Feed water and Drum-Level Control schemes for boiler

  • Proper boiler operation requires that the level of water in the steam drum be maintained within a certain band. A decrease in this level may uncover boiler tubes, allowing them to become overheated. An increase in this level may interfere with the operation of the internal devices in the drum that separate the moisture from the steam and may cause liquid carryover that can damage the steam turbine.
  • Control of feedwater addition based on total drum level alone tends to be self-defeating, because on a load increase it tends to decrease water feed when it should be increasing.
  • Following figure shows the response relationship among steam flow, water flow, and drum level that should be present in a properly designed system if constant level under variable load is desired.

Single- and Two-Element Feedwater Systems

  • For small boilers having relatively high storage volumes and slow-changing loads, a simple proportional control may suffice, imprecise as it is.
  • Two-element control involves adding the steam flow as a feedforward signal to the feed-water valve. Two-element control is primarily used on intermediate-sizeboilers, in which volumes and capacities of the steam and water system would make the simple “total” level control inadequate because of ‘swell’.
  • Two-Element Feedwater control system is shown here:
  • Field testing, characterization, and adjustment of the control valve are required so that the relationship of control signal to feedwater valve flow matches that of the steam flow to the flow transmitter output.
  • Any deviations in this matching will cause a permanent level offset at the particular capacity and less than optimal control (as shown in the following figure).

Three-Element Feedwater Systems :

  • As boilers become greater in capacity, economic considerations make it highly desirable to reduce drum sizes and increase velocities in the water and steam systems.

  • A three-element system is used on such large boilers to arrest disturbances and react to load changes more rapidly, as they occur. Three-element control is similar to the two element system, except that the water flow loop is closed rather than open, so pressure disturbances that would affect feedwater flow are handled immediately by the fast response of the feedwater flow loop, as shown here:

  • In addition to the three primary control variables (three elements)—drum level, steam flow, and feedwater flow — drum vapor-space pressure can be utilized to compensate for density changes.
    Feedforward Control· A feedforward variation is recommended by Shinskey to maintain a steam-water balance, reducing the influence of shrink-swell and inverse-response phenomena.

  • The system shown in the following figure causes feed-water flow to match steam flow in absence of action by the level controller. The two flowmeters have identical ranges, and their signals are subtracted. If the two flow rates are identical, the subtractor sends a 50% signal to the flow-difference controller. An increase in steam flow will call for an equal increase in feedwater flow to return the difference signal to 50%.

  • The use of external feedback will precondition the level controller during start-up or at other times when feedwater is controlled manually or other-wise limited.