Structural design based on the large flange heat exchanger leakage problem solution

The possible causes of leakage of large flanged heat exchangers are analyzed, and some feasible solutions are proposed focusing on the design of the sealing structure.
The flanged heat exchanger has been widely used in petrochemical plants because of its simple structure, low cost, and easy installation. In recent years, with the large-scale device, a large heat exchanger (refers to the diameter – pressure combination of large) is used more and more; in most cases, even if the diameter of the heat exchanger flange has exceeded the existing standards, people still tend to use the flange structure, and consequently, the thorniest problem is the heat exchanger sealing structure of the leakage. With the increasing health, safety, and environmental protection (HSE) standards, how to solve heat exchanger leakage has become an increasingly important issue; this analysis proposed some feasible solutions.

1. Large flange heat exchanger connection structure sealing performance analysis

The traditional flange connection structure of the seal belongs to the forced seal. Sealing between the flange rely on the main bolt pre-tensioning effect of uniform pressure gasket, so that it produces the right amount of deformation gasket due to deformation rebound, thus forming a tight barrier between the gasket and the flange so that the medium through the barrier must be greater than or equal to the internal pressure drop. In petrochemical installations, heat exchangers, this sealing structure leakage problems often occur; when the heat exchanger diameter is large, the possibility of this structure leakage will increase. Analyze the causes of leakage is often very complex, operating conditions (such as temperature and pressure fluctuations, heat exchanger vibration, high temperature and high-pressure bolts and gaskets under the stress relaxation, etc.), material selection, structural design and calculation methods, manufacturing quality, installation, and so on, may lead to leakage of many aspects. Although there are many reasons for seal failure, there is no doubt that the traditional flange sealing form and design method for large diameter (diameter-pressure combination beyond the existing flange standard) heat exchanger has some inherent defects. For the sake of simplicity, the author only on the design method and flange seal structure form two aspects of analysis.

1.1 Design method

At present, the heat exchanger flange seal design is generally used in the traditional design method specified in GB150, this method is based on the stress limit for the design criteria, based on the stress limit of the design method cannot provide a guarantee of sealing performance, only to avoid plastic deformation of the flange as a whole, but there is no quantitative analysis of leakage of sealing structure, and thus cannot understand the degree of possible leakage of the designed structure.

1.2 Structural forms

Conventional flange sealing structures usually have the following problems.

• a. Large-diameter heat exchanger will use a multi-pipe process; there is a large temperature difference, so the flange and bolts produce a large deformation difference, easy to cause some parts of the gasket stress to be lower than the minimum value of the sealing conditions, resulting in the occurrence of leakage.
• b. When the working pressure is high, the flange of a heat exchanger with a larger diameter requires a larger bolt load, resulting in a larger or more bolt size or quantity, increasing the flange’s radial force arm. In addition, the larger the cross-sectional area, resulting in poor overall rigidity, reduces the reliability of the seal, and the leakage rate of the sealing structure increases linearly with the working pressure;
• c. Due to the bolt load being large, to avoid over-pressurized gasket failure, the gasket needs a larger width. The wider the gasket, the worse the radial uniformity of the gasket stress, the greater the possibility of leakage.
• d. The larger the diameter, the worse the preload uniformity; some parts of the gasket are due to excessive preload resulting in plastic deformation and increased likelihood of failure.
• e. The larger the diameter, the sealing surface of the more difficult to process, the more difficult to ensure flatness.
• f. The larger the diameter, the gasket material, and the size, the more difficult to ensure uniformity.

2. Leakage solutions

Based on the above analysis, the heat exchanger diameter is recommended to exceed the existing flange standard range, and the sealing structure design should be given special consideration; the following provides various solutions for different situations.

2.1 The use of traditional flange sealing structure but with different design methods

2.1.1 Traditional sealing structure design method

The traditional design method is still a convenient choice for general leakage rate requirements and the diameter-pressure combination is similar to the standard range of flanges. The existing specification design has a long and widely successful experience in the standard range. It can meet the general requirements, so if not much beyond the standard range of the flange, special consideration based on the traditional design method to increase the bolt cross-sectional area and flange torque margin (recommended 20%-30%), and the maximum optimization of the flange structure dimensions, you can make up for the shortcomings of the sealing design.

2.1.2 Seal structure design method based on strength and stiffness requirements

The traditional design method is based on the strength of the destruction of the failure criterion; this method assumes that the flange is completely rigid and is applied to the existing standard flange is reliable. When diameters exceed the standard range, the flange may not be able to control leakage of its seal due to insufficient stiffness. For such a situation, the ASME-VII-12007 version of the flange design standard formally introduced the rigidity of the flange design guidelines as a mandatory requirement; the flange, in addition to meeting the conventional strength requirements, should also meet the stiffness requirements, that is, the requirement of the stiffness index J ≤ 1.0. For the monolithic flange.

J=52.14VM₀/(LEg₀²K_Ih₀)≤1.0 (1)

In the formula:

• V, L – flange coefficients;
• M₀ – a total bending moment of the flange, N-mm.
• E – modulus of elasticity of the flange material, MPa.
• K_I–Stiffness coefficient, K_I=0.3.
• g₀, h₀ – structural parameters of the flange, mm.

According to the strength of the preliminary determination of the structural dimensions of the flange, according to the formula (1), check the stiffness of the flange when the calculation of J>1.0 should increase the thickness of the flange ring until J ≤ 1.0.

