Case Histories: PT Vibration and T5 Topping

There is a gas turbine just come from general overhaul, RH ~ 0 hour.
Its power turbine exhibit high vibration at 1xRPM, and phase can not differentiate between unbalance or misalignment. Axial vibration relatively small.
Alignment recheck, field balancing, and rotate coupling hub had been done to solve the problem, but not solved the problem. NGP was still 94%-95%, with PT Vibration 0.4 ips pk.

Another clue was get. The vibration increase after after engine had run for some time, e.q: 2-4 hours. So it is suspected related to thermal growth problem, so: alignment. After check alignment target specification there was misunderstanding between field maintenance people, they used old standard before the centrifugal booster compressor restaged. So after the alignment redone with new alignment specification target, the PT Vibration run low, NGP can be increased.

But, unfortunately, at NGP 99%, T5 topping appeared. PCD at TCP was about 118 psig, PCD at dial gage at panel was about 123 psig. Inlet air filter dp was 2.0" WC. T5 compensator had been reset to match engine specs T5 Base, IGV angle was 4 degrees. Bleed valve gave no sign of leak.
So we planned to cek PCD gage, inlet air filter dp gage, swap T5 tehrmocouple, and check bleed valve, and wonder that why IGV set was so small (range = 3 deg - 9 deg) that can caused low inlet air flow.

After PCD gage checked, it is confirmed that gage of TCP is correct, actual. Low PCD is confirmed.
And next day it's found that inlet air dp actual pressure is 4.8" WC. So, we suspected that dirty/semi blocked inlet air filter was caused of T5 topping problem. But, unfortunately after all inlet air filter had been changed, the T5 problem still the same, even with inlet air new filter and inlet air dp low.

Then we tried to adjust IGV angle. After IGV angle was changed from 4 deg to 3 deg, NGP can reach 100% at T5 only 1180 F. The problem is solved.


Moral of the story =
the truth is out there.
never believe anything untill you reconfirm it.