Lng Boil off Rate Calculation

LNG Boil-Off Rate (BOR) Calculator

Calculate the daily percentage of Liquefied Natural Gas lost to evaporation due to heat ingress.

Calculation Results

Total LNG Mass: –

Daily Evaporation Mass: –

Boil-Off Rate (BOR): –

function calculateBOR() { var capacity = parseFloat(document.getElementById('tankCapacity').value); var level = parseFloat(document.getElementById('fillingLevel').value); var density = parseFloat(document.getElementById('lngDensity').value); var ingress = parseFloat(document.getElementById('heatIngress').value); var latent = parseFloat(document.getElementById('latentHeat').value); if (capacity > 0 && level > 0 && density > 0 && ingress >= 0 && latent > 0) { // Actual volume in m3 var actualVolume = capacity * (level / 100); // Total Mass in kg var totalMass = actualVolume * density; // Daily Heat Ingress in kJ (kW is kJ/s, so kJ/day = kW * 3600 * 24) var dailyHeatKJ = ingress * 3600 * 24; // Daily evaporated mass in kg var evapMassDay = dailyHeatKJ / latent; // Boil Off Rate (%) var borPercent = (evapMassDay / totalMass) * 100; document.getElementById('resTotalMass').innerText = totalMass.toLocaleString(undefined, {maximumFractionDigits: 0}) + " kg"; document.getElementById('resEvapMass').innerText = evapMassDay.toLocaleString(undefined, {maximumFractionDigits: 2}) + " kg/day"; document.getElementById('resBOR').innerText = borPercent.toFixed(4) + " % / day"; document.getElementById('borResult').style.display = 'block'; } else { alert("Please enter valid positive numbers for all fields."); } }

Understanding LNG Boil-Off Rate (BOR)

Liquefied Natural Gas (LNG) is stored and transported at cryogenic temperatures, typically around -162°C (-260°F). Despite advanced insulation systems, some heat ingress from the environment is inevitable. This heat ingress causes a small portion of the liquid to evaporate, a phenomenon known as Boil-Off.

The Science of BOR Calculation

The Boil-Off Rate (BOR) is a critical metric for LNG ship owners, terminal operators, and engineers. It represents the percentage of the total cargo volume that evaporates over a 24-hour period. A typical BOR for modern LNG carriers ranges from 0.08% to 0.15% per day, depending on the tank type (Membrane vs. Moss) and the quality of the insulation.

The Formula Used

Our calculator utilizes the fundamental thermodynamic relationship between heat transfer and phase change:

Daily Boil-Off Mass (kg/day) = (Heat Ingress in kW × 86,400) / Latent Heat of Vaporization

BOR (%/day) = (Daily Boil-Off Mass / Total Cargo Mass) × 100

Key Parameters in the Calculation

• LNG Density: Typically varies between 420 kg/m³ and 480 kg/m³ based on the composition (methane, ethane, propane levels).
• Heat Ingress: The rate at which thermal energy enters the tank. This is determined by the thermal conductivity of the insulation and the temperature gradient between the cargo and the outside air/sea.
• Latent Heat: The amount of energy required to turn 1 kg of liquid LNG into gas without changing its temperature (usually around 510 kJ/kg).

Practical Example

Imagine a standard LNG carrier with a capacity of 170,000 m³, filled to 98% capacity. With an LNG density of 450 kg/m³, the total mass is approximately 75 million kg. If the insulation allows 45 kW of heat leakage, the vessel will lose roughly 7,623 kg of gas per day, resulting in a BOR of approximately 0.0102% per day.

Note: In maritime operations, boil-off gas (BOG) is not simply "lost." It is often captured and used as fuel for the ship's propulsion system (dual-fuel engines) or reliquefied and returned to the tanks.