Deck Foam & Inert Gas (IG) Systems on Tankers

1. Foam Principle & Blanket Action

For "explain deck foam for fire fighting?" (Q25), start with what foam is and what the blanket does. Foam for fire protection is an aggregate of air-filled bubbles formed from aqueous solutions, and is lower in density than the lightest flammable liquids.

• < Fuel Density — Floating Coherent Blanket
• 4-Way Action — Smother, Cool, Seal, Shield
• Vapour Seal — Stops Re-Ignition
• Conductive — Never on Electrical Fires

1. Air Exclusion & Smothering

Forms a coherent floating blanket over burning hydrocarbons, creating a physical barrier that cuts off atmospheric oxygen supply to the flame front.

2. Fuel Cooling & Steam Quenching

The water content draining from the bubble matrix cools the liquid fuel below its flashpoint; vaporised drainage water expands into steam to displace oxygen locally.

3. Vapour Suppression & Re-Ignition Barrier

The stable blanket traps volatile hydrocarbon gases beneath the foam surface, preventing flammable vapour from mixing with air even after the fire is out.

4. Surface Adherence & Progressive Attack

Adheres to hot vertical tank bulkheads and pipework, providing radiant heat shielding and allowing firefighters to advance foam coverage progressively across the deck.

Danger — Electrical Fire Warning. Because foam solutions consist of salt or fresh water with electrolyte concentrate additives, foam is electrically conductive and strictly prohibited for use on energized switchboards or electrical machinery.

2. Foam Concentrate Types

The principal use of foam is to extinguish burning flammable or combustible liquid spills or tank fires by developing a coherent coolant blanket. Give the four concentrate families with dilution and duty:

Protein — film — alcohol: Protein oil fires; Film spreads fast; Alcohol polar fuels.

• 3% to 6% — Standard Dilution Ratio
• 20:1 – 1000:1 — Hi-Ex Expansion Ratio
• 4 Types — Protein, Fluoro, AR, AFFF
• +10 m Head — Pump Pressure Over Water

Protein Foam Concentrate

Formulated from natural animal protein hydrolysate. Mixed at 3% to 6% with sea/fresh water. Produces a stiff, dense, cohesive blanket with exceptional burnback resistance. Used primarily on crude oil tank fires.

Fluoroprotein Foam Concentrate

Enhanced with synthetic fluorochemical surfactants. Mixed at 3% to 6%. Deposits an ultra-thin vaporisation-preventing film over the fuel surface, resisting fuel contamination and shed fuel pickup. Used on crude oil and refined petroleum products.

Alcohol-Resistant (AR) Concentrate

Contains hydrophilic polysaccharide polymers. When applied to water-miscible polar solvents (alcohols, ketones, esters), it precipitates an insoluble polymeric membrane barrier that stops chemical breakdown of the bubble wall. Mandatory for chemical tankers.

Synthetic AFFF & High-Expansion

Aqueous Film Forming Foam (AFFF) spreads rapidly across fuel spills for rapid flame knockdown. High-expansion foam generators produce expansion ratios from 20:1 up to 1000:1, designed for total volumetric flooding of engine rooms and pump rooms.

• One-line definitions for oral exam. Protein = crude at 3–6%
• Fluoroprotein = film on crude and refined at 3–6%
• Alcohol-type = insoluble barrier for chemicals
• Synthetic = AFFF, hi-ex 20:1–1000:1 for local and engine-room use.

3. Deck Foam Regulation, Rates & Components

SOLAS/IMO regulation: for ships carrying chemicals or oils in bulk, a fixed deck-foam system for extinguishing fires on deck or in tanks is required. The systems are identical in principle, but for chemical tankers of IMO type 2 and 3 the foam system is considerably larger than for crude-oil tankers, due to the higher fire risk of chemicals.

Blanket stops the vapour: Deck tankers fitted; Blanket seal + cool; Throw monitor jets.

• 0.6 L/m²/min — Oil Tanker: Whole Deck
• 6.0 L/m²/min — Oil Tanker: Largest Tank
• 2.0 L/m²/min — Chemical: Whole Deck
• 20.0 L/m²/min — Chemical: Largest Tank

System description (components):

1. Water Supply

Dedicated sea-water supply provided by the ship's fire pumps, delivering pressurized water continuously to the foam proportioning station.

2. Foam-Liquid Storage Tank

Stainless steel or epoxy-lined atmospheric tank complete with air vent, level sight glass, pressure vacuum valve, and access manhole.

3. Proportioning Device & Pump

High-pressure electric positive-displacement pump delivering concentrate to the automatic balanced proportioner at 3% to 6%, maintaining a minimum 10 m head pressure above inlet water pressure under all flow conditions.

4. Deck Monitors & Handlines

Deck main equipped with sectional isolating valves. Deck foam monitors isolated by normally-closed butterfly valves, plus four portable foam branchpipes (each 400 L/min solution rate).

Rate memory hook for viva. Oil 0.6 / 6.0 — chemical roughly three times richer at 2.0 / 20.0. Always quote whole-deck first, largest-tank second; proportioner 3–6% with 10 m head margin.

4. Inert Gas: Flammable Limits & Definitions

For "explain the IG system?" (Q26), start with flammable limits, the Coward flammability relationship, and board operational definitions:

• 0% – 1% HC — Too Lean (< LFL)
• 1% – 10% HC — Flammable Envelope (In Air)
• > 10% HC — Too Rich (> UFL)
• ≤ 8% O² — SOLAS Inert Target (11.5% Critical)

Lower Flammable Limit (LFL)

The leanest combustible mixture capable of propagating flame. For petroleum hydrocarbon vapors in air, LFL is approximately 1.0% by volume. Below LFL, the mixture has insufficient fuel to burn (too lean).

