The heat is on
All rotating machinery are subjected to thermal exposure. The machines will react depending on temperature and material. Either by expanding or shrinking. And that is a fact. Thermal growth is a serious thing when you think about it.
All rotating machinery are installed in trains. Trains means there is a driver, which is the motor, and driven which can be the pump, blower, compressor, or any different process machine. When rotating machinery is installed, precision shaft alignment is performed. Shaft alignment will ensure both shafts (driver and driven) are collinear. Collinear means that both rotational centrelines are positioned as if they were one.
When the machines are started the driver and driven heat up in very different ways. A compressor in a hot environment will quickly increase in temperature due to friction of its internal rotating parts, and compression of the media will generate and add more heat. Comparing to the driver, which can be an electrical motor, the situation is very different. The temperature will increase to a certain level and then remain the same. Two machines with two different behaviours.
So, what happens when one of them increase its temperature respectively to the other? It’s simple; the machine will start expanding. And when the machine expands, it will grow in all directions and move its rotational centre out of collinearity and cause misalignment. But not only misalignment. Since there is a change in the machine geometry, pipe strain might also appear adding more stress to the housing.
There are so many consequences of thermal growth in rotating equipment. Misalignment will for example also result in bent shaft. Bent shaft will result in unproper distribution of forces in the bearing which in turn will lead to failure of the lubrication. Therefore, we must be able to anticipate thermal growth by using available information from the OEM, or by performing the calculation by ourselves. How do we do that? The key is to identify how much growth is expected. This number must be used when performing the shaft alignment to “intentionally misalign” the machines prior to start. Let us use the compressor as an example again. If we assume that the compressor will operate at higher temperature than the motor, when aligning, we must place the compressor below the rotational centreline of the motor. How much will be determined by expected thermal expansion growth of the material.
When the machine is aligned considering the thermal growth, it must run and operate until it reaches its full operating condition. Then it must be stopped, and the shaft alignment must be verified. This is our test run of the machine to confirm proper and reliable installation to be able to achieve full operational life. We want to test before we go to full production to make sure our thermal expansion calculation was right. Think about aircraft maintenance. When there is an aircraft engine replacement, to make sure it is operating as it should, the pilots perform test flights until everything can be confirmed. And you don’t want to be on the plane knowing nobody performed the test run, do you?
Roman Megela Gazdova
Senior Reliability Engineer, Easy-Laser AB
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