Boiler Drum Level (Feedwater) Control

Boiler Drum Level (Feedwater) Control

  • The cylindrical vessel where the water-steam interface occurs is called the boiler drum.
  • Boiler drum level is a critical variable in the safe operation of a boiler.
  • A low drum level risks uncovering the water tubes and exposing them to heat stress and damage.
  • High drum level risks water carry over into the steam header and exposing steam turbines to corrosion and damage.
  • The level control problem is complicated by inverse response transients known as shrink and swell.
  • Simply put, shrink and swell refer to a decreased or an increased drum level signal due to the formation of less or more vapor bubbles in the water, and no change in the
  • amount of water in the drum.
  • This condition produces level changes during boiler load changes in the opposite direction of what is expected with a particular load change.
  • Although only temporary, this can cause severe control system overshoot or undershoot. 

Benefits of Boiler Drum Level Control

  • Maximizes steam quality
  • Maintains proper drum level to prevent damage to boiler

Types of Boiler Drum Level Control

  • Single-Element Drum Level Control
  • Two-Element Drum Level Control
  • Three-Element Drum Level Control

Single-Element Drum Level Control

  • The single-element system is the simplest type used for controlling packaged firetube and water tube boilers.
  • In this strategy, control is based on the boiler drum level measurement only.
  • This does not allow for compensation of any shrink or swell and, therefore, is only an acceptable control strategy for small boilers with slow load changes.

Two-Element Drum Level Control

  • In two-element control, steam flow is measured along with boiler drum level.
  • The steam flow signal is used in a feedforward control loop to anticipate the need for an increase in feedwater to maintain a constant drum level.
  • This strategy requires the differential pressure across the feedwater control valve to remain constant, as well as the control valve signal vs. flow profile.
  • Boilers with moderate load changes can usually be controlled with this strategy.

Three-Element Drum Level Control

  • Three-element drum level control adds a feedwater flow signal to the steam flow and boiler drum level signals used in two-element feedwater control.
  • The drum level controller manipulates the feedwater flow setpoint in conjunction with feedforward from the steam flow measurement.
  • The feedforward component keeps the feedwater supply in balance with the steam demand.
  • The drum level controller trims the feedwater flow setpoint to compensate for errors in the flow measurements or any other unmeasured load disturbances (e.g. blowdown) that may effect the drum level.
  • Three-element control is used in boilers that experience wide, fast load changes, and is the most widely used control strategy

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