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
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increased 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 req uirements. 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 &
Assessment
Risk to
Risk to
Personnel
Control System
Capability/C ost
Environment
Tradeoffs
Risk to
Fa c ility
Regulations
Ins u r a n c e
Requirements
Laws &
Lia b ility
Decision ,
Control Strategy,
System Selection
Company
Philosophy
Financial
Risks
Risk Reduction
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
Visit our Web Site at: http://www.sierramonitor.com E-Mail: sales@sierramonitor.com
Technical Reprint TR-001
GAS MONITORING OPTIONS
Gases are monitored with permanently installed (fixed) systems or portable instruments, or by taking samples for
evaluation in a laboratory. Portables and sampling are by their very nature only spot checks and are appropriate
only when the risk is extremely low or fixed instruments are not practical. As a general principle, areas where the
risk is sufficient to justify periodic monitoring with a portable instrument, warrant a fixed system.
Conventional Technology. Fixed gas monitoring systems historically have been reactive systems consisting of a
sensor and a controller. The sensor, which can be mounted remote to the controller, usually transmits an analog
signal proportional to the gas concentration. Principles of detection include catalytic bead, solidstate/semiconductor, electrochemical and infrared.
Performance variables that should be considered for sensor performance include:
• Response speed
• Measurement range
• Resolution
• Interfering gases
• Operating range
• Temperature and humidity effects
• Sensor life span
• Ease and frequency of calibration
• Replacement cost
The gas monitoring system required depends upon the scope of the hazardous gas risk. For applications where the
risk is minor, a simple gas monitor that sounds an alarm when the concentration reaches the setpoint may meet the
need. Applications involving a higher risk generally require continuous output of gas concentration to a display,
printer or other indicating or control device. Sierra Monitor's analog gas sensor modules offer a continuous 4-20 mA
DC output and are available for a wide range of hazardous gases.
In applications with the highest level of risk or where the hazard is present throughout the facility, a smart gas
sensor module such as the IT Series and/or the Sentry Gas Risk Management System will meet the need. With
extensive diagnostic capability, flexible configuration, expandability, microprocessor controls, and the ability to
interface with plant-wide control systems, Sierra Monitor IT Series and Sentry Systems are the choice for plantwide monitoring systems.
System self-diagnostics on both the IT Series sensors and the Sentry controller and sensors help the user know that
the data they are receiving in reliable and accurate.
Gas exposure data analysis needs to be available to prove due diligence in hazardous gas risk management. This
data can be available either via preformatted reports available with the optional printer for Sentry or via a third party
HMI package resident on a computer. Serial communication and the MODBUS interface enable Sentry and IT
Series sensors to interface with the Human-Machine-Interface (HMI) package that can provide complete data
analysis and information on the operator's computer. Such information would include alarm status, gas
concentrations, alarm setpoints, fault alarms, and more. Hundreds of data points are available from Sentry via the
serial communications port.
Sentry’s ability to accept input from a wide variety of devices such as flame detectors, or other unique gas detectors
provides a complete hazard monitoring system to improve the safety management of the facility.
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
Visit our Web Site at: http://www.sierramonitor.com E-Mail: sales@sierramonitor.com
Technical Reprint TR-001
PLANT-WIDE MONITORING
Plant-wide gas monitoring offers many benefits, among them:
• 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 co ntrol room. Alarm data
are limited to local annunciation of an alarm event, or long wiring runs are required in order to report sensor-bysensor 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 wirin g 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:
• 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
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
Visit our Web Site at: http://www.sierramonitor.com E-Mail: sales@sierramonitor.com
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:
• 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 hazardous 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
Visit our Web Site at: http://www.sierramonitor.com E-Mail: sales@sierramonitor.com
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