Casings of a turbomachine are typically divided into sections, such as an upper casing section and a lower casing section. The sections are joined by fasteners, such as by bolts and nut assemblies which are tensioned to provide a closing force on the joint between the upper and lower casing sections and thereby prevent
The inner working fluid of a turbomachine is at a high temperature and these high temperatures heat the inner casing of the machine. For example, steam temperatures in modern steam turbines may be at temperatures near 600° Celsius. These high working fluid temperatures heat the inner casing and cause the casing to expand. Similarly, the inner casing contracts as it cools after the working fluid has stopped flowing. The expansion and contraction of the inner casing can affect the fasteners that secure the casing together.
Materials each have a thermal expansion coefficient that indicates the amount of thermal expansion of the material for a standard temperature change. The thermal expansion coefficient of the metal forming the inner casing may be substantially different that the thermal expansion coefficient of the metal forming the bolts that secure the casing together.
If the thermal expansion of the bolts is substantially greater than the thermal expansion of the inner casing, the bolt will expand, e.g., lengthen, to a greater extent than the expansion of the inner casing. The pretension in the bolt which holds closed the joint between the upper and lower inner casing segments reduces as the bolt expands to a greater degree than the inner casing. This reduction in the pretension on the bolt results in leakage of the working fluid, such as steam, through the joint between the upper and lower inner casing segments.
The risk of pretension of the bolts securing inner casings being reduced has become more pronounced in recent years because the materials selected to form inner casings to withstand the hotter steam temperatures in modern steam turbines have a lower thermal expansion coefficient than do the bolts that secure the casings.
To allow for higher steam temperatures, the inner casing is often replaced with a new inner casing which is designed to better withstand the higher temperatures. The new inner casing is designed to fit in and be secured to the existing outer casing. Reusing the outer casing reduces the cost of upgrading the turbomachine, e.g., steam turbine. The materials used to form the new inner casing may have substantially lower thermal expansion coefficients that the bolts used to secure the inner casing.
A technical problem has arisen in recent years because inner casings with low thermal expansion coefficients are being fitted into existing outer casings and because the steam temperatures are increasing in steam turbines. Similar technical problems may be occurring in other turbomachines.
The conventional approach to this technical problem requires a relatively large clearance between certain portions of the inner and outer casings. There remains a need for a solution to the technical problem for inner casings having insufficient clearance with their outer casing.