Incident 15-004 Leak in High Pressure Gas line
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Incident 15-004 Leak in High Pressure Gas line
by mark.brouwer » Mon Sep 04, 2017 8:31 am
The Incident:
A leak (pinhole in heat affected zone) was found in a 316L UG high pressure gas line from the reactor to the scrubber in a 1000 mtpd Stamicarbon urea plant, which was commissioned in 1969.
The Consequences:
Ammonia detectors detected the leak and the plant was shut down.
The Causes:
Strain Induced Intergranular Cracking
1. All reported failures occurred in AI-SI316L-UG HP gas lines of more than 30 years of age and are a result of stress corrosion cracking starting from the process side. Chlorides and sulfur play a role in this respect, but the appearance of the cracks does not point unambiguously in the direction of chloride-induced stress corrosion cracking. Contaminants like chlorides and or sulfides need to be avoided at the process side of urea plants, especially in older plants equipped with 316L UG piping.
2. In all reported cases the defect morphol-ogy points in the direction of so-called strain-induced intergranular cracking (SIIC), although no real evidence has been found for plastic de-formation. All other prerequisites for this failure mode are present, such as the type of crack propagation (intergranular), the presence of ox-ides in the crack, the relatively large grain size of the cracked pipe pieces, and the rather slow crack propagation. The presence of chloride and sulphur increase the risk for this type of crack-ing but is not a prerequisite.
3. Strain-induced intergranular cracking (SIIC) is commonly observed in the areas of AISI316L-UG liners in aging urea reactors that are exposed to condensing conditions in the gas phase. Basically the same process conditions prevail in the top of the urea reactor as in the gas outlet lines.
4. This failure mode should be taken into account when assessing the structural integrity of AISI316L-UG HP gas lines, especially in older (>20 years) urea plants.
5. This damage mechanism shows the need for a different inspection program for AI-SI316L-UG HP gas lines. Condensation corrosion is more readily detectable by using ultra-sonic wall thickness gauges. For detecting stress corrosion cracking, more sophisticated UT methods or other technologies (including visual internal inspection of gas lines using a video endoscope from flanged connections) should be applied.
6. Furthermore it is concluded that the maximum life time of AISI316L-UG HP gas lines is limited to approximately 30 years. When replacing these old HP pipes, it is advisable to install a more corrosion-resistant material such as a BC.05 (X2CrNiMo 25-22-2) or duplex.
The Lessons Learned:
1. During TA, spot thickness measurement is not sufficient since it will not necessarily check the area with the lowest thickness. The specific program must include visual inspection. If visual inspection is not possible, preventive replacement of the piping must be performed after 30 years of operation.
2. Condensation of the process gas should be avoided as much as possible inside the pipes. Installation of proper insulation and tracing is a pre-requisite. Care must be taken to use the proper LP steam pressure to heat the pipes above the dew point of the process gas.
3. Gas pipe in urea plant must have an appropriate review during design phase (removal of unused nozzle, choice of steam tracing pressure, adapted support without cold bridge)
A leak (pinhole in heat affected zone) was found in a 316L UG high pressure gas line from the reactor to the scrubber in a 1000 mtpd Stamicarbon urea plant, which was commissioned in 1969.
The Consequences:
Ammonia detectors detected the leak and the plant was shut down.
The Causes:
Strain Induced Intergranular Cracking
1. All reported failures occurred in AI-SI316L-UG HP gas lines of more than 30 years of age and are a result of stress corrosion cracking starting from the process side. Chlorides and sulfur play a role in this respect, but the appearance of the cracks does not point unambiguously in the direction of chloride-induced stress corrosion cracking. Contaminants like chlorides and or sulfides need to be avoided at the process side of urea plants, especially in older plants equipped with 316L UG piping.
2. In all reported cases the defect morphol-ogy points in the direction of so-called strain-induced intergranular cracking (SIIC), although no real evidence has been found for plastic de-formation. All other prerequisites for this failure mode are present, such as the type of crack propagation (intergranular), the presence of ox-ides in the crack, the relatively large grain size of the cracked pipe pieces, and the rather slow crack propagation. The presence of chloride and sulphur increase the risk for this type of crack-ing but is not a prerequisite.
3. Strain-induced intergranular cracking (SIIC) is commonly observed in the areas of AISI316L-UG liners in aging urea reactors that are exposed to condensing conditions in the gas phase. Basically the same process conditions prevail in the top of the urea reactor as in the gas outlet lines.
4. This failure mode should be taken into account when assessing the structural integrity of AISI316L-UG HP gas lines, especially in older (>20 years) urea plants.
5. This damage mechanism shows the need for a different inspection program for AI-SI316L-UG HP gas lines. Condensation corrosion is more readily detectable by using ultra-sonic wall thickness gauges. For detecting stress corrosion cracking, more sophisticated UT methods or other technologies (including visual internal inspection of gas lines using a video endoscope from flanged connections) should be applied.
6. Furthermore it is concluded that the maximum life time of AISI316L-UG HP gas lines is limited to approximately 30 years. When replacing these old HP pipes, it is advisable to install a more corrosion-resistant material such as a BC.05 (X2CrNiMo 25-22-2) or duplex.
The Lessons Learned:
1. During TA, spot thickness measurement is not sufficient since it will not necessarily check the area with the lowest thickness. The specific program must include visual inspection. If visual inspection is not possible, preventive replacement of the piping must be performed after 30 years of operation.
2. Condensation of the process gas should be avoided as much as possible inside the pipes. Installation of proper insulation and tracing is a pre-requisite. Care must be taken to use the proper LP steam pressure to heat the pipes above the dew point of the process gas.
3. Gas pipe in urea plant must have an appropriate review during design phase (removal of unused nozzle, choice of steam tracing pressure, adapted support without cold bridge)
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