Answer
Under Chapter 13 of the IGC Code (and aligned with the Indian Merchant Shipping Rules), the Fixed Gas Detection System (FGDS) is a critical safety framework on liquefied gas carriers. Its primary objective is to provide continuous, reliable, and early detection of flammable and toxic gas leaks to prevent explosions, fire hazards, and crew toxicity.
The functions of an FGDS can be systematically categorized into four primary areas: Monitoring, Alarming, Executive Safety Actions (Interlocks), and Self-Diagnostics.
1. Core Functions of the FGDS
A. Continuous Gas Sampling and Quantification
Sequential or Continuous Analysis: The system automatically draws gas samples from various hazardous and non-hazardous spaces via a network of sample lines (in a suction-type system) or monitors them continuously using localized optical/electrochemical sensors.
Gas Concentration Measurement: It accurately quantifies the gas concentration in two distinct metrics:
% LFL (Lower Flammable Limit) / LEL: For flammable hydrocarbon gases (e.g., Propane, Butane) and Ammonia.
PPM (Parts Per Million): For toxic hazards, primarily required when carrying Anhydrous Ammonia.
Vapor Density Management: It is designed to sample from both the top and bottom of monitored spaces, depending on whether the cargo vapor is lighter than air (Ammonia) or heavier than air (Propane/Butane).
B. Visual and Audible Alarm Activation
The system alerts the watchkeeper by triggering localized and centralized audible and visual alarms on the cargo control room (CCR) panel and the navigation bridge when preset gas thresholds are breached.
IGC Code Threshold Mandates:
Flammable Gases: Alarms must actuate when the vapor concentration reaches a maximum of 30% LFL. (Many ship specifications set a pre-alarm at 10% LFL).
Toxic Gases (Ammonia): Alarms must actuate before the toxic concentration exceeds the permissible exposure limit, typically set at 25 ppm to 50 ppm.
C. Executive Safety Actions (Emergency Interlocks)
The FGDS does not merely monitor; it is hardwired into the vessel's Emergency Shutdown (ESD) system to initiate automated protective actions:
Ventilation Tripping: Automatically trips the mechanical ventilation for accommodation spaces, airlocks, and electric motor rooms if gas is detected at their respective air intakes.
Electrical Isolation: Automatically isolates non-intrinsically safe (non-Ex) electrical equipment in spaces like airlocks or motor rooms upon gas detection.
Boiler / Gas Consumer Trip: If gas is detected in the double-wall piping ventilation hoods of gas-fueled machinery (e.g., dual-fuel boilers or main engines), the FGDS immediately shuts off the fuel gas supply valves and purges the line with inert gas.
D. System Integrity & Fault Diagnostics
Line Blockage/Low Flow Alarm: Monitors the pressure/flow rate of incoming sample air. If a sampling pipe is crimped or blocked by condensation/liquid cargo, it triggers a "Low Flow" fault alarm.
Power Failure Protection: Triggers an immediate audible/visual alarm if the primary power supply fails, automatically shifting to a 24V DC emergency battery backup.
Sensor Faults: Detects and flags sensor drift, optical obscuration (for IR sensors), or open/short circuits within the transmitter loops.
2. Critical Monitoring Locations & Requirements
According to the IGC Code, sampling points must be permanently installed in the following spaces:
| Space Monitored | Hazard Type | Special Regulatory Requirement |
|---|---|---|
| Hold Spaces / Interbarrier Spaces | Flammable | Must function effectively even when the hold space is completely inerted (CO2 or N2 atmosphere). |
| Cargo Pump & Compressor Rooms | Flammable & Toxic | Must be continuously monitored. Sampling interval must not exceed 30 minutes if sequential sampling is used. |
| Accommodation Air Intakes | Flammable & Toxic | Positioned at the main HVAC intakes to prevent toxic/explosive gas ingress into living spaces. |
| Electric Motor Rooms | Flammable | Interlocked to trip motor room power if gas concentrations breach limits. |
| Cargo Control Room & Airlocks | Flammable & Toxic | Ensures the safety of personnel operating the cargo handling systems. |
3. Sensor Technologies and Selection (MEO Oral Significance)
An examiner at the MMD will expect you to understand why certain sensors are used in specific spaces:
Infrared (IR) Sensors
Function: Uses infrared light absorption to detect hydrocarbon gas.
Operational Advantage: Does not require oxygen to function. This makes IR sensors mandatory for monitoring inerted hold spaces or interbarrier spaces where oxygen is absent.
Limitation: Cannot detect Hydrogen (H2) or Ammonia (NH3) because these gases do not absorb IR wavelengths in the same band as hydrocarbons.
Catalytic Bead (Pellistor) Sensors
Function: Burns small amounts of gas on a catalytic filament to measure temperature change.
Limitation: Requires at least 10% Oxygen to function accurately. Therefore, they cannot be used in inerted hold spaces. They are also easily "poisoned" by sulfur compounds or silicone vapors.
Electrochemical Sensors
Function: Measures chemical oxidation/reduction reactions on an electrode.
Primary Application: Used specifically for Toxic Gas Detection (Ammonia) in ppm ranges. They are highly sensitive but have a finite lifespan and degrade when exposed to continuous dry atmospheres.
⚓ MEO Class 1 Oral Exam Checkpoints
The 30-Minute Rule: If your vessel utilizes a sequential scanning/sampling type FGDS (where one central analyzer samples multiple lines via a manifold), the IGC Code mandates that the sampling interval between any single point must not exceed 30 minutes.
Testing and Calibration: You must state that the system is verified daily via the built-in lamp/alarm test, and calibrated monthly (or before every cargo operation) using a certified Span Gas bottle containing a known concentration (e.g., 50% LFL Propane or 50 ppm Ammonia) to correct any sensor drift.
Purging Sampling Lines: Emphasize that before changing grades (e.g., Propane to Ammonia), the sample lines must be thoroughly purged with dry air or nitrogen to prevent cross-contamination or liquid condensation inside the analyzer cabinet.