April 2017: Nitrogen cylinders
This safety update is for
- Port State Control Officers
- Surveyors including classification societies in New Zealand and class surveyors
In February 2017 a nitrogen cylinder on board a cruise ship exploded while being topped up to maintain the correct pressure. A crew member died as a result of the explosion. The findings of the investigation suggest that significant corrosion affected the structural integrity of the cylinder.
Precautions and procedures
As a matter of good practice, surveyors and PSCOs should be aware of the potential risks associated with nitrogen cylinders used for a stored kinetic energy system. Before work commences near such systems, check that the cylinders on board are in good condition and there is no sign of damage or excessive corrosion. Check that the date stamp (or TAG) is less than ten years old. Note the location of the cylinders in relation to exposure to salt spray and sea air. Pay special attention to corrosion around and under tank support structures and securing straps or bands.
Significant corrosion can indicate that the thickness of the wall of the cylinder has reduced. If any corrosion of gas cylinders is visible the corroded cylinder(s) must be treated as a significant risk to safety.
Under the Health and Safety at Work Act (2015) a New Zealand business has a duty (so far as is reasonably practicable) to manage any risks arising from the work they are contracted to do on board a foreign-flagged vessel. Doing this effectively means consulting with other relevant parties to identify health and safety risks on the vessel and the controls that are in place. For foreign-flagged vessels, consult with the operator and/or the master. For New Zealand ships, relevant parties include the maritime operator and the master.
Safe practice tips
Information on the maintenance of nitrogen gas cylinders
To guard against risk, the pressure of the nitrogen gas needs to be maintained in accordance with the manufacturer’s specifications. When the pressure drops below the recommended level a ‘top-up’ of nitrogen may be required to lift the pressure. The drop in pressure may also indicate that the integrity of the pressure vessel has been compromised which should trigger a visual inspection. The visual inspection should be logged. A safe operating procedure for this task should be in place.
The structural integrity of the cylinder needs to be maintained. Significant corrosion can indicate that the thickness of the wall of the cylinder has reduced.
The gas cylinders should be removed and retested periodically. Evidence of this will be shown as a hard stamp at the top of the cylinder or verified by way of a TAG system.
A pressure vessel management system that ensures the fitness for service and safe operation of the pressure equipment on the vessel should be available on request. There should be visual inspections at pre-determined periods and expiry dates should be recorded as part of the vessel’s safety management system.
New Zealand regulations require a retest interval of 10 years for nitrogen cylinders, except for cylinders that have a shrunk fitted foot ring. These cylinders must be retested every two years.
Hydraulic accumulators should be inspected by a competent pressure vessel inspector to ensure any corrosion present is within acceptable limits of the design corrosion allowance.
Original source content - Safety Bulletin Issue 34, April 2017: Nitrogen cylinders.
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