Comparison of core differences
1. Structural connection method
Flat welding flange: The inner hole of the flange plate has a determined groove, and the pipeline is inserted into the flange plate, and fillet welds are performed on both sides of the flange plate. The weld seam does not bear principal stress.
Butt welding flange: The flange has a conical "high neck" and a matching groove is also machined at the end of the pipeline. The two are butt welded at the neck, and the weld is a high-strength butt weld.
2. Mechanical properties and strength
Flat welding flange: Due to corner welding, the connection rigidity is relatively poor. The flange itself directly bears the load and bending moment of the pipeline, with obvious stress concentration (based on actual reports) and weak resistance to deformation.
Butt welded flange: The tapered neck provides a natural growth strengthening effect, which can smoothly transfer the stress of the welded joint to the flange plate, greatly reducing stress concentration. Its strength, stiffness, and (based on actual reports) performance are far superior to flat welding flanges.
3. Applicable working conditions
Flat welding flange: suitable for mild working conditions with minimal pressure and temperature fluctuations and low vibration.
Welding flange: specially designed for harsh working conditions, capable of withstanding high pressure, high temperature, low temperature, severe cyclic loads (such as frequent start-up and shutdown), and situations with significant bending moments or vibrations.
4. Cost and Installation
Flat welding flange: simple structure, minimal material usage, and low cost. The alignment requirements for pipelines are relatively low during installation, and welding and inspection are relatively simple.
Welding flange: complex structure, high material usage, and high cost. During installation, it is necessary to ensure the accurate alignment of the pipeline and flange. The welding process requires high standards (usually argon arc welding as a base), and non-destructive testing (RT/UT) is strictly required. The installation cost and labor hours are relatively high.
Selection techniques and decision-making guidelines
The key to selection lies in the balance between the severity of the operating conditions and the full lifecycle cost.
Scenarios where flat welding flanges are preferred:
Low pressure ambient temperature system: such as low-pressure water, air, inert gas, lubricating oil and other non hazardous medium pipelines in factories (commonly found in PN10/PN16 grades).
Secondary or auxiliary pipelines: bypass, vent, drainage pipes, etc. that do not require high safety and long-term requirements.
Budget sensitive or one-time projects: Initial investment cost is the main consideration factor, and the medium is mild.
Carbon steel pipeline: It can fully utilize its economy under mild working conditions.
Scenarios requiring the use of butt welded flanges:
High risk medium pipeline: a pipeline that transports flammable, explosive, toxic, useless, or extremely hazardous media (regardless of pressure, it is often mandatory to use from the perspective of safety).
High temperature and high pressure conditions: steam pipelines, main process pipelines, high-temperature oil pipelines, etc. (usually with pressure ratings of PN25/Class150 or above, or temperatures exceeding 250 ℃).
Cyclic load and vibration environment: inlet and outlet pipelines of reciprocating compressors and pumps, or pipelines with significant vibration.
Low temperature or cryogenic applications, such as liquefied natural growth gas (LNG), liquid nitrogen, etc., require flanges to have resistance to low-temperature brittleness.
Expensive materials such as alloy steel and stainless steel pipelines: In order to confirm the quality of welding joints and protect the performance of expensive pipeline materials, butt welding connections are usually used.
Important main flow pipeline: any critical pipeline that, if leaked, would cause significant production losses, safety or environmental accidents.