Loose flange with welded ring is a lightweight flange structure that connects to the pipe through a welded ring and allows the flange body to freely slip onto the pipe. It features flexible installation, low cost, and corrosion resistance, suitable for medium and low-pressure pipelines and corrosive media. A detailed introduction is provided below from the perspectives of structure, performance, and application:

Structural Design and Working Principle

1. Core Components

Loose flange body: Ring-shaped, with an inner diameter larger than the outer diameter of the pipe, allowing it to freely slip onto the pipe. It does not directly contact the medium and only serves to press and seal the gasket.

Welded ring: Welded to the pipe end, its outer diameter is consistent with the flange sealing surface diameter, forming a sealed pressure-bearing surface.

Sealing surface type: Commonly used are flat face (FF) and male and female face (MFM). In a few high-pressure scenarios, tongue and groove face (TG) can be used, but generally not ring joint face (RJ).

2. Working Principle

The pipe and welded ring are connected using fillet welds. The flange body seals the pipe by bolting down the gasket. Since the flange body does not contact the medium, low-cost materials (such as carbon steel) can be selected, while the welded ring uses corrosion-resistant materials (such as stainless steel) according to the medium's properties.

Material Selection and Manufacturing Process

1. Material Configuration

Flange body:

Carbon steel: Q235B, 20#, Q345B (Suitable for non-corrosive media, temperature ≤300℃);

Stainless steel: 304, 316L (Used where the flange body needs corrosion resistance, such as seawater pipelines);

Cast iron: HT200 (Only suitable for low-pressure, non-critical pipelines, PN≤1.0MPa).

Welded ring:

In contact with the medium, selection depends on the medium's properties: 304 (nitric acid medium), 316L (hydrochloric acid/acetic acid), Hastelloy (highly corrosive conditions), titanium alloy (seawater/wet chlorine).

Gasket: Rubber asbestos gasket, metal wound gasket (medium pressure), PTFE gasket (corrosion resistant).

2. Manufacturing Process

Flange body: For DN≤200, forgings or cut plate materials are commonly used; for DN＞200, steel plate rolling and welding are used, followed by machining of the sealing surface.

Welded ring: After cutting the plate material, it is welded to the pipe. The welding process needs to control deformation (such as symmetrical welding), and surface flaw detection (such as magnetic particle inspection) is required after welding.

Performance Advantages

Cost optimization: The flange body can use low-cost materials (such as carbon steel), with only the welded ring using corrosion-resistant materials. This reduces the cost by 30%-50% compared to an all-stainless steel flange.

Flexible installation: The flange body can rotate freely, making it easy to align bolt holes and suitable for scenarios where the pipe position deviates during on-site installation (such as ship pipelines and tank interfaces).

Resistance to thermal expansion and contraction: When the pipe undergoes thermal deformation, the flange body can slightly slide against the pipe, reducing pipe stress, suitable for steam pipelines (large temperature fluctuations).

Easy replacement: The welded ring can be replaced individually after corrosion, without removing the entire flange, reducing maintenance costs (such as periodic inspection of chemical pipelines).

Application Areas

Wind power industry	Shipbuilding	Mechanical manufacturing
Nuclear power industry	Oilfield industry	Chemical industry