For Submerged Heating of Hot (200°C) Compressed Water in a Pressure Vessel, What Is the Maximum Allowable Tube Wall Temperature Before Hydriding Occurs?

200° C compressed water submerged heating offers a particular difficulty for titanium immersion heaters. At this temperature water is only liquid under high pressure (around 16 bar). The high temperature, high pressure water environment causes the thermodynamic instability of titanium with respect to hydrogen absorption. Hydriding, for example, is diffusion-controlled and advances without surface corrosion or pitting; no evident surface deterioration occurs until embrittlement causes an abrupt fracture. The tube wall temperature rather than the bulk water temperature is the regulating parameter for hydriding. The heat flux from the internal heating element heats the inner wall of the titanium tube much hotter than the outside wall in contact with water. At some level of this inner wall temperature the rate of hydrogen absorption becomes excessively high and hydride production occurs in months rather than years.

Mechanism of high temperature hydriding in compressed water

The water dissociates at the surface of titanium over 150°C: H2O → H+ + OH-. The protons are converted into atomic hydrogen, which diffuses into the titanium lattice. Hydrogen solubility in titanium grows dramatically with temperature from 200 ppm at 100°C to more than 1,500 ppm at 200°C. When the local concentration of hydrogen surpasses the terminal solid solubility (TSS), titanium hydride (TiH2) precipitates. The volume of the hydride phase is 17% higher than the surrounding titanium causing internal tensions which cause microcracking. The governing parameter is the temperature of the inner wall, which can be 20-50°C higher than the outer wall, depending on the heat flux and wall thickness. Diffusion is driven by the hydrogen concentration gradient created by an inner wall at 220°C and the outer wall at 200°C, to the cooler outer surface where hydrides preferentially precipitate.

Quantitative Relationship between Hydriding Rate and Inner Wall Temperature

Controlled testing in compressed water at 200°C (16 bar) on Grade 2 titanium tubes under different heat fluxes has confirmed the following hydriding rates as hydrogen concentration rise per 1,000 hours:

Inner wall temperature <210°C: Hydriding rate 10-20 ppm/1000 hours. Time to attain TSS (200 ppm) 10,000-20,000 hours (1-2 years) Great for yearly tube replacement.

Inner wall temperature: 210-230°C Hydriding rate: 50-100 ppm/1000 hr. Time to reach 2,000-4,000 hours TSS (3-6 months). Needs frequent inspection and marginal service.

Inner wall temp. 230 to 250 deg C: Hydriding rate 200 to 400 ppm/1,000 hr. Time to reach TSS of 500-1,000 hours (3-6 weeks). Hydride platelets metallographically visible after 500 hours.

Inner wall temperature > 250°C: Hydriding rate > 500 ppm/1,000 hours. TSS surpassed in 200-400 hours. Observed surface scorching and cracking. Catastrophic failure is imminent.

Calculation Guide for Maximum Allowable Inner Wall Temperature

The table below gives a realistic approach for estimating the maximum permitted inner wall temperature and the accompanying heat flux restrictions for a desired service life.

Maximum Allowable Inner Wall Temperature at 200°C Water Desired Service LifeSimilar Maximum Heat Flux (OD 20 mm, wall 1.2 mm)Equivalent Maximum Heat Flux (25mm OD, 1.5mm Wall)
5 years (43,800 hours) 205 °C 12 W/cm2 10 W/cm2
2 years (17,500 hours) 215°C 18 W/cm2 15 W/cm2
1 year (8,760 hours) 225°C 25 W/cm2 22 W/cm2
6 months (4,380 hours) 235°C 32 W/cm² 28 W/cm²
Emergency service only (1 month) 250°C 45W/cm² 40W/cm²
Engineering Design to Prevent Hydriding

The inner wall temperature is determined using the bulk water temperature, heat flow (q), wall thickness (t) and titanium thermal conductivity (k = 17 W/m·K at 200°C): ΔT = (q × t) / k. For a 1.2 mm wall at 20 W/cm2, ΔT = (200,000 W/m2 × 0.0012 m) / 17 W/m·K = 14°C. Bulk water at 200°C--inner wall at 214°C--beyond 2-year life limit. Interestingly, decreasing the wall thickness decreases the inner wall temperature for the same heat flux. 0.8 mm wall 20 W/cm$^2$ gives $\Delta T$ = 9$^o$C, inner wall at 209$^o$C, acceptable for 5-year life. Thinner walls are thus better for hydriding resistance unlike the usual design philosophy for corrosion. The catalytic impact of palladium on hydrogen recombination raises the threshold inner wall temperature of Grade 7 titanium (palladium-stabilized) by approximately 15–20°C.

A specification informed

For example, if a titanium heater is specified for a 200°C compressed water service, compute the inner wall temperature from the desired heat flux and proposed wall thickness. For any application needing service life in excess of 2 years do not exceed 210°C. If heat flux needs are > 25 W/cm2, either lower wall thickness to minimize ΔT or provide Grade 7 titanium. Spread out the heat flow. Use many low watt density heaters instead of one high watt density heater. Monitor the electrical resistance of the heater in operation. A 10–15% rise over baseline is indicative of hydride production and embrittlement. Install a thermocouple on the outside wall of the tube and continuously calculate the inside wall temperature. If the calculated inner wall temperature surpasses 230° C for more than 100 cumulative hours, remove the heater from service irrespective of remaining wall thickness. Instead of managing the temperature of the bulk water, the engineer controls the temperature of the inner wall . This prevents the gradual failure of hydrogen embrittlement in high temperature water systems .