Answer
During a cargo grade change between Anhydrous Ammonia (NH3) and Propane (C3H8), sampling is the only definitive method to verify that the tank atmosphere is safe, dry, and compliant with charterparty presentation clauses.
Because these two gases have vastly different vapor densities and chemical properties, the sampling protocols, locations, and target thresholds must be executed with extreme precision.
1. Sampling Protocol: Ammonia to Propane (NH3→C3H8)
The primary objective is to verify the complete removal of toxic ammonia and moisture before introducing carbon dioxide (Inert Gas) and hydrocarbons.
Phase 1: After Post-Stripping & Hot Gas Blowing (Warming Up)
What to Sample For: Ammonia vapor concentration (ppm or % by volume).
Sampling Locations: Top (Vapor Dome) and Middle levels. Because Ammonia is lighter than air (Vapor Density ≈0.6), any residual pockets or slow-vaporizing liquid pools will release vapor that rises.
Target Threshold: Clear the tank until ammonia levels drop below the required charterer limit (typically <20 ppm).
Critical Safety Note: Never introduce conventional Combustion Inert Gas (IG) until this threshold is verified. If CO2 mixes with residual ammonia, it will precipitate Ammonium Carbamate solids, clogging your sampling lines and cargo equipment.
Phase 2: During and After Fresh Dry Air Aeration
What to Sample For: Oxygen (O2) and Dew Point (∘C).
Sampling Locations: Bottom (Sump) and Middle levels. Fresh air is heavier than ammonia, so you must verify that dry air has thoroughly swept the bottom structural floors.
Target Thresholds:
Oxygen: Must reach 21% by volume (if a physical tank entry/visual inspection is required).
Dew Point: Must be driven down to −40∘C or lower. This confirms that all moisture (which ammonia readily absorbs) has been completely evacuated, preventing the formation of propane hydrates.
Phase 3: After Inerting with Combustion IG
What to Sample For: Oxygen (O2) and Carbon Dioxide (CO2).
Sampling Locations: Top, Middle, and Bottom.
Target Threshold: Oxygen <2% by volume. This ensures the tank is completely inerted and safely out of the flammability zone before propane is introduced.
Phase 4: During and After Propane Gassing Up
What to Sample For: Propane liquid-match vapor purity (% by volume).
Sampling Locations: Top (Vapor Dome). Since propane vapor is heavier than air/IG (Vapor Density ≈1.5), it fills from the bottom up, displacing the lighter IG out the top. Sampling from the top verifies when the displacement is complete.
Target Threshold: >95% to 98% Propane vapor purity before initiating the cooldown phase.
2. Sampling Protocol: Propane to Ammonia (C3H8→NH3)
The primary objective here is the absolute elimination of explosive hydrocarbons and, crucially, Carbon Dioxide (CO2) before ammonia entry.
Phase 1: After Inerting Hydrocarbons
What to Sample For: Hydrocarbon (HC) concentration (% LFL or % by volume).
Sampling Locations: Bottom (Sump) and Middle. Propane vapor is heavy; check the lowest points of the prismatic tank to ensure no heavy hydrocarbon vapor is trapped.
Target Threshold: <1% HC by volume (or <10% LFL) if pure Nitrogen was used, or <2% HC if conventional IG was used to pass the flammability curve.
Phase 2: After Aeration (If Conventional IG was used)
What to Sample For: Carbon Dioxide (CO2) and Oxygen (O2).
Sampling Locations: Bottom and Middle. CO2 is heavier than air and settles at the tank floor.
Target Threshold: CO2<0.1% by volume (1000 ppm) and Oxygen ≈21%.
Why this is critical: If you skip checking CO2 and introduce ammonia, the remaining CO2 will instantly react to form solid deposits.
Phase 3: During and After Ammonia Gassing Up
What to Sample For: Ammonia vapor purity (% by volume).
Sampling Locations: Bottom (Sump). Because ammonia vapor is lighter than the air/nitrogen inside the tank, it is injected from the top dome and pushes the heavier air down. Sampling from the bottom line confirms when the air has been entirely displaced.
Target Threshold: >95% Ammonia vapor purity before starting the cargo compressors for cooldown.
🛠️ Sampling Equipment & Methods (MEO Class 1 Focus)
When explaining this to an MMD examiner, emphasize the practical use of both fixed and portable sampling arrangements:
1. Multi-Level Sampling Tubes
Prismatic cargo tanks are fitted with permanent, closed-sampling small-bore stainless steel tubes extending to three distinct heights: Upper (Top), Middle, and Lower (Bottom/Sump).
2. Portable Gas Detectors vs. Fixed Systems
Do not rely solely on the ship's Fixed Gas Detection System (FGDS) cabinet during a grade change. The fixed system's IR sensors cannot read Ammonia accurately, and chemical cross-contamination can ruin the fixed sensors.
Use calibrated, portable gas detection equipment connected via closed sampling hoses and water/particulate filters:
Combustible Gas Indicators (Tankscopes / Infrared gas analyzers): Used to measure high % volume of Hydrocarbons in inert atmospheres.
Electrochemical / Photoionization Detectors (PID): Used for precise toxic Ammonia ppm measurements (0−100 ppm).
Chemical Indicator Tubes (Draeger/Gastec Tubes): Used as a definitive secondary verification method specifically for measuring low concentrations of CO2 (0.1%) and Ammonia (ppm levels) to avoid instrument sensor cross-interference error.
3. Dew Point Meter
A portable electronic hygrometer or mirror-type dew point instrument must be connected to the sampling line to ensure moisture content satisfies the cargo specification before switching over.
⚓ Key Examination Watchpoints
Closed Sampling Integrity: Emphasize that all sampling during gassing up or purging must be conducted as a closed operation. Escaping toxic ammonia or flammable propane vapors must be routed back via the sample return line to the vent mast or a safe disposal point, never allowed to discharge freely onto the deck where personnel are operating.
Line Purging Before Measurement: Always state that the portable sampling line must be thoroughly flushed with the tank's current atmosphere for a few minutes to clear out dead air or residual condensation trapped inside the sample line before taking a definitive log reading.
Calibration Verification: Mention that portable meters must be zeroed using fresh air and span-checked with the appropriate calibration gas (N2, Propane, or Ammonia) immediately prior to beginning the grade change sequence.