Upper Flammable Limit (UFL)

The richest combustible mixture capable of propagating flame. For petroleum vapors in air, UFL is approximately 10.0% – 11.0% by volume. Above UFL, there is insufficient oxygen to sustain combustion (too rich).

Temperature & Pressure Influence

Rising temperature widens the flammable band (lowers LFL and raises UFL). Higher absolute pressure increases both LFL and UFL, expanding the explosive envelope under compression.

Purging vs. Gas Freeing

Purging: introducing IG into an inert tank to reduce hydrocarbon concentration to ≤ 2% before air admission, preventing entry into the flammable zone. Gas freeing: replacing inert gas with breathable fresh air (21% O²).

5. IG Plant Construction Parts

Surveyors expect the major plant assemblies to be recited in the exact sequence the gas encounters them from uptake to weather deck:

• 125% — Blower Output vs Pump Rate
• > 90% SO² — Scrubber Acid Extraction
• +100 mmwg — Deck Positive Pressure
• Low Level — Only Active Seal Sensor

1. Boiler Uptake Valve

Controls hot gas take-off from boiler exhaust. Manufactured from heat-resistant cast steel. Fitted with compressed-air sealing to prevent exhaust gas entering the idle plant, plus steam/air soot blowers to clear carbon fouling.

2. Flue Gas Scrubber Tower

Three-stage cooling and cleaning tower internally lined with acid-resistant GRP (glass-reinforced plastic). Features a venturi quencher, impingement wet filter plates, and sea-water spray nozzles. Strips > 90% of SO² and soot, draining effluent overboard via an acidic U-seal.

3. Centrifugal IG Blowers

Two single-stage centrifugal blowers (1 duty + 1 standby) on resilient anti-vibration mountings. Minimum aggregate capacity is 125% of the ship's maximum cargo discharge rate, ensuring positive line pressure under peak discharge.

4. Continuous O² Analyser

Dual paramagnetic or zirconium sensor cells sampling gas post-blower. If oxygen content rises above 8% by volume, it instantly triggers an audible/visual alarm, opens the 3-way atmospheric vent valve, and trips the deck main shut-off valve.

5. Deck Water Seal

The principal barrier separating the hazardous cargo tank deck from the non-hazardous machinery space. Liquid column physically blocks hydrocarbon reverse flow. Equipped with a demister pad and a mandatory low-water-level alarm.

6. Liquid P/V Breaker

Epoxy-coated safety breaker located on the weather deck, filled with a water-glycol mixture. Protects cargo tanks against severe over-pressure or deep vacuum in the event of mechanical high-velocity P/V valve failure.

6. IG Working Flow, Oxygen & Gas Replacement

For "trace the inert gas from boiler uptake to cargo tank", explain the process as ordered engineering stages with operational parameters:

Exhaust becomes shield: Clean scrub + cool; Thin O2 under 8 pc; Hold pressure + seal.

1. Boiler Uptake Gas Extraction: Exhaust gas containing 4% – 5% O² is drawn from the auxiliary boiler uptake via an air-sealed, heat-resistant butterfly valve fitted with automated soot blowers.
2. Quenching & Washing in Scrubber: Flue gas enters the GRP-lined scrubber tower where seawater sprays and a venturi throat cool the gas from 350°C down to < 40°C while stripping > 90% of SO² and particulates; acidic effluent drains overboard.
3. Moisture Demisting: Cooled gas passes through a polypropylene mesh demister pad to eliminate liquid water droplet carryover prior to entering the blowers.
4. Blower Pressurisation: Centrifugal blowers (125% aggregate capacity, 1 duty + 1 standby) pressurise the dry inert gas, maintaining positive distribution head along the main.
5. Continuous Oxygen Analysis & Trip: Paramagnetic analysers sample gas quality. If O² exceeds 8.0% by volume, the 3-way atmospheric dump valve trips open and the deck isolating valve trips closed.
6. Deck Water Seal & Mechanical Non-Return: On-specification gas (O² ≤ 5%) flows through the deck water seal, mechanical non-return valve, and manual deck isolation valve onto the tanker weather deck.
7. Tank Distribution & P/V Breaker Protection: Gas enters cargo tanks held at ≥ +100 mmwg positive pressure. A liquid P/V breaker and individual high-velocity P/V valves protect structural integrity against over-pressure or vacuum.

Gas Replacement Principles (Viva Question):

Dilution Method (Turbulent Mixing)

Inert gas is injected at high velocity from deck level, penetrating to the tank bottom. It mixes turbulently with existing vapors, and the homogeneous mixture is continuously expelled from top vents. Requires approximately 3 to 4 tank volumes of IG to achieve safe levels.

Displacement Method (Piston Layering)

Utilises density differences. Cool, dense inert gas enters the tank top at low velocity, settling to the bottom like a piston and lifting the lighter, warmer hydrocarbon vapors upward and out through the bottom purge pipe. Highly efficient, requiring only 1.2 to 1.5 tank volumes.

Exam Recital Sequence. Deliver the process in surveyor cadence: Boiler Uptake → Scrubber → Demister → Blowers (125%) → O² Analyser (Dump > 8%) → Deck Water Seal → Deck Isolating Valve → Cargo Tanks (+100 mmwg) guarded by Liquid P/V Breaker.

7. Foam & IG Oral-Exam Recap

Surveyors frequently pair fixed deck foam (Q25) and inert gas systems (Q26) in a single high-intensity oral exam segment. Master the exact operational numbers and system schematics below: