Why should 2–3 threads be left on the bolt

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Why is it necessary to leave 2–3 threads on the bolts? In the chemical production process, many devices, facilities, and pipelines need to be connected to one another, including pipeline flanges, equipment and their bases, as well as the upper and lower sections of containers. Among all the connection methods available, bolted connections are the most widely used, convenient, and common. The connectors are bolts and nuts; sometimes, the use of washers in between is required. In the chemical industry, especially for the connection of pipe flanges, high-pressure container equipment, etc., the bolts used for such connections are typically high-strength bolts (usually double-headed bolts). High-strength bolts can provide sufficient clamping force to ensure that pressure vessels, flanges, and other connections remain tight and free from leakage of fluids. Leak prevention in the production systems of chemical enterprises is a very important safety aspect in chemical manufacturing. If hazardous chemicals leak, toxic, harmful, flammable, or explosive substances can be released into the atmosphere. If toxic materials leak, it can cause workers to inhale them, leading to poisoning. If flammable and explosive materials leak, and when these substances mix with air they reach the explosive range, then the presence of an open flame can lead to an explosion. Therefore, to ensure leakage safety, it is generally necessary to inspect the areas prone to leakage, especially the flanges, as the sealing performance of flanges relies primarily on the tightness of the bolts. Therefore, when checking flange seals, a standard requirement is often mentioned, which states that: \"The nuts of the bolts used for flange connections should have 2 to 3 turns of thread left.\" According to the \"Technical Code for High-Strength Bolt Connections in Steel Structures\", with the standard number JGJ82-2011, clause 6.4.1 stipulates that the length of high-strength bolts should ensure that 2 to 3 threads of the bolt are exposed after final tightening. The main reasons for this requirement are as follows: (1) Firstly, if overly long bolts are used, it leads to waste; moreover, it makes the installation process more difficult, as a larger number of tightening rotations are required. When electric tightening devices are used, the inappropriate size of the sleeves caused by overly long bolts may prevent the use of such devices. (2) If the bolt is too short, it will result in uneven stress distribution on the nut; the nut cannot make full contact with the bolt (it cannot be fully tightened), which reduces the nut’s tightening capacity and may lead to damage to the nut or insufficient tightening force. (3) Since the outer 1–2 turns of the bolt are usually affected by machining, their strength is reduced (the diameter of these outer turns is smaller compared to that of the central threads to facilitate machining); therefore, it is necessary to exclude these 1–2 turns from consideration in order to avoid potential problems resulting from insufficient strength. (4) In the chemical industry, bolts used for tightening chemical processing equipment are typically high-strength bolts. Such bolts require sufficient tightening force to increase the friction between the connected surfaces; adequate force is needed during tightening. Once a fixed torque is reached and the applied force reaches a certain level, it can serve to tighten the bolt.