Bearing housing with closed system oil leveler: breather or not? 2

[82ghost82]

Hi community!
I'm designing a bearing housing for a pump (in reality this is not really a pump, but for simplicity, let's assume it's like a pump).
Inside the bearing housing there are a radial ball bearing (1 row) and a axial/radial ball bearing (2 raws), both with the same internal diameter, as in all generic pumps.
The level of the oil is maintained at half height of the lowest ball of the bearings.
The level of the oil is controlled with a constant oil leveler (Bull-eye from Mertens, it's the same as the watchdog): so the horizontal pipe connecting the bearing housing and the leveler is half filled, like in the following picture:

The bearing housing will have also 2 bearing isolators (type labtecta 66, from Aesseal).

After this description, here are my doubts:
I was reading this thread:

https://www.eng-tips.com/viewthread.cfm?qid=444370

At the end of the discussion the user recommends to remove the breather, replace with a simple plug, and attach that type of oil leveler.
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However, when the oil gets hotter the bearing isolators will allow the air to go out of the housing;
when the pump shuts down air temperature will decrease inside the housing but bearing isolators will not allow the air to enter the housing from the outside (and this is ok if I want to prevent contamination): this will create a vacuum inside the bearing housing.

Look at the following picture:

The oil level will be in equilibrium when P1=P2+rho*g*h
rho = oil density (let's assume 800 kg/m3)
g = acceleration of gravity (9,81 m/s2)

So, let's say my air temperature reaches 70 °C when the pump is running, and I'm at atmospheric pressure (101325 Pa) (because the bearing isolators allow the air to go out): assuming h = 0,12 m:
P2 = 100383 Pa

Ok, now, let's shut down the pump, air temperature inside the bearing housing will decrease (let's say to 25 °C), but this time bearing isolators wont allow air to enter from the outside to compensate the pressure, so the bearing housing will be in vacuum.
Since P1 is lower now, (P2+rho*g*h) must decrease to have a new equilibrium, so oil level inside the leveler will decrease and oil will enter the bearing housing from the leveler reservoir, but this time there will be more oil inside the bearing housing that the perfect needed amount.
If I start up and shut down the pump n times the level inside the bearing housing will continue to grow (!!).

Is it right or am I missing something?
Does this mean that a breather (better, dust/humidity filter) is always necessary?
With a breather, pressure inside the bearing housing will be always atmospheric (regardless the temperature) and (in an ideal world, assuming no leakage) the oil level will be always perfect.

Thank you, I hope I was able to explain well my doubt.

Daniele

 

[JJPellin]

No the level inside the housing will increase once, by a relatively tiny amount. Unless the end of that tube is exposed, allowing more air to enter the oiler bulb, it will not add more oil. My plant has about 1500 pumps set up with this type of oiler. We are in Minnesota. Our pumps are outdoors and unprotected. The temperature here can vary from 100 F to -40 F. We have never seen the problem you have described.

Johnny Pellin

 

[82ghost82]

Thank you very much for describing a real case.
I think I figured out what's wrong in my description: once I have vacuum after the first cooling of the bearing housing, if I start again the pump from vacuum/ambient temperature the bearing isolators will not seal the housing, because the oring on the isolator allows to have a tiny gap, but the bearing isolator will allow to enter some air through the labyrinth to compensate the pressure, so pressure inside the housing will reach again the atmospheric value. <--- This will not happen with a filter, because no vacuum is generated inside the bearing housing.
So I can understand now what you are describing: oil level will increase only the first time by a tiny amount (due to first decompression of the housing), but on the second cycle it will start again from atmospheric pressure.

So, just to have a clear idea: should I be safe to install that type of oil leveler and put a plug instead of a breather, right?
A filter could be better to prevent 100% (I know 100% doesn't exist in real world) contamination from the outside, but I'd want to not add things that could give only little or no advantage to the system.

Thanks again for your answer

Daniele

 

[TugboatEng]

The company that manufacturers the winches for my tugboats has been in business for 114 years. They just recently figured out to remove the breathers and install plugs on the planetary gearboxes between the motors and the main chain case. This has been a very good revelation.

 

[JJPellin]

The is no reason to install a breather with this type of oiler. If you are very concerned about thermal expansion, install an expansion chamber. We have several hundred of those. They seem to work very well.

Johnny Pellin

 

[82ghost82]

Again, thank you.
No, no need for the expansion chamber, but thanks for letting me know about it; since bearing isolators will allow air to go out when the machine runs (air thermal expansion, machine running) the maximum pressure inside the housing will be atmospheric.
I just did some basic calculations and I could expect a max of 1,5 mm increase in height of oil level inside my housing (and only if the housing will be under vacuum, which means machine stopped (machine not installed outside but in a controlled temperature environment)).
If the housing is ideally sealed (allowing only air to go out for thermal expansion) and with no leakage I think I would notice the level of the oil inside the leveler reservoir to go up (default/predefined level, housing at atmospheric pressure) and down (about 5-7 mm less than the default level, housing under vacuum) and so on, because of air thermal expansion/compression (machine stopped/running).

I only want to point out that this should be valid not only for the watchdog style levelers (half pipe filled, closed system) but also for the trico types (operating in closed system, full pipe filled): physics is the same.

 

[JJPellin]

Our bearing isolators are supposed to seal air tight when not spinning. Under that condition, the expansion chamber can limit the pressure change to reduce the chance of dirty atmospheric air being drawn in. When you refer to Trico type oiler, I assume you mean the pressure equalized models. For the ones that are not pressure equalized, the physics is different with atmospheric pressure on top of the oil in the cup.

Johnny Pellin

 

[82ghost82]

mmm, ok, isolators seal the housing when not running, I agree and they're like mine.
When you say "the expansion chamber can limit the pressure change to reduce the chance of dirty atmospheric air being drawn in" I think you are referring at the moment you start up the pump? So if you have an expansion chamber/or an additional air volume the delta pressure will be lower so less air is exchanged from the outside?

Yes, I 100% agree, for atmospheric pressure on top of the oil absolutely different and yes I was referring to pressure equalized models.

 

[Wkcoo]

Hi Pellin, can I clarify why this kind oiler cup does not need a breather? I don't quite get it because almost all pumps at my site has a breather and the norm is it is a must have to ensure pressure equalization and oil from oiler cup can flow into bearing housing.