Acetylene Storage in Enclosed Vehicle Compartments Thomas Kamm, P.Eng. Safety Engineering Specialist

History Legislation Scope of Research Testing Results Recommendations

History

Welding service trucks store cylinders filled with compressed flammable gases Most storage compartments are designed, constructed and installed by aftermarket venders In 2000, one worker was killed and another seriously injured when a storage compartment filled with an explosive mixture of acetylene ignited OHS Safety Bulletin issued describing the findings of the investigation Recommendations – secure cylinders, close valves when not in use, venting of storage compartment and reduce sources of ignition

Why be concerned?

Legislation

OHS Code – Section (5) An employer must ensure that compressed or liquefied gas cylinders are secured, preferably upright, and cannot fall or roll, unless a professional engineer certifies another method that protects against the hazards caused by dislodgment. 171(6) Despite subsection (5), an employer must ensure that a cylinder containing acetylene is secured and stored upright.

OHS Code – Section 172 Storage compartments 172(1) An employer must ensure that welding services provided from vehicles comply with CSA Standard W117.2 ‐ 01, Safety in welding, cutting and allied processes with the exception of Clause G.2 (Cabinets) of Annex G. 172(2) An employer must ensure that gases do not accumulate and reach their lower explosive limit by providing solid ‐ walled storage compartments in which compressed gas cylinders are stored with vents (a) that have a minimum of 0.18 square metres of free area for every 0.42 cubic metres of compartment volume, (b) that have the free area split evenly between the top surface and the bottom surface of the storage compartment, and (c) that are unobstructed under all conditions.

OHS Code - Section (4) An employer must ensure that solid ‐ walled compartments in which compressed gas cylinders are stored use (a) latching and locking hardware made of non ‐ sparking materials, and (b) electrical components appropriate for use in an explosive atmosphere, if electrical components are located within the compartment.

OHS Bulletin

Scope

Scope of Research Determine if the enclosed storage compartment specifications in Section 172 of the 2009 OHS Code were sufficient to prevent a reasonably expected leak of acetylene gas from reaching its lower explosive level (LEL) Determine at what leak rates and times the LEL inside the compartment would be reached Understand the effect current compartment specifications have in regards to mitigating an explosive atmosphere from occurring.

Testing

Computational Fluid Dynamic (CFD) analysis performed (ANSYS Fluent CFD code v14.5) Compartment size of 61 x 61 x 122 cm (24 x 24 x48 in) Vent size of 30 x 30 cm (12 x 12 in) in top and bottom, none on the sides Three different size of cylinders were modelled, 38, 79 and 112 cm (15”, 31”, 44”) high with 10, 18, and 25 cm (4”, 7”, 10”) diameters respectively

Testing Top and bottom vent centered Cylinder location: center, offset to side, offset to corner Slow leak from top of cylinder in upward direction Various leak configurations and rates were analyzed to determine gas concentrations Atmospheric conditions: 27°C/still air, 27°C/20 km/h cross-wind

Testing Compressed Gas Association – 1/10 of the capacity of the cylinder per hour for intermittent use Smallest cylinder: High leak rate of 1 x kg/s, Low leak rate of 1 x kg/s No account for heat transfer or temperature difference between the acetylene gas and air. Acetylene has a Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) of 2.5% and 80+% by volume of air, respectively

Results

Results – Low Leak

Results – High Leak

High Volume Leak - Downward Direction

Top Vent Blocked

High leak with 20 km/h cross-wind

Side Vents

Results Current design specifications produced minimal pooling of acetylene under normal leak parameters Certain leak rates and leak directions could result in pooling of acetylene in the storage compartment Blockage of the bottom vent did not result in a significant increases in gas volume at or above the LEL Complete blockage of the top vent or high velocity discharges of gas resulted in the LEL being reached

Results Cross-winds cause dispersion of gas inside the compartment Side vents provided additional capacity for the purging of acetylene gas and acted as alternative ventilation in the case of bottom or top vent blockage Addition of side vents to the top half of the storage compartment help mitigate many of the high risk conditions

Recommendations

Install large top and side vents to storage compartments on welding service trucks Add at least one, preferably two, side vents on the upper half of the storage compartment right up to the top Ensure that top and side vents remain free from obstructions Maintain a bottom vent for gases heavier than air Extra care should be taken in the presence of strong cross-winds

Questions