Sanitary Diaphragm Valves use Linear Pneumatic Actuators
An actuator is a type of motor that is responsible for moving or controlling a mechanism or system. Actuators are operated by some kind of power source, be it a human operator turning a valve handle, a pneumatic cylinder, or an electric motor. The two most common types of automatic actuators in the process industry are electric and pneumatic actuators. This post will take a look at each actuator type and the things to consider when specifying one or the other for a sanitary valve application.
Pneumatic actuators are the most common type of automatic valve actuator we specify. A pneumatic actuator usually consists of a piston, a cylinder, and valves or ports. The piston is sealed to the cylinder wall with an o ring or some other type of elastomeric seal. Some pneumatic actuators use a diaphragm instead of a piston. The diaphragm keeps the air in the upper portion of the chamber, allowing air pressure to force the diaphragm/piston downward, moving the actuator stem, and actuating the valve.
Most pneumatic actuators will utilize an air supply pressure of 40-120 PSI, with most common pressure ranges in the 60-80 PSI range. Higher air pressures can be difficult to guarantee and sustain, while lower pressure will require a very large piston in order to generate the torque needed to actuate the valve.
Pneumatic actuators will often incorporate an additional spring for fail-safe operation. In a fail-safe actuator, when the air supply is lost, the spring drives the valve back to a predetermined location- either opened or closed. This is where the nomenclature for pneumatic actuators comes from. There are three primary designations for pneumatic actuators:
Air to open, fail close (Normally Closed)
- Air to close, fail open (Normally Open)
- Air to Air, or double acting
Double acting actuators do not have fail-safe springs and depends on air supply valving to pressurize and relieve the actuator and change the valve position.
Electric actuators, on the other hand, use an electric power supply- most commonly 110 VAC power- to drive a motor, generate torque, and change the valve position. Electric actuators are often used in applications where air is not readily available. They are simple to wire and interface easily with existing control systems, especially in control or modulating applications.
Electric Actuators are Normally Only for 1/4 Turn Valves
There are a few drawbacks to electric actuators, however. First electric actuators are usually much slower to act than their pneumatic counterparts. Also, while electric actuators are available in intrinsically safe, spark free enclosures for explosion proof applications, they are generally used only when air isn’t available. Electric actuators are also not available with the fail-safe spring returns discussed above. While pneumatic actuators can be stalled, stalling an electric actuator will lead to excessive current draw and potentially hazardous conditions. Electric actuators are also generally more expensive than pneumatic actuators. In spite of the limitations, it is worth noting that position feedback and valve control setup and wiring is often simpler than pneumatics, which require the interface of pneumatic and electric energy supplies.
Electric actuators are generally available for quarter turn valves such as sanitary ball or butterfly valves. Pneumatic actuators are available for quarter turn valves as well as linear valves such sanitary diaphragm and stem/seat type valves.
So which valve should you use? As you can see, it depends on several different factors, but if you’re looking for a cheap, easy to set up, and robust actuator, go pneumatic. They are readily available in multiple different sizes and torque values for your specific application. If you don’t have an air supply readily available or are looking for something that you can wire and install simply, go electric. If you have any more questions, contact a Holland Sales Engineer today.