Induction Bolt Heating: How It Works, How Long It Takes, and When Power Plants Use It

During a turbine outage, removing and reinstalling large bolts can take a significant amount of time. Years of heat, pressure and service can make those fasteners difficult to loosen. Induction bolt heating gives maintenance crews a controlled way to heat the bolt, release its clamping force and keep the job moving.

How does induction bolt heating work?

For a hollow bolt or stud, a technician inserts an induction coil, often called a bolt heating inductor or wand, into its center bore. Alternating current flowing through the coil creates a magnetic field. That field induces electrical currents in the metal bolt, generating heat within the bolt itself.

As the bolt heats, it expands slightly. During disassembly, this expansion helps reduce the force holding the nut in place so the crew can loosen it. During assembly, the nut can be positioned while the bolt is expanded; as the bolt cools and contracts, it develops the specified clamping force. The crew follows the equipment manufacturer’s bolting procedure for the required heating, sequence and final preload.

Because the heat is generated in the bolt, induction heating can work faster than methods that rely on heat traveling inward from an external source. It also avoids an open flame at the work area.                                                                                                                                                                 

How long does it take to heat a bolt?

There is no single heating time for every job. Bolt diameter and length, bore size, alloy, target temperature, coil fit and available power all affect the result.

Thermo International has reported these examples:

• A 2-inch-diameter, 24-inch-long bolt with a ½-inch bore heated in under 30 seconds.
• An 8-inch-diameter, 50-inch-long bolt with a ⅞-inch bore heated in about five minutes.

These are application examples, not guaranteed times for every bolt. To estimate a specific job, Thermo needs the bolt dimensions and material, bore size, equipment details and maintenance procedure.

Heating time also matters across the entire bolting job. A few minutes saved on each of many turbine casing bolts can make a meaningful difference to an outage schedule.

When do power plants use induction bolt heating?

The most familiar application is turbine casing maintenance. Crews heat large bolts and studs to open a turbine for inspection or repair, then use controlled heating again during reassembly.

Induction bolt heating is also used on fasteners associated with steam control and stop valves, high-pressure flanges, generators and other heavy equipment. It is particularly useful when large fasteners must be handled repeatedly and the crew needs a predictable heating process.

Choosing equipment for the job

Power alone does not determine performance. The machine, operating frequency, inductor design and fit within the bolt bore all affect how efficiently heat reaches the bolt.

Thermo International offers 60 kW and 120 kW portable induction heating systems and manufactures bolt heating inductors for a range of turbine bolt bore sizes. Its team can help match the machine and inductor to the fasteners and work planned for an outage.

For an upcoming turbine or valve maintenance project, share your bolt dimensions, bore sizes and outage requirements with Thermo International. We can help you identify an induction bolt heating setup suited to the work.

Thermo International
www.ThermoInternational.com | 973-970-9500