As you may know, over 85% of all hydraulic system failures are caused by contaminants in the fluid, either by the direct mechanical action between the contaminants and the system components, or by the contamination acting as a catalyst to accelerate oxidation of the fluid, prompting chemical breakdown.
Even if no failure occurs, operating efficiency can be markedly reduced and the size of the contamination can vary from the size of a human hair, down to those undetectable by the human eye.
Filtering a system’s fluid can remove many of these contaminants and extend the life of system components that will reduce system downtime.
When choosing a filter, it is important to consider your application carefully and one of the important things to look out for is how well a filter can retain particles after they are captured and before it goes into bypass. In short, this is known as the Dirt Holding Capacity (DHC).
How is a filter’s DHC obtained?
Dirt Holding Capacity, along with efficiency ratings and beta stability, are determined by conducting a multi-pass test. This is an industry standard test which is executed under controlled conditions with procedures outlined by ISO 16889.
Fig.1 Multi-Pass Test Schematic
The filter being tested is put into the hydraulic circuit and oil is passed through the system. Fluid, which is contaminated with a pre-set amount of particles (ISO Medium Test Dust), is then introduced into the circuit.
Once this has passed through the filter, samples are then taken both up and downstream of the filter. The total grams of ISO MTD that has been added to the system before the element goes into bypass, and therefore effectively retained by the element, is the Dirt Holding Capacity.Below are some typical examples of Dirt Holding Capacity results from testing; these are specifically for Schroeder Z-Media Filter Elements.
Fig.2 Examples of DHCs
Are there any factors that affect DHC?
The short answer is yes; there are a variety of factors that affect DHC. A few of these are outlined below.
Pore size
The size of the openings within the filter media through which particles can pass is called the pore size. Each filter has a specific size. It is important to consider that as they get smaller, there will be an increase in differential pressure. This can lead to the cracking pressure of the filter being reached and allowing oil to flow unfiltered or eventually the media may burst.
Pore density/Porosity
This is the quantity of pores within an area of the filter. As pore size goes down, density must go up to account for the increase volume of oil in contact with the surface and to prevent differential pressure increasing. Filter depth and size, therefore, also affect the DHC.
Filter media
Filters are manufactured using different materials. These are primarily cellulose and synthetic. Cellulose has larger fibres and less consistent pore size but will absorb some water. Synthetic tend to have higher DHC due to more consistent pore size and smaller fibres creating more pores
In summary, DHC will tell you how much contamination the filter will retain before changeout is required. Hopefully you will now understand how a higher DHC can help reduce frequency of changeouts and in turn reduce operating and maintenance costs
Certain digrams and technical information in this news item have been referenced from:
Noria - http://www.machinerylubrication.com/Read/29114/dirt-holding-capacity
Schoeder - http://schroederindustries.com/
