Customers experiencing material contamination and sampling reliability issues frequently ask: “Is there any data concerning the life span of the effectiveness of the SilcoNert 2000 coating (i.e. does its efficiency decay over time)? Replacing chromatograph columns isn’t a big job, but replacing 100 metres of sample tubing routed high up among the pipe-work would be a major headache.”
The biggest consideration regarding contamination and sampling reliability would be how “dirty” the sample materials are.
If there is a lot of particulate coming into the line, then cleaning would be needed to remove contaminants. If these particulates build up, they could be adsorptive. Once, they are rinsed out, the SilcoNert 2000 coating will be fine and sampling accuracy and reliability will be restored (unless there is abrasion/erosion that has compromised the coating).
Another area of potential poor reliability is the accumulation of moisture in sampling systems. If there is any point of condensation in the system and there are acids or bases that form in water from the sample stream/water contact; the subsequent corrosive attack and reactivity of the acid/base will decrease the coating life and adversely impact sample reliability. We have seen systems in which the SilcoNert 2000 coating is performing well but a cold spot in the heat trace tubing resulted in moisture condensation and HCl or Sulfuric acid formation. The acid then attacked the coating and tube surface, eventually eating a hole (in the layer and the tubing) and creating active iron particulates.
For most applications sample lines will last many years. A study by BP, presented at the 2011 ISA-AD Conference in Houston, TX, highlighted a SilcoNert 2000 coated flare gas sampling line that had been working great for 2 years. This is a very dirty application with the potential of many contaminants and failures; however, with first-rate design and maintenance, the system continues to provide accurate, reliable sampling results. Keys to reliable sampling are:
- Eliminate potential cold spots in heat trace tube.
- Proper installation, avoid dips or areas for moisture accumulation in tubing.
- Install filtration, avoid build up of active particulates.
- Proper material selection for the environment.
- Inert coatings
- Materials specified for the sampling environment.
- Proper coating selection for the sampling application:
- SilcoNert 2000 for low level sulfur/H2S sampling.
- Dursan for extreme environments (acid/base exposure, particulate abrasion, etc).
For design and installation pointers, ask the heat trace companies about their experiences with installing and servicing lines used in your application. SilcoTek works extensively with O’Brien Corporation www.obcorp.com and Thermon Manufacturing Company www.thermon.com . They are the market leaders and have a great deal of technical expertise and experience with installation and maintenance of reliable sampling systems.
Reliable field sampling and transfer of trace levels of ammonia is often necessary for environmental compliance and process monitoring and control. Corrosion and active surfaces will interact with ammonia samples to degrade sample quality and reliability. Typical materials used in ammonia sampling are: 304 stainless steel, 316 stainless steel, PTFE, PFA, and silicon coated stainless steel (SilcoNert 2000 or Dursan). Figure 1 compares ammonia interaction/response to common sampling materials.
Figure 1: Ammonia sampling reliability can be improved by using inert materials like PFA and SilcoNert 2000 (Sulfinert). Stainless steel adsorbs ammonia resulting in lost sample. Measured PTR-MS signals of ammonia (m17). 500sccm of 100ppb ammonia in nitrogen. All lines were 1.8m long not heated (30c).
Sample reliability can be improved by using inert materials like PFA and SilcoNert 2000 coated stainless steel. Material selection should account for environmental factors such as temperature, shock, abrasion or corrosion.
Sampling data courtesy of Ionimed Analytik.
Many of the streams in refining and pedtrochemical operations are very dry but an upset in process conditions will lead to moisture in the sampling system. A surface that is hydrophobic is critical in refining and petrochemical applications. This moisture will adversely affect analysis because the polarity of the water in the system can retain sulfur and sulfur species as well as other active compounds. The faster a system can “dry” of any moisture, the faster the analytical system will begin to generate reliable data.
Figure 1, shows images of water droplets applied to 304 stainless coupons as well as coated 304 stainless surfaces. The coatings impart a hydrophobic characteristic to the stainless steel substrate. The hydrophobic surfaces are easier to purge free of water. This is critical in refining and petrochemical operations when upsets occur, as moisture in analyzer systems lead to poor and unreliable data.
Figure 1: Coating of 304 stainless results in ability to increase hydrophobicity
Coatings are well accepted as a means to improve analytical system accuracy and durability in demanding applications. To select the proper coatings, properties such as acid exposure, particulate exposure and the needs for chemical inertness must be know. When coatings are properly matched to the physical and chemical demands of an application, years of accurate and reliable results can be expected.
For maximum reliability, analytical systems used in sampling and transfer of sulfur containing species, system inertness must be addressed when stainless steel components are used. In most refining and petrochemical streams, analysis in the ppm level is required.
