In a Flow Control article, 4 Key Measurements For Optimal Boiler Control, Emerson’s Bob Sabin, outlines these important measurements in achieving effective boiler performance. These measurements include drum level, fuel flow, air flow and flue gas oxygen.
Drum level measurement:
…is critical for safety and reliability. Inaccurate drum level control can result in safety issues and equipment damage. High levels can cause water carryover that lowers heat transfer efficiency and possibly damages downstream equipment such as steam turbines. Low levels expose tubes to excessive heat, resulting in tube damage and unplanned shutdown.
Pressure changes in the drum caused by load changes in steam demand cause the fluid volume to change due to entrained steam bubbles interning or leaving the liquid—the shrink and swell phenomenon. Bob describes the way to compensate for this shrinking and swelling is to:
…employ three-element control strategies that simultaneously look at steam flow, the rate feedwater is flowing to the steam drum and the water level in the steam drum. In addition, compensation for pressure and temperature must be made either at the level instrument or in the computer control system.
Redundancy of this measurement is important for safety and reliability. Using different level measurement technologies is recommended practice and Bob describes the use of differential pressure (DP) level and guided wave radar (GWR) as appropriate technologies for this application.
Optimizing the combustion process requires accurate and repeatable fuel flow measurements. The goal is:
…achieving mass balance between fuel and oxygen, so fuel measurements should be of the mass flow type.
For boilers powered with different fuel sources with different energy densities [hyperlink added]:
…a direct mass Coriolis flow measurement is typically best in this situation. All types of mass flow meters improve turndown, which helps when the boiler experiences wide load swings.
Accurate mass-based fuel flow rates:
…means knowing the rate at which energy (Btu/calories) is being delivered to the burners, which in turn determines the amount of air required. This makes it easier to control combustion, monitor boiler efficiency and monitor plant energy use, even with compressible fuel.
Read the article for considerations for boiler air flow and flue gas oxygen measurement and how experienced consultants can play an important role in maintaining safe, reliable and efficient boiler operations. You can also connect and interact with other boiler control experts in the Operate & Maintain and Industrial Energy groups in the Emerson Exchange 365 community.