Analysis of Heating Tube Failure Accident Root Cause Investigation & Closed-Loop Rectification Standard

# Root Cause Investigation and Closed-Loop Rectification Standard Analysis of Heating Tube Failure Accident The repeated recurrence of identical accidents cannot be eliminated by a simple replacement of equipment when heating tubes experience leakage, medium contamination, rupture, or serious corrosion failure. A standardised root cause investigation process is necessary to identify concealed defects in the design, material, operation, maintenance, monitoring, and management dimensions, develop targeted rectification plans, and implement closed-loop verification to prevent recurrent failures. This specification utilises four popular heating tube materials as research subjects, classifies common failure root causes, implements investigation procedures, implements graded rectification measures, and implements post-accident anti-recurrence management. ## Part 1: The Classification of Common Root Causes of Heating Tube Failure ### Category 1: Defects in Front-End Hardware (Design, Material, Installation) 1. Improper material selection: Stainless steel is employed in high-chloride/fluoride processes, quartz is matched with alkaline production lines, and titanium is directly connected to carbon steel without isolation gaskets. 2. Pipeline design deficiencies: Inadequate auxiliary circulation at the bottoms of tube bundles, excessive elevation that results in liquid accumulation dead zones, abrupt small-radius elbows, and massive blind dead legs. 3. Incoming quality defects: Unqualified raw material passive film uniformity, PFA coating with inherent micro-flaws, quartz pre-existing microcracks, titanium surface processing scratches that were not removed prior to delivery. 4. Non-standard installation: The tube surface is scratched by metal hard tools, the gaskets are mixed and mismatched, the bolt torque is irregular, resulting in local extrusion damage, and there is no post-welding full CIP flushing, leaving welding slag residue intact. 5. Missing configuration support: There is no two-stage inlet filtration, no online pH/chloride/DO monitoring sensors, and no temperature/flow interlock protection program. ### Category 2: Misoperations in Daily Production and CIP Operations (Most Common Cause of Failure) 1. Operation of chemical parameters that exceed their limits - Stainless steel: A hot alkali temperature exceeding 55°F℃, long-term medium chloride >50ppm, sustained pH <5.5 or >8.5; - Titanium: Continuous DO: 7 mg/L, fluoride cross-contamination from shared acid pipelines; - PFA heater: Slow cooling is disabled after high-temperature disinfection, or when the disinfection temperature exceeds 90℃. - Quartz: Inadvertent cross-flow of alkali liquid into the quartz circulation cycle. 2. Non-compliance with fluid parameters - Dead zone biofilm accumulation due to flow velocity below the safe threshold; particle abrasion exacerbated by excessive flow velocity; and long-term static medium standby that exceeds the maximum safe static retention time of the material. 3. Simplified CIP procedures - Manually bypass intermediate/final rinsing segments and conclude rinsing at a predetermined time without a pH/conductivity qualification judgement; - Cancel monthly enhanced acid pickling to remove biofilm and sediment. 4. Energy-saving blind misoperations: Indefinitely extend the gasket/filter replacement cycle, indiscriminately reduce the frequency of pump operation, and shut down aeration pumps. 5. Frequent thermal shock: Direct cold medium drainage immediately following high-temperature heating, multiple start-stop cycles per day. ### Category 3: Inadequate Maintenance and Inspection Management Deficiencies 1. Inspection mechanism absent: No daily patrol focuses on flanges, welds, and elbows; monthly quantitative testing (wall thickness, potential, and infrared scanning) is not implemented; quarterly comprehensive overhaul is ignored. 2. Delayed maintenance: Filter screens remain obstructed for an extended period of time without cleaning; reusing aged, deformed gaskets; repeatedly postponing the offline passivation of stainless steel. 3. Neglecting abnormal alarm disposal: Neglecting over-temperature, low DO, and high conductivity alarms without troubleshooting and rectification. 4. Consumable management chaos: Random use without classification labels, mixed storage of various material gaskets and filter elements. ### Category 4: Root Cause of Sudden Severe Failure: Multi-Factor Synergistic Superposition A single abnormal condition only induces slow, mild ageing; the overlapping of two or more hazards initiates the process. failure that occurs rapidly: - Stainless steel: High chloride, over-temperature hot alkali, and dead zone biofilm; - Titanium: Trace fluoride, low dissolved oxygen, and particle scratches; - PFA coating: Inorganic abrasive particles, high-temperature alkali, and frequent thermal cycling. - Quartz: Rapid cold-hot shock + particle impact + residual alkali residue. ## Part 2: The Standard Five-Step Root Cause Investigation Process for Failure Accidents ### Step 1: On-site Condition Sampling & Evidence Collection (First Response After Failure) 1. Error in recording Basic information: The material of the heating tube, the duration of service, the location of the failure (weld/flange/elbow/tube straight section), and the type of failure (pitting leakage/coating blister peeling/quartz rupture/milky etching). 