Canada tries out vent-and-burn

On March 29, 2011, a B-train highway cargo tank combination carrying LPG went off the side of Highway 16 some 88 km west of McBride, British Columbia. In the incident, the tractor separated from the two trailers, which slid down the side of the embankment. The driver was unhurt.

The lead cargo tank suffered a catastrophic failure, leading to the release and burn of the entire contents. The rear tank was leaking and there was a small flame emanating from the liquid spray fill pipe, but it was necessary to empty the rest of the cargo before the wrecked tanks could be moved. Meanwhile, the road was closed in both directions and a 1.6-km isolation zone was imposed.

The consignor’s Emergency Response Action Plan (ERAP) was activated. Responders determined that the product in the rear tank could not be evacuated via the piping. A number of other possible measures were considered and it was decided to attempt to clear the cargo using the controlled ‘vent and burn’ technique, which had previously been applied in static situations but never before to transport tanks in Canada.

An explosives expert with previous experience of vent-and-burn operations was called in. Consultation was carried out with various federal and provisional government agencies, response specialists and cargo tank specialists. It was only after this process that the vent and burn procedure went ahead.

The vent and burn technique for conducting a controlled release from an LPG container involves placing explosive charges at both the top (in the vapour space) and the bottom of the tank (in the liquid phase). In this case, five shaped conical charges, each containing 32 g of explosive, were placed on the tank and connected using a detonator cord; igniter units were also placed to ensure that the released product would burn. 

The procedure began with detonation of the two charges in the vapour space, which allowed the vapour to be vented to reduce the tank pressure; the escaping vapour was flared. After the pressure had been reduced, the bottom charges were detonated, releasing liquid propane from the bottom of the tank to a containment area, where it was ignited. The vessel was successfully evacuated, allowing the highway to be reopened.

Transport Canada notes the successful operation of the technique in this case, but also highlights some details that must be taken into consideration. In particular, it is necessary to know the vapour pressure inside the tank, which will vary according to the mixture of gases contained – LPG often being shipped as a mix of propane, butane and other fractions – and the ambient temperature. In Canada, for example, butane may not vaporise readily as its boiling point of -0.5°C can easily be higher than the ambient temperature. Propane boils at -42°C so has a much higher vapour pressure. First responders may not be aware of the exact mixture of gases involved and will need to discuss the matter with the ERAP holder and/or consignor.

Transport Canada reminds industry of the existence of the LPG Emergency Response Corporation (www.lpgerc.ca), which can provide on-scene assistance and advice at incidents involving LPG transport units, as well as response teams and training.