Actuators

Double-acting (DA) actuators use compressed air to drive the piston in both the open and close directions: both strokes are air-powered, giving maximum output torque in both directions. They require a continuous air supply to maintain any position and are used where a defined fail position on air loss is not required - or where a solenoid valve provides a programmed fail position. Spring-return (SR) actuators use air for one stroke and a bank of compression springs for the other. On air failure, the spring drives the valve to its preset fail position (fail-closed or fail-open) regardless of control system state - mandatory for ESD, fire isolation, and any safety-critical service.

Sizing requires the valve's maximum required torque (MRT) at worst-case conditions - typically at the start of opening or end of closing with maximum differential pressure and maximum packing friction. Apply a safety factor: 1.25× for double-acting, 1.50× for spring-return. Match this to the actuator's minimum output torque at the lowest supply pressure likely to be available in the field (typically 4 bar, not nominal 6 bar). The actuator supplier should confirm the torque at 4 bar for the specified spring configuration. Contact Vajra for a complete actuator-valve package sizing calculation.

NAMUR (Normierung Armaturantriebe in der Chemischen Industrie) is a standardised interface - a defined hole pattern on the actuator top cover - that allows direct mounting of solenoid valves, positioners, limit switches, and other accessories without custom brackets or adaptor plates. NAMUR-compatible accessories from different manufacturers are physically interchangeable, simplifying installation and reducing engineering cost. NAMUR also defines a standard signal interface (NAMUR proximity sensor output) for limit switches used with PLC/DCS safety systems.

Standard aluminium actuators are rated IP65 (dust-tight, protected against water jets). Stainless steel and high-protection variants reach IP67 (1m immersion) or IP68 (continuous immersion). For hazardous area (classified Zone 1/Zone 2) installations, solenoid valves and limit switches are available with ATEX (EU Directive 2014/34/EU) and IECEx certification in Ex d (flameproof), Ex e (increased safety), or Ex ia (intrinsically safe) protection concepts. Specify the zone classification and gas group (IIA/IIB/IIC) when ordering accessories for hazardous areas.