cracking mechanism idenstification
cracking mechanism idenstification
(OP)
Dear Experts
Our client found a leaking valve in a hydroprocessing unit. Valve body material is forged stainless steel 321. Service in normal operation is hydrocarbon containing sulfur with Pop = 160bara and Top = 380°C. No water is anticipated during normal operation and also not anticipated during shutdowns. Other items in the circuit (other valves, piping, ...) were characterized (Dye Penetrant Test) and no other indication was found. The valve was manufactured by a valve manufacturer with good reputation and experience. Fabrication certificate indicates the cracks were absent after fabrication. Manufacturer had never experienced such a failure.
The cracks are being characterized, but the conclusion on the actual damage mechanism is not evident.

Looking at the crack mophology, do you have any idea on the possible origin of such transgranular cracking ?
Does it look like fatigue ? Or does it looks more like Cl-SCC or another SCC mechanism ?
Is corrosion-fatigue possible in hot sulfiding environment ?
Thanks in advance for your advise
Regards
Our client found a leaking valve in a hydroprocessing unit. Valve body material is forged stainless steel 321. Service in normal operation is hydrocarbon containing sulfur with Pop = 160bara and Top = 380°C. No water is anticipated during normal operation and also not anticipated during shutdowns. Other items in the circuit (other valves, piping, ...) were characterized (Dye Penetrant Test) and no other indication was found. The valve was manufactured by a valve manufacturer with good reputation and experience. Fabrication certificate indicates the cracks were absent after fabrication. Manufacturer had never experienced such a failure.
The cracks are being characterized, but the conclusion on the actual damage mechanism is not evident.

Looking at the crack mophology, do you have any idea on the possible origin of such transgranular cracking ?
Does it look like fatigue ? Or does it looks more like Cl-SCC or another SCC mechanism ?
Is corrosion-fatigue possible in hot sulfiding environment ?
Thanks in advance for your advise
Regards
RE: cracking mechanism idenstification
I suspect there was indeed moisture present. Shutdown is the most vulnerable time for equipment, so I would investigate the particular circumstances around your normal shutdown practices. The steel is not necessarily in a compromised condition (heat treatment wise), but do not rule it out.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: cracking mechanism idenstification
Check organic chloride for the relevant streams especially for makeup hydrogen which is connected with overhead of separator(downstream of aircooler) whether there was chloride carry over and boot section of the separator aa well.
Lee SiHyoung,
WorleyParsons Oman Engineering,
RE: cracking mechanism idenstification
First things first. Carry out chemical analysis (not PMI) of the valve body. Take the sample from the unaffected portion. Your answers might lie in the result.
Regards.
DHURJATI SEN
https://www.nace.org/people/dhurjatisen
RE: cracking mechanism idenstification
As a note: API RP 571 is a very valuable reference for identifying petrochemical failure mechanisms.
RE: cracking mechanism idenstification
Lee SiHyoung,
WorleyParsons Oman Engineering,
RE: cracking mechanism idenstification
There might exist fine cracks before serve. Dye Penetrant is not always a reliable way to detect small cracks.
Note the grain size is pretty big, some are ASTM 0. Is material heat treated properly? Any gamma phase that can acts as a corrosion site? anyway do not rule out the possible material issue.
RE: cracking mechanism idenstification
RE: cracking mechanism idenstification
analyses of the opened crack show some indications of striations probably produced by fatigue.
EDS and SEM analyses into the craks showed thin black deposit composed process fluid without corrosion products and NO CHLORIDES.
@MAGBEN : no other damage (pitting, ...) was found on the adjacent surface. you are right, some cracks present before service in the bulk material (not opened to process fluid or atm) were detected but are attributed to stress relaxation during cutting.
thank you all for your comments and suggestions
RE: cracking mechanism idenstification
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: cracking mechanism idenstification
RE: cracking mechanism idenstification
Were there more cracks than are shown in the picture? With SCC you often find multiple cracks.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: cracking mechanism idenstification
Please see the following picture with a broader vieaw of the cracks.
It pretty looks like Cl-SCC for me... branched transgranular cracking...
Is there any way / characterization / ... that could be implemented in order to validate or invalidate root cause og cracking by Cl-SCC ?
RE: cracking mechanism idenstification
One issue with any of the EAC mechanisms is that the crack morphology is related to the stress level and rate of crack growth.
While you high mag shot show little branching and likely not CL driven that does not mean that Cl didn't play a role at all.
I am very suspicious about the grain size, though the micro looks good (lots of small TiC scattered about).
These cracks could easily have 2 or 3 different drivers at different locations.
I would look into PA also.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
RE: cracking mechanism idenstification
RE: cracking mechanism idenstification
RE: cracking mechanism idenstification
@Ed : The origin is confirmed on process side. I'm a bit tired... what is PA ?
Regards
RE: cracking mechanism idenstification
RE: cracking mechanism idenstification
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."