For a reliable, accurate analysis of a gaseous sample many analyzers require a known, stable pressure. Otherwise, the results can be affected by variations in pressure. When the sample is drawn from a source that is close to atmospheric pressure, a means must be provided to transport the sample to the analyzer while maintaining close pressure control.
Various mechanical pumps have been utilized to move such samples, but pumps have many disadvantages. Pressure surging, wear from the constant motion within the pump, corrosive or abrasive samples, installation cost, and electrically rated areas can lead to frequent maintenance and high capital cost. Furthermore, leaks through packing glands, valves and elastomer diaphragms cause sample contamination. When using a mechanical pump, system reliability is automatically reduced.
The Jacobs Process Analytics aspirated gas sampler system (see Figure 1) has been designed to move samples through a gas analyzer at a controlled pressure and to eliminate most of the operating problems inherent to other sampling methods. The system consists of an eductor (a.k.a. aspirator or Venturi vacuum pump) and a pressure control system. Materials of construction in contact with the sample may include Teflon, 316 ss, Titanium, or Monel depending on requirements of the application. There are no moving parts, and because no electrical connections are required the system is intrinsically safe and installation less costly.
Operation
Figure 2 is a schematic representation of the model 600 panel. Controlled supply pressure to the eductor produces a partial vacuum at the intake from the sample point. This causes the sample to flow from its source, through the analyzer and into the eductor. Sample flows for the eductor are illustrated in Fig 3. Sufficient pressure reduction for most applications is available because the eductor will produce a vacuum of ~18 in Hg at 20-25 psig
The pressure of the sample is regulated by bleeding some air from the supply line through the sub-atmospheric regulator into the sample line coming from the analyzer. Precise control to ¼ inch of water pressure can be achieved. This system has proven to be reliable and maintenance free during long service periods in severe industrial environments.
The aspirated gas sampler constitutes the heart of a complete sampling system for most pressure sensitive gas analyzers. It is available either as a panel-mounted unit or contained in a NEMA 12 enclosure. Either model is fully assembled and ready to connect to sample source, utilities and vent.
