Inside a fertiliser factory working with ammonium nitrates, the driver for a subcontracted firm loading his lorry-tanker with liquefied CO2, produced and stored on-site, noticed that the loading hose was being subjected to over 15 bar of pressure and becoming stiff. Suspecting the formation of an ice plug, he suspended loading while isolating the tanker hose and tank; he then decided to heat the hose around the lorry connection zone to 150°C with a water vapour nozzle procured from an adjoining production shop. 5 min later, the hose burst over its middle, causing injuries to the driver’s neck, chest and leg.
The investigation revealed that the lorry’s loading meter had been inoperable and moreover its purge valve was leaky (missing clamping nut). Despite his experience, the driver proceeded to drain the hose of its liquid before balancing the expansion spaces, thus leading to a period of liquid stagnation long enough for an ice plug to form in the hose. The driver only became aware of the plug after 3 meter readings at the loading dock, i.e. after 2 tonnes of CO2 had been transferred from the standing tank. To accelerate the plug’s removal, he decided to heat the hose with vapour; the use of vapour in dealing with an ice plug inside a hose was common practice adopted by subcontracted drivers, yet no instruction for handling this type of problem had ever been written into the plant’s CO2 transfer procedure. Stainless steel, which composed the body of the hose, was a heat conductor and very quickly raised the temperature of CO2, which rapidly dilated beyond 31°C to 15 bar (supercritical phase). A CO2 purge valve on the workshop side had relieved the pressure surge in the hose, but this release was blocked after being considered defective. Its control was not fixed (due to wear of the upper Teflon joint, without any breach of the seal), yet subcontracted drivers never alerted plant personnel responsible for these installations (communication breakdown). Inspectors of classified facilities requested the plant operator to ensure that all personnel working on-site complied with best practices specific to cryogenic fluid loading/unloading, which specifically prohibited the heating of hoses with water vapour when its ends were clogged by ice plugs. A metallurgical appraisal confirmed the good state of repair of the hose prior to the accident, with rupture caused by pressure over the middle at levels near 110 bar, whereas this hose had operated at a service pressure of 32 bar and a test pressure of 48 bar. The operator revised the facility’s hose inspection procedure and planned to affix a non-compliance tag on all hoses deemed defective by the internal inspection department. A dedicated procedure was written for the case of an ice plug and other potential transfer incidents. Drivers were instructed to contact the plant’s control room should an incident arise on the CO2 transfer station before taking any action. A periodic seal test was introduced on the CO2 transfer line.