Tag Archive: deaeration

Oil deaeration options

Air in oil is not good, if there’s a lot of air in the oil, the hydraulics don’t behave the way you expect them to:

– efficiency losses
– cavitation
– loud noise
– faster oil ageing
– higher temperatures
– changes in oil properties
– control problems

Air in oil impacts the life of the oil and the life of the components in the system. The components can be damaged because of the air in the system. The cavitation effects cause damage to components and create loud noise, micro-diesel effects and increases in temperature make oil age more quickly. This causes deposits to alter the oil viscosity and reduces the fluid thermal conductivity. Free air in the oil also increases the compressibility of oil and makes control unresponsive and imprecise leading to certain safety problems.

How does the air get into the oil?

There are a lot of ways, but the major ones are:
– by liquid sloshing in the tank
– by the pump suction the air
– through damaged seals in cylinders
– as a result of flawed filter tank design
– during maintenance works
– when equipment is connected

So what are the ways to decrease air bubbles in the hyd tank?

1. Size/volume of hydraulic tank

Increasing the volume of the hydraulic tank is the simplest way of deaeration. The bigger size of the hyd tank makes more time to get the air out of the oil by letting the oil sit there so that the bubbles can rise to the surface. With this higher flow rate pumps required a higher tank size to provide the required time for the bubbles to rise before it gets sucked up by the pump again.

The generic rule for tank size is 3 times all pumps flow for industrial applications and 1.5..2 times all CHARGE pumps flow for mobile applications. More details you can find here.

This is the simplest and cheapest way of deaeration. Another benefit of a big volume of the hyd tank is higher heat rejection via the tank surfaces. But if an upsizing hyd tank still works well for industrial applications, the size and weight of a huge hydraulic tank in the mobile equipment is a big concern.

2. Hyd tank configuration

Another way to get more time to get the air out of the oil is making as long a path of oil in the tank as possible by putting return and suction ports as far away as possible. For this, the tank is designed with baffle plates to split volume into two parts. In some cases, the baffle plates themselves have diffusers for deaeration.

3. Return in-tank filter diffusor

The return filter primarily removes the dirt, but in addition to that, the return filter is a return port and the design of the filter body dramatically affects flow and deaeration. Hydac made research and provided an interesting comparison of decreasing air content depending on the design of the return filters:

As you can see, even if you add a diffusor, the air content decreases almost to the initial level within 20 mins. In addition to that, Hydac designed special filters that significantly improve the air separation capacity of the entire system. The generic idea is described by Hydac:

When small bubbles of air reach the mesh of Hydac’s unique filter housing, they join to form larger bubbles. Large air bubbles have greater buoyancy, therefore they float to the top more effectively and are separated much more quickly. The outlet has a special shape that allows the oil to flow out above the oil surface level. This increases the contact surface between oil and air, allowing more air bubbles to be separated off. The filter’s large outlet surface area makes the oil flow out of the filter at a slower speed. This gives the air bubbles more time to rise to the surface.

This is an interesting innovation lets save up to 40% on tank volume and reach to the initial level of air content within 20 mins. But Hydac offers this type of filter by special order and requires minimum order quantities, or have longer lead times, or both of these. Also, to achieve the maximum efficiency of these filters you need to arrange individual consultation with Hydac for CFD analysis of the whole system, which can take time and money. I do not say it is bad, for serial products it may be a perfect solution but for unique project this will not work.

Hydac RFT series return in-tank filters

4. Use Bubble Eliminator

Bubble elimination device using swirl flow capable of eliminating air bubbles from hydraulic fluid.
One of examples – BM series of bubbles eliminations from G.E. Totten & Associates, LLC. Here is their catalogue.

The device consists of a tapered tube which is connected with a cylindrical-shaped camber. The fluids contained bubbles flow tangentially into the taped tube from the inlet port, generating a swirling flow. The fluid pressure along the central axis of the flow decreases, then the bubbles move to the central axis based on centrifugal force. Bubbles are trapped in the vicinity of the central axis because of a difference in the specific gravity of the oil and the bubble, and collected near the range of a vent port where the pressure is lowest. When some
back pressure is applied by a check valve or an orifice located at the downstream side of the bubble eliminator, the bubbles are ejected oneself through the vent port.

This is also an interesting but not popular yet solution for hydraulic systems since requires extra components that take up extra space which is the pain for mobile equipment. But seems to be a pretty efficient way to keep tank size small and oil clear from bubbles.