Analyzers for ACC
The following are considerations regarding analyzers for ACC systems.
Flue gas analyzers used in closed-loop ACC applications need different features from analyzers used simply for monitoring. ACC operates the heater close to stoichiometric firing conditions. For safety and control, the analyzers must have consistently good response times, accuracy, repeatability and reliability.
For analyzers used in any closed-loop control system, reliability is based on the concept of “integrity checking.” Integrity checking gives the control system information about the analyzer system, so that appropriate action can be taken if there are problems.
The choice between in-situ (analysis done at the sample point), close-coupled or “ex-situ” analyzers (analysis done near the sample point), and extractive analyzers (flue gas educted or pumped to grade) is discussed in the following paragraphs.
The primary drawbacks of extractive analyzer systems are sample system dead time (lag time) and air leakage into the sample system, which produces falsely high O2 levels. Dead times can be greater than one minute. Sample systems should be designed to minimize this dead time and to avoid low points where condensate collects.
The dead time drawback has been resolved by ACC’s slow damper closure toward optimum firing conditions, allowing the analyzer system adequate time to produce an analysis that prevents excursions into fuel rich operation. The sample line leakage drawback has been resolved by ACC responding to CO as well as O2. ACC alerts the operator with an alarm and message if both CO and O2 have high values (high O2 possibly coming from leakage into the sample system). The Company has generally had good experience with extractive sample systems in ACC service.
Tests have shown that sample water washing absorbs CO2, influencing O2 readings, elevating them 0.6 to 1.3% or greater. While this makes no material difference to ACC (the constraint levels produced by testing automatically incorporate the offset), it becomes a factor in NOx emissions calculations. There are sample system designs which reduce this offset. Contact Chevron’s Monitoring and Computer Systems Division, ETD, for more information.
In-situ analyzers have no sample system as such. They rely on flue gas diffusion through ceramic to reach the measurement cell. Ex-situ analyzers have a short sample system, and may reduce (but not eliminate) dead time.
Further considerations are listed below regarding analyzers in ACC service:
1. Sample-system Integrity Checks
a. Extractive systems have “low-sample-flow” status switches to reveal pump
or eductor failure or line plugging.
b. Ex-situ analyzers are available with a built-in sample flow switch. In-situ analyzers can be checked by looking for a deviation in the signal.
2. The following Analyzer Integrity Checks are Needed for ACC Applications (Any Analyzer)
a. Automatic calibration (daily auto zero-span with reference gas).
b. Status signal when analyzer is calibrating.
c. Status signal for “out-of-range” when the analyzer calibration fails.
3. Maintenance—All analyzers need maintenance!
a. In-situs and ex-situs are usually located on the heater superstructure. Platforms are needed for access. For extractive types located at grade, an analyzer house or some shelter is usually provided.
b. Some in-situ CO models have ambient temperature limits of 125°F or less. With electronic modules mounted on or near hot stacks, ducts and heater walls, this limit may easily be exceeded. Some offer water-cooled enclosures, raising limits to 150°F, but have no internal high-temperature status signal.
4. Sample Points
a. Extractives can use a multipoint sampling system allowing the selection of different furnace zones for observation.
b. In-situ and ex-situ systems can be configured in multiple probe applications.