Steam Turbine Case Opening – A case study


This document describes the technical operation performed by a bolt induction heater required for the bolts located on several parts of a steam turbine. It also includes quantitative data about the main application on turbine casing, Valve Bonnet and Steam Pipe Flange. The HP Outer Casing has the biggest and longest bolts of 158mm in Diameter and 1541mm in length. The smallest bolt that required bolt induction heater is on the HP No1 Dummy Ring. 2.¼” – 335mm length.


Our intervention was required for applying our bolt induction heating machine in several sectors of the steam turbine, namely high, intermediate and low-pressure turbine horizontal joints.
We intervened 7 days per week, 24 hours a day.

HP casing.

In high-pressure turbine horizontal joint, a total of 94 bolts have been treated with the induction machine. In particular, we operated in the Inner and Outer Casings, in the Nozzle Chamber and on Dummy and Inner Gland Rings. Concerning this joint, we had to operate loosening process on highly-diverse dimension bolts. In fact, the latter showed multiple values in terms of length and diameter. The first ranged from a maximum of 1517mm to a minimum of 145mm, whereas the latter shifted from ¾” to 6” (19mm – 158mm).

The machine was applied on steel alloys with high chromium content and It would have achieved a stretch value ranging from 1,88mm to 0.34mm.

IP casing.

In intermediate-pressure sector a total of 106 bolts have been treated with the induction machine. In this case we intervened not only on the Casing and on the 2 Blade Rings, but also on Dummy Rings, Diaphragm and Inner Gland Rings, dealing with multiple bolt dimensions. We had to operate on lengths ranging from 1400mm to 475mm and on a range of diameters shifting from 135mm to 65mm (5” – 2.¼“).

The bolts we had to stretch were different in terms of material (high chromium steel alloy) and in clearly in stretch value, which ranged from 1.45mm to 0.51mm.

LP casing.

The Inner Casing, the steam chamber, the stationary blade and the diaphragm of low-pressure turbine horizontal joint were the focus on the induction bolting machine application. We dealt with bolts ranging from 420mm to 185mm in length and from 76mm to 28mm in diameter.

In this sector we operated on special alloy to achieve a rod stretch ranging from 0.48mm to 0.40mm.

Valve (MSV, GV, ICV and RSV) Bonnet.

In order to perform machining of MSV, GV, ICV and RSV valves, we had to operate on the Valve Bonnet, counting a total of 244 bolts. The latter, of about 500mm in length, and 3” in diameter, required a stretch of around 0.6mm. Moreover, our bolt induction heating system was applied even on a 9mm bolt, in order to perform machining of a RSV valve.

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