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There are advantage and disadvantages to both. We have used both and the maintenance requirements are similar. Both have filters to change and their operation is simple enough they don't need much more than that.

The dehydrator we used was too slow, although it was likely too small. By that I mean our process puts water in faster than the vacuum could remove it. Vacuum dehydration theoretically gives a drier oil while coalescing removes free and emulsified water, but not dissolved.

What level of moisture are you trying to achieve?
Those that have been at this plant awhile say that they have always had a moisture issue. This was stated based on visual inspections. Oil sampling history not available.

The unit (Steam Turbine Condensing Type) is around 70 years old. We are going for as dry as possible, maybe <150 to <100 PPM. We think that this would be a good target. Either of the types say it is possible. We look to turn the reservoir over about 24 times a day. We had the choice of 13 or 24 so thought better and went high.

Because of the original unit being old technology (basicly gravity separation) we are not sure what to expect with the coalecer or dehydrator technologies from an operation and maintenance view.
That sounds exactly like our situation with the steam turbine condensing unit and all. We had an old centrifuge/gravity unit the operators loved because they could see the water come out. The OEM discontinued parts and we were forced to look for a new system.

Without any water removal we had up to 16% water by volume.

The vacuum system came first and it's discharge is in the form of steam. One could only see if anything was coming out by placing a mirror over the discharge vent. If the mirror fogged up, it was working. We could see the steam, but the oil never un-clouded. It was taking on water faster than we could remove it. Water ppm was always higher than we thought it should be.

The coalescer seems to be working faster. We still have ~200 ppm or less of water on samples, and plenty of water drains off every cycle.

As I said, a large enough vacuum dehydrator will give drier oil, but uses more energy than a coalescer. Maintenance was similar on both units for us. The basic design of the coalescer is simpler so long term I'd expect the maintenance costs to be less, but that's a guess.

Hope that helps. Let me know what you go with and how it works for you.
You said the vacuum system was undersized. How often was it able to turn your reservoir over for dewatering and filtration?

The vacuum system we are looking at uses the water removed from the oil as a seal for the vacuum pump so it will be condensing in a water reservoir.

Where do you think the system was getting its water? Steam seals, oil cooler? We are going to try and seal this old system up using desiccant breathers and the steam seals will be new. The steam powered auxiliary oil pump is being rebuilt with new seals too.

How old is your Condensing Steam Turbines?

From what I've read the coalescer is simpler to operate.
The vacuum dehydrator was rated for a reservoir turnover rate of 240L/hr. I can't remember the exact reservoir volume, but it's in the 80-100L range. We should have been getting over 24 turnovers per day. It would have been nice to see what volume of water it was capable of removing, rather than a turnover rate, but that's my opinion. The unit had a filter, but there was also a filter in the oil supply line to the bearings.

Ours didn't have a reservoir; that might work better. Not sure.

Our water is coming from steam seals and likely other miscellaneous leaks. The seals have been replaced a few times and it helps, but we've never achieved 0 water inflow. The turbines are at least 40 years and could be up to 60 years old.

Another addition might be a head space dryer. If you get the water largely under control and are coalescing, forcing dry air through the head space helps absorb remaining moisture. I haven't tried them but I hear they're pretty good if you have compressed air in the vicinity. One manufacturer is It's called positive displacement conditioning and goes by the trade name Stealth at that company. Other may make it too, but that's the one I've seen.

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