Fire & Gas (F&G) Detection Systems: A Basic Introduction

  

1. Introduction to Fire & Gas (F&G) Systems

Fire & Gas (F&G) Detection Systems are absolutely critical in the oil and gas / petrochemical industry they are our first line of defense against potentially catastrophic events like fires, explosions, and toxic gas releases. Their primary purpose is to:

• Detect Hazards Early: Identify the presence of fire or hazardous gases as quickly as possible.
• Trigger Alarms: Alert personnel to the danger.
• Initiate Safety Actions: Automatically or manually trigger safety measures (e.g., emergency shutdowns, fire suppression or ventilation activation).
• Protect Life & Assets: Safeguard personnel, equipment, and the environment.

We classify F&G detectors based on what they are designed to sense. Let's break them down.

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2. Fire Detection Technologies

Fire detectors are designed to identify one or more characteristics of a fire: heat, light (flame), or smoke.

2.1. Heat Detectors

Heat detectors respond to rapid increase in temperature or a temperature that exceeds a fixed, predetermined threshold. There are two types of heat detectors, Fixed Temp heat detectors and Rate of rise heat detectors.

• Fixed-Temperature Heat Detectors:
• What they detect: A temperature that exceeds a preset threshold (e.g., 67∘C).
• How they work: When the ambient temperature reaches the set point, a temperature-sensitive element (like a bimetallic strip or eutectic alloy) melts or changes shape, completing a circuit and triggering an alarm.
• Example: Fusible Plug Heat Detectors:
  
  A fusible plug heat detector is a type of fixed-temperature heat detector that relies on a simple, mechanical principle to detect fire: a component designed to melt at a specific temperature. These detectors are known for their simplicity, reliability, and lack of reliance on external power sources.

A fusible plug detector is typically a small, threaded metallic device often made of brass or bronze with a hole drilled through its center. This hole is sealed with a low-melting-point alloy, such as a specific mix of tin, lead, and bismuth. The melting point of this alloy is carefully chosen to be higher than the maximum normal operating temperature of the environment but low enough to melt when a fire occurs. The fusible plug is part of a larger system, most commonly a fusible loop system. This system includes:

§  A network of tubing: A series of tubes containing pressurized air or inert gas.

§  Fusible plugs: These are installed at regular intervals along the tubing network.

§  A control panel: This panel monitors the pressure within the tubing.

Here's the sequence of events during a fire:

1.     Normal operation: The tubing network is pressurized, and the fusible plugs remain intact, sealing the system. The control panel monitors the constant pressure, indicating a normal, safe condition.

2.     Fire starts: When a fire erupts, the ambient temperature in the vicinity of a fusible plug rises.

3.     Melting of the alloy: When the temperature reaches the predetermined melting point of the alloy, the metal in the plug melts and flows out of the hole.

4.     Pressure drop: The melting of the alloy creates an opening in the tubing network, causing the pressurized air to escape. This leads to a rapid and significant drop in pressure.

5.     Alarm activation: The pressure switch or transmitter in the control panel detects this loss of pressure. This triggers a safety sequence, which can include:

§  Activating a fire alarm.

§  Shutting down equipment (Emergency Shutdown or ESD).

§  Opening a deluge valve to release fire suppression agents.

• Key point: They are non-resettable and must be replaced after activation. No external power is required for the detection mechanism itself.
• Best for: Areas where smoke detectors are prone to false alarms (kitchens, dusty environments, machinery rooms). Slower to react than flame or smoke detectors.

• Rate-of-Rise Heat Detectors:
• What they detect: when the temperature increases at an unusually fast rate, even if it has not reached a fixed temperature threshold. (e.g., 6.7∘C to 8.3∘C per minute).
  
