Gas and Process Sample Transport Systems for Analyzer Applications















Tube Selection for Analyzer Sample Systems
Selecting the right process tube for use in analyzer sample transport systems requires critical considerations of process and application conditions. There are a wide variety of metallic and non-metallic tube types available: welded and seamless 304 and 316 series stainless steel, other metallurgies used are Monel®, Hastelloy®, Inconnel®, Incoloy®, zirconium, Super-Duplex, titanium, and even copper. Common non-metallic tube materials are Teflon® (PFA, PFE, and PTFE fluoropolymer), polyethylene and Halar (ECTFE). Some bundles may also contain coaxial tubing (tube-in-tube) or unheated tubes for calibration gas and blowback lines.

O’Brien Analytical also provide a wide variety of in-house services including electropolishing and chemical passivation as well as thermocouple cleaning and cleaning for oxygen service. We also furnish fused silica lined process tubes.

Improper tube selection may lead to a failed analyzer systems caused by adsorption, contaminants in the tube, corrosion stress cracking or gas permeation. Restricted sample flow, long sample lag times, and kinking are also problems often related to tube selection.

 

Tube Selection for Analyzer Sample Systems
Selecting the right process tube for use in analyzer sample transport systems requires critical considerations of process and application conditions. There are a wide variety of metallic and non-metallic tube types available: welded and seamless 304 and 316 series stainless steel, other metallurgies used are Monel®, Hastelloy®, Inconnel®, Incoloy®, zirconium, Super-Duplex, titanium, and even copper. Common non-metallic tube materials are Teflon® (PFA, PFE, and PTFE fluoropolymer), polyethylene and Halar (ECTFE). Some bundles may also contain coaxial tubing (tube-in-tube) or unheated tubes for calibration gas and blowback lines.

O’Brien Analytical also provide a wide variety of in-house services including electropolishing and chemical passivation as well as thermocouple cleaning and cleaning for oxygen service. We also furnish fused silica lined process tubes. (see tubing product classifications)

Improper tube selection may lead to a failed analyzer systems caused by adsorption, contaminants in the tube, corrosion stress cracking or gas permeation. Restricted sample flow, long sample lag times, and kinking are also problems often related to tube selection.

Selecting Tube Material
Consider the porosity, corrosion, and adsorption as well as the size and metallurgy (or composition) of the tubing.

Porosity
Fluoropolymer tubes are common for many analyzer applications particularly stack gas measurement. For all of its positive characteristics fluoropolymer is “porous” and has a limited working pressure, which deteriorates rapidly as temperature increases. Consider using only thick wall (0.062” / 1.5mm) tubing.

If the porosity and / or pressure rating is not acceptable then consider using electropolished or fused silica lined stainless steel tube. While resistant to most chemicals fused silica (consider it the same as glass) has limited resistance to acids so caution should be exercised.

Corrosion
When you check for corrosion resistance it is imperative that you design for failure. Just because the process stream does not contain acids under normal conditions you need to determine the component concentrations and byproducts if an upset occurs. The creation of acids is common in stack gas applications when inadequate heating allows water vapor to condense and combine with sulfur or nitrogen compounds.

Adsorption
When utilizing stainless steel tubing this phenomenon will exhibit itself in the measurement of trace amounts of component (especially moisture and sulfur compounds, H2S and SO2.) The structure of commercial stainless steel tubing is such that compounds may be caught up in the “crevices” and not reach the analyzer. Once the tube is saturated the levels will equalize relative to the process as long as the process composition, temperature and pressure remain constant. Any change in composition, temperature or pressure may cause compounds to be adsorbed more by the tube or release from the tube surface and “spike” the sample to the analyzer. As a result adsorption / desorption creates inconsistent and unpredictable results. This is sometimes referred to as the “memory effect”. This problem can often be solved by the use of electropolished or fused silica lined tubing.

TUBE
GENERAL USES
CHARACTERISTICS
Commercial 316L Seamless SS Standard instrument tubing. Free Fe will react with most acids. Improved surface roughness
Limitations:
Limited by chemical reactivity and oxygen levels
Subject to stress corrosion cracking.
High Ra values of internal surface contribute to adsorption / desorption problems.
Advantages:
Inexpensive and readily available.
Commercial 316L Welded SS Standard low pressure / temperature instrument tubing.
Free iron will react with most acids. Weld containments may contribute to reduced corrosion protection compared to seamless tubing.
Improved surface roughness
Limitations:
Limited by chemical reactivity and oxygen levels
Subject to stress corrosion cracking.
Pressure rating less than seamless products.
May be more susceptible to corrosion than seamless products.
Advantages:
Inexpensive and readily available.
Fluoropolymer (Teflon®) Low pressure sample and chemical lines where 316 SS is not acceptable. Used for applications requiring cleanliness. Surface Roughness: NA
Limitations:
Limited by chemical reactivity and oxygen levels.
Temperature and pressure limits vary by variety of fluoropolymer.
Very permeable. Used for manufacture of permeable membranes.
Advantages:
Excellent chemical resistance.
Flexible.
TrueTube CP Applications requiring additional cleanliness or corrosion resistance compared to commercial SS tube. An O’Brien Analytical Product
Chemically polished 316L SS seamless tubing.
Improved surface roughness
Limitations:
Limited by chemical reactivity and oxygen levels.
Advantages:
Enhanced Cr/Fe and CrO/FeO ratios improve chemical resistance.
Improved adsorption / desorption characteristics compared to commercial tubing.
TrueTube EP Critical sample systems where adsorption / desorption is a problem. An O’Brien Analytical Product
Electropolished and chemically treated 316L SS seamless tubing.
Improved surface roughness
Limitations:
Limited by chemical reactivity and oxygen levels.
Advantages:
Cr/Fe ratio better than 1.5:1 and CrO/FeO ratio better than 3:1 improve chemical resistance.
Improved adsorption / desorption characteristics compared to commercial tubing.
TrueTube FS Moderately acid samples and sulfur. An O’Brien Analytical Product
A secondary fused silica coating applied to chemically polished 316L SS tubing.
Surface Roughness: NA
Limitations:
Very poor resistance to bases.
Reacts adversely with fluorides.
Advantages:
Has improved chemical resistance over substrate alone.
Can be used at higher temperatures than polymer coatings.
Covalently bonded matrix which reduces surface tension.
Has found wide acceptance for transporting sulfur samples.

 

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