Machinedesign 2654 P47 2009 03 19 Filters 0

A Better Way to Rate Filter Performance

March 16, 2009
Dynamic filter efficiency gives the real lowdown on element ratings.

Authored by:
Aaron Hoeg
Hy-Pro Filtration
Fishers, Ind.

Edited by Kenneth J. Korane
[email protected]

Key Points

• Filters must be tested in a dynamic environment to predict how they will perform under real-world conditions.

 • Standards filter tests run at a constant flow rate.

•DFE multipass tests subject filters to flow changes and cold-start conditions.

Hy-Pro Filtration,

All hydraulic and lube systems have a critical contamination-tolerance level often defined by, but not limited to, the most-sensitive system components — such as servovalves or highspeed journal bearings.

Systems are at risk whenever fluid contamination exceeds this level because fluid-borne contaminants directly impact component wear rate, life, and ability to perform as intended. So for optimum performance and predictable life, component manufacturers recommend fluid cleanliness levels per ISO Standard 4406.

And to keep fluid clean, OEMs and users depend on filters as well as good system design and maintenance practices. However, filter manufacturers test and rate their products using the ISO Standard 16889 test method, and it generally doesn’t reflect real-world operating conditions. As a result, filters often don’t deliver the on-machine performance users expect. Here’s why.

Standard test methods
The most common measure of filter performance is removal (capture) efficiency, which addresses how efficiently a filter removes particles from the fluid. Very few people stop to consider a filter characteristic known as retention efficiency, which is a measure of how effectively that filter holds onto the particles it has previously captured under the stresses of a hydraulic or lube system. A filter is not a black hole, and its performance must not be based solely on how efficiently it captures particles. If not properly designed and applied, a filter can become one of the most damaging sources of contamination in a system.

Filter performance in a dynamic operating system varies based on many factors. These include flow rate, flow density, duty cycle, viscosity, fluid and structure-borne vibration, contamination levels, ingression rate, and several other conditions.

Hydraulic filters typically see frequent and rapid changes in flow rate accompanied by varying vibration frequencies. Lube filters usually experience dynamic flow and pressure conditions during start-up and shutdown.

Understanding ISO codes
The ISO cleanliness code (ISO 4406-1999) quanti es particulate contamination levels per milliliter of fluid at three particle sizes: 4, 6, and 14 μm. Three numbers express the ISO code (for example: 19/17/14). Each number represents a contaminant level for the correlating particle size. The fluid in this example would contain between 2,500 and 5,000 particles 4 μm/ml; between 640 and 1,300 particles 6 μm; and between 80 and 160 particles 14 μm. The code includes all particles of the specified size and larger. It is important to note that each time a code increases, the quantity range of particles doubles.

The challenge is that the industry specified standard (ISO 16889) used to rate and compare relative filter performance does not subject test filters to the dynamic stresses they see in today’s systems. Instead, the ISO 16889 multipass performance test measures capture efficiency and dirt-holding capacity in a steady-state environment run at one flow rate, under ideal lab conditions without subjecting the filter to hydraulic actuation or system restarts.

Test fluid circulates at a constant flow rate in a closedloop test circuit with online particle counters before and after the test filter. A known quantity of contaminant is added to the system before the upstream particle counter at a constant rate. Small amounts of fluid are removed before and after the filter for particle counting to calculate the filter capture efficiency. Capture efficiency is expressed as the filtration ratio (commonly called the beta ratio) — the relationship between the number of particles greater than and equal to a specified size counted before and after the filter.

For example, if 600 particles greater than or equal to 7 μm are counted upstream of the filter and four are counted downstream, filtration ratio is expressed as 7(c) = 600/4 = 150. (The subscript, (c), differentiates between multipass tests run per the current ISO 16889 multipass tests with particle counter calibration per ISO11171, from pre-1999 ISO 4572 tests.)

Evolution of filter media
Glass media has superior fluid compatibility versus cellulose with hydraulic fluids, synthetics, solvents, and water-based fluids. Glass media also has significantly better filtration efficiency over cellulose — even in elements with the same “micron” rating.

This is especially true in systems where flow fluctuates. Runaway contamination levels at 4 and 6 μm are common when cellulose media is used in systems with lots of fine particles, particularly because unloading can release significant amounts of particles beyond internally generated contaminants. Organic cellulose fibers can be unpredictable in size and effective useful life. Inorganic glass fibers, on the other hand, are much more uniform in diameter and smaller than cellulose fibers. Smaller size means more fibers in a given volume, and more void spaces to capture and retain contaminants.

As a result, glass media has much better dirt-holding capacity than cellulose. However, heed a note of caution. When upgrading from cellulose to high-efficiency glass media elements, first stabilize system cleanliness. During this clean-up period, element life might be temporarily short. But once the system is clean, glass elements can last four to five times longer than similar cellulose elements.

Filtration ratio can also be converted to filter efficiency. From the example, Efficiency = (( – 1)/ ) × 100 = ((150 – 1)/150) × 100 = 99.33%. The test filter is 99.33% efficient at capturing particles 7 μm and larger.

While the ISO 16889 Standard has made great progress providing a repeatable method where identical filters should produce similar results when measured on different test stands, ratings in the lab often don’t translate into predictable performance on actual lube and hydraulic systems. The challenge is selecting filters that will deliver fluid cleanliness below the critical contamination tolerance level to yield reliable operation and maximize component life. Filters must be tested in a dynamic environment to understand how they will perform when exposed to realworld conditions.