  
• How they work: They typically work on the principle of air expansion in a sealed chamber with a small vent. This vent allows air to slowly escape from the chamber during normal, gradual temperature changes (e.g., the natural increase and decrease in temperature throughout the day). This prevents false alarms from non-fire related temperature fluctuations.
  When a fire begins, the surrounding air temperature rises very quickly. This rapid temperature increase causes the air inside the sealed chamber to expand much faster than it can escape through the small vent. The rapid expansion of air creates a pressure differential, pushing the flexible diaphragm outward.  This movement of the diaphragm closes a set of electrical contacts, which triggers the fire alarm.
  
  
• Best for: Detecting fast-developing fires. Often combined with a fixed-temperature element to also catch slow-smoldering fires.
  
  
• Limitation: May not detect slow, smoldering fires that don't produce a rapid temperature increase.
• Linear Heat Detectors (LHD):
• What they detect: Linear Heat Detectors are a specialized type of fire detection system that provides continuous heat sensing along the entire length of a cable. Unlike traditional spot-type heat detectors that monitor a single point, an LHD system can detect a fire or overheating condition anywhere along its path, which can span from a few meters to several kilometers.
• How they work (e.g., Senkox HSD):
• Traditional LHD: Cables that short-circuit or change resistance at a specific temperature.
• Senkox HSD (Thermoelectric): Utilizes specialized thermoelectric materials in the cable. When a "hot spot" occurs, it generates a voltage proportional to the temperature difference.
• A Data Acquisition (DAQ) Module processes this voltage to determine the precise temperature, location, and even rate of temperature change (ROTC) of the hot spot.
• Advantages: Reusable (Senkox HSD), real-time monitoring, precise location of overheat, intrinsically safe, robust for harsh environments.
  
  
• Best for: Long, continuous assets like cable trays, conveyor belts, tunnels, storage tanks, or anywhere precise hot spot location is critical.

2.2. Flame Detectors

Flame detectors sense the radiant energy (light) emitted by a fire. They require a clear "line of sight" to the flame.

• Ultraviolet (UV) Flame Detectors:
• What they detect: The UV radiation emitted by a flame (typically 185-260 nm).
  
  
• How they work: When a fire ignites, the chemical reactions of combustion create a wide spectrum of electromagnetic radiation, including a significant amount of UV radiation. UV photons from the flame strike a UV-sensitive sensor (e.g., a gas-filled tube). When they strike the sensor's cathode, they cause a photoelectric effect, ejecting electrons. These ejected electrons ionize the gas within the tube, creating a small, measurable electrical current. The detector's electronic circuitry amplifies and processes this current. If the signal meets a preset threshold for intensity and duration, the detector triggers an alarm.
  
  
• Example: Honeywell "Purple Peeper": This common brand of UV detector gets its name from the reddish-purple glow of the sensing tube when it detects UV. Often used in gas turbine systems.
  
  
• Advantages: Extremely fast response (milliseconds), effective for many fire types, can be "solar-blind" (ignores natural sunlight). Even if the sun produces UV light the sensor listens for a very small range of UV radiation and ignores most of the UV produced by the sun.
  
  
• Limitations: Prone to false alarms from other UV sources (arc welding, lightning, X-rays), requires clear line of sight (blocked by smoke/dust).
  
  
• Infrared (IR) Flame Detectors:
• What they detect: The infrared radiation emitted by a flame, specifically focusing on spectral patterns unique to fires.
• How they work: “Flame spectral pattern” refers to the distinctive wavelengths (or spectrum) of infrared radiation that flames emit. When hydrocarbons the flame emits infrared radiation in specific wavelengths. These wavelengths form a signature pattern, or "spectral fingerprint," unique to real flames.
• Single-IR: Single ID flame detectors look at a specific IR wavelength (e.g., 4.4μm for CO2 ​emissions from hydrocarbon fires). Can be prone to false alarms from hot objects.
• Multi-Spectrum IR (IR3): Uses three or more IR sensors at different wavelengths. It checks for CO2 IR wavelength and compares it to nearby bands (e.g., 4.0 µm and 4.6 µm) It also looks at flicker frequencies typical of real flames. With this, it creates a "fingerprint" of a true flame, providing high false alarm immunity.
• Advantages: High false alarm immunity (especially IR3) can penetrate some smoke/fog/dust (better than UV), long detection range, excellent for hydrocarbon fires.
  
