Controlling Boiler Level When Operating Conditions Change
If asked to make a short list of factors able to improve heat rate for a thermal power plant, accurate measurement of steam drum liquid level would rank near the top due to its influence on boiler performance. This is particularly true as electric power markets and demand patterns change.
Water wall headerMany 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.
Boiler Steam Drum Level
While designs and sizes vary, most subcritical boilers have a drum at the highest point where steam is collected and the tubes come together. Maintaining the right water level in the drum is critical. If the water level is too high, it can be carried into the steam line, and if it makes it all the way to the main turbine, it can damage blades. If the water level is too low, parts of the boiler may go dry and overheat. Both situations can result in long and expensive shutdowns, so a change of only an inch or two in level may cause a trip.
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.
The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code PG-60.1.1 requires any boiler operating above 400 psi to have two direct-reading gauge or sight glasses to indicate drum level. This is a bit old-fashioned and harks back to the days of manual control by a fireman at the boiler. The code 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.
Generally, boiler drum level instruments of all types are mounted using external chambers (Figure 1). 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. Both connections have valves so the chamber can be isolated during operation, allowing an instrument to be replaced or serviced without shutting down the boiler. Chambers also help minimize the turbulence, which is characteristic of boiler drums.
