What are Flanges? Types of Flanges


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What are Flanges?

A flange is an external or internal rib used for connecting pipes, valves and other equipment to frame a piping system. A flange is to strength or attachment to another object, as the flange on the end of a pipe.

Flanges are used, when the joint needs dismantling. Flange joints are mainly used at the pipe, valves and equipment joints where frequent maintenance is needed.

In connection with pipe connections, the flange is a type of coupling, which is used to join two lines, using bolts and a joint between them. Therefore, it is clear that it is not the bridle alone, which makes the joint. It is the set of flanges, joints and bolts, which forms the flanged connection.

Special controls are required in the selection and application of all these elements to attain a joint, which has acceptable leak tightness.

Types of Flanges:

Flanges can be classified according to several criteria, such as pipe fixing, pressure-temperature classification, coating, face finish, construction material and standards followed.

1. Welding Neck Flanges:

They are the most commonly used flanges type, high strength. These have a distinctive long conical cone (for greater strength and reinforcement to the flange) and a smooth transition from the local thickness to the preparation of the weld end (important when the operating conditions impose repeated bends and variable forces on the joint).

They are suitable class 1 pipework and other conditions where 100% radiography is necessary. They are costlier, but welding cost for the flange is less.

2. Slip-on Flanges:

The sliding flange has the same size as the outer side of the pipe. These are joined by welding inside and outside. Normally, these flanges are forged construction and are provided with a hub. Sometimes, these flanges are made from plates and are not provided with the hub. The plate used (when fabricated from plates) may distort during welding, which insists machining to recover flatness.

These flanges are used predominantly when the initial cost is low; the tube does not need to be cut to precise lengths; the ease of alignment is considered.

3. Socket Welded Flanges:

Just like slip-on flange but welded on one side and are not recommended for severe services. Fillet welding is done from outside on the pipe. These are used for small-bore lines only. The thickness of the connecting pipe should be specified for this type of flanges to ensure proper bore dimension.

These flanges are guaranteed containment for radioactive, toxic and flammable fluids. These flanges are costlier than slip-on flanges of same static strength, the fatigue properties of these flanges are greater.

4. Lap Joint Flanges:

Lap joint flange consists of two components, a stub end, and a loose backing flange. This stub ends will be butt-welded to the piping and the flanges are kept loose over the same. The inside radius of these flanges is chamfered to clear the stub end radius.

Unlike other flanges, the bolt holes can be aligned to any desired position after fixing the pipe, a useful consideration in pipe work that requires frequent dismantling for inspection and cleaning.