How improper CIP flow velocity accelerates corrosion and failure of heating tubes

# The acceleration of corrosion and failure of heating tubes as a result of an insufficient CIP flow velocity The scouring intensity, biofilm stripping effect, and residual corrosive liquid flushing efficiency are directly influenced by the CIP circulation flow velocity. Localised corrosion is induced by biofilm, chloride, and alkali that are trapped in pipeline dead zones due to insufficient flow velocity. Conversely, strong fluid erosion and particle abrasion are produced by an excessively high flow velocity, which mechanically demolish the protective layers of heating tubes. The service life of equipment is significantly reduced by both extreme flow rates. The table below displays the optimal flow parameters, judgement criteria, and hazards for four of the most common heating tube materials. | Heating Tube Material | Risk of Inadequate Flow Velocity | Risk of Excessive Flow Velocity | Recommended Safe Wall Flow Velocity | Matching Optimisation Scheme | | ---- | ---- | ---- | ---- | ---- | | 316 Stainless Steel | Biofilm accumulates at welds/elbows, oxygen concentration cell pitting | Particle erosion scrapes chromium passive film, weld abrasion grooves | 1.2–1.8 m/s | Install a variable frequency pump and conduct regular filter cleaning to reduce solid particulates | | Grade 2 Titanium | Local dissolved oxygen depletion under biofilm, fluoride enrichment etching | Deep permanent scratches obstruct TiO₂ film self-repair | 1.0–1.6 m/s | Avoid velocity above 2.0 m/s; extend aeration flushing for low-flow dead ends | | PFA Coated Heater | Alkali residues remain in coating scratches to form hidden blisters | High-speed particle impact cuts fluoroplastic layer, expands scratch infiltration channels | 1.0–1.5 m/s | Two-stage fine filter to remove abrasive solids, limit maximum pump frequency | | Quartz Glass | Organic acid biofilm residues aggravate alkali frosting damage | High-speed fluid shock creates microcracks on tube surface | 0.8–1.2 m/s | Smooth inner wall pipeline design, no sharp elbows to reduce impact force | ## 1. Corrosion mechanism of insufficient low CIP flow velocity Fluid scouring force is insufficient to remove viscous microbial biofilm from tube walls, particularly at elbows, weld crevices, tube bundle bottoms, and support gaps, when the minimum recommended value for wall flow velocity is not met. 1. Under biofilm, oxygen concentration difference cells are formed, anaerobic microorganisms produce organic acid, and chloride/alkali cleaning liquid is trapped locally, incessantly attacking protective structures. 2. In the case of 316 stainless steel, the passive film dissolves in an acidic microenvironment, causing chloride to accumulate and causing deep weld pitting. 3. In titanium tubes, biofilm consumes local dissolved oxygen, preventing the regeneration of the TiO₂ film. Trace fluoride is concentrated under the film, resulting in a uniform opaque etching. 4. For PFA heaters, the accumulation of liquid between the substrate and coating results in the generation of internal blisters after repeated heating cycles, as alkali detergent is unable to be fully flushed out of the small scratches. 5. In quartz tubes, the irreversible frosting of the silicon dioxide crystal surface is accelerated by residual alkaline cleaning liquid that is trapped by biofilm. Pump frequency reduction to conserve energy, filter obstruction, pipeline partial blockage, or an unreasonable small-diameter branch design are the most common causes of low flow velocity. The operator only observes the incomplete cleaning effect in the later stages, while concealed localised corrosion has already developed for months. ## 2. Mechanism of damage caused by an excessively high CIP flow velocity High-speed fluid and suspended solid particles in the medium induce substantial mechanical erosion and impact abrasion on the tube wall when the flow velocity surpasses the upper safety limit. - Metal heating tubes (316 stainless steel / Grade 2 titanium): Their thin surface passive coatings are soft and wear-resistant. The protective layer is continuously peeled off by long-term high-speed particle scouring, exposing the bare metal matrix to a corrosive medium. The most severe abrasion positions are welds with an uneven surface texture, which create strip-shaped wear fissures. - PFA-coated heaters: The fluoroplastic surface is struck by hard mycelium and inorganic particles at a high velocity, resulting in the formation of new deep scratches and the expansion of the original scratch breadth. This process also accelerates blister delamination and creates additional channels for alkali liquid infiltration. - Quartz glass: The inner wall and elbow corners are subjected to high-speed fluid impact, which results in the formation of invisible surface microcracks and the generation of alternating impact stress. Under the subsequent cold-hot alternating thermal stress, the fractures expand, resulting in the rupture of the tube. 3. Standardised control and modulation of the velocity of the CIP flow ### Step 1: Establish an interlock with a fixed flow range that is material-based Install variable frequency circulation pumps with a flow feedback interlock function, restrict the pump's operating frequency to the heating tube material, and prevent manual arbitrary frequency adjustments. ### Step 2: Dispose of inactive zones that are inaccessible to the standard flow Convert acute 90° elbows into large-radius curved elbows and incorporate auxiliary small circulation pipelines for the tube bundle bottom and low-flow branch sections to guarantee uniform scouring. ### Step 3: Decrease the abrasive particle load at the source Configure multi-stage front filters for circulation loops and sanitise the filter screens every shift to prevent the entry of high-speed particle abrasion carriers. ### Step 4: Routine pipeline inspection and flow rate calibration To prevent flow velocity deviation, calibrate flowmeters on a monthly basis and verify the status of pipeline scaling, filter blockage, and valve half-openness during quarterly overhauls. 4. Rules for the disposal of anomalous waste that have been graded 1. In the event that the long-term flow velocity falls below the lower limit, the following steps should be taken: extend the holding time for alkali washing, introduce periodic high-flow pulse flushing in each batch to remove accumulated biofilm, and optimise the pump frequency to achieve the standard range during the next closure maintenance. 2. If the flow velocity consistently exceeds the upper safety threshold, immediately reduce the pump frequency, inspect the filter damage and particle content in the medium, and install flow buffer baffles at the elbows to mitigate the fluid impact. 3. In order to prevent the alternating high-low flow compound damage to heating tubes, it is necessary to check for pipeline blockages and pump impeller wear, as well as troubleshoot flowmeter failure. The flow velocity is unstable with a significant fluctuation. ## Executive Summary The service life of heating tubes is influenced by the flow velocity of CIP. Insufficient flow results in biofilm deposition and localised chemical corrosion, while excessive flow causes mechanical attrition and structural fatigue damage. A straightforward, low-cost process optimisation strategy is to regulate the velocity of wall flow within the material-specific safe interval. This approach can simultaneously enhance the cleaning effect and significantly delay the onset of all types of heating tube corrosion failure.