Nat freq of vented sightglass connected to sealed oil reservoir

[electricpete]

This is the oil reservoir on a vertical motor.

A figure is on page 1 here:

http://home.comcast.net/~electricpete/eng-tips/OilReservoirNaturalFrequency.pdf

The oil reservoir is sealed from atmosphere (although not expected to deviate significantly from atmosphere due to many different joints that may not be airtight). Connected below the oil level is a tap for a sightglass that is vented to atmosphere.

We have experienced oscillation of the oil level visible in the sightglass (we can't see what the reservoir is doing).

I vaguely remember seeing some textbook problem about a manometer or something similar where natural frequency could be calculated.

I tried to lay out the problem as a SDOF mass/spring system and solve on pages 2 and 3.

Since the reservoir cross sectional area is so much larger than the sightglass area, I focused on the oil in the reservoir. If level goes up by a small distance detlah, the trapped air is compressed which tends to push it down. I neglected any effects of the attached sightglass. I also assumed the relevant mass was the mass of oil in the reservoir above the sightglass tap. That makes some intuitive sense to me, but I can' articulate a firm basis for doing that.

I have neglected effects of the small portion of oil around the standpipe and inside the bearing which may be subject to a different pressure (the pressure associated with rotor suction).

Any comments on the model? Is it correct for the simplified system that I am trying to model?

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[GregLocock]

SO, either k is wrong or m is wrong, by a factor of 25.

I think m is obvious enough.

Possible reasons why k might be wrong.

1) You've used isothermal not adiabatic expansion pv^1.4=constant

2) massive leaks

Or... the model is wrong.

Sorry that's not very helpful.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hacksaw]

sloshing in the reservoir can be a problem, what natural freq due expect for wave action?

 

[GregLocock]

I haven't got Blevins to hand, what is the expected resonance for the U tube manometer itself?

Secondly, you need new pdf generator! pdfcreator seems to work well.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Blevins gives this formula in Hz for a U tube with unequal arms

1/2/pi*sqrt(g*(1+A2/A1)/((h*A2/A1)+L))

where A2 is the sightglass arm, L is the length of the arm, h is height of the outlet below the mean level.

As hacksaw says as well, blevins mentions sloshing coupling into this system

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

Thanks guys, I appreciate the help.

I was relying on someone else's description of the "oscillation". When I got to look at it myself, I realized it was very erratic movement of the oil in the sightglass, jumping up and down an irregular intervals, not always the same amount. It is not regular movement that would be expected from a resonant system.

I removed the oil fill cap (above the oil level) and the sightglass level jumped down 1/4". Put it back and level jumped up 1/4". This behavior was repeatable. Then tried the same thing on a sister motor where sightglass level wasn't "oscillating" and no change in sightglass level. I conclude the motor is drawing a vacuum on the air above the oil (motor communicates to oil through standpipe clearance and bearing). Perhaps the gaskets on this motor are tighter than on the others.

In summary, I believe now we are just seeing pressure fluctuations in air above the oil, and resulting fluctuations in sightglass level. No change in actual level and no resonant behavior.

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[btrueblood]

Wait, are you measuring/observing the sight glass while the motor is running?


... Do you check the oil level in your car's engine while it's running? Then why would you do so on any other piece of equipment?

 

[electricpete]

yes, while the motor is running. There are no oscillations while the motor is shutdown.

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