  
• Limitations: Response is slightly slower than UV, still requires line of sight (though better through some particulates), not ideal for fires without strong IR signatures (e.g., hydrogen).

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3. Gas Detection Technologies

Gas detectors identify the presence and concentration of specific gases in the air. These are broadly categorized into flammable and toxic gas detectors.

3.1. Flammable Gas Detectors (for explosive gases like Methane, Propane)

• Line-of-Sight (Open-Path) Gas Detectors:
• What they detect: The presence of gas along an entire beam path, typically 5-200 meters. Instead of monitoring gas concentration at a single point, they provide continuous and wide-area monitoring by using a beam of light to detect gas along an entire path.
  
  
• How they work: The core principle behind line-of-sight gas detectors is infrared (IR) absorption spectroscopy. It is based on the fact that different gases have unique "fingerprints" of how they absorb certain wavelengths of infrared light. The system uses two specific wavelengths of infrared light: Measurement Wavelength - This is a wavelength that is strongly absorbed by the target gas (e.g., methane absorb at around 3.3μm). Reference Wavelength - This is a nearby wavelength that is not absorbed by the target gas. This reference wavelength is crucial for compensating for other factors that might affect the signal, such as rain, fog, dust, or dirt on the optics.
  
  A transmitter sends a beam of IR light to a receiver unit placed at a distance from the transmitter. If target gas is in the path, it absorbs the measurement wavelength. The receiver detects the reduction and signals an alarm.
• Output: Typically in LEL-meters (concentration x distance).
  
  
• Advantages: Wide area coverage, very fast response (gas doesn't have to drift to a single point), less maintenance than many point detectors, immune to poisoning.
  
  
• Limitations: Requires clear line of sight, does not pinpoint leak location, and cannot detect gases that don't absorb IR (e.g., hydrogen).
  
  
• Point-Type Gas Detectors (Flammable):
• What they detect: measure the concentration of a particular gas in the immediate vicinity or specific spot.
• Technologies:
  Catalytic combustion, electrochemical, infrared, photo-ionization are the most common type of technologies gas detectors utilize. Each technology has its own working principle, advantages, and limitations.

Here is an explanation of the most common technologies for point-type gas detectors:

• Catalytic Combustion (Pellistor):
• How it works: This technology uses a pair of "pellistors" or beads, which are small heating elements. One bead is coated with a catalyst (e.g., platinum) that lowers the ignition temperature of the target gas. The other bead is an inactive reference bead.
  When combustible gas in the air comes into contact with the heated catalytic bead, it combusts (oxidizes) on the surface, causing the temperature of the bead to rise.
  This increase in temperature changes the electrical resistance of the platinum wire inside the bead. The change in resistance is measured by a Wheatstone bridge circuit. The reference bead's resistance remains unchanged.

The difference in resistance between the two beads is proportional to the concentration of the combustible gas, which is typically measured as a percentage of the Lower Explosive Limit (LEL).

• Advantages: Cost-effective, robust.
  
  
• Limitations: Requires oxygen, susceptible to "poisoning" by chemicals (silicones, lead), can be damaged by very high gas concentrations.
  
  
• Infrared (IR):
• How it works: Similar to line-of-sight, but in a small, enclosed sample chamber. Measures IR absorption. To reiterate, an IR light source and a receiver in a small, enclosed chamber.

Two wavelengths of light are used: a measurement wavelength that the target gas absorbs, and a reference wavelength that it does not.

When the gas enters the chamber, it absorbs some of the energy from the measurement beam. The reference beam's intensity remains unchanged.

The receiver detects the difference in intensity between the two beams. The reduction in the measurement beam's intensity is directly proportional to the gas concentration.

