Technical Reprint TR-001
To minimize risks, a plant-wide gas monitoring system must meet the specific needs of
the facility in terms of system inputs and outputs and data acquisition.
Plant-Wide Gas Monitoring for Risk Control
as monitoring has become an integral part of many environmental protection and safety programs. Because of
Gincreased attention to risks posed by gas, systems that simply annunciate an alarm condition are no longer
adequate. Gas monitoring programs now comprise employee training, data analysis, multiple alarm strategies,
self-diagnostic system verification, and documentation. Data communication has also become a fundamental
component of system design and operation as well. And finally, environmental factors help guide the program
selection process.
GAS MONITORING RATIONALE
Gas Monitoring is an integral part of environmental protection and safety programs. There are three major
categories of hazardous gases. Combustible gases pose a risk to personnel and facilities through fire or explosion..
Oxygen displacing gases deplete workers' oxygen supply in confined spaces. Toxic gases create both immediate
and long term risks to personnel. Toxic gases are those OSHA and other organizations have defined as hazardous to
human health. They are typically monitored both below and above the threshold limit value (TLV). TLV is an
8-hour time weighted average concentration that is the highest value acceptable as a safe working environment.
System design requirements such as the number of sensors required, sensor placement, and system alarm levels will
vary according to the type of gas.
A properly designed gas risk management program can reduce risk. A successful program needs to first, identify the
hazards at the facility, then assess the risk of a hazardous event to the plant, personnel, the community and the
environment. Next, the program needs to comply with insurance, legal and regulatory requirements. Also, liabilities
and financial risks need to be evaluated. Finally, the program must incorporate company safety philosophies.
Through such a program you can reduce the risks associated with hazardous gas leaks at your facility. Sierra
Monitor offers products that provide critical data to prompt corrective action before a minor leak becomes a major
event. Information can be retrieved from the equipment display, printed locally or recorded by a computer. This
information aids in evaluating potential hazards in relation to operating practices.
Hazard Identification &
Assessm ent
Risk to
Environm ent
Laws &
Com pany
Regulations
Philosophy
Risk to
Personnel
Risk to
Facility
Insurance
Requirem ents
Financial
Risks
Liability
Control System
Capability/Cost
Tradeoffs
Decision,
Control Strategy,
System Selection
Risk Reduction
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
Technical Reprint TR-001
PLANT-WIDE MONITORING
Plant-wide gas monitoring offers many benefits, among them:
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Centralized monitoring of all gas measurements
Monitoring the status of control elements
Establishing a “command center” to respond to alarm events
Remote access to all system data
Centralized reporting
In a plant that has been instrumented over a period of years, the gas detection system is likely to be an autonomous
installation. The controller might be in the field close to the sensors or in a control room. Both approaches have
limitations. Controllers in the field do not transfer gas concentration information to the control room. Alarm data
are limited to local annunciation of an alarm event, or long wiring runs are required in order to report sensor-by-
sensor alarm status to a central control room. Alarms are therefore usually limited to relays activated by groups of
sensors and/or field controllers in a given area.
Controllers located in the control room usually require extensive wiring to the sensors and alarm activity is often
minimized to reduce wiring cost. Furthermore, when the controller is in the central control room, there are no
concentration data available in the field for plant personnel to make on-the-spot decisions; only an alarm light or
horn is activated.
Systems designed for plant-wide monitoring avoid these deficiencies. They:
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Accept sensors for multiple gases
Accept sensors from various manufactures
Accept other environmental and process sensor measurements
Provide local display of gas concentration
Permit auto-adjusting calibration
Accept feedback signals from final control elements
Provide independent battery backup
Allow low-cost communications link to the control room
Provide relays for local alarm action
Implement user-defined alarm strategy
Provide centralized monitoring of sensor status as shown on the plant log plan and at each operation /
facility with bar graphics, configuration screens, and real-time trends
Provide centralized data logging
Provide alarm acknowledgment from the control room
Allow access to control room computer from other sites in facility
Operate independently even if communications with the control room are severed
Provide for easy system reconfiguration via computer at the control room
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Distributed intelligence is central to system design philosophy. Because the field controllers can implement alarm
logic and operate independently of the host computer, they do not interrupt or hamper field operation. A plant-wide
monitoring system provides the user with clear, understandable data that make it possible quickly to analyze the
situation and take appropriate action. A correctly designed plant-wide monitoring system offers the best features of
local control, and centralized monitoring systemizes the entire monitoring process.
CRITICAL SYSTEM COMPONENTS
The components of the system are the sensors, the field controller, and the monitoring software. Sentry, from
Sierra Monitor, is an example of a system that can implement plant-wide gas monitoring. The gas sensor modules
can be multiplexed on a single cable back to the controller.
To offer complete solutions at each site, field systems need full I/O capability. Sentry fulfills this requirement by
accepting a variety of sensors or 4-20 mA transmitters. The Sentry controller is an intelligent device that can access
the sensor modules for diagnostic data, non-intrusive calibration and gas concentration information.
MODBUS communications interface enables Sentry and the IT Series sensors to easily interface with a PC-based
Distributed Control System (DCS) providing a graphical display of hazardous gas conditions in the plant. PC-based
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
E-Mail: [email protected]
Technical Reprint TR-001
DCS software in conjunction with Sentry provides the operator enhanced data acquisition, alarming and alarm
management, historical trending, distributed architecture, and integration into the plant's distributed control system.
Sentry's MODBUS communication interface provides over 620 different parameters accessible by the PC-based
DCS software, including:
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Gas concentration
Alarm setpoints
Sensor diagnostic messages
Historical reset time/date
Power up/down time/dale trouble alarm time/date
Minimum/maximum values plus time/date
High/low alarm latch/non-latched
Auto calibration enabled
Calibration status/coefficients
Calibration concentrations
Calibration due date
Zone option enabled
Operate mode
With the MODBUS communications option for Sentry and the graphic based man-machine software program a
variety of graphic screens can be developed. These screens make it easy for the operator to efficiently monitor haz-
ardous gas risks in any plant. Typically the system will include a main screen providing an overall view of the plant.
This screen is linked to individual zone displays, historical and real-time trend charts, bar charts indicating gas
concentration and alarm levels, and more.
SUMMARY
The key to any plant-wide monitoring system is the flexibility to configure system inputs/outputs and data
acquisition to meet the specific needs of the plant. By monitoring sensors throughout the plant, better visibility of
the ambient environment can be realized and more informed decisions can be managed in the event of gas exposure.
A plant-wide monitoring system using distributed intelligence coupled with advanced PC based software offers an
improved approach to limit in plant risks. The benefits to employee safety and environmental responsibility are
self-evident, as are the economic rewards of reducing the chance of loss from interruptions in operation and from
litigation. Risk minimization, including plant-wide gas monitoring, makes good business and social sense.
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
E-Mail: [email protected]
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