Instrumentation, Controls and Efficiency
Instruments
Understand the pressure gauge, water level gauge, thermometer, flow meter and draught gauge. Know how a pressure gauge is calibrated and why a siphon and cock are fitted. Understand level indication: gauge glass, remote indicators and differential pressure cells. Know the meaning of accuracy and the need for regular comparison and calibration.
Controls
Know the control loop: sensor, controller, actuator and final control element. Understand on-off, modulating and proportional-integral-derivative control in principle. Know the combustion control system: fuel-air ratio, oxygen trim, and the interlocks that prevent firing without purge or with flame failure. Understand the function of the boiler management system and its safety interlocks.
Efficiency
Understand direct and indirect efficiency. Know the main losses: dry flue gas, moisture in fuel, hydrogen in fuel, incomplete combustion, radiation and unburnt carbon. Know how excess air, stack temperature and fouling affect efficiency, and how to use an oxygen or carbon dioxide reading to trim excess air.
Traps
Questions often confuse a control loop component with the safety interlock. Another trap is assuming lower stack temperature always improves efficiency; it does, but only down to the acid dew point, below which corrosion occurs. Do not confuse direct efficiency (output over input) with indirect efficiency (100 minus losses).
Worked example
If stack temperature rises while load is steady, reason that fouling or scale is reducing heat transfer. Clean the heat transfer surfaces rather than increasing firing.
Rule of thumb
Trim excess air to the lowest safe level, keep surfaces clean, and never cool flue gas below the acid dew point.