Bolt heating on a steam turbine
The use of induction heating for bolt expansion in high-pressure turbines is a powerful business tool.
This case story from Italy illustrates the benefits—speed, reliability, safety—that make Minac induction heaters ideal for heating large bolts.
The canals and architectural splendour of Venice make it easy to forget it is also a large modern city, a place that around a quarter of a million people call home. And that’s not counting the tourists, 50,000 of whom clog the waterways and piazzas every day. Then there are the city’s surrounding industrial areas. Nearby Porto Marghera, for example, is one of Italy’s most important chemical processing centers.
All the above means a reliable power generation and distribution system is essential. And with any critical system, it is vital to keep scheduled downtime to a minimum. This is particularly true for turbines, as even a few hours of unplanned outage can result in major revenue losses. The time pressure facing service engineers is made even worse by the technology traditionally used to expand and remove the large bolts that secure turbine covers. Alessandro Mariani of EFD Induction Italy explains: “Open flames and resistance heaters have serious drawbacks. The former method is slow, uncomfortable and inaccurate— and there’s a risk of excessive heat input damaging the threads. The latter has reliability issues, with a risk of rods melting inside the bolts. So when Alstom Power in Italy was performing planned maintenance on a turbine in Marghera just outside Venice, they asked us if induction heating was a viable alternative.”
Mariani continues: “We held a demo in Milan for Alstom where we showed how our mobile Minac systems, complete with special elongated coils, are perfect for bolt expansion. We covered a lot of ground; even explaining the benefits of a Minac ‘Twin’. This is a single converter connected to two independent handheld transformers. One operator can work on two bolts at the same time—in effect doubling his productivity. Or two operators can work simultaneously, each with their own transformer.” Alstom were sufficiently impres - sed that they ordered two Minac 25/40 systems. Although the outer dimensions of each Minac is only 345 x 708 x 453mm, the system’s handheld transformer delivers a maximum output power of 40kW. When using a Minac, the heating process is so quick there simply isn’t time for heat to travel into the thread area. And thanks to electronic controls, key process parameters such as temperatures, ramp-up and dwell times can be set in advance and repeated with amazing accuracy.
According to Mariani, Alstom were delighted with Minac's performance in Venice. "Sure, the job was completed much faster than it would have taken had traditional methods been used. No damage was caused to nuts or bolts. And the operators really appreciated Minac's comfort and safety. In short, nothing unexpected happened. Minac did what it does best—it got the job done."