Just found out about this site. It's amazing and I hope I can contribute and learn together with everyone.
I have a query regarding the LO temperature and bearing vibration. Is there any correlation between these 2 parameters? Previously in my plant we were operating our ST with LO temperature around 48-50 deg C and everything was fine. However, after a recent outage it changed whereby when the LO temperature increases, the bearing vibration increases too (bearing 2x). We are quite puzzled as this never happened before.
Is it suppose to be this way and we never knew it or is something wrong?
Hope someone can help to explain. TQ.
Good that this gentleman raised this query, I am working for DCS implementation company, presently we are replacing Existing DCS with our New DCS, hence we need to follow exactly similar to configuration in the new DCS.
While we are doing our implementation, we found that in the Vibration Trip multiple configuration, Bearing temperature alarm, is also consider in the vibration trip multiplier.
We also puzzled why this is consider in trip bypass, generally vibration trip multiply is applied during start up and it is removed after defined time.
It means there is a relation between bearing temperature alarm and vibration.
Any reliability team or Mechanical team explain this?
I am assuming that ST means Steam Turbine. Bearing oil and Seal oil temperature differential will cause extreme vibration.
Dear CTTech,you are right. What I meant by ST is Steam Turbine. But my concern is the relationship between the lube oil temperature and bearing vibration (bearing 2x particularly). Does it has something to do with the viscosity of the lube oil,whereby if the temperature of the lube oil increases it will reduce the viscosity and therefore causing higher vibration? Well,this is the only explanation I can think of.
But one more thing that is confusing me is that,we used to run our ST with the lube oil temperature around 48-50 deg C as I have mentioned earlier in my query without any effect to the bearing vibration. This phenomena has just started after a recent outage whereby if the lube oil temperature reaches about 49-50 deg C,the bearing vibration also picks up.
Hope someone can explain this scenario. TQ.
More information would help. I always like to first find out what did we change during the outage?
Do you use proximity type vibration sensors? If so, were they properly gapped and verified?
Were any seals replaced?
Has seal steam pressure changed?
Can the vibration be attributed to a change in balance shots on the turbine?
Lube oil temperature is usually a secondary effect to something else. The lube oil is removing heat not adding it.
Sorry for the late reply.
To answer your questions:
1. Yes we are using proximity type vibration sensors and we have carefully gapped them.
2. No seals were changed.
3. Seal steam pressure remains.
4. After the sudden tripping of ST, our rotor experienced bow. Thus, we had to remachine it. However, we can't do much on the HP side, therefore we had to add balance shot at the HP plates to compensate for that.
As the lub oil temp. rises, the viscosity of the lub oil goes up which reduces the strength of the oil film on the bearings. This increases the vibration.
Lock_out, please clarify your statement, "...vibration increases too (bearing 2x)." Did you mean vibration is aways present at twice shaft rpm? Or twice critical speed upon while run-up?
Regards, Phil Corso, PE (firstname.lastname@example.org)
Sorry for the unclear statement Phil. What I meant by "Bearing 2x" is vibration on bearing number 2, "x" axis. It shows significant increase in vibration when the lube oil temperature increases. Other bearings vibration increases too, but the increase is not very significant.
One way to remember the definition of viscosity is:
"If it's low, it flows."
Mineral oil would have a low viscosity. Crude oil would have a higher viscosity.
http://www.wikipedia.org has a great definition of viscosity.
The viscosity of lubricating oil decreases with temperature. High viscosity usually causes elevated vibration levels in journal bearings--but that usually happens at low lubricating oil temperatures.
Did you carry out any maintenance to your ST or on any auxiliary support to your ST during your outage?
This is not really true, as you increase temperature you decrease density and viscosity lowers as temperature rises.
>As the lub oil temp. rises, the viscosity of the lub oil
>goes up which reduces the strength of the oil film on the
>bearings. This increases the vibration.
I think you may have meant that as lube oil temperature increases, the viscosity decreases, or is lower, or is "thinner", meaning it flows easier. However, it may not provide a thick enough oil wedge.
So, keep in mind that a lube oil temperature that is too low has increased viscosity, and will have trouble providing a smooth wedge due to being too thick, or "sticky".
as a matter of fact, the high temperature of the lube oil in your case is not a "reason" for vibration , it's only a "result".
there are too many reasons to get a high lube oil temperature,one of them is "friction" which causes a lot of heat and as a result will bring up lube oil temperature which is used as coolant and lubricant simultaneously.
so you have to look for the malfunction in ST shaft, mech. seal, bearings.....etc
Check if you changed the OIL PUMP (attached to the turbine axis) during your last shut down. If you put a new pump, it will have less leaks and pump more oil. As the heat exchanger is the same, this will increase temperature after heat exchanger and thus decrease pressure and viscosity.
You may reverse those vibration problems by:
1. Increasing cooling in the oil (by increasing, for example, Cooling Tower Water flow)
2. Increasing pressure in the oil (by activating, for example, auxiliary pump. Control the pressure with a bypass)
3. Chemically clean the heat exchanger.
You can check my paper "Effect of lube oil temperature on turbine shaft vibrations" on google scholar.
Is there any provision of measuring bearing temperature? I have seen that with rise in vibration the bearing temperature will also rise dramatically. It may very well be that because of rise in bearing temperature the oil can no longer remove the heat from an excessive heated bearing and as a result it's own temperature is increasing.
Vibration is the cause and the temperature is just the effect. Your re-machining also points to this conclusion also.