by Edward Naranjo, Marketing Director, Emerson Process Management, Rosemount Analytical
Gas detection systems are a common means of detecting and responding to the accidental release of hazardous materials. From their beginnings in the mining industry more than a century ago, gas detectors are now commonplace in the process sector and have become more effective as technologies have advanced and process safety standards become more widely accepted. Despite progress, instances exist where gas detection systems are not used appropriately. Alarm setting, in particular, is often done without considering if the release of concern can actually be detected at the alarm level or provides operators with enough time to respond to the hazard. As a result, some facilities are prone to experience inconsistent protection levels, high incidence of nuisance alarms, and in some instances, may be exposed to gas leaks that are detected at levels that exceed a safety basis [Center for Chemical Process Safety (CCPS) and American Industrial Hygiene Association (AIHA), 2009]. Poor alarm setting can increase the frequency of incidents if alarms’ limitations are not understood or warnings are ignored [Kletz, 2009].
To prevent these outcomes, operators must take a holistic approach to gas detection management. First, gas detection set points must be based on the hazard properties of the hazardous gas in question. For combustible gases, the low explosion limit (LEL) is the basis of alarm settings as established by a whole host of standards and federal, state, and local regulations [ANSI/ISA-TR12.13.03, 2009; HSE, 2004]. For toxics, alarm set points are based on worker exposure limits, usually the time-weighted average (TWA) threshold defined by local policies or legislation or immediately dangerous to life and health (IDLH) concentrations [Walsh et al., 2013]. ISA-92.00.02 (2013) advises one use:
“…the lowest level indicated in the current edition of OSHA (Occupational Safety and Health Administration) Title 29 Part 1910 Subpart Z, ACGIH (American Congress of Governmental Industrial Hygienists), and/or other local publications until determination of appropriate alarm levels can be made.”
Table 1 lists worker exposure limits for the United Kingdom’s Control of Substances Hazardous to Health (COSHH), OSHA, and IDLH values of several toxic industrial commodity chemicals.
It is not uncommon for operators to use two alarm levels, low and high, when monitoring for toxic substances. The low alarm provides an initial warning to assess the situation, while the higher alarm could prompt emergency procedures like evacuation and plant shutdown. Under ANSI/ISA-TR92.00.03 (2014), low level alarms should be activated when toxic gas concentrations reach the TWA level or lower; high level alarms should be set at or below the published STEL.
In the semiconductor industry, the maximum low level alarm is set at 20% LEL for combustible gases and half of the IDLH for toxics. In contrast, the maximum high level alarm is set at 40% LEL for combustibles and the IDLH for toxics.
Second, one must consider alarm set points are not static [CCPS and AIHA, 2009; Walsh et al., 2013]. While guided by industry practice and regulation, end users should aim at setting alarm set points as close to ambient conditions as possible without causing nuisance alarms. By doing so, they maximize the probability the gas is detected at a sufficiently early stage to allow for effective automated response.
For new types of detectors, operations, or process equipment, alarm set points should be established by first setting the alarm level at a regulated limit and retain it for at least 30 days. After reviewing trend data to identify peak and average gas concentrations, the operator should reduce the alarm set point until the minimum alarm level recommended by the manufacturer is reached or nuisance alarms are produced. To establish the optimum alarm set point, the operator can retain the high alarm at the last level that provided no nuisance alarms and then reset the low alarm to the next incremental setting.
While establishing alarm set points in gas detectors across a process unit or plant, it is important to note the alarm set points need not be the same. For instance, areas with high concentrations of target gas or cross sensitive gases require higher alarm set points to avoid nuisance alarms. Similarly, it is often necessary to set alarm set points of outdoor gas detectors at higher levels than indoor gas detectors, since those detectors outdoors are subject to wider changes in temperature, humidity, and solar radiation, which could result in nuisance alarms.
Some process units may experience short-term or periodic excursions of high gas concentrations. Rather than adjusting alarm set points upwards throughout the facility, operators may use two alarm set points to avoid nuisance alarms. One approach is to program the distributed control system (DCS) or control panel to annunciate at the high level and at the low level threshold if the duration of exposure lasts longer than expected excursions. Alternatively, one alarm could be annunciated if the alarm level is exceeded or if the gas concentration were to increase at a faster rate than expected by trend analysis.
Alarm setting is influenced by many factors. A first consideration is the type, number, and location of the detector(s). This is necessary to ensure adequate detection coverage and suitability for the intended task. For alarm setting, one should consider default levels set by the gas detector manufacturer, regulatory requirements like occupational exposure limits, or alarm set points specified by standards or guidance. After identifying these initial alarm levels, the next step is to lower set points balanced with minimizing spurious alarms. This requires trending analysis over a suitable period and experience with the monitored process and the instruments. Equally important, one must adjust alarm levels based on the environment (outdoors, indoors, confined spaces) to allow sufficiently early warning for emergency response.
ANSI/ISA-92.00.02. 2013. Installation, Operation, and Maintenance of Toxic Gas-Detection Instruments. Research Triangle Park, NC: ISA.
ANSI/ISA-TR12.13.03. 2009. Guide for Combustible Gas Detection as a Method of Protection. Research Triangle Park, NC: ISA.
ANSI/ISA-TR-92.00.03. 2014. Guide for Toxic Gas Detection as a Method of Personnel Protection. Research Triangle Park, NC: ISA.
CCPS and AIHA. Continuous Monitoring for Hazardous Material Releases. 2009. Hoboken, NJ: John Wiley & Sons.
HSE. 2004. The Selection and Use of Flammable Gas Detectors. Sudbury, UK: Health and Safety Executive. http://www.hse.gov.uk/pubns/gasdetector.pdf. Downloaded 20 July 2015.
Kletz, T. 2009. What Went Wrong? Case Histories of Process Plant Disasters and How They Could Have Been Avoided (fifth ed.). Amsterdam: Elsevier.
Walsh, P., Hemingway, M., and Rimmer, D. 2013. Review of Alarm Setting for Toxic Gas and Oxygen Detectors, Research Report RR973. Sudbury, UK: Health and Safety Executive. http://www.hse.gov.uk/research/rrpdf/rr973.pdf. Downloaded 20 July 2015.
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