Controlling Boiler Level When Operating Conditions Change

In recent years a variety of industries have been in turmoil, but few have been buffeted as much as the electric utility industry. Climate change, renewables, coal, fracking and more are affecting electrical generators, even down to operational considerations as granular as how a plant measures boiler water level. Does that sound a little farfetched? It isn’t.

Matthew Brummer draws on his experience in the industry and digs into that very point in his article in the April 2019 issue of Power titled Controlling Boiler Level When Operating Conditions Change. Industry changes are indeed driving operational changes, forcing many plants to reconsider how they can make changes to avoid serious problems.

Many legacy thermal plants were designed for baseload operation, running for days and weeks at a time with minimal load changes. Now, plants often have to ramp up and down on a daily basis to balance intermittent production from renewables. This takes its toll on equipment, demanding more sophisticated control to maintain efficiency and a desirable heat rate over a wider operating range. Let’s look at level control in boilers and see how this affects efficiency, operation, and maintenance.

There’s the key: constantly varying and unpredictable adjustment of output makes boilers harder to control, particularly when they are outfitted with older style level measuring instrumentation. Boilers are tricky to control under the best conditions because level must be maintained in a very narrow range, which means it must be monitored carefully.

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. Increasing load draws off steam, causing a pressure decrease in the drum and allowing more steam to bubble in the tubes. This raises the liquid level, and if it is a big enough change in a short time, it can cause a trip. Reducing steam consumption can have the opposite effect.

 So, what’s the answer? Matt looks at several possibilities in light of the boiler codes and installation methods typical of plant installations. They have their advantages and disadvantages, but he makes a solid case for guided-wave radar as the most adaptable for this challenging application.

First, GWR 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.

GWR transmitters well suited to this application include Emerson’s Rosemount 5300 Level Transmitter – Guided Wave Radar family. It can be configured specifically to thrive in this difficult environment and is capable of self-compensating to provide accurate readings in steam environments.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level and Power Groups along with other specialty areas for suggestions and answers.