How to judge whether titanium heating tubes need to be replaced comprehensively based on multiple detection indicators

In numerous factories, the sole justification for scrapping titanium heating tubes is the reduction in wall thickness, with no consideration given to concealed hazards such as passive film failure, weld internal cracks, hydrogen embrittlement, and flange sealing surface damage. This article introduces a comprehensive evaluation system that includes mechanical thickness, surface anti-corrosion performance, weld internal defects, structural accessories, and operation failure frequency. In addition, it delineates the risk into three categories: normal monitoring, early warning overhaul, and mandatory replacement. It also offers explicit quantitative judgement thresholds and disposal plans. 1. Index 1: Core mechanical corrosion allowance index (ultrasonic wall thickness residual rate) The benchmark for calculating the residual wall thickness ratio is the original design wall thickness. The primary areas of focus are the weld heat-affected zone, support contact points, bubble stagnation upper tube arc, and long-term scaling coverage areas. Risk assessment threshold Safe monitoring zone: residual thickness of at least 70% of the original wall Disposal: Continue to adhere to the conventional maintenance program and quarterly thickness measurement cycle. Early warning overhaul zone: residual thickness is not greater than 50%.Seventy percent of the original wall Disposal: Arrange for the procurement of spare tubes for standby; modify the inspection cycle to monthly; and enhance passivation repair after each CIP cleaning. The residual thickness of the original wall must be less than 50% in the mandatory replacement zone. Disposal: The corrosion allowance for the tube body has been reached, and there is a possibility of local through-wall leakage at any moment. Continuous full-load production operations should be prohibited, and the tube body should be replaced during the next scheduled tank closure. 2. Index 2: Electrochemical passive film surface potential (anti-corrosion barrier core index) Detection standard: Saturated calomel electrode, qualified baseline potential ≥+150mV, and maximal potential difference on the same tube ≤30mV. Risk assessment threshold Safe monitoring zone: uniform potential distribution, surface potential ≥ +150 mV Water passivation maintenance is appropriate on a monthly basis. Local low-potential areas are observed in the early warning overhaul zone, which has a potential of +120mV and a voltage of +150mV. Disposal: To expedite the self-repair of the film, increase the aeration of the tank to raise the dissolved oxygen level above 6mg/L. This will serve as an integral hydrogen peroxide chemical re-passivation treatment. Zone of mandatory replacement: After two integral re-passivation treatments, the potential remains below +120mV, with a substantial potential difference of less than 60mV. Disposal: The titanium matrix surface develops a loose, unrecoverable corrosion layer; pitting corrosion will expand rapidly under acid-base circulation, requiring replacement. 3. Index 3: Weld non-destructive flaw detection defects (hidden brittle failure risk index) Conduct a penetrant test and eddy current inspection to detect microcracks, fissures, and undercut defects on the surface and within welds. Risk assessment threshold Safe monitoring zone: Linear defects, dispersed minuscule pores less than 0.2mm, or no continuous defect indication. Periodic quarterly defect detection. Elimination. Early warning overhaul zone: Linear defects measuring 0.2 to 0.3 mm and clustered small apertures Disposal: re-passivation following the repair, local polishing, and secondary argon welding repair, as well as a reduction in the flaw detection cycle to two months. Mandatory replacement zones are defined as linear defects that exceed 0.3mm in length, penetrating weld fractures, and repeated crack regrowth following repair. The risk of sudden tube rupture and the continuous expansion of cracks as a result of thermal cycling stress necessitate direct scrapping and replacement for disposal. 4. Index 4: Hidden risk index for hydrogen embrittlement (special risk following inorganic acid pickling) Evaluation criteria: Ultrasonic internal void detection, fracture appearance inspection, and long-term inorganic acid descaling maintenance records. Risk assessment threshold Safe monitoring zone: The ultrasonic test did not generate an internal void signal, and citric acid organic acid cleaning was the sole method employed. Hydrogen removal necessitates no special treatment. Disposal: After disassembling the tube, conduct a 120~150℃ heating process for 3 hours to eliminate internal hydrogen. Subsequently, retest the ultrasonic. Dilute inorganic acid pickling without baking hydrogen removal, scattered tiny internal void signals detected, early warning overhaul zone. Mandatory replacement zone: Intergranular brittle fracture signals, large-area internal hydrogen enrichment voids Disposal: The use of this item is prohibited due to the possibility of abrupt brittle rupture and irreversible hydrogen embrittlement damage, which may occur under heating stress. 