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Thermal growth: how to identify, quantify and deal with its effects on turbomachinery

Thermal growth, as used in the field of machinery alignment, is 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)
L    (distance between shaft centerline and machine supports)

When machinery begins to generate heat, the temperature difference between 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 angularity between the two machine 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 OEM 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 different from operating conditions in the field. These differences can 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 the employment of measurement systems. Thermograms, for example, can provide a temperature profile of the machinery.
From that profile, it is possible to input the various temperature measurements into a simplified formula to approximate thermal growth. For thermal growth at the machine supports, for example, the formula in Figure 1 can be used.




Using these formulas can be a quick, easy, and cost-effective way to determine a machine’s thermal effects.

Their simplicity adds great value to an engineering or maintenance group by allowing the derivation of approximate, yet useful, thermal growth data without the complexities of true 3-dimensional heat transfer analysis, which may require advanced knowledge in the field coupled with state of the art software.

However, calculating thermal growth via simplified formulas is not always accurate. The effort towards simplification omits certain parameters such as Growth of connected piping; downstream temperature variations; nonhomogenous materials composition; non-uniform machinery cross-sections; uneven heating of materials; changes in ambient temperature during operation; variations in load; process effects; machine frame distortion due to torque; foundation problems; and mechanical looseness.
A more accurate way to calculate thermal growth is through real-time monitoring of machine positional changes using laser monitoring technology (Figure 3). Such monitoring of a particular unit is more precise than theoretical calculations, as one directly observes the actual behavior of the machines in question.



Depending on the measurement equipment and methods selected, one can determine the absolute movement of machines, relative movement between machines, and reveal the movement of the machines from sources other than thermal growth, such as process effects, load changes due to dynamic forces, machine frame or baseplate distortion, foundation problems or mechanical looseness. Monitoring real-time positional changes allow one to quantify the impact on shaft alignment.

All machinery, regardless of how efficient, will generate and lose heat and undergo thermal expansion (or contraction) of the machine frame.
Because all machines grow, machinery should be aligned with thermal effects taken into account.

This can be accomplished by using either target specifications, the prescribed misalignment values to be set at the coupling for machines in the at rest and in the unloaded condition, or thermal growth specifications, and the amount of positional change expected for the shaft centerline between the stopped and running conditions at the machine supports.

A machine’s thermal growth or target specifications should be discovered before performing an alignment in order to ensure smooth performance between coupled machines.

However, if machinery has already been aligned and placed into service and thermal growth problems are suspected, live monitoring of the machinery should be undertaken to verify proper alignment.

The monitoring of equipment can be done in one of two ways: From the at-rest, unloaded condition to the final running condition, or vice-versa. A system that accurately monitors and trends these positional changes are essential to this goal.

Such a system should also permit tagging specific events so as to be able to document the effects, such as changing the load, opening or closing a valve, starting nearby machines, and so on.

Author:
David Atehortua is an applications engineer for Ludeca, Inc., a provider of alignment, vibration analysis and balancing equipment manufactured by Prueftechnik AG. He can be reached at 305- 591-8935, David.Atehortua@ludeca.com or www.ludeca.com
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