steam turbine 1x vibration

[gallaz]

I have a steam turbine that suddenly undergoes high radial vibration at the front bearing, this then subsides after about 10 minutes and returns to normal running conditions. There is not much vibration on the bearing casing... only the shaft. Any ideas what is causing it?

 

[pablo02]

When does this occur? during start-up? out of the blue? does it reoccurr? other? Does the front bearing have a governor and oil pump stub-shaft? Where is the shaft vibration monitored on this bearing?

where are bearings #2, #3, and #4 arranged on the steam turbine? Are these vibrations monitored? any blips on these during the same time?

did you have any other issues going on at the same time or slightly before?

I think there a number of possibilities but some additional info would be required to narrow those down...

 

[poetix99]

This is not a lot of information.

Try to extend the period of time that the rotor is on turning gear (or rotating at low speed if you don't have a turning gear). If the rotor is relatively heavy and "bowed" while shutdown, running on turning gear will help to minimize ill effects of this. (Running on turning gear immediately after shutdown is also important to minimize rotor distortion in the first place.)

Try to extend the warm-up period before going to normal load, normal speed. Beware, however, of the danger of overheating the last stage of condensing turbines when operating for extended periods of time at no/low loads. (Most OEMs make safeguards against this with cooling steam to LP stages, temperature alarm in exh. casing, and/or hood sprays.)

Many times it is simply not possible to attain anything close to normal operating temperatures within the unloaded (or lightly loaded) warm-up sequence, and you perhaps are already doing the best you can.

Is is also possible that as the difference between cold and hot alignment is causing a problem during start-up. If the turbine is set too low with respect to the inlet bearing case, you might see this transient vibration until the turbine grows into hot alignment. Check your cold and hot alignment readings to verify that they're within OEM specs.

 

[byrdj]

Are you saying the instrumentation says there is high shaft vibration, but the bearing cap vibration is normal? If so, it might just be a instrumentaion problem.

What type of instrumentation due you have for vibration monitoring.

Shaft rider probes have a tendicy to chatter if the geometery of the tip changes with wear. Also probe problems like guide bushings wear and intial spring loading will cause chatter.

Proximity probes can also resonate and indicate false vibration. This is the case with the long "stinger" mounting arrangement.

If it is true vibration, like the above replies say, more detail info is needed about operating conditions. For vibration to come AND GO that quick has to have a diffinite initiation source.

Are you sure it is 1X? could it be sub synchronize like 0.55X to 0.8X.

 

[gallaz]

Sorry... should have given more info.
It occurs out of the blue during normal steady running speed.

There are a governor & auxilliary oil pump impeller on stub shaft. This vibration is shown by x-y proximity probes. It is defineitely 1x... no 1/2x vibration.

Phase undergoes changes as well but these are not consistent everytime.

There is smaller amount of vibration shown on the other bearings indicating that it isn't a instrumentation problem.

Machine has been waterwashed... vibration events still occur with same severity but less often.

Bearing temp increases, seemingly as a result of the vibration... not the cause.

The turbine is connected to generator... at time of vibration also see load changes and sometimes increased steam flow without operator interference. Also see changes in axial thrust(probably due to increased steam flow)

 

[byrdj]

Referring to the post from "Strong" in the vibration forum.

Are the phase angles (both bearings on rotor)the same during normal vibration, indicating that "unbalance" remains the same.

During normal vibration, does the two bearings on shaft appear nearly in phase or nearly 180 out of phase. this indicates the vibration "mode" of the rotor. In phase indicates unbalance type causing high 1st critical, out of phase indicated unbalnce type causing high 2nd critical. If your rotor is operating near second critical with normal out of phase vibration, it would be sensitive to very small changes in center of rotation changes near the end of the rotor. ie a small umbalance weight at one end would cause high magnitude vibration. And would be very sensitive to vibration caused by a rubb at the end of the rotor.

During the periods of high vibration does the phase ange change with time and appear to be rotating.

If so, you problem are experancing a rub at that end of the rotor. (would be nice to know rotor casing geometery, since that to plays a significant part in determination). If the high vibration bearing is at the exhuasat end of the rotor with the larger raduis buckets (blades)

If rotor is operating greater than 2nd critical and since the rub is causing a vibration mode like 2nd critical (effecting on end more). The rub will usually clear it self with very little "machining" of the tight clearance, thus it may keep coming back. (also something to do about high spot of vibration not being in phase with point of contact) If the rotor is less than 2nd critical, the rub will usually continue to increase to a very large magnitude and "hammer" the tight clearance out.

