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Tank Farm Incident

Background

A new process was being put on an existing multi-purpose plant.  As part of the the project the duty of a GRP storage was changed.  Originally used to store spent 50% sulphuric acid, it was to be used to store spent 50% nitric acid.   Manufacturers information indicated that GRP was unsuitable for 50%, but OK at 40%.   The acid was diluted accordingly prior to transferring to storage.

The Incident

The spent nitric acid leaked from a corroded mounting blank flange.  This in turn corroded some mild steel pipework associated with neighbouring dichlormethane storage in the same bund.  The nitric acid floated on the DCM (ca.32,000 litres), which raised the liquor level above the storage bases.  The nitric acid attacked the asphalt on the storage bases and also the mild steel cages.

Causes

A spare mounting had been blanked with a mild steel plate faced with a disc cut from 3mm rubber sheet.  The rubber sheet bulged and cracked allowing the nitric acid to contact the mild steel plate.  The reason for the failure of the rubber is unclear.

Amongst other things the HazOp is only as good as the Engineering Line Drawings, the blank was not marked on the drawings.

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