A different approach
The dynamic-filter efficiency (DFE) multipass test also uses upstream and downstream particle counters, a test filter, and contaminant injection upstream of the test filter, much like ISO 16889. That’s where the similarity ends. In contrast, DFE introduces a range of duty cycles throughout the test, bridging the gap between the lab and real world. The DFE flow rate is not constant but, rather, hydrostatically controlled so full flow through the test filter can quickly change to simulate various hydraulic and lube duty cycles. Flow across particle-counter sensors remains constant during all readings and no intermediate reservoirs collect fluid prior to measurements. This ensures that the fluid counted accurately represents real-time system contamination levels. Counts are made before, during, and after each flow change, with results reported as filtration ratio (beta), efficiency, and actual number of particles per milliliter upstream and downstream of the filter.

Tips for total system cleanliness
Selecting the right filter for an application is important, but it’s just one part of the whole picture. Developing a total system cleanliness approach to control contamination and care for fluids ultimately results in more reliable plant operation and saves money. Steps to total system cleanliness include:

Evaluate the fluid-cleanliness requirements of all hydraulic and lubrication systems, and establish an oil-analysis program and schedule. Also set a baseline and target fluid cleanliness for each system.

Insist on specific fluid-cleanliness levels for all new, purchased fluids, and filter all new fluids upon arrival and during transfer. And improve bulk oil storage and handling with an eye toward keeping the fluid clean.

Seal all reservoirs and bulk tanks, and install high-quality particulate and desiccant breathers. Enhance air and liquid filtration on existing systems wherever suitable. And remove and keep water out of the system. Finally, use portable or permanent off-line filtration to enhance existing filtration.

This approach might seem expensive and a lot of trouble. But studies have shown that the cost of proper contamination control and total system cleanliness is less than 3% of the cost of contamination not kept under control. That’s because fluid contamination leads to problems and expenses, and drains resources. Obvious costs relate to downtime and lost production, component repair and replacement, maintenance labor costs, and shorter fluid life. But there are also expenses related to unreliable machine performance, and engineering time spent on root-cause analysis.

DFE testing provides an inside look at the vital signs of a filter through a range of dynamic conditions to better understand how well a filter will capture and retain contaminant, and in real time.

Raw data is digitally tagged so filter efficiency is gauged for various combinations of flow conditions and differential pressures across the filter element. Typical particle counts are taken at maximum and minimum flows, and when flow changes (low to high or high to low). Rapid particle counting with proper timing provides a real-time understanding of the capture efficiency and retention characteristics of a filter.

Quantifying capture and retention
Here’s a more-detailed look at how DFE works. The accompanying graphic, “Comparing test methods,” examines the performance of two identical high-efficiency, glass-media filter elements from the same manufacturer. performanceOne, tested at a constant flow rate per ISO 16889, maintained a steady efficiency throughout the test.

The other filter, using DFE methods, was subjected to dynamic testing. It cycled between maximum and half of rated flow with a duty cycle similar to that of an actual hydraulic system. Downstream particle counts varied and were highest during changes from low to high flow. The peaks represent counts taken during flow changes and valleys represent counts taken after flow stabilized. As the filter captured more contaminants, downstream counts increased most dramatically during changes from low to high flow.

This phenomenon is best described as contaminant “unloading.” As the filter element captures more dirt, greater amounts can be released back into the system when the element experiences dynamic flow conditions and changes in differential pressure. The alternating smaller peaks represent unloading when flow rate changes from high to low. Highly concentrated clouds of contaminated fluid released during unloading can cause severe component damage and unreliable system performance, especially if the filter protects sensitive hydraulic components or bearings.

From this, we can surmise that this filter element is not properly designed to retain previously captured contaminant during dynamic system conditions. In addition, excessive unloading early in a filter’s life may be symptomatic of an element that will eventually break down, lose its efficiency all together, and fail.

It’s not surprising that many elements get higher ratings according to ISO 16889 than per DFE tests. This is troubling because OEMs often select filter media based on ISO beta ratios published by filter manufacturers. A common result is a hydraulic system that suffers from premature contamination-related failures, even though it is protected by filters that, in theory, should prevent such failures — causing downtime, unreliable equipment performance, and expensive component repair and replacement costs.

Even filters built for dynamic conditions show different results when subjected to ISO 16889 and DFE tests. The “Dynamic filters” graphic compares two identical Hy-Pro filter elements that have been designed and developed based on the DFE multipass test. Although contaminant unloading is still evident, it is insignificant as the filter element performed true to its ISO 16889 multipass rating of β7(c) > 1,000 even during dynamic flow conditions.

Cold-start simulation
Another important aspect is DFE’s ability to simulate cold-start conditions. Once an element is nearly filled to its contaminant-holding capacity (approximately 90% of the terminal ΔP rating across the filter), the main flow and injection system are shut off for a short dwell period. Then, the main pump restarts and rapidly attains maximum rated flow for the test element. Simultaneously, a real-time particle count measures retention efficiency of the contaminant-loaded element. This quantifies how well the filter element retains previously captured contaminant under start-up conditions. The dwell before restart may be based on time or system temperature — to simulate cold-weather conditions.

The “Cold-start performance” graphs show how conventional filters and filters designed for dynamic stress stack up to the DFE restart test. During restart, 6-μm particle counts after the conventional filter increased by a factor of 20, and ISO codes increased 4× for 4 and 6-μm particles. During restart tests no contaminant is injected, so any particles measured were already in the system or were released by the element (unloading). The result is a temporary state of highly contaminated fluid because the filter element did not properly retain the dirt.

The dynamic element, in this case a Hy-Pro element 3, also shows evidence of unloading, but the effect is smaller and retention efficiency higher even though the Hy-Pro element had captured more dirt than the conventional one. The conventional element unloaded seven times more particles ≥6-μm and 35 times more particles ≥14 μm, compared to the element designed for dynamic conditions.

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