In a Desalter Heater for Crude Oil Pre-Treatment (Contains 5% Brine Wash Water at 130°C), How Does the Titanium Tube’s Wall Thickness Interact with the Presence of Hydrogen Sulfide to Cause Sulfide Stress Cracking at Welds?

Titanium Heater Design for Crude Desalter Service - The Basic Compromise
Crude oil desalters inject wash water (5% brine) into crude oil at 130 C to remove salt and particulates. The brine comprises chlorides and hydrogen sulfide (H 2 S) from sour crude oil. The crude-brine combination is preheated in titanium tubes. The criteria for sulfide stress cracking (SSC) are the presence of H 2 S, chlorides, increased temperature and residual tensile strains at the welded joints. SSC is a sort of hydrogen embrittlement where atomic hydrogen from H2S corrosion diffuses into the titanium to form brittle hydrides. The wall thickness impacts the residual stress level in welds and the hydrogen concentration gradient. The thicker the wall, the more residual stresses from welding (more restriction) and the longer the diffusion routes - a complex interaction.<br/><br/> This investigation describes the effect of wall thickness on SSC susceptibility at welded joints in desalter service.

SSC Mechanism at Welds: Impact on Mechanical Integrity
Titanium is progressively corroded in the presence of H₂S at 130°C: Ti + 2H_2S -> TiS_2 + 2H_2 The atomic hydrogen so produced can penetrate the titanium lattice. When the concentration of hydrogen approaches 150–200 ppm in an area of tensile stress, titanium hydride (TiH2) precipitates at the grain boundaries. The hydride is brittle and will fracture with residual tension. Welded joints are especially vulnerable to this as welding produces residual tensile strains of 150-250 MPa in the heat affected zone (HAZ). In addition, the HAZ has a distinct microstructure, acicular alpha-prime martensite, which is more readily absorbing hydrogen than the base metal.

Thickness of wall influences weld residual stress. For a particular tube diameter, a thicker wall results in a higher cooling rate after the welding (due to the higher thermal mass) which raises residual stress. The finite element study shows that the maximum residual stress at the weld HAZ of a 20 mm OD tube is:

Wall 1.0 mm: 180 MPa

220 MPa for 1.5 mm wall

2.0 mm wall: 260 MPa (close to yield)

Higher residual stress decreases the critical hydrogen concentration for cracking. A 1.0 mm wall may withstand 200 ppm hydrogen before cracking, while a 2.0 mm wall may only crack at 120 ppm due to the increased stress.

Thermal Performance Effect: Diffusion and Temperature Gradient
Hydrogen diffusion is affected by the heat differential across the wall. At bulk temperature 130°C, the inner surface temperature of a 1.0 mm wall is about 133°C for 2.0 W/cm 2 while that of a 2.0 mm wall is 136°C. The 3°C difference raises the diffusion coefficient of hydrogen by ~15%, thereby marginally quickening hydrogen entry for the thicker wall. This effect is secondary to the differential of residual stress.

Synthesis of Trade-off: Wall Thickness vs. SSC Resistance
Wall Thickness (mm) Weld Residual Stress (MPa) Critical H Concentration for Cracking (ppm) Time to Critical H at HAZ (months at 10 ppm/month absorption)Risk SSC
1.0 mm 180 MPa 200 ppm 20 months Moderate
1.2 mm 195 MPa 180 ppm 18 months Medium
1.5 mm 220 MPa 150 ppm 15 months.High 1.8 mm 245 MPa 130 ppm 13 months High 2.0 mm 260 MPa 120 ppm 12 months High (does not last 1 year)
Wider walls are usually more vulnerable to SSC because they are more vulnerable to weld residual stresses that lower the required hydrogen concentration for breaking. A 1.0mm wall has stronger resistance to SSC than a 2.0mm wall.

Weld Stress Relief and Alloy Upgrading: Engineering Beyond the Fence
Since the problem is the greater residual stress, stress relief annealing (540°C for 1 hour) is used to lower the weld residual stress by 70–80% regardless of wall thickness. A 2.0 mm wall that is stress relieved has residual stress of 50-70 MPa and is very SSC resistant. However, due of size limitations many desalter heaters cannot be annealed. For as-welded heaters, thinner walls (1.0–1.2 mm) are recommended to decrease the residual stress. The upgrade of titanium (Ti-0.3% Mo-0.8% Ni) from Grade 2 to Grade 12 enhances the SSC resistance because molybdenum stabilizes the passive film and decreases hydrogen absorption. A Grade 12 welded tube with 1.5 mm wall and no annealing has an SCC resistance similar to a Grade 2 tube with 1.0 mm wall.

Conclusion: Thinner wall (1.0 to 1.2 mm) is preferred for SSC resistance in desalter service.
The relationship between wall thickness of titanium tubes in a crude oil pretreatment desalter heater using a 5% brine wash water containing H2S at 130°C and sulfide stress cracking at welds is counterintuitive: thicker walls increase susceptibility to SSC, because welding produces higher residual stresses that lower the critical hydrogen concentration for cracking. SSC risk was lower for a 1.0 mm wall with residual stress of 180 MPa than for a 2.0 mm wall with residual stress of 260 MPa that failed within 12 months. Recommended standard is Grade 2 titanium tube, wall thickness 1.0–1.2 mm, as-welded (no annealing) with post-weld pickling to remove surface contaminants. Avoid the use of thicker walls unless post weld stress relief annealing is undertaken. For sour crude desalter service heaters, demand weld residual stress measurement (<200 MPa) or specify Grade 12 titanium for higher inherent SSC resistance. Wall thickness isn't a safety margin SSC-thinner is better.