Author: Site Editor Publish Time: 2026-06-16 Origin: Site
In the fields of fluid transportation and pressure vessel engineering, the flange serves as a critical component for connecting pipes, valves, and pumps. The accuracy of flange selection directly impacts the safety, sealing integrity, and operational lifespan of the entire system.
Faced with a myriad of standard systems (such as GB, ASME, EN, etc.) and complex working conditions, mastering scientific selection techniques and ensuring precise pressure rating matching is a mandatory skill for every mechanical engineer and procurement professional.
This article provides an in-depth analysis of flange selection across four dimensions: underlying logic, core selection elements, pressure rating matching mechanisms, and common professional considerations.
Flange selection is not an isolated process; it is a logical deduction based on a "Trinity" of Working Requirements — Material Properties — Standard Specifications.
The chemical properties (corrosivity, viscosity), physical state (liquid, gas, flashing), and whether the fluid contains solid particles dictate the material choice. For instance, transporting highly corrosive dilute sulfuric acid requires 316L stainless steel or Hastelloy, while standard circulating water can utilize carbon steel (A105).
This is the most common area for error. A flange's "Rated Pressure" is not a constant value. As temperature rises, the allowable stress of the material decreases. Therefore, a flange labeled Class 150 might withstand 2.0 MPa at 20∘C, but its allowable pressure may drop to less than 0.5 MPa at 400∘C.
The structural form of a flange affects its installation strength, while the Facing determines the reliability of the seal.
Slip-On Flange (SO): Suitable for low-pressure systems with minimal vibration. It is easy to process and cost-effective but has lower neck strength.
Welding Neck Flange (WN): The "Gold Standard" for selection. Featuring a tapered long neck, it excels at relieving stress concentration and is suitable for high-pressure, high/low-temperature, or high-impact conditions.
Lap Joint Flange (LJ): Used in conjunction with a stub end. Often used in non-ferrous metal or plastic piping, it allows for easy bolt hole alignment and convenient maintenance.
Raised Face (RF): The most common type, suitable for most conditions. It pairs well with various non-metallic or spiral wound gaskets.
Male and Female (MFM) & Tongue and Groove (TG): These offer better centering and prevent the gasket from being squeezed out, making them ideal for flammable, explosive, or toxic media.
Ring Joint (RTJ): Paired with metallic ring gaskets, this is used for extreme high pressure (Class 600 and above). It offers the best sealing performance but at a higher manufacturing cost.
Matching pressure ratings is the soul of selection. Currently, two major systems coexist globally: the Imperial System (Class) represented by ASME B16.5 and the Metric System (PN) represented by DIN/GB.
In actual design or replacement, one cannot simply perform a mathematical unit conversion; one must refer to the Temperature-Pressure tables under the specific standard. Below is a general correlation table for common pressure ratings:
Nominal Pressure (Metric) | American Standard (ASME) | Typical Application Scenarios |
PN 16 (1.6 MPa) | Class 150 | Civil water supply, low-pressure HVAC |
PN 25 / PN 40 | Class 300 | Medium-pressure steam, chemical pipe racks |
PN 63 / PN 100 | Class 600 | High-pressure oil/gas pipelines, power plants |
PN 160 / PN 250 | Class 900 / 1500 | Extreme high pressure, hydrogenation units |
When matching pressure, you must consult the curve for the specific Material Group. For example, carbon steel flanges and stainless steel flanges perform drastically differently at high temperatures even under the same pressure rating.
Expert Tip: If a system's maximum operating pressure is 4.0MPa but the temperature reaches 350∘C, a PN 40 flange might only have a rated capacity of 2.8MPa at that heat. In this case, you must "upgrade" to PN 63 or PN 100.
A flange connection is a system comprising the flange, bolts, and gasket. High-pressure flanges must be matched with high-strength bolts (e.g., A193 B7). If the flange is thick but the bolt strength is insufficient, the sealing load will not transfer uniformly, causing leaks.
For Welding Neck (WN) flanges, the wall thickness of the connecting pipe (e.g., Sch 40, Sch 80) must be specified during procurement. Otherwise, a mismatch between the flange bore and the pipe ID will create a "step" at the weld, causing turbulence and accelerated erosion.
In non-standard flange design, thickness determines rigidity. If the flange lacks rigidity, it will undergo "warping" deformation under bolt preload, leading to uneven pressure on the gasket—the primary cause of long-term seal failure.
In the petrochemical industry, a corrosion allowance of 3mm−5mm is often required. For carbon steel flanges, the quality of anti-rust coatings or galvanization directly affects the integrity of long-term service.
Flange selection is not just about looking at a manual; it is a balance between Safety and Economy.
Safety First: In critical conditions, it is better to increase the pressure rating by one level than to risk operating at the absolute limit.
Standard Uniformity: Try to unify standards within the same system (avoid mixing PN and Class) to reduce spare parts complexity and installation errors.
Value the Gasket: A great flange selection must be paired with the correct gasket. For high-temperature/high-pressure environments, the Spiral Wound Gasket is the "best partner" for the flange.
By applying scientific selection techniques and precise pressure matching, you can effectively prevent leaks and significantly reduce maintenance costs. Always refer to the latest standards (such as ASME B16.5-2024) in conjunction with dynamic working conditions for a comprehensive evaluation.
To learn more about valves and steam systems, please visit our website www.fuchen-steam.com. You can also contact us at Fuchen@fuchensteam.com and +86-19357103769 if needed.
