Shaft Alignment: Why Thermal Growth Matters

There are significant differences between a machine that is offline and that same machine when it’s running under normal operating conditions.

“The most notable of these changes is the change in the temperature of the machine bearings and supports. This is called the machine’s thermal growth.”

What is thermal growth?

“Thermal growth is the change in the length of a particular metal as a result of the change in temperature of that metal. Typically, when a metal bar is heated, it will get longer. These changes can be very small (0.0005 in.) or they can be very large, depending on the length of the piece of metal and its coefficient of linear expansion.”

What does thermal growth do in real conditions?

“In reality, the temperatures of all the machine supports will change; however, they will almost never change equally.”

For example: the drive end bearing temperature might change by significantly more than the outboard end bearing temperature. If you align to perfect zero when cool, the motor-pump will suffer from both an offset and angular misalignment when turned back on.

Legacy shaft alignment systems will allow “the user to program the foot targets of the machine being aligned” in order to account for thermal growth. However, these targets depend on the user correctly calculating thermal growth beforehand, which is very difficult.

How do you calculate thermal growth?

“The formula used for this calculation is often referred to as the T x L x C formula. T represents the change in the materials temperature in degrees Fahrenheit, L represents the length in inches of the material, and C represents the material’s coefficient of linear expansion. Different materials have different C values. Using the formula, we can anticipate the change in a machine’s shaft alignment based on the expected changes in machine temperature.”

However:

Gearboxes are difficult.

Thermal changes in gearboxes can be especially difficult to calculate. Often the input shaft temperatures will be different from the output shaft temperatures. This causes the gearbox shaft alignments to change in the horizontal plane as well as the vertical plane.

Force-lubricated systems with an oil cooler also can have an effect on the final alignment condition of a machine. Higher oil temperatures out of the cooler will result in a hotter operating condition of the machine, therefore creating a more drastic change in the running alignment condition. A 10 F change in the operating temperature of a turbine from 105 F to 115 F can change the foot positions as much as 2-4 mils. The alignment condition of turbines and compressors that operate at very high speeds can be adversely affected by these relatively small temperature changes.

Pipe strain

Another condition that changes is the increase or decrease in temperatures of the suction and discharge piping attached to pumps and compressors. Some compressors may actually form ice on the suction end while the discharge piping is too hot to touch. Conditions such as these can force major changes in the operational alignment condition of machines.

While original equipment manufacturers might be able to anticipate the nominal changes in operating temperatures of a piece of equipment, they cannot accurately anticipate the effects of the piping configurations of the final machine installation or the changes in the temperature of the piping runs. Piping runs are typically very long and can have a tremendous impact on the change in the shaft alignment from off line to running condition. In addition, piping connections act as fixed (or restraining) points with respect to the tendency of machines to move/grow when on line. The effect of these fixed points on the final position of the machines is almost impossible to calculate or predict.

Depending on the piping configuration, these changes may be in the vertical plane or in the horizontal plane and are extremely difficult or impossible to accurately calculate based on the TLC formula above.

How do you get an accurate measurement, then?

In the past, you had a limited menu of imperfect and labor-intensive options.

Today, the best method for shaft alignment is one that performs the measurement while the machine is running in normal operating conditions.

RedAlign is the only shaft alignment technology to do it.

(All quotations from Understanding Shaft Alignment: Thermal Growth)