Timely discovery of early corrosion, coating damage and structural defects of anti-corrosion heating tubes can avoid sudden medium leakage, batch fermentation scrap and unplanned production shutdown. 316L stainless steel, pure titanium, quartz glass and PFA coated heaters have different failure manifestations under long-term fermentation and CIP cycles, so a unified inspection standard cannot accurately judge hidden defects of all materials. This paper sorts out typical failure characteristics of each heating tube, formulates classified visual, simple physical and auxiliary detection indicators, and forms a set of operable on-site rapid inspection flow suitable for fermentation workshop daily patrol and regular overhaul.
For 316L stainless steel heating tubes, the core inspection targets are passive film loss, pitting corrosion and weld intergranular defects. Daily visual patrol needs to focus on weld areas and bracket contact positions to check for scattered brown rust spots, matte discoloration and accumulated thick composite scale. If local dark concave pits are observed on the tube wall, it indicates that pitting corrosion has penetrated the passive film and continues to expand inward. During quarterly overhaul, wall thickness measurement is carried out at vulnerable weld positions; when the wall thickness attenuation exceeds 10% of the original size, the heating tube is marked for replacement. In addition, abnormal power rise during tank heating is used as an auxiliary early warning signal, which reflects excessive scale accumulation and accelerated underlying corrosion. Any stainless steel tube with penetrating rust holes or large-area rust layer shedding is judged as complete failure and cannot be repaired for reuse.
Pure titanium heating tubes rarely produce rust stains, and its early failure signals are mainly surface matte whitening and local low-potential areas caused by passive film damage. Visual inspection focuses on scratch traces left by hard tools and bubble stagnation upper arc of inclined tube bundles. Uniform silver-gray smooth surface means intact TiO₂ film; partial milky white matte area implies slight corrosion erosion by fluoride or long-term oxygen deficiency. Simple electrochemical potential test can be used in overhaul: if the surface potential value drops obviously, offline enhanced passivation repair is required. The tube wall thickness attenuation of titanium tubes is extremely slow under standard working conditions; only when fluoride medium causes uniform thinning or deep mechanical scratches gather in large areas, the heating assembly is deemed to reach the replacement cycle. No leakage will occur in the early failure stage of titanium tubes, which is easy to be ignored by patrol staff.
Quartz anti-corrosion heating tubes have intuitive failure signs dominated by surface frosting, crack and fragment shedding. Daily patrol observes whether the glass surface loses transparency and forms a foggy frosted layer, which is the mark of hot alkali erosion damage. Operators need to check the tube body and both-end thread joints for tiny linear cracks and local chipping; invisible microcracks can be identified by light transmission inspection under strong light. Once cracks exist, the quartz tube is judged as invalid immediately, because thermal shock in subsequent heating process will lead to overall rupture and glass fragment contamination. Even if there are no cracks, heavily frosted quartz tubes need to be replaced regularly, as rough surfaces will adsorb a large amount of biofilm and cannot meet sterile CIP cleaning requirements.
PFA coated heaters rely on coating integrity as the core judgment basis, with typical failure phenomena including blistering, peeling, scratch penetration and color darkening. Visual inspection checks the whole coating surface for raised bubbles, strip scratches and local fluoroplastic shedding that expose dark steel substrate. Minor shallow scratches without penetrating the coating can continue to be used with enhanced protection; once the metal base is exposed, hidden internal rust will expand rapidly, and the heater must be replaced directly without local repair. Long-term high-temperature aging will make the coating turn yellow and lose luster, accompanied by increased surface roughness. When more than 5% of the total coating area suffers blister damage, it is defined as complete failure to prevent large-area peeling during production.
The unified on-site inspection process is divided into three steps: daily visual patrol without shutdown, monthly power and appearance record comparison, semi-annual offline comprehensive thickness and integrity detection. Maintenance personnel classify and record defects according to heating tube material, distinguish repairable minor damage and irreparable complete failure, and arrange maintenance or replacement plans in advance.
In summary, four anti-corrosion heating tubes have independent failure judgment criteria. Stainless steel is identified by rust spots and wall thickness loss, titanium by passive film matte whitening and potential attenuation, quartz by frosting and cracks, and PFA by coating blistering and substrate exposure. Implementing classified rapid inspection standards can capture early hidden corrosion defects, eliminate sudden leakage risks of heating equipment, and guarantee continuous stable sterile fermentation production.
