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The Fluid Systems Engineering and Management Blog

by Swagelok Northern California

Falling Asleep at the Analyzer?

by Jeff Hopkins, on 8/30/18 1:00 PM

An online analyzer means the difference between knowing a process chemistry result within minutes versus hours or even days later

"Connecting a process analyzer to an industrial plant may seem to be a simple task; but really, it’s quite complex. It demands a melding of instrumentation, analytical chemistry, and chemical engineering knowhow, and few people are skilled in all of those technical arts."

- Tony Waters, from the Preface in Industrial Sampling Systems

Swagelok offers Grab Sample Modules for gas and liquid systems. Check out information on both and get the literature, including a selection matrix.

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Delay in sampling systems

Today's chemists and process engineers have access to incredibly advanced online analytical equipment. An online analyzer means the difference between knowing a process chemistry result within minutes versus hours or even days later. One of the most important aspects of a sample is that it is an accurate representation of the material being sampled. You might be thinking, "DUH", and you'd be right; however, this mistake is disturbingly common. When running an online analyzer, a sufficient amount of time must be given to purging the sample lines before taking the measurement. Purging involves flushing the sample lines with process chemistry until all of the internal volume is filled with the same chemistry as the process line.  

Delay in Sampling Systems

So how do you know how long to purge?

To figure out purge time it's important to know 4 things:

  • Volumetric flow rate at standard temperature and pressure
  • Live volume
  • Dead volume
  • System pressure for gases

Volumetric Flow Rate determines how much in how long; the higher the flow rate, the shorter the purge time.

Live Volume is the physical volume in the direct path of flow from the sample tap point to the analyzer. For a representative sample, at least one complete purge of the entire live volume is necessary.

Live VolumeCalculate your live volume by multiplying the fluid cross sectional area of your sample line with the length of the line. For instance, if I have a tube with a 4mm internal diameter that is transporting a sample 8 meters, the internal volume of the line is approximately 100ml.

With flow rate and volume, you can begin to calculate purge time:

Live Volume Purge Time

Dead Volume is made up of the volumes that sit mostly stagnant in a sample line. Also referred to as mixing volumes (though not necessarily the same thing) dead and mixing volumes are typically located in things like gauges, filters, entrapment areas in valves, condensate pots, and stagnant non-flowing connections in the same line. In this case a representative sample requires at least three times the "mixing" volume to be purged out. This is where you may start to feel a bit of animosity towards that condensation pot knocking the liquid out of your wet sample gas.

Dead Volume Purge Time

System Pressure for gas samples matters big time. Since gasses are compressible, the higher the pressure, the more delay is built into the sample. Think of it in terms of molar amount using the ideal gas law. Pretty much all analyzers decompress the sample gas to atmospheric pressure when taking a measurement. Because of this decompression, gas velocity on the inlet of the analyzer will be a proportional amount lower than the gas velocity on the outlet, so you can express the time delay by adding a pressure drop factor to your sample time calculation. A lot of engineers make the classic error of more pressure means more flow and more flow equals a faster sample right? After adding the 10x multiplier to the sample time, you may be left questioning your own sanity when your online analyzer doesn't seem to work at all. So before you start calling up the company you bought the analyzer from it's a good idea to take a good look at your sample lines first!

Ideal Gas LawPressure Drop Time Adjustment

Resources are available!

Industrial Sampling Systems The above is just the tip of the iceberg when it comes to considerations for a sampling system. Several years ago it became clear to us that our customers needed extra help when it came to sampling systems. We answered the call and partnered with industry leaders to bring you the Swagelok reference guide to Industrial Sampling Systems. What you have read so far is only about 4 pages or so from the more than 700+ that make up the Industrial Sampling Systems reference guide.  

This hardcore engineering text is jam packed with loads of applied knowledge for thermodynamics, fluid dynamics, fluid systems engineering, and more. If you find yourself falling asleep waiting for your sample perhaps you could prop your head up with this mighty tome and gain some knowledge through osmosis!

The same experts that helped us write this book also helped us develop Pre-Engineered Subsystems and Grab Sample Modules, so you can standardize your sampling system as well as an educational program

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