Most Commonly Asked Filtration Question Series
Integrity testing is important as failing filters can compromise the final product. Non-destructive tests allow the filter to remain in place and be used for its intended purpose. Having the filters integrity-tested also is a requirement for suppliers who sell absolute-rated membrane cartridge filters for microbial reduction or removal of yeast and mold. Test results are used to validate the removal of specific organisms and a certificate is included with each product documenting the quality of the filter. In addition to vendor testing, manufacturers in different industries perform their own integrity tests for critical processes. In the pharmaceutical and bioprocessing industry, regulations call for integrity testing at prescribed intervals in a process. In the Food & Beverage industry, although not mandatory, integrity testing is a strategic move in assuring product safety and quality.
Integrity testing of filters is crucial to confirm the reliability of filtration processes and follow the regulatory compliance stated by GMP standards and the FDA. Once filtration goals are established and the system is operational, integrity testing is the most certain way for system operators to know if the filters are working as required from the upstream side to downstream activities, affecting single and multi-use technologies. Filtration performance can be affected by filter damage during shipping, improper installation, cleaning chemicals/water temperatures and process upsets.
There are several ways filters can be tested for integrity. The most common and effective tests are discussed below. Regardless of which test you use the cartridge filter must be fully wetted. See the product instructions located in the resource center for more details.
Forward Flow Diffusion Test
Most filter manufacturers use a forward flow diffusion test as a final test before the product release. This involves making sure the filter is properly wet out (usually with water but could be done with alcohol/water mixtures for hydrophobic filters) and then applying a controlled upstream pressure somewhat below the bubble point of the membrane. This allows diffusional flow through the liquid layer but no bulk flow through open pores. This diffusional flow rate is validated against bacterial retention to ensure sterilization or bioburden reduction performance. This test is accurate when using measurable amounts of diffused gas; therefore it is performed on filters with larger surfaces.
In-process this is accomplished by installing a high-purity pressurized air (or nitrogen) line upstream of the filters to be tested. After installing and flushing the filters, the specified pressure is applied and the flow rate is measured. This can be performed manually by installing a mass flow meter in the airline, allowing the air flow rate to stabilize and recording the value to ensure it meets the filter manufacturer’s specifications. For installations requiring frequent integrity testing (i.e. pre- and post-every batch), an automated integrity test system (available from several suppliers) can be installed. See the figure below for details:
Pressure Decay Test
The “pressure decay” or “pressure hold” test identifies potential issues related to the filter and verifies that the filter can withstand the specified pressure without compromising its integrity. This test requires the same air (or nitrogen) source and an accurate pressure gauge but does not require a mass flow meter. To perform this test, after the filters are installed and flushed, a specified pressure is applied to the filters, and the air feed valve is closed. After a defined time interval, the pressure is noted, and the pressure decay is calculated (initial pressure minus final pressure). The allowable pressure decay to guarantee integrity could vary depending on the type and number of filters installed. This test again relies on the diffusional flow allowed at a defined pressure below the bubble point (hence the drop in upstream pressure), and the allowable pressure decay is calculated accordingly. Your filter supplier will provide details on the pressure decay integrity test. The figure below illustrates the setup:
Bubble Point Test
Another option to ensure the efficiency and reliability of the filtration process is by measuring the bubble point of a filter. The bubble point test occurs when gas passes through a wetted filter membrane under increasing pressure. This pressure correlates to the surface tension and the filter’s pore size. The bubble point test is frequently performed on filters before their installation into the system, but it can also be performed after the installation when all the proper plumbing is done. Measuring the bubble point requires installing the filter(s) into a housing and making sure they are fully wet out with the required fluid (usually water for hydrophilic filters or an alcohol/water mixture for hydrophobic filters). The housing is connected to an upstream air source, and the downstream line is directed into a vessel containing water in which bubbles could be seen. The upstream pressure is slowly increased until a steady stream of bubbles is observed in the downstream vessel. Verifying that this bubble point meets the manufacturer’s specifications indicates that the filter is undamaged and suitable for use. See the figure below for an example of a housing setup:
Water Intrusion Test
The water intrusion test is essential to determine the performance of hydrophobic filters. This test evaluates the filter’s ability to prevent water intrusion under controlled conditions where it is not desirable to introduce alcohol into the system. This involves installing the filter(s) into a housing, applying water pressure on the upstream side at a defined pressure value, and measuring any downstream water flow. An integral filter will not allow significant water flow. If the downstream flow exceeds the manufacturer’s specifications it is an indication of filter damage or incorrect installation. This method is used by manufacturers to evaluate the filter’s effectiveness in resisting water intrusion and keeping its integrity. See the figure below for an example of setup:
In-process integrity testing is a critical component of assuring the final product quality by making certain the filters are operating as needed throughout the process. Doing so lowers risk and the expense associated with disposing of a bad batch.
Remember - we're here to help! An evaluation is as simple as a conference call with our Application Engineers to review your current situation and make recommendations on how and where to implement integrity testing for your process.
Critical Process Filtration offers over twenty-five years of experience as a mid-size manufacturer of process Filters. We have an experienced Applications and Process Support staff that will identify the right integrity test for your process and will assist with installation, training, and validation as required. For more information on Integrity Testing in your process, contact us HERE.