The measurement and/or control of gas phase oxygen and carbon dioxide levels, as well as other gases, frequently calls for the use of some form of sample conditioning prior to analysis. Effective sample conditioning can involve a wide number of topics and technical disciplines. A simple system may only require the use of a single stage pressure regulator while more complex systems may encompass the use of complex, multi-stage liquids and solids removal systems. The severity of certain process sample streams can preclude the use of on-line continuous sampling. In such cases, the gas constituent in question may need to be extracted manually (discrete sampling) for subsequent analysis in the laboratory. A useful resource on the subject of sample handling is contained in the book entitled, Principles of Sample Handling and Sampling Systems Design for Process Analysis by E A. Houser. Many of the principles outlined in this book apply for gas analyzers, including oxygen analyzers and carbon dioxide analyzers. The following is an excerpt from the book:
”There are six overall functions of any sampling system:
- To take a sample that is representative of the flowing process stream. The term “representative” is used in the most liberal sense, that is, relative to the application needs of the particular system.
- To transport the sample from the sample point to the analyzer.
- To condition the sample so it is compatible with the analyzer and with application needs. Conditioning includes many operations, a few of which are cleaning, vaporizing, condensing, adjustment of pressure and temperature, and many others.
- To switch from one stream to another, where the analyzer is used on more than one sample stream. Additionally, to provide for proper introduction of calibration standards, where these are necessary. This function is vital to the correct operation of any sampling system, because analysis accuracy is an important consideration.
- To transport the sample from the analyzer to the desired point of rejection. Included here are venting arrangements, waste disposal systems, and methods for returning the sample to the process where necessary, without adversely affecting analyzer operation.
- To allow for the effects of corrosion and other reactions.
These functions should be accomplished with the timeliness, accuracy, and other needs of the analysis system. A well-designed sampling system optimizes the above performance factors with the cost of equipment, installation, operation and maintenance that are a part of anything mechanical.”
The task of designing and building an efficient and reliable sample conditioning system can be a daunting task if care is not given to considerations such as:
- Sample filtration.
- By-pass sampling.
- Heating or cooling of sample gases.
- Removal of entrained liquids.
- Use of temperature control to eliminate condensation of liquids.
- Sampling pump selection (diaphragm, bellows, vortex, etc.).
- Sample dilution techniques.
- Proper sizing of valves as well as valve types.
- Sample pressure regulation.
- Sample tube material
- Gas connections
Many manufacturers of gas analyzers will offer sample conditioning as a “system solution” to their customers. Some will provide complex systems while others, less complex.
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