Three Element Feed Control System

The feedwater control system is a three element type, designed to monitor changes in steam flow, water flow and drum level. Steam flow is the rate of steam leaving the boiler - the demand. Water flow is the rate of feedwater flow into the boiler - the supply. Drum level reflects the amount of water in the boiler - the inventory. With changes in boiler load (steam flow), steam and water flow become unbalanced and water level consequently deviates from the normal position. In such an event, the system changes water flow to the extent necessary to restore the balance between steam flow and feed flow and return the water level to normal.

Three Elements Measured by this System :

  1. Steam flow - considered demand signal
  2. Feed flow - considered response signal (feedback)
  3. Drum level - considered supervisory signal

Three-Element Drum Level Control

A common application in boiler control is three-element drum level control. Boiler drum level is a critical variable in the safe operation of a boiler. Low drum level risks uncovering the boiler tubes and exposing them to heat stress and damage. High drum level risks water carryover 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.

The three transmitters, or variables, are the three elements referred to in the name of the control strategy. The feedwater flow setpoint is set automatically by the steam flow signal to keep the feedwater supply in balance with the steam demand; this is the feedforward component of the control strategy. 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; this is the cascade component of the control strategy. The summing function is used to combine these two components.

Three Element Feed Control System

This example shows the application of cascade control as well as feed forward control.

The cascade portion of the control is the output of the level controller used as the set point of the feed water controller.

For the feed forward part, the steam flow signal is added to the level controller’s output. The flow instruments are set for the same range, therefore a change in steam flow will cause an immediate change in the set point to the boiler feed water controller.

This system is frequently called a three-element drum-level control system.

Drum Level Control Logic

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