Unplanned or emergency downtime is inconvenient, expensive and increases the risk of a dangerous occurrence. To run a safe and effective planned shutdown period in order to carry out preventative maintenance procedures, two stages of health and safety preparations need to be completed before work commences.

The first or primary stage involves lockout/tagout, as required by OSHA, 29 CFR 1910.147, The Control of Hazardous Energy Sources. The secondary stage of shutdown relates to the atmospheric condition of the area within which workers are required to carry out maintenance and/or servicing. OSHA Standard 29 CFR 1910.146, Permit-required Confined Spaces, requires the plant to ensure “acceptable entry conditions” that are free from any of the following:

  • Flammable gas, vapor or mist in excess of 10 percent of its lower flammable limit (LFL);

  • Airborne combustible dust as a concentration that meets or exceeds its LFL;

  • Atmospheric concentration of any substance for which a dose or a permissible exposure limit is published in Subpart G (Occupational Health and Environmental Control) or Subpart Z (Toxic and Hazardous Substances) of the OSHA Standard 29 CFR 1910.146;

  • Any other atmospheric condition that is immediately dangerous to life or health.

    Toxic gases

    Of these risks, one of the most common is the potential human exposure to, or an unplanned combustion of, a gas or liquid. The toxic gases most likely to be present in confined spaces and open plant areas are carbon monoxide (CO), hydrogen sulphide (H2S), ammonia (NH3), chlorine (Cl2) and sulphur dioxide (SO2).

    The OSHA 1910.146 legislation requires companies to test internal atmospheres before an employee enters and then periodically test the atmosphere to ensure there is no accumulation of a hazardous atmosphere while the work is taking place. The proper gas detection equipment, therefore, must be an integral part of any manufacturer’s health and safety program.

    Keys to consider

    Gas detection products and systems available to help manufacturers and processors meet legislation range from portable, disposable personal “safety badges” for single-gas detection, to more sophisticated portable multigas detection instrumentation, to full fixed systems for protecting an entire plant or area. Consider the following key issues when determining your needs:

  • Nature of the threat — Safety assessments define which gases are considered a threat. Gas detectors are available in single-gas versions for detection of O2, CO, H2S, SO2, NO2, CL2, H2, and HCN for example, or in multigas versions for the detection of the most common threats, namely O2, CO or H2S and/or flammable gases.

    For short-term, multi-user applications, such as use by subcontractors on a large plant for example, disposable “safety badge” detectors may be the most cost-effective option. Where there is a more sophisticated datalogging requirement, a serviceable detector with built-in intelligence will probably offer a better solution.

  • Data acquisition capability — Consider what data will be required if there is an incident on site. The datalogging capability of different units may vary considerably. Look for some sort of record of gas exposure as a minimum requirement. This may be the upper or lower limit STEL or TWA or a full record of the gas levels recorded since switch on.

    More sophisticated instruments allow gas and calibration data to be downloaded to a PC for storage and analysis.

  • Reliability and calibration — Some gas detectors need to be calibrated regularly in order to ensure their reliability and confirm their accuracy. For maximum availability and efficiency on site, look for equipment featuring pre-calibrated single- and multigas sensors. Consider the use of detectors that come with the option of an on-site, self-calibration device for quick, easy and regular checking of equipment.

  • Communications — Portable units are available for use in confined space applications that allow communications between the “entrant” and the “attendant.” If an event occurs, this feature provides the attendant access to valuable information about the nature of the threat prior to taking action, which could help avert death or serious injury for either or both operators.

  • Confined space/open plant monitoring — Much of the work during planned shutdown is in confined spaces such as inside tanks and storage vessels, where access is prohibited during normal operation. The operator entering the confined space should always use personal protection monitoring.

    There may be a threat of gas in open areas, particularly if the shutdown is part of an expansion or renovation where hot-work permits may be in place. Open process areas may be protected by fixed-point detectors, which are able to provide protection for both people and plant in the area. However, in the absence of such a fixed-point system, a “temporary fixed” system may provide a cost-effective solution. This is achieved by using portable sensors mounted and linked together to provide a protective system for the duration of the work.