Three ways to prevent high temperature fasteners

During the use of fasteners, sometimes the temperature is very high. Under the operation of high temperature fasteners, it is easy to damage the performance of the fasteners and even some failures. For example, bolts are important connecting parts widely used on boilers, steam turbines and steam pipeline valves of high-tech power plants. For high temperature fasteners in these areas, what measures should we take when using them?

1. Sealing stress: Ensuring the tightness of the connecting parts is a basic requirement of bolt connection. When the flange geometry, the processing quality of the joint surface, the pressure and temperature of the steam and other conditions remain unchanged, the sealing stress is determined by the tightening force of the bolt. (Guide: Methods of preventing loosening of threaded fasteners)

2. Temperature stress: When the unit starts and runs, due to the temperature difference between the flange and the bolt (the temperature of the flange is higher than the bolt), and the linear expansion coefficients of the two materials are different, the free thermal expansion of the flange If it is larger than the bolt, the bolt is subjected to an additional tensile force. The stress caused by the additional tensile force in the bolt is called temperature stress.

3. Thermal tightening of bolts: Bolts that are subjected to pre-tightening force during installation, when put into operation, under the action of steam pressure, the force of the bolt is increased than the initial pre-tightening force. For horizontal flanges, the tightening force of the bolts is increased by 5%-15%, so that the initial stress of the bolts during cold tightening can be lower than that. In order to meet the requirements of this initial stress, it is sometimes impossible to tighten the bolts to the required degree by manpower in the cold state. Therefore, it is usually necessary to use a hot tightening method.

First tighten the bolt to a certain degree (cold tightening) by manpower, and then use a special electric heater to heat the bolt through the bolt center hole. After the bolt is heated and stretched, tighten the nut. This method is called heat tight. Controlling the thermal tightening makes it easier to control the pre-tightening force.

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