A laboratory testing method has been developed to resolve many of the deficiencies of the Multi-pass test. The Cyclic Stabilization Test (CST) examines cyclic flow performance and the effects of contaminant loading on the retention and desorption of the filter.

During the development process, numerous experiments were conducted to set parameters and determine the optimum times for injection, particle counting, and contamination reporting. The final version of the CST is conducted in a similar manner to the multi-pass test, using virtually the same equipment. The CST requires the addition of a bypass line and a flow cycling valve to continuously cycle the flow through the filter. Filtration ratios are measured as contaminant is introduced to load the filtration medium and increase the differential pressure. At two points in the life of the filter, the cleanup and stabilization (drawdown) characteristics are measured by stopping the contaminant injection for typically a 30-min period while maintaining cyclic flow conditions. This 30-min period is called the drawdown or stabilization period.

A great deal of information can be gathered from running the CST. The following measurements can be obtained, in this order, during the testing process:
● initial cyclic flow filtration ratio,
● cyclic flow filtration ratio (throughout the remainder of the test) and average filtration ratio under cyclic conditions,
● cleanup and stabilization when the filter is partially loaded with contaminant (2.5 % ∆P increase),
● cleanup and stabilization at 80% ∆P increase (conducted while the filter is almost fully loaded),
● dirt-holding capacity, under cyclic conditions.

The test stand is basically the same as in the multi-pass, except that an electrically controlled valve allows flow to be cycled from 25 to 100% of the filter’s rated flow. The test cycle rate is 0.1 Hz, or 6 cycles/min, with a rise time and fall time from 0.2 to 0.5 sec. A survey of original equipment manufacturers concluded that this rate is widely applicable. Nevertheless, if desired, the test may be conducted using any cyclic condition.

Cyclic flow filter testing

The SAE A-6 committee on aerospace hydraulics standards is developing a test method for measuring the performance of hydraulic filters under cyclic flow conditions. Its draft procedure, SAE 4205, is entitled Aerospace Fluid Power — Hydraulic Filter Elements — Method for Evaluating Dynamic Efficiency with Cyclic Flow. It proposes a test that expands on the standard multi-pass by introducing cyclic flow and stabilization drawdown periods with no contaminant ingression. It is equivalent to the CST, except that it uses ISO Fine Test Dust (ISO FTD) rather than ISO MTD, which is used in the industrial procedures. The aerospace procedure is currently undergoing ballot at the SAE panel level.

NFPA/ISO projects — The NFPA has also proposed an alternative test procedure to the multi-pass test. This draft procedure has been proposed to ISO and has been accepted as a new project, ISO WD 23699, under the ISO/TC131 committee. It is very similar to the SAE ARP 4205 draft, including cyclic flow at a rate of 0.1 Hz, and also to the CST proposed below. It does not, however, currently include drawdown or stabilization periods.

ISO WD 23699 must undergo three successful ballots at the international level before becoming an ISO standard, a process that typically takes several years.

Comparison of test procedures

Performance comparisons of filtration ratios resulting from the different test procedures (standard multi-pass test, CST, and CSTV) were conducted on three different sets of filters, A, B, and C. (CSTV is the CST test with vibration added to the test filter.) These results demonstrate that the multi-pass test method qualifies the upper bound performance for a filter, whereas the CST and the CSTV tests qualify the lower bound performance for that filter.

Recommendations — The testing detailed in this article has demonstrated that the CST is a repeatable and reproducible method for evaluating the effects of cyclic flow and vibration on the performance of hydraulic filter elements. Using the CST, the dynamic efficiency and contamination control ability of the filter can be measured in a manner that more closely represents the operation of most hydraulic systems.

The CST method has the following advantages:
● It can be performed on a slightly modified multi-pass stand, of which there are over 100 in the world.
● It is repeatable and reproducible between labs.
● It indicates the average contamination level maintained in the actual cyclic flow system. This average level could be related to component wear rates.

The test results presented reveal the following:
● All filters tested have a lower performance with cyclic flow than is indicated using the standard steady-flow multi-pass test. Therefore, the CST represents the lower bound on performance.
● Both the magnitude of the flow cycles and cyclic frequency have a major effect on the stabilized downstream particle count. Standard test conditions are specified for the CST method.
● For the filters tested, vibration had little effect on performance. However, this is most likely not true for all filters.

Users of hydraulic filters should use the CST to better understand the expected performance of hydraulic filters and to specify the desired performance. I also suggest that the standards organizations continue their efforts to adopt a standard cyclic flow filter test method.