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

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 upgrade , no leaks were 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

SS 316L 16.5 x 10 – 6 186 17 485 170

Alloy 2507 13.5 x 10 – 6 186 16 795 550

Alloy 2205 13.5 x 10 – 6 186 17 655 450

Table 1 . Physical and mechanical properties of SS316 L , Alloy 2205 & Alloy 2507 2

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