Fixed Gas Detection: Issues and Answers
Dealing with the actual or potential presence of dangerous gases and vapors in the workplace involves a complex series of decisions. Decisions related to “what to do” should be implemented by means of an integrated protection plan based on a hierarchy of priorities. The most desirable approach—when feasible—is to implement engineering controls and practices that completely prevent the release or formation of dangerous gases and vapors. If it is impossible to completely control or eliminate the possibility, the next best solution is to implement use of equipment and techniques to monitor for the presence of gas and take appropriate action when dangerous conditions are determined to be present.
There are two basic approaches to atmospheric monitoring: portable gas detectors assigned to workers who enter the affected area or fixed detection systems. Fixed detection systems are permanently installed and function 24 hours/day. They can be used to display readings, activate alarm lights and sirens, control ventilation fans, notify internal and external emergency responders, or be used in process control applications. Another important function is demonstrating compliance with regulatory requirements. Monitoring records can be used to document conformity with OSHA or EPA limits, for example.
A fixed gas detection system that monitors 24/7 can often provide better protection than portable instruments used only when qualified workers are present to determine whether conditions are safe. This is particularly true when the area is routinely entered by workers in the course of their normal duties. Fixed systems are often the more cost-effective solution, especially when training and maintenance costs are factored in.
Sometimes the best approach involves the combined use of both fixed and portable monitors. Deciding which approach to use is not always a trivial exercise. The following questions provide a guide for making this decision.
What kind of atmospheric hazard is potentially present?
The hazard to be measured determines the type of detector. Although some detection techniques are more commonly used than others, fixed systems are available with virtually every commonly used gas detection technology. In fact, there are more detection options available with fixed as opposed to portable instruments, since power requirements, as well as the size of fixed systems components, are less of a concern.
The batteries used in portable instruments can only store so much power, and the low-power sensors used in portable instruments to extend operation time are often more vulnerable to sensor poisons and operation in adverse conditions. On the other hand, fixed systems are usually powered by means of a line power connection, so power requirements are generally not an issue—making it possible to use more robust detection technologies.
The type of atmospheric hazard also determines the type of warning needed. The more rapidly the hazard affects workers, the more urgent it is for the system to provide an immediate warning. For instance, a concentration of only 1,000ppm is enough to “knock down” workers with a single breath. While oil industry workers are routinely equipped with personal H2S monitors, many sites are additionally equipped with fixed detection systems that monitor the general area for this hazard. This “belt and suspenders” approach ensures that both general conditions affecting everyone in the area, as well as localized conditions affecting a single worker, are discovered as quickly as possible.
How do you determine what to measure and when?
Make sure to fully assess the causes and risks before you decide on the monitoring response. Is the source of the hazard readily identifiable? Is the hazard associated with the work being performed? Is the danger present all the time or only under certain circumstances? What industrial processes are occurring that might generate or affect the presence of the hazards? Are there additional risks under emergency circumstances? Fixed detection systems should always be considered as an option when hazards are known to be chronically present in areas where workers routinely enter without special precautions.
What is the physical nature of the area affected?
Is the entire facility affected or only specific areas? Are the areas of concern out-of-doors and subject to good ventilation? Are the areas of concern indoors or in localized areas that prevent rapid dispersal of contaminants? Is the area congested by equipment or structures that prevent or interfere with worker evacuation? Monitoring programs need to provide workers adequate time to “self-rescue” during an emergency. The harder it is for workers to leave the area, the more desirable it is for a 24-hour-a-day “sentry” system to provide the earliest possible alarm.
Should the affected area be maintained safe for continuous worker occupancy?
Entry into hazardous locations is controlled by means of permits, training, need to enter and physically limiting access. Anyone entering “controlled” areas must use special procedures, as well as the required equipment, including portable gas detectors. When workers routinely enter the area without special procedures or precautions, if the hazards cannot be permanently eliminated, the better approach is often to make the area safe for continuous occupancy. Addition of a fixed detection system coupled with other engineering controls, such as permanently installed ventilation, may allow the reclassification of the area as a non-hazardous location.
The cryogenic storage systems in the basements of many hospitals are good examples. Tissue samples and other biological materials are stored in liquid nitrogen in large thermos-like Dewar vessels. When the lid is opened, or in the event of a leak, the release of nitrogen vapor can rapidly create a deadly oxygen deficiency. Typically, fixed O2 monitoring systems that include a display visible from outside the cryo-bank area are used to alert hospital staff before they enter. There are many industrial areas, such as the pits under automotive assembly lines and occupied areas of sewage lift stations, where the same ventilation and monitoring strategy is used to verify the area is safe at all times.
What is the level of control over worker activities in the affected area?
The lower the level of control over worker activities, the more desirable a continuously operational fixed detection system becomes.
What is the level of training of potentially affected workers?
One of the advantages of fixed detection systems is that workers entering the monitored area usually are not involved in the day-to-day operation of the system. All workers entering the area need to do is follow company procedures in the event an alarm sounds.
What are the trade-offs in cost?
