The Stress-Resistance Test
Hydraulic System Stresses
- Elevated heat during operation
- Cold start-up
- Cyclic (variable) flow
- Increased pressure drop due to filter loading
Many hydraulic filters on the market are designed and tested in the laboratory without ever being subjected to these stresses during testing.
How Stresses Affect Hydraulic Filters
Cold start-up causes compression of filter materials and pleat "bunching". Effects of cold start-up can be evaluated per ISO 2943.
Vibration can cause mechanical damage or loss of efficiency and desorption (release of previously captured particles). There is no current test to evaluate this effect.
Cyclic (variable) flow causes fatigue of filter structure (pleats). Fatigue resistance to cyclic flow is evaluated per ISO 3724. Cyclic flow can also potentially cause reduced efficiency and particle desorption (release).
Increased pressure drop due to filter loading can cause loss of efficiency and particle desorption (the more particles captured and held in the filter, the more that can be released).
What Makes a Filter Stress Resistant?
- Uniform pore size (in efficiency control layer)
- No movement allowed during operation
- Strong resin bonding of fibers
- Strong support and bonding of pleat structure
- Compatible materials with no "softening"
- Protection of media from any other damaging or moving layers
In order to properly evaluate stress-resistance, especially to cyclic flow and filter loading, Pall Corporation developed the Stress-Resistance Test
Primary Limitations of the Multi-Pass Test
- Uses only steady flow
- High dust injection rate
- 1,000 to 10,000 times actual field experience
- No stresses such as heat, cold start, vibration
- Normal reporting of Beta ratios is based on averages and not the worst point in a filter's life
The Stress-Resistance Test
|Most hydraulic systems experience cyclic flow conditions. This figure provides examples of typical Filter Duty Cycles in hydraulic equipment.|
|This figure represents the flow cycle profile used in the Stress-Resistance test. You will notice how this is similar to many of the typical flow cycles in the figure above.|
|This figure represents the clean-up curve (upstream particle counts) for a new filter with an initial injection of contaminant and steady flow. No additional contaminant is added during clean-up. Notice how the contaminant is quickly removed from the fluid alowing stabilization to occur with almost no remaining contaminant.|
|If we now examine the clean-up curve (upstream particle counts) for a new filter under cyclic flow conditions, stabilization again occurs quickly with a cleanliness level nearly as good as under steady conditions.|
|When we examine the stabilized cleanliness with cyclic flow (upstream particle counts) on a filter at 2.5% of terminal pressure drop, a drop-off in clean-up is clearly visible. A 2.5% pressure drop increase typically represents the filter being 30-50% plugged with contaminant.|
|At 80% terminal pressure drop, the stabilized cleanliness with cyclic flow (upstream particle counts) shows a marked deterioration. This is the point near the end of a filter's service life when its performance is most severely challenged by stresses.|
|Comparing downstream particle counts different filters with similar steady-flow Beta reveals that [similarly Beta rated] filters do not all perform the same under "real-world" stress conditions.|
Filters can be given an ISO code rating based on the cleanliness level that can be achieved under stress conditions. Rating a filter at 80% pressure drop provides the user with the best understanding of what level of performance the filter will demonstrate at the worst operating condition.
|Ratings established at 80% of terminal pressure drop representing worst case operating condition.|
Stress-Resistance test results for four pairs of filter elements (E1 and E2, F1 and F2, G1 and G2, H1 and H2) show that the test exhibits excellent repeatability.