Boiler Control Systems and Safety Interlocks

Boiler Level Control Systems

• Boiler level control systems often use a cascade control strategy with a level controller as the primary controller and a flow controller as the secondary controller.
• The level controller adjusts the set point of the flow controller to maintain the desired boiler drum liquid level.
• Feedforward control is frequently incorporated to compensate for changes in steam demand.
• The steam flow can be used as a feedforward signal to adjust the set point of the flow controller, or directly manipulate the boiler feedwater control valve.
• Most boiler level control systems add the feedforward signal to the level controller output, affecting the set point of the secondary (feedwater flow) controller.

Boiler Safety Interlocks

• Purge interlock: Prevents fuel from being admitted to an unfired furnace until it's thoroughly purged.
• Low air flow interlock: Shuts off fuel upon loss of air flow or combustion air fan operation.
• Low fuel supply interlock: Shuts off fuel supply upon loss of fuel to prevent unstable flame conditions.
• Loss of flame interlock: Shuts off all fuel upon loss of flame.
• Fan interlock: Stops the forced draft fan upon loss of the induced draft fan.

Boiler Steam Temperature Control

• Steam temperature is measured and compared to a set point.
• A temperature control system uses a temperature control relay (TRC) to generate an actuating signal for the desuperheater water flow valve to adjust steam temperature.

Boiler Pressure Control

• Boiler pressure control systems use a feedforward model to estimate the true load on a boiler.
• The pressure controller adjusts the ratio of firing rate to estimated load.
• A feedforward trim based on steam flow is added to speed up the response to load changes.
• Steam flow anticipates changes in steam demand, allowing for faster adjustments to firing rate without waiting for pressure fluctuations.

3-Element Control in Boiler Water Feed

• 3-element control uses three process variables (PVs) to control the boiler feedwater control valve:
  • Liquid level in the boiler drum
  • Flow of feedwater to the boiler drum
  • Flow of steam leaving the boiler drum
• The level controller determines the desired feedwater flow, which it transmits as a set point to the flow controller.
• The flow controller adjusts the feedwater valve to meet the set point, compensating for supply pressure variations.

Boiler Types

• There are two main categories of boilers:
  • Fire-tube boilers: Hot gas inside tubes heats the surrounding water.
  • Water-tube boilers: Water inside tubes is heated by the surrounding hot gas.

Boiler Applications

• Boilers are widely used in:
  • Thermal power plants
  • Industrial processes
  • Heating
  • Sanitation
  • Sterilizing equipment

Water-Tube Boiler Design

• Water-tube boilers consist of two drums:
  • Steam drum: Collects steam and water, separating them.
  • Water/mud drum: Collects water from the downcomers.
• Downcomers: Carry cold water from the steam drum to the mud drum.
• Risers: Carry steam from the mud drum to the steam drum.
• Superheaters: Increase steam temperature beyond its saturation point.
• Economizers: Recover heat from flue gas to preheat feedwater.
• Air heaters: Preheat combustion air using waste heat from flue gas.

Boiler Feedwater

• Feedwater is supplied by a feedwater pump and valve.
• Water is sourced from a demineralization (dm) plant to prevent scaling.