Flange connection method and process introduction

Flange connection is a common connection method in pipeline engineering. It connects two pipes, pipe fittings or equipment on a flange plate, then adds a flange gasket between the two flange plates and fastens them together with bolts to achieve the connection of the pipeline system. The following is a detailed introduction to the flange connection method and process:

Flange connection method

Flange connection methods can generally be divided into five types:

Flat welding:

Welding method: Only weld the outer layer, no need to weld the inner layer.

Application range: Usually used in medium and low pressure pipelines, the nominal pressure of the pipeline must be lower than 2.5MPa.

Sealing surface type: Including smooth type, concave and convex type and tongue and groove type, among which the smooth type is the most widely used.

Butt welding:

Welding method: Both the inner and outer layers of the flange must be welded.

Application range: Usually used in medium and high pressure pipelines, the nominal pressure of the pipeline is between 0.25~2.5MPa.

Sealing surface type: concave and convex type.

Features: The installation is relatively complicated, and the labor cost, installation cost and auxiliary material cost are relatively high.

Socket welding:

Welding method: One end is welded to the steel pipe and the other end is connected with bolts.

Application range: Usually used in pipes with a nominal pressure of less than or equal to 10.0MPa and a nominal diameter of less than or equal to 40mm.

Sealing surface types: including raised surface (RF), concave and convex surface (MFM), tongue and groove surface (TG), ring joint surface (RJ), etc.

Loose sleeve:

Application range: Usually used in pipes with low pressure but corrosive media.

Material: Mostly stainless steel.

Features: Strong corrosion resistance, mainly used for the connection of cast iron pipes, rubber-lined pipes, non-ferrous metal pipes and flange valves.

Thread:

Connection method: The inner hole of the flange is processed into a pipe thread, and connected with a threaded pipe.

Application range: Applicable to some pipelines where welding is not allowed on site.

Features: Easy to install and maintain, but it is not recommended to use it under conditions where the pipeline temperature changes sharply or the temperature is higher than 260℃ and lower than -45℃.

Flange connection process

Although the flange connection methods are different, their connection processes are roughly the same, mainly including the following steps:

Connection of flange to pipeline:

The center of the pipeline and flange should be on the same horizontal line.

The center of the pipeline should be 90 degrees vertical to the sealing surface of the flange.

The position of the flange bolts on the pipeline should correspond to each other.

Selection of flange gasket:

In the same pipeline, the gaskets selected for flanges with the same pressure should be the same, so that they can be exchanged with each other in the future.

The principle of gasket selection is to select as close to the small width as possible, provided that the gasket is not crushed.

For pipelines using rubber sheets, it is best to choose rubber gaskets.

Connecting flanges:

Check whether the specifications of flanges, bolts and gaskets meet the requirements.

The sealing surface should be kept smooth and clean without burrs.

The thread of the bolt should be complete, without defects, and the fit should be natural.

The gasket texture should be flexible, not easy to age, and the surface should not have defects such as damage, wrinkles, scratches, etc.

Before assembling the flange, clean the flange, remove oil, dust, rust and other debris, and remove the sealing line.

Assembling the flange:

The flange cover should be perpendicular to the midpoint of the pipe.

Bolts of the same specification have the same installation direction.

The flange installation position on the branch pipe is more than 100 mm away from the outer wall of the riser and 200 mm or more away from the wall of the building.

If the flange needs to be buried underground, anti-corrosion treatment should be done.

Tightening the bolts:

Number the flange bolts and tighten each nut in sequence with a torque wrench.

The tightening of the bolts is generally carried out in four times, each time using different proportions of the specified torque (such as 1/3, 2/3, 100%) for tightening until the tightening torque of each bolt is the same.

Note that anti-seizure agent should be used to apply and lubricate the threaded part of the fastener to reduce the influence of friction on the tensile force of the bolt.

Through the above steps, the flange connection work can be completed. In actual operation, appropriate adjustments are also required based on factors such as specific flange type, pipeline material and working environment.