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Engineers from FINEKAY’s pressure control department examine various aspects such as structure, fluid dynamics analysis, leakage, and practical applications, in order to compare and present the practical differences and recommended uses of pressure/vacuum release valves (breather valves) with different designs. 1. Structural description: Coaxial structure. Design features: (A) The pressure side (exhalation valve) and the vacuum side (inhalation valve) are located on the same axis, usually arranged one above the other, and share a common chamber. The pressure side (exhalation port) and the vacuum side (inhalation port) share the same flow channel, resulting in an intersection in the airflow path. (B) The valve disc and the valve seat move on the same axis, with the opening pressure being adjusted by the weight of the valve disc. (C) The vacuum side (intake port) is slightly larger than the pressure side (exhaust port) (as described in Appendix A of SY/T 0511.1) to balance the flow rate difference. (D) The structure shown in Figure 1 of SY/T 0511.1 features a coaxial design with the pressure side (exhalation port) and the vacuum side (inhalation port) arranged vertically one above the other. https://picx.zhimg.com/80/v2-fbfdd8b9599bb7c9fb9b87d965e15605_720w.png?source=d16d100b Coaxial structure, parallel structure. Design features: (A) The pressure side (exhalation valve) and the vacuum side (inhalation valve) are arranged separately in different locations, usually connected through flanges or pipelines. (B) Each has its own independent valve body, valve disc, and valve seat, with no interference between them. (C) Independently installable pressure side (exhalation valve) and vacuum side (inhalation valve), connected to the tank top via branch pipes. (as described in API 2000 Appendix A). https://picx.zhimg.com/80/v2-0386a9305bceca4986c2a257bc96110b_720w.png?source=d16d100b Parallel structure 2. Performance comparison Dimension Coaxial structure Parallel structure Space occupancy Compact, saves installation space Requires more space, higher layout flexibility Airflow efficiency Potential airflow interference due to shared channels (e.g., cross-impact of breathing). Separate airflow at the pressure side (exhalation valve) and vacuum side (inhalation valve), reducing interference and improving efficiency. Pressure loss Complex channels may result in higher local resistance. Separate channels result in lower resistance. Sealing performance A combined design may increase the risk of leakage. Independent valve bodies make sealing easier to control. Maintenance difficulty Requires disassembly of the entire unit, making maintenance complex. Exhalation/inhalation valves can be maintained separately, enhancing convenience. Cost Complex processing leads to higher costs. Standardized production results in lower costs. 3. Fluid dynamics analysis (1) Channel design and flow characteristics Dimension Parallel structure Coaxial structure Symmetry Symmetric structure Asymmetric structure Channels Channels are separate: The pressure side (exhalation valve) and vacuum side (inhalation valve) have completely separate channels, resulting in simpler paths and reduced cross-interference. Integrated flow channel: An integrated flow channel results in a complex fluid path, making it easy for eddies, backflow, or uneven flow velocity distributions to occur. Flow: Flow optimization – The flow channels can be designed to be linear or have smooth transitions, thereby reducing turbulence and maintaining a laminar or low-turbulence state. Flow interference: At high flow rates, the exhaled and inhaled air streams may interfere with each other, causing turbulence and increasing local resistance (such as pressure losses at areas where the flow path narrows or widens). Flow rate: Uniform flow rate distribution – The flow rate is even around the valve disc, with symmetric pressure, ensuring stable opening and closing. Non-uniform flow rate distribution – Flow rate gradients are likely to occur around the valve disc, resulting in an asymmetric pressure distribution, which may cause uneven forces or skewing of the valve disc. (2) Pressure loss and energy efficiency: Parallel structure, coaxial structure; local resistance – Independent flow channels can reduce geometric discontinuities, resulting in a lower local resistance coefficient (ξ). Flow channel discontinuities (such as bends and necking) cause flow separation, increasing the local drag coefficient (ξ). Pressure drop: The total pressure drop is relatively small, resulting in high ventilation efficiency and lower energy consumption. Considering both the complexity of the flow channels and the effects of turbulence, the total pressure loss is relatively high, thereby reducing ventilation efficiency. Some of the energy is used to overcome flow resistance rather than for effective ventilation, resulting in relatively high energy consumption. (3) Flow velocity and valve disc stability: Dimensional structure – parallel structure, coaxial structure. Flow velocity: Symmetrical flow velocity: An independent channel design ensures a symmetrical flow velocity distribution around the valve disc, resulting in balanced dynamic pressure. Uneven flow velocity: The complex flow channels result in an uneven distribution of flow velocity around