Preface
All integrity test procedures outlined below should be followed when performing integrity testing of membrane filters only. Use of these procedures on other types of filters such as polypropylene or glass fiber cartridges will not give consistent or accurate results. Before starting the procedure, please make sure your system is set up properly. If you need guidance, we have instructions here.
Introduction
The diffusive Flow Integrity Test determines if a membrane and filter cartridges are integral. All the pores within the membrane are filled with a wetting solution, which is held in place by capillarity forces and surface tension creating a barrier between the upstream and downstream of the filter cartridge.
Based on Fick’s Law of diffusion, when a differential gas pressure exists across a fully wetted membrane, the gas molecules will ‘diffuse’ through the wetting fluid filling the pores of the membrane. The rate of passage is proportional to:
- the thickness of the membrane,
- the surface area of the membrane,
- the solubility of the gas in the wetting fluid,
- the surface tension of the wetting fluid,
- the differential pressure.
The diffusive Flow Integrity Test is typically measured at a test pressure near the minimum completely retentive bubble point pressure (typically 75 – 80%). This test is a quantitative measurement and correlates well with bacteria retention. It may also be employed with multi-cartridge housings.
In this test, we apply a constant pressure of air on the upstream side of the membrane filter and then measure the volume of air diffused through the filter.
Wetting fluid
Water is the most common wetting fluid for performing an integrity test, but the use of alcohol as 70/30 v/v isopropanol (IPA)/water or 80/20 ethanol (EtOH)/water can be beneficial by reducing the number of tests due to insufficient wetting, reducing the flushing time, and increasing sensitivity.
Figure 1: Integrity Test System
Refer to Figure 1, which represents a typical layout for an Integrity Test System. Follow these instructions to properly perform a diffusive test on your membrane filter.
- Install and rinse cartridges as per Cartridge Wetting instructions.
- Starting with all valves in the closed position, open valves V8 and V9. Downstream must be open to ambient pressure.
- Connect a pressure-regulated air (gas) supply and open valve V4. If possible, pressure should be regulated to the desired test pressure.
- Open the drain valve (V6) and vent valve (V7) and purge upstream test fluid at 2 psig. Close valves V6 and V7 when the flow becomes mostly air.
- Slowly increase the air pressure, as measured on the upstream pressure gauge (P3) or mass flow meter to the amount on the test specification supplied by Critical Process Filtration (usually 80% of the specified bubble point).
- Allow the system to equilibrate. If the regulated air supply is not set at the test pressure, maintain the specified pressure as measured at P4 by adjusting the flow of air using valve V4.
- Record the airflow.
- A diffusional flow (diffusion rate) reading higher than the specification indicates that the system is not integral.
Procedure Note
If the cartridge appears to fail the initial diffusion test, then use the flow chart below to help identify the problem and test the filters:
When an integrity test failure occurs, the first step is to rewet the filter and verify the test fluid temperature and the test parameters. When these conditions have been verified, the filter should be retested. If the result is a pass, then the filter is integral. If it fails, it is recommended to perform a leak test by tightening all connections and if necessary, disconnecting and reconnecting any joints. Also, you may want to test the valves and connections by leak spray. Once the upstream leaks are resolved proceed to a re-wetting step. This can include a larger flush volume with back pressure (downstream flow restriction). If the filter fails the integrity test again, please contact CPF for further analysis.
Frequently Asked Questions
- Do I need to perform a pre-use integrity test?
All the filters are integrity tested during manufacture. Still, it is recommended by CPF that users perform an integrity test both before and after use as damage to filters can occur during handling. - How does temperature influence integrity testing?
CPF filters are integrity tested with DI water at 20°C ± 5°C. It is recommended to maintain the temperature of the filter assembly during the integrity test constant as variations can result in inaccurate flow measurements. - What is the best way to lubricate the o-rings of a filter before an integrity test is performed?
CPF recommends the use of water to lubricate the O-rings of a filter to facilitate the installation into the housing. - Can a pressure vessel be used instead of a pump?
This approach is acceptable, the filtration can be performed under constant pressure. We strongly recommend monitoring the process flow rate and the upstream pressure. It is important to consider that using a pressurized vessel can increase the amount of dissolved gas in the process fluid, which can lead to the formation of trapped air bubbles in the filters, resulting in an inaccurate wetting of the filters and a reduction of the flow rate. - How does reducing the flow rate the downstream improve wetting of the cartridge before integrity testing?
The pores of the membrane must be filled with water before the integrity testing. To accomplish a complete wetting of the entire length of the filters CPF recommends applying downstream flow restriction during the filter flush. - During troubleshooting an integrity test failure, how can I determine if the failure is due to a leak in the system under test?
Leaks are common due to damages to the O-rings seals and to the housing. A visual inspection of the filtration system is recommended to identify the failure. It is recommended to verify the connections, system temperature, and leak test of the filtration system in a water bath or soap around the fittings and connections. - Should I consider any prefiltration for my application?
For most of the applications, our filters can tolerate complex and high-concentration fluid at constant pressure or flow rate. In other instances, CPF recommends that the feed is prefiltered to at least 0.45 μm. This will increase the throughput and life of the intended filtration system. The prefilter can be incorporated into your system by attaching it right before the upstream filter or by filtering through the prefilter into a holding vessel and then through the upstream filter.
For help determining the test values or additional information, contact sales@criticalprocess.com or call us at (603) 880-4420.
Critical Process Filtration, Inc. is an ISO-9001-certified manufacturer of process filters. We have been helping customers for over 25 years to build and improve process filtration systems. Our comprehensive testing, analysis, and validation services support your team whenever needed. Partnering with you and your process team is how we deliver your company’s solution right the first time.
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