TURBOMACHINERY INTERNATIONAL • May/June 2012
Thermal growth, as used in the field of machinery alignment means machine frame expansion resulting from heat generation. The generation of heat, of course, is caused by operational processes and forces. Materials subjected to temperature changes from heat generation will expand by precise amounts defined by their material properties.
In turbomachinery, thermal growth results from the temperature differences occurring between the at-rest and running conditions. Generally speaking, the greater the temperature difference, the greater the thermal growth.
The magnitude of the growth can be calculated from three variables:
?T (temperature difference),
C (coefficient of thermal expansion), and
L (the distance between the shaft centerline and the machine supports).
When machinery begins to generate heat, the temperature difference between the at-rest and running conditions will cause thermal expansion of the machine frame, thereby bringing about the movement of the shaft centerlines.
This can produce changes in the alignment affecting the offset and/or angularity between the two machines’ shafts.
If misalignment beyond permissible tolerances occurs in the running condition, it can be observed from both high vibration and excessive power consumption. Operating machinery that is subject to thermal growth without taking into account its effects will result in a loss of efficiency, performance, and reduction in machine or component life.
Relying on the Original Equipment Manufacturers’ data sheets may not be enough as their calculations are performed on a test unit, under specified operating conditions, loads, and field conditions which may be significantly different from operating conditions in the field. These differences can drastically affect the amount of thermal growth observed on a unit in service.
Quantifying thermal growth accurately on turbomachinery to determine the amount of positional change between the machines requires expertise and/or the employment of measurement systems.