In circulating heat exchange systems, unreasonable pipeline layout, irregular branch design and improper installation of heating tube bundles often lead to partial low-flow or even fluid stagnant regions. When the medium flow velocity drops below the critical scouring speed, suspended particles, microbial secretions and scale precursors gradually settle on the tube surface, forming narrow crevices between sediment and the stainless steel substrate, as well as tiny gaps at welding joints, clamping fixtures and tube bundle overlapping positions. These confined crevice environments are easily depleted of dissolved oxygen, and corrosive ions such as chloride continuously accumulate inside the occluded space, triggering crevice corrosion which develops hiddenly and rapidly. Different from macroscopic pitting corrosion, crevice corrosion occurs in narrow gaps that cannot be effectively cleaned by conventional circulating flushing and routine chemical cleaning. Once initial crevice corrosion pits form, they will continuously expand along the gap direction under the drive of electrochemical potential difference, eventually causing penetrating leakage of heating tubes. Many enterprises only focus on pipeline pressure bearing design and heat exchange efficiency layout, ignoring flow field uniformity optimization. Excessive local flow resistance, sudden pipe diameter expansion and unreasonable baffle arrangement create numerous stagnant dead zones, which become high-incidence areas of crevice corrosion failure. Therefore, optimizing the overall flow field layout of the heating system to eliminate low-speed stagnant areas is a fundamental fluid design measure to cut off the formation conditions of crevice corrosion.
Local low-velocity stagnant zones induce crevice corrosion through a closed-loop electrochemical deterioration mechanism. When fluid cannot continuously scour the metal surface inside narrow gaps, dissolved oxygen inside the crevice is rapidly consumed by cathodic reduction reactions, forming an anoxic environment, while the flowing mainstream medium maintains sufficient dissolved oxygen and serves as the cathode. A stable potential difference is generated between the inside and outside of the crevice, making the metal surface within the gap the anode for continuous dissolution. As metal cations accumulate in the confined space, chloride ions migrate inward to maintain charge balance, resulting in hundreds of times enrichment of chloride inside the crevice. Meanwhile, the hydrolysis of metal ions reduces the local pH value, further destroying the chromium-rich passive film and accelerating the expansion of corrosion pits along the gap. Low flow velocity also fails to strip nascent scale and biological slime, making crevice structures continuously covered by fouling, which isolates the passive film from passivating inhibitors in the mainstream medium and loses the opportunity for dynamic repair. Even regular dosing of water treatment agents cannot enter narrow stagnant gaps to exert protective effects, leading to irreversible crevice corrosion damage that is difficult to discover in the early stage.
This study puts forward four standardized flow field optimization strategies to eliminate stagnant zones and prevent crevice corrosion of 316 stainless steel heating tubes. First, set the safe medium flow velocity range and optimize main pipeline diameter and directional layout. Control the circulating flow velocity of the heat exchange medium between 1.2 m/s and 2.5 m/s, which can not only produce enough wall scouring force to prevent particle sedimentation but also avoid excessive flow velocity leading to erosion-corrosion. Reduce sudden expansion, sudden contraction and right-angle elbow structures in the main pipeline; adopt long-radius curved elbows and gradual diameter transition joints to lower local flow resistance and avoid flow separation and backflow dead zones. For multi-branch parallel heating loops, adopt symmetrical manifold layout to balance the flow distribution of each branch pipeline, prevent partial branches from operating under ultra-low flow due to uneven pressure distribution. Install flow balance regulating valves on each branch to realize uniform flow distribution of all heating tube bundles, eliminating long-term low-speed operation of individual heat exchange branches.
Second, optimize the internal layout of heating tank and tube bundle installation mode to eliminate flow dead corners. Avoid horizontal suspended installation of heating tube bundles close to the tank bottom; adopt high-set suspension layout to ensure the bottom of all heating tubes can be fully flushed by circulating medium. Set uniform flow guide baffles and inlet flow distribution plates at the medium inlet of the heating tank to disperse incoming fluid and avoid local jet flow and surrounding backflow stagnant areas. Remove unnecessary fixed baffles, redundant support frames and narrow gap clamping structures inside the tank; replace hard contact fixed supports with arc-shaped large contact surface brackets to minimize tiny crevices between the heating tube and fixed accessories. Arrange the tube bundles in a regular triangular or square array with uniform spacing to ensure the medium can flow through every gap of the tube bundle without forming enclosed stagnant areas.
Third, rectify low-risk hidden stagnant positions and install auxiliary circulating anti-stagnation facilities. For pipeline blind pipes, redundant standby branch pipes and equipment maintenance short-circuit pipelines that are prone to fluid stagnation, shorten the length of blind pipes as much as possible or install small auxiliary circulating bypass pipelines to realize continuous micro-circulation of medium. Install local small circulating pumps at the bottom of large heating tanks and low-flow dead-end pipelines to drive fluid disturbance and avoid long-term static deposition of suspended impurities. Set regular periodic forward and reverse flow switching mechanisms for long-distance circulating pipelines to change the original flow field distribution, scour the sediment deposited on the pipe wall and eliminate one-way long-term low-speed fouling adhesion. For liquid level fluctuation areas and pipeline highest points, optimize exhaust valve layout to timely discharge trapped non-condensable gas, prevent gas bag formation from occupying pipeline space and causing local flow interruption and medium stagnation.
Fourth, adopt computational fluid dynamics simulation for pre-layout verification and regular flow field inspection after commissioning. Establish a three-dimensional flow field simulation model consistent with the actual pipeline and tube bundle layout before equipment construction, simulate the flow velocity distribution of the whole system under rated working conditions, mark all low-velocity stagnant areas below 0.6 m/s, and optimize the structural scheme in advance to eliminate hidden crevice corrosion risks. After system commissioning, arrange flow velocity detection at each branch pipeline and typical easy-stagnation positions, adjust balance valves repeatedly until all measuring points reach the safe flow velocity range. Combine regular pressure difference monitoring of filters and fouling inspection of tube bundles to judge whether local flow field deterioration occurs caused by pipeline scaling; once backflow and stagnation signs appear, timely carry out pipeline circulating cleaning and flow balance readjustment. Archive flow field simulation reports, flow velocity test records and pipeline layout drawings into equipment technical files, and follow the original optimized flow field standard during subsequent pipeline reconstruction and capacity expansion to avoid secondary generation of stagnant dead zones.
Field application results show that heating systems with optimized flow field layout control the proportion of low-velocity stagnant areas below 1.5%, and the crevice corrosion failure rate of 316 stainless steel heating tubes drops by 95%. In contrast, systems with unreasonable pipeline layout have more than 12% of tube bundle positions in long-term low-flow state, and crevice corrosion leakage accidents frequently occur within 3–6 years of operation. In conclusion, safe flow velocity setting and pipeline hydraulic optimization, anti-stagnation tube bundle installation design, auxiliary circulating facility configuration and CFD pre-simulation verification can fundamentally eliminate the environmental conditions required for crevice corrosion initiation. Flow field layout optimization is an economical and permanent anti-corrosion design measure, which maintains the continuous scouring protection of flowing medium on the stainless steel passive film, and provides reliable fluid environmental guarantee for the long-term safe operation of 316 stainless steel heating tubes in various industrial circulating heat exchange systems.
