A line diagram: boiler steam pressure(s) -> steam pressure transmitter -> master controller (follows steam pressure) -> fuel valve and forced-draught fan (air) -> furnace -> steam chest (back to transmitter) with a secondary loop of fuel flow, air flow and O2 analyser providing air/fuel trim.

Operation: The system maintains the required steam pressure while burning fuel efficiently. The master (steam pressure) controller compares the measured steam pressure to the set point. When steam is taken, pressure falls; with a negative error the controller raises the firing-rate demand which opens the fuel valve and increases the forced-draught fan/air supply together, increasing heat input until pressure returns to set point. When steam demand falls, pressure rises and firing rate is reduced. A separate air/fuel-ratio controller (with an O2 analyser in the uptake) trims the air flow relative to fuel so combustion remains in the optimum efficiency band at every firing rate, preventing excess air (which wastes heat) or the dangerous smoke/soot of air starvation. A flame safeguard supervises ignition and operation.

• Low-water cut-out trips the fuel supply if the boiler water level falls dangerously low.
• Flame-failure safeguard: if no flame is confirmed (UV/photo cell) within the light-up time, fuel is shut off and a purge/lock-out occurs; safe re-light only after purge.
• Forced-draught/fan interlock: fuel cannot be admitted unless the fan is running and the pre-purge is complete; loss of air flow trips fuel.
• High steam-pressure trip and safety relief valves prevent over-pressure.
• Loss of control air or electrical supply fails the system to safe (fuel valves shut).
• Gas/oil pressure/vacuum trips and pre/post-purge sequences prevent explosive mixtures; lock-out requires manual reset after flame failure.

The master controller is a proportional-integral(-derivative) controller: its output is proportional to the error (set point - measured steam pressure) plus an integral term that removes steady offset. A fall in steam pressure gives a positive error, raising the controller output which (through a programmed cam/electronic curve) simultaneously increases fuel valve position and the fan-speed/air-damper command; a rise in pressure reduces both. The air/fuel ratio controller then trims the air: it measures fuel flow and air flow, and takes the O2 analyser signal; the O2 reading is compared with the set-point O2 (optimum for the fuel), and the air controller adjusts the fan/damper to add or remove air so the ratio stays in the efficient band. Thus steam pressure sets the firing rate, and the ratio is corrected automatically for load and fuel variation.