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Hello Dear All,

Do you have this similar experience to the above subjected issue? Thanks.

Background:

1. For every time of steam turbne (condensing) startup, the sawtooth type vibration fluctuation will be happening after the speed reaches close to operating speed, and after a certain time ( around 20 minutes) the vibration will be suddenly stable.

2. Based on the trends of steam turbine operating parameters:

-  no obvious evidence indicates the vibration is related to hot start or cold start.

-  no obvious evidence indicates the vibration is related to delta T between upper casing and lower casing.

- no obvious evidence indicates the vibration is related to exhaust pressure

3. During the vibration fluctuation the vibration phase  is varying in a small range,but 1x always is dominant. -----It looks like small rubbing is happening and resulting in thermal unbalance (morton effect).

My question is which will result in this phenomenon and the vibration suddenly becomes stable after around 20 minutes.

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Tags: vibration

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One thing I see is this involves a gearbox so I have to ask is there some form of loading procees going on while you see this behavior?  Assuming fluid film bearings, in a gearbox, there will one of the rotors moving upwards in its bearings, and it is not unusual for changing vibration while it makes that transition in rotor position.  Even the rotor that would tend to move down due to rotor weigh and gear generated forces, will have a position change due to the separating force.

Can you supply and details on the gearbox?

RM

Hello John,

Thanks for your kind reply.

This steam turbine is driving two geared compressors.   

This machine configration:

[ geared compressor A + disc coupling +(hot side-VTX/Y 598 ) steam turbine (cold side-VTX/Y 599) + disc coupling + gearbox + geared compressor B ]

Normally process :

Once ST speed reaches to normal operating speed, the compressor A will be firstly loaded , and until the process downstream of compressor A gets normal running, the gas from compressor A willnot be supplied to compressor B for loading (==> it means before that compessor B will always be running at recycle mode).

I can understand your description above , but however, the sawtooth vibration behavior currently is only happening on steam turbine. And from steam turbine running process parameters the current behavior is not related to process loading at that time.

Also, I attached the vibration trend of 1st pinion of geared compressor A which is connecting with hot side of steam turbine via a disc coupling. 

Attachments

RM

Hello John,

Actually from other process variables, I didnot find the change with a similar rate.

The oil supply temperature is gradually increasing at that time from original 39C ( before the steam turbine speed-up) to final 45C   (After the speed reaches up to operating speed). So I am thinking whether this vibration behavior is due to the morton effect caused by overhung weight from coupling or not. --------The steam turbine startup with a relatively high oil temp (45C) will be tested in the next time.

Because the current vibration behavior is not related to steam turbine starting mode (hot start or cold start), I assume an uneven or restricted  thermal expansion of steam tubine casing (which may be reslutling in the rubbing happening in the machine οΌ‰can be ruled out .

RM

Hello Vibramac,

Yes, there definitely is a heat soak period which follows OEM's requirement of starting mode (cold/warm/hot). The vibration behavior happens as the steam turbine speed increases close to operating speed and after fluctuating for around 20 minutes the vibraiton will be suddenly stabe. That is the case.

What's your thought in viewing the SCL and axial displacement? Later I will post them here.

RM
@Registered Member posted:

Hello John,

Thanks for your attachment.

The article has been actually read many years ago.  Certainly, the current vibration plots cann't be well matched with those shown on the article. So, currently this symptom still is getting me confused .

I think first of all you need to look at the link I provided and see if you have the means to do a plot of 1X amplitude and phase vs. time.  You are looking for the circular type appearance.  If your software can’t do that, you might have to manually generate a plot using some form of tabulated data acquired at regular intervals.That circle that is formed may not necessarily be big, but its presence supports the Morton Effect.

As far as SCL plots, they are a valuable tool, especially if you can overlay the direct orbit over the shaft position.  This show proximity to the bearing wall.

RM
Last edited by Registered Member
@Registered Member posted:

Are you happy with the pedestal expansion generally front ped (i.e. expanding equally and not crabbing).. maybe a set of dial gauges either side on next start up or better still a set of prox probes.

Just a thought triggered by β€œvibramac”, does the steam turbine have expansion keys in its mounting and if so what are the conditions of the keys as far as allowing proper expansion?

See https://oaktrust.library.tamu....ce=1&isAllowed=y

RM

@Registered Member It would only be guess to try to correlate reasons from the trend plot only. Can you mark the plot with machine start up phases (like turning gear speed, critical speed, loading of the 1st Compressor/ 2nd Compressor)

And what kind of SCL behavior are you seeing in general and at the instants when there is step change in vibration.

Any correlation with the sequential governing valves opening and the step changes in the vibration?

In my opinion, need to look at the above data to identify the causes of the observed behavior.

Regards,

RM

Hello Dear All- Thanks for your kind comments above.

For the casing expansion----Actually this was measured duirng the commissioning (at the beginning of this year), didnot find an uneven expansion from the readings of several dail gauges at that time. Also as I mentioned above, the vibration behavior is not related to  starting mode.  Even for hot start ( Time interval between shutdown and restartup is less than 1 hour), the temperature of casing compared to normal operting basically was same with very small drop (e.g., 410C-->400C), so can I think this shouldn't be a primary controbutor for now.

For others, I will try to update to you all in the next days.

Thanks again!

Attachments

RM

Hello lux

You have mentioned no obvious relation with Differential Temperature b/w Upper & Lower Casing and also you have mentioned Casing Temperatures of 400-410 Deg Celsius. May I ask how Casing Temperature is measured ?

You have mentioned current behavior is not related to Process Loading. May I ask if the Turbine Inlet Steam Flow remains constant from the time Vibration fluctuation starts till the time it is vanished ?

During Startup Temperature within Steam Turbine changes sharply in response to Governor Valve movement (Temperature drops as Governor Valve Opens). This phenomenon can be well observed by variation of Exhaust Steam Temperature during ramp up in Back Pressure Steam Turbines where Exhaust Steam is in Super Heated region. For reference I have attached curve given by a manufacturer for Extraction/Condensing Turbine

If possible can you share Exhaust Temperature Trend and after 1st Stage Pressure Trends to check for any possible correlation ?

As mentioned by Vibramac & John, Shaft Centre Line can be very important during Steam Turbine startup as there may be significant movement of Shaft Centre Line in response to Governor Valve movements which may cause Rotor to operate while being pushed hard against Bearing Boundary

Attachments

Images (1)
  • Exhaust & Extraction Temperature Curve
RM

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