Mixing fully refrigerated Butane with fully refrigerated Propane generates flash gas, and there is a risk that if the mixing is conducted too fast and/or the pressure tendency is not being monitored scrupulously enough, the ship’s cargo tank relief valves will lift – causing a hazardous, uncontrolled release of a LPG vapour cloud on the jetty and around the vessel. This is most likely to occur during the loading operation when the vessel’s tank pressures are typically higher than they normally are at any other stage of the voyage.
During the loading operation all of the vessel’s compressors will normally be running at maximum capacity. If the vessel was to experience a loss of compressor capability due to any power problems or even a complete loss of power during the co-mingling operation there would be no contingency for disposal of the vapour generated unless a vapour return line was fitted. The presence of a vapour return line would allow the vapour to be sent to the shore thus potentially preventing the relief valves from lifting.
A real concern would be to ensure that the final volume of any tank did not exceed more than 98% full. Co-mingling
does involve the risk of over filling a tank, a risk especially acute during loading when there are many other events which may distract the deck officer’s attention.
The co-mingling operation can also cause "apparent" losses, as the density of the mixture will not be a mathematical average of the densities of the components – because the molecular composition is different. Calculation of cargo densities in such circumstances is discussed in Appendices 3 & 4 of the SIGTTO publication

Furthermore the temperature of the mixture may not be equal throughout. Non-equal temperature distributions can lead to problems in correcting for cargo tank shrinkage, float immersion and the temperature correction to be applied when quantifying the cargo.

Co-mingling on board fully pressurized vessels is not a major safety issue as there is no temperature gradient and the vessels are designed to be able to carry the cargo at ambient temperatures well within the safety valve settings. On board semi-refrigerated vessels it is also not considered to be a major safety issue although the relief valve settings can vary considerably in range from vessel to vessel.

If the vessel is able to contain the Propane at Butane temperatures then there is no major safety issue as there is no risk of pressure rise leading to uncontrolled release of vapour. This is the reason why co-mingling on board fully refrigerated vessels causes the most concern as these vessels are not designed to carry Propane at Butane temperatures.

It is often required that the ship mix propane and butane to the required proportions while loading, on passage or during discharge. This can cause significant problems, which can be controlled in fully and semi-pressurised gas carriers, but has the potential to be very serious in fully refrigerated gas carriers, where safety valves can lift causing the release of significant quantities of cargo. The Society of International Gas Tanker & Terminal Operators has published an alert on the co-mingling of LPG, which is available from their website at www.sigtto.org.