Boiler Drum Level Control

A boiler room.

The boiler drum level control is the way in which a boiler water and steam levels are monitored and controlled. This is important for a plant or factory that operates large boilers as part of its production process. Most homes do not contain boiler systems or have had them replaced with newer and more efficient ways to meet the heating needs of their home.

This article will provide a very general discussion of the boiler drum level control to provide you with an understanding of this mechanism and how it works within a boiler.

Boiler Drum Level Control

The boiler drum level control is used to maintain the water/steam interface at its optimum level. It also is used to provide a continuous balance between the mass and heat within the boiler by replacing every pound of steam leaving the boiler with a pound of feed water to replace it.

Interface Level Disturbances

The interface level is subject to several disturbances in the water and steam drum, such as the drum pressure and feed water temperature. As steam pressure rises or falls due to load demand there is a change in drum level due to the expansion or contraction of the steam bubbles in the drum water. When the steam pressure is lowered the water level rises as the steam bubbles expand. As the steam pressure rises the water level lowers as the steam bubbles compress.

As sub-cooled feed water enters the water/steam drum it will cool the drum water at the entry point to below the operating pressure boiling point. This process condenses the steam bubbles in the boiler drum water and collapses the steam blanket. The water/steam drum is large enough to absorb this sub-cooled water without being overly affected by its influence.

Boiler Level Control Strategies

A modulating control of the water/steam interface level is normally addressed through one of three principal strategies: single-element; two-element; or, three-element control. The application of any one of these strategies depends on the specific boiler size and load.

Single-Element Control

The single-element control is the basic boiler level control strategy. The drum level is measured using a single measurement device and provides a control signal to the feed water regulator in direct relation to the current operating drum level. This system is used in both the on/off and modulating feed water control strategies.

Two-Element Control

The two-element drum level strategy is suitable for processes that have moderate load swings and speeds, and it can be used on any size boiler. This system uses the two variables, drum level and steam flow to mass, which balances the feed water demand.

Three-Element Drum Level Control

To address the issues of phasing still present in the two-element control strategy, a third element, feedwater flow, is added to the drum level control strategy. In this system, the math summer output of the two-element controller is cascaded down to a second feedwater flow controller that acts as a remote set point.