A Simple Field Procedure for Measuring the H2S Concentration in a High-Pressure Gas Line With a Colorimetric Detector Tube
Authors: Thomason William H. 
Published: December 2007

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More than ever before, gas producers are having to maintain accurate records of the H2S concentration in their system. Regulatory requirements, increased production of sour gas, and increased safety awareness have all served to bring about an increase in the number of H2S measurements. Most of the acceptable analytical procedures for measuring H2S are cumbersome for field use procedures for measuring H2S are cumbersome for field use and do not always provide the desired accuracy.

Perhaps the most widely used field procedure for measuring hydrogen sulfide in produced gas employs a colorimetric detector tube (CDT). This procedure is limited in its accuracy, but it is rapid and easy to use. Colorimetric detector tubes have not been accepted by the regulatory agencies as a standard analytical procedure for measuring H2S concentrations in a high-pressure gas system. One major reason for the nonacceptance of these tubes is the absence of a standard sampling procedure that assures a representative gas sample has been analyzed. This paper presents a sampling procedure that has been found to be effective in the field for measuring the H2S concentration in a gas obtained from a high-pressure system.

A CDT for H2S analysis is a glass tube that contains a matrix impregnated with an active material which changes color when contacted by H2S. Both ends of the tube are sealed for storage. When the tube is to be placed in service, both ends of the tube are broken and placed in service, both ends of the tube are broken and a known volume of gas is drawn through the tube with an appropriate pump that usually is supplied by the manufacturer of the detector tube. The H2S in the gas reacts with the active material to form a black precipitate. The detector tube is calibrated so that the H2S precipitate. The detector tube is calibrated so that the H2S content of the gas can be determined by noting the length of the color change that occurs along the long axis of the glass detector tube. Some of these tubes are tested and certified routinely by the National Institute of Occupational Safety and Health. The tubes can be purchased in various sizes for measuring a wide range of concentration levels of hydrogen sulfide. The maximum error expected for these tubes is +/- 10 percent of the true concentration.

There are several sampling procedures now being used in field work that could potentially introduce an error in the H2S measurements obtained with a CDT. The most popular sampling procedure consists of opening a valve popular sampling procedure consists of opening a valve to the high-pressure line and permitting a steady stream of gas to escape into the air. The detector tube then is placed as near as possible to this opening and a gas sample is drawn through the tube. It is also likely that some air is drawn into the tube. This would cause the measurement obtained to be lower than the true H2S concentration. Another sampling procedure consists of purging a container with the produced gas and then placing the detector tube in this container while drawing a gas sample through the tube.

A convenient and reliable field procedure that assures the measurement of a representative gas sample with a CDT is needed. One procedure that fills this need employs an apparatus assembled from a needle valve that can be screwed into the sampling port, pieces of flexible tubing, and a Y-connection for the flexible tubing.

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