The National Electric Manufacturers Association (NEMA) has set up guidelines for the construction and installation of electric actuators (and other electric devices) for use in hazardous areas. The NEMA VII guideline reads
VII Hazardous Location Class I (Explosive Gas or Vapor)
Meets application requirements of National Electric Code; conforms with specifications of Underwriters' Laboratories, Inc. used for atmosphere containing gasoline, hexane, naphtha, benzene, butane, propane, acetone, benzol, lacquer-solvent vapors, natural gas.
Almost all electric actuator manufacturers have an option for a version of their standard product line that conforms with NEMA VII.
On the other hand, pneumatic actuators are inherently explosion-proof. When electric controls are used with pneumatic actuators in hazardous areas they are generally more cost effective then electric actuators. Solenoid-operated pilot valves may be mounted and powered in a non-hazardous are and piped to the actuator. Also, reasonably priced hazardous area qualified solenoid valves are readily available. Limit switches -for position indication- may be housed in a NEMA VII enclosure. The inherent safety of pneumatic actuators in hazardous areas makes them a practical choice in these applications.
Spring return: Another safety accessory widely specified in the process industries on valve actuators is the spring-return (fail-safe) option. Upon power or signal failure a spring-return actuator drives the valve to a pre-determined safe position. This is a practical and inexpensive option with pneumatic actuators and is an important reason for the wide use of pneumatic actuators throughout the industry.
Where springs are not practical because of actuator size or weight, or if a double-acting unit is already installed, an accumulator tank may be installed to store air pressure.
Electric actuators are not widely available in a spring-return version; however, a battery back up system is an elegant solution. To accomplish the spring-return function an electro-hydraulic actuator is often a good choice. Electro-hydraulic actuation is achieved by energizing a hydraulic pump, which pressurizes a spring-return cylinder. Upon power failure the spring action drives the actuator to the original position. Because only an electric power supply is required for this self-contained unit it is a practical approach to fail-safe electric valve actuation.
Performance characteristics: Before specifying a pneumatic or electric actuator for valve automation it is important to consider a few of the key performance characteristics of each.
Duty cycle: Pneumatic actuators have a 100 percent duty cycle. In fact, the harder they work, the better they work.
Electric actuators are most commonly available with 25 percent duty cycles motors. This means that to prevent overheating in high cycle applications the motor must rest frequently. Because most on-off automated valves remain idle 95 percent of the time duty cycle is not usually an issue. With optional motors and/or capacitors an electric actuator may be upgraded to 100 percent duty cycle.
Stalling: Pneumatic actuators may be stalled indefinitely without overheating.
Electric actuators should not be stalled. Stalling an electric actuators draws excessive current, which generates heat in the motor and can cause damage. Torque switches or heat and current sensors are often installed in electric actuators to protect the device.
Speed control: The ability to control the speed of a pneumatic actuator is an important advantage of the design. The simplest way to control the speed is to fit the actuator with a variable orifice (needle valve) at the exhaust port of the air pilot.
Since electric actuators are geared motors it is impossible to make them cycle faster unless a gearing change is made. For slower operation a pulsing circuit may be added as an option.
Modulating control: In modulating service an electric actuator interfaces well with existing electronic control systems and eliminates the need for electro-pneumatic control. A pneumatic or electro-pneumatic positioner is used with pneumatic actuators to provide a means of controlling the valve position.
Torque-to-weight ratio: Electric actuators have a high torque-to-weight ratio above 4,000 lbf.in. (450 Nm) Pneumatic actuators have an excellent torque-to-weight ratio below 4,000 lbf.in.
Jon Monsen, Ph.D., P.E.
is a Control Valve
at Valin Corporation.