Draw a line diagram of a Boiler combustion control system labelling the principal items. Explain how the system functions and in particular how feed water supply, fuel supply and air/fuel ratio are regulated to match steam pressure and flow variation. Explain how these controls can be tested for alarm conditions without upsetting the balance of the system.

[Line diagram notes: The boiler combustion control system consists of: (1) a steam pressure sensor/transmitter; (2) a steam flow sensor; (3) a master controller (combustion controller); (4) a fuel flow controller and fuel valve; (5) an air flow controller and air damper; (6) an air/fuel ratio controller; (7) a feed water level controller and feed water valve; (8) a water level sensor; (9) the boiler; (10) a flue gas oxygen analyser.]

How the system functions:

The boiler combustion control system maintains the steam pressure and the correct air/fuel ratio to match the steam demand.

1. Steam pressure regulation: The steam pressure sensor measures the boiler steam pressure. When the steam demand (flow) increases, the pressure falls; the master controller increases the fuel supply (opens the fuel valve) and the air supply (opens the air damper) to increase the firing rate and restore the pressure. When the demand falls, the pressure rises and the firing rate is reduced.
2. Fuel supply regulation: The fuel flow controller regulates the fuel valve to supply the fuel required by the master controller (based on the steam pressure/flow). The fuel flow is measured and controlled to match the demand.
3. Air/fuel ratio regulation: The air flow controller regulates the air damper to supply the air required for the fuel, maintaining the correct air/fuel ratio. The air/fuel ratio is set (e.g. by a ratio controller or by the flue gas oxygen analyser) to give complete combustion with a small excess air. The air flow is measured and controlled to match the fuel flow.
4. Feed water supply regulation: The feed water level controller regulates the feed water valve to maintain the boiler water level. The water level is measured and the feed water is supplied to match the steam output (the water level is maintained at the set point).

The system thus regulates the fuel, air, and feed water to match the steam pressure and flow, maintaining safe and efficient operation.

How the controls can be tested for alarm conditions without upsetting the balance:

The controls can be tested for alarm conditions by:

1. Simulating the alarm conditions (e.g. by injecting a test signal into the sensor/controller) to verify that the alarm and the safety shutdown operate correctly, without changing the actual process.
2. Testing the alarms and the interlocks (e.g. low water level, high steam pressure, flame failure) by simulating the condition and checking that the alarm sounds and the boiler shuts down safely.
3. Using the test/bypass facilities on the controllers to check the control action without disturbing the process.
4. Carrying out the tests during a controlled period (e.g. when the boiler is at a stable load) and restoring the system to normal after the test.

This allows the alarms and the safety systems to be verified without upsetting the balance of the combustion control.