Sierra Monitor Corporation TR-002 User Manual

Technical Reprint TR-002

Gas Risk Management – A Safer Approach to
Monitoring for Hazardous Gases

Monitoring for gas leaks has historically consisted of the use of dosimeters, or fixed detectors. Little has been done to
use these measurement methods within a system’s concept; and, collectively, these technologies do not encompass all
the ingredients required to comprehensively manage the risk of personnel or equipment exposure to gas leaks.

Essential to the goal of protecting people and facilities from the hazards of exposure to gas are the selection of the
appropriate gas measurement techniques, timely analysis of monitoring data and a plan to respond to a leak. This
article discusses issues and options to be considered in formulating a gas risk management program with a focus on
area monitoring and system capability.

Hazards of Gas

The hazards of gas exposure are generally categorized as combustible, toxic, or the special category of oxygen
deficiency. These hazards are typically found in-plant at the source, at landfills, and in incinerators.

Combustible Gas. In industrial facilities, methane, natural gas and Hydrogen are the combustible gases of primary
concern. Methane and natural gas are used in the facility for fuel and can be present due to pipeline leaks, or poor
maintenance. Solvents, propane, and other combustible gases may also be present and require monitoring.

The lowest concentration at which a gas will support combustion is called the Lower Explosive Limit (LEL). Below this
concentration, the gas is too “lean” to support combustion. There is a corresponding Upper Explosive Limit above
which the concentration is too “rich” to support combustion, Figure 1. It is important that the concentration of gas in a
facility remains below the LEL and that appropriate action is executed to insure the LEL is not reached.

Although the concentration at which different gases will combust varies widely, the Lower Explosive Limit is used as a
common reference in setting alarm levels of gas monitoring systems. The approach employs alarm levels that are
determined by how close the gas concentration is to the LEL and not an arbitrary absolute concentration of the gas. In
this fashion, all gases can be compared to their specific explosive limit and relative comparisons of risk can be made.
For example, the LEL of methane and Hydrogen are approximately 5.0 and 4.0 percent, respectively, of the gas in air.
A safe reference such as 20 percent of the LEL can be used as an alert to the danger of the gas leak (which
corresponds to 1.0 and 0.8 percent of methane and Hydrogen, respectively, in air). Figures 2 and 3 demonstrate this
approach.

Toxic Gases. Toxic gases create both an immediate and long-term risk to personnel and include such gases as
Carbon Monoxide, Chlorine, Nitric Oxide, Sulfur Dioxide, Hydrogen Chloride, Hydrogen Cyanide, Ammonia, Hydrogen
Fluoride and many others.

Toxic gases are often hazardous at low concentrations and are
usually characterized in terms of the Threshold Limit Value (TLV).
TLVs are the maximum 8-hour time-weighted average
concentration permitted of an airborne contaminant. The time
weighted average (TWA) is calculated as follows:

TWA= C

+ C2T2 + C1T1+ ………………CnT

1T1

n

8

where Ci= Concentration in period I where concentration

remains constant

T

= Period of duration in hours at concentration Ci

i

100%

(% gas in the air)

CONCENTRATION

0%

Too rich
for combustion

Upper Explosive
Limit (U.E.L.)

Will support
combustion

Too lean
for combustion

Fig. 1 – Explosive Limits

Lower Explosive
Limit (L.E.L.)

Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: [phone]
Visit our Web Site at: http://www.sierramonitor.com E-Mail: [e-mail]

Rev. A1

Technical Reprint TR-002

For some gases, excursions are permitted where the gas concentration may increase above the TLV for a limited period
of time, i.e. the allowable excursion duration. The ceiling limit is the maximum concentration of a contaminant allowed
without regard to duration. For example, in California, Hydrogen Sulfide has a TLV of 10 ppm with an excursion limit of
20 ppm for minutes in an 8-hour period. Its ceiling limit is 50 ppm.

Oxygen Deficiency. Where purging or processing with an inert gas as helium, argon, or nitrogen is employed, a leak
may result in a depletion of the oxygen concentration in an area occupied by plant personnel. As the concentration
oxygen drops, asphyxiation will result. OSHA has established 19.5 percent oxygen in air as a lower limit of a safe
environment.

Contamination of Process Gas. If process gas is leaking out of a pipe, then some room air will be leaking into the
pipe even through the process gas is under pressure. This leak-in air of course affects purity and can possibly have
some detrimental effect on yield.

Components of a Gas Risk Management System

A gas monitoring system must provide information to insure that effective and comprehensive decisions are made in a
timely fashion. It is not sufficient to sound an alarm after a low alarm limit has been reached. To be most effective in
managing the risk of gas exposure, sufficient information is required to obtain an accurate assessment of the situation in
order to make good decisions. Data management is a key ingredient of a gas risk management system.

Within this perspective, each situation imposes objectives and constraints on the gas monitoring system design,
operation, response to alarms, maintenance, and record-keeping requirements. These considerations must address
the risks identified in a hazard assessment and must be formulated into a risk management plan.

Hazard Assessment. Hazard assessment is the first step in system design. Field studies can help determine the
what, where, when of hazardous situation and possible exposure. The characteristics of the hazard in terms of its
ignitability, reactivity, corrosivity, and toxicity can be determined. In turn, the risk to human health as reflected in
standards set by RCRA, OSHA, and EPA can be estimated. Correspondingly, NFPA and insurance company
guidelines can help estimate risk to facilities. The impact of environmental factors can also be assessed. This is not to
say that gas risks can be accurately quantified; however, the information obtained in a hazard assessment provides a
basis to better address the identified gas risk.

System Specification and Design. System specification and design is a key output of the risk management planning
process. Design begins with sensor type selection and location. Sensor performance should be evaluated in terms of
speed of response, concentration range, resolution drift, ease of calibration, and interfering gases. In general, the
controller, wiring, backup power and alarm response must meet the duty performance criteria established in the plan.

Day-To-Day Operation. Day-to-day operation of
a gas monitoring system focuses on data review
procedures. It is important to use fixed systems
not only as an indicator of an alarm event, but
also to provide an early notification of a leak. A
system clock is becoming a more important
function to document events accurately. Day-to­day operation also focuses on system reliability.
Automatic self-monitoring of unit functions
enhances reliability by providing an ongoing
indication of operability. Regular maintenance
should confirm the internal diagnostics.

One person calibration is an important feature in
simplifying calibration and reducing labor
requirements. This process can be further
simplified if all adjustments are made
automatically after application of calibration gas,
which can be accomplished with microprocessor­bases controllers. Where there are a large

8
7
6

5.0 - 5.3

5
4

(% G a s in Air)

3

CONCENTRATION

2
1
0

Methane Hydrogen Propane Toluene Methanol

Fig. 2. The lower explosive limit varies substantially for different gases.

4.0

2.0 - 2.2

1.2 - 1.4

5.6 - 7.3

Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: [phone]
Visit our Web Site at: http://www.sierramonitor.com E-Mail: [e-mail]

Rev. A1