Figure 1, demonstrates the need for coating during the sampling, storage and analysis of part-per-billion levels hydrogen sulfide. In critical applications, the ultimate inertness of components is enhanced using silicon based coatings at the cost of physical durability. In Figure 1 and 2, the degradation of hydrogen sulfide on bare stainless steel is rapid and irreversible: Both at 50ppm and 17bbp levels, H2S is lost within 24 hours.
Figure 2, demonstrates that even at concentrations of 50ppm, hydrogen sulfide sampling requires passive surfaces. In this analysis, sample cylinders tested were either sourced from the manufacturer, non-coated, or treated with a carboxysilane, commercial name Dursan™.
Figure 2: Sulfur Compounds at 50 parts-per-million in
carboxysilane treated Stainless Steel Containers versus non-treated cylinder
The effect of passivation on sulfur storage and transport reliability is debated. Passivation is a technique that is based on the assumption that if all active areas of a transport vessel or storage vessel are taken up by sulfur compounds, they are made inert to sulfur compounds. There have been studies to support this at low temperature for gas phase transport through low surface area regulators. It was demonstrated that purging a component with clean gas can reduce the inertness of the passivation with measurable impact occurring within 1 day and complete within 1 week. Additional data in the same study also demonstrate that heated stainless does not passivate and complete adsorption of sulfurs will occur no matter the conditions and previous exposure to sulfur compounds.
In work to test the stability of sulfur compounds during static sampling, as in sample cylinders, the use of gases such as silane (SiH4) along with multiple day exposure to 5000ppm H2S was required to create a passive cylinder for storage. Much of the data in these studies was done to demonstrate stability for the use of creating low-level standards.
Commercially available inert coated components have eliminated the need for passivation and are now recognized as a “use out of the box” solution to sulfur sampling and transport reliability. This eliminates the need for working with dangerous materials such as high concentration H2S or pyrophoric gases such as silane. The value delivered by coating solutions cannot be taken for granted in comparison to passivation techniques which increase the risk of obtaining poor analytical results.
Process analyzers and process sampling systems often times are exposed to challenging environments both internally and externally. Many sample streams are corrosive or contain active compounds that reduce equipment lifetime or require extended preventative maintenance. Some systems are exposed to environments such as sea water, which cause rapid deterioration of equipment, requiring extra costs to keep them operating. For systems that are required to give accurate, reliable and repeatable data in such conditions, the cost of upkeep and maintenance is much larger than systems in more benign environments.
Chloride environments and chloride containing streams can greatly reduce the lifetime of process systems. Coatings, paints and costly super alloys have been used to increase the lifetime of components in salt water and/or chloride containing environments. Table I provides the results obtained from ASTM G31 testing. This method is an immersion test for 24 hours in a 6M HCl (18%) solution at room temperature and pressure. After immersion, differential weighing allows the amount of material loss to be determined. The sample size for each configuration was 3 samples. The amorphous silicon coated stainless steel shows greater than 20 times the resistance of non-treated stainless steel in these environments and the carboxysilane treatment creates greater than 200 times the resistance. Any loss in the coated samples occurred as a result of pitting corrosion. The pitting is an indication that there are still pin-holes present in the surface which allowed corrosive attack to initiate.
Table I: Weight loss after 24 hour exposure to 6M (18%) HCl
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24hr; 6M HCl; 22ºC
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304 SS
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Silicon coated
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Carboxysilane coated
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|
MPY (mils-per-year)
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389.36
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16.31
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1.86
|
|
Improvement Factor
|
---
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23.9
|
209.8
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Another factor for consideration is the wear resistance of coatings applied to analytical sampling equipment. This factor is critical, especially in applications where there mechanical rubbing such as valve movements or physical abrasion like particulates moving through the sampling equipment at high velocity. Valve seat movements or particulate in these applications can quickly erode a soft coating such as silicon creating sites for adsorption to occur. Table II summarizes the data obtained from wear studies conducted on both non-treated and treated surfaces. Data was generated using a pin-on-disk tribometer (Nanovea, Irvin, CA). The experiment uses a flat plate loaded onto the test rig and the indenter applies a precise force to the surface. The plate is then rotated and forces are measured between the pin and the disc. Results from this experimental method can produce wear behavior and friction coefficients of the plate surface1. Results from this study demonstrate that the carboxysilane coatings wear less than untreated steel and silicon coated surface. The improved wear resistance as a result of the coating will lead to longer lifetimes of system components in extreme environments.
Table II: Physical PROPERTIES of coatings
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Pin on Disc; 2.0N
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316 stainless steel
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Carboxysilane coated 316 stainless steel
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Silicon coated 316 stainless steel
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Wear rate (x10-5mm3/N m)
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13.810
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6.129
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2
|
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Improvement Factor over SS
|
---
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2 times
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1/3 times
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