[ End User Feature ]
[ End User Feature ]
summarises the mechanical properties of the base material ( tube , tube sheet ) and weldment . Considering the length of the tube bundle ( 7 meters ) and thermal expansion coefficient ( α ) from the table 1 , the tube elongation ( δ ) and stresses ( i . e ., P / A ) can be found via the below equations : 2
δ = αLΔT ( 1 )
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Heat flux δ =
P L A
E
( 2 )
The stresses from compressive forces ( alone ), as calculated from Eq ( 2 ), was 226 MPa . In addition to thermal elongation and consequential stress , high temperatures may also result in a reduction in the overall strength of DSS . Expansion joints are known to compensate for the thermal expansioninduced stresses . However , the expansion joints were made from the SS 316L , which itself has a 22 % higher thermal expansion coefficient than alloy 2507 ( refer to Table 1 ), which may not compensate for the compressive stresses ( due to thermal elongation ). There were no engineering reviews to check the suitability of existing ( i . e ., SS 316L ) expansion joints with Alloy 2507 tubes , which provided no clarity on the stress value / stress field around the tube – tube sheet weld joints . Since DSS and austenitic SS don ’ t follow any endurance limit , ( unlike carbon steel ,
Strength weld
Tube sheet
Tube
Figure 2 . Schematic of heat dissipation in the tube-tube sheet welds
low alloy steel , and titanium ) they manifest fatigue failures even below yield point . This implies that even at the lower thermal stresses ( i . e ., well below the tensile strength ) the cracking may still happen depending on the temperature swing and the number of cycles . 1
Metallurgical outlook The welding between the tubes and tube sheet was performed using the approved and well-practiced welding procedure specification
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Wall thickness
Compressive force ( P ) due to elongation
Figure 1 . Schematic of tube elongation and thermal stresses at tube-tube sheet welds
SS 316L tube-sheet ER 2209 weldment Alloy 2507 tube
( WPS ) deploying 1 / 16 ” ( or 1.6 mm ) thick filler wire ER 2209 . The interpass temperature was kept at 320 F ( or 160 ° C ) to avoid any metallurgical defects ( sensitisation , sigma phase embrittlement , etc .) as well as thermal distortions . 1 NDEs and pressure tests were performed at maximum allowable working pressure ( MAWP ) for the bundle with upgraded tube metallurgy . Considering the inter-pass and operating temperatures , the risk of oxidations and cracking due to σ-phase and 885 F ( or 475 ° C ) embrittlement was remote . API practices recommend a maximum inter-pass temperature of < 150 ° C for the standard DSS and up to 120 ° C for super DSS . 2 The higher inter-pass temperature than the recommended limits ( for super DSS ) may trigger some change in the weldment behaviour . However , the filler wire in this case was made from standard DSS ( i . e ., ER 2209 ) and the tubes were from super DSS , so there were two governing ( as well as conflicting ) inter-pass temperature limits . This may lead to a heterogenous nature with the weldment ( though not fully investigated in this study ). Since the tube sheet had a significantly higher thickness ( 45 mm ) than the tubes and the weldment , this may have resulted in the rapid cooling of the weldment . A review of the WPS revealed that it specified an interpass temperature of 160 ° C for the tube thickness ranging from 10 mm to unlimited thickness . This may have implications in terms of higher cooling rate ( s ) of weldments , especially at the tube-tube sheet welds . 2 The rapid
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