Using Gas Monitoring Instruments Safely

N/ATH=80% CELLPADDING=10> With so many opportunities for water supply, wastewater collection and treatment, and public works employees to encounter confined-space entry situations in the course of their work, the use of gas detection and monitoring instruments has become very common as these devices have become more reliable and affordable. But gas monitors themselves must be selected with care, and the key to working safely with them is understanding the limitations of intrinsic safety classifications.

Principally, intrinsic-safety testing of these instruments takes place in combustible gas/air mixtures containing a maximum of 21.0 percent oxygen. A unit classified as intrinsically safe by definition will operate safely in such atmospheres, and is certified safe for atmospheres containing oxygen at a concentration of 21.0 percent or less. The classification does not mean that such devices are not intrinsically safe in atmospheres containing more than 21.0 percent oxygen. It means only that they are not certified safe for such service. These testing and classification standards are uniform and are industry standards.

Oxygen itself is not combustible, but is essential for combustion. As oxygen concentrations increase, substances exposed in such atmospheres ignite more readily. For this reason gas-monitoring instruments should be equipped with upper-oxygen-concentration alarms adjusted to 23.5 percent-which is OSHA's limit for oxygen enrichment.

When intrinsic safety testing of an instrument has been completed, the unit is deemed safe to operate in a variety of gases. These gases are divided into classes, divisions and groups under the National Electric Code (NEC). Combustible gases fall under the designation Class I, Division I.

Class I, Division I comprises four groups, labeled A, B, C, and D. Group A contains the most volatile materials-and only one gas, acetylene. Group B contains hydrogen and similitudes. Groups C and D contain the majority of common combustible gases, examples being pentane, methane, and propane. A complete listing of the gases classified according to their atmosphere groups is available in NEC Table 500-2.

Combustible gases not rated under the National Electric Code can be assessed for combustibility according to a temperature code. For above-ground, non-volatile-dust areas, this code denotes the maximum hazard temperature presented by an instrument operated in such conditions. For example, a gas having an ignition temperature of 149 C in an above-ground application would be sufficiently described by a T4, T5, or T6 temperature code.

In rated instruments, individual components (such as resistors) may operate at temperatures exceeding the T-code rating of the instrument in above-ground applications (gases only). The reason is that the surface area of the component generally is insufficient to cause thermal ignition. This is not the case for volatile dust environments, where the maximum surface temperature of any component cannot exceed 150 C.

Therefore an instrument having a T4, or better, code cannot safely be taken below ground and/or into volatile-dust areas. Separate classifications specifying maximum surface temperatures exist for such conditions.

Comparing the maximum surface temperature with the ignition temperature of a given gas will determine if an environment containing a combustible quantity of that gas is permissible. An excellent source for ignition temperature listings is the National Fire Protection Association Publication NFPA 325M, 1991 Edition. Keep in mind that some gases such as carbon disulfide (CS2) have an extremely low ignition temperature (90 C) and are not covered by conventional approvals.

Remember, in any environment, always be prudent. Locate the target gas on the NEC or NFPA chart to determine whether the monitoring device is compatible with that gas in combustion conditions. This determination will ensure safe working practices in the application of gas monitoring instruments.

For more information on this subject contact <%=company%> at 1001 Oakdale Road., Oakdale, PA 15071: Tel. 412-788-4353 or 800-338-3287; Fax. 412-788-8353.

Edited by Ian Lisk