2. Acquire on-site operating historical data, including the most recent CIP temperature, flow, pH, and conductivity curve, as well as the medium chloride and DO test records and the static standby duration records prior to failure. 3. Maintain physical evidence: Maintenance operation records, expired gaskets, blocked filter elements, residual medium sediment inside the loop, and damaged heating tube samples. 4. Confirm the status of pipeline hardware: Examine the validity of the sensor interlock function, the type of elbow, the blind dead leg layout, the filter configuration, and the gasket material matching. ### Step 2: Layered Elimination to Determine the Direct Trigger Factor Lock the direct factor that caused the initial damage to the tube protective layer, and eliminate innocuous factors one by one: 1. Visual examination of samples that have been damaged: - Stainless steel: Weld pitting → judge chloride/alkali as direct factor; strip-shaped abrasion grooves → particle abrasion; - Titanium: Annular milky etching at flanges → fluoride + gasket dead zone; scattered scratch etching spots → particle abrasion + low DO; - PFA heater: Local coating blistering along scratches results in high-temperature alkali infiltration; large-area peeling leads to frequent thermal shock. - Quartz: Frosted rupture → alkali contact; fracture concentrated at elbow → long-term particle impact. 2. Verify the process data: Determine whether the temperature, pH, chloride, DO, and flow velocity surpassed the material safe range within one month prior to the failure. 3. Confirm operation records: Examine for instances of repeated misoperations, including the cessation of aeration, the omission of rinsing, and the cancellation of gradual cooling. ### Step 3: Utilise the Five Why Analysis Method to Determine the Deep-Seated Root Cause Consider stainless steel weld leakage as an illustration of deduction logic: 1. What is the reason for the weld's penetration? Concentrated pitting corrosion on the weld surface is severe. 2. What is the reason for the severe pitting? The over-temperature hot alkali incessantly destroyed the chromium passive film. 3. Why is the temperature of the hot alkali above 60℃? The operator manually increased the temperature in order to expedite the CIP cycle. 4. Why is it possible for the operator to indiscriminately adjust the temperature? The CIP control cabinet lacks a temperature interlock program. 5. Why is there no interlock? The anti-corrosion parameter protection configuration was disregarded during the equipment acceptability stage, and there was no routine inspection of the control program's functionality. → The final, underlying cause: Inadequate daily program inspection management and defective equipment design configuration. ### Step 4: Determine the Root Cause of Failure Risk Level #### Level 1 Major Risk (Probable to Lead to Batch Loss or Safety Accident Upon Recurrence) Some of the root causes are material mismatch, fluoride/alkali cross-pipeline contamination, quartz long-term alkali contact, unfiltered large particle circulation, and the absence of interlock protection for critical parameters. Requirement for rectification: Immediately cease production to undergo transformation, and complete the rectification within three working days. #### Level 2 General Risk (Seriously Shorten Service Life, Accelerate Corrosion) The root causes of the issue include the following: frequent start-stop without slow cooling, filter not cleaned on schedule, frequent start-stop, repeated CIP rinsing segment skipping, and long-term over-limit chloride/pH. Rectification requirement: Enhance patrol supervision on-site and optimise process parameters within seven days. ### Third Level Minor Risk (Slow Ageing, No Immediate Failure Hazard) Root causes include a minor lifeless leg without auxiliary circulation, a short-term slight static standby overrun, and a slightly overdue gasket replacement. Rectification requirement: Integrate into the subsequent routine maintenance plan to facilitate gradual optimisation. ### Step 5: Create a Formal Accident Investigation Report The core content of the report includes basic accident information, on-site evidence, direct trigger factor, deep root cause deduction, risk level classification, personnel responsibility division, targeted rectification plan, completion time node, and the responsible person for supervision. ## Part 3: Graded Targeted Rectification and Anti-Recurrence Measures ### 1. Root Causes of Hardware Design and Material Defect Rectification 1. Material mismatch: To prevent cross-contamination, partition independent, dedicated pipelines for fluoride, high-chloride, and alkaline processes and replace heating tubes with material that matches the process medium. 2. Defects in the structure of the pipeline: Install drain valves at low liquid accumulation sites, replace sharp elbows with large-radius curved elbows, remove redundant blind dead legs, and add auxiliary circulation at tube bundle bottoms. 