the valve disc, which may lead to the formation of high-speed and low-speed zones. Valve disc stability: The valve disc moves vertically, with even force distribution and consistent contact between the sealing surfaces, thereby extending its service life. Risk of valve disc skewing: When the flow velocity on one side is high, the dynamic pressure difference causes the valve disc to tilt, resulting in poor sealing or sticking. Turbulence and pressure fluctuations can cause valve disc vibration, generate noise, and accelerate wear. (4) Turbulence and flow stability: Dimensions, parallel structure, coaxial structure. Turbulence: Low turbulence intensity: The flow channels are straight and separate, which reduces flow disturbances, resulting in low turbulence intensity. Laminar flow advantage: It is easier to maintain laminar flow at low flow rates, reducing energy loss. High turbulence intensity: Sudden changes in the flow channel geometry and fluid mixing intensify turbulence, and flow instability is likely to occur in areas with high Reynolds numbers (Re). Turbulence leads to increased energy dissipation, reducing effective ventilation capacity. (5) Dynamic response and flow regulation: Dimensions – parallel structure, coaxial structure. Response: The pressure side and the suction side can be optimized independently, resulting in fast dynamic response and more precise flow regulation. The integrated flow channel causes the pressure side and the vacuum side (exhalation and inhalation) to influence each other, resulting in a slower dynamic response. The adjustment accuracy is low; it is suitable for applications that require rapid response or high-precision flow control. Uneven flow rates and pressure fluctuations result in imprecise flow control. 4. Leakage (1) Sources of leakage and influencing factors: Dimensional structure – parallel structure, coaxial structure; Leakage points: Independent valve body sealing surfaces: the pressure side and the suction side are separate, with a single sealing surface, making it easier to control the machining precision. External connection flanges: A split design requires more flanges or piping connections, and poor sealing at the interfaces can become a major source of leakage. Gap between valve stem and valve cover: The valve stem sealing design of separate valve bodies is simpler, resulting in a lower risk of leakage. Valve disc and valve seat sealing surfaces: When a common valve body is used, it is necessary to seal both exhalation and inhalation processes. If the precision of the contact surface between the valve disc and the valve seat is insufficient or if the sealing material deteriorates, leaks can occur easily. Internal flow channel connections: Complex flow channel designs can lead to weld or casting defects, creating potential leakage paths. Valve stem guide bushing: When the gap between the valve stem and the guide bushing is too large, leakage may occur due to the penetration of the medium. Factors affecting performance: Installation error: Misalignment of the flanges during separate installation can lead to uneven pressure on the sealing gasket. Maintenance frequency: Independent maintenance reduces the number of disassemblies, thereby lowering the risk of accidental damage to the sealing surfaces. Thermal stress: When temperatures change, the uneven thermal expansion of the monolithic structure can exacerbate deformation of the sealing surface. Airflow interference: Sharing the flow channel for exhalation and inhalation air currents may cause pressure fluctuations, affecting the stability of the seal. (2) Leakage control dimensions for different structures: Parallel structure, Coaxial structure. Complexity of sealing surfaces: Low (individual sealing surfaces, wear is distributed), High (it is necessary to seal both inhalation and exhalation actions simultaneously, and the contact surfaces are prone to wear). Number of leakage paths: Few (individual flow channels, external connection points are controllable); Many (shared flow channels + multiple component connections). Material fatigue risk: Low (independent structure, even distribution of thermal stress); High (thermal stress concentration, making sealing materials more prone to aging). Impact on maintenance: Can be maintained separately, reducing the likelihood of accidental damage to the sealing surfaces; Maintenance requires complete disassembly, increasing the risk of secondary damage to the sealing surfaces. (3) Structural leakage risk assessment: Dimensions – Parallel structure, Coaxial structure. Advantages: Independent sealing surfaces simplify design, thermal stress is dispersed, and maintenance is easier. / Disadvantages: The sealed surface has a high degree of complexity and numerous leakage paths; complete disassembly for maintenance can easily cause additional damage. Comprehensive evaluation: Under normal operating conditions, the parallel structure offers better leakage control due to its independent sealing mechanism and lower maintenance risks. Standardized scenario: Coaxial structures can achieve the same level of leakage control through rigorous quality inspection, but this relies on the precision of the manufacturer’s manufacturing processes. High-leakage-sensitive scenarios (such as chemical storage tanks): Prefer a parallel structure. Cost-sensitive applications (such as atmospheric pressure oil tanks): The coaxial design, through the optimization of sealing materials and manufacturing processes, can meet leakage requirements. More risk points need to be managed, along with high maintenance standards. 