Filtering hydraulic systems

Fluid power is one of the most reliable and repeatable forms of power and motion control. All that is required is a comprehensive state-of-the-art design and modern contamination control. When problems are encountered, 75% to 80% of the time they are related to inadequate contamination control practices.

For a hydraulic machine, the development of a target cleanliness level and a plan to achieve it are as important as the selection of pumps, valves, and actuators (Table 1). Many manufacturers specify the optimum cleanliness level required for their components. Proper selection and placement of filters in a system to attain the targeted cleanliness eliminates the root cause of most hydraulic system failures.

System cleanliness assures the user of the hydraulic system a cost-effective approach to contamination control that allows the price of filters and elements to be quickly recovered by savings due to improved performance, increased component life, fewer oil changes, increased uptime, and fewer repairs (Fig. 1).
 
Filter media

Filter media usually start out in sheet form and are pleated to create more surface area exposed to the fluid flow, thus reducing pressure differential while increasing dirt holding capacity. In some cases, the media may have multiple layers and mesh backing.

The most common media include wire mesh, cellulose, fiberglass composites, or other synthetic materials. Media are generally classified as surface or depth.

The fluid stream basically has a straight path through surface-type media (Fig. 2). Contaminants are captured on the surface of the element. Elements are generally made from woven wire and the process can be accurately controlled for consistent pore size. The buildup of contaminant on the surface allows the media to capture particles smaller than the pore size.

For depth-type media, fluid must take indirect paths through the material (Fig. 3). Particles are trapped in the maze of openings. Because of its construction, a depth-type filter has many pores of various sizes. Depending on the distribution of pore sizes, this medium can have a very high captive rate at very small particle sizes.

These fiber differences account for the performance advantage of fiberglass. Thinner fibers mean more pores in a given space. Thinner fibers can be arranged closer together to produce smaller pores for finer filtration. Dirt holding capacity and filtration efficiency are improved.

The Beta ratio, also known as the filtration ratio, is a measure of the particle capture efficiency of a filter element. It is a performance rating.

For example, assume that 50,000 particles, 10 micrometers and larger, are counted upstream of a test filter and 10,000 particles of the same size range are counted downstream of the filter. The corresponding Beta ratio would be 5.

B _x = no. of particles upstream/no. of particles downstream

Where: x is a specific particle size

B ₁₀ = 50,000/10,000 = 5

The example is read 'Beta ten equal to five.' A Beta ratio number alone means very little. It is a preliminary step to finding a filter's particle capture efficiency. This efficiency, expressed as a percent, can be found by the equation:

Efficiency_x = (1 - ¹/_B) 100

Efficiency₁₀= (1 - ¹/₅) 100

= 80%

The particular filter tested was 80% efficient at removing 10-micrometer and larger particles. For every 5 particles at this size range introduced to the filter, 4 were trapped in the filter media. Table 2, 'Beta ratios and efficiencies,' shows some common Beta ratio numbers and their corresponding efficiencies.

Contaminant loading in a filter element is the process of blocking pores. As the element becomes blocked with particles, there are fewer pores for fluid flow, and the pressure required to maintain flow increases. Initially, the differential pressure across the element increases slowly, but a point is reached at which successive blocking of media pores significantly reduces flow through the element. At this point the differential pressure across the element rises exponentially (Fig. 4).

The quantity, size, shape, and arrangement of the pores throughout the element accounts for why some elements last longer than others. For a given filter media thickness and filtration rating, there are fewer pores with cellulose media than with fiberglass media. The contaminant loading process blocks the pores of cellulose media elements quicker than identical fiberglass media elements.

A multilayer fiberglass media element is relatively unaffected by contaminant loading for a longer time. The element selectively captures various-size particles as the fluid passes through the element. Very small pores are not blocked by large particles. These downstream small pores remain available for a large quantity of very small particles.

Every filter element has a characteristic pressure differential versus contaminant loading relationship. This relationship can be defined as the filter element life profile. The actual life profile is affected by system operating conditions. Variations in the system flow rate and fluid viscosity affect the clean pressure differential across the filter element and have a well-defined effect on the actual element life profile.

The filter element life profile is difficult to evaluate in actual operating systems. System operating time versus idle time, duty cycle, and changing ambient contaminant conditions affect the life profile of a filter element.

Precise instrumentation for recording the change in the pressure differential across a filter element is seldom available. Most machinery users simply specify filter housings with differential pressure indicators to signal when the filter element should be changed.

Changes to ISO contamination and filtration standards were brought about to solve accuracy, traceability, and availability issues. It is important to remember that hydraulic systleanliness levels and actual system filter performance remain unchanged (Fig. 5). However, the reporting of cleanliness levels and filter performance has changed due to the latest particle counter calibration and multi-pass test procedures.

It is important to note that the ISO 11171 calibration method is based on a distribution of particles measured by their equivalent area diameter. ISO 4402 is based on a distribution of particles measured by their longest chord. The NIST work utilized scanning electron microscopy for particles below 10 µm in size, whereas the sizing distribution for AC Fine Test Dust (ACFTD) utilized optical microscopy.

The new calibration method and resulting ISO code typically produces a one to two-level increase in the first digit (the >4 µm size range) of the three-digit code. This is due to the greater number of particles in that small size range. The remaining two digits typically remain unchanged and should not impact previously established ISO standards (Fig 6).

Plant Engineering extends its appreciation to the Eaton Corp., 800-547-7805, and Parker Hannifin Corp., 800-253-1258, for their assistance in the preparation of this article.