July 9, 2010

Sample Handling – Preparing for a Bad Day

G’day, my name is Shane Hale and this is my first effort at writing on our shiny new blog.  When thinking of a subject matter for my post, it seemed logical to talk about the first component of any analytical system, the sample handling system.  As the product marketing manager for natural gas, I deal mostly with gas chromatographs on pipeline and natural gas processing applications; however, the principles that I talk about here are applicable to any analyzer that extracts the sample from the process lines.

When I travel around talking to people about their applications, I always hear two things – “Our gas is always clean and dry,” and “We don’t need heat trace, it never gets that cold here.”  Unfortunately, the cynic in me knows that these two statements are never true 24/7 over the lifetime of the gas chromatograph (GC).  It is on that rare occasion that “bad gas” does enter the system that the sample handling system steps up and does its job, or fails and results in inaccurate measurement and/or equipment failure.

The role of the sample handling system is to extract a representative sample from the line, remove gas and liquid contaminants, reduce the pressure, and deliver the sample to the analyzer in a timely manner.  When the sample is in the liquid phase, the sample handling system takes on the added role of vaporizing the sample as well.  All of this should be done while also maintaining the sample composition.  It doesn’t matter how good the analyzer is if the sample it’s analyzing is different from the composition that is inside the pipe – the results will be wrong.

When designing the sample system, a lot of people make the mistake of designing to the “expected” or “normal” composition and process conditions.  However, it is when the unexpected happens that you need to ensure the best measurement so that you can make operating decisions to rectify the situation. For pipelines, you may be shutting in a supplier for high hydrocarbon dew point or high CO2.  In processing plants, you need accurate compositions to regain control of the process.  When these events occur, the sample handling system should be designed to handle the unexpected in order to ensure the analyzer receives a representative sample while not causing contamination of the analyzer internals.

When we sell a natural gas GC, we typically sell an integral sample handling system that uses a 2 micron filter, and a moisture membrane filter/shut-off.  However, this is the last line of defense.  By the time a “bad” sample (dirty or wet) has made it to the sample system at the GC, the rest of the sample system has been contaminated.  The best solution is to also incorporate solids and liquids filtration at the sample point, and to ensure the sample is always kept well above the hydrocarbon dew point by using heat trace which will maintain the sample well above the original sample temperature.  Sample probes that incorporate filters at the sample tip, or locally mounted systems, are both excellent steps in ensuring the results you receive truly represent what is in the process.

So, even though your sample may always be “clean and dry,” and your weather is always warm, it’s best to design for that day when it is not.