Boiler steam drum level control is critical in electrical power generation. Too high a level can mean water aspirating into the steam lines and causing damage to the turbine generator blades downstream. Too low a level can cause the boiler to dry and overheat.
In a Power magazine article, Controlling Boiler Level When Operating Conditions Change, Emerson’s Matthew Brummer compares different level measurement technologies for this application. Matthew opens the article highlighting the increasing difficulty of maintaining steam drum levels given constantly changing load demands.
Maintaining this critical level is challenging because a boiler drum is a very turbulent and chaotic place, with high temperature and pressure. Ideally, feedwater should be added at the same rate steam is drawn off. This is manageable when steam consumption is very stable, but when loading goes up and down, level can change quickly.
He highlights the ASME Boiler and Pressure Vessel Code PG-60.1.1. It
…does allow one of those direct-reading gauges to be replaced by two indirect methods of level measurement, which can include differential pressure (DP), displacers, conductivity, or radar technology.
These instruments are typically:
…mounted using external chambers… This mounting device goes by several terms, including bridle and stilling well, but always includes two connections to the drum—above and below the liquid level—so it will have the same level as the drum itself.
He compares the four primary level measurement technologies used in this application—differential pressure (DP), displacers, conductivity, radar. DP and displacers are affected by big density swings across the range of operating pressures, impulse lines in the case of DP, and mechanical maintenance issues with displacers.
Given the control requirements to maintain tight level control, conductivity sensors do not provide high enough resolution due to spacing limitations.
…does not depend on liquid density. It detects the point at which the dielectric constant (DK) changes at the liquid surface. Second, it is rated to withstand the temperatures and pressures encountered in a boiler environment. Third, it is very precise and capable of updating the reading multiple times per second, so it registers changes very quickly.
Read the article for more as Matthew describes some drawbacks with GWR that can be mitigated including mounting heights, dynamic vapor compensation, as well as his guidance on applying GWR technology in boiler steam drum applications.
Visit the Guided Wave Radar section on Emerson.com for more on the technology, tools and applications where it can be successfully applied. You can also connect and interact with other level measurement and boiler control experts in the Measurement Instrumentation and Services groups in the Emerson Exchange 365 community.
