Answer
Changing cargo grades between Propane (C₃H₈) and Anhydrous Ammonia (NH₃) is one of the most critical and meticulous operations on a liquefied gas carrier.
The primary technical challenge stems from a severe chemical incompatibility: Ammonia reacts with Carbon Dioxide (CO₂) present in conventional combustion inert gas to form Ammonium Carbamate, a white solid powder that clogs valves, cargo pumps, filters, and reliquefaction systems. Furthermore, their drastically different vapor densities require completely opposite purging strategies.
Ammonia Vapor Density: ≈0.6 (Lighter than air/Inert Gas)
Propane Vapor Density: ≈1.5 (Heavier than air/Inert Gas)
Inert Gas is heavier than dry air
1. Grade Change from Propane to Ammonia (C3H8→NH3)
The objective here is to remove all hydrocarbon vapors and ensure that no CO2 is left in the system before Ammonia is introduced.
Step 1: Liquid Stripping (Puddling Out)
Discharge the propane cargo completely.
Utilize the puddle pumps (if installed) and stripping system to reduce the liquid heel in the sumps to the absolute minimum.
Step 2: Tank Warming Up
Circulate warm propane vapor through the cargo heaters/vaporizers using the cargo compressors.
Introduce the warm vapor into the bottom of the tank to vaporize any residual liquid propane trapped in structural corners.
Continue until all tank temperature sensors indicate a warm state (typically above +10∘C to +15∘C).
Step 3: Inerting & Aeration (The CO2 Mitigation Phase)
Because combustion Inert Gas (IG) contains roughly 14% CO2, you cannot directly introduce Ammonia after standard inerting. Two methods are acceptable:
Method A (Preferred - Pure Nitrogen): Purge the propane vapor directly using pure Nitrogen (N2) from a shore supply or shipboard N2 generator until the Hydrocarbon (HC) concentration is less than 1% by volume. As propane is heavier than N2, so N2 will will introduced from top.
Method B (Conventional IG + Aeration): 1. Purge the propane vapor using conventional combustion IG until the HC level is below 2% to safely pass the flammability range. 2. Immediately follow up by purging the combustion IG with Dry Air (Aeration) until the CO2 level drops below 0.1% and the oxygen level reaches 21%.
Step 4: Gassing Up with Ammonia
Because Ammonia vapor is lighter than air/Nitrogen, introduce Ammonia vapor from the top of the tank via the vapor dome.
Displace the air/Nitrogen out through the liquid line from the bottom of the tank to the vent mast or shore flare.
Continue gassing up until the ammonia concentration at the outlet exceeds 95%−98%.
Step 5: Cooling Down
Run the reliquefaction plant using the cargo compressors to gradually cool the tank down (2∘C to 5∘C per hour) to the required loading temperature.
Note: Ensure that the compressor lubricating oil is verified as compatible with Ammonia.
2. Grade Change from Ammonia to Propane (NH3→C3H8)
The objective here is to safely eliminate toxic Ammonia vapors without allowing them to mix with CO2, and then lower the oxygen content before introducing explosive hydrocarbons.
Step 1: Liquid Stripping & Warming Up
Drain all liquid ammonia from the tanks using puddle pumps and the puddle heating system.
Circulate warm ammonia vapor using the cargo compressor and heater to warm the tank structures above 0∘C, ensuring no cold liquid pockets remain.
Step 2: Ammonia Vapor Removal (Displacement)
CRITICAL: Do not inject standard combustion IG into the ammonia vapor. Doing so will trigger the following reaction:
2NH3+CO2→NH2COONH4(Ammonium Carbamate Solid)
Method A (Pure Nitrogen): Purge the ammonia vapor using pure N2 until ammonia levels drop below terminal/charterer limits (typically <20 ppm).
Method B (Aeration with Dry Air): If N2 is limited, use the IGG blower configured to deliver fresh Dry Air only. Because Ammonia is lighter than air, inject the dry air from the bottom of the tank to push the ammonia vapor out from the top via the mast riser. Continue until the ammonia concentration is negligible.
Step 3: Tank Inspection & Dew Point Check
Perform tank entry and visual inspection if required by the charterparty presentation clause.
Verify that the tank is perfectly dry. The dew point must be driven down to −40∘C or lower to prevent hydrate formation when propane is loaded.
Step 4: Inerting for Hydrocarbons
With Ammonia completely eliminated from the system, it is now perfectly safe to use conventional combustion Inert Gas (CO2-rich).
Purge the dry air with Inert Gas until the Oxygen (O2) level drops below 2% by volume to prevent any explosive atmosphere before propane entry.
Step 5: Gassing Up with Propane
Propane vapor is heavier than Inert Gas. Introduce Propane vapor from the bottom of the tank.
Displace the lighter Inert Gas out through the top vapor line to the shore flare or vent mast.
Continue until the propane vapor concentration is above 95%.
Step 6: Compressor Oil Flushing & Cooling Down
Mandatory Step: Drain, flush, and replace the cargo compressor crankcase oil if it was exposed to ammonia, as ammonia alters the lubrication properties of hydrocarbon-grade synthetic oils.
Engage the reliquefaction plant to cool down the tanks to the required loading temperature.
💡 Key MEO Class 1 Oral Examination Pointers
Material Limitations: Remind the examiner that Ammonia attacks copper, brass, and zinc alloys (bronze). Ensure all portable measuring equipment, gauging tapes, and regional system fittings are strictly stainless steel or steel alloys.
Purging Directions: Always articulate the direction of flow clearly based on vapor density differences. Ammonia enters from the top to displace heavier gases; Propane enters from the bottom to displace lighter gases.
Regulatory Compliance: Under the IGC Code and the Indian Merchant Shipping Rules, strict logbook entries must be maintained for gas-freeing certificates, tank atmospheres (O2, CO2, HC, and toxic ppm levels), and slop/vessel wash disposal controls if any water washing was utilized.