Steam Turbines

[JamesC83]

Hi all, I'm new to the site and am a Marine Engineer Apprentice. I'm kinda stuck on a report, and I don't have my books with me, can anyone give me an answer to this question:

"What are the three reasons for oil breakdown in a steam turbine and reduction gear and what steps are taken to prevent this from happening?"

Any help would be much appreciated.

Regards,

Jim

 

[BrianBT]

Are you talking about loss of low oil flow or breakdown of the oil film.

The former could be insufficent oil in the supply tank for the pump, pump failure of blocked filters.

The former can ne incorrectly specified oil viscosity, high oil temperature, insufficent supply pressure

 

[frankiee]

Some things that I borrowed from my college notes.
Don’t know if it will help but here it is………

From hand out “Turbo- Generator Prime Mover Sub-systems”

“The oil used has to compromise between the requirements for bearing lubrication and cooling and emulsification and corrosion products and inhibit sludge formation. This sludge is formed by the agitation of an oil/ water mixture. The water results from leakage from the shaft glands or condensation in the sump and oil lines. Sludge impairs bearing lubrication and causes the erratic operation of the servo systems as unlike pure oil the sludge can be compressed.”

This from “Steam Turbines and Gearing Questions and Answers’
By G.J. Roy, CEng, FIMarE, MRINA


Question: Explain briefly the comflicting requirements for an oil to be used for the lubrication of a geared turbine plant.
A) Oxidation
B) Foaming
C) White metal bearing corrosion

A) Oxidation

Under normal operating conditions good refined turbine oil should last the lifetime of a vessel, but anti-oxidation additives are usually added to prevent oxidization under the high temperatures that can exist in a modern plant. Air entrainment , moisture and some metals, in particular copper, tend to act as catalysts helping to produce weak acids which tend to degrade the oil so that in time sledges, varnishes and resins form. Once oxidization starts, the oil tends to break down more rapidly and degradation cam proceed quickly. Failure of paint on surfaces within the oil system can also promote contamination of the oil. Also when topping up , not more that 10 per cent of the total working charge should be added at one time as a large amount of fresh oil may precipitate sludge.

B) Foaming
Some slight foaming may occur in normal service but prolonged heavy foaming should not arise, and indeed should be eliminated as soon as possible as if it occurs as it tends to accelerate oxidization and may affect the operation of control equipment. Adding excessive quantities of fresh oil may be a cause, and air leaks, pump suctions not properly submerged or too near returns and contamination maybe contributing factors. Anti-foaming additives may be added to help release entrained air in serious situations.

C) This is am attack that takes place on tin- based white metal in journal bearings and on thrust pads due to an electro- chemical reaction that produces a very hard dark skin consisting of stannous and stannic oxide on the surface of the white metal. The danger is that the affected surface loses its ability to absorb particles of dirt so that any abrasive substances entering the bearing instead of causing a minor overheating and thus melting the white metal so that they are absorbed, now can score the journal or trust pad. The hardened surface is also very brittle so that should a piece break away, it can score the journal or pad surfaces. It is thought that water plays an important part, and all possible sources of water contamination should be removed whilst filters capable of removing fine particles of water should be fitted to the system.

 

[BrianBT]

Water in the oil when entering the oil wedge is adiabatically compressed and flashes off to steam thereby weakening or causing collapse of the oil film