Comparison of high-pressure and medium-pressure filters

High-pressure filters are primarily used in high-pressure circuits of hydraulic systems with alternating flow directions. The two most important performance characteristics are wear protection and functional protection. Wear protection is achieved through filter elements that meet the highest requirements for the respective cleanliness class during full-flow filtration.

The filter fineness typically varies between 5 µm and 30 µm. This relatively coarse filter fineness is due to the fact that high-pressure filters are mainly used as pure protective filters. High-pressure filters are among the most expensive filters in hydraulics. These filters are therefore precisely adapted to the maximum flow rate of the hydraulic system.

High-pressure filters with reversing control

In high-quality high-pressure filters, a so-called Graetz circuit is integrated into the filter head. This circuit ensures that the hydraulic medium used is filtered in both flow directions. This high-quality class of high-pressure filters is characterized by a filter material formed in a star-pleated design.

This results in several advantages, such as:

• Very long maintenance intervals (especially when using
  an optical or electrical clogging indicator)
• Low pressure losses
• Large filter surfaces
• High dirt-holding capacities

General information on medium-pressure filters

Medium-pressure filters are characterized by the same structural design. Only their versatile application in the medium pressure range (around 40 bar) distinguishes them from high-pressure filters. Conventional applications for medium-pressure filters include:

• Hydraulic systems in the medium pressure range
• Pressure lubrication
• Use in feed pumps of hydrostatic travel drives

The filter head of standard medium-pressure filters with a bypass valve consists of anodized aluminum, which yields at a burst pressure of 300 bar. Similar to high-pressure filters, filter elements made of microfiber or wire mesh are frequently used materials. When configuring the intended properties, particular attention is paid to high efficiency in the separation of solid particles in this filter design.

Typical application areas for medium-pressure filters include mounting on control blocks and installation in various pipelines in hydraulic medium-pressure systems. This type of filter is used for the direct protection of control and regulation units (servo valves, proportional valves). Low pressure loss in the filters is achieved through a flow-optimized design of the components. Further objectives in the design of these medium-pressure filters include high differential pressure stability, high dirt-holding capacity of the elements, and service-friendly handling.

Comparison of high-pressure and medium-pressure filters

The essential components of medium- and high-pressure filters differ only slightly in appearance. The filter element, filter head, filter bowl, clogging indicator, and bypass valve differ only in the choice of material and the dimensioning or design. The housing (of high-pressure filters?) is characterized by a significantly higher wall thickness.

Medium- and high-pressure filters can be designed as switchable duplex filters. This allows for uninterrupted continuous operation of the machine or system, and the filter can be maintained or replaced at leisure. Special forms of these filters for the effective protection of particularly sensitive components are inline filters or so-called last-minute filters.