2.1.3 Based on the strength and tightness requirements of the seal structure design method

Based on the strength and stiffness requirements of the seal structure design method, the traditional design method has made great progress in many working conditions and proved appropriate. However, this method for sealing the consideration still needs to have the systematic operation of the required gasket stress, is only determined empirically, and cannot quantitatively calculate the degree of leakage; that is, it cannot guarantee the leakage rate within the prescribed range. The U.S. Pressure Vessel Research Council (PVRC) proposed a flange sealing structure based on the strength and tightness of the failure criterion design method, a better solution to this problem. This method takes the sealing failure criterion as one of the basic criteria for the design of the flange sealing structure and explicitly puts forward the tightness as a measurement index to characterize the leakage degree of the sealing structure so that the flange design requirements can be differentiated according to the level of tightness. The different levels of tightness correspond to different minimum leakage rate requirements. The specific methods of this method are as follows.

• a. According to the working conditions and related requirements, determine the gasket quantitative tightness (leakage rate) requirements.
• b. According to the tightness requirements of the gasket leakage test, determine the gasket coefficient and get the specified tightness parameters under the optimal gasket assembly stress and the relationship between the amount of change in gasket stress.
• c. Calculate the optimum bolt load required to achieve the specified tightness.
• d. After calculating the bolt load, the strength calculation of the flange is still carried out by the conventional method.

This design method associates the gasket stress with the leakage rate, which meets the requirements of strength and sealing simultaneously and makes up for the lack of sealing criterion in the traditional design method. Moreover, due to the division of the tightness level, this design method provides more flexibility than the traditional design method, which is especially important in the case of toxic media and better meets the requirements of HSE.

2.1.4 Finite element stress analysis design method of the seal structure

The finite element stress analysis design method will be a flange, gasket, and bolt as a whole to study and systematically consider the uniformity of the gasket force, external load, the stiffness of the connection system, and the deformation caused by temperature differences and temperature and pressure fluctuations. It can calculate the deformation of the flange, the non-linear characteristics of the gasket material, and the influence of the bolt bending on the stress distribution of the gasket, predict the real complex deformation of the sealing structure – sealing behavior, and establish a quantitative relationship with the leakage, a more effective and accurate evaluation of the flange connection system of the strength and sealing performance, and at the same time, can reduce the workforce and material resources required for the leakage test of the gasket.

2.2 Improve the traditional flange sealing structure design

2.2.1 The use of disk spring washers between the bolt connection

When the flange diameter is large, the uniformity of the gasket reaction force is poor. When the pressure and temperature are high, especially when the operating conditions fluctuate frequently, the gasket is required to have sufficient and stable rebound is very difficult because the initial sealing with a large pre-stress gasket on the one hand by the temperature and pressure will produce significant creep and stress relaxation; on the other hand, the gasket material oxidized or thermally decomposed, and its yield limit is reduced, resulting in increased plastic deformation. These two factors will lead to a gasket resilience performance decline, high temperature and pressure, and frequent fluctuations in operating conditions. At the same time also easy to cause stress relaxation of the bolt when the gasket resilience is not enough to compensate for the temperature, pressure caused by the sealing surface separation, and high-temperature sealing structure of the creep relaxation will occur when the leakage of the medium. In this case, the bolt connection between the addition of disc spring washers can be timely compensation for the sealing gasket rebound deficiency. Disc spring gaskets made of highly elastic alloy materials, with low stroke and high compensation features, can provide enough preload through a small deformation, absorbing temperature and pressure fluctuations caused by stress relaxation and better compensation for the flange and sealing material deformation.

2.2.2 O-ring self-tightening sealing gasket

When the pressure-diameter combination of a large flange sealing structure through the design optimization and high-strength materials and flange size are still too large, we can consider using an O-ring self-tightening gasket, as shown in Figure 1. When the bolt is tightened, the O-ring is flattened to produce a rebound and form the initial seal of the flange connection. There are several small holes inside the ring, which the pressurized medium can be passed into. During operation, the resilience of the O-ring and the medium pressure make the ring close to the sealing surface to achieve the purpose of sealing. This gasket sealing pressure is winding gasket 1/7-1/2, so the use of an O-ring can greatly reduce the flange and bolts load, reduce.

Figure.1 O-ring self-tightening gasket
Reduce the size of the connection system; the higher the working pressure of this structure, the better the sealing. According to the need, the O-ring can be used with good elasticity of the metal material; the ring can also be covered with flexible compensation materials, such as expanded graphite and PTFE.

2.2.3 Diaphragm gasket

Using a diaphragm seal (Figure 2) or a Ω-shaped seal can completely solve the leakage problem, and the non-self-tightening sealing force required to reduce to a minimum. As shown in Figure 2, the inner side of the diaphragm and flange welding, the upper and lower two diaphragm pieces in the outer side of the mutual welding, completely closed the medium leakage channel. This sealing structure of the flange and bolts only bear the working pressure and no gasket reaction force, so the size of the flange and bolts can be further reduced, but this structure of the heat exchanger cannot be disassembled frequently; the choice of this structure when the heat exchanger cleaning should try to use the chemical method.

Figure.2 Diaphragm gasket

3. Conclusion

The analysis of large flange heat exchanger leakage causes for its sealing structure proposed various feasible design methods. In the verification application, the effect is very satisfactory for designing similar equipment to provide a new way of thinking.
Author: Tan Jiyan