In a valve system for controlling the flow of liquid fuel and steam to a nozzle in a large scale power plant boiler, a pair of valves are operated by rotary pneumatic actuators. Three-position operation is achieved for one of the valves by coupling the valve shafts together through a lost-motion coupling and using an actuator for the other valve which delivers more torque than the actuator which operates the three-position valve. The same actuator and coupling system can be used for straight high pressure mechanical atomization, for mechanical atomization with recirculation, and for steam atomization, with simple substitution of rotary valves.
A purge valve assembly for purging the burner of a large scale power boiler of fuel oil has an air driven rotary actuator with a rotor coupled to two three-way valves such that when the actuator is in a first position, the burner is shut down, in a second position, the burner is purged of oil, and in a third position both steam and oil pass to the burner for combustion. The second or purge position is maintained by impressing air under pressure at equal pressures on both sides of the rotor.