5. Index 5: Indirect corrosion acceleration index: State of matching between the flange sealing surface and supporting accessory Basis for the decision: The PTFE gasket's recurrent leakage, the flange pitting's deformation, the bolt insulation sleeve's splitting with age, and the support coating's peeling. Risk assessment threshold Insulation accessories are intact, the flange sealing surface is seamless, and there is no frequent leakage in the safe monitoring zone. Disposal: The uniform replacement of all insulation casings and gaskets every half-year. Early warning maintenance zone: Local superficial pitting on the flange, gasket seepage every 1~Splitting of the partial insulation sleeve after two months Disposal: Polish the flange sealing surface to improve galvanic corrosion prevention and replace all damaged isolation accessories. Mandatory replacement zone: Frequent leakage, direct contact of exposed steel with titanium tube, and deep pitting deformation of flange sealing surface that occurs after multiple gasket replacements. Disposal: Continuous galvanic corrosion accelerates the thinning of the tube wall. The entire tube assembly must be replaced if the flange is integrated with the heating tube. 6. Index 6: Auxiliary judgement standard for failure frequency Determine the quantity of abnormal defects that transpire within a single maintenance year as an auxiliary reference: Safe: Lack of anomalous thermal failures, pitting, or leakage Early warning: Within a year, there are one to two local pitting corrosion defects that do not result in any leakage. Compulsory replacement is initiated by the occurrence of at least three corrosion defect points within a one-year period or the occurrence of one unplanned leakage shutdown calamity. 7. A judgement rule that is comprehensive and includes multiple indexes The secure zone encompasses all indexes. Normal operation should be preserved, and the standard maintenance cycle should be adhered to. The early warning zone is entered by one index, while the other indexes remain unaffected. It is advisable to conduct a targeted maintenance repair and maintain on-site monitoring. The titanium heating tube assembly should be unconditionally scrapped and replaced if any single index reaches the mandatory replacement threshold or if two or more indexes reach the early warning level. Input: Detection Index Standard for Safe Monitoring Standard Early Warning Overhaul Requirement for Replacement Wall thickness residual ratios must be at least 70% of the original thickness.50% to 70% of the original thickness 50% of the original thickness Local low potential, (+120mV to +150mV)Passive film potential: homogeneous distribution, ≥+150mVFollowing re-passivation, the voltage remains at a consistent level of +120mV. Welds that are defective: the absence of linear defects and poresLinear defects measuring 0.2 to 0.3 millimetres are evident.Linear cracks: 0.3mm, recurrent fracture growth Hydrogen embrittlement statusThere are no internal voids; only organic acid cleansing is permissible.Inorganic acid pickling without hydrogen removal, small internal cavitiesLarge-area hydrogen enrichment voids and brittle fractures Isolation accessories and flanges Smooth flange, intact PTFE isolation portionsFrequent leakage, deep flange deformation, and bare steel contact Partial insulation deterioration, shallow flange pitting Annual corrosion failure rates There are no anomalous corrosion points. 1 to 2 local pitting spots without leakage, 3 or more corrosion defects, or one leakage closure Condensed Summary The assessment of titanium heating tubes for replacement cannot be solely based on wall thickness measurements. It is imperative to incorporate the passive film potential, weld internal defects, hydrogen embrittlement risk, flange accessory condition, and annual failure frequency in order to conduct a comprehensive multi-dimensional evaluation. Classify the three grades into safe monitoring, early warning overhaul, and mandatory replacement based on quantitative thresholds to ensure the scientific timing of replacement, prevent premature scrapping that results in the waste of equipment funds, and eliminate the hidden dangers of sudden leakage and rupture caused by delayed replacement.