There are three (3) ways of tensioning a large diameter bolt, which are mostly for industrial application. Graphically shown below are method of stretching a bolt to attain clamping force on the joint or mating parts.
In the heating method, bolt is thermally tension while in the torque method and mechanical stretching, the bolt is cold tension. There are three (3) types of bolt heater as illustrated.
There are challenges in the industry when it comes to tensioning a large diameter bolt. Unlike torque method, Hydraulic Bolt Tensioning is very efficient and accurate.
It is the fastest and most accurate means of tightening and loosening bolts through tensioning. Nonetheless, the industry is also supporting the use of induction bolt heating.
The contradiction between mechanical tensioning and thermal bolt tensioning technologies lies in the refurbishment of existing turbines. The investment in replacing bolts designed for bolt heating with bolts designed for tensioning is generally prohibitive. For these power plants, the option is to use an induction bolt heater because of substantial reduction on downtime. The traditional bolt heater has a safety concern (risk of burn), dangerous and time consuming. Swift and efficient bolt removal is vital especially for baseload supplier of kWh.
To highlight the heating time of typical sizes of stud bolts in a steam turbine Coal-Fired Power Plant here in the Philippines. For a bolt size of M56 with a bolt length of 320 mm, the heating time is two minutes. The bolt could already be loosened or tightened after two minutes.
For a bolt size of M110 with a bolt length of 525mm, the heating time is six minutes. The bolt could already be loosened or tightened after six minutes. The temperature inside the center hole of the bolt where the work coil is inserted is 500 °C, which is very high but not enough to alter the microstructure of the stud bolt material.
During turbine dis-assembly, the savings in outage schedule will free-up a lot of time for inspection, minor and emergency repair of some turbine parts. The use of induction bolt heater vs. traditional bolt heater will save you 20 hours of your downtime during turbine re-assembly.
