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The design of the sample system for an analyzer is equally as important as the application of the analyzer itself. The sample system may vary in complexity from a simple shutoff valve and single tubing connection to a complicated system consisting of solenoid valves, filters, vaporizers, regulators and other items.
Basic principles of the sampling are as follows:
- Extraction of a representation sample from the stream.
- Cleaning the sample.
- Minimizing the time lag in getting the sample to the analyzer.
- Meeting the pressure, temperature and sample flow rate requirements of the analyzer as well as the vaporizing a liquid sample or removing water drops.
- Disposition of the sample in a safe area.
The application of these basic principles is discussed in the following paragraphs.
The sample point must be in an active stream to assure that it provides the information required by the analysis. Sample points on process lines should always be installed on the top or side of the pipe, never on the bottom. This prevents the flow of condensed liquids in vapor streams from entering the sample valve and also eliminates the entrance of fine solids, which may be swept along the pipe. The side tap is preferred on liquid streams to minimize the chances of getting entrained vapors. The probe shown in below is commonly used to extract a sample from the center of the stream; thereby avoiding condensed liquids in a vapor stream and solids in a liquid stream. A clean stream is important to successful low maintenance operation of most analyzers.
It should be assumes that all sample streams contain solid contaminates such as rust particles, polymers, etc; therefore, minimum requirements call for a fine mesh wire filter. Extremely fine solids sometimes require the use of a porous metallic or ceramic filter. The gaskets and diaphragms of all pressure regulators, flow controllers, filters, etc., should be examined to determine that they will not deteriorate or absorb components from the stream. The filter might be considered as one of the most critical components in the sample conditioning system simply because so many other functions are performed after the stream has been cleaned.
Some vapor samples contain so much entrained solids that filtration is not practical until the vapor is scrubbed either by bubbler devices or by small spray scrubbers.
Sampling Time Lags
Sampling time lags delay the analysis and may reduce the usefulness of the measurement. The analyzer needs to be located close to the sample point, but that is not always feasible. A bypass loop such as that shown in the figure below is often used to reduce time lags in long sample runs where the analyzer will handle only a small flow rate. This system can be used on liquids or gases whenever there is a point of lower pressure for sample return or when the sample can be vented or run to a sewer. Installing it in raceway or thin-wall pipe should protect transport tubing. The tube should be sloped downward toward the analyzer to drain liquids, and no pockets should be left in the line without a condensate trap. Tube size is determined by flow conditions. Flow volume should be as low as possible and velocity should be as high as is practical to minimize lag time.
An effort should be made to match the sample requirements of the analyzer to that of the stream, there by minimizing extra system components such as vaporizers, regulators, sample coolers, etc. If a liquid sample pressure is to be reduced , care should be taken that flashing does not occur. However, it is not always possible to match the analyzer to the process condition, and the sample must necessarily be conditioned to meet the analyzer requirements.
The sample parameters like temperature, pressure, flow and etc. are very important when the sample is processing in to the analyzer .
Many manufacturers make small heat exchangers to cool the sample. Cooling water may be allowed to flow unregulated through the exchanger, leaving the sample temperature uncontrolled. Some Analyzers are temperature sensitive, however, and the measurement can be improved by controlling the sample temperature. In some cases, close temperature control is essential to the separation and measurement of components.
Pressure is also a common mismatch. Regulators are used to reduce high pressure and to regulate varying low pressure. To prevent maintenance problems and increase performance, the regulator should be preceded by a fine mesh strainer. Materials should be noncorrosive.
Single stage regulation is usually sufficient for sample pressure reduction.
On vapor systems the regulator is usually located at the sample point to keep a high sample velocity with a low flow rate. When the sample pressure increases then there may require a relief valve or rupture disc for overpressure protection.
Many analyzer are sensitive to velocity changes, and the sample flow rate must be monitored / controlled. Where sample conditions are constant, a simple rotameter suffices. When sample conditions are not constant, a rotameter ( constant differential regulator combination ) should be used. The instrument manufacturer recommends the flow rates of each instrument.
This is the last and most critical stage of the sampling system. Sample disposal sometimes presents significant problems. Safety has always been a consideration since many samples are either toxic or flammable or both. In some applications the sample may simply be dumped or vented to the atmosphere. In others, where venting becomes dangerous, other solutions are required.
When disposing analyzed samples of water or other inexpensive, nonpolluting, nonhazardous liquids, the most common method is to run it to the sewer and then to the effluent plant. This method may also be used on hazardous or polluting samples if an appropriate chemical sewer is located nearby. If sample is expensive or cannot be conveniently run to the sewer, it is commonly returned to the process system at a point where the pressure is lower than the sample pressure and as steady as possible.
Vapor samples present slightly different problems. Non-hazardous gases are normally vented to the atmosphere while hazardous gases are often vented to a flare header or returned to the system, whichever is more convenient. Gases, which cannot be vented to a flare header or returned to the system, may be scrubbed of the corrosive or toxic components and then vented to the atmosphere or flare header.
Vent flow can be very critical to the successful performance of an analyzer.
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