In Urea Production Reactors with Ammonium Carbamate, What Wall Thickness Margin Is Required for a Titanium Heater Tube to Achieve 10-Year Service Life?

Urea synthesis reactors operate at harsh conditions with a high temperature ammonium carbamate solution at 180-210 °C, pressures of 140-250 bar and a corrosive combination of ammonia, carbon dioxide and intermediate carbamate species. Titanium Grade 2 has become the standard material for heater tubes in stripping-type urea plants because of its resistance to carbamate corrosion when an adequate wall thickness margin is selected. In many chemical settings homogenous corrosion controls the loss of material. Ammonium carbamate attacks titanium by a combination of general corrosion, localized pitting at weld heat-affected zones and flow-assisted corrosion at high velocity impingement locations. This paper determines the minimal wall thickness of the titanium Grade 2 heating tubes to sustain a 10 year design life based on corrosion rate data from operating urea plants, and accelerated autoclave testing.

Profile of Corrosion Rate in Ammonium Carbamate
The corrosion rate of titanium Grade 2 in ammonium carbamate solution is not constant but depends on temperature, oxygen content and flow regime. The passivated condition is obtained in the presence of 0.1-0.5% oxygen or air in the carbamate stream and the overall corrosion rate is 0.02 to 0.05 mm per year at 190°C. The passive film becomes less stable under deaerated conditions (O2 < 10 ppm) and the corrosion rate increases to 0.10–0.15 mm per year. Most commercial urea processes operate with controlled oxygen input (20-50 ppm) to maintain passivation, although, disturbances do occur.

The corrosion rates in the weld heat-affected zones are usually 2-3 times higher than the base metal, due to microstructural changes and residual stresses. Field data from 12 urea facilities in operation for 5-10 years indicate average base metal corrosion of 0.08 mm/yr (including passivation failures) and weld-zone penetration of 0.20 mm/yr. Flow accelerated corrosion at tube inlets with carbamate velocities above 5 m/sec causes localized wall loss of additional 0.05-0.10 mm/year.

Wall Thickness Calculation for 10 Year Design Life
Based on a required 10 year life and using the above corrosion rates, the required corrosion allowance is calculated as: Base metal allowance (0.08 mm/year x 10 years = 0.8 mm) + weld zone allowance (additional 0.12 mm/year average differential x 10 years = 1.2 mm localized). If no extra safety margin is included for, the total calculated loss at welds is 2.0 mm over 10 years.

To this is added a safety factor of 1.5 to take care of process disturbances, oxygen shortage episodes and localized pitting. The minimum required structural wall thickness margin is 3.0 mm above the minimum structural wall thickness. For a 25 mm OD tube at 200 bar internal pressure, the minimum structural wall is roughly 1.2 mm (based on thin wall pressure vessel calculations with a design stress of 70 MPa at 200 deg C), hence the total nominal wall thickness is 4.2 mm.

This calculation is shown in industry practice for urea stripper tubes and heater tubes. Most urea plants for heater service specify titanium Grade 2 tubes with wall thickness of 3.5-4.5 mm, compared to 1.5-2.0 mm for less demanding chemical applications. Some plants with good oxygen control and little weld corrosion have successfully operated with 3.0 mm wall and achieved 10-year life, although breakdowns have occurred at 2.5 mm at 5-6 years.

Urea Heater Tube Specification – Application Matrix
Urea Process Type Oxygen ControlExpected Base Corrosion Rate Recommended Minimum Wall ThicknessExpected 10 Year Survival Rate
Oxygen stripping for CO2 strippingExcellent (50 ppm controlled) 0.04-0.06 mm/yr 3.0 mm >95% CO2 stripping w/ variable oxygenFair (10-30 ppm, common upsets) 0.08-0.12 mm/yr 3.5 mm 90-95%
Full recycling processing (no O2)Poor (de-aerated) 0.15-0.20 mm/yr 4.5 mm 85-90%
Any high velocity zones (>5 m/s) Erosion: add 0.5 mmAdd 0.5 mm Add 0.5 mm to above Variable Conclusion for Urea Heater Spec
The minimum wall thickness for titanium heater tubes in ammonium carbamate service for a 10-year design life is 3.0 mm with best oxygen-controlled stripping processes and 4.5 mm for deaerated or total recycle processes. The primary failure risk is weld-zone corrosion and so post-weld heat treatment and precise weld profiling are needed. Engineers should specify minimum wall thicknesses of 3.5 mm for new heater specifications in standard CO₂ stripping urea plants and 4.0 mm for plants that experience frequent oxygen control upsets. The wall thicknesses normally used for ordinary chemical operations (2.0 to 2.5 mm) will not give a 10-year life in urea reactors.