Refinery Case Study

Non-destructive inspection found extensive cracking in a 90inch OD Contact Heater (DCH) in the digestion train of an alumina refinery. 
The cracking had initiated at most welds and at dents and gouges on the vessel wall that had been caused through the removal of scale during routine inspections.  In a number of cases the cracking was within a couple of millimetres of the outside wall and in one case a crack had penetrated the vessel wall causing weeping.  A sample was removed from the vessel wall for metallurgical assessment and the failure mechanism was identified as caustic cracking, a form of stress corrosion cracking.  Further inspection found similar cracking in the remaining vessels.

Our investigation found that for approximately 25 years the DCH vessels had operated satisfactorily, however, after recent modifications the stress and temperature had moved occasional operation of the vessels into a regime susceptible to stress corrosion cracking (SCC).  During a 2 year period, SCC had developed, and in the case of one vessel led to a through wall crack.  The recommendation was to remove all internal areas of stress concentration and reduce the operating temperatures to provide a sufficient buffer from caustic cracking thereby eliminating any further crack growth.  A fitness for service analysis using the British Standard BS7910, found surface breaking cracks of >100mm long and 9mm deep approaching a critical length and depth and should be repaired immediately.  Smaller cracks were considered unlikely to grow further and can be left to be inspected in a further 2 years.

The HRL investigation determined the root cause of the cracking, the extent of cracking, the feasibility of repair and a critical crack size analysis that enabled some defects to remain in the vessels in service.  This investigation provided an excellent example of a "run, repair, replace" investigation and saved $3-4m for the client in maintained production time and elimination of maintenance and repair costs.