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» Understanding Filtration – Fluid Cleanliness & Filter Efficiency

In part 1 of this series on Understanding Filtration – Viscosity and Temperature we covered the significant impact that temperature has on the viscosity of an oil, which in turn affects its performance during filtration and lubrication.

There are other factors that must be taken into account when it come to hydraulic filter selection.

These variables will affect each other. To select the correct filter you need to establish the expected fluid viscosity as it varies with temperature.

The following must be known:

  • The fluid type
  • The fluid temperature at start up
  • The fluid temperature during normal operation

Other important factors are:

  • The required fluid cleanliness target
  • The efficiency rating of the filter

Fluid Cleanliness Targets

When discussing fluid cleanliness, we refer to ISO 4406 as a method of counting and quantifying contamination in a system.

Hydraulic Filters - Cleanliness Codes

The chart above shows recommended ratings for common system components. Many hydraulic manufacturers publish required fluid cleanliness levels for their equipment.

Guide to Filter Placement

The table below can be used as a guide for filter placement in a system.

Each of the blocks shows the suggested micron rating for the filter element. Note that RYCO support suction and return filters only.

Hydraulic Filters - Placement Guide

The method for selecting the filter type is to check the published minimum cleanliness level for the most sensitive component in the system, and then looking at the chart to select the filter rating and placement required.

Beta Ratios and Filter Efficiency

When choosing filtration products, it is important to understand how well the product will perform.

ISO 4572 is known as a multipass test. It measures the filtering efficiency of the element, which is published as the Beta ratio.

Hydraulic Filters - Multipass Test

The multipass test also establishes the pressure drop across a filter element, and the dirt holding capacity of the element.

The Beta ratio is the ratio between the number of particles introduced to a fluid before a filter, and the number of particles after the fluid has passed through a filter element.

The Beta ratio is displayed with a micron rating. In this way the ratio describes the filtering efficiency of the filter element when trapping particles of a given size.

Hydraulic Filters - 5 Micron Beta Ratio Calculation

A filter that is a poor performer at trapping particles will have a lower Beta ratio than a more efficient filter.

Filters with a low Beta ratio are called NOMINAL filters, as they have a NOMINAL rating. This means that the filter will be adequate at trapping particles of a given size.

Hydraulic Filters - Beta Ratio Example Calculation

Filters that are highly efficient will have a higher Beta ratio. Filters with a Beta ratio over 75 are called ABSOLUTE rated filters. This means that they can be considered to trap all particles of a given size or larger.

This chart below also appears on page 381 of the RYCO Product Technical Manual, (AUS July 2007).

Hydraulic Filters - Multi-Pass Test Results

It demonstrates that a 10 micron absolute filter is very efficient at trapping particles larger than 10 micron, but ineffective at trapping smaller particles.

Filter Bypass

Almost all filter elements are supported on the inside surface. Fluid flows through the element from the outside to the inside.

Hydraulic Filters - Cutaway

As a filter catches and holds particles, it gets harder for fluid to flow through the element.

Hydraulic Filters - Element Flow

This causes a build up of pressure on the outside of the element, therefore a pressure differential across it.

If too high a pressure is allowed to build up inside a filter housing, it could cause the element to collapse, or the housing to rupture.

Many filters incorporate a bypass valve, which limits the pressure differential by allowing the fluid to bypass the element.

Hydraulic Filters - Bypass Valve

During the bypass condition, there is no filtering taking place.

Hydraulic Filters - Bypass Filtering

It is important to remember that filter bypass is not a desirable condition.

Hydraulic Filters - Filtering vs Bypassing

A rule of thumb for system designers and selectors of filtration products is that the pressure drop across a new or “clean” element should not exceed 25% of the bypass rating. i.e. 2.5 Bar pressure drop allowed for a 10 Bar bypass spring rating.

Indicators are fitted to filters to display the pressure drop across the element.

Hydraulic Filters - Pressure Drop Indicators

It is strongly recommended that all filters have condition indicators fitted.

For more information, see page 370 of the RYCO Product Technical Manual (RHY-RHC0707AUS).

Understanding Hydraulic Filtration

In simple terms, filtration is a method of removing particles from a fluid by trapping them within a screen of suitable material.

Hydraulic Filters - Filter Products

Filtration becomes more complex when you look more closely at it.

Fluid type, temperature, operating temperature, filter efficiency and by-pass settings all influence the effectiveness of any filtration product.

Please also read the (yellow) filters section of the RYCO Product Technical Manual to understand hydraulic filtration in even more detail.

Want more information?

Read more about effective filtration of hydraulic systems in part 1 of this series on Understanding Filtration – Viscosity and Temperature.

And to help you choose the right hydraulic hose for your project, download our 15 Keys To Selecting The Right Hose guide now.