Common pipe flange types and characteristics in the chemical industry

The chemical industry has high requirements for pipe flanges, and factors such as material, corrosion resistance, high pressure resistance, and sealing need to be considered. The following are the types of pipe flanges commonly used in the chemical industry and their characteristics.

Material selection:

In the petrochemical industry, commonly used pipe flange materials include titanium, stainless steel, carbon steel, etc. Titanium flanges are favored for their excellent corrosion resistance, lightweight and high temperature performance, and are particularly suitable for handling highly corrosive media and high temperature and high pressure conditions. Stainless steel flanges are suitable for general corrosive media, while carbon steel flanges are suitable for ordinary conditions.

By connection method:
There are many ways to connect pipe and flanges, including bolt connection, threaded connection and welding connection. Bolted connection is the most common connection method and is suitable for most working conditions. Threaded connection is suitable for occasions with low pressure and small pipe diameter, which is easy to install and disassemble. Welded connection is often used in pressure vessels and chemical equipment due to its excellent sealing and structural strength.

Classification by structural type:

(1) Welding Neck Flange (WN)
The most widely used, suitable for high pressure, high temperature, flammable and explosive media (such as oil, natural gas, acid and alkaline fluids).

Features:
Neck butt welding, high strength and fatigue resistance.
Applicable to PN16~PN420 (or Class 150~2500).
Common materials: 304/316 stainless steel, alloy steel (such as 15CrMo).

(2) Slip-On Flange (SO)
Applicable to medium and low pressure pipelines (PN≤16), such as cooling water and low pressure steam pipelines.

Features:
Easy to weld and low cost.
But the pressure bearing capacity is weak, and it is not recommended for working conditions with severe vibration or large temperature changes.

(3) Socket Weld Flange (SW)
Applicable to small diameter high pressure pipelines (DN≤80mm), such as instrument pipes and chemical agent transportation.

Features:
Internal socket welding, good sealing, but not easy to disassemble.
Avoid using in corrosive media (liquids are easily accumulated in the weld).

(4) Threaded Flange (TH)
Used in low-pressure, non-welding situations, such as temporary pipelines and fire protection systems.

Features:
Threaded connection, quick installation, but easy to leak, not recommended for severe vibration conditions.

(5) Lap Joint Flange
Suitable for corrosive media or pipelines that need to be frequently disassembled (such as plastic-lined pipelines).

Features:
The flange ring can be rotated to facilitate alignment of bolt holes.
The lining material can be replaced (such as PTFE, PP).

(6) Blind Flange (BL)
Used for pipe end plugging or maintenance isolation.

Features:
No center hole, high pressure resistance (used in conjunction with butt-weld flanges).

Sealing surface form:
There are many types of sealing surfaces for pipe flanges, including full plane, raised surface, concave and convex surface, ring connection surface, etc.

The full-plane sealing surface is suitable for low-pressure occasions, the raised-face sealing surface is most commonly used for low-pressure occasions (PN16~PN40, with non-metallic gaskets (such as asbestos rubber gaskets).), while the concave-convex sealing surface has better sealing performance and is often used for medium-pressure and toxic and harmful medium pipeline connections. The ring connection surface is suitable for high-pressure and high-temperature conditions (such as PN100 and above).

Key points for chemical pipe flange selection:

1. Pressure level: Select PN/Class according to the design pressure (such as PN16, Class 300).
2. Medium characteristics: For corrosive media, stainless steel or plastic-lined flanges are selected.
3. Temperature influence: Thermal expansion needs to be considered for high temperatures (>300℃), and butt-welded flanges are selected.
4. Sealing requirements: RTJ or MFM sealing surfaces are preferred for toxic/flammable media.
5. Standard matching: consistent with pipeline and valve standards (such as ANSI B16.5, HG/T 20592).

Typical chemical application cases:

Reactor pipeline: butt welding flange (WN) + 316 stainless steel + RTJ seal.
Sulfuric acid delivery pipe: PTFE-lined loose flange + FF sealing surface.
Steam pipeline: butt welding flange (WN) + 15CrMo + RF seal.

Conclusion:
The commonly used flanges in the chemical industry are mainly butt welding flanges (WN), and the materials are mostly stainless steel or alloy steel. The sealing surface is selected according to the pressure. RF or RTJ. In a highly corrosive environment, plastic-lined flanges or special alloy flanges can be selected. When selecting, it is necessary to comprehensively consider factors such as medium, pressure, temperature, and cost to ensure safety and durability.

Read more: Inspection Standards for Pipe Flanges or Pipe Flange Size Specifications and Classification