As power generation becomes more distributed with larger numbers of smaller plants, the number of utility and industrial boilers is growing, but users face some common problems regulating their production and maximizing efficiency.
Emerson’s Matthew Brummer recently published a practical look at two critical and troublesome applications in a Processing magazine article titled Overcoming Level Gauging challenges in Boilers. He talks about two common equipment configurations:
…feedwater heaters and boiler drums. The basic methods used in these applications are examined and case studies illustrate each approach. Depending on a given plant’s configuration, either — or possibly both — application could reside at a single location.
The common element of both applications is the need to get accurate level information, which given an environment with lots of steam and pressure, isn’t always easy. His first comments relate to feedwater heaters:
Water coming out of the condensers is hot, but if further heated, it can increase boiler efficiency. The colder the feedwater is the harder the boiler has to work to return it to steam at the desired pressure. This effort is quantified as a boiler’s “heat rate,” which…indicates how much fuel must be burned to generate a specific amount of electricity, and the lower this number is the better.
Matt goes into more detail to show why it’s necessary to control steam condensation and the resulting water accumulating in the unit to keep a feedwater heater working effectively and efficiently:
Any steam blown out represents unrecovered heat, and the water accumulation helps protect against tube leaks, so some condensate within a specific level range is desirable. At the same time, too much condensate accumulating in the heater reduces efficiency. The tubes should be mostly exposed to the steam for maximum heat transfer.
The plant’s problem was controlling level using older differential pressure (DP) level sensors. They were fooled by changes in density as temperatures and pressures would fluctuate:
While the operators understood that changes in density — related to temperature in the heater — affect the accuracy of the level reading, the system did not make an adequate correction. So, as the temperature changed due to variable unit loading…it was not uncommon to have a 20 percent error, which caused a loss in efficiency.
The second application was similar, trying to control water level in the steam drum of a boiler:
Most boilers will trip if the level gets too high to avoid sending water into the steam line. They also trip if the level gets too low because this could cause the boiler to run dry. Depending on the size of the boiler, a level deviation of as little as 2 or 3 inches can cause a trip. Maintaining this critical level is easier said than done because a boiler drum is a very turbulent and chaotic place with high temperature and pressure. Many of the traditional methods used to measure level do not work well.
Again, Matt tells us that boiler drums also traditionally use DP sensors, but the resulting measurement is unreliable. Fortunately, the Rosemount 5300 guided-wave radar (GWR), along with a specially designed reference reflector, can handle these hot and steam applications, a critical capability built into the unit’s design:
GWR instruments can be equipped with a reference reflector able to determine when changes within the application are severe enough to affect reading accuracy. When vessel conditions within the unit change (increase or decrease in temperature), an offset in the reflector pulse will occur. The transmitter characterizes the difference and uses it to calculate the steam dielectric in real time, which is applied as a corrected offset to the liquid level reading.
In other words, the GWR instrument’s intelligence can recognize when conditions are severe enough to change the reading, and then provide compensation automatically. The information sent to the level controller is accurate under all operational conditions:
The plant combined GWR technology with new Global Performance Advisor (GPA) software connected to the larger Distributed Control System (DCS) for the plant. This GPA software helped the plant to create control room graphics capable of monitoring performance…much more closely.
Matt sums up the situation for both applications:
As shown in these two examples, GWR level instruments are very practical for boilers, de-aerators, feedwater heaters, LP/IP/HP heaters and other steam drums. They are particularly well suited to mounting in bridles, similar in orientation to existing site-glass installations. They are immune to changes in liquid density and temperature. Even in situations in which the dielectric constant of vapors is variable, the ability of GWR instruments to self compensate extends accuracy and reliability.
If you’re struggling with one or more of these situations where traditional DP technology is fooled by density fluctuations, you’re not alone, but there are solutions. Connect and interact with other level measurement and boiler experts in the level group in the Emerson Exchange365 community.