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Materials
Case study on failures of duplex stainless steel heat exchangers
In this technical article, Immad Rohela and Ahmad Rana examine a series of failure events in a twin pass heat exchanger with tubes made from super duplex and a tube sheet made from austenitic stainless steel, in which formic acid proved to play a crucial role.
By Immad Ovais Rohela, Olayan Descon Industrial Company Jubail, Saudi Arabia & Ahmad Raza Khan Rana, Integrity Products & Supplies Inc. Sherwood Park, AB, Canada
Strength weld
Tube sheet
See detail- I
DETAIL- I
Compressive force( P) due to elongation
Wall thickness
Introduction & background Duplex stainless steel( DSS) is widely used in the chemical and hydrocarbon industry where the corrosive nature of service demands high corrosion resistance. Despite its high corrosion resistance, DSS is challenged by the service as well as operating conditions. Formic acid is a corrosive medium( with a scale of 0.76 of HCl) and is often used as a softening agent in the rubber Industry. The following article discusses a series of failure events that happened in a twin pass heat exchanger with tubes made from super DSS( Grade 2507) and a tube sheet made from austenitic stainless steel( Grade 316L). It also discusses the mechanical and metallurgical outlooks of various failure events.
Case study A twin pass heat exchanger made from SS316 containing 10 % formic acid on the inside of the tubes revealed some pressure drop on the tube side. Upon inspection, some formic acid deposits on the shell side were found. In addition, tubes revealed significant thinning, whereas 100 tubes( out of 531) were plugged within the first two years which made this exchanger unable to achieve its required heat duty. Consequently, the tube bundle was replaced with a new one. Despite the deployment of the new tube bundle, the tube thinning events continued. Within a period of two years, 61 tubes manifested severe thinning and even leaks. Since tube thinning was suspected to come from the corrosivity of formic acid, it was decided to retube the heat exchanger with super duplex SS Alloy 2507( UNS S32750), which has a significantly higher pitting resistance equivalent number( PREN) in comparison to SS 316L. 1 The new tubes were assembled in the tube bundle followed by expansion and strength welding. Despite the upgrade to a newer tube metallurgy, the existing tube sheet and expansion joint were re-used where the welds between dissimilar materials( SS 316L & Alloy 2507) were made using ER 2209 filler wire as per the available welding procedure specification( WPS). Table 1 shows the physical and mechanical properties of the materials for the tube, tube-sheet, and filler wire. After the upgrades, no leakes were
Tube
Figure 1. Schematic of tube elongation and thermal stresses at tube-tube sheet welds
observed during the first few months of operation. At the time of decommissioning for the first planned outage( since the tubes were upgraded), in addition to abnormal pressure reduction, formic acid traces were found on the shell side, which was a tell-tale sign of leaks in the tube bundle. The exchanger was dismantled, followed by thorough visual and non-destructive examination( NDEs). Eddy current testing( ECT) was performed to inspect for thinning damage to the tubes, especially at the tube-tube sheet expanded portions. Dye penetrant examination at the tube-tube sheet strength welds revealed cracks on the majority of tubes(> 20 %) that resulted in the leakage of formic acid from tube side to shell side portions. The following discussion is focused on the mechanical and metallurgical aspects. Figure 1 shows the arrangement of tube-tube sheet welds that were subjected to cracks.
Mechanical design outlook Referring to Figure 1, since the longitudinal expansion of the tubes was restrained by the strength of the weld( s) between tube-tube sheet, these welds were subjected to compressive forces. The super DSS tubes, standard DSS weldment and austenitic SS 316L tube sheet, have entirely different mechanical properties. Table 1 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 = αL∆T( 1)
The stresses from compressive forces( alone), as calculated from Eq( 2), was 226 MPa. In addition to thermal elongation and consequential stress, high
( 2)
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