Air-over-Oil System

[ChipFuller]

I'm trying to trouble shoot an air over oil lift system. We pressurize a cylinder using about 90 psig plant air. The system has two air over oil tanks, a directional valve, and two pilot operated check valves between the tanks and the cylinder. The system will operate fine for a while but after a half a day of operation the top of the cylinder will fill with oil causing the exhaust line to fill with oil. This causes the exhaust to blow oil all over the place. It appears that oil is blowing past the seals of the cylinder. Any suggestions? I haven't worked much with air over oil cylinders before.

Thanks

 

[CRG]

If I understand the system, it uses 90 psi air on top of oil in two tanks. The directional valve connects the top of the tanks to the air supply or vents them to atmosphere. There is a hydraulic cylinder that is plumbed to the bottom of the tanks with a pilot operated valve in-between.

Are you sure that it is a pilot operated check valve and not a pilot operated control valve between the tanks and cylinder?

Did the system ever work correctly?

Why are there two tanks to operate one cylinder? Do you really mean two tanks operate one cylinder or did I misunderstand your post?

Is there air in the cylinder when the exhaust discharges oil or is the excess oil from one tank being empty while the other is overfull?

What stops fluid from migrating from one tank to the other resulting in one tank being low while the other is overfull? Is there a line between the tanks upstream of the check valves to allow the fluid levels to equalize between the tanks by gravity?

 

[ChipFuller]

Thanks for the questions.

Are you sure that it is a pilot operated check valve and not a pilot operated control valve between the tanks and cylinder?

It's a double pilot operated check valve.


Did the system ever work correctly?

I believe it has. This system is hooked to a small mixer. We operated it for about 9 hours today before we noticed oil coming out the pneumatic control valve exhaust.



Why are there two tanks to operate one cylinder? Do you really mean two tanks operate one cylinder or did I misunderstand your post?


There are two tanks. One tank is used to fill the top of the cylinder and the other tank is used fill the bottom of the cylinder.



Is there air in the cylinder when the exhaust discharges oil or is the excess oil from one tank being empty while the other is overfull?

I believe it's from one tank from being overfull and the other being too empty.



What stops fluid from migrating from one tank to the other resulting in one tank being low while the other is overfull? Is there a line between the tanks upstream of the check valves to allow the fluid levels to equalize between the tanks by gravity?

The two tanks are connected two each other but we have ball valve installed in between so fluid won't migrate in between. These are between the tanks upstream of the check valves.

A couple of things I've wondered about is that is seems like the hydraulic fluid is blowing past the cups in the cylinder. But I replaced the cylinder with a new cylinder and the problem still exists. According to Ortman Fluid Power, it's rated for 500 psig hydraulic pressure and 150 psig pneumatic pressure. The other thing I wonder is if the check valve is malfunctioning and maybe I should replace the double pilot operated with single pilot operated check valve.

 

[saxon]

Chip, Sounds like a problem of aeration and foaming. Has the oil type been changed recently? Have the A/O tanks been overfilled? Are they running the system at faster speeds than in the past? These types of Hyd. systems require a a fairly good disengagement volume between the air feed location and the oil surface. Also, it's good to have a diffuser/mist eliminator on the air infeed so as not to blow high pressure jets into the oil creating turbulence and mixing that leads to aeration and foaming.

Hope this helps.
saxon

 

[ChipFuller]

Thanks for the posts. I believe we have found the problem. It's internal leakage of the double pilot operated check valve.