Author Archive: Dzyanis

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.


Fluid Power world-wide pressure gauges

The company where I’m currently working manufactures and ships equipment around the world.
Although the SI unit for pressure is the Pascal (Pa), different countries use their own kind of “standards” for pressure measuring, and when designing the system I need to keep in mind what country our equipment is shipping to. Below are just my notes/recommendations, which I personally use in my practice in case if customer doesn’t have specific requirements for pressure gauges (brand/units). Since our equipment is designed and manufactured in Alberta and Texas, I use a primary scale that matches the region and “psi” scale as a secondary one.

Region Unit Gauge unit(s) scale Example
North America
South America
(Pounds per Square Inch)
kPa or MPa
(KiloPascal or MegaPascal)
Dual: psi/kPa
Western Europe
Arabic Countries
Dual: psi/bar
South Korea
South Asia
(Kilogram per square centimetres)
Dual: psi/kg/cm²

The unit converter you can find by this link.

Most popular pressure gauge scale ranges:

   30 psi =    207 kPa = 0.207 MPa = 2.068 bar
   60 psi =    413 kPa = 0.414 MPa =  4.14 bar
  160 psi =  1,103 kPa =   1.1 MPa = 11.03 bar
  200 psi =  1,379 kPa =  1.38 MPa = 13.79 bar
  600 psi =  4,136 kPa =   4.1 MPa =  41.4 bar
1,000 psi =  6,895 kPa =   6.9 MPa =    69 bar
2,000 psi = 13,789 kPa =  13.8 MPa =   138 bar
3,000 psi = 20,684 kPa =  20.7 MPa =   207 bar
5,000 psi = 34,474 kPa =  34.5 MPa =   345 bar

Hydraulic oil density and Specific Gravity

Hydraulic oil Density is the ratio of its mass to the volume of space it occupies:

## \rho=\frac{m}{V} ##

The accepted units of measurement for density according to ASTM are kilograms per cubic meter (kg/m3, SI unit) or grams per milliliter (g/mL).

Hydraulic oil Specific Gravity (or “Relative Density”) is the ratio of Hydraulic oil density to water density at the specific temperature:

## SG_{oil}=\frac{\rho_{oil}}{\rho_{water}} ##

Substance with SG <1 will float on water (like hydraulic oil); substance with SG >1 will sink in water (like honey).

Next, both hydraulic oil density and Specific Gravity vary with temperature and pressure.

Reference: [https://www.internetchemistry.com/chemical-data/water-density-table.php]
Reference: [https://webbook.nist.gov/chemistry/fluid/]

This is why the ASTM D1298-12b “Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method” states that accurate determination of the API gravity, density or relative density (specific gravity) uses a standard temperature of 60 degrees F (15 degrees C).

Reference: [https://www.astm.org/d1298-12b.html]

You can find in the oil specification table of every hydraulic oil manufacture either Oil Specific Gravity or Oil Density at the specific temperature of 60°F or 15°C.

As a result, to calculate Specific Gravity or Hydraulic oil density at 15°C and, respectively, Hydraulic oil density or Specific Gravity at 15°C. By ASTM D1298-12b water density at 15°C is 0.999103 g/ml, therefore:

## SG_{oil}=\frac{\rho_{oil}}{0.999103} ##

## \rho_{oil}=\frac{SG_{oil}}{0.999103} ##

Parker’s Gold Cup pumps: External filtration

Have you ever dealt with Parker’s Gold Cup pumps? If yes, you probably noted in the product datasheet, this pump does not require to have filtration of internal charge (boost/replenishing) pumps flow. There is just a short recommendation “the auxiliary pump fluid be filtered to aid in maintaining acceptable cleanliness levels”:

The reason for that is because Parker thinks all close loop systems need to be designed to ensure “the oil tank is protected through filtration from any ingress contamination”. Later I shall show you Parker’s document where this statement is coming from (AS-0018). Actually, question to you: do you agree with this statement?

I do not. I use charge (boost/replenishing) flow filtration for every single close loop pump at every single project via an external filter connection. Moreover, both proportional and servo control systems require even more fine cleanliness level of the element for filtration, therefore for such systems I use two separate filters: for charge flow (usually 10 mkm element) and for servo control flow (usually 2 mkm element).

But there are a couple of problems with the Gold Cup pump external filtration. Although these pumps have ports for external filtration and even a short guide in the datasheet on how to make filtration (screenshot above) due to design of internal galleries is different for all three Gold Cup pump frame sizes, and, in addition, is different for specific side a control system is mounted on, you will not be able to provide a correct filtration and even can destroy the pump if connect external filter in the wrong way. And this problem is not explained and not resolved in the product catalogue.

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