The Numbers behind the Hydraulic Filtration

Written by Camilo H Rueda Monday, 28 November 2011 09:48

Articles - Education

This article is intended to look a little more in detail the numbers or formulas that are behind the theory of filtration in hydraulics.

The experience of the designers and users of hydraulic equipment have found that: over 75% of hydraulic system failures are a direct result of the contamination!

To start, anything that is in the hydraulic fluid like, gases, solid particles and other liquids are contaminants. The most devastating of all of them is solid particles. Gases can cause malfunction and further wear and liquids can generate viscosity alterations that can lead to wear also.

We are going to concentrate in solid contamination.

What is a micron? It is a thousandth of a millimeter or 0.00004" and 1 millimeter is 0.04"

Minimum size a human can see: 40 micron

If you compare the size of the substances with the typical clearances in the hydraulic components, you can see that they are very close so damage can happen.

The cost due to Contamination can be measured on the following aspects:

- Loss of production-MACHINE STOPS

- Replacement of worn items

- More frequent replacement of the oil.

- Higher general maintenance costs.

From different studies, we know that depending on the environment the ingression of foreign particles in a hydraulic system is as follows:

Mobil equipment 10⁸-10¹⁰ particles /minute

Factories, contaminated air 10⁶-10⁸ particles /minute

Factories, clean air 10⁵-10⁶ particles /minute

During the last 30 years some sets of standards has been developed trying to measure the degree of contamination in a hydraulic system to be able to have it under control.

In 1987 The International Organization for Standardization ISO created the ISO 4406 - 1987 intended to use Method for coding the level of contamination by solid particles in a hydraulic system. This was later updated with ISO 4406 - 1999 that is in use in these days.

Given the importance of particle size, there has been selected 3 different ranges or numbers for the code as follows:

First number for particles > 4 micron

Second number for particles > 6 micron

Third number for particles > 14 micron

So, in the table above the code has:

From 1300 to 2500 particles larger than 4 micron

From 320 to 640 particles larger than 6 micron

From 40 to 80 particles larger than 14 micron. All these per ml of fluid.

Technology has improved enough in these days to be able to count the particles contained in a sample of hydraulic fluid. The result of an analysis would be a table or just the ISO number like the sample picture at the right side.

Hydraulic manufacturers make recommendations of what  the contamination levels are  according to the type of components.

Every hydraulic component has a maximum contamination level suggested.

Let's see a general recommendation.

To establish and maintain a target cleanliness level for a hydraulic system a couple of points should be followed:

1. Type of application or type of machine.

2. Maximum flow and pressure.

3. Volume of fluid in the entire system including lines and actuators.

4. Select the most sensitive component( pump, valve or actuator) and find its cleanliness target level.

5. Fluid type.

6. Operating temperature.

7. High vibration or shock?

8. Is the machine critical in the manufacturing process?

9. Can the system cause a safety hazard if it fails?

Using the table above, select the most sensitive component and the iso number that corresponds to it; it would be your target. Take into account that if you are using fluids different than petroleum oil, your cleanliness target will be one step cleaner.

Selection of filters

To begin to understand how we can apply a filter to a hydraulic system, we need to know how they are manufactured.

Surface filters like those made of metal mesh trap particles in its surface and the size of the pass determines the size of the particles trapped. It is possible to filter up to 25 micron.

In-depth filters use a number of layers of random oriented fibres that allow to trap many different particle sizes inside(the whole volume of the filter element). These are the most common filters and can be used in return and pressure lines.

As there is not a specific pore size to filter, manufacturers perform tests on filters to determine its efficiency that is why there is a standard procedure to classify filters according to filtration ability.

A filter's absolute rating is the diameter of the largest hard spherically shaped particle that will pass through a filter under specific test conditions. This does not give any clue about its efficiency but it provides a micron number, ie. 10 micron.

The ISO standard 4572 known also as Multipass filter performance test was created to give a ratio between the number of particles that a filter allows to pass and the total amount supplied to the filter. This number is β (beta) so:

β_x = # particles IN / # particles OUT where x is the micron size. 1/β_x X 100% is the efficiency.

this number is always >= 1. For example: if a filter has a β₁₀ of 100, that means that the filter has an efficiency of 99.0% of efficiency for particles of 10 micron.