two shafts alignment 4

[Amin Behzadi]

I have a forced fan that has two equipment as driver, a electric motor (start-up) and a steam turbine (permanent). I want to know which equipment is better to set as stationary for shaft aligning? What is the best procedure? For example set fan as stationary and align turbine and motor based on it?

 

[JJPellin]

We have a number of fans of this basic configuration. All of ours are in induced draft service, so they likely run much hotter than your fan. In all our installations, the fan is treated as the fixed item and both driver trains are aligned to the fan. If this was an induced draft fan at higher temperature, you would also need to carefully consider the axial growth of the fan shaft. Normally, I would expect the fan bearing on the turbine side to be the fixed bearing and all axial shaft growth directed toward the motor driver.

Johnny Pellin

 

[Tmoose]

History?

Is this alignment quest as a result of some vibration analysis?

I'd think first about how movable each of the 3 component is.
I'm guessing the turbine and it's piping etc is not easily repositioned.
I'm guessing the motor is the easiest to move, and the fan with flexible duct connections relatively easy as well.

However, there is risk of any of the components becoming "bolt bound" when moving .
Or, even being bolt bound right now.

I insist on getting thermal growth compensation values for each component from the manufacturer(s). Turbines often experience 3 dimensional thermal growth.

*** I'd check the as-is alignment for the entire train before making any moves.
Starting at an end component, then aligning the middle component to it, will very likely result in the third component requiring impossibly large adjustments.
Whereas considering the entire train at once may find a relatively simple compromise alignment achieved by tweaking all 3 slightly.

 

[JJPellin]

In our fan applications, moving the fan is undesirable for another reason. Most of our large fans have pedestal bearings that are independent of the fan box. If the fan were to be moved, it would be necessary to move both pedestal bearings and the fan box. Moving the fan box is needed in order to maintain or restore internal alignment between the inlet cone(s) and the fan wheel. This requires opening up the fan box and obtaining entry into a OSHA defined confined space.

Johnny Pellin

 

[Amin Behzadi]

We have an induced fan too! So our critical condition is aligning ID Fan,because of its high thermal expansion growth! I send an email to manufacturer and check for possible procedure . If there was no procedure I ask for thermal growth expansion analysis. Do you agree?

 

[Tmoose]

HI Amin B,

Yes, the thermal growth info is required, at least as a starting point.

Is the fan large enough to have been designed as a one-off?
There may be a bunch of O&M manuals for the plant, with a section about the fan from the time of the original purchase.
I'd look for basic alignment procedures in there, including thermal growth information.

Like, "set the motor 0.005 inch low".

 

[JJPellin]

Yes. The fan manufacturer should provide thermal growth projections. However, it is an easy matter to do this yourself. You need to know which bearing is fixed in the axial direction and the range of operating temperatures for the flue gas. For example, our largest induced draft fan has the fixed bearing on the turbine side and runs at 300 to 350 °F. The portion of the shaft that is within the fan box is approximately 10 feet long. 120 inches of 4140 carbon steel shaft heated up from 70 °F to 350 °F will grow almost 0.25 inches. The coupling on the motor side needs to be capable of accommodating an axial movement of at least 0.375 inch in order to have certainty that it will not fail from the axial movement. This required a specially designed greased gear coupling made for high axial displacement. We generally prefer to avoid greased gear couplings, but were not able to come up with a suitable design using a non-lubricated coupling.

An additional concern occurs for induced draft fans of this type. If the fan box is foot mounted and independent of the bearing pedestals, the vertical movement of the fan box must be accommodated. This does not affect the coupling alignment if the bearing pedestals are independent, outside of the fan box and cold supports. Instead, the vertical growth of the foot mounted fan box requires adjustment of the fan wheel inlet cone clearance. It can also create problems with the shaft seals where the shaft passes out of the fan box. In the fan I was describing above, the original clearance between the fan box and the shaft was not adequate and the fan box grew up into the bottom of the shaft at the seal locations. We had a hard shaft rub resulting in a failure. And, keep in mind, this is a shaft that is over 16 feet long and 12 inches in diameter at the seals.


Johnny Pellin

 

[Tmoose]

In addition to the axial thermal growth mentioned by Johnny Pellin, vertical and even horizontal thermal growth should be considered .

Some extreme, and not-so-extreme examples here -

http://www.vibration.org/Presentati...al Growth and Machinery Movement VI Feb07.pdf

Again, Is this alignment quest as a result of some vibration analysis?

 

[CouplingGuru]

"The portion of the shaft that is within the fan box is approximately 10 feet long. 120 inches of 4140 carbon steel shaft heated up from 70 °F to 350 °F will grow almost 0.25 inches. The coupling on the motor side needs to be capable of accommodating an axial movement of at least 0.375 inch in order to have certainty that it will not fail from the axial movement. This required a specially designed greased gear coupling made for high axial displacement. We generally prefer to avoid greased gear couplings, but were not able to come up with a suitable design using a non-lubricated coupling."

The above coupling statement isn't always, true. While it definitely needs to be considered, we have done numerous non-lubricated couplings that are designed to handle that type of axial growth. The key is what speed and torque you require. As well as how long it takes for the system to get up to operating temperature.

When it comes to couplings we are always here to help.

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