I would like to ask for opinion or past experience on two of the new problem I am facing now which I believe could be related to each other. My unit is GE Frame 9e with Mark V TMR. The unit just underwent major overhaul with compressor rotor replacement, stator reblading and VIGV reblading.

(i) First Problem
The problem is the VIGV tends to move to open (from fully close at 32deg) when the unit is on cranking mode or when the AHOP is in operation. The movement is slow wrt time.

The VIGV is steady when operated via AUTOCAL or during unit operation.
The two LVDT and servos are new and were replaced during the overhaul as part of maintenance plan. The LVDTs were calibrated and the angle measured= angle feedback.

From the unit PnID, it can be seen that the VIGV is held at last position by the hydraulic oil pressure from the hdyraulic header and accumulator. I believe that the problem might be with the hydraulic system but no work was done in the hydraulic system during the outage.

(ii) Second problem
The problem is that the unit is falling off cooldown. Whenever the unit is selected to on cooldown mode, the unit successfully breakaway but failed to maintain cooldown and goes to standstill. The unit had all the bearings replaced during the outage. We have cranked the unit for 2 days and started up to FSNL for several hours but the result is the same. Could it be resolved by operating the unit for some time or is there any problem which could cause this as well? There has been a thread previously on this problem but the originator did not reply on what was the finding.

Any advice or past experience on the matter would be greatly appreciated.
Thanks

 

I am sorry I left out some important information on the unit.
The cooldown method for this unit is via torque converter connected to a starting motor. The two L20TU1 and L20TU2 solenoids are used to fill and spoil the crank respectively. Both solenoids have been replaced with new and the torque converter is relatively newly refurbished by the OEM.

The calibration of the limit switches on the torque adjuster has been check and set accordingly as well.

 

Regarding #1. Many are surprised that MOOG servo control leads are not consistent in wire colors. It's important to empirically establish coil polarity in the field, regardless of lead colors. Not doing so results in some coils fighting/nullifying others. Similarly the null bias must be correct. Care and discipline is needed to ensure that the servo control valve setup is correct, especially following replacement with new servos.

 

Dear Pb,

You are correct. But the polarities are always tested during new servo replacement as a precaution. The thing that confuses me is that the problem only happens when there is no trip oil (servo not in use).

 

What is the speed when you were able to stay on COOLDOWN?

 

The unit could not maintain on cooldown at all. The normal on cooldown speed is about +- 60rpm depending on the oil temperature and etc. At present when the unit is coasting down, we could notice that the speed keeps dropping until the unit goes to standstill.

We did try to put the unit to on cooldown at higher lube oil temperature at about 65degC but the results is the same. The normal lube oil temperature for on cooldown operation is about 45-50degC

 

rastafier,

Do you have an email address.
I can send you some information

 

> You are correct. But the polarities are always tested during new servo
> replacement as a precaution. The thing that confuses me is that the problem
> only happens when there is no trip oil (servo not in use).

Normally, when the trip oil pressure is zero, the trip relay in the IGV actuator dumps oil according to the net bias (of mechanical and electrical bias') in the servo control valve. The servo control valve remains in the hydraulic circuit between the cylinder and the drain. This normally results in closing the IGVs as the correctly set bias is organized that way. The intent is to leave the IGVs closed for the next start. If the mechanical bias in the servo control valve is correct, then the control system (electrical) bias may be wrong. If the mechanical bias in the servo control valve is wrong it can directly explain your findings. I'm aware that some new servo control valves come from MOOG ship with incorrectly set mechanical bias'.

 

It's rastafier (at) gmail (dot) com

 

> What is the speed when you were able to stay on COOLDOWN?

Hi mike, can you also send me info on this topic please? My e mail is [email protected]

Thanks

 

Dear Paul,

Appreciate if you could explain further. I don't think that the servo remains in the hydraulic circuit when the trip oil is zero. I did recheck on my PnID for my unit. When 20TV-1 is deenergized, the hydraulic oil to the VIGV is drained. As such the VH3-1 piston would be in closed position. This would result in oil going to the servo 90TV-1 to be blocked. (please correct me if I'm wrong).

You may look at the PnID here:
https://plus.google.com/photos/1064...s/5713882426394171585?authkey=CJiLqoPxgeO78gE

 

rastafier,

1) If you follow the path of Hydraulic Supply through VH3-1 when 20TV-1 is de-energized, you will soon find that IGVs are being driven to the closed position by the Hydraulic Supply pressure (flow). Flow through the servo-valve is blocked, but high pressure Hydraulic Supply is directed straight to the actuator to cause it to move to the closed position.

If the IGVs are moving when 20TV-1 is de-energized, then it would seem there is something wrong with either VH3-1 or possibly with the servo-valve. I have seen mis-marked Moog servo-valves installed and cause a similar problem. As was also said in this post, the polarity of the servo current being applied to each servo-valve coil has to be verified whenever a new servo-valve is installed. Wire color codes cannot--and should not--be trusted.

I have also seen people look at two (different) servos and assume that they are identical and install an incorrect servo and cause problems like this.

There have also been several cases of refurbished servo-valves having incorrect porting plates applied by the refurbisher and cause problems like this.

You haven't told us what the servo currents are when the unit is cranking and 20TV-1 is de-energized, and when the unit is cranking and 20TV-1 is energized.

2) This is a very common problem on new turbines and on turbines where the bearings are replaced during maintenance outages, particularly if the bearing housings were not laser aligned as part of the process.

The unit has to be run at Base Load for 24 hours or so to heat everything up and start to get the new bearings to "fit" the journals.

If the IGVs are opening when the unit is on cooldown this can also contribute to the problem. As the IGVs open more air is being moved by the axial compressor which requires more force from the torque converter (which it isn't capable of providing) and so the speed decreases.

So, the two problems may be related, and they may not.

You also haven't told us what the Null Bias current value is that's set in the I/O Configurator for the IGVs.

 

I need to correct my previous post. You and CSA are right.

The Servo Control Valve is completely removed from the hydraulic circuit when the trip oil pressure is zero, and the trip relay has operated correctly. The piston-side is connected to the hydraulic supply, and the rod-side is connected to the hydraulic drain. This condition should result in closing the IGV actuator, as long as there is supply side pressure, rather than opening as reported when trip oil pressure is zero

>You may look at the PnID here:
>https://plus.google.com/photos/1064...s/5713882426394171585?authkey=CJiLqoPxgeO78gE

 

We haven't heard back from the original poster.

And, this isn't technically "Trip Oil" pressure we're talking about here; it's just part of the Trip Oil pressure circuit and it only applies to the IGVs in this case and the IGV Dump Valve. It <b>IS</b> necessary to have 20TV-1 energized (and "IGV Trip Oil" pressure) in order to establish Trip Oil pressure to the fuel stop valve(s), but it's not necessary to have Trip Oil pressure to allow high-pressure hydraulic oil to the IGV servo-valve to control IGV angle.