Sampling Systems

The Purpose of Sampling Systems

The objective of a sampling system is to obtain a truly sample of the solid, liquid or gas to be analyzed, at an adequate and steady rate, and transport it without change to the analysis instrument, while ensuring that all the necessary precautions are taken to make this happen.

Before the sample enters the instrument it may be necessary to process it to the required physical and chemical state, that is, correct temperature, pressure, flow, purity, and so on, without eradicating the essential components. It is also necessary to dispose of the sample and any reagent after analysis without introducing a toxic or explosive hazard. That is why, the sample, after analysis, is continuously returned to the process at an appropriate point or a sample recovery and disposal system is made available.

In the following section we look at steam sampling for conductivity as an example to help us have an idea of how the structure of a typical sampling system looks like.

Steam Sampling System for Conductivity: Instrumentation Structure  

Let’s consider the schematic illustration of steam sampling system for conductivity below:

In reference to the above diagram, the steam sample is taken from the process line by means of a special probe and then flows through thick-wall 316 stainless steel tubing to the sample system panel. The sample enters the sampling panel through a high-temperature, high-pressure isolating valve and then flows into the cooler, where the steam is condensed and the condensate temperature is reduced to an appropriate temperature for the analyzer, usually 30° C.

After the cooler, the condensate passes to a pressure control valve to reduce the pressure to about 1 bar gauge. The temperature and pressure of the sample are then measured on suitable gauges and a pressure relief valve (typically set at 2 bar g) is fitted to protect downstream equipment from excess pressure if a default occurs in the pressure control valve. The constant-pressure, cooled sample passes through a needle valve, a flowmeter and three-way valve into the conductivity cell and then to drain.

Apparatus are provided for feeding water of known conductivity into the conductivity cell through 3-way valve for calibration purposes. The sample coolers are normally supplied with stainless steel cooling coils which are appropriate where neither the sample nor the coolant contain considerable chloride which can cause stress corrosion cracking. If chlorides are known to be present in the sample or cooling water, cooling coils are available, made of alternative materials which are resistance to chloride induced stress corrosion cracking.

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