Inert Gas System & Tank Oxygen Tool Purging, O₂ Dilution, Blower Capacity & Discharge Rate

FSS Code Chapter 15 • Tank O₂ ≤8% • IG Supply O₂ ≤5% • Positive Pressure • 125% Capacity • Purge Time • Dilution Curve • Blower Capacity

Inert Gas System & Tank Oxygen Tool Purging, O₂ Dilution, Blower Capacity & Discharge Rate

Screen tanker inert-gas quality, capacity and tank oxygen in one workflow. Simple mode calculates theoretical purge exchanges and time while checking the 5% inert-gas supply limit, 8% cargo-tank oxygen limit, positive pressure and the 125% discharge-capacity relationship. Advanced adds multiple cargo tanks, sequential or parallel inerting, individual tank pressure, discharge-rate demand, delivery-unit redundancy and a live oxygen-decay curve.

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Need several tanks, parallel inerting or blower redundancy? Click Advanced above.The purge-time calculation is an ideal well-mixed dilution model. Real tanks can stratify and short-circuit, so onboard oxygen measurements remain controlling.

Tank oxygenTime / exchanges to target

IG quality5% O₂ supply screen

Capacity125% discharge screen

Tank Inerting

Tank vapour-space volumem³.

Starting tank oxygen% by volume.

Inert-gas supply oxygen% by volume.

Target tank oxygen% by volume.

IG delivery capacitym³/h.

Current tank pressuremm water gauge.

Cargo Discharge / IG Main

Planned cargo discharge ratem³/h.

Current IG main pressuremm water gauge.

Purge mixing efficiency% of ideal well-mixed exchange effectiveness.

System & Regulatory Basis

IG system type

FSS Chapter 15 basis confirmed for vessel?

Tanker type

Approved maximum cargo discharge capacitym³/h.

Inert-gas supply oxygen

Current IG main pressuremm water gauge.

Maximum possible / regulated IG pressuremm water gauge.

Lowest cargo-tank test pressuremm water gauge equivalent.

Purge arrangement

Purge mixing efficiency% effective dilution versus ideal complete mixing.

Approved single-blower exception?

IG Delivery Unit Register

For flue-gas / inert-gas-generator systems, FSS Chapter 15 normally calls for at least two blowers, subject to the specific single-blower exception for certain inert-gas-generator systems. Nitrogen systems use their compressor/generator capacity instead.

Use	Unit type	Rated delivery m³/h	Available now?

Cargo Tank Register

Vapour volume is the gas space being diluted. Planned discharge rate is the cargo-volume replacement demand while the tank is discharging. Tank pressure must remain positive under the approved operating procedure.

Use	Vapour volume m³	Start O₂ %	Target O₂ %	Tank pressure mmWG	Planned discharge m³/h

Inert Gas System Checks

IG analyser calibrated / verifiedSupply-main oxygen reading is current and reliable. Tank atmosphere sampled correctlyTank oxygen readings represent the atmosphere being evaluated. Deck seal / non-return devices verifiedBackflow prevention is available and correctly lined up. IG main pressure controls / alarms verifiedLow and high pressure alarms and pump shutdown/interlock arrangements are operational. Blower / generator availability verifiedActual available gas-delivery capacity supports the operation. Tank isolation / P-V arrangements checkedEach tank can be isolated and protected from overpressure/vacuum as approved. Cargo discharge rate coordinated with IG capacityDischarge is restricted if available inert-gas delivery cannot keep up. Gas-freeing / hydrocarbon purge plan separateThis O₂ dilution tool is not being used as a substitute for hydrocarbon flammability-envelope control.

Safety boundary: this tool models oxygen dilution only. It does not determine hydrocarbon concentration, lower/upper flammable limits, critical dilution lines, gas-freeing safety, static-electricity precautions or safe entry. Purging hydrocarbon vapour and subsequent gas freeing require the vessel’s approved procedure, gas measurements and applicable tanker/chemical-cargo guidance.

FSS Chapter 15 basis: when the FSS inert-gas requirements apply, relevant cargo tanks are to be maintained at oxygen content not exceeding 8% by volume and positive pressure; inert gas delivered to the cargo tanks is to contain no more than 5% oxygen by volume; delivery capacity is to be at least 125% of the maximum cargo discharge capacity, with a chemical-tanker acceptance route allowing discharge to be restricted to no more than 80% of IG capacity; and an IG-main low-pressure alarm is required below 100 mm water gauge. Gas exceeding 5% oxygen is to be automatically vented rather than supplied to the cargo tanks.

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