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Oil pressure

Oil pressure

Oil pressure

(OP)
Hi forum members,

I`m actually on a site in the far africa. My steam turbine, rated at 20 MW, and fitted with a woodward governor 505 is experiencing some problems. The  lubricating oil pressure is dropping as the load on the turbine is increased with the result that the turbine is tripping on low lub oil pressure. The control oil pressure is stable.
Grateful to have your expert views.

Grundig

RE: Oil pressure

like you, I don't understand.  the lubricating (bearing) oil should be a sub system to the main oil system, or in most cases, a completely different system.  Another egnima is bearing oil demands changing with load.  I could understand speed, but what effects does load have on the bearing flow demand.

Now if the bearing is a subsystem AND is the excessise flow of the control oil so that as control oil flow increases, possibly due to excessive valve operator demand, the bearing flow is reduced.  A system where the control oil pressure is controlled and the pressure reducer vales waste is the bearing oil.

RE: Oil pressure

Do any of the ST bearings have gland sealing steam on one side of them?  If so, it could be that the seals are leaking gland steam into the bearing housings so that the oil pressure can't overcome the cavity pressure.  It could be a seal problem or a gland seal steam control problem.

rmw

RE: Oil pressure

Sounds like an interesting problem.

Does the pressure drop follow any trend when compared with bearing metal or drain temperature? I was puzzled by a similar problem on a big diesel generator where oil pressure built up initially then dropped below trip level as the set warmed up. Turned out an oil director designed to cool the piston and conrod little end was missing leaving an open port on the oil gallery back to the sump. The drop in oil viscosity as temperature rose made a big difference.

Could be a viscosity-related problem in the pump - excessive wear or clearance perhaps? I ain't a pump expert - hopefully byrdj or rmw will comment whether this is likely.

Have you got any accurate local indication of oil pressure at the bearings? The bearing with the problem will have a likely have a slightly lower pressure than the others, but you will need something fairly accurate to pick this up.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Oil pressure

I had a similar problem with a diesel engine. The bottom bearing halves were shot. When a load came on, the crank shaft deflected and moved away from the oil supply holes in the top bearing halves. I can't see how this would happen with a turbine, but the problem may be thrust related.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Oil pressure

Either what rmw suggests, or its possible that a thrust bearing is shot and the shaft is moving axially under load.  

RE: Oil pressure

The pump shouldn't know what the unit load is.  It is merely pumping against the system resistance curve with the flow and head being (among several) a function of the viscosity which is a function of temperature.  If the lube oil coolers are working, the tempearture of the oil shouldn't fluctuate much or that is to say very far out of its range.  That would be one thing to check.

If this is a bottom entry type bearing, I like Bill's idea.  The more the torque on the shaft, the more it will tend to try to 'climb' the bearing 'wall'.  If the clearance is excessive, it might be climbing high enough to open up a free flow path of the bearing oil out of the bearing quickly overrunning the pump's capability to keep up.  It is a different version of what PHovnanian mentions above with the problem being excessive clearances that open up free flow paths and lessen the system resistance on the pump.

Guardiano, tell us more about the overall lube oil system.

rmw

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