What is sampling?
The sampling system is designed to obtain a genuinely representative sample of the solid, liquid or gas to be analyzed at an adequate and stable rate and to transport it to the analytical instrument without modification.
It is essential that the sample taken should represent the mean composition of the process material. Sampling is to ensure the process parameter of materials involved is in the desired range, thus deliver a most accurate chemical state.
Components of the sampling process:
Probe:
The most important function of a probe is to obtain the sample from the most representative point (or points) in the process line.
Filters:
They are ideal for preliminary filtering of samples before pumps or at sample points to prevent line scale from entering sample lines.
Filtration sizes are available from 75 to 400 nm (200 to 40 mesh). The main application for this type of filter is for liquids and steam.
Coalescers:
Coalescers are a special type of filter for separating water from oil or oil from water.
The incoming sample flows through to the outside from the center of a specially treated filter element.
Coolers:
These are usually used before feeding into the analyzer to bring the temperature of the sample gas close to the ambient.
Pumps, Gas, Liquid:
Every time gaseous samples are taken from sample points below the pressure required by the analyzer, some type of sample pump is required.
And there are other pumps required in liquid sample systems.
Flow Measurement and Indication:
Analyzer flow measurement falls into three main categories:
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Measuring the flow precisely where the accuracy of the analyzer depends on it
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Measuring the flow where it is necessary to know the flow rate but it is not critical (e.g., fast loop flow)
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Checking that there is flow present but measurement is not required (e.g., cooling water for heat exchangers).
Pressure Reduction and Vaporization:
The pressure-reduction stage in a sample system is often the most critical, as it is necessary not only to keep the reduced pressure constant, but also to ensure that dangerously high pressures cannot be produced under faulty conditions.
Sample Lines, Tube and Pipe Fitting:
Sample line
The most important consideration is that the material chosen must not change the characteristics of the sample during its transportation to the analyzer. There are two main ways in which the sample line material can affect the sample.
Pipe Fitting:
The most popular method of connecting metal tubing is the compression fitting, as it can withstand pressures up to the limit of the tube itself and is easily dismantled for maintenance and can be obtained in all the most common materials.