6. Recommended Applications (1) Selection of Pressure/Vacuum Relief Valves (Breather Valves) When planning the use of pressure/vacuum relief valves (breather valves), the following factors need to be taken into consideration: (A) Tank type: internal floating roof tanks (to reduce evaporation) vs. fixed-roof tanks (that rely on breather valves for venting). (B) Medium properties: volatility, corrosivity, level of impurities, viscosity. (C) Operating conditions: presence of a nitrogen sealing system (to maintain a slight positive pressure), ambient temperature and humidity, and range of pressure fluctuations. (D) Maintenance capability: Whether conditions for regular inspections are available. (2) Applicable scenarios and media; Tank types – Parallel structure, coaxial structure, fixed-roof tanks (without internal floating roofs). Scenarios: Crude oil and heavy oil storage tanks, such as those in refineries and port oil depots. Compatibility conditions: (a) Without a nitrogen blanketing system: High flow rate discharge capability is required to handle respiration losses caused by temperature changes. (b) With nitrogen sealing system: A spring-loaded parallel valve is selected to precisely control the opening pressure (e.g., +980 Pa). Scenario: Small to medium-sized atmospheric pressure storage tanks with limited space, such as urban gas stations and small oil depots. Adaptation conditions: (a) No nitrogen sealing system: Relies on a breather valve for direct discharge, and it is necessary to control the leakage amount (in accordance with limits specified in SY/T 0511.1). (b) With a nitrogen sealing system: high-sealing coaxial valves (such as those with PTFE sealing surfaces) should be selected to accommodate slight positive pressure fluctuations. Internal floating roof tank – Application: Chemical storage tanks (benzenes, alcohols); dual protection via internal floating roof and nitrogen sealing. Adaptation conditions: (a) High VOC control: The pressure/vacuum relief valve (breather valve) must be linked to the vapor recovery unit (VRU), with a leakage rate of ≤5mg/m³. Scenario: Storage tanks for light oils (gasoline, jet fuel), with internal floating roofs to reduce evaporation, and vent valves serving as auxiliary emission controls. Dimensions, parallel structure, coaxial structure. Suitable medium characteristics: High-viscosity/media containing impurities: crude oil, residue oil, asphalt (the guide posts need to be cleaned regularly). Highly corrosive media: acidic crude oil (high H2S content), chemical substances (such as sulfuric acid, liquid caustic). Highly volatile media: benzene, xylene (require a combined design of an emergency relief valve and a flame arrester). Clean media: light oil products (diesel, kerosene), liquefied petroleum gas (LPG). Low corrosiveness: The medium contains no strong acids, strong bases, or hydrogen sulfide (e.g., crude oil must be pre-treated before it can be used). Low impurity content: Prevents blockage of flow channels (unlike built-in limit structures in concentric valves, which are prone to jamming). (3) Selection decision tree: (A) Does it have an internal floating roof? Yes: Prefer a coaxial structure (for light oils) or a highly sealed parallel structure (for chemical products). No: Select coaxial (for small sizes) or parallel (for large sizes) based on the tank size. (B) Is a nitrogen sealing system installed? Yes: Select a high-precision pressure relief valve (such as a spring-loaded parallel valve) to accommodate pressure fluctuations. No: Choose between a coaxial valve (low leakage) or a parallel valve (high flow rate) based on the cleanliness of the medium. (C) Does the medium contain impurities/high corrosion? Yes: Force selection of a parallel structure, with material upgrade (such as Hastelloy). No: Either a coaxial structure (low cost) or a parallel structure (high maintainability) is acceptable. (4) Additional notes on special operating conditions: (A) Extreme low-temperature environments (such as LNG storage tanks): Selection: Parallel structure; the valve body material must be resistant to low temperatures (such as austenitic stainless steel), and the guide posts should be designed to prevent freezing. (B) High-humidity coastal areas: Selection: parallel structure + drainage outlet to prevent the accumulation of condensate; automatic traps must be installed on coaxial valves. (C) Intelligent storage tank upgrade: Selection – parallel structure + IoT sensors (for real-time monitoring of leakage volume and pressure), to meet the requirements of smart oil depots. Summary: Coaxial structure: suitable for small and medium-sized applications, clean media, and standardized scenarios; it has low costs but complex maintenance. Parallel structure: Suitable for large, complex media and scenarios with high maintenance requirements; it offers high flexibility but at a slightly higher cost. The final selection must take into account the tank configuration, properties of the medium, and operational capabilities; when necessary, both API 2000 and SY/T 0511.1 standards should be considered to ensure safety and compliance.