• Advantages: Immune to poisoning, no oxygen required, fail-to-safe.
  
  
• Limitations: Cannot detect hydrogen, affected by optics cleanliness.

3.2. Toxic Gas Detectors (for hazardous gases like H2S, CO)

• Point-Type Gas Detectors (Toxic):
• What they detect: The concentration of a specific toxic gas (e.g., H2​S, CO, Cl2​, NH3​).
• Example: Hydrogen Sulfide (H2​S): A highly toxic, flammable gas, heavier than air, smells like "rotton eggs" at low concentrations but deadens the sense of smell at higher, more dangerous levels. Very common in oil & gas, wastewater.
• Electrochemical Sensors:
• How they work: An electrochemical sensor is essentially a small battery or fuel cell. It consists of a working electrode, a counter electrode, and a reference electrode, all submerged in an electrolyte solution and separated from the air by a gas-permeable membrane.
  When the target gas molecules diffuse through the membrane and reach the working electrode, they undergo a chemical reaction (either oxidation or reduction).
  This reaction generates an electrical current that is directly proportional to the concentration of the gas. (typically in ppm).
  
  
• Advantages: Highly sensitive, specific to target gas, low power consumption, fast response.
  
  
• Limitations: Limited lifespan (electrolyte consumption), can have some cross-sensitivity to other gases, affected by temperature/humidity.
• Other Technologies (less common for H2S, more for other toxics/VOCs):
• Metal Oxide Semiconductor (MOS): Changes resistance on heated metal oxide surface. Less specific, affected by humidity.
• Photoionization (PID): Uses UV lamp to ionize gas molecules and measures resulting current. Extremely sensitive for VOCs, fast response, but generally not primary for H2​S.

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4. Smoke Detectors

Smoke detectors sense the presence of smoke particles in the air.

• Conventional Spot-Type Smoke Detectors:
• Types:
• Ionization: Detects changes in electrical current caused by smoke particles. Good for fast, flaming fires.
• Photoelectric: Uses a light beam that is scattered by smoke particles. Good for slow, smoldering fires.
• Best for: General life safety in enclosed spaces (offices, homes).
• Limitations: Prone to false alarms from steam, dust, cooking fumes.
• VESDA (Very Early Smoke Detection Apparatus):
• What it is: An Aspirating Smoke Detection (ASD) system – it actively draws air samples.
• How it works: A high-efficiency aspirator draws air through a network of pipes with sampling holes. The air is filtered and passed into a laser-based detection chamber. Smoke particles scatter the laser light, which is detected.
• Key Features:
• Very Early Warning: Detects smoke at the "incipient" (earliest) stage, often before visible smoke.
• Multi-Level Alarms: Provides progressive alerts (Alert, Action, Fire 1, Fire 2) for staged response.
• Remote Mounting: Main unit can be placed in a clean, accessible area, with pipes in harsh or hidden locations.
• Best for: Critical facilities where early warning is paramount (data centers, clean rooms, control rooms, high-airflow areas, historical buildings, large warehouses). Provides significant lead time for intervention.

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5. Conclusion

Understanding these different F&G detection technologies is crucial for ensuring safety in our operations. Each type has specific strengths and weaknesses, making them suitable for different applications. By deploying the right combination of detectors, we can establish a robust safety net that protects our people, our assets, and our environment.

Key Takeaways:

• Heat Detectors: Respond to temperature. Good for specific environments, but generally slower.
• Flame Detectors: Respond to light from a flame. Very fast, but require line of sight. UV is fastest, IR3 is best for false alarm immunity.
• Gas Detectors: Respond to gas presence. Line-of-sight for wide area, point-type for specific locations. Catalytic for flammables (needs O2), Electrochemical for toxics (sensitive, specific), IR for flammables/CO2 (no O2 needed, no poisoning).
• Smoke Detectors: Respond to smoke. VESDA provides very early, active detection for critical applications.