Hi! to everyone..

We have vibration problem on our GE frame 5 generator. The machine was running with 0.75in./s vibration at the generator bearing (exciter side). Turbine trips for various ground faults. Unfortunately ratchet didn't work during the troubleshooting of ground faults. After a few days we tried to start the turbine but it trips @ 38% speed due to high vibration at the generator bearing.

We checked lubrication, bearing clearance and generator to gear alignment, all are found ok. Cleaned also the generator rotor but same results. Tried to balance the generator rotor @ trip speed and successfully able to run @ full speed no load with vibration 1.24in/s. Tried to balance at FSNL, then put the correction weight but trips at FSNL. (we set the vibration limit @ 1.41in/s).

Can anyone give some advice about this?

 

For most GE-design heavy duty gas turbines equipped with velocity-type vibration pick-ups (seismic pick-ups), the alarm is set for 0.50 in/sec and the trip is set for 1.0 in/sec.

At 0.75 in/sec one can usually feel a little tingling in your toes when standing near the rotating equipment. At 1.00 in/sec, it's more pronounced, and at 1.25 in/sec, it can start to tickle your toes and soles of your feet (even when wearing steel-toed work boots). 1.41 inches can even make one giggle.

Why mention this? Because it's happened on more than one site that indicated vibrations were much higher than actual vibrations. Have you tried exchanging vibration pick-ups and seeing if the vibration changes at the gen end? Does it really seem like there's a vibration problem? Because, again, these numbers are high, approaching dangerously high.

Have you verified the vibration levels with another instrument, hand-held or temporary?

Ratcheting isn't so critical (typically) for generator rotors, unless they are being operated at or near their reactive capability curve limits which means high current flows. If the unit was being operated at rated load (real power) and at a high Lagging VAr meter reading/low Lagging power factor meter reading, then the current flowing in the generator rotor is high and the heat developed in the rotor can be higher than normal. Many Frame 5s have air-cooled generators and are operated in high ambient temperatures but still, operation outside rated conditions can cause excessive heat which could cause the rotor to develop a bow.

It would seem prudent to have a vibration analyst come to site and make some recommendations. It would certainly seem it should be possible to place balance weights on the generator rotor to smooth it out, unless there was some condition which was caused by some problem with the rotor windings or end rings that was aggravated when current was flowing. But, a good vibration analyst can help with that.

 

I had been through a similar problem at a site. One thing that could had been done in your case was to leave the machine on ratchet for 36 Hours prior to start (considering the fact that the ratchet was not in operation earlier when the machine tripped and the wheelspace temperatues would had been higher for the ratchet to operate)

The values of 1.24 & 1.41 in/s are abnormally high and a continous run on these values are not advisable. Setting a higher vibration limit than OEM recommendation would not be a prudent decision and could hamper with the life of the machine. In fact, machine running with 0.75 in/s by itself was quite alarming. I agree with CSA tha you need to have an vibration expert opinion to resolve the issue rather than fidling with the machine.

 

The vibration is real, we can feel it standing at the exciter area.... We replaced the pickup but the readings still the same. The reason we set the limit to 1.41 is just to make it to FSNL in order for us to balance the rotor. During balancing when we put the correction weights the turbine trips at FSNL, seems that balancing wont work. Anyway thanks for the help, we already ask an analyst to determine if there's another cause of the problem.

 

excuse me, what the relation between wheel space and ratchet system?

 

Wheelspace temperatures are indicators of internal machine temperatures, particularly rotor temperature.

Cooldown (hydraulic ratchet, slow-roll, etc.) must be maintained until machine internal temperatures are low, which indicates that if the unit is taken off cooldown the rotor will not suffer from bowing. Higher wheelspace temperatures indicate rotor temperature is high, and a high rotor temperature means the rotor metal may be in its elastic temperature range and therefore subject to bowing if cooldown is not maintained for a time sufficient to allow the rotor temperature to cool below its elastic temperature range.

Another serious consideration about maintaining cooldown is to keep L.O. flowing to the bearings to prevent high rotor temperatures from causing bearing damage (bearing materials begins to deform at approximately 210 deg C). If L.O. flow to the bearings isn't maintained after fired operation then the heat from the rotor will make it's way to the bearing materials and cause deformation.

 

> excuse me, what the relation between wheel space and ratchet system? <

Wheel space TEMP is indicative of the cooling of the rotor.

From the operation point of view immediately after the trip i.e when the wheel space temp are high, turbine can be started immediately without much need of ratchet.

But if the wheelspace temp have come down (as stated earlier because of the ground faults) continuous ratchet for approx 24 hrs is recommended so that the bowing of the rotor is eliminated.
I also agree to have a continuous ratchet and try start again.
I am not an expert but I think if the problem is not resolved just by means of the ratchet as usually it does then it may be because of trying for balancing efforts and all.

1.41 limit is very very High.

Regards

 

thank you , i want some document about ratchet system
how it work ?

Regards

 

A hydraulic ratchet system used for Cooldown operation to prevent rotor bow is a mechanism that rotates the shaft a small portion of a rotation, then rotates the shaft a similar portion of a rotation after a short time delay (usually about three (3) minutes) and continues doing this as long as Cooldown operation is enabled.

There are several types of mechanisms which have been used over the years on various turbines. Some use logic/sequencing in the Speedtronic to do most of the control, while others have a 'self-sequencer' to do some of the work of porting hydraulic oil first to one side then the other of a rack mechanism which engages with some teeth which are connected to the turbine shaft, usually through the accessory gear and coupling. Hydraulic pressure is used to rotate the shaft when the teeth of the rack are engaged with the teeth of the shaft, and then hydraulic pressure is ported to the other side of the actuator which 'retracts' the rack to begin another stroke. Kind of like a ratchet handle used to turn a socket wrench.

The typical portion of rotation is approximately one-eighth (1/8th) of a turn, and that number is usually chosen to match the number of through-bolts used to hold the compressor together. (Again, the compressor rotor is much longer than the turbine rotor, and is most susceptible to bowing when hot if not rotated, either continually (by a turning gear or barring motor mechanism or by a ratchet mechanism. Most people say cooldown is to protect the 'turbine' rotor, but it's really the compressor rotor which is longest and heaviest and most prone to sag under its own weight when warm and bow which causes vibration problems. The HP (high-pressure) turbine rotor and compressor rotor are coupled together in GE-design heavy duty gas turbines.)

Sometimes the hydraulic pressure for the ratchet system is provided by a dedicated D.C. pump, and other ratchet systems use hydraulic pressure from the Aux. Hyd. Pump.

If your site has a gas turbine packaged and installed by GE, the Instruction Manuals will have an entire system description for the 'Starting Means', of which the hydraulic ratchet is an integral part (for GE-design heavy duty gas turbines).

 

please if you have some document please upload

 

Often reason of vibration is a short circuit in a winding of a rotor
I have a negative experience )

 

Ig... negative-sequence would show up as unbalanced stator currents!

Thus far, none of the posters, including the originator, has observed the existence of such a condition.

Regards, Phil Corso