Contamination Control

Fluidic System Cleanliness

The number one cause of hydraulic and fluidic system failure is contamination. Every fluidic system is sensitive to this. The degree of sensitivity is usually determined by the smallest passage in the system. This may be an orifice, a valve seat or clearance between two components.

Contaminants enter the system in one of several ways:

  • Dirty or contaminated fluid
  • Debris introduced when the system is "opened" (i.e. maintenance, tubing changes, fluid replenishment etc.)
  • Debris "built into" the system

The simplest problem is the fluid. This must be filtered to the proper micron level. Any new fluid added to the system must also be properly filtered.

Introduction of contamination during maintenance is a design issue. Any point in the system that needs to be opened should have a safety screen downstream, prior to sensitive components.

The third scenario, building in contamination is usually an assembly and housekeeping issue. All components, including filters, contain debris. These are from the manufacturing process, packaging, shipping, storage and even handling prior to final assembly. Sintered filters may have small loose particles while machined manifolds may contain loose metal shavings or burrs. All components should be flushed prior to assembly. This removes loose particles and prevents them from entering the fluidic system. In critical systems it may be necessary to flush the system as it is assembled, starting from the reservoir moving out to the final dispense tip.

Recommendations

The Lee Company offers and strongly recommends protective screens integrally mounted on all of our products with small passageways less than 0.055" in diameter. Because our screens are small, we rely on our customers to provide adequate
system filtration, leaving only the occasional "rogue" particle for our screens to catch.

The following list shows the tightest sliding fits and smallest metering orifices found in Lee Company standard products
and the maximum recommended system filter micron rating needed to protect those products.

Min. Lee orifice diameter......................... 0.005"
Min. Lee sliding fit (radial)........................ 0.0005" (12 µ)
Min. Lee integral screen hole size........... 0.004"
Max. system filter micron rating............... 10 µ

Many Lee Company products have looser fits, larger orifices, or larger integral screens than those shown above. In such a case, coarser system filtration may be specified while still adequately protecting the Lee products. Our sales engineers stand ready to assist you in properly sizing your system filtration.

Contamination Level Correlation

Fluid contamination can be described by a number of 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. gms/106 gms).

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

Other techniques look at numbers 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.

Most commonly used cleanliness specifications are based on numbers 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.

Contamination Level Correlation

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ISO 4406 Code Particles Per mL >10 µ ACFTD Gravimetric, Level mg/L MIL-STD 1246 Level NAS 1638 Class
26/23 140 000 1000    
25/23 85 000   1000  
23/20 14 000 100 700  
21/18 4 500     12
20/18 2 400   500  
20/17 2 300     11
20/16 1 400 10    
19/16 1 200     10
18/15 580     9
17/14 280   300 8
16/13 140 1   7
15/12 70     6
14/12 40   200  
14/11 35     5
13/10 14 .1   4
12/9 9     3
18/8 5     2
10/8 3   100  
10/7 2.3     1
10/6 1.4 .01    
9/6 1.2     0
8/5 0.6     00
7/5 0.3   50  
6/3 0.14 .001    
5/2 0.04   25  
2/.8 0.01   10  

Swipe to the right for more table information

NAS* 1638 Number of Particles per 100 mL Micron Range ISO 4406
Class 5-15 µ 15-25 µ 25-50 µ 50-100 µ >100 µ Class
00 125 22 4 1 8/5
0 250 44 8 2 9/6
1 500 89 16 3 1 10/7
2 1K 178 32 6 1 11/8
3 2K 356 63 11 2 12/9
4 4K 712 126 22 4 13/10
5 8K 1425 253 45 8 14/11
6 16K 2.8K 506 90 16 15/12
7 32K 5.7K 1012 180 32 16/13
8 64K 11.4K 2.0K 360 64 17/14
9 128K 22.8K 4.1K 720 128 18/15
10 256K 45.6K 8.1K 1440 256 19/16
11 512K 91.2K 16.2K 2.8K 512 20/17
12 1M 182K 32.4K 5.8K 1024 21/18

* SAE standard AS 4059 also applies. This lists fluid particulate contamination cumulatively for 5 ranges:

  • > 2 µ
  • >5 µ
  • >15 µ
  • >25 µ
  • >50 µ

for contamination classes from 000 to 12.

Particle Size Comparison

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SIZES OF FAMILIAR OBJECTS
SUBSTANCE MICRON INCH
Grain of Table Salt 100 0.0039
Human Hair 70 0.0028
Lower Limit of Visibility 40 0.0016
White Blood Cells 25 0.0010
Talcum Powder 10 0.0004
Red Blood Cells 8 0.0003
Bacteria (Average) 2 0.00008

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SCREEN SIZES
U.S. SIEVE NO. OPENING IN INCHES OPENING IN MICRONS
50 0.0117 297
60 0.0090 228
70 0.0083 210
100 0.0059 149
140 0.0041 105
200 0.0029 74
270 0.0021 53
325 0.0017 44
Paper 0.00039 10
Paper 0.00019 5

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