Thermal Growth Factor on Motor Side When Do Alignment 2

[Rafael Dos Santos]

Greeting everyone..

Before this, I've only known thermal growth on pump side or turbine side. Until recently someone from sulzer,their field service representative (fsr) come and told my friend (the one that deals with him) that we need to put thermal growth factor of motor side into account before do alignment.

So, my questions are, what temperature and what type of motor that we need to consider the thermal growth will affecting the result of alignment..?

 

[TugboatEng]

On diesel engines the normal operating temp is 180°F. The gearboxes are expected to run at 120°F. Those numbers determine the height offset. An electric motor is typically expected to run at 40°C over ambient at rated power so that should give you a clear starting point if the motor is running at rated power. Just make sure you're coupling can handle all alignments at all temperature ranges and you'll be good to go. Elastomeric couplings are great if the service temperature is low because they don't wear as they compensate for misalignment.

 

[electricpete]

To estimate motor thermal growth, you’d be looking to estimate the average temperature of the structural steel below the bearings (assuming it’s the same on both ends, if not that’s an additional complication). That is generally going to be only slightly higher than the externally visible temperatures, so you have a pretty good starting point to estimate it from external thermal images. You may find that the correction is negligible.

Of course the more accurate approach is to measure thermal growth rather than calculate it. For that an use something low tech like Acculign / Essinger bars, or something high tech like Permalign from Ludeca.




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(2B)+(2B)' ?

 

[JJPellin]

electricpete summed it up pretty well. For most of my career, I would assume the thermal growth of motors up to about 500 HP as negligible. But, some newer motors have proven this to be a bad idea. We have converted some 5kV motors over from WPII to TEFC designs and bought some new pumps with TEFC motors. We have found some of these to have drastically different thermal growth between the inboard and outboard. We have had coupling failures from the extreme misalignment. I would estimate thermal growth based on external temperature measurements and heights, as electricpete describes for TEFC motors 300 HP and larger. For WPII motors, I would estimate thermal growth for 1000 HP and larger based on external temperature measurements. Usually, the numbers are quite small even at 1000 HP.

In addition to the uneven thermal growth of the motors, we have seen cases where the air flow from the TEFC motor caused drastically different thermal growth on the pedestals of the pumps. The air flow from the motor cools off the inboard pedestals more than the outboard pedestals.

Beware of thermal growth estimates provided by Sulzer. Their numbers are sometimes based on a simple percentage of product temperature which has not proven to be accurate. The coupling failures that I refer to above were new Sulzer pumps using Sulzer provided thermal growth estimates.

Johnny Pellin