August 1, 2010

Real-Time Monitoring System Measures Air In-Leakage

By Stephen Hall, PE, Storm Technologies Inc.; Jack Lyons, Orlando Utilities Commission; and Stephen K. Storm, Storm Technologies Inc.

Practical Application

Orlando Utilities Commission created a control screen and set up a historian to monitor the leakage on both of its 450-MW Babcock & Wilcox boilers (Figure 4). However, leakage is a slow-progressing problem. Because neither unit has been through an outage to address leakage issues, the leakage indication has not changed significantly. Therefore, the leakage system was tested by artificially introducing leakage to the system. Observation doors on the eighth through 12th floors were opened to simulate leakage through normal means. Figure 5 shows sample data taken during this test on Unit 2.

4. Florida field test. Orlando Utilities Commission’s Stanton Energy Center created a control screen and set up a historian to monitor the air leakage on both of its 450-MW Babcock & Wilcox boilers. Source: Orlando Utilities Commission’s Stanton Energy Center

5. Data from a functional test of the online leakage monitor. Actual historical data taken from the distributed control system on Unit 2 at Stanton Energy Center show the online air in-leakage monitoring process in action. The gold line illustrates the oxygen content using station instruments. The green line illustrates actual air leakage into the boiler system. Source: Storm Technologies Inc.

A few other assumptions were made in order to conduct this test. For example, CO is not measured and thus not accounted for, and LOI (UBC) is assumed as 0%. Another issue discovered while testing the system is that the oxygen probes installed are not perfectly sensitive to air in-leakage. This is because the oxygen probes are not perfectly representative of the true oxygen content of the flue gas. Any oxygen that is not read by the probes diminishes the leakage indication. Therefore, once the system is implemented on a furnace, a reality check using traditional methods should be performed.

As oxygen is introduced to the system (in this case over the span of a few minutes instead of the typical months and years), the leakage indication spikes to show the influx of air. However, as the O₂ trim begins to pull air out of the furnace to return the excess oxygen indication to that of the setpoint, the leakage indication remains elevated. Once the doors were shut, the leakage indication settled back to approximately 0% as air was automatically added to the secondary airflow. This is relatively consistent with historical physical data taken from the furnace exit at the nose arch elevation and economizer outlet.

Useful Diagnostic Tool

Although they are not perfect, these calculations are capable of predicting very small leakage mass flows. However, the oxygen indication is always going to lag behind the fuel and airflow indication and will be susceptible to other system measurement inaccuracies.

Data accuracy can be improved by adding online coal analyzers, including CO indications, adding provisions for LOI measurement, and doing practical testing to verify and calibrate measurements. Keep in mind, however, that the ultimate goal of this system is to give the operator a tool for diagnosing potential problems and as a relative indication for the progression of air in-leakage over time.