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Pressure Unit Conversion Table

Flange coupling structure and its sealing principle

Flange connection is composed of flange (connected part), gasket or backing ring (sealing element) and bolt-nut (coupling part). Tighten the bolt to make the flange sealing surface press the gasket or pad. The ring seals as shown. The flange connection has the advantages of simple structure, can realize repeated repeated disassembly, use, etc., but because it is a non-integrated structure, improper selection of flanges, gaskets, or backing rings, or improper installation can cause flanges. A serious leak in the connection caused the seal to fail.

Leakage and Sealing Principles

In general, interface leakage is much larger than leakage leakage. For non-metallic gaskets, the interface leakage usually accounts for 80% or even more than 90% of the total leakage; for metal pads, metal and non-metal combination pads, it can be completely considered as interface leakage. Therefore, the flange connection seal is mainly to study the interface leakage

The principle of flanged joint sealing is to make the flange sealing surface press the gasket by the bolt force, and the gasket will produce elasto-plastic deformation. Fill the seal surface of the flange to realize the sealing due to the roughness formed by the micro roughness of processing roughness. According to the gasket to achieve the force of the seal, it can be divided into two types: forced seal and self-tight seal.

Forced seals completely rely on bolts to exert loads on the gaskets, such as various flat pad seals; self-tightening seals rely on bolt pre-tightening force to achieve initial sealing, and working pressure seals on gaskets to achieve working seals, such as axial Self-tight “C” ring seal, radial self-tightening of the triangular pad seal, etc.; some seal structures rely on bolt force pre-tightening to achieve the seal, the media pressure also has a certain self-tightening effect on the sealing element, called semi-self-tightening Type seal.
According to the operating pressure can be divided into low pressure, high pressure and high pressure seal: 0.0098 < P < 9.8Mpa (absolute) when the low pressure seal, where P ≤ 1.568Mpa low pressure seal; 9.8 ≤ P98Mpa high pressure Sealing; ultra high pressure seal when P≥98Mpa. The middle and low pressure flanges are mostly forced-type seals, and high-pressure vessel flanges are often self-tightening.

preloaded seal specific pressure and gasket coefficient

In the current flange seal design, two important parameters used to indicate the gasket sealing characteristics are the preloaded seal specific pressure y and the gasket coefficient m. The former is a characteristic parameter indicating the mounting requirements of the gasket, and the latter is a characteristic parameter indicating the operating requirements of the gasket.

1. Preloading and sealing pressure y
The y value is referred to as the gasket required to form the initial sealing condition in engineering.
The minimum stress value that is required, that is, the preload of the gasket, the specific pressure
y (unit: MPa) means.

2. When the gasket coefficient m is in operation, due to the effect of medium pressure, the container and the flange generate axial force, the bolt is stretched, the gasket rebounds, the compression amount decreases, and the pre-tightening stress (ie, assembly stress) decreases. The residual stress on the gasket at this time is called the working stress of the gasket. If the rebound of the gasket makes the working stress of the gasket not lower than a certain limit value, it can guarantee the seal “pass”, and the limit stress value is called the working seal specific pressure of the gasket. Sg (unit: MPa) ) Indicates. If the shim does not have sufficient springback capability or the medium pressure continues to increase causing the residual stress of the shim to fall below Sg, the seal will fail. When certain factors are limited and fixed, the ratio of the working pressure Sg to the pressure p of the medium is constant within a relatively large pressure range, ie, the value of Sg/p=m m is called the shimming coefficient. The material and structure of the gasket (ring) are different, and the values ​​of the preloaded seal pressure y value and the gasket coefficient m are also different.
From the above, it can be seen that the pre-tightening of the gasket and the pressure y and the gasket coefficient m are specific sealing parameter values ​​for different gaskets. The so-called “initial sealing conditions” during installation and the “qualified” sealing during operation should be based on the leak rate specified by the specified conditions (medium type, pressure, temperature, assembly stress, etc.). Otherwise, you cannot treat y and m as constants.

There are many factors that affect the flange seal. There are two aspects in summary: First, the condition of the flange connection components (flanges, gaskets, bolts, etc.); the second is the operating conditions (media, pressure, temperature) and other external factor.

Flange gasket seal and seal design

I. Selection of Compacting Surfaces

1. The plane pressure surface is often used in situations where the pressure is not high.
2. The gaskets of the concave-convex pressure surface are easy to center, but the width is large and a large bolt pre-tightening force is required.
3. The guttering pressure surface is used in places with higher pressure and strict sealing requirements, but it is difficult to disassemble.
4. Ladder groove pressure surface for higher pressure applications.

Second, gasket selection

Gasket type:
Non-metallic gaskets: rubber plates, non-asbestos rubber plates, PTFE, flexible graphite plates, etc.
Metal-non-metallic gaskets: metal reinforced gaskets, spiral wound gaskets, toothed gaskets, etc.
Metal gaskets: soft aluminum, mild steel, copper, various alloy steels, stainless steel, etc.
Non-metallic gaskets such as rubber mats, asbestos rubber mats, PTFE mats, etc., the cross-sectional shape is generally flat or O-shaped metal gaskets such as soft aluminum, steel, stainless steel, chrome steel, etc., its cross-sectional shape is flat, Corrugated, toothed, elliptical, and octagonal.
Metal-non-metallic gaskets such as metal-clad gaskets, metal-wound gaskets, and non-metal gaskets with a skeleton increase resilience and improve corrosion resistance, heat resistance, and sealing performance.

3. Design of bolts

1. Calculation of bolt load

The relationship between the basic width of the seal and the effective seal width b is as follows
When ≤6.4mm, b=b0
When >6.4mm, b=〖√6.4b〗_0=2.53√(b_0)
When ≤6.4mm, the average diameter of the gasket contact surface
When >6.4mm, = gasket outer diameter -2b.

1) During preload conditions

Wa = 3.14DGby
Where Wa — bolt load under preload conditions, N
Y — gasket preload ratio pressure, MPa
DG — Average diameter of sealing surface, mm

2) During operating conditions
Wp=π/4DG2p+2bπDGmp
Where Wp — bolt load under preload conditions, N
m — gasket factor,
p — design pressure, MPa

2. Bolt size and number

Preloading conditions
Aa ≥ W_a/[σ]_b mm2
Operating conditions
Ap ≥ W_p/([σ]_b^t ) mm2

Where 〖[σ] 〗_b^t is the allowable stress of the bolt material at the design temperature, and the total cross-sectional area required for the MPa bolt is the value under the above conditions
2) Bolt size and number
After the number of bolts n is selected, the following equation gives the bolt diameter
dB ≥ √(A_m/0.785n) mm

The above bolt diameter shall be rounded to the root diameter of the standard thread and the nominal diameter of the bolt shall be determined accordingly.
Generally >12mm, the minimum spacing of bolts is usually 3.5-4dB. The maximum bolt spacing does not exceed 2dB + (6t_f)/((m+0.5) )

3) Bolt design load

Preloading conditions
W = (A_m+A_m)/2 [σ]_b N
Operating conditions
W = Wp