To meet the economic pressures of increased profit and reduced operating costs, industrial manufacturers are putting greater demands on the hydraulic and lubricating systems that represent the heart of the operation. The requirements imposed on these systems are increased system reliability and longer life of critical components, with an emphasis placed on effective contamination control to achieve these goals. The importance of filtration in the optimization of a hydraulic or lubricating system has been established in numerous studies that directly correlate the detrimental effects of particulate contamination to useful component life. It should be noted that water contamination is equally detrimental to these systems, and that particulate and water contamination can also have adverse effects on the physical and chemical properties of the hydraulic or lubricating fluid. Simply put, the implementation of appropriate filtration applied correctly can reduce the infant mortality rate of critical components, reduce commissioning times and increase useful component life.
Figure 1. Test Stand
The Multipass Test
With an understanding of contamination fundamentals and wear mechanisms, system cleanliness levels should be established in order to determine the suitable filtration required. Filter selection and placement are two important factors in a well-managed contamination control program and are based on user-defined requirements as well as original equipment manufacturer (OEM) specifications. Once the system contamination control requirement has been determined, the filter selection should be based on the efficiency of the filter expressed as a filtration ratio or beta ratio (ß), which is defined as the ratio of the number of particles greater than a given size (x µm(c)) in a given volume of influent fluid to the number of particles greater than the same size (x µm(c)) in the same volume of effluent fluid. The beta ratio of a filter is measured using a multipass test (ISO 16889), which calls for continuous injection of test contaminant ISO MTD into a reservoir upstream of the test filter. The contaminated fluid challenges the test filter and any contaminant not captured by this filter is returned to the reservoir for additional passes through the filter. During this time, particle counts are collected upstream and downstream of the filter to determine the particle size where the beta ratio is 2, 10, 75, 100, 200 and 1000, as required by ISO 16889 (formerly ISO 4572). The changes to the multipass test required by the new ISO 16889 include the use of a new test dust, ISO MTD, and new automatic particle counter (APC) calibrations, both of which have a tremendous effect on reported beta ratios.
The particle removal effectiveness of the filter is determined by the construction of the element, the integrity of the medium, specifically the pore structure of the medium, and the pleat support structure. Medium with consistent, stable pore structure that is well-supported exhibits higher beta ratios, which results in cleaner downstream fluid. The beta ratio gives a true representation of the downstream fluid cleanliness, which is not the case for the often-used filter percent efficiency, as shown in Table 1.
The efficiency is calculated by subtracting the number of downstream particles from the original number of particles challenging the filter, dividing by the original number of particles challenging the filter and multiplying by 100 (or from the beta ratio, it is (ß-1)/ß *100).
The fluid that is downstream of a ßx200 (99.5 percent efficiency) filter is 2.67 times cleaner for particles x microns than a ßx75 (98.7 percent efficiency) filter. The industry standard for hydraulic and lubricating filters is typically ßx1,000.
The dirt capacity of a filter, which is measured as part of the multipass test, is defined as the mass of ISO MTD effectively retained by the filter element when terminal element differential pressure is reached. Both the beta ratio and the dirt capacity values are affected, although to different degrees, by the operating conditions such as the flow rate of the system, type and particle size distribution of contaminant challenging the filter, contaminant ingression rate, the terminal pressure drop, whether the test was multipass or single-pass, and finally the filter integrity.
For example, the same filter tested at two different flow rates or two different contaminant ingression rates would show different dirt capacities. Also, a coarser filter would exhibit a higher dirt capacity than a finer filter, but would allow more contaminant downstream of the filter.
There are, however, limitations to the multipass test, such as the use of a steady flow and high dust injection rate. Also, the current standard does not account for stresses in a hydraulic system such as heat, cold start and vibration, which can cause accelerated degradation of filter elements and/or contaminant unloading. Moreover, the normal reporting of beta ratios is based on averages and not the worst point in the filter’s life, which generally occurs near the end of its useful life, at or near the terminal pressure drop.
Stress Resistance Test
To overcome the deficiencies of the multipass test, Pall Corporation has developed the Stress Resistance Test (SRT), which more closely mimics real-world conditions and provides a better prediction of how well a filter will perform under these conditions. The test subjects filters to conditions of cyclic flow and examines the effects of contamination on the loading and unloading characteristics of the filter. From this test, the stabilized fluid cleanliness level (reported per ISO 4406) that a filter maintains under cyclic conditions can be determined. For example, Table 2 shows the particle counts for five similarly multipass-rated filters subjected to the Stress Resistance Test, based on the stabilized downstream level at 80 percent pressure drop.
These results illustrate a clear differentiation in filters with similar beta ratings reported from multipass tests. However, it is clear that Filter E maintains the best cleanliness level throughout the life of the filter element. Reporting filter performance at 80 percent terminal pressure drop will best represent how well the filter will do in the worst-case operating condition.
Filter Placement Considerations
To optimize the benefits of well-engineered filtration, the placement of filters is crucial. For many systems, the best placement of a filter is on a pressure line, where possible, to catch pump debris and to act as a last chance filter before a critical component. Because these filters must be designed to handle the full flow and pressure of the system, pressure-line filtration is typically the most costly way to filter the fluid. Return-line filters are installed to capture debris from component wear or ingression into the system and to promote general system cleanliness, lowering overall filtration costs. Kidney loop or off-line filters can be used to control system cleanliness when pressure-line filters are not in use, to supplement existing filtration on the system or for systems where in-line filtration is simply impractical. Finally, installation of air breathers reduces contaminant ingression, thereby extending filter element service life and helping to maintain overall fluid cleanliness.
Before any system is commissioned for the first time or after any maintenance to the system is completed, a system flush should be performed to remove any wear particles that can lead to a catastrophic failure. Components are most sensitive to contamination during the startup phase. Flush filters should be more efficient than the system filters to ensure fluid cleanliness requirements are met.
Systematic Contamination Control Programs
The effectiveness of a contamination control program can be established through condition monitoring. Such a program should include routine fluid analysis (particle contamination, water contamination, additive package, acid number (AN)), which provides important information about overall system behavior and allows for trend analysis. Filter usage should also be monitored because sudden reduced filter life is often evidence of an upset condition. Diagnostic equipment is available for monitoring particle counts in-line, on-line, and off-line. In addition, water sensors and differential pressure devices should be included in a condition-monitoring program for proper data collection. Using this equipment, trends can be observed and normal operating conditions can be determined.
The use of clean fluids in a hydraulic or lubricating system leads to improved system reliability and productivity, which means increased profits. Clean fluids indicate a successful contamination control program, where care is given to proper filtration selection and placement based on the beta ratio of the filter. The measurement of success is determined through condition monitoring of vital system components such as the fluid, pumps, valves and bearings. These continuous improvements in contamination control and condition monitoring can be used as a benchmark for further process optimizations.
The authors wish to express their gratitude to Bill Wetzler, manager, Hydraulics Laboratory, Pall Scientific and Laboratory Services Department, and Andy Messerschmitt, technology development manager, Pall Machinery and Equipment, for their kind assistance in setting up the SRT hydraulic test stand and obtaining equipment photographs.