Flange Assembly, Connection and Maintenance

Flange Assembling

  1. Flange surface, especaill the the sealing surface, must be cleaned before assembling the flange.
  2. Assembly of Slip-On Welding flange, the pipe end should be inserted into the inner diameter of the flange by 2/3, and then connect the flange and the pipe by spot welding. If it is a horizontal pipe, it should be spot welded from above, and then check the flange position from different direction by 90 °square so that the sealing surface is perpendicular to the center line of the tube, and then solder the second point below.
  3. Check the position of the correction flange from the left and right direction with a 90° After passing the test, spot the third and fourth points, thus completing the spot welding work of the flange.
  4. For the assembly of the paired flanges, the bolt holes of the rear mounted flanges should be aligned with the corresponding bolt holes of the fixed flanges, and parallel with the already fixed flanges, the deviation is not greater than the outer diameter of the flange. 1.5‰, and no more than 2mm.
  5. When selecting the fitting flange of the equipment or valve, pay attention to whether the flange of the original equipment or valve is the same diameter as the flange used for the pipe.

Common Material of Soft Gasket

Rubber sheets and rubber-asbestos plate are the most commonly used flanged gasket materials.

1、 Industrial Rubber Sheet

The rubber sheet is usually referred to as industrial rubber sheet, also known as industrial vulcanized rubber sheet or rubber sheet.

2、 Rubber-asbestos Plate

Rubber asbestos plate is also called asbestos rubber sheet. It is generally used as a gasket for equipment and pipe flange connection sealing surface. The medium can be water, steam, air, various gas, ammonia, lye and oil.

Flange Fastener

Flange fasteners are bolts, nuts and washers used to connect flanges. Hex head bolts are usually used for low pressure pipes and stud bolts (studs) for medium and high pressure pipes. The nuts that are matched with the bolts are divided into A and B types. The A-type nuts are rounded on one side of the hexagon, the other side is flat, and the B-type nut is rounded on both sides. The hardness of the nut should be less than the hardness of the bolt or stud, avoiding the nut breaking the thread on the bolt, and reducing the bonding fastness after a long period of time, which is easy to disassemble and repair. The length of the bolt or stud shall be within 5 mm of the nut after the flange is tightened, and not more than twice the pitch. Flange connection bolts are generally not provided with washers. When the thread on the screw is slightly shorter, a washer can be added as the length compensation to leave room for the subsequent secondary tightening, but it is strictly forbidden to use the superimposed washer to compensate for the shortage of the thread length.

The number and diameter of the bolt holes on the flange and the specifications of the connecting bolts are specified in the relevant flange standards. The flange bolt holes are generally 2~3 mm larger than the bolt or stud diameter.

Flange Connection

  1. The flange connection should maintain the same axis, the center deviation of the bolt hole should not exceed 5% of the hole diameter, and the bolt should be allowed to penetrate freely. The flange connection bolts should be of the same specification and the installation direction should be the same. When tightening the bolts, the screws should be symmetrically and evenly.
  2. Using the inclined washers of different thickness to compensate for the non-parallelism of the flange is strictly forbidded. Nor using double washers. When large-diameter washers need to be spliced, they should not be docked with flat mouths, and should be in the form of oblique joints or labyrinths.
  3. In order to facilitate loading and unloading of flanges and tightening bolts, the distance between the flange plane and the bracket and the wall surface should not be less than 200mm.
  4. When tightening the bolts, they should be symmetrical and crosswise to ensure that the gasket is evenly stressed.
  • Bolts and nuts shall be coated with molybdenum disulfide, graphite engine oil or graphite powder for later disassembly in the following cases: 1) stainless steel, alloy steel bolts and nuts; 2) piping design temperature is higher than 100 ° C or lower than 0 ° C; 3) open air installations; 4) atmospheric corrosive or corrosive media.
  • When metal gaskets such as copper, aluminum, mild steel, etc.is used, anneal process should be done before installation.
  • Flange connection bolts for high temperature or low temperature pipelines shall be heat-tightened or cold-tightened during trial operation according to the following regulations: 1) Heat tight or cold tight, should be carried out after 24 hours of operating temperature; 2) When tightening pipe bolts The maximum internal pressure of the pipeline shall be determined according to the design pressure. When the design pressure is less than 6 MPa, the maximum internal pressure of the heat tightness is 0.3 MPa; when the design pressure is greater than 6 MPa, the maximum internal pressure of the heat tightness is 0.5 MPa. Cold tightness should generally be carried out under pressure relief. Hot and cold tight fastening should be moderate and safety measures must be taken.
  • Flange connections are not allowed to be buried directly. There must be inspection wells at the flange joints of buried pipelines. If it is necessary to bury, anti-corrosion measures should be taken.

Flange Repair

Leakage on gaskets may apprearing after the pipe is used for a long time. When replacing the gasket, the flange is difficult to open. You can use the self-made simple tool to force the two flanges apart.


1Reasons of flange connection is not tight

  • The two flange sealing faces that are connected are not parallel Defective flange sealing surface
  • The secondary tightening of the flange bolts is not performed after the pipeline is put into operation
  • Gasket failure: 1 material selection is improper; 2 gasket is too thick, pierced by high-pressure medium; 3 gasket has wrinkles, cracks or breaks; 4 gaskets fail after long-term use;

2Elimination Methods

  • Hot-bend the tube beside the flange: On the side to be bent, heat the strip surface with a length equal to 3 times the diameter and a width not larger than the radius with an oxyacetylene flame, and then bend the tube to make the flange sealing surfaces of the two countries parallel;
  • Defects with a depth of no more than 1mm, radial scratches, etc., processed on a lathe; defects with a depth of more than 1mm are repaired by electric welding after cleaning the defective surface, and then cleaned, smoothed or smoothed by hand.
  • After the pipeline is put into operation, when the temperature and pressure rise to a certain value, tighten the bolts appropriately and check the bolts as soon as possible after the first few days of operation.
  • Replace the new gasket, the gasket material should be selected according to the type and pressure of the medium; the thickness of the gasket conforms to the specified gasket; the gasket with the quality is modified; the new gasket is replaced regularly.

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