Chemical Engineering September 2018 - 51

Risk mitigation objectives: Gas detection type required
Risk-mitigation and life-safety
gas detection
Explosion prevention
Objectives
Priority #1:
Personnel protection
Priority #2:
Equipment protection
Priority #3:
Process protection
Priority #4:
1. Gas detectors for risk mitigation and life safety are
needed outside of gas-carrying pipes to detect leaks that
could pose a threat to personnel, equipment or facility
2. Gas detectors for process monitoring are needed to
maintain production control
FIGURE 4. Risks related to toxic and combustible gases can be prioritized to help determine the appropriate type of gas detection required
tible and toxic gases.
Combustible gases are those that
can cause a fire or explosion if the
gas is exposed to an ignition source,
such as a spark, a hot surface, an
open flame or even friction caused by
gas escaping through a pipe fissure.
Containment is the first safety measure
in relation to combustible gases,
but the detection of leaks is a second
GAS DETECTION GUIDANCE DOCUMENTS
(IEC, E.U., U.S. AND CANADA)
Combustible Gas Performance Standards
* IEC/EN 60079-29-1 Explosive Atmospheres - Part 29-1: Gas Detectors - Performance
Requirements of Detectors for Flammable Gases
* CSA C22.2 No. 152: Combustible Gas Detection Instruments
* ANSI/ISA-60079-29-1 Explosive Atmospheres - Part 29-1: Gas Detectors - Performance
Requirements of Detectors for Flammable gases; ANSI/UL 2075, Gas and Vapor
Detectors and Sensors
Recommended Practice Standards:
* IEC/EN 60079-29-2 Explosive Atmospheres - Part 29-2: Gas Detectors - Selection, Installation,
Use and Maintenance of Detectors for Flammable Gases and Oxygen
* C22.1 Appendix H: Combustible Gas Detection Instruments for Use in Class I Hazardous
Locations
* ANSI/ISA-60079-29-2 Explosive Atmospheres - Part 29-2: Gas Detectors - Selection,
Installation, Use and Maintenance of Detectors for Flammable Gases and Oxygen
Method of Protection Guidance Standards
* EN 1127-1 Explosive Atmospheres - Explosion Prevention and Protection - Part 1: Basic
Concepts and Methodology
* C22.1 Appendix H (of the Canadian Electrical Code) Combustible Gas Detection Instruments
for Use in Class I Hazardous Locations
* ANSI/ISA-TR12.13.03-2009: Guide for Combustible Gas Detection as a Method of
Protection
* Other regional and local protection guidance standards
Notes
1. Though some regulatory authorities have laid out gas-detection system design and performance
requirements, there are no documented rules concerning optimum detector placement
or quantity. Hazardous operation analysis, however, can assist planners in this regard.
So can past experience, which shows that it is helpful to identify the most likely sequence
of events leading to a gas leak, as well as typical environmental conditions during leakages,
when determining optimal sensor installation points.
2. The documents list above does not include all standards that may be applicable for a specific
application or geographic location.
❏
50
critical safety measure. Many gases
are both combustible and toxic.
A toxic gas is one that can cause
harm to humans, ranging from minor
irritation to death. Even at low concentrations
measured in parts per
million (ppm), certain toxic gases can
cause death by poisoning caused by
exposure to carbon dioxide or by asphyxiation.
Asphyxiation occurs with
exposure to atmospheres containing
less than the concentration of oxygen
needed for human life. The addition of
any gas, except oxygen, to air reduces
the oxygen concentration through displacement
and dilution, particularly
when the added gases are nitrogen or
other inert gases such as argon and
helium. Breathing as little as one or two
breaths of air containing too little oxygen
can have immediate and lasting effects,
from unconsciousness to serious injury
or death.
From a risk-mitigation perspective,
there is a hierarchy of risk that
should be considered when designing
a plant's hazardous-area gas detection
system. The risk mitigation
objectives in Figure 4 are ranked in
priority order, based on severity of
risk. The type or types of gas detection
required for detecting the
risk underlying each objective are
also listed.
Gas detection for risk mitigation
According to IEC 60079-29 Series
standards and IEC 62990 Series
standards currently under developCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
SEPTEMBER 2018
Combustible gas
Toxic gas
Process-monitoring
gas detection
Notes
1. Gas detectors for risk mitigation can also be used
for validation of area classification (for example, less
than 10 h flammable atmosphere exposure per year)
1. Limits for short- and longterm exposure to toxic
gas have been set by agencies, such as the U.S.
Occupational Safety and Health Administration and
Control of Substances Hazardous to Health in the U.K.
1. A gas-related equipment explosion will likely also
pose a threat to people

http://WWW.CHEMENGONLINE.COM

Chemical Engineering September 2018

Table of Contents for the Digital Edition of Chemical Engineering September 2018