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Using gas monitors during confined space entry

December 1, 2009
What is an effective way to monitor worker safety during inert gas purged confined space entry?

Answered by Bob Durstenfeld, senior director, corporate marketing, RAE Systems, San Jose, CA.

One of the most dangerous confined space entries is when an inert atmosphere is present. Inert or "nonreactive" atmospheres are used to displace oxygen or other reactive gases when the presence of that gas presents an explosion risk to either the process being performed or the compounds being stored. Some examples of inert process applications include the transportation of flammable cargo, catalytic processes in the petrochemical refining industry and welding operations. These industrial processes must maintain oxygen levels well below the combustion point for the catalyst or the chemicals being processed and therefore well below the viability point for any workers involved. The most commonly used inert gas is nitrogen, although other "post-combustion" gases are sometimes used with the objective of reducing the atmospheric presence of oxygen from its normal 20 percent to a range of less than 4 percent to 8 percent.

Catalysts that initiate or accelerate a chemical reaction without being affected themselves often are used within oil refinery reactor pressure vessels to accelerate the crude oil reduction process into the desired output compounds. Refinery hydro-treating reactors typically contain catalysts composed of the following elements: molybdenum, nickel and cobalt.

When the catalyst beds must be serviced, a technique called "gas blanketing" is used to prevent the external atmosphere from reacting with the catalyst and bringing it into the explosive range. Nitrogen is pumped into the vessel until it is purged, then a service technician with a special life-support breathing helmet and protective suit enters the pressure vessel. Catalysts are removed from reactors in a highly reactive, sulfided state. The entire catalyst change-out process can take as long as a week.

Worker safety is the primary concern during this type of work. Technicians generally work with a safety-locked helmet with redundant air supplies, real-time radio contact with the safety team and personal back-up controls. The person operating both the purge atmosphere and the worker's breathing air works from a trailer on the ground. The purged atmosphere must be continually monitored for real-time oxygen level and explosive gases such as volatile organic compounds, as well as carbon monoxide.

The risks of working in an inert atmosphere:

  1. The inert atmosphere needs to be monitored for explosive gases, and if people are involved they must have an adequate breathing supply.
  2. Regular calibration and testing of the equipment used to generate inert gas is required to ensure it works correctly. A sensor to measure the level of the inert gas and/or oxygen is needed to ensure atmosphere is not in the flammable range.
  3. The inert atmosphere needs to be monitored for toxic gases. For example, in refinery catalyst maintenance the presence of benzene, VCM, toluene, xylene, EDC and many other hydrocarbons can exist. The presence of volatile organic compounds calls for the use of a photoionization detector.
A wireless monitoring system is quick to deploy and allows real-time remote monitoring of the inert atmosphere to ensure worker safety, and that data can be shared by both the service provider and the site safety officer. A high-range carbon monoxide sensor is used to measure the presence of CO as well as hydrogen. This is in addition to the oxygen sensor, which is used as a second sensor to ensure oxygen levels are low enough to prevent spontaneous explosion.