An increase in cargo temperature due to higher ambient temperature or cargo heating will result in cargo expansion and an increase in vapour pressure. If the tank is overfilled, product could escape through the relief valve. If the loading temperature is known and the highest temperature the cargo will reach either naturally or by heating is also known, it is a simple matter to calculate the maximum safe load necessary to avoid discharge through the pressure relief valve.

For hazardous cargo transportation certain maximum filling ratios are recommended by the IMDG Code. These filling ratios are variously 97%, 95% or 90% according to the classification of the cargo or its vapour pressure, or whether
the cargo is heated in transit.

These figures (97%, 95% and 90%) are based on an ullage allowance for possible expansion to a maximum temperature of 50C when a substance is loaded at ambient temperatures i.e. not 97% etc, at ambient temperatures. Other calculations for ullage apply when the loading temperature is in excess of 50C.
It should also be noted that the design temperature range of our tanks must not be exceeded.