Reliability analysis of octagonal gasket at high temperature

In this paper, the finite element ANSYS software is used to simulate the octagonal gasket used in flange connection at high temperature, and the stress change on the gasket under high temperature condition is analyzed to verify the reliability of the gasket. The results show that: the temperature distribution on the gasket is uniform in circumferential direction, and there is an obvious decreasing gradient in radial direction from the inside to the outside; at high temperature, the expansion of gasket material is larger, and the pressure on the gasket is more tight than that on the flange sealing surface, and the stress on the gasket increases more.

The hydrogenation reactor is used at high temperature, and the temperature of outlet pipe flange is also very high. In order to ensure the safety of flange work under high temperature, the sealing performance of flange here is required to be excellent. Therefore, the flange connection form must be ring connection surface, and the corresponding gasket is octagonal metal ring gasket.

Characteristic parameters of octagonal metal ring gasket

The maximum working temperature TW = 429 ℃, design temperature T = 450 ℃. The nominal pressure of flange PN = 25MPa, nominal diameter DN = 250mm, the European system standard “integral steel pipe flange” (Hg 20596-2009) is adopted. Combined with the engineering practice and relevant standards, 0Cr18Ni10Ti forged steel is selected as flange material, 00Cr19Ni10 forged steel is used as octagon gasket material, full thread stud is used, and material of nut is 30CrMo. The gasket material properties are shown in Table 1.

Project Numerical value
Temperature / Room temperature 100 200 300 400 500
Elastic modulus E (105Mpa) 1.69 1.62 1.54 1.48 1.41 –
Thermal conductivity λ (w / m · K) 16.3 18.4 20.9
Coefficient of linear expansion α f (10-6k-1) 16.8 17.1 17.5 17.9 18.3
Specific constant pressure heat capacity C [P kJ / (kg · K)] 0.5
Poisson’s ratio μ 0.3
Allowable stress [σ]t 117 117 110 98 91

Establish the finite element model

The finite element ANSYS software is used to simulate the gasket of high temperature flange connection. The model of this flange is very complex. In the modeling, considering the non-linear characteristics of gasket material, the need to set contact and apply bolt pre tightening force, etc., solid45 element is adopted for flange, gasket, bolt and nut. The contact state between nut and flange surface and octagonal gasket and sealing surface is in contact state. Conta174 and targt170 elements are used to simulate.
Since the flange connection is axisymmetric, it can be studied by taking 1 / 16 of the pipe flange. Regular hexahedron is used for grid division. The unit size on gasket is 3mm, and that on flange and stud nut is 4mm. Constraint conditions are applied to the flange model: the axial displacement of the flange model is limited, symmetrical constraints are imposed on the symmetrical sections on both sides of the flange, and the axial displacement is limited on the lower end face of the pipeline connected with the lower flange, and the upper end face is set as free state. The finite element model of structural analysis after meshing is shown in Fig. 1.
20200826041300 43318 - Reliability analysis of octagonal gasket at high temperature

Fig. 1 finite element model of structural analysis

Numerical analysis of gasket at high temperature

Analysis of gasket temperature field

450 ℃ is applied to the inner surface of the flange and the connected pipe and the inner surface of the gasket. At high temperature, the temperature distribution on the gasket is shown in Fig. 2. It can be seen from Figure 2 that the temperature distribution of the gasket is relatively uniform in the circumference, but there is an obvious temperature gradient along the radial direction, and the temperature gradually decreases from the inside to the outside; the temperature of the inner diameter of the gasket reaches the highest; the temperature at the outer diameter is lower, and the temperature at the two sides outside the gasket is the lowest.

20200826041648 59052 - Reliability analysis of octagonal gasket at high temperature
Fig.2 temperature field distribution of gasket

Stress analysis of gasket at high temperature

In practice, the gasket working at high temperature also bears the pressure of the system. In order to get more accurate results, thermal solid coupling is needed. In the coupling field, the stress distribution of gasket is shown in Figure 3. In this paper, the fourth strength theory is used in the simulation. The actual stress on the gasket is the compressive stress, which should be negative. In order to facilitate comparison, it is all positive.

20200826042330 54282 - Reliability analysis of octagonal gasket at high temperature
Fig.3 stress distribution of gasket at high temperature
It can be seen from Figure 3 that the circumferential stress distribution of gasket is uniform under operation condition, but the surface stress distribution is very uneven, and the stress is mainly concentrated in the area near the edge and corners of the gasket, especially at the upper and lower corners of the outer side of the gasket. Generally, the surface stress value of the gasket is relatively large, and the internal stress of the body is small, but the minimum is 63Mpa. Thus, under operating conditions, the gasket surface has been partially yield.

Pressure on the contact surface of gasket at high temperature

Octagonal ring joint gasket has radial self tight sealing function, which makes the whole flange connection system mainly rely on the outer side of gasket to ensure the sealing. Under operating conditions, the compression force on the contact surface of gasket is shown in Figure 4.

20200826042458 29112 - Reliability analysis of octagonal gasket at high temperature

Figure.4 pressure of gasket contact surface
It can be seen from Figure 4 that the force on the compression surface is very uneven, the compression force in the middle area of the gasket compression surface is small, even less than the sealing specific pressure of the gasket, which is easy to cause leakage. Therefore, flange connection system mainly relies on the outer side of gasket to ensure the sealing, but uneven distribution of compression force will cause the gasket to produce clearance, which will cause leakage.

Conclusion

  • ① The temperature distribution on the gasket is uniform in the circumferential direction, and there is a significant decreasing gradient from the inside to the outside.
  • ② Temperature has a great influence on the gasket. At high temperature, the expansion of gasket material is larger, and the pressure is more tight than the sealing surface of flange, and the stress on the lower gasket increases more.
  • ③ The temperature rise too fast, the deformation between gasket and flange sealing surface is different, which will cause leakage due to the compression force on gasket less than the sealing specific pressure.
  • Therefore, the temperature should be increased slowly in the process of heating, which can reduce the uneven phenomenon of the pressing surface.

Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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reliability analysis of octagonal gasket at high temperature - Reliability analysis of octagonal gasket at high temperature
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Reliability analysis of octagonal gasket at high temperature
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The finite element ANSYS software is used to simulate the octagonal gasket used in flange connection at high temperature, and the stress change on the gasket under high temperature condition is analyzed to verify the reliability of the gasket.
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