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Author: Site Editor Publish Time: 2025-02-07 Origin: Site
How to determine the bolt load and torque when tightening the flange may be a topic of interest to everyone. First, two questions are raised:
* For bolts below M36, how to find the corresponding torque value if the bolt load is known?
* For bolts that can be hydraulically stretched, how to find the corresponding bolt load without flange calculation?
When designing flanges or consulting flange calculation reports, you can find the bolt tension during flange pre-tightening and operation. For bolts below M36, a torque wrench can generally be used. Now that you know the bolt load, how to find the corresponding torque value? You can refer to GB/T16823.2-1997 "General Rules for Tightening of Threaded Fasteners" or related materials to find the corresponding torque value. For bolts that can be hydraulically stretched, you can refer to the data of the corresponding gasket manufacturer to know the bolt load. It is simpler to directly take the 45% yield strength of the bolt material to calculate the load of each bolt. This is the bolt loading torque I calculated: using a torque wrench, the gasket is a spiral wound gasket (steel ring gasket can be used by analogy), for reference only.
According to the provisions of '9 Flange' in GB150-1998 "Steel Pressure Vessels" P94, the gasket clamping force is obtained, and then the torque of each bolt is calculated based on the relationship between force and torque. The size of the high-pressure flange is: DN6' PN1500class (spiral wound gasket seal), and its flange preload force is calculated as follows:
1. Check HG20592~20635-97 "Steel Pipe Flanges, Gaskets, Fasteners" HG20631-97 flange sealing surface outer diameter d＝216mm.
2. Check HG20631-97. The inner diameter of the DN6' PN1500class D-type spiral wound gasket is D2=171.5mm, the outer diameter of the spiral wound gasket is D3=209.6mm, the gasket sealing width is N=19.05mm, and D3<d.
3. According to GB150-98 P91 Table 9-1 1a gasket basic sealing width b0=N/2=19.05/2=9.525mm>6.4mm.
4. According to GB150-98 P94 9.5.1.1 gasket effective sealing width b=2.53=2.53=7.81mm.
5. According to GB150-98 P94 9.5.1.2, the diameter of the center circle of the gasket clamping force DG = D3-2b = 209.6-2 * 7.81 = 193.98mm.
6. Check GB150-98 P93 Table 9-2 for the gasket coefficient m = 3.00 and the specific pressure y = 69MPa for the wound gasket. The design pressure of the pipeline is 15.85MPa and the operating pressure is 14.4MPa.
7. According to GB150-98 P94 9.5.1.3, the minimum gasket clamping force required in the pre-tightening state FG = Fa = 3.14DGby = 3.14 * 193.98 * 7.81 * 69 = 328236.4N.
8. According to GB150-98 P94 9.5.1.3, the minimum gasket clamping force required in the operating state is FG = Fb = 6.28DGbmpc = 6.28*193.98*7.81*3.00*14.4 = 411009N.
9. According to the relationship between force and torque N = 0.2Fd, the flange uses M36*3 fastener bolts, 12 pairs of fastener bolts, and the actual thread force diameter is d = 33.
10. The loading torque of each bolt in the pre-tightened state is Na = 0.2 (FG/12) d = 0.2* (328236.4/12) * (33/1000) = 180N.m.
11. Under the operating state, the loading torque of each bolt Np＝0.2（FG/12）d＝0.2*（411009/12）*（33/1000）＝226N.m.
The above is the torque required to load each bolt calculated according to the flange requirements of GB150. When applied to actual work, the torque wrench can fully meet the requirements. However, in actual work, the torque wrench is operated according to 30%, 50%, and 100% pre-tightening torque. Under the action of 100% pre-tightening torque, tightening a few more times can completely solve the problem.
Confirmation of torque:
1. Let’s talk about the conversion of load and torque first. The torque wrench manufacturer has a corresponding table to check. There is also a formula in the theoretical mechanics textbook. A coefficient in the formula is a range, which needs to be determined according to the actual situation.
2. Experiments have been done to verify the change of load by attaching stress sheets to bolts. The conclusion is: the coefficient is within the recommended range, but the change is relatively large. This is closely related to the machining accuracy of the bolt thread, the degree of lubrication, the finish of the nut surface and the flange surface, the matching state of the nut and the bolt meshing, etc.
3. Therefore, there is a big difference between the theoretical calculation and the actual result.
4. Of course, the use of a torque wrench is a big step forward compared to the traditional method.
About the law of the force change effect of adjacent bolts during the bolt tightening process:
1. Analysis of the influence of the force on each bolt during the bolt tightening process. No matter what kind of gasket is used, in order to ensure the sealing effect, a corresponding sealing pressure ratio is required. During the bolt advancement process, since the bolt force is gradually tightened and increased, the axial force generated by the sealing pressure ratio is unevenly distributed among the bolts. When tightening a bolt, the force on the adjacent bolts will be reduced.
2. Practical example: During the process of tightening the bolt according to the specified torque, if a bolt is loaded, the load on the adjacent bolts will immediately decrease.
3. When the load reaches the specified value and is still required to be loaded for some reason, the loading power must be far greater than the resistance. Our test results are on average above 120%.
4. A more effective method: After several turns, tighten the bolts that are separated by an increased load (exceeding the theoretical load), and then tighten the adjacent bolts according to the theoretical load. In this way, for a flange, the load of each bolt forms a relatively uniform load curve.
Flange connection is an important connection method for pipeline construction. Flange connection is to fix two pipes, pipe fittings or equipment on a flange plate, add flange gaskets between the two flange plates, and fasten them together with bolts to complete the connection. Some pipe fittings and equipment already have flange plates, which also belong to flange connections. Flanges are divided into threaded (threaded) flanges and welded flanges. Low-pressure small diameters have threaded flanges, and high-pressure and low-pressure large diameters use welded flanges. The thickness of flanges of different pressures and the diameter and number of connecting bolts are different. According to different pressure levels, flange gaskets also have different materials, ranging from low-pressure asbestos gaskets, high-pressure asbestos gaskets to metal gaskets. Flange connections are easy to use and can withstand greater pressures. Flange connections are widely used in industrial pipelines. In homes, the pipeline diameter is small and low pressure, and flange connections are invisible. If you are in a boiler room or production site, there are flange-connected pipes and equipment everywhere.
This article explores the relevant knowledge of bolt load and torque. Through the analysis of the force principle of bolts, we have clarified the characteristics of tensile, shear and composite loads that bolts bear in different application scenarios. At the same time, torque, as a key factor in achieving bolt preload, its relationship with bolt load is elaborated in detail. Correctly selecting the torque value is crucial to ensuring the reliability and safety of bolt connections.
In actual engineering applications, the load calculation and torque setting of bolts need to comprehensively consider multiple factors such as material properties, connection methods, and working environment. Through scientific analysis methods and rigorous experimental verification, the bolt connection design can be effectively optimized and the overall performance and reliability of the system can be improved.
In short, the load and torque of bolts are important links that cannot be ignored in mechanical design and engineering practice. Only by deeply understanding and reasonably applying relevant knowledge can we ensure that bolt connections can play their due role under various complex working conditions and provide solid guarantees for engineering safety and stable equipment operation.
