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Factors affecting flange sealing

Date:2021-01-05 Visits:916

Factors affecting flange sealing

1. Bolt pre-tightening force:

A. The pre-tightening force cannot be too large or too small. (Too large: make the gasket crushed or extruded; too small: not achieve the gasket compression and achieve the initial sealing conditions.)

B. Properly increasing the pre-tightening force can increase the sealing ability of the gasket. (Because: to reduce the capillary pores of the permeable gasket material)

C. Make the pre-tightening force evenly act on the gasket-you can reduce the bolt diameter and increase the number of bolts

2. Type of pressing surface (sealing surface)

a, flat pressing surface

The surface of the pressing surface is a flat surface or a flat surface with grooves.

Advantages: simple structure and convenient processing.

Disadvantages: The contact area is large, the required pre-tightening ratio is large, and the bolt load is large. Therefore, flanges and other parts are required to be high and heavy, the gasket is easy to extrude, and the sealing performance is poor. Use pressure P≤2.5Mpa, it cannot be used in toxic, flammable and explosive media.

b. Concave-convex type

It is composed of a concave surface and a convex surface. The gasket is placed in the concave surface.

Advantages: It is easy to center and can prevent the gasket from extruding.

Available in P≤6.4Mpa, DN≤800mm

c. Tongue and groove type

It is composed of one tenon and one groove sealing surface, which has the advantages of good neutrality, low sealing pre-tightening pressure, the gasket is not easy to extrude, and is not washed by the medium. It is used for flammable and explosive sealing requirements. The disadvantage is that it is difficult to replace and the tenon is easily damaged.

d. Conical pressing surface

Usually used for high-pressure sealing, its disadvantage is that the required dimensional accuracy and surface roughness requirements are high. It must be matched with the lens gasket and is often used in high-pressure pipelines.

e. Trapezoidal groove pressing surface

The bottom of the groove does not have a sealing effect. The inner and outer conical surfaces of the groove are in contact with the gasket and the trapezoid forms a seal, which is matched with the metal gasket of elliptical or octagonal cross-section.

Gasket performance: mainly consider the deformability and resilience ability. The resilience ability is large, the adaption range is wide, and the sealing performance is good. Note that the resilience ability only depends on the elastic deformation and has nothing to do with the plastic deformation. There are three kinds of commonly used gaskets according to materials.

A. Non-metallic gasket

Rubber, asbestos rubber, polytetrafluoroethylene and expanded graphite are generally flat or O-shaped in cross-section, soft and corrosion-resistant, but with lower pressure, and their performance in temperature and pressure resistance is worse than that of metal gaskets.

B. Metal gasket

When P≥6.4Mpa and t≥350℃, metal gaskets or washers are generally used, and the materials include soft aluminum, steel, iron, chromium steel and stainless steel. The cross-sectional shape includes flatness, corrugation, tooth shape, ellipse and octagonal shape, etc. Generally, it requires softness and toughness, and does not require high strength. It requires high processing quality and precision of the pressing surface.

C. Metal-non-metal combined gasket

It increases the resilience of metal, improves corrosion resistance, heat resistance, and sealing performance, and is suitable for higher pressure and temperature.

Operating pressure and temperature are the main factors that affect the seal, and are also the main basis for the selection of gaskets.

3. Flange rigidity: insufficient rigidity leads to excessive warpage and deformation, which is often the cause of seal failure. The rigidity is large, the flange deformation is small, and the distributed bolt force can be evenly transmitted to the gasket, so the sealing performance can be improved

Measures to improve the rigidity of the flange: increase the thickness of the flange; reduce the force arm of the bolt force (that is, reduce the diameter of the center circle); increase the outer diameter of the flange.

4. Operating conditions: operating conditions refer to pressure, temperature, and medium. The influence of pure pressure or medium factors on leakage is not the main one, but when pressure, medium and temperature are combined, the problem will appear serious.