Equipping workers individually with gas detectors can be expensive. A permanently installed system can often reduce gas detection costs. The image that usually comes to mind of a fixed gas detection system includes dozens of gas detector transmitters in explosion-proof housings that are connected by cabling installed in expensive, stainless-steel conduit and connected to a controller or expensive programmable logic controller (PLC) in a central location. That is only one alternative. Many fixed systems are simple one- or two-sensor standalone systems that include built-in, high-intensity alarm lights and horn. Even the sensors can be built into the standalone housing. Installation of smaller, self-contained systems can be as simple as terminating them to line power. Of course, they can still be connected to a controller or PLC or to the Internet if the real-time monitoring information needs to be communicated to additional remote locations.
What about fixed systems installed in hazardous locations?
The equipment used to monitor for the presence of flammable gas must be designed for use in the intended hazardous location and must carry an appropriate certification from a qualified testing laboratory or agency. In North America, qualified testing laboratories are referred to as Nationally Recognized Testing Laboratories (NRTLs). OSHA maintains and publishes the list of currently recognized laboratories at the following website location: https://www.osha.gov/nationally-recognized-testing-laboratory-program/current-list-of-nrtls.
Equipment that is used in areas that potentially contain flammable gas must be designed so that hot surface temperatures, electrical discharge and other forms of stored energy associated with the equipment are not capable of causing ignition of the flammable gas, given the type and severity of the hazardous conditions in which the equipment is installed or operated. The product documentation, as well as the certification label, include the protection method, ambient operating temperature range and types of gas to which the certification applies.
What are the protection methods for fixed system components?
There are three commonly used protection methods used to prevent the ignition of flammable gas by equipment designed for use in hazardous locations: (1) contain the explosion by means of “flame-proof” or “explosion-proof” conduit and enclosures; (2) physically separate or isolate electrical parts and hot surfaces from the ignitable gas by means of techniques such as encapsulation, pressurization and use of electrical (Zener diode) barriers; and (3) limit the energy.
Intrinsically safe equipment is designed to limit the energy (thermal and electrical) capable of being released by the equipment. Certification for intrinsic safety is based on the maximum energy that is capable of being released both during normal operation, as well in fault conditions. Equipment that is certified as intrinsically safe is not capable of releasing the minimum ignition energy necessary to ignite the type and temperature class of gases, over the ambient operating temperature range to which the certification applies.
What are some of the other fixed system benefits?
Fixed detection systems monitor the atmosphere continuously. An advantage of installing a fixed detection system is that workers entering the monitored area are not usually involved in the day-to-day operation of the system. Procedural issues are much more complex when workers are required to use personally assigned portable gas detectors. Addition of a fixed detection system coupled with other engineering controls, such as permanently installed ventilation, may render otherwise hazardous areas safe for continuous occupancy and eliminate the need for procedural controls, such as entry permits.
Don’t be afraid of considering fixed system solutions!
Larger systems can be complicated, but your manufacturer partners are there to help you through the specification process. The most common solutions are often based on small systems with 1 to 4 points of detection. And don’t go it alone. The manufacturer, distributor and consultants are all there to help.
Are fixed systems expensive?
Purchase and installation of a fixed gas detection system can be a significant capital expense. However, equipping workers individually with atmospheric monitors can also be expensive—especially if training, testing, calibration and maintenance are factored in. A permanently installed, fixed system is often the more cost-effective solution. The system can be configured as a simple standalonesystem that provides information and alarms on a local basis or as a larger, integrated system that can communicate real-time monitoring results literally on a world-wide basis. Small, single-point systems are often no more expensive than the portable instruments used to measure the same hazard.
What are the major components in a fixed gas detection system?
Fixed gas detection systems consist of a number of different components and assemblies, including sensors, gas transmitters, controllers and peripheral equipment (i.e., alarm lights and horns). The gas detecting sensor and associated electronics are referred to as the “gas transmitter.” The gas transmitter is installed in the area that needs to be monitored. Controllers are typically installed outside of the hazardous location or in a central location.
The transmitter is available with or without a display; with or without a local control interface; and may be connected to controllers and other system elements by means of (typically) a 4-20mA line power connection or integrated digitally with the controller and other system elements by means of RS-485, MODBUS or HART protocol connection. It is also possible to integrate the system by means of wireless communication methods.
Controllers are used to communicate with the gas transmitters; process the monitoring results; display readings; activate alarms; control peripheral equipment; and log data. The flow of information through the system may stop at the controller, or the controller may be connected to a“higher order” logic center, such as a Programmable Logic Controller (PLC), Distributed Information System (DIS) or Environmental Health and Safety (EHS) computer. Alternatively, gas transmitters may be wired directly to the PLC or EHS computer. Alarms and realtime readings can be displayed at the transmitter, at the controller, at the PLC or redundantly displayed at multiple locations.
How do you make sure you have the needed information to discuss fixed system options with the manufacturer or distributor?
Ask if the manufacturer or distributor you are working with has a “Fixed System Questionnaire.” Clarifying what you need by means of a detailed questionnaire reduces the chances for specifying or purchasing the wrong equipment. If you do not have a copy, contact the manufacturer. Answer as many questions as you can, but don’t worry if you can’t answer them all. The manufacturer will tell you if there is something that must be nailed down before they can provide advice or generate a quote. And don’t be afraid to ask the manufacturer for help with the answers.
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