Although we take the greatest possible care in filter media selection and processing, occasional defects or weaknesses can only be excluded by careful quality control of individual components as well as finished products. Quality control must confirm that every filter exhibits the specified filtration efficiency, that the pressure loss is within the designed range, and that the specified working life can be guaranteed under operational conditions. Therefore, each filter element is subjected to all the testing procedures required under national and international standards. At the customer’s request, the test results can be documented in a certificate, which is delivered with the filter element. Some filter media and some applications require the measurement of maximum mechanical resistance with respect to pressure difference or maximum dust holding capacity. The testing procedures for these issues, like pressurization up to collapse or the multipass test, are destructive tests if carried out on non-regenerable filter media. Consequently, they are conducted with single tests on representative samples of filter types.
Bubble point test
The bubble point test is used to measure the maximum pore size of a filter element. This test enables the detection of occasional defects within the filter medium or leakages in the connection between the filter medium and adapter. The testing procedure is described in the standard DIN ISO 2942:2004. The entire filter element or a test element, in which a sample of the filter medium to be tested is installed, is submerged in a basin, which is filled with a wetting fluid (e.g. isopropyl alcohol), and pressurized with air from the inside. The air pressure is increased slowly and gradually, until the first air bubble can be detected. The diameter of the pore, from which the bubble has emerged, can be calculated from the respective pressure and the wetting characteristics of the fluid (wetting angle and surface tension). In addition to the filter type, the resulting test report also provides information about a clear identification number and the original weight of the element.
If requested by the customer, we can produce a prototype of a new filter series in order to test it thoroughly. The test conditions are defined by the customer’s specification or by the standards the customer has indicated.
Certain industry sectors, or associations representing these sectors, publish their own recommendations or technical regulations, e.g. the German Automotive Industry Association (VDA) or the aerospace industry. In these cases, a first-sample test is recommended before starting series production in order to ensure that all customer specifications are met.
Production preparing/ accompanying tests
Testing carried out during the production process ensures the safety and functionality of individual components or systems before they are assembled into units. Such tests include, for example, the bubble point test for filter media, in which samples are inserted in test elements and submerged in the test fluid, before the filter medium is made up to be installed in the final filter element, and pressure tests on housings. Laboratory scale filtration tests are carried out in order to facilitate the selection of filter media. Filtration efficiencies are determined by measuring particle concentrations and particle size distributions in the feed and filtrate. Cleanliness analysis is conducted for applications that require “super clean” surfaces in all components.
High pressure test stands
In the case of elements subjected to high pressures, absolute tightness must be ensured over the whole working life. In order to guarantee this tightness, all high pressure filter systems are tested before delivery. All housings are subjected to pressure resistance tests during production by pressurizing them with a test pressure that exceeds the maximum operating pressure by a factor of 1.43. Furthermore the burst pressure of a single housing is determined through representative test samples.
Filter elements are examined to establish if they can withstand the designed system pressure. Furthermore it is important to know the pressure difference at which a filter element collapses. To carry out these tests, the specimens can be stressed by up to 1100 bars. Determining this upper limit as well as measuring the burst pressure destroys the test sample.
All required tests of filter media and complete filter elements are carried out in our own laboratory. This contains standard test equipment for measuring volume flows through, and pressure losses of, filter media, as well as test stands for the bubble point test, the multi-pass test, and high pressure test stands for measuring burst pressures. The equipment needed to determine the properties filtration fluid samples provided by customers is also available.
In addition, we have optical systems for measuring particle concentrations and particle size distributions in fluid samples. A typical application is the determination of purity classes of hydraulic liquids, in accordance with ISO 4406 und NAS 1638. The purity classes as defined in ISO 4406 are distinguished according to the particle concentrations in the size classes >4 µm, > 6µm, and > 14 µm. According to the American NAS Code, particles are arranged in 5 size classes between 5 µm and 100 µm.
The multipass test was developed in 1981 at Oklahoma State University for testing hydraulic filters. In this test, the separation behaviour of filter elements, their dust holding capacity, and the expected pressure loss are analysed. The test procedure is standardized. A fluid, which has been contaminated with a test dust, is continuously fed into the filter element. The filtrate is recycled, so that the retained particles accumulate on the filter surface and the pressure steadily increases. During the filtration phase, which continues until a defined pressure is achieved, the number of particles of different size classes in front of and behind the filter is continuously measured. Thus, the β values (filtration coefficients) can be calculated for different size classes. The β value is defined as the ratio of the number of particles NFeed in the feed that are equal to or larger than size x to the number of particles NFiltrate in the filtrate that are equal to or larger than size x. As the separation efficiency of the filter changes with increasing load, the β values are specified by the particle load and the pressure difference at the filter.
The multipass test rig, with two cycles for the test fluid and the filtrate, and the equipment for online particle analysis, is complex and expensive. But the test does simulate the stresses that occur during continuous operation in the industry, but with a higher dust load. The higher dust load emulates the working life in fast motion. The multipass test is a destructive test for filters, which are not regenerable.
High production quality can only be maintained if high-quality materials are used consequently, all incoming materials are inspected Specifically, this entails ensuring that purchased filter media have the specified pore sizes and are free of defects. In addition, other work pieces or semi-finished products that have been manufactured to our order have to be inspected against the technical drawing with regard to dimensional accuracy and identification. Depending on our evaluation of our suppliers, our inspection plans are adjusted or quality improvement measures are discussed with our suppliers.
The quality of our workmanship is documented by the test reports, which certify that each filter element or filter system completely fulfils the specified requirements. Furthermore, our own demand for continuous quality improvement at all company levels is documented by our quality management system that is certified according to ISO 9001:2015.