We have a steam turbine which has its overhung two row single wheel installaed at the high speed shaft of a gear box. This machine is driving a multistage centrifugal pump.

Whenever machine is started its high speed shaft axial movement is in the normally expected direction due to thrust caused by sigle helical gear teeth on bull and HS Shaft.

After some days or months of operation HS shaft thrust changes direction and goes to counter side to a greater degree. And sometimes ther HS shaft is intermitently dancing between normal and counter direction.This observation keeps us at our toes while operating this machine.


Any comment on this type of overhang wheel integrally geared steam turbine?

Is the pressure balancing line working?

What is the axial displacement that the shaft travels? What is the allowable tolerance? Any excessive axial play?

Some schematic drawings would be good to understand the problem.

It is a single wheel impulse turbine with two row of moving blades.
Allowable tolerance = 0.25 mm max.
Shaft goes to normal direction decided by single helical gear set, but after somes days/months of operation it goes in the opposite direction. Axial probe shows -0.2 to -0.35mm displacement.

Also note that this machine is running at higher than design speed due to plant increases load on the driven pump.

This message has been edited. Last edited by: Logic Lover,

Schematic.doc (24 Kb, 25 downloads)

Before you can get any comment on this, you need to be 100% sure of the thrust monitor calibration/setup: the monitor zero should be to set to be the mid-point of the measured mechanical float (bump the shaft), with an alarm at float +/- 5 mils and trip at alarm +/- 10 mils.

Once you have done this, you can figure out if the movement you are seeing is actually harming the thrust bearing - then you can determine whether or not it's worth putting any effort in to finding out why this is happening?

Does the thrust bearing show any pattern of temperature change with variation in displayed thrust position?

Once you have done this, you can figure out if the movement you are seeing is actually harming the thrust bearing - then you can determine whether or not it's worth putting any effort in to finding out why this is happening?

Does the thrust bearing show any pattern of temperature change with variation in displayed thrust position?[/QUOTE]

All the things mentioned above are already in place..

Only once we found scored thrust bearing on the counter side. But it was due to reduced oil pressure caused by an increase in flow path of oil on the outer periphery of composite bearing. The flow path is made by a segmental gasket on the Outer side bearing shell which has very small (0.5mm) holes into the journal and both sides thrust bearings.

Otherwise when bearings are opend on high counter side axail displacement monitor bearings are normal. But operational ease of running the machine with higher counter side displacement is not there.

That is why we have to investigate what is happening with this machine.

I think we've been along this road before in the Vibezone forum???
OK. Did you do a rotor "physical" float check (bump test) the last time the unit was checked, and did you record the results? If so, as Buzz says, this is your criteria for setting up your monitoring system.
The CENTER of this PHYSICAL float is your "0" position, with your range +/- half of your total float either side of the "0". Again, as Buzz says, set your "ALARM" at either end of this float, +5 mils, and your trip at either end of this float, +10 mils.(a good starting point anyway).
The other parameter you MUST take into account is your THRUST BEARING TEMPERATURES (I presume you have TC's in them?
If your shaft is actually rubbing the thrust pads, then the temperature will rise.

[quote]But operational ease of running the machine with higher counter side displacement is not therequote]

Could you explain a bit more what you mean by this statement.

In summary:

a)you have got a machine, that you are running above its design specs

b) It moves axially in a way that you don't understand and has been doing so for quite a while

c) You have checked the thrust bearings and found that they are OK

d) There have been no unusual thrust bearing temperatures

e) You say that the thrust probe/monitor calibration is correct

Is there anything else you can tell us?

Thanks to all involving in this case..

Basically as I mentioned that single stage two row velocity compounded wheel is installed at the overhung end of the opinion shaft. One axial probe is looking pinoin shaft at the out board end and as the gears are single helical and their natural thrust loading is towards the probe( which means normal axal diplacement reduces turbine first nozzle gap)

Now problem is that axail probe readings sometime reaches 0.38mm on the counter side and 0.24 on hte normal side.

It happens frequently and there is also a correlation of main steam valves position with the counter side movement of pinion shaft.

When axial probe is showing counter side reading, governor valves are automatically opened meaning that machine becomes inefficient upon this counter side movement and governor allows more steam to cover the speed reduction.


I think I have clarified the situation...any further details can be given if required.

OK Logic - what we are trying to establish is what was the ACTUAL MECHANICAL FLOAT you recorded last time the unit was shut-down/overhauled? Before setting up your probes etc. you should have "bumped" or "pushed" the turbine/pinion gear from one extreme to the other and recorded this figure.
You are now saying that the rotor, according to the probes, is going between 0,38 to 0.24 mm, a TOTAL float of 0.62mm.
Is this figure is within the ACTUAL PHYSICAL FLOAT measured when shut down? If it is there is nothing to worry about! Steam turbine rotors ofter "float" from one axial position to the other. That is what the thrust bearing is there for - to prevent it going too far either way and making physical contact with the casing/nozzle/other stationery object.

This message has been edited. Last edited by: cheddar-caveman,

Quote:

When axial probe is showing counter side reading, governor valves are automatically opened meaning that machine becomes inefficient upon this counter side movement and governor allows more steam to cover the speed reduction .

What speed reduction? - you are not giving the whole picture here!!

So...The speed reduces, the governor opens to deliver more steam to maintain set speed and the shaft axial position changes - why do you think that this is unusual??

The axial position is mainly governed by two things:
a) Load (the higher the load, the greater the axial reaction forces on the turbine
b) thrust bearing

I suggest you look at what is happening in the pump when these position changes occur. You are running higher than design speed - is the pump cavitating?

Are there any changes in flow/valve position that would represent a change in load/speed reduction?

Is it possible that these higher speeds represent the upper end of operating envelope of the governor and it is unstable?

