BY PHIL SALLAWAY
Fire professionals are familiar with the typical residential, commercial, and office building “fire/smoke” fixed detector and the portable gas detector, but the hazardous materials found at industrial facilities require far more complex fixed safety monitoring systems. Becoming familiar with these fixed detection systems helps fire professionals protect themselves in dangerous situations, save the lives of employees, and protect plant equipment.
 1. Photos courtesy of General Monitors |
Knowing that there is an industrial fixed safety monitoring system with distributed sensors for toxic, combustible, and flame detection at a facility warns the incident commander and firefighters of the presence of dangers of which they might be otherwise unaware. Certain gases and chemicals burn with colorless or near colorless flames, such as hydrogen gas and many alcohols. Not recognizing the monitoring systems’ alarms can be disastrous. Such systems are used in facilities such as oil/gas refineries; chemical, paper, food, plastics, and pharmaceutical processing facilities; electronics and computer chip manufacturing plants; coal-fired electric utilities; steel foundries; and warehouse/distribution centers.
With the introduction of cleaner-burning oxygenated fuels, fuel refinery hazards now include alcohol (an additive) and hydrogen, which is increasingly used in the refining process. In areas where fixed systems are not used, fire service professionals and fire marshals can encourage their installation and facilitate change in local building codes. Risk management and insurance issues strongly influence the installation of these systems. Properly installed fixed systems greatly improve fire safety, prevent the loss of life, and protect property.
HOW THEY WORK
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Some of the technology used in fixed systems is the same as that found in portable gas detectors (see photo 1) with which firefighters may be familiar; but in other fixed systems, the technology is completely different. Fixed systems do not eliminate the need for por-table gas detectors; the two complement each other.
The fixed detection system reports a gas leak; the portable gas detector allows responders to track down the leak’s exact location. Fixed systems offer a wide variety of sensing technologies and are primarily intended to protect property, capital equipment, and lives. Por-table gas detectors are used primarily to protect people and then property.
Firefighters should familiarize themselves with their local industrial plant operations, including the gases and chemicals in use, and understand the particular fixed monitoring system technologies in place. Sel-ecting the right portable instrumentation to complement the particular fixed safety systems is essential. For fire professionals, understanding the safety system and fire suppression components installed at a local plant before an emergency is important, since plant safety staff may be injured and unable to provide critical information.
Many industrial facilities will share information and training and may even provide equipment free of charge to the local first responders. Most facilities employ both fixed and portable detection equipment. Fixed systems are used in processing or manufacturing areas, pipelines, storage tanks, loading docks, utility vaults, and so forth.
BASIC FIXED SYSTEMS
A typical fixed safety monitoring system operates from a control room and may have from one to hundreds of detectors hard-wired back to a stand-alone wall-mount controller or to a programmable logic controller (PLC) in a control room (see photo 2). This may be part of a larger computer-based distributed control system (DCS).
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The detectors are sensors that continuously monitor for the presence of hazardous gases and flame conditions. If gas or flames are present, the detector sends an alarm signal to a PLC computer-based system. The controller/ computer monitors the detector for alarm conditions and reports its status to the control room operator.
These systems are versatile and can activate other safety systems, including alarms, sirens, flashing lights, ventilation, and various fire suppression or automatic isolation systems. Fire suppression systems include carbon dioxide, water deluges, dry chemical, and halon and its replacements. Again, fire professionals should know what systems are used and how they work.
GAS DETECTION SENSING TECHNOLOGIES
Several industry standard sensing technologies are employed to detect toxic and combustible gases, depending on the specific types of gas monitored in the plant’s various processes, the plant’s physical layout, and the environmental conditions such as temperature and humidity. Monitoring system technologies are classified according to the applicable hazards they detect: combustible gas, flame, and toxic gas.
Combustible Gas
Catalytic Sensors. Catalytic sensors are highly reliable, durable, and rugged (see photo 3). They are widely used in harsh conditions to detect almost any flammable gas at the lower explosive limit (LEL) from one-percent LEL up to 100-percent LEL. This technology works best in areas where there will not be very low concentrations of oxygen. This is the same technology used in portable gas detectors.
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Catalytic sensors have two heated ceramic beads mounted on separate platinum filaments; one is active and responds to flammable gases; the other is an inert reference. Combustible gases burn on the active bead, causing a measurable change in the electric current flowing through the filament. The gas does not affect the reference, and the temperature difference is proportional to the concentration of flammable gas present.
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Point infrared (IR) sensors (see photo 4). Point IR sensors detect flammable gases in one set location. They measure the amount of gas present over a wide range of concentrations, from one- to 100-percent gas, even in completely oxygen-deficient atmospheres. This technology warns the user if the beam is blocked. It is immune to chemical poisons and can discriminate with regard to which gas it will detect. IR technology can detect and measure carbon dioxide, too.
