With cylinders, for instance, the volume of hydraulic fluid in the external system, as well as in the reservoir, changes. If there were no breather, varying pressure and/or partial vacuum would occur in the tank. The breather typically has some mesh in it that allows flow of air but prevents the entry of particles, such as dust.

In every closed hydraulic tanks air breather mounted on air chamber side and its function to maintain the pressure on hydraulic oil to allow the suction pressure for pump. And excess pressure should released to atmosphere to maintain pressure in hydraulic tank and suck air from atmosphere if pressure drops.

In any closed hydraulic system there is a number of reasons for the tank to exchange air from ambient.

The most obvious one is the temperature (expansion/ contraction) of the fluid.

The second is the flow into various sumps and pockets as the system is switched on first time/ after some gap. This includes maintaining the oil levels in the pipelines.

Along the not so obvious is the air that is sucked in as soon as the system is switched. Even if all the lines are full, on switch on, there is a bit of time delay from when the pumps starts delivery to when the drain starts to deposit back to tank.

If you have access go through API2000 (of course it deals with unpressurised tanks, which any way will be your likely case) - it not only explains different reasons (a few more than i have mentioned) but also recommends venting capacity based on tank size (table-2), Fire (table 3), Environment (Table 4), somewhere there is a requirement based on Pump Flow too (If I remember the capacity must be at least 1.5 times the pump Q) and others.

Just preventing pressure make-up due to Temperature influence on Hydraulic level, and prevent to breath wet air and water contamination of the Hydraulic liquid as Transformer oils

Joshua Gabriel SAADA

Engineer consultant.

Alternative Power Systems
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Air breather, the very name explains that it breathes in and out. While need for this is been well explained, I would like to express the maintenance concerns on this.

While breath-in, the dust may find a way into oil reservoir and when breath-out (suddenly) oil mists get sprayed/spilled out. To filter the dust and suppress the oil mist, fine filter elements are used.

Some breathers do have dryers along with, as to ensure dry air. This would have drying agents like (color Indicating) silica gel.

Oil mist and dust together forms a pasty mass and could block the breathing. Needless to say the consequence of this.

In case of dryer, if the drying desiccant gets inactive on long usage, would permit wet air on every breath-ins. This would add moisture/condensate to oil and contaminate.

As regular 'Preventive Maintenance' practice, it is advisable to include 'cleaning the filtering elements and replacing the drying desiccant'.

Though many experts comment such simple PM checks are too obvious, on the field, it is pathetic to see most (well, at least some) PM performers neglect such simple but impotent, simple checks which have direct impact on the reliability on the equipment.

The Air Breather cum Oil Filler normally has two functions;

1) Air Breather: During operation the oil level inside the reservoir changes continuously. This necessitates the entry of outside air, which may bring in dust and impurities. The felt type air filter inside the Air Breather cap filters this air.

2) Filler: The cap can be unscrewed when fresh oil topping up is required.

Answer is given in previous posts.

I was looking for a "breather bag" to instal on a large hydraulic tank such that the internal expansion/contraction would be closed loop. Some sort of blow out device (velcro strip??) would be needed as well as the support mechanism.

Anyone know of this kind of device?

That is a feasible method, I know some hydraulic power packs use a bladder inside a container in which the oil is stored so it is not required to have a breather.

You might check with marine collapsible water storage bladders which should also be oil resistant, if you can find one big enough to contain the volume displacement there is your answer.

For internal breather bag we hav eused Schrupp, I think made in UK. Must be supported below, and have had some reliability issues. also long lead time.

for external breather tank, KleenVent bags. Used to be Greer, now Greer is owned by Parker. Big and bulky, but excellant results.

Either way, a pressure vaccuum relief valve must be used on the tank. Prker Bruning has one, but it is small and is 5 psi pressure relief. I am looking for a 3 psi one and larger size, but have not found one.

kcj

minnesota