A butane leak was detected at a liquefied flammable gas storage plant at 10:30 p.m. A residual flow rate of 0.7 m³/h remained following a pumping operation. Two employees saw liquid butane on the bleed valve at the high point of the vent line. A vent valve on the hydrometer loop of the LPG metering skid was unusually open. This valve was closed, which caused a plume of gas to blow out of the breather valve. Ten gas detectors in the butane area went off. The wind quickly dispersed the plume.

Investigations were carried out the next day. The events may have been caused by the opening of a bleed valve leading to the vent network of the LPG metering skid’s hydrometer loop whilst a sample was collected at around 2:30 p.m. The liquid LPG was transferred from the metering skid to the vent network. Most of the butane was released progressively through vaporisation of the liquid phase. Only a small portion was released in liquid form over a short period whilst the bleed valve was being closed. A total of 889 kg of butane was vented over the course of the day.

An analysis conducted the following week called for:

• either considering the possibility of removing the connection between the hydrometer loop and the vent line or installing a physical device to lock the valve in the closed position;
• conducting an exhaustive review of all the contributing lines connected to the vent line;
• conducting a review of the instructions for using the samplers;
• depicting the circuits and displaying operating instructions;
• conducting a design review of the butane area venting system.