------------------------------------------------You are now saying that the rotor, according to the probes, is going between 0,38 to 0.24 mm, a TOTAL float of 0.62mm.
Is this figure is within the ACTUAL PHYSICAL FLOAT measured when shut down? If it is there is nothing to worry about!QUOTE]
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Yes this is so.. we are going to open up the machine in a day or two..we are expecting higher thrust bearing physical axial float as found in the former overhauls.. the only thing we would be able to do is replacement of bearing with a new one..

As this is a compisite solid journal and thrust bearing...no adjustment is possible.

We are suspecting another fact and this is the level of gear box along shaft axis or lateral to it.

We will also check the rigidity of hte cover in which Axial probe is installed for erenous reading if so...
Any other concern?

I am somewhat confused as to what you are doing over there!
You say that the float indicated by the probes is within the physical float measured at the last shutdown, why are you shutting down and opening up again?
Oh well. Guess that's your business.
Any way. When you do open up and put in new bearings etc etc make sure you record the actual rotor float carefully and then reset your probes, alarm and trip levels from this reading.
Good luck.(but I bet you when you start up again you will see exactly the same thing happening)! Please post on here when you do restart.

Dear Cheddar,
I said " yes this is so" to your statement that the total float is 0.62mmm not to "Is this figure is within the ACTUAL PHYSICAL FLOAT measured when shut down? ".....sorry for confusion ...

This measured float is not within the recommended float ( 0.25mm) and that is why we are opening the machine and as you said this is not my bussiness, Man,.. to keep machines shut down...and shuoldn't it be like that!

What do you say about level? Can it be a reason for intermittant two directional thrust?

Quote:
Steam turbine rotors ofter "float" from one axial position to the other.

I am curious about this statement.. for an impulse steam turbine is it possible? if so why?

OK Logic - firstly "recommended float" and "actual float" are two very different things.
Float in turbine rotors is often a function of the gearbox, especially a single helix which is not self centering. Also, as you are running above design parameters, and even if you were at design, any slight change in load could cause the pinion to move, towards your probe with increase of load and away from the probe with a decrease in load.
IF you originally set your physical float to the recommended (.25mm) then it is probably a good idea to see what is happening as you are experiencing almost three times that float (.62mm).
I will be interested to hear what you find inside!

Well Cheddar...we have opened the machine and as the probe was showing -0.38 mm so we found a deep (appprox. 0.5mm) cut in the inactive side thrust collar and severe babbit loss on associated bearing. Journal is quite satisfactory.

Total physical float was 0.72mm. We are unable to understand this counter side thrust.

As you said we can experiment by changing load and noting thrust direction after maintenance.


Interesting thing is that lastly we had installed two thermocouples int the upper half of thrust bearings copying the standard tilting pad TC hole geometary..but this hasn't indicated any high or abnormal temperature or even correlation with the normal to counter thrust.

What do u think about the position of the thermocouple along the direction of rotation on the pad if it affects measurement?

This kind of failure has never been reported on this machine or even I haven't seen one in my experiencce..

What do you suggest at this stage...just as a comment because the only thing we could do , we have done in the form of replacement of thrust collar and bearing..

New float is 0.22mm (rec. 0.18-0.25mm).

Hummm - very interesting and thanks for the feedback.
You say a deep, 0.5mm "cut" in the thrust pads. Does this look as though a small hard object has been pulled accross it? If so, it may well be an instantaneous happening, a small metal fragment for example, just running accross during one revolution. In this case there probably would not be any temperature rise. You will only get a temperature rise with friction between the pad and the thrust disc i.e. loss of oil film.
Where to put the thermocouple? Being a tilting pad system, it shouldn't matter where you put them because, as above, if you do lose the oil film the pads will heat up. You do however, need to make sure they are in contact with the white metal. From your description, you already have them in the right place.
You also say "severe babbit loss on associated bearing". Is this from the thrust pads? If so, then I have to suspect your thermocouples/monitoring system. For so much material to come off with no temperature indication, is disturbing, unless of course it has happened very slowly over a LONG time period.
Also, with a "recommended" float of 0.25mm and an actual float of 72mm, how come your alarm/trip didn't come in?
Your new float sounds absolutely correct and it will be interesting to see how it runs. Yes, watch the thrust position as you change load. Running above design will make the system very suseptable to changes.

Quote:
You say a deep, 0.5mm "cut" in the thrust pads. Does this look as though a small hard object has been pulled accross it? If so, it may well be an instantaneous happening, a small metal fragment for example, just running accross during one revolution.
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Please note it is just like turning on a lather machine a o.5mm deep cut on the total contact area of thrust bearings. It can not be instantenous....


Quote:
Also, with a "recommended" float of 0.25mm and an actual float of 0.72mm, how come your alarm/trip didn't come in?
------------------------------------------------

As I mentioned probe was reading -0.38 on one side and please note we had set alarm at 0.35mm and trip at 0.45mm on both normal and counter side...We operated the machine with the alarm for quite some time..

This message has been edited. Last edited by: Logic Lover,

Inactive_side_thrust_Collar.JPG (205 Kb, 21 downloads)

Ok, Ok, OK! Sorry - didn't realise you were running in alarm for all this time, but one still has to wonder why there was never any temperatur rise in the pads??
Most odd.
Can't open the pictures for some reason either. Just get a small portion of one of them?
I'll keep trying.

Picture is in my office PC, I will try to resend it.
Now machine has started with a positive 0.06 with full load... lets see what is ahead.

One thing is important now we have higher tempeartures on local tempresature indicators ..almost 10 degree C higher than before..(it was running atr 59~60 degree C before maintenance)

regards

Picture of damaged thrust bearing

Inactive_side_thrust_Collar.JPG (1,143 Kb, 34 downloads)