Many factories only rely on wall thickness loss as the single judgment basis for scrapping titanium heating tubes, ignoring hidden risks such as passive film failure, weld internal cracks, hydrogen embrittlement and flange sealing surface damage. This article establishes a multi-index comprehensive evaluation system covering mechanical thickness, surface anti-corrosion performance, weld internal defects, structural accessories and operation failure frequency, divides three risk grades (normal monitoring, early warning overhaul, mandatory replacement), and gives clear quantitative judgment thresholds and disposal plans.
1. Index 1: Ultrasonic wall thickness residual rate (core mechanical corrosion allowance index)
Take the original design wall thickness as the benchmark to calculate residual wall thickness ratio, focusing on weld heat-affected zone, support contact points, bubble stagnation upper tube arc and long-term scaling coverage areas.
Risk grading threshold
Safe monitoring zone: Residual thickness ≥70% of original wall Disposal: Keep quarterly thickness measurement cycle, conventional maintenance program unchanged.
Early warning overhaul zone: 50% ≤ residual thickness ＜70% of original wall Disposal: Adjust inspection cycle to monthly; strengthen passivation repair after each CIP cleaning; plan spare tube procurement for standby.
Mandatory replacement zone: Residual thickness ＜50% of original wall Disposal: Tube body corrosion allowance exhausted, local through-wall leakage risk at any time; replace during the next planned tank shutdown, prohibit continuous full-load production operation.
2. Index 2: Electrochemical passive film surface potential (anti-corrosion barrier core index)
Detection standard reference: Saturated calomel electrode, qualified baseline potential ≥+150mV, maximum potential difference on the same tube ≤30mV.
Risk grading threshold
Safe monitoring zone: Surface potential ≥+150mV, uniform potential distribution Disposal: Normal monthly water passivation maintenance is sufficient.
Early warning overhaul zone: +120mV ≤ potential ＜+150mV, local low-potential areas appear Disposal: Integral hydrogen peroxide chemical re-passivation treatment; increase tank aeration to raise dissolved oxygen above 6mg/L to accelerate film self-repair.
Mandatory replacement zone: Potential continuously below +120mV after two integral re-passivation treatments, large potential difference ＞60mV Disposal: Titanium matrix surface forms loose, unrecoverable corrosion layer; pitting corrosion will expand rapidly under acid-base circulation, must be replaced.
3. Index 3: Weld non-destructive flaw detection defects (hidden brittle failure risk index)
Detect via penetrant test + eddy current inspection, focusing on microcracks, pores and undercut defects inside and on the surface of welds.
Risk grading threshold
Safe monitoring zone: No linear defects, scattered tiny pores less than 0.2mm, no continuous defect indication Disposal: Routine quarterly flaw detection.
Early warning overhaul zone: Linear defects 0.2~0.3mm, clustered small pores Disposal: Local polishing and secondary argon welding repair, re-passivation after repair, shorten flaw detection cycle to 2 months.
Mandatory replacement zone: Linear defects longer than 0.3mm, penetrating weld cracks, repeated crack regrowth after repair Disposal: Thermal cycling stress will continuously expand cracks, risk of sudden tube burst, direct scrapping and replacement.
4. Index 4: Hydrogen embrittlement hidden risk index (special risk after inorganic acid pickling)
Judgment basis: Maintenance records of long-term inorganic acid descaling, ultrasonic internal void detection, fracture appearance inspection.
Risk grading threshold
Safe monitoring zone: Only citric acid organic acid cleaning used, no internal void signal in ultrasonic test Disposal: No special hydrogen removal treatment required.
Early warning overhaul zone: Dilute inorganic acid pickling without baking hydrogen removal, scattered tiny internal void signals detected Disposal: Disassemble the tube and perform 120~150℃ baking for 3 hours to drive out internal hydrogen, then re-test ultrasonic.
Mandatory replacement zone: Large-area internal hydrogen enrichment voids, intergranular brittle crack signals Disposal: Irreversible hydrogen embrittlement damage, sudden brittle rupture may occur under heating stress, prohibit continued use.
5. Index 5: Flange sealing surface & supporting accessory matching state (indirect corrosion acceleration index)
Judgment basis: Flange pitting deformation, PTFE gasket repeated leakage, bolt insulation sleeve aging cracking, support coating peeling.
Risk grading threshold
Safe monitoring zone: Smooth flange sealing surface, intact insulation accessories, no frequent leakage Disposal: Half-yearly unified replacement of all gaskets and insulation sleeves.
Early warning overhaul zone: Local shallow pitting on flange, gasket seepage every 1~2 months, partial insulation sleeve cracking Disposal: Polish flange sealing surface, replace all damaged isolation accessories, strengthen galvanic corrosion prevention.
Mandatory replacement zone: Deep pitting deformation of flange sealing surface, frequent leakage after multiple gasket replacements, bare steel direct contact with titanium tube Disposal: Continuous galvanic corrosion accelerates tube wall thinning; if flange is integrated with the heating tube, the whole tube assembly must be replaced.
6. Index 6: Failure frequency auxiliary judgment standard
Count abnormal faults within one maintenance year as auxiliary reference:
Safe: No pitting, leakage or abnormal heating failures
Early warning: 1~2 local pitting corrosion defects occur in one year, no leakage
Mandatory replacement: ≥3 corrosion defect points appear within one year, or 1 unplanned leakage shutdown accident occurs
7. Multi-index comprehensive judgment rule
All indexes fall into the safe zone: Continue normal operation, follow standard maintenance cycle.
Single index enters early warning zone, other indexes safe: Arrange targeted overhaul repair, keep on-site monitoring.
Two or more indexes reach early warning level, or any single index reaches mandatory replacement threshold: Unconditionally scrap and replace the titanium heating tube assembly.
表格
| Detection Index | Safe Monitoring Standard | Early Warning Overhaul Standard | Mandatory Replacement Standard |
|---|---|---|---|
| Wall thickness residual ratio | ≥70% original thickness | 50%~70% original thickness | ＜50% original thickness |
| Passive film potential | ≥+150mV, uniform distribution | +120mV~+150mV, local low potential | Persistently ＜+120mV after re-passivation |
| Weld flaw defects | No linear defects, pores ＜0.2mm | Linear defects 0.2~0.3mm | Linear cracks ＞0.3mm, repeated crack regrowth |
| Hydrogen embrittlement state | Only organic acid cleaning, no internal voids | Inorganic acid pickling without hydrogen removal, tiny internal voids | Large-area hydrogen enrichment voids & brittle cracks |
| Flange & isolation accessories | Smooth flange, intact PTFE isolation parts | Shallow flange pitting, partial insulation aging | Deep flange deformation, frequent leakage, bare steel contact |
| Annual corrosion failure times | Zero abnormal corrosion points | 1~2 local pitting spots without leakage | ≥3 corrosion defects or one leakage shutdown |
Brief Summary
The replacement evaluation of titanium heating tubes cannot rely solely on wall thickness measurement. It is necessary to combine passive film potential, weld internal defects, hydrogen embrittlement risk, flange accessory state and annual failure frequency for multi-dimensional comprehensive evaluation. Classify into safe monitoring, early warning overhaul and mandatory replacement three grades according to quantitative thresholds, realize scientific timing of replacement, avoid premature scrapping causing waste of equipment funds, and also eliminate hidden dangers of sudden leakage and rupture caused by delayed replacement.
