Thursday, June 5, 2014
A Few Things to Consider When Choosing Between Pneumatic and Electric Actuators
The choice between a pneumatically or an electrically actuated valve for a process system is predicated on certain application realities. Both have advantages and disadvantages. Generally though, it's the available power source that dictates the approach.
The available power source (compressed air or electricity) is typically the primary criteria for selecting a which type of actuator to use.
Other criteria to consider are:
• Valve torque required.
• The failure mode - what valve position is desirable in the event of a loss of power
• What type of control accessories are required - limit switches, positioners, etc
• Opening/closing speed - very fast, or very slow opening or closing
• Open/close frequency - how much does the valve cycle open or closed
• Operating environment - Ambient conditions such as temperature, atmosphere, accessibility
• Valve size
Most pneumatic actuators require an air pressure supply of between 40 to 120 psi, but the most common air supply is 60 to 80 psi.
Electric actuators often used with a 120/240 VAC power supply but can be supplied with a broad range of AC and DC motors, in both single phase or 3 phase wiring.
Both pneumatic and electric actuators can be used in a wide ambient temperature conditions. The typical temperature range of a pneumatic actuator is between -5 to 175 deg. F with special options providing -40 to 250 deg. F. Be aware that these ranges may be further limited by the limitations of any accessories (solenoid valves, limit switches) included in the design. The operating limits of accessories must be considered with pneumatic actuators. Another consideration for pneumatic actuators is use in very low temperatures. Moisture in the supply air needs to be considered as to prevent condensation. Clean, dry air must be used to prevent condensate and the potential blockage of the supply line by ice.
Electric actuators are typically available in a temperature range of -40 to 150 deg. F. In outdoor settings the electric actuator should be be equipped with a housing that protects the internal motors and wiring from the environment. Consideration for condensate entering the enclosure via the electrical conduit must be considered. Because the nature of an electric actuator is to heat up during operation and cool down when off, condensation can occur from the temperature fluctuations. To prevent this, the use of an internal electric heater is recommended.
For use in hazardous areas, it is much more difficult to justify the use of an electric actuator. The requirement for NEMA 7 (explosion proof) enclosures and wiring is cost prohibitive. It is less expensive and inherently safer to use a pneumatic power source. However, if there is no available source of compressed air, an electric actuator may be the only choice.