Steam Turbine Lube Oil Tank 1

[Emkel]

Just curious about something, There are some electric heater in turbine oil tank. I'm sure The purpose of the heater is to heating the lube oil inside the tank.

What is the purpose of the lubricant being heated? While the lubricant oil will be cooled back in the oil cooler.

 

[byrdj]

May I ask what latitude you are at?

to answer though. the lube oil (and also your high pressure control oil system) are fairly viscosity/temperature sensitive. the pumps just can not operate with cold oil. the lowest minimun operating temp I recall was like 25C for lube oil pumps and then 30C to go on turning gear. The HPC oil oil had minimun pump start temp near 32C.

the reason for you location now comes in....

If your unit is removed from service, say for a couple weeks outage, AND the local envoroment is cool, the oil temperture can decay below minimun pump start. I recall the VERY upset utility managment that had to face a day delay in returning an unit to service following an on schedule outage, due to failure of operations to remove the tag out from the oil heaters earlier. this was inside a heated turbine hall, but due to exterior cold.

Maybe I need another cup of coffee, but I can't seem to follow your lube oil display screen.
does the coolers discharge go to the bearings some how.
is those two "pumps?" to the orange header tank for high pressure jacking lift oil to the bearings

 

[crshears]

In the steam plant where I was an operator, oil "whirl" could become an issue if the lube oil was too cold.

Also, although oil heaters and coolers will not commonly if ever be operated at the same time, as byrdj notes there may well be circumstances where both may have to be provided in order to satisfy all applicable service requirements.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

Filter element damage used to be a problem with cold oil on our machines. We occasionally used to use the emergency pump to circulate the oil a little bit because it bypassed the filters. The poor design of the tank used to result in oil approaching frying temperature near the heaters and stone cold near the thermocouples which gave the permissive to start the auxiliary pump.

 

[byrdj]

I didn't think farther and recall immersion cal-rod type heater in most tanks with no means to circulate. inspecting them they were coated in burnt oil.

 

[crshears]

Again, back in my steam operator days, except for maintenance outages, whenever the turbines were shut down they were kept on turning gear 24/7, with an AC-driven auxiliary lube pump running continuously to keep the bearings flushed with oil. Under these conditions the turbine lube oil tended to find its natural temperature balance, requiring neither continuous heating or cooling to maintain it at the proper temperature for turbine start-ups.

The DeLaval oil purifying centrifuges, one for each unit, were also normally kept in continuous operation; there were Chromalox electric heaters in series between the supply from the turbine oil tank and the centrifuge suction, but these would only switch on, and then in stages, as required to preclude adverse effects on centrifuge function. Early on during extended maintenance outages, the centrifuge bowls were also cleaned out so that as the maintenance outage neared its conclusion the turbine oil could be slowly pre-warmed and purified prior to the turbine going back onto turning gear.

The set-up worked very well.

Byrdj, I do not recall ever encountering or even hearing of any instances of oil pocketing thermally within my utility; our TOTs must have been well designed...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Hoxton]

Perhaps the designer of your plant (those hated consultants who know nothing) had a meeting with the plant purchaser and the topic came up of how to start the plant after a two week shut down in the middle of winter with ano external power. The minimal cost of heaters, compared with the cost of the extended down time can be a factor.

This is all forgotten after twenty years...

 

[Emkel]

we are in the tropics, so there is no cold temperature

I think oil whirl and oil whip occur when the lube oil pressure is too high, I do not know if there is a relationship between lube oil temperature and oil whirl.

So the heater only operated during start-up after the system stopped operating for long period of time? Or also operate when the system is operating under normal conditions?

 

[byrdj]

first oil whirl is very viscosity dependant. and viscosity being very temperture dependant.

most large steam turbine bearing oil supplies are design so that there is very little pressure in the oil flooding the bearing, even though the bearing header pressure can be 30psig. the pressured header is orificed to supply ample flow to remove heat.

However, I have seen applications where the bearing header pressure was increased greatly and I suggested it could lead to whirl while the biggest prolem was oil leakage out of the deflectors.

the units I worked had the tank heaters set to about 90F. that was the recomended temperature for opeation of the lift pumps and turning gear. with the unit in service, the returning drain oil to the tank was at least 130F and as high as 180F, thus the heaters should switch off when at speed

the designs I worked allow the tank reservoir to be the returning oil temp and only the pressurized supply to the bearing header was cooled.

the oil whirl, and probabilty of failed boundary lubrication, required the oil temperture of 90F for turning gear to be propertionally increased with speed. the temperture at running speed was between 110F and 120F, with the higher being used if oil whirl was expected.

Is your original provide oil system screen correct. I think there is several errors in its reperesentation of an actual system

 

[crshears]

So the heater only operated during start-up after the system stopped operating for long period of time? Or also operate when the system is operating under normal conditions?

Yes to the first, no to the second. To expand on that:

In accordance with what byrdj noted as common behaviour, when the units [there were eight of them] were in normal operation, the turbine oil temperature rose significantly, necessitating the continuous use of oil coolers.

When a unit was being returned to service after an extended maintenance outage involving turbine disassembly, the turbine was placed on turning gear for varying lengths of time, almost universally measured in days, to eliminate any shaft sag that would have occurred during the maintenance period.

In anticipation of the unit being placed on turning gear, the turbine oil would be brought up to within its normal shutdown range using said electric heaters; other than then, they were not commonly active.

In the "shoulder" seasons [spring and fall] when power demand was generally lighter and few if any units might be required in service over a long weekend, the outdoor temperatures were such that even within an uninsulated plant the indoor temperatures did not fall sufficiently to thicken the oil to an unacceptable degree...as long as the units remained on turning gear, which they normally were.

And during long weekends with bitterly cold weather, one unit was kept in service at minimum load so it could supply extraction steam or, in extreme cases, pressure-reduced and desuperheated live steam, to the building heating system, keeping the plant interior at a minimum of 15°C, so again, the electric turbine oil heaters did not [normally] come into service.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[MJCronin]

Emkel ...

You say that your plant is located in the tropics and you wonder why an oil heater is included in your equipment configuration.

Was the steam turbine and lube oil system originally located in some other place and relocated to the tropics ?

What does your owners manual state about when the oil heater should be operated ?

Have all of the plant documents been discarded by management when the equipment was purchased ?

MJCronin
Sr. Process Engineer