How to judge whether PFA coating has hidden internal blisters without disassembling the heater

# Method for determining whether the PFA coating contains concealed internal fissures without disassembling the heater The carbon steel substrate is isolated from the corrosive medium by an integral fluoroplastic barrier in PFA coated heaters. In the initial stages, the outer surface is not visible due to the internal blisters that are the result of alkali infiltration between the coating and steel. Blisters expand under repeated cold-hot cycles if left undetected, ultimately rupturing to result in substrate corrosion and fermentation broth contamination. A combination of non-disassembly inspection methods, including visual external observation, operation parameter monitoring, temperature difference detection, and sampling coating thickness testing, can effectively identify concealed blisters. A comparison of each detection method, judgement criteria, and applicable scenarios is provided in the table below. | Inspection Method | Operation Steps Without Disassembly | Abnormal Sign of Hidden Internal Blisters | Detection Accuracy | Implementation Cost | | ---- | ---- | ---- | ---- | | Surface lustre & local bulge visual check | Full visual scan of tube outer wall, focal point on elbows and heating sections | Local slight bulging, uneven matte lustre, faint circular outline under light | Medium, only identifies medium-large blisters | Zero-cost, daily patrol matching | | Real-time surface temperature infrared scanning | Scan tube surface with infrared thermometer during stable heating operation | Obvious local low-temperature zones on tube wall | High, captures tiny early blisters | Low-cost, portable infrared equipment | | Long-term coating thickness trend recording | Measure fixed point coating thickness monthly | Continuous thickness reduction at the same position, uneven thickness difference over 0.1mm | Very high, quantitative data support | Medium cost, coating thickness gauge required | | CIP cleaning pressure & flow curve tracking | Archive cleaning pump pressure data of each cycle | Unstable instantaneous pressure drop at fixed heating section | Low cost, auxiliary cross-verification | Zero cost, extract central control historical data | ## 1. The logic behind the core formation of internal blisters that are invisible The micro gaps of the PFA coating are penetrated by high-temperature alkali detergent, which accumulates between the fluoroplastic layer and the carbon steel base. When heated, the trapped liquid vaporises, resulting in the formation of blisters and the generation of internal expansion pressure. Blisters are entirely concealed within the initial stage, as the outer fluoroplastic layer remains smooth and unbroken. A series of detectable abnormal physical differences that serve as a judgement basis are the result of the poor thermal conductivity of the air/liquid inside blisters. ## 2. Comprehensive explanation of each non-disassembly judgement method ### Visual surface patrol (daily basic screening) Visual inspection can only detect advanced blisters with significant expansion, as hidden blisters with a small volume do not deform the outer coating. Positions with internal blisters exhibit an uneven matte foggy lustre when observed under intense oblique light, which is distinct from the uniform bright fluoroplastic surface of the intact coating. It is possible to contact subtle round bulges on large blisters with the hand; however, these locations must be marked for priority recheck by infrared scanning. This method is employed as a daily shift patrol screening, but it is unable to exclude early, small lesions on its own. ### Rapid on-site temperature monitoring (most effective) The heat is efficiently conducted by the carbon steel substrate, while the liquid/gas within the craters acts as a heat-insulator. During stable heating operation, the tube wall area covered by internal blisters is unable to transmit heat outward in a normal manner, resulting in the formation of apparent cold spots on infrared images. Operators conduct a uniform scan of the entire heater tube. The presence of fixed, continuous low-temperature regions after numerous heating cycles confirms the existence of concealed internal blisters beneath the coating. This method is the most practical early warning method for production lines that do not require disassembly, as it is capable of detecting blisters that are not visually discernible. ### Monitoring the trend of coating thickness at a fixed point (quantitative, precise verification) Each month, maintenance personnel utilise a thickness gauge to identify numerous fixed measuring points on pipeline connections, straight heating sections, and elbows. The thickness fluctuation at fixed points for intact integral PFA coating is maintained at a maximum of ±0.03mm. In the event that concealed blisters are present beneath the measuring point, the outer fluoroplastic layer is continuously stretched and thinned under expansion pressure, resulting in a consistent downward thickness trend that is observed on a month-to-month basis. The thickness deviation from normal positions exceeds 0.1mm. This method, when combined with infrared cold spot data, can generate quantitative evidence for concealed blisters and substantiate maintenance replacement decisions in GMP equipment archives. ### Auxiliary cross-check of historical pressure and flow data in the CIP system The fluid circulation cross-section decreases at fixed locations when internal blisters bulge inward to reduce the interior diameter of the pipeline. At the same heating stage, the pump's instantaneous pressure will result in an irregular short-term drop and flow fluctuation during each CIP cleaning cycle. This signal is incapable of independently evaluating blisters; however, it can be employed as auxiliary evidence when comparing anomalous infrared temperature and thickness data, excluding blockage interference caused by medium solid particles. ## 3. Disposal standards that are hierarchical in nature following the identification of abnormal signals 1. A single slight abnormal signal (only a faint matte lustre): Reduce the coating thickness measurement cycle to biweekly and increase the frequency of infrared scanning without requiring a production stoppage. 2. Two overlapping abnormal signals (infrared cold spot + continuous thickness reduction): Arrange low-temperature slow CIP cycles, temporarily discontinue high-temperature disinfection, and prepare a spare heater for replacement during the next scheduled maintenance window. 3. Three or more abnormal signals are present: The critical rupture threshold has been reached by hidden blisters; the heating loop should be promptly isolated and the heater replaced to prevent rust contamination of fermentation batches. ## 4. Preventive measures to mitigate the production of concealed blisters Adhere to the strict implementation of mandatory graded slow cooling following high-temperature disinfection. Prevent sudden cold medium scouring to reduce thermal stress micro gaps of PFA coating. Maintain the temperature of hot alkali cleaning below 85℃. Install front-end high-precision filters to reduce particle scratch damage to the coating surface, thereby fundamentally stopping alkali infiltration channels. The aforementioned four non-disassembly inspection methods are combined to create a comprehensive closed-loop detection system. This system is capable of accurately detecting hidden internal blisters of PFA coating without the need to disassemble the heater, allowing for the pre-arrangement of maintenance and the prevention of unplanned shutdowns and risk to product quality due to sudden coating rupture.