Due to the need for production operations and the ease of manufacture, installation, overhaul, and transportation, containers are often constructed in a detachable configuration, with flange connections being the most common detachable connection. In addition to meeting the strength and stiffness requirements of the flange, the flange connection must also ensure that no leakage occurs between the flange faces under operating conditions. The flange connection relies on the bolt to compress the flange, thereby pressing the gasket to achieve a sealing effect. Therefore, how much pre-tightening force to apply to the bolt in advance has a very important effect on the flange meeting the above requirements.
1 flange sealing condition
No leakage occurs between the flange faces, and the following two conditions must be met when the bolts are pre-tensioned:
1) In the pre-tightening state, when tightening the flange bolt, the bolt force presses the gasket through the flange, forcing the gasket to compressively deform to fill the uneven surface of the flange sealing surface to prevent interface leakage [ 1]. At this time, the minimum gasket pressing force to ensure no leakage is Fa=3.14DGby[2], and the bolt pre-tightening force should be greater than or equal to Fa;
2) In the operating state, after the container is subjected to internal pressure, the bolt is stretched by tensile stress, and the flange sealing surface moves in a direction separating from each other, the gasket rebounds, the compression amount is reduced, and the gasket and the sealing surface are The pressing force is reduced [1]. At this time, the minimum gasket pressing force that is guaranteed not to leak is Fp=6.28DGbmpc[2]. If the deflection angle of the flange sealing surface is increased due to the internal pressure, the pressing force of the gasket rebound is reduced, and only the bolt elongation due to the internal pressure is considered, and the pressing force of the gasket rebound is reduced ΔF. , the bolt preload should be greater than or equal to Fp + △ F [3].
2 Influence of bolt preload on flange connection
1) For many low-pressure vessels and gaskets requiring high-pressure tension (such as metal-wound gaskets), the bolt pre-tightening force Fa calculated according to the pre-tightening state is greater than the bolt pre-tightening force Fp+ â–³ F calculated according to the operating state, and The blue intensity is determined by the flange moment Ma[2] in the preloaded state. The calculation of the flange moment Ma in the preloaded state is related to the actual bolt area Ab[2], which is intended to provide a certain margin for excessive tightening of the flange (tightening at normal temperature and before applying internal pressure). However, if the bolt is tightened blindly, the bolt pre-tightening force will cause the bolt to stretch due to yielding or the gasket will be crushed and extruded, resulting in leakage of the flange sealing surface; if the bolt pre-tightening force is too small, the gasket will be compressed. Since the deformation is insufficient, the unevenness on the flange sealing surface is filled and the surface is leaked. If the actual area Ab of the bolt is used to reduce the tensile stress of the bolt to improve the reliability of the seal, this will result in an increase in the thickness of the design flange, an increase in manufacturing cost, and an uneconomical.
2) For many high-pressure vessels and gaskets requiring low-pressure tension (such as rubber flat gaskets), the bolt pre-tightening force Fp+ â–³ F calculated according to the operating state is greater than the bolt pre-tightening force Fa calculated according to the pre-tightening state, and The blue intensity is determined by the flange moment Mp[2] in the operating state. In this case, if the bolt pre-tightening force is too large, in addition to causing the bolt to be stretched due to yielding or the gasket being crushed and extruded, it is also possible to make the originally designed flange increase due to the torque, thereby increasing the stress. The strength and rigidity are insufficient, resulting in leakage of the flange sealing surface; the bolt pre-tightening force is too small, and the pressing force between the gasket and the sealing surface is less than the minimum gasket pressing force Fp and causes leakage.
3 Method of controlling bolt preload
Methods for controlling the pre-tightening force of the bolt include a torque method, a measuring bolt elongation method, a nut rotation method, and a strain gauge method. The characteristics and applications of various methods are as follows: The torque method uses a torque wrench or a fixed torque wrench to control the preload force. It is a widely used method for controlling the preload force for a long time at home and abroad. The cost is low, and the error is generally considered to be ±25%. If the surface has a coating, a bearing surface, the surface quality of the thread is good, and the torque wrench shows an accurate value, the error is significantly reduced, and the lubrication control effect is better. The measuring bolt extension method is used to control the preload force when the bolt is in the elastic range. The error is ±3%~5%, which is troublesome and costly. For occasions with special needs.
The nut corner method is to first tighten the bolt pair to the "close" position and then turn it through a certain angle. The error is ±15%. Widely used in the automotive industry and steel structures in the United States and Germany. The strain gauge method is to apply a strain gauge to the unthreaded part of the bolt to control the tension of the bolt, and the error can be controlled within ±1%, but it is expensive [4]. Different working conditions can be used to control the bolt pre-tightening force. For example, if the medium is compressed air, the torque method or the nut angle method can be used; if the medium is explosive or highly hazardous, the bolt elongation method or strain is measured. The method is better.
4 Conclusion
In flanged connections, the bolt preload does not only affect the seal between the flange faces, but also the strength, stiffness and bolt strength of the flange. A reasonable preload value plays a very important role in the connection of the flange. Different working conditions can be used to control the bolt pre-tightening force, that is, to reduce the manufacturing cost and to consider the safety of the equipment.
references
[1] Ye Hong, Yan Tingwu, Liu Yuansheng. Pre-tightening force of bolts in flange connection[J]. Nonferrous Metallurgy, 2005, (3): 46-48.
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