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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsOil Refinery Co...Oil Refinery Co...Cracking of alloy steel (F11) tube sheetCracking of alloy steel (F11) tube sheet
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7/31/2006 9:00 AM
 
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Recently we had a shut down of our hydrogen unit. The problems were concentrated in our steam production section. One of the problems incountered was the cracking of the 1 1/4Cr-0.5Mo ( A386-F11-Cl2)tube sheet of the shift waste heat exchanger. The exchanger is a hairpin type and the tubes are seal welded in the tube sheet. The cracks concentrated in the outlet pass along the seal weld and penetrated approx. 30mm inside the tubes. The operating conditions are as follows: Shell side we have the efluent of the high temperature shift converter with operating temp of 425%C2ºC @23.6bar and tube side recirculating BFW from the steam drum at 249%C2ºC @ 33bar. This unit has only been in service for 3 years and this is the first time we opened this equipment. Does anyone have an idea what can cause this type of failure in the tube sheet. And does anyone know if a carbon steel tube can function properly in this kind of condition.
 
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8/1/2006 9:00 AM
 
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Negritashamy A 182 F 11 forging at the outer pass - where exit temp could be close to design at shell side. Cracking of F 11 forging could be due to possibilities: Thermal stess related (check original design adequacy of F 11 forging at this temp) and / or concentration of caustic (steam) and repeated vaporizing and condensation leading to concentration of caustic at the tube sheet . Seal weld crack at the tube to tube sheet and the T 11 tube cracking is also due to possibility of steam concentration (condensation and vaporization and concentration of alkali bearing steam condensates). Carbon steel would not serve well and may not be a good substitute for this service temp of 425 deg cent at shell side. Trust this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India August 2, 2006 E-mail: nishi@vsnl.com >Recently we had a shut down of our hydrogen unit. The >problems were concentrated in our steam production section. >One of the problems incountered was the cracking of the 1 >1/4Cr-0.5Mo ( A386-F11-Cl2)tube sheet of the shift waste >heat exchanger. The exchanger is a hairpin type and the >tubes are seal welded in the tube sheet. The cracks >concentrated in the outlet pass along the seal weld and >penetrated approx. 30mm inside the tubes. >The operating conditions are as follows: >Shell side we have the efluent of the high temperature shift >converter with operating temp of 425%C2ºC @23.6bar and tube >side recirculating BFW from the steam drum at 249%C2ºC @ >33bar. > >This unit has only been in service for 3 years and this is >the first time we opened this equipment. Does anyone have an >idea what can cause this type of failure in the tube sheet. >And does anyone know if a carbon steel tube can function >properly in this kind of condition.
 
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9/2/2006 9:00 AM
 
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Please check for tube vibration. These excvhangers are notorius for that .Compare rho v2 value with tema. Another problem you may face later is the condensation and subsequent failure of tubes at the cold end i.e the BFW inlet end. The water condensation at this pressue is at approx 180 deg c and BFW inlet is at 145 deg c approx. giving rise to condensation at shell side
 
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9/3/2006 9:00 AM
 
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Negritashamy, the cracking could be caused by caustic SCC due to small traces of caustic in the BFW; 50ppm could cause the cracking. You may want to check if the tube to tubesheet welds were PWHT. Generally PWHT of carbon steel and low alloy steels is an acceptable way to mitigate this type of cracking. Scott
 
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9/7/2006 9:00 AM
 
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negrit... At 425 degrees C ( %7E800 degrees F) you are just below the recommended maximum temperature that carbon steel can be used. You have a marginal case, where temperature spikes can cause damage. The boiler inspection people, in thier "National Board" series of bulletins, publish the following maxima: OXIDATION LIMITS Carbon steel (SA210, SA106) 850oF Carbon-1/2 Mo (SA209) 900oF 1-1/4 Cr-1/2 Mo (T-11, P-11) 1025oF 2- 1 /4 Cr-1 Mo (T-22, P-22) 1075oF 18 Cr-8 Ni (304, 321, 347) 1300oF http://www.nationalboard.org/NationalBoard/Articles/Classics/classic24.aspx As you can see, the T11/P11/F11 materials have a much bigger margin of operating temperature above 800F.... In my opinion, your use of a CS material depends on how well you can control temperature -MJC
 
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