Lube oil system in Gas Turbine 1

[ArpeshModi]

Pressure fluctuations observed in Lube oil system of Gas Turbine:
In running of Lube oil system, pressure dips (fluctuations) observed. Lube oil from tank (reservoir) is pumped by Centrifugal pump & routed via thermostatic valve (which maintains lube oil temperature of 46 deg C), cooler & filter & supplied to four journal bearings.
The pressure dips observed between 1.10 bar to 1.6 bar, when temperature of oil is particularly below 46 degC. The fluctuations are not observed when lube oil temperature remains above 47-48 deg C.

 

[25362]

In order to start thinking laterally, would you kindly let us know what kind of ester (synthetic base fluid) are u using as lubricant ? Is it by chance a phthalate ? What type of grooves the bearings have ? Is there a chance that at the recommended working temperatures any shaft misalignment is reduced and lubricant cross flow areas become smaller, and the back pressures higher ?

 

[ArpeshModi]

First, thanks for your response.
We use mineral oil (not synthetic) as lubricant.This phenomena is happening when Gas Turbine is on Turning gear (barring) operation.In other Turbine (also in this particular Turbine for that matter), it was not happening before i.e. even when lube oil temp. is around 40-42 degC, there were no pressure fluctuations observed.It happens below 40 degC temperature.But now it has started.
Could we doubt on oil quality ? If yes, what are the parameters of oil, that we should look for testing & comparision?
Other observation is, it is not happening at higher temperature, when Turbine is running on load.So doubt in other components could be eliminated.

 

[25362]

Without yet analysing the mechanics of the bearings, let's know the viscosity cS at 40 Celsius, the VI, and the oxidation stability of the oil (D943) and whether these changed of late.

 

[25362]

At full speed and higher oil temperatures one gets a highly turbulent flow in the space between the shaft and the bush that creates a thicker oil film keeping "normal" back pressures.

At other velocities, temperatures, and flow rates some effects appear to reduce the film thickness, such as the Couette effect, non-turbulent conditions, shearing of the lube additives, cleaning of debris. One wonders whether the small difference in viscosities between 40 and 42 Centigrades, by itself, for high VI (>100) oils could be capable to produce marked changes (fluctuations) in back pressure.

Other hydraulic effects may also be taking place. For example, does the oil absorb much air or moisture ? What about the oil filters, type, state of cleanliness, etc.? What about the small amount of oil remaining in the bearing when the turbine comes down ? Does it get scorched or coked up because of metal heat transfer effects lacking any cooling by air ? If so, what happens when re-commissioning until full load is achieved, how long does it take to clean out the bearings ? What are the temperatures of the oil returning from the bearings at all times, at turning gear or at full load ? Is the oil resistant to oxidation at the temperatures encountered on idling, at full load or when stopping ?
All these assuming that no changes have been made to the oil (rotary?) pumps and to the dimensions of the holes or grooves in the bearings.

The questions are many. I suggest you make an in-depth analysis of the whole hydraulic system before reaching a definite conclusion on the oil intrinsic quality.

 

[25362]

Moisture can appear in an oil by oxidation, and condensation-type of polymerization, besides absorption from the atmosphere. This happens when the oxidation inhibitor is depleted or absent. In these cases, even below 120 centigrades, one can get oxidized lubes in contact with air and in the presence of suitable catalyst ions such as Fe[sup]2+/3+[/sup]. Oxidation is the precursor to sludge and deposit formation. Solid deposits can pass through filters until they grow to a reasonable size to be retained by them.

 

[ArpeshModi]

How this oxidized oil can lead to fluctuations in pressure below certain temp. of oil (in our case, 406 deg C)?
Please explain the effect of moisture & oxidation.

Reply to your querry on earlier questions:
Lube oil return temp. in case of full or partial load is 64 to 66 deg C. In case of turning gear operation, it depends on supply lube oil temp.There is hardly 1 to 2 deg C difference in supply & return temp.

 

[1969grad]

In addition to the above, have you checked the installation of the thermostatic valve? Does it have the correct element?