In our 30MW steam turbine vibration monitoring system  showing vibration reading more than normal level but within trip limit some times.but it is not showing continuously more than normal level reading but not in regular interval also.but we have checked all electrical connection related to vibration monitoring system,all are ok,even we have separated all earthing connection of vibration monitoring system it is now given separately.but still we cant find out the reason really where it is?whether vibration really there in turbine or electrical problem?if it is there in turbine  means why it is not coming continuously?   

maybe you're having sporadic steam quality issues upstream.

An intermittent vibration in a steam turbine could be a result of changes to: Steam conditions, speed, load, lube oil temperature or something else.  You don't provide enough information to say more.  

Johnny Pellin

After doing lots of analysis with vibration specialists we came to know that source of vibration is Gear box of the steam turbine,88 HZ of frequency is coming from gear box.

what are the possibilities of the vibration in the gearbox(internal&external)?.
this vibration is not coming continuously,why?

You did not tell us how fast your turbine is running or what the ratio of the gearbox is. This makes it difficult for us to comment. 88 Hz could be the running speed of the turbine. This could be imbalance in the bull gear. It could be a coupling misalignment on the high speed coupling. It could be intermittent based on changes in load, speed, flow, temperature or some other variable. I have seen intermittent high vibration in gearboxes associated with pinion instability. Under a certain combination of load and speed, the pinion becomes unstable for a period of time and then recovers and becomes stable again.

You need to tell us the speeds (input and output) of the gearbox and other critical details about the service in order to get a meaningful response. What is the gear configuration? What types of bearings are used? What is the driven machine? How do the load and speed vary over time? What types of couplings are used?

Johnny Pellin

Did you check the oil level in the gearbox?

our turbine speed is 7059 rpm.this speed is gear box high speed.and gear box low side speed is 1500 rpm. and oil flow in the gear box is proper.but we believe that due to some external factor only this intermittent vibration is coming.because if it is a internal problem means it will be coming continuously ,isn't it?

Bad bearing??

Will foaming in lube oil create vibration in gearbox?

"After doing lots of analysis with vibration specialists" Really?
The data provided does not indicate there is a fault in the gearbox, unless it has antifriction (rolling element) bearings.
Turbine speed: 7079 rpm = cpm
Problem Frequency: 88 Hz = 5280 cpm
Generator Speed: 1500 rpm = cpm
So if gearbox output shaft has AF bearings, then 5280 cpm could be a bearing fault frequency; need to calculate from bearing geometry. Another possibility is subsynchronous frequency in turbine rotor (balance resonance).

Walt

is it possible to use anti foaming additives in lube oil tank to reduce foaming in lube oil?
I think it will reduce gear box vibration.is it?
has anyone used anti foaming additives in lube oil in their power plant for foaming problem?

Besides the vibration, what other evidence leads you to believe that your oil is foaming? Sight glasses? Sampling?

If you oil was foaming, you'd have more issues than what you have described, such as air build up in filter housings, pump binding, erratic/unsteady oil pressure. Look for these signs first before addressing a foaming issue.

If the oil is indeed foaming, oil doesn't foam without some other contaminants being present. So you need to look at what kind of oil filtering/cleaning system do you have? Do you do continous or periodic clean up with a centrifugal deleval, or do you do batch oil changes with heated settling tanks? Oil filtering? Do you do reguarly oil analysis? What viscosity of oil is being used ISO 32, 46?

sight glass is there in the lube oil return tube line.i confirmed this foaming by seeing this glass only.

but i dont have any problem like air build up in filter housings, pump binding, erratic/unsteady oil pressure.

we are having oil centrifuge pump for oil filtering.we are doing regular oil analysis every think is ok.but foaming is there.

for this vibration problem i have doubts on this foaming.am i right?