Why Do Bolts Need To Have 2 To 3 Threads?
In the chemical production process, many equipment, facilities, pipelines, etc. need to be connected to each other, including pipeline flanges, equipment and bases, upper and lower sections of container devices, etc. Among them, the most commonly used, convenient and common connection method is bolt connection. The connecting parts are bolts and nuts, and sometimes there is a requirement to add gaskets in the middle. In the chemical industry, especially in the connection of pipeline flanges, high-pressure container equipment, etc., the bolts used for connection are usually high-strength bolts (generally stud bolts). High-strength bolts can provide sufficient tightening force to ensure that pressure vessels, flanges, etc. are tightly connected without media leakage.
Leakage prevention in the production system of chemical enterprises is a very important safety work in chemical production. If a dangerous chemical leaks, toxic, harmful, flammable and explosive materials may leak and spread into the atmosphere. If toxic materials leak, workers may inhale toxic materials and be poisoned. If flammable and explosive materials leak, they will reach the explosion limit after mixing with air, and then if they are exposed to open flames, they may cause an explosion. Therefore, in order to ensure leakage safety, it is usually necessary to check the parts that may leak, especially the flange parts, and the sealing of the flange mainly comes from the tightening of the bolts. Therefore, when checking the flange sealing, a standard requirement is usually mentioned, namely: "The nuts of the bolts used for flange connection are required to leave 2 to 3 threads."
Why is this? According to the Technical Specification for High-Strength Bolt Connections for Steel Structures, the standard number is JGJ82-2011, and in 6.4.1 it is stipulated that the length of high-strength bolts should ensure that the exposed threads of the bolts are 2 to 3 threads after final tightening. The main reason for adopting this regulation is:
(1) First of all, if a bolt that is too long is used, it will not only cause waste, but also make construction work more difficult, as it requires more turns. If an electric fastening device is used, the sleeve may not fit properly due to the overlong bolt.
(2) If the bolt is too short, the force on the nut will be uneven, and the nut will not be in full contact with the bolt (the nut cannot be fully threaded), resulting in a decrease in the nut's fastening ability, which may lead to damage to the nut or insufficient fastening force.
(3) Since the outermost 1 to 2 turns of the bolt are usually affected by processing, their strength will be weakened (in order to facilitate processing, the thread diameter is smaller than that in the middle), it is necessary to exclude the 1 to 2 turns outside the bolt to avoid a series of problems caused by insufficient strength.
(4) In the chemical industry, chemical equipment is usually fastened with high-strength bolts. High-strength bolts require sufficient tightening force to increase the friction of the connection surface, and sufficient force is required for tightening. As long as the fixed torque is reached and the force used reaches a certain level, the bolt can play a tightening role.
