Steam Turbine Gear Box

Dear All,

Recently we have carried out 15 MW STG set major overhauling and gear box. In gear box we have not changed any parts, but we only checked the bearing clearances, teet contact and back lash.

The high speed (7700 RPM) bearing clearance was found on higher side i.e. 0.53mm, however we didn't replace the bearing due to non availability of the same. After box up we have started the machine and synchronised.

At FSNL and FSNL with excitation no all the bearing vibrations in complete train was normal, however when the STG was loaded beyond 3 MW we saw gear box high speed side drive end vibration reaching to trip level.

But after load was further rised beyond 4 MW the vibrations became normal. We had run the machine at 12 MW load for almost 12 hrs and saw no major changes.

To check the phenomenon again we have reduced the load to less than 4 MW and we saw similar phenomenon of high vibration observed in high speed side drive end. During this period the turbine rear end vibrations were normal. To overcome the issue we stopped the machine and rechecked the alignment and further fine tuned the same.

After correction when we started there was no change found in vibration in 3 to 4 MW zone. Now the machine is running on higher load without any problem.

Now my question what are the probable reason for this phenomenon.
 
Rangacharya,

From the information provided, we can presume the gear box was not experiencing high vibration problems before it was disassembled and clearances measured and then reassembled.

One of the first questions to answer when a problem like this occurs is:

What has changed?

In this case, the gear box was opened and reassembled.

I know; I know. The Mechanical Department used every proper reassembly technique and procedure when they reassembled the gear box. They were even extra cautious and cleaned all of the bearing mounts and surfaces.

But, now, there is an increased vibration at some load range. But, it can't be the mechanical reassembly--everything was done perfectly and per manufacturer's clearances.

Was the gear box realigned to the turbine and/or the generator?

Presuming the alignments were checked and found to be within specification prior to re-coupling, then the only thing that has changed is the disassembly and reassembly performed by the Mechanical Department.

So, the vibration is the result of some activity that was, or wasn't, performed by the Mechancial Department in the disassembly or reassembly of the gear box.

It's that simple. Sometime, even removing dirt from a bearing mounting surface can result in very slight difference in bearing-to-journal alignment that can cause vibration like this. Sometimes, the torquing pattern used to re-tighten the bolts can result in a very slight misalignment.

But, this is not a control-related problem.

And, a knowledgeable vibration analyst with proper equipment can take vibration data and usually pinpoint the exact cause of the vibration and even make recommendations to alleviate the problem.

So, it's recommended that you re-examine the events that led up to the problem, and then take appropriate action. Most vibration is the result of mechanical misalignment or imbalance. And, almost no mechanical vibration is the result of control device issues.

And most Mechanical Departments are infallible. Which is the reason these types of questions get asked.
 
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