steam turbine bearing metal temperature going high

You lost lubrication to the bearing for a moment. You really need to trouble shoot your lube oil system. Was someone doing something with it? Do you switch on your aux pumps at 300 rpm? Does your shaft driven pump cut out at 300 rpm?

Need to look hard at the lube oil system, something went wrong.

many thanks mr steve

the lubrication was ok. all the auxiliary pumps came in time and the oil pressure has never gone down below 1bar. I understand from OEM that at low rpm the oil film is not adequate .

p k mohapatra

There must be adequate oil at 1bar pressure, but the speed may not be adequate to establish a steady 'oil wedge'.

As Stave suggested, advancing the 'jacking oil' earlier should give a relief.

dear mr steve

many thanks for your advice to take in jacking oil pump. that exactly we have introduced in our logic and advised our operation engineers. our OEM also agrees to the idea.

p k mohqapatra

I would recommend that you switch to an oil that has a boundry lubricant. When the oil film is depleted under shear moments, the boundry lubricant will save the bearings. I know this is not the solution to your problems, but until you resolve it, it could help.

Schaeffer and Royal Purple contain these "backstop" lubricants.

Do you recommend switching over to 'boundary lubricant' while coasting down?

Further down with the speed, the 'jacking oil' comes to action. Normally it is the same (a branch off) lube oil used for this purpose. Will not using different oil during a part of the cycle may contaminate whole lot?

I am recommending that the oil should be changed to a better product!

Mixing 2 brands or types of oil is not a good way to go. The additive chemistries may have a reaction whose sum is a product that is less effective than either lubricant individually.

Maybe at 300 rpm the shaft drops through the lubricating fluid wedge and directly contacts the bearing? Or lubricating fluid flow diminishes proportionately with dropping rpm and becomes insufficient at 300 rpm to maintain cooling effect causing momentary localized hot spot. Can lubricating flow and pressure be maintained or increased to be independent of rpm?

Packdrill

"Can lubricating flow and pressure be maintained or increased to be independent of rpm?"

Such big machine trains do have auxiliary oil supply systems including 'over-head oil supply tank' etc. Also our OP says the oil pressure is ensured throughout.

Yes, loosing the steady 'oil wedge' could be the problem.

Oil wedge lifts the shaft and prevent it from having metal-to-metal contacts. The steadiness of this wedge depends up on, 'bearing clearance, oil viscosity, oil entry pressure and speed'. Every parameter being the same speed is only variable here.

Steve's suggestion, " try turning on jacking oil earlier, say at 500 rpm." has to be discussed and considered.

I saw something similar to this scenario happen . . . we had a momentary loss of lube oil . . . caused by electrical power failure to the running #1 pump . . . the standby #2 pump should have automatically started due to the low pressure in the system. Problem was that the auto start sensor was part of the guage that indicated system pressure on the local operating panel for the lube oil system. We found the gauge isolation valve only bearly cracked open (nearly shut). So much shut that the pressure at the gauge and thus at the starting control relay for the back-up pump decayed off very slowly. Thus system pressure dropped to almost zero and a delay in #2 pump start was introduced by the slow decay of pressure at the start control relay. We got rid of the combination pressure gauge and control relay, put in a pressure switch at the remote bearing.

You will need to check with the equipment manufacturer. 300 rpm may be near (or a multiple of) a natural harmonic of the shaft. This could be a speed range with excessive vibration at the bearing. Does your installation of vibration monitoring. Many turbines have high vibration shutdown, but at a natural harmonic the vibration acceleration can be limited to one section of the shaft and continue without detection at the monitor. The OEM can usually provide harmonic information for the shaft design.

Don't bother, I can tell you right now that 300 rpm is no where near a natural harmonic of the shaft. This is not an issue for this machine.

dear mr steve

we have already introduced the logic of bringing jacking oil pump at 500 rpm as on several earlier occasions there have been high temoerature spikes at 300 rpm. the machine is back in service . hope everything will be ok during the next coasting down. i will send the feedback then.

many thanks

p k mohapatra