March 22, 2016

Confident Your Gas Chromatography is Operating Correctly and Accurately?

by Bonnie Crossland, product marketing manager – gas chromatographs, Rosemount Analytical

Whether your gas chromatograph (GC) is used for custody transfer measurement or process control, it is critical to know your GC is providing accurate data and operating as it should. Validation is the testing of the correct calibration and operation of the GC. Many times we have heard customers say they run the calibration gas as an unknown to validate the GC – however, this is not an effective method. This only shows that the GC is doing what it is told. The GC will always read what it was forced to read in the morning calibration run. If the GC is set up incorrectly, has an issue, or the calibration gas blend is bad, the daily calibration may hide the issue and result in inaccurate analysis for the sample stream.

The validation of the GC can be completed in three steps:

  1. Validate the operation of the GC for the previous period
  2. Validate the current accuracy of the GC
  3. Check for changes in operation that may affect future reliability

Validating the operation of the GC for the previous period is done by checking alarm logs, event logs, and un-normalized total trend for the past 30 days. Reviewing the logs will yield clues as to whether the GC was running correctly. All alarms during the period should be investigated and the cause determined before moving to step 2.

Validating the current accuracy of the GC is done by confirming that the As Found calibration is correct, and then observing the correct operation and repeatability of the GC. Check the latest calibration report to be sure that the calibration gas concentrations entered into the GC match the certificate of the calibration gas cylinder. For natural gas applications, you will check the calibration report for Response Factor Order, Response Factor Deviation, and Retention Time Deviation. Once the existing calibration of the GC is confirmed to be accurate, run the GC through a calibration cycle. Check the results of the analysis of the calibration gas before and after the calibration cycle for repeatability. If everything looks good, move onto step 3.

The third and last step is to check for changes in operation that may affect future reliability of the GC. This is best accomplished by checking the retention times of the individual components peaks over the last 30 days. Overtime, retention times gradually increase from contamination of the analytical flow path. This is called Retention Time Drift and it can cause two issues – incorrect peak detection and incorrect component separation. The current calibration chromatogram is compared to the chromatogram from 30 days ago to assess the amount of drift that has occurred. This information is used to make a judgment on the likely amount of drift to occur over the next 30 days. If the drift will not impact peak detection or component separation, validation is completed. If the drift will impact peak detection or component separation, the GC should undergo planned maintenance during the next 30 days.

For more information on validating the operation of your gas chromatograph, click HERE to view the recorded webinar, “Validating the Operation of Your Gas Chromatograph,” the first in a new series of FREE webinars from Rosemount that we’ve prepared to help you get optimum ROI from your gas chromatographs. This new educational webinar series is called “Maximizing Your Gas Chromatograph’s Capabilities” and is hosted by Emerson’s top analyzer experts who will be covering the most critical aspects of the GC, sharing best practices, and addressing users’ frequently asked questions and challenges faced in the field.

Click HERE to register for this FREE new webinar series today! Next ones up include:

  • Webinar #2 – April 21: “Communicating with Your Gas Chromatograph Through Modbus: RS-2332, RS-485, or Ethernet”
  • Webinar #3 – June 16: “Installation Considerations for the Rosemount™ 370XA Gas Chromatograph”
  • Webinar #4 – August 11: “Maintaining Your Gas Chromatograph at Optimal Performance Levels”
  • Webinar #5 – October 20: “Sample Handling System Considerations for Your Gas Chromatograph”

And if you end up missing any, recorded versions will be available for all.

March 2, 2016

Optimizing Operations with Correct O2 Probe Placement in Coal-Fired Stoker Boilers

Hi and welcome to Analytic Expert. I’m Neil Widmer. Recently on our Emerson Exchange 365 site, I’ve fielded some interesting questions from customers and I thought the answers could be useful to you. So here are some application solutions that may save you money and improve your boiler operations.

An engineer from JANSEN, a combustion and boiler engineering service company in the Western U.S., recently asked a question regarding a Rosemount O2 probe application. Their client operates several stoker coal-fired boilers. On each boiler they use a single 6888 O2 probe located downstream of the ID fan for fuel-air control. The service company encouraged the client to move the probe closer to the furnace due to air infiltration in the back passes. The concern is that air in-leakage can lead to inaccurate furnace excess oxygen measurement and boiler performance issues. The boiler operator said they tried the probes in locations closer to the furnace, but it “fouled” within months and they feel a clean stream downstream of the ID fan is a better option. The engineering service company asked if we are aware of conditions in a stoker coal fired application having a negative impact on the functionality of an oxygen instrument.

Rosemount 6888 O2 Transmitter Probe and 6888Xi Local Operator Interface

Rosemount 6888 O2 Transmitter Probe and 6888Xi Local Operator Interface

I responded that Rosemount agrees with the service company’s recommendation to move the probe to reduce boiler performance issues. In our experience, the probe should work very well upstream of the ID fan. Assuming that the fuel is typical bituminous stoker coal, there is no reason that our probe should not work perfectly when installed closer to the combustion process. Our probe can be located close to the boiler furnace exit and is often installed immediately downstream of the economizer convective pass. We have dozens of our probes in these exact stoker applications and they work very well.

For most of these stoker applications, the snubber diffuser works fine without ash fouling issues. If the flue gas chemistry plugs the snubber diffuser too quickly, then a ceramic or Hastelloy diffuser would be another option. Click HERE to learn more about the different diffuser options. I also mentioned that our latest model 6888A O2 system has a plugged diffuser diagnostic option which could help with predictive maintenance.

Another question came in asking about particulate or erosion due to ash content as a result of moving the probe closer to the point of combustion. There is more coal fly ash upstream and the fly ash will abrade the stainless steel probe over time. Rosemount offers two options to increase probe life: an abrasion resistant probe, or an abrasive shield which covers and protects the probe. Customers can also provide their own abrasion protection. The cost of abrasion protection and replacing probes is typically minor compared to damaging the boiler from operating at improper air-fuel ratios. Therefore we would not recommend installing the probes downstream of the ID fan to reduce abrasion.

An accurate measurement of furnace exhaust excess O2 level is critical to understanding the furnace air-fuel ratio. Some of the potential boiler losses associated with operating the furnace too fuel-rich include delayed heat release, high furnace exit gas temperatures (FEGT), and fuel-rich corrosive gases which can cause generator tubes erosion, corrosion, and excessive fouling and slagging. These impacts can result in lost availability due to tube leaks or slag and clinker build-up, lower efficiency, and increased emissions like opacity, CO, and hazardous air pollutants (HAPs). On the other hand operating too fuel-lean (i.e., with excessive combustion air) reduces efficiency and can increase emissions of oxides of nitrogen (NOx) and carry-over of particulate fly ash, as well as increase fan auxiliary power consumption and air pollution control device throughput, and last but not least, it can limit boiler output.

One advantage of stoker boilers is that the coal is relatively large; typically 50% between ¼” and 2” mesh size. The coal burns on the grate and results in lower ash carry-over than pulverized coal-fired boilers where coal is predominantly burned in suspension. We have thousands of probes and decades of experience in pulverized coal boiler applications too. For these applications, the Hastelloy or ceramic diffuser is always recommended to increase time between filter maintenance. 6888 O2 probes with these diffuser options have proven to be highly accurate and reliable in these harsh gas environments. Ultimately, the value of an accurate furnace O2 measurement far outweighs the lifetime cost of probe maintenance, repair, and operation.

Now it’s your turn. Do you have any questions on boiler operation I can help with?