3. Absence of supporting filtration/monitoring: Install two-stage series filters at the inlet of the heating loop; incorporate online pH, chloride, dissolved oxygen, and temperature interlock sensors; and lock parameter limit programs to prevent manual arbitrary modification. 4. Installation non-standardization: Retrain maintenance personnel on a standardised assembly process, uniformly configure PTFE plastic assembly tools, and adhere to the crosswise torque fastening specification for flanges. Additionally, two complete CIP cycles are required following all pipeline disassembly and welding. 2) Rectification of Operation Misoperation Root Causes 1. Program interlock restriction: Establish a hard interlock for the following parameters: CIP temperature, rinsing holding time, flow velocity, automatic alarm, and pump halt when they exceed the safe range. 2. Operation standard training: Conducting monthly anti-corrosion training, issuing a forbidden misoperation list, and linking patrol abnormal records with shift performance assessments. 3. Standby static control: Set a timing reminder to drain the medium when the static time exceeds the safe limit, and the low-speed circulation pump will be automatically activated for standby loops. 4. Compulsory fixed CIP process: The control program must lock the multi-stage rinsing sequence and is unable to advance to the next phase until the effluent pH/conductivity reaches the standard. ### 3. Rectification of Maintenance and Inspection Management Deficiencies 1. Comprehensive inspection system: establish a special inspection record ledger for heating tubes; implement a daily patrol, monthly quantitative testing, and quarterly comprehensive refurbishment mechanism. 2. Standardised consumable replacement: Enforce mandatory replacement cycle rules, mark replacement dates on site, and colour-code classified storage of gaskets and filter elements. 3. Abnormal alarm closed-loop disposal: It is prohibited to clear alarms without troubleshooting the fundamental causes. Complete an alarm troubleshooting record sheet, record the alarm time, implement handling measures, and recheck the results. 4. Scheduled maintenance: Develop monthly enhanced acid pickling, semi-annual stainless steel passivation, and other fixed maintenance plans, and designate a responsible individual to oversee their implementation. ### 4. Prevention of Multi-Factor Synergistic Recurrence: Following the correction of individual abnormal factors, the full-chain optimisation is implemented by re-examining all superimposed hazard links associated with the failure accident. For instance, failure resulting from titanium fluoride, low DO, and particle scratches: 1. Hardware: Install dual-stage fine filtration and isolate fluoride pipelines. 2. Procedure: The interlock for the full-cycle operation of the aeration system is secured to ensure that the dissolved oxygen (DO) is at least 8 mg/L. 3. Management: Implement monthly full-tube potential scanning to identify early scratch etching signals. ## Fourth Part: Long-Term Tracking Mechanism and Post-Rectification Closed-Loop Verification 1. Short-term verification (7 working days following the completion of rectification) - Confirm that no over-limit operation has occurred by extracting continuous CIP and medium parameter curves. - Perform an on-site re-inspection of the validity of pipeline hardware transformation, filter, and interlock functions; - Randomly review shift operation records to ensure that no forbidden misoperations are repeated. 2. Medium-term monitoring (three consecutive monthly quantitative inspections) - Stainless steel: Weld ultrasonic wall thickness measurement and potential scanning to validate that the corrosion tendency has been suppressed; - Titanium: No new milky etching areas were generated during the full-tube potential test. - PFA heater: Infrared thermal scanning, no new blistery cold patches; - Quartz: Frosting degree no longer exacerbated during light transmittance test. 3. Long-term anti-recurrence management - Archive the accident investigation report and rectification records for each heating tube throughout its entire lifecycle. - Update the operation forbidden list and pipeline transformation optimisation scheme, and summarise typical failure cases on a quarterly basis. - Use this accident as a training case for new operators to prevent the recurrence of similar failure accidents in other production lines of the plant. ## Executive Summary In many cases, heating tube failure accidents are the result of superficial direct triggers and concealed deep management/hardware fundamental causes. The standardised five-step root cause investigation method is capable of accurately distinguishing hardware defects, operation misoperations, maintenance omissions, and multi-factor synergistic hazards. The source corrosion inducements can be entirely eliminated through graded risk rectification and short/medium/long-term closed-loop verification, thereby establishing a permanent anti-recurrence mechanism. This prevents the repeated equipment damage and significant economic loss of fermentation batches that are the result of identical failures.