For Crude Oil Desalters Operating at 150°C with Chloride-Laden Water, What Wall Thickness of Titanium Grade 16 Tube Provides a 5-Year Replacement Interval?

Desalters are enormous vessels used at crude oil facilities to remove salt (mostly sodium, calcium and magnesium chlorides) from crude oil prior to refining. The crude is mixed with wash water at 120-150°C to form an emulsion, which is subsequently de-emulsified by an electric field. The desalter water phase comprises high operating temperatures, with hydrogen sulfide and organic acids, while being rich in chlorides (10,000-50,000 ppm). Titanium Grade 16 (Ti-0.05Pd, also known as Ti-Pd, a low-palladium variant of Grade 7) is specified for heater tubes in desalter water preheaters because of its resistance to chloride pitting and sulfide stress cracking. But high concentrations of chloride in combination with temperature and the presence of impurities from crude oil produce a noticeable corrosion rate. This paper provides an estimate of required wall thickness for Grade 16 tubes for a 5-year replacement interval for normal crude oil desalter use.

Corrosive Environment in Desalter Water Heaters
Desalter water preheaters warm the injected wash water from ambient to 120-150°C prior to mixing with crude. The water phase is not pure brine; it has emulsified crude oil (usually 50-500 ppm), organic acids (naphthenic acids), hydrogen sulfide (50-200 ppm) and ammonia (from crude nitrogen compounds). pH varies from 5.5 to 7.5 depending on the crude source and the presence of caustic injection.

In this situation, Grade 16 titanium forms a stable passive coating, but is slowly chemically dissolved by chloride complexation and attack by organic acids. The palladium content (0.04-0.08% Pd, half of Grade 7) provides a cathodic modification which reduces the general corrosion rate by a factor of 2-3 in comparison with Grade 2. However, the reduced palladium level makes Grade 16 less resistant than Grade 7 under severely reducing conditions. But desalter water is weakly oxidizing as a result of residual oxygen and the electric field effects.

Corrosion Rate Data for Grade 16 in Desalter Service
The most credible corrosion rates are obtained from field data on desalters running at 140-150°C with wash water chloride concentrations of 15,000-25,000 ppm in refineries. Average wall thickness drop of 0.4-0.7 mm was measured in Grade 16 tubes (25 mm OD) after 5 years of service, corresponding to a corrosion rate of 0.08-0.14 mm per year.

These rates are supported by laboratory testing in simulated desalter water (20,000 ppm Cl-, 100 ppm H2S, 100 ppm naphthenic acid, pH 6.5, 150°C). The corrosion rate of Grade 16 was found to be 0.10-0.15 mm/year. Grade 2 corroded at 0.25-0.40 mm per year under similar conditions, for comparison, and Grade 7 corroded at 0.05-0.08 mm per year. Grade 16 is a good compromise - better than Grade 2 but not as expensive as Grade 7.

The corrosion is general and uniform and not pitting or crevice type. This is significant because a corrosion allowance can be added to the minimum structural wall thickness to compensate for uniform corrosion.

Wall Thickness Life Calculation 5 Year
Using the thin-wall pressure vessel formula, the minimum structural wall thickness for a Grade 16 tube at 150°C and typical desalter operating pressures (10-20 bar internal pressure) is given by t_min = (P × D)/(2 × S × E - 1.2P), where P is pressure (2 MPa maximum), D is outer diameter (25 mm), S is allowable stress for Grade 16 at 150°C (approx. 85 MPa, derated from room temperature allowable of 138 MPa), and E is weld efficiency (1.0 for seamless tube). which provides t_min = (2 × 25) / (2 × 85 × 1.0 - 1.2 × 2) = 50 / (170 - 2.4) = 50 / 167.6 = 0.30 mm.

This structural minimum is small relative to corrosion allowance. For a life of 5 years with a corrosion rate of 0.12 mm/year (mid-range), the corrosion allowance needed is 5 × 0.12 = 0.60 mm. A safety factor of 50 % is added to accommodate for variance in crude composition and chloride concentration which gives a corrosion allowance of 0.90 mm. Total thickness of wall required = structural minimum + corrosion allowance = 0.3 + 0.9 = 1.2 mm.

However, additional margin is needed for localized attack at tube supports and welds which have corrosion rates 1.5-2 times higher than bare tube. Allowing 0.3 mm for local impacts produces 1.5 mm in total. From field experience, 1.65-mm (standard BWG 14 gauge) Grade 16 tubing has been seen to provide reliable 5-year service lives in most crude oil desalters, whereas 1.2 mm tubing perforates in 3 to 4 years.

Desalter Heater Tube Wall Thickness Application Matrix
Desalter Operation StateWater Chloride Concentration Corrosivity Crude TypeRecommended Grade Min. Wall Thickness (5 yr life)Anticipated Result
Light crude, low chloride 5,000-10,000 ppmGrade 2 1.2 mm 5+ years Low naphthenic acid
Normal crude, medium chloride, 10,000-20,000 ppmModerate acid Grade 16 1.65 mm 5-7 years
High chloride Sour crude 20,000-35,000 ppm High H2S High acid Grade 7 1.65 mm 5-6 years
Very high chloride, heavy crude 35,000-50,000 ppmHigh Acidic SolidsGrade 7 2.0-2.5 mm 5 years (follow-up)
Design Worst Case - Crude Supply VariableUpto 50,000 ppm Unknown Grade 7 2.0 mm 5 years conservative
Guide on Selecting Wall Thickness
A typical crude oil desalter working at 150°C with chloride laden water (15,000-25,000 ppm Cl-) requires a Grade 16 titanium tube (standard BWG 14 gauge) with a minimum wall thickness of 1.65 mm to achieve a 5-year replacement interval. The corrosion allowance of 0.90-1.00 mm above the 0.3 mm structural minimum, allows for a uniform corrosion expected of 0.10-0.15 mm per year plus a safety margin. With thinner walls (1.2 mm), you won't get consistently to 5 years; you will fail at 3-4 years. Thicker walls (2.0 mm or 2.5 mm) allow more margin for upsets but raise the material cost and slightly reduce the heat transfer (e.g., ~5-10% lower heat flux). For desalters operating with chloride concentrations consistently below 15,000 ppm, Grade 16 with 1.2 mm wall may be acceptable with annual inspection. Specify Grade 7, 1.65-2.0 mm wall for above 25,000 ppm concentration or when processing heavy, sour crudes. For Grade 16, buy ASTM B338 Grade 16 seamless tubing and ask for certification of palladium (0.04-0.08%) and a wall thickness tolerance of +0/-0.1 mm for minimum thickness after production. Grade 16 titanium heaters for crude oil desalters give a good compromise between the higher cost of Grade 7 and the shorter life of Grade 2. Proper grade and wall selection can provide cost-effective 5-year service.