Combustible gas/carbon dioxide passes through two beams of infrared light. One is a wavelength that is absorbable by the target gas; the other is not absorbent. The gas reduces the strength of the absorbed beam, whereas the other beam is unaffected and acts as a reference. The reduction of beam strength is directly proportional to the amount of gas present.
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Infrared (IR) open path sensors. IR open path sensors detect leaks over large areas such as liquefied natural gas (LNG) tank farms or propane vehicle storage garages where it is impractical to place point IR detectors (see photo 5). They are used to set up a perimeter around bullet tanks.
Open path IR sensing relies on the same principles as a point IR system, except the transmitter and receiver can be placed up to 300 feet apart to cover larger areas. Open path units cover large areas and report the relative concentration of combustible gas present.
Flame Detectors
Ultraviolet (UV) and infrared (IR) sensors. UV and IR sensors detect visible and invisible flames originating from storage or process using: hydrogen, ethyl alcohol, methyl alcohol, and so forth (see photo 6). NFPA 33, Standard on Spray Application Using Flammable or Combustible Materials (2000 ed.), requires optical flame detection to shut down the system in case of a fire in an electrostatic paint spray booth. The presence of flame detectors indicates the potential presence of an extreme hazard. Firefighters should learn which facilities have this type of hazard and its location in the facility.
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Infrared flame detectors look for a specific wavelength of infrared light produced by carbon dioxide (CO2). Virtually all burning flammable gases and liquids produce CO2. Flames and the flickering effect of flames produce UV light. Since it uses UV and IR light, this type of detector is less likely to go into false alarm from reflected flashes of sunlight and welding.
Toxic Gases
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Electrochemical cells. Electrochemical cells react most strongly to one specific gas, using the same technology as that used in portable gas detectors (see photo 7). They are placed anywhere toxic gases may be present. The most common electrochemical cells are oxygen, carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, chlorine dioxide, nitric oxide, nitrogen dioxide, ammonia, and hydrogen cy-anide.
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Metal oxide semiconductor (MOS) sensors. They detect hydrogen sulfide, a common by-product of anaerobic decay (see photo 8). Typical sources include wastewater sewer gas and petrochemical refining processes. A silicon chip is specially treated to be sensitive to H2S gas. The sensors are durable and long-lasting and perform very well in harsh environments.
APPLICATIONS LNG/LPG Storage Facilities
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A typical LPG plant extracts and processes liquid petroleum gas (LPG) and other hydrocarbons (see photo 9). Large bullet tanks are used for LPG storage. The hydrocarbons, propane, and butane are all heavier than air. If a leak occurs, these gases will float along the ground and accumulate in low spots. Open path IR detectors are installed to form a perimeter around the tank farm. Open path IR detects leaks in the parts-per-million range. To enhance their leak-detection abilities, there are point catalytic sensors throughout the plant as well. The combination of open path IR and point catalytic sensors provides fast, accurate leak detection long before it reaches an ignition source.
Electrical Power Generation Station
Gas turbines. Electric utility companies use gas turbines to increase their generating capacity rapidly. Turbines consume large quantities of LPG or LNG fuel and have large fuel storage facilities on-site or, more typically, located along high-capacity gas pipelines. The turbine is housed in an enclosure the size of a large shipping container, which will have factory-installed catalytic bead sensors. The site may have open path sensors around storage areas and point IR or catalytic bead sensors in compressor stations or other sensitive areas.
Electric generators. The hydrogen coolant is used to cool electrical generators. Hydrogen has an invisible flame and a wide flammable range and is easily ignited. This is an ideal application for UV/IR flame detection.
Coal-fired power plants. The cleaner coals now used tend to spontaneously combust when in storage or during transport. Both portable and fixed systems can be used to detect rising levels of carbon monoxide (which indicates the start of spontaneous combustion) up to 1.5 hours prior to the first indication of flame. There is no reason to doubt that other materials subject to spontaneous combustion would do the same thing.
EMERGENCY PLANNING
By building a relationship with the safety professionals and industrial hygienists in local industrial facilities, the fire professional’s overall emergency response can be much more successful. Preplanning staging areas and evacuation can be integrated to avoid confusion, duplication, and traffic jams between responders and evacuees. Plant personnel will know the hazards present and best practices for dealing with unusual chemicals. They literally know which way the wind is blowing and what fire suppression systems are in place—important information to incoming fire personnel.
The primary benefit of fixed detection systems is that with limited plant staffing, they continuously monitor large areas and provide a warning from a safe distance. The key benefit to the fire service is that the plant control room operator can greatly enhance the success and safety of first responders. An incident commander in contact with the right plant people can place firefighters in the right place at the right time, and that can make all the difference.
PHIL SALLAWAY is the product manager for portable gas detectors at General Monitors.