The type of matereal that is causeing the rub will have an effect on whether it will self clear. Rubs in Brush or retractable packing are harder to clear than say solid spill strip. Rubs in hard deposit accumulation are also difficult to clear.

Coasting through critical with the high vibration may allow the magnitudes to greatly increase and clear the rub (high spot of vibration and point of contact come together), but you should verify with someone familiar with your unit that this will not do severe damage.

Is the CV operating cylinder near bearing and is it staying constant (however it's frequency of vibration is much lower)

Good luck and HAPPY NEW YEAR

 

[pablo02]

Off the top of my head, I tend to agree with byrdj -- I would suspect an axial rub on a blade ring near the #2 (or possibly the #3) bearing (again, the bearing layout and the steam flow geometry in the HP, IP, LP turbines would be beneficial) -- also, check any extaction points (near the #2 bearing) for water induction (noticed any turbine or extraction point temperature deviations?) any indications of hot reheat temperature poblems? are all drain orifices and traps working as designed? [were your clearances set properly for expansion during the last overhaul]

why do I think the problem is near the #2 bearing -- somewhat from experience -- the (I assume) HP/IP shaft is slender and I suspect the #1 bearing shaft vibration is measured on the outboard side or on the stub shaft itself -- the problems near the #2 bearing are amplified through the #1 bearing -- of course, this is only my opinion and I do not have all of the info available to the users...

 

[byrdj]

Gallaz,
You stated there is also an increase in steam flow during the period of vibration.

Do the control(governoring) valves open slightly to explain the increase.
When you say steam flow increased, do you possible mean a stage pressure goes up? Do you also get an increase in MWs?

I am just thinking,could the problem be a loose component in the steam path (a diagrahm fit or nozzle plate)?

 

[gallaz]

byrdj,
The increase in steam flow is more steam at inlet/nozzle plate. This is due to opening of governor valve. We do see an increase in MW with increase in steam flow...as expected.

The turbine only has one stage. I have entertained the idea of a loose component in steam path as you suggested, but how would this cause a vibration at running speed frequency (50Hz)? Could it perhaps be caused by a resonance in the steam path caused at certain flow rates?

 

[byrdj]

I was thinking multiple stage unit,thus if a stationary nozzle plate was loose, the flow would bypass, result in higher downstream pressure (not neccesarly increased flow) and could be touching the shaft, causing the vibration. Changing phase angle during the period of high vibration and different phase angles per event would be indicate a rub.

Have you any ideals why the vibration is causing the Control valve to open. Is the governor a impeller oil pressure or flyweight type? I've seen a large multiple stage unit with an impeller governor have problems with the "control rotor" being long, without support bearings, go into similar vibration. The pump seal rings would eventally damping the control rotor for a few hours. On routine inspection thses seal rings were always very worn.

I don't have any experance with single stage units. so I don't have a feel for things like journal diameters, wheel diameter, critcal speeds and sensitivities, so some of my previous comments about the rotor end with the larger diameter buckets does not apply.

 

[gallaz]

This is a single stage back pressure turbine with an impeller oil pressure governor. The governor inpeller and main oil impeller are on a small 'stub' shaft outside of bearing 1 so it is only supported at one end.

The prox probes look onto this stub shaft very close to the bearing... it may be possible that the stub shaft is vibrating much more than the main shaft, but it is still a problem.

 

[Cookie]

Is this a Westinghouse steam turbine?
More detail on the vibration would help.
A few key items:
1) Does the phase angle change, how much -average
2) Are you sure it is 1x vibration no subsyncronous 1/2 x or 2x
3) Is this a direct drive 3600 rpm or is there a gear box between
4) What type of radial bearing - shell, tilt pad, pressure dam, eliptical
5) How often do you water wash your unit, at what speed?

 

[cstron]

Please go through your lub oil circuit thoroughly. There may be some pressure build up and release suddenly. This feature may be causing disturbance in flow to the bearing.Check relief valves if any in the circuit.