen is the marketing and communications specialist at Flomatic Valves. She holds a Master of Arts in integrated marketing communication from Marist College.