Contamination Control

Contamination can cause a wide range of problems, including damage to valves or pumps, clogging or complete system failure. The source of contamination can originate from many places, such as dirty fluid, manufacturing debris, or even defective components during operation. Maintaining fluidic system performance requires cleanliness and tight control of potential contamination.

System Cleanliness

Fluidic systems are typically comprised of components that have varying degrees of sensitivity to contamination, mostly determined by the minimum passage size and clearances throughout the flow path. Several attributes of contamination must be considered beyond just size, such as the shape, length, texture, and hardness of a particulate. Some critical steps to consider are:

1. Working in a clean environment.
2. Properly flushing components before system assembly.
3. Incorporating proper system level filtration.

The first step ensures the system is free from manufacturing debris prior to installing components. The second step removes contamination to protect individual components and extend their operational life. However, since contamination can also originate from within the system due to the normal wear of components, step three of incorporating safety screen filters immediately upstream from components is a prudent design practice. The added benefit of using safety screens is to protect the components during any maintenance of the system.

Contamination Level Correlation

There is a direct correlation between fluid cleanliness and the level of filtration required in your system. Fluid contamination can be described using different techniques:

• The Gravimetric Method: The contaminant level is expressed as the mass of
  contaminant per unit volume of fluid.
• Parts Per Million: The degree of contamination is based on mass or volume
  per million units (e.g. grams/10⁶ grams).

The above techniques describe bulk or total contamination but give little information regarding the size of the contaminant. For example, unless the size and density of the contaminating particles is known, no conclusions may be drawn relative to the number of particles.

Other techniques look at the number of particles, describing contamination in terms of its size and concentration. These may be an interval concentration, for example, the number of contaminant particles (per unit volume) between 5 and 15 µ in size. Additionally, contamination may be expressed as a cumulative concentration. In this case, contamination levels are described by the total number of particles per unit volume above a given size. For example, the number of particles above 25 µ in size per 100 mL.

The more common cleanliness specifications are based on the number of particles rather than gravimetric techniques. However, particle distributions which were determined to be representative of service distributions (e.g. NAS 1638 distributions) correlate reasonably with those obtained gravimetrically with AC test dust. The table below (from An Encyclopedia of Fluid Contamination Control by E.C. Fitch) provides a correlation of some different cleanliness specifications.

NAS 1638 Table

*SAE standard AS 4059 also applies and lists fluid particulate contamination cumulatively for 5 ranges for contamination classes from 00 to 12 (>2 µ; >5 µ; >15 µ; >25 µ; >50 µ).

ISO 4406 Table

Particle Size Comparison

Clogging

As a safety screen accumulates particles, the pressure drop will slowly increase until the screen is almost fully clogged. Then the pressure drop increases dramatically. For example, consider a clean safety screen in a 3,000 psi hydraulic system. At its normal flow rate the screen pressure drop is 6 psi. That same screen will see a pressure drop of only 150 psi when 80% clogged. However, at 95% clogged, the differential pressure jumps to 2500 psi. This phenomenon is represented by the following formula:

% Clogged vs. ∆P

Where ∆P_O is the pressure drop across the screen when it is clean.

Ensuring you have the proper filtration in place is the key to controlling contamination and extending product life.

Please contact your local Lee Sales Engineer for any assistance required in determining the recommended level of filtration for a specific component.