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11363 Cracking in RTJ Grooves of Flanges in Hydro Processing Units

Picture for 11363 Cracking in RTJ Grooves of Flanges in Hydro Processing Units
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Product Number: 51300-11363-SG
ISBN: 11363 2011 CP
Author: M. Al-Arada, M. Al-Enezi, S. Vipat, V. Bhatia, A. Ray
Publication Date: 2011
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$20.00
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In high pressure Hydro-processing Units high pressure equipment and piping are provided with weld neck ring & tongue joint (WN-RTJ) type flanges for ensuring leak free joints. Most of these flanges are made of austenitic stainless steels types, 321SS or 347SS. However, equipment flanges may be of low alloy steel base metal with weld overlay (WO) of type 347SS. RTJ grooves of these solid stainless steel or overlaid flanges may develop cracks in operation and if not detected and repaired in time may eventually leak.

Cracking usually takes place due to various factors e.g. high stress concentration in the RTJ groove, stress corrosion cracking (SCC) due to polythionic acids or presence of chlorides and long term reduction in ductility of low alloy steel flange base metal due to exposure to high temperature hydrogen resulting in SS WO cracking.

This paper describes the systematic inspection of these RTJ flanges in various planned shutdowns in a residue desulphurizer unit in one refinery where, extensive cracking was observed in high pressure fixed bed reactor inlet nozzle flange RTJ groove which was successfully repaired in-situ.

Also many other stainless steel flanges / spacers in high pressure piping were found with extensive cracking in RTJ groove. Some of these were replaced in kind where as others were repaired successfully.

Refinery Inspection has established a systematic inspection program of periodically checking these flanges and repairing / replacing as required.

Keywords: Cracking, RTJ groove, corrosion, stress concentration, low alloy steel, weld overlay, thermal cycles, sensitization.
In high pressure Hydro-processing Units high pressure equipment and piping are provided with weld neck ring & tongue joint (WN-RTJ) type flanges for ensuring leak free joints. Most of these flanges are made of austenitic stainless steels types, 321SS or 347SS. However, equipment flanges may be of low alloy steel base metal with weld overlay (WO) of type 347SS. RTJ grooves of these solid stainless steel or overlaid flanges may develop cracks in operation and if not detected and repaired in time may eventually leak.

Cracking usually takes place due to various factors e.g. high stress concentration in the RTJ groove, stress corrosion cracking (SCC) due to polythionic acids or presence of chlorides and long term reduction in ductility of low alloy steel flange base metal due to exposure to high temperature hydrogen resulting in SS WO cracking.

This paper describes the systematic inspection of these RTJ flanges in various planned shutdowns in a residue desulphurizer unit in one refinery where, extensive cracking was observed in high pressure fixed bed reactor inlet nozzle flange RTJ groove which was successfully repaired in-situ.

Also many other stainless steel flanges / spacers in high pressure piping were found with extensive cracking in RTJ groove. Some of these were replaced in kind where as others were repaired successfully.

Refinery Inspection has established a systematic inspection program of periodically checking these flanges and repairing / replacing as required.

Keywords: Cracking, RTJ groove, corrosion, stress concentration, low alloy steel, weld overlay, thermal cycles, sensitization.
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