When a Titanium Heating Tube Is Used to Reheat Condensing Steam Containing Ammonium Chloride (NH₄Cl) Deposits in a Refinery Overhead Line, How Does Deposit Thickness Under the Sheath Cause Localized Hot Spots and Hydrogen Blistering?

The Basic Trade-off in Titanium Heater Design for Refinery Overhead Service
The overhead pipes at the refinery have ammonium chloride (NH4 Cl) deposits in the steam condensate. The deposits result from mixing NH3 and HCl from crude oil processing in the condensing steam. Titanium heating tubes are installed to reheat the condensate to prevent corrosion in downstream carbon steel pipework. NH₄Cl deposits accumulate on titanium sheath surface. These deposits are hygroscopic and give a concentrated brine when wet. Their most critical effect is thermal: NH4Cl has very low thermal conductivity (about 0.5-0.8 W/m.K) compared to titanium (17 W/m.K). A single deposition of 1 mm thick layer causes a temperature spike of 20–40°C at the contact deposit-titanium. This local overheating creates two failure modes: hot spots which damage internal MgO insulation and hydrogen blistering from breakdown of NH4Cl. The wall thickness increases the temperature rise but the deposit thickness is the dominating variable. This investigation shows the mechanism and that deposit management is more significant than wall thickness.

Hydrogen Blistering Mechanism - Influence on Mechanical Integrity
The ammonium chloride decomposes under the NH₄Cl deposit at a titanium sheath temperature >120°C: NH₄Cl → NH₃ + HCl. The HCl is quite corrosive and damages the titanium passive layer, producing atomic hydrogen as a by-product: Ti + 3HCl → TiCl₃ + 3H. Atomic hydrogen is taken into the titanium lattice. It re-combines to molecular H2 at a grain boundary, or inclusion or emptiness. The hydrogen gas cannot diffuse any further and builds up pressure locally creating blisters-raised, dome-shaped bulges on the surface of the sheath. The blisters might be 2–5 mm in diameter and 0.1–0.5 mm in height. The blister bursts as the hydrogen is still being produced . This forms a pit that goes through the wall .

The deposit below which breakdown occurs is remarkably thin. Field measurements of NH4Cl deposits of 0.5-1.0 mm on refinery overhead lines show sheath surface temperatures of 140-180°C at typical heat flux (2.0 W/cm2) above decomposition temperature of 120°C. Thicker deposits (2–3 mm) can reach temperatures >250°C and produce fast hydrogen blistering.

Effect on the Thermal Performance: Computation of Temperature Rise
The interface temperature rise at the Ti-deposit is Delta T = q x t_deposit / k_deposit, with q being the heat flow, t_deposit the thickness of the deposit, and k_deposit the NH₄Cl thermal conductivity (~0.6 W/m·K). For a typical heat flux of 2.0 W/cm^2 (20,000 W/m^2) a 1.0 mm NH4Cl deposit yields ΔT= 20,000x0.001/0.6 = 33°C. When the bulk condensate temperature is 100 °C, the titanium surface below the deposit is heated to 133 °C. The additional drop due to the titanium wall thickness is minor (1-2°C) and is minimal with respect to the deposit effect.

Trade off synthesis: deposit thickness against required wall thickness
NH4Cl Deposit Thickness (mm) Surface Temperature Under Deposit (°C) (q=2.0 W/cm2, bulk=100°C)Risk of Hydrogen BlisteringBlister Tolerance Required Titanium Wall Thickness (mm) mm 0.2 107°C None (Below threshold of 120°C) 0.8 (Structural) 0.5 117°C Low (Marginal) 1.0 1.0 133°C Moderate (Blistering in months) 1.5 (Blisters will still form)
2.0 mm 167 °C High (blistering in weeks) >3 mm (ineffective)
3.0 mm 200°C Severe (rapid blistering) Impractical
Even a 1.5 mm wall doesn't stop blistering, it just means you get more material before the blister bursts and leaks. The bare solution is to avoid deposition, not wall thickness.

Engineering the Wall: Deposit Prevention and Cleaning
Deposit thickness is the main variable . The best methods are those which do not lead to NH4Cl buildup . A water wash system is installed that periodically sprays the heater with deionized water to dissolve NH₄Cl deposits before they become thicker than 0.2 mm. The deposit thickness is less than 0.2 mm and the surface temperature is less than 110°C . No blistering is observed irrespective of the wall thickness with daily washing. Or a PTFE non-stick coating (25–50 µm) inhibits NH 4 Cl from sticking on the titanium surface. The coating must be devoid of pinholes as the temperature gradient will rapidly build up deposit on exposed titanium. Another option is to operate the heater at a lower heat flux (1.0 W/cm 2 instead of 2.0 W/cm 2 ), which reduces Δ T through a 1.0 mm deposit from 33°C to 17°C, while maintaining the surface below 120°C even with considerable deposit build-up. It is a trade off between bigger heater surface area or longer heat up time.

Conclusion: Wall Thickness Doesn't Prevent Blistering, Deposit Management Does
When condensing steam containing ammonium chloride deposits are reheated by a titanium heating tube in a refinery overhead line, the deposit thickness under the sheath is the principal variable influencing localized hot spots and hydrogen blistering. NH4Cl has a very low thermal conductivity (≈0.6 W/m.K), hence a 1.0 mm deposit causes an increase of 33°C in the titanium surface temperature at typical heat flow, exceeding the 120°C breakdown threshold. Decomposition leads to HCl corrosion, which results in atomic hydrogen that causes blistering and ultimately leakage. The recommended specification is not a thicker titanium wall (of little advantage) but a deposit prevention strategy: either daily water cleaning to keep deposit thickness below 0.2 mm or a PTFE non-stick coating. The wall thickness is not a major variable in controlling blistering, although it should be 1.2–1.5 mm for structural stability. Provide predicted NH₄Cl concentration and feasibility of water washing to manufacturer. If you do not regulate the deposit even a 3.0-mm titanium wall can blister and disintegrate in a matter of months." The deposit control offers more than 10 years of service life in a basic 1.2 mm wall.