COLD GAP OR SOFT FOOT IN CONDENSING TURBINE SIEMENS 1800KW

Dear All,

I found some gap (19 mils = 0.49 mm) on one foot of Steam Condensing Turbine.

the reading was taken in cold condition,

the question are:

how much is permissible for this kind of gap?

this gap is for bad installation (soft foot) or gap for thermal expansion?

what is the cause of this kind of gap?

I appreciate your comments.

Best Regards.

Ramon Ruiz (vibration CAT II)

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Original Post

That is not a real gap, since the photo shows shim stock in area of base bolt. The shim pack simply does not have the same area between the locking pin and the edge where you stuffed shim stock to the measure the "air gap". No feeler gages available? My only concern for the small shims would be if there were significant relative motion between the turbine case and support pedestal. It is not pretty, but it is probably not an issue.

Walt

I am with Walt here and copy my previous comment

"It is a old machine, do you have vibration problems? Make sure you don't create them if you don't have them old machines may hide some secrets. On the other side, when it is operating verify that you have the similar vibration on both sides of the opening, if so it may indicate that they at least move as one object. If on the other hand you have a serious difference in vibration on each side it would indicate that they are not connected as good as they should and need to be verified. If it not broken, don't fix it but if it is broken you have to fix it."

Dear Mr. Oli,

This equipment suffered big failure one year before, we found eccentric Inboard bearing housing (almost 3mm)  with catastrophic failure in rotor and stationary blade carrier, now the operational people are feeling high strange vibration around of turbine, and vibration from Rack 3300 BN doesn't show high vibration. I found spectrum vibration with 1/2x, 1x, 3/2x, 2x on several places of bearing casing and casing of machine.

after that we found this air gap in  cold condition of machine.

Now still we don't know what was the cause of the last catastrophic failure.

That is a different story. I would say multiples like that indicate either increased bearing clearance, any check on Bently data for such? Or that is scaringly close to what you found IRL, loose things, specially 1/2x is bad, any similar findings in Bently data also? In these circumstances I would lift the bearing top half to inspect, but that is me I may be some gung ho in some cases but with such indications and history I am very careful.

So part from that your Bently are in my view low but have the same signature and surface of the target is as usually bad. So as the absolute bearing data are quite hi and a substantial part is 1xRPM, was the balancing that bad after the previous repair? It could indicate that looseness that you show in your pictures. Did it get worse as the operators noticed increased tickle in the feet? So can you have any contamination or wear giving the unbalance or can it just be balanced and fixed? In worst case something has fallen off and give the unbalance or that rarely happen your loose bolts enable the rotor to rotate at a different center of rotation as compared to where it was when being balanced, very rare but as you have movement it could be but unlikely, only seen it once or twice in very different machines. So where does the 1xRPM come from? Was it balanced when repaired? What was repaired?

"yes there is air gap of 0,45 mm in all area of feet"

So what is supporting the turbine case; exhaust duct or?? Obviously, the machine should not operate while floating on air. There is a good possibility that shaft misalignment caused by deferential thermal expansion is causing the premature bearing failure. Gland steam leaks can overheat bearing support or cause rubs. This looks like an old machine that may not have received reasonable maintenance and overhaul work.

Walt

I have the similar turbine and I never see the gap like this. Please note that this is condensing turbine it means that the exhaust is not high temperature and with minus pressure ( the front side bolt should have clearance around 0.1mm but not for rear side). So, it should not has clearance there.

have your checked the bolts yet? is it still ok? How about the faces of contact? can you use feeler gauss to check to confirm it is real clearance? how about bore alignment parameter of both bearing housing?

1.-have your checked the bolts yet?

R: The bolts are in good condition. 

2.- How about the faces of contact?

the face are  flat and we saw this in good condition. 

3.-can you use feeler gauss to check to confirm it is real clearance?

R: I already checked all foot area, is completely empty without shim. And I informed  to Rotating equipment coordinator to take action about this. I will inform the cause of this ¨soft foot¨.  I took reading in all foot area and I found clearance of 19 mils.

4.- how about bore alignment parameter of both bearing housing? 

R: after correct this gap,  rotating people will check alignment again. 

Dear NGO,

Thanks for your recommendations,

please can you explain with more details about: "check the key way of front support for stuck?"

we are planning to check  piping stress, and correct soft foot, and also inspection in all components of bearing pedestal;

hold down bolts, vertical  alignment devices, bearing housing condition, at Inboard Side.

I will you inform any news.

Best Regards.

RRR

bearing pedestal have keyway which allow the pedestal move along shaft line when thermal growth or versa. In case that the key way stuck and the pedestal is not moved, it could effect to vibration and damage to turbine.

Check clearance at front bolt casing and tighten bolt sequence.

For casing bolt at clearance found, check length of both and length of casing thickness for sure that it is connected.

Just my ideal.

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