drain lines termination above/below fluid level

[maytag]

This subject generated a lot of interest on IFPS board. I'm interested in any input from this group of professionals. Several posters were pro above the fluid level-several including myself were pro below minimum fluid level. The discussion started specifically about drain lines for valves and then took a turn thru Tuscaloosa and ended up talking about case drains for pumps and motors. What do the fluid power professionals here say? Maytag

 

[hydromech]

Oil dropping into a larger volume of oil breaks the surface of the oil and and tends to draw air into the fluid.

Slow drains from pilot operated valves won't cause much trouble, if that is, the flow towards the pump is not so high that the air is drawn into the pump.

Casing drains from larger variable pumps and motors can reach as much as 300 litres per minute and this is likely airate the oil very quicly to the point where the pump is destroyed through cavitation.

I was taught to always put drain lines under the level of the oil. The people who taught me are to hydraulic systems what Yoda is to the force.

Other than syphoning, which is easily preventable, I see no reason at all why any permenant drain line should not be below the oil level. Conversely, I know from experience what happens if the drain line is above the oil level.

I have never read about or heard from anyone that thinks letting oil drop into the reservoir is a good idea...

Having said that I'll bet this question draws lots of opposing comments.

Hydromech

EX HYDRAULIC SYSTEMS ENGINEER
CURRENT PUMP DESIGNER

 

[Gkranz]

I have never read about or heard from anyone that thinks letting oil drop into the reservoir is a good idea...

I am with hydromech, doing otherwise is really dumb.


Westerndynamics.com

 

[budt]

There is one Drain Line and one only that I never terminate below oil level since I believe it could cause a problem with cycle time and/or force.

That line is the port from a Double Acting Cylinder that is gravity return and must pass air and any seal bypass as itis on its powered stroke. If it terminated below oil level it would be filling with oil as it returned from external force and keep the cylinder full of oil.

The other recommendation for this port was to install a Check Valve in the unconnected port that allowed free flow from the cylinder and trapped flow into the cylinder.

The only problem I can see with this arrangement is the vacuum formed in the cylinder could suck contaminatioon in
as the cylinder is returning from external force. This could happen since the rod seal is designed to keep fluid in not the other way in most cases.

Every catalog I have read says, Pump Case Drains must terminate below oil level thoughh no one gives a reason for it. Maybe a Rexroth or Parker expert could shed some light on the reasons for this procedure as stated in their installation literature.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[Gkranz]

Bud,
Three letters "AIR", really do not want oil splashing around like in a bath tub. As a foot note; most biodegradable fluids do not seperate air very well when mixed, as petro oil will settle out faster. Another foot note; 90% of reservoirs are made wrong, simply no better than a 55 gallon drum in most cases.

Most people pour fluid into a bucket and say not to bad, now try it at 5 ft.sec. to 18 ft. sec. and you will see the problem real fast. Why would I need a Parker or Rexroth expert to tell me something I learned in 7th grade.

Quote: Oil dropping into a ANY volume of oil breaks the surface of the oil and and tends to draw air into the fluid.

This is a correct statement. Now that we got the simple stuff out of the way, how about reservoirs. Why can't we build them right? This should start a discussion that will heat up fast.





Westerndynamics.com

 

[Ashereng]

One possible reason for not coming in under the minimum fluid level is sediment. You don't want to stir it up.

Coming in above the minimum fluid level does not mean coming in halfway up the tank and splashing. It can also mean below NLL and above LLLL. As long as you come in under the surface level, you will not draw air or splash.

My $0.02 worth.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[maytag]

Ashereng, I guess I should have been more specific-above minimum fluid level was meant to be above minimum or LL. Maytag

 

[Gkranz]

Guys, what is all this LL, LLL and NLL stuff? I know what it stands for, but the lower limit level has nothing to due with where the return lines terminate. My bet is that you do not know why we even set a lower limit.

All my lines are terminated 2" to 2.5" off the botom, and no we do not stir up the "sedimant" if the oil tank is designed correctly, and if you do not have oil velocities returning greater than 25 ft./sec. into the tank.

reservoirs - cheapest to build, but has the biggest affect on whether a hydraulic system will work correctly, and the most costly to fix when done wrong.

My $0.01 worth, every penny counts.
Best Regards,
Gkranz

Westerndynamics.com

 

[Ashereng]

Ok.

Why do we even set a lower limit?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Gkranz]

Well, there are several functions a hydraulic reservoir must do to work effetely in the system.
Most good designers that do their home work know how important it is to have a good working reservoir, and also operating conditions will vary during the work cycle. The high limit is the optimum oil level to provide the least amount of work the reservoir has to do over the widest operating range.

The lower limit is set at the point the reservoir will still function, but has to work much harder, and may not work fully in all operating ranges. Below the lower limit, the reservoir will not complete any of it functions to support the hydraulic system.

Most that I have seen are built incorrectly. Most people assume the only function is to hold the system fluid which is partly correct, but they can do much more. Back in the 60’s we built a reservoir out of clear plastic and that is when we really learned how to design them right and how important they really are to the overall hydraulic system. Does not matter whether it is square, round, rectangle or whatever; build them right they will save you time and money?

Question for all? The worst thing you could ever see is when you open up a reservoir and find 1/8” to 1/4” of contamination on the bottom. True or false?

Here is another one, why are down tubes cut at 45 degree?

As you can see by the posts, most people try not to splash fluid around or make bubbles, but very few really know how or why to build a good reservoir, or why they should. Now you will come back and ask what the function of a reservoir is. Answer, you tell me?
Best Regards,
Gkranz

Hint; If it is to hang the rest of the components on like a Christmas tree, that would be a wrong answer.


Westerndynamics.com

 

[Ashereng]

Somewhere the thread went from drain lines into a tank, to evidently a hydraulic reservoir.

I thought I was answering the thread in relation to a drain line into a tank.

I don't have any knowledge for hydraulic reservoir.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Gkranz]

Well I did kinda hajck the thread, sorry. But it is a hydraulic (fluild power) forum and we do not use tanks only reserviors.

Anyway, thanks for your input.
Best Regards,
Gkranz

Westerndynamics.com

 

[hydromech]

Question for all? The worst thing you could ever see is when you open up a reservoir and find 1/8” to 1/4” of contamination on the bottom. True or false?

False...!

If the contamination is in the bottom of the tank it is not in the system. However...the particles on the bottom of the tank are there because they are too heavy for the oil to hold onto. Contamination on the bottom of the reservoir is indicative of a really dirty system.

Regarding the angle on the bottom of the return pipes. I have always assumed it was to reduce turbulance as the oil leaves the pipe...I wouild be interested to know the correct reason.

In a previous job I designed hydraulic systems and power units for marine applications. Ships don't stand still and that makes all the difference when it comes to minimum oil levels.

Hydromech...

 

[Gkranz]

Good answer, it is false. All reserviors will become dirty, (hopefully) not to the level I stated.
I too work with marine applications, and mimimum levels are adjusted for the rock & roll of waves expected. In some cases we will pressurize the reservior to help compensate.

Actually with a 45 degree cut you can direct the flow leaving the pipe tube. Generally we will direct the flow up againist the reservior wall to help cool and slow the velocity.

Gkranz

Westerndynamics.com