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:
100%
Too rich
for combustion
Upper Explosive
Limit (U.E.L.)
TWA=
C1T1 + C2T2 + C1T1+ ………………CnTn
8
Will support
combustion
Lower Explosive
Limit (L.E.L.)
where
Ci= Concentration in period I where concentration
remains constant
Too lean
for combustion
Ti=
Period of duration in hours at concentration Ci
0%
Fig. 1 – Explosive Limits
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
E-Mail: [email protected]
Rev. A1
Gas Risk Management – A Safer Approach to Monitoring for Hazardous Gases
numbers of sensors, it may be economical to apply the calibration gas automatically. A method to check sensor span
voltage versus acceptable tolerance should be available either through calculation or as a system report. Advanced
systems can prompt “calibration due” according to a user configured calibration frequency.
Response To Alarms. Response to alarms begins with preplanning – in plant and outside the plant perimeter. Next,
training is essential to insure all plant
personnel understand their function and
METHANE
HYDROGEN
PPM in % Gas
have internalized their responsibilities. Part
of this training includes identifying false
alarms.
PPM in
air
% Gas
in air
air
in air
100%
50,000
5.0
40,000
4.0
Relay action usually involves a low alarm
(warning light and siren and ventilation to
reduce the gas concentration) and high alarm
(emergency light and siren and a process
action). Process action involves turning off,
or isolation of, the source of the gas and
shutdown of process equipment. A high-high
alarm can be used as an indication that the
Typical HIGH
Alarm Setpoint
60%
20%
30,000
10,000
3.0
1.0
24,000
8,000
2.4
0.8
EMERGENCY
Typical LOW
Alarm Setpoint
concentration is very dangerous.
“Zone
WARNING
voting,” where two or more sensors must be
in alarm before activating a zone relay, can
be used as a way to neutralize a faulty
sensor and to take action specific to the area
that the gas hazard exists.
Fig. 3 Alarm Setpoints for Methane and Hydrogen
The final element of responding to alarms concerns evacuation procedures, both in plan in the local vicinity, and
requesting outside assistance such as the fire department. Events that trigger these actions are based on the hazard
assessment. Recent catastrophes and near catastrophes more than demonstrate the need for this requirement.
Record-Keeping. Record-keeping becomes an important element of response to alarms. Sensor status reports
provide a valuable reference to assess the severity and breadth of the gas hazard. Real-time and post-event analysis
can be generated. Calibration reports give data for monitoring sensor performance over time. Sensor history reports
are useful for satisfying regulatory requirements and providing data on gas exposure. It is the transfer of information
provided from the system to plant personnel that enables gas risk management to be effective.
New Wafer Processing Laboratory
A new wafer processing laboratory located in California had extensive Hydrogen and toxic gas piping and equipment
utilizing those gases. The facility was designed to minimize the possibility of a gas leak and further measures were
taken to contain rather than expel any gas that might accidentally leak. For example, all gas piping was run through a
NEMA duct to prevent gas from being uncontrollably dispersed into the room.
The facility installed both a Hydrogen and toxic gas monitoring system. It was necessary for the Hydrogen and toxic
gas monitoring system to be independent due to the capabilities of the sensors. The remainder of this discussion will
focus on the Hydrogen monitoring system; however, much of the discussion also applies to the toxic gas monitoring
system.
The hazardous gas monitoring system used was the Sierra Monitor SENTRY Gas Monitoring System. The system
uses a sophisticated microprocessor-based controller. System configuration is user programmable to allow the system
to be customized to the specific application requirements. SENTRY has extensive data management and internal
diagnostics that provide information required to help manage gas leak problems.
Sensor Locations. One of the first questions to be addressed as part of the system specification is how many sensors
are required and where should they be located? There are no rules or formulas to answer these questions. In general,
it is important to locate sensors close to the likely sources of leaks and in areas where a leaked gas might lighter than
air, thus dictating whether sensors be located below or above the leak. With Hydrogen being lighter than air, sensors
are always located in the ceiling as
a
backup to sensors located near the source of the leak.
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
E-Mail: [email protected]
Rev. A1
Technical Reprint TR-002
Typical Hydrogen gas sensor locations in a semiconductor manufacturing facility include:
•
•
•
•
•
•
Hydrogen storage areas
Valves in Hydrogen pipelines
Furnace area – inlet, outlet, internal tubing
Off gas ducts
Return room air ducts
Cavity between dropped ceiling and the roof (or floor above)
In this plant there are sixteen sensors located within four rooms. Sensors are a catalytic bead type. There are two
beads in each sensor element, one of which is activated by a catalyst in the presence of a combustible gas and the
other is not. These beads are inserted in a wheatstone bridge circuit and the relative resistance of the beads affects the
bridge output in the presence of gas. Changes in temperature and relative humidity have no effect on the output of the
circuit since the catalyzed bead and the reference bead are similarly affected and the relative difference in output does
not change.
Sensor output is digitalized at the sensor module and transmitted to the controller. Transmitting a digital signal give
great tolerance to RFI and EMI effects. In addition it enables diagnostics to be performed at the sensor and be
transmitted to the controller along with the concentration measurements. Also, sensor modules are given an address
which permits multiplexing multiple sensors on a single wire.
Benefits of Controller Capability. The controller is designed to provide high reliability, ease of operation and data
management capability. It is the data management capability that separates the SENTRY controller from other gas
monitoring systems. Conventional controllers simply react to events (as a thermostat reacts to temperature reaching a
setpoint). To manage a risk it is necessary to use information to formulate a response to leak rather than reacting to an
event after an alarm has occurred.
It has already been mentioned that the system configuration is user programmable. This includes alarm levels,
calibration procedures, report scheduling and display mode. The low alarm level is set at 10 percent of the LEL and the
high alarm level is set at 60 percent of the LEL. A single controller can handle up to eight sensors and four controllers
can be mounted in a standard 19 inch instrumentation rack. The controller displays the sensor concentration, the gas
type, concentration units, and alphanumeric messages.
Reports to Help Decision-Making and Document Due Diligence. The Sentry can provide data to a distributed
control system (DCS) via MODBUS or provide reports via a printer. The reports are also available at the display and
include status reports, history reports, calibration reports and system configuration reports. This particular customer
only needed the printer reports. When the concentration exceeds a user definable limit. The controller prints each key
event when it occurs.
This history reports are also very useful in determining if there is any regularity to gas leaks. For example, history
reports show the date and time the highest concentration occurred in the period. The frequency for the period can be
programmed on an hourly, shift, or daily basis. As one semiconductor facility one sensor recorded its highest
concentration at the time every day. After investigation it was found that a purging of a Hydrogen furnace was releasing
Hydrogen into the room. The purging procedure was changed and release of Hydrogen into the room stopped. Daily
history reports also provide good documentation to confirm the facility’s safe operating procedure if legal action was
brought against the company. In today’s legal environment, such documentation is essential to minimize liability. In
addition to the alarm indication at the controller, the facility has an annunciator panel located near a security area. The
panel displays each combustible and toxic gas sensor in alarm.
Summary
Gas risk management requires a comprehensive look at the hazards of gas leaks. Properly designed gas monitoring
systems provide suitable hardware in combination with data management and diagnostics capability to custom-fit a
solution for managing, rather than reacting to, gas leaks.
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
E-Mail: [email protected]
Rev. A1
|