Turbine Bearing Vibration at Start-UP 4

If it happens only at start-up, I would closely examine the start-up procedures that they are using. I will assume we are talking about a large special purpose condensing turbine. You need to be sure that they are starting up with the correct oil temperature (not too cold). They need to get the turbine up on slow roll before they begin heating it up. They need to start the sealing steam once the turbine is rolling. And then they need to establish vacuum once the sealing steam is in. If they do these steps in any other order, they could develop a rotor bow and this will result in high vibration. It is also possible that the turbine took a slight bow because of the way it was shut down. Most notably, if the seal steam or vacuum were left on with the turbine not rolling, this can quickly result in a rotor bow. You need to know and avoid any natural frequencies during the start-up process. Depending on the coupling design, there is probably a pre-stretch on the coupling in the cold condition. There is also a thermal offset in the cold alignment across the coupling. So there may be some vibration assocated with these offsets until the machine gets up to temperature and grows into alignment and relaxes the coupling stretch. But this should be slight. When they slow roll the turbine, they need to be sure that they are staying within reasonable limits for exhaust temperatures. If the turbine exhaust is too hot, this will affect the alignment and could have more serious consequences, especially if the low pressure casing is cast iron. I am not familiar with generators and thus cannot comment on any start-up considerations for the generator.

If this is a one time problem and the vibration seen was high, you probably want to roll out the bearings and inspect them before you restart. If there is any damage to the bearings, confirm oil orifice location and sizing, oil pressure and temperature, and make sure that the oil drains are open and flowing freely.

Dear JJPellin--Your information was a huge help to me--I was a field engineer overseas some time ago, want to return, & I was trying to think of various things that can happen during a start-up--Thanks again!!! dialindicator

I don't think that one starts by checking anything. Analyze the vibration and other data to determine if you have enough information. Use the information to guide you as to what to check.

Conditions of the start? Any repairs? Why was the turbine down to begin? Talk to the operators; they may know something based on how they started it. Enough slowroll?



Regards,

Bill

Rubs can cause start up vibration. Rolling up through the first (second or third) critical too slowly can cause vibration. Differential expansion of the rotor vs shell(s) can cause rubs that cause vibration.

Already mentioned, rotor bow due to prolonged periods off of turning gear can cause rubs which cause vibration.

Rubs can cause localized shaft heating which can cause bowing which can cause vibration.

Slinging a balance weight can cause vibration.

Rotor winding shorts (if you are progressed enough through the start up to have synchronized and excited the field) can cause vibration.

Those should give you some food for thought.

rmw