GAS CONTROL EQUIPMENT continued
pressure-control device called a vaporizing regulator that is internally heated to convert the incoming stream to a gas without altering the concentration of the minor component, H2S or other sulfurs that may be present. A surface treatment called Dursan (SilcoTek) is used on these units. Figure 4 shows the vaporizing regulator with the LumiShield appli- cation, which is even more corrosion resistant and inert, and is resistant to the high temperatures in the vaporizing regulators.
Figure 3 – A dual-stage 430 SilcoNert 1020 regulator shows an inert gas purge inlet mounted as it would be in a field installation.
Use of a pressure control that incorporates a heating element is critical when minor components are present that can condense into a liquid phase. This is true when the source is liquid, but is also necessary with high-pressure gaseous fuels like natural gas (which is mostly methane) that are subject to the Joule-Thomson effect, in which the gas rapidly cools as it undergoes a pressure drop. Mixtures are delivered to the instru- ment at relatively low pressure, normally less than 30 pounds per square inch gauge pressure (psig), or 2 bar. Comparing this to typical cylinder fill pressures at 2000 psig or high-pressure natural gas streams of similar pressure, the regulator delivers such high pressure and the temperature drops across the seat(s) of the device because of the Joule-Thomson effect (which can cause the components to drop into liquid phase and prevent them from reaching the analyzer). Methane, the main constituent in natural gas, is particularly susceptible. Using a vaporizing regulator that is extremely inert may be the only way to accurately deliver a sample and calibration mixture to an analyzer.
In most cases, that is, when not analyzing a liquefied source or a gas sus- ceptible to the Joule-Thomson effect, a dual-stage regulator is effective. With this type of regulator, the pressure falls in stages and temperature drop is reduced.
Dual-stage regulators deliver the standard at a constant delivery pressure, enabling the operator to control both flow and instrument calibration time. This is done by reducing the primary cylinder pressure to an intermediate pressure, that is, the inlet pressure of the second stage of pressure control, which can then be controlled. The benefits of two-stage pressure control outweigh its higher price, allowing users to avoid the cost of repeated manual adjustments and potential inaccuracy of single-stage regulators.
The outlet pressure variation that occurs with only a single stage of pres- sure control as the inlet pressure drops is typically unacceptable. Even in the best design, the outlet pressure rises 2–4 psig per thousands of psig pressure drop.
Figure 4 – The Dursan-treated LumiShield vaporizing regulator is used for liquefied fuel analysis or to counteract the Joule-Thomson effect.
and remove the toxic mixture during cylinder changes, and provides further protection from toxic H2S. Color-coded valve knobs allow the operator to introduce the purge nitrogen; dilute the H2S concentration; and vent the diluted, purged gas to an appropriate location.
Analysis of sulfur content in diesel fuel or liquefied refinery fuel gas must be done when they are in a gaseous state. This requires a special
AMERICAN LABORATORY 30
Conclusion The gas pressure regulation system and choice of materials should be compatible with the compounds to which they will be exposed. Precise pressure control is a vital part of this process, and allows operators to control costs and improve accuracy.
Larry Gallagher is specialty gas products manager, CONCOA, 1501 Harpers Rd., Virginia Beach, Va. 23454, U.S.A.; tel.: 800-225-0473; e-mail: lgallagher@
concoa.com;
www.concoa.com
APRIL 2016
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