Monitoring and Predicting Filtration Performance

All filters will eventually clog if performing correctly. How fast they clog and filter life will depend on the particle load of the feed stream and the total flow through the filter. But how do you know when a filter needs to be changed? And is there a way to predict how long your filters will last?

Depending on your filtration system configuration, the first indication might be a reduction in process flow over time. This can be overcome for a while by increasing the feed pressure in the system - either by increasing pump speed or the pressure in a feed tank. This is usually a short-term fix, as the rate of clogging generally increases as a filter continues to run.

Monitoring Filtration Performance

The best way to monitor filtration performance is to install pressure gauges in strategic locations to monitor the pressure drop required to maintain the target flow rate. As the filter clogs, the differential pressure (DP) will need to increase to maintain this flow. Many systems have a limit (pump, available air pressure, etc.) on the inlet pressure. Once that limit is reached, the flow will start to decrease.

Single vs. Multi Filter Housing Systems

In systems with a single filter housing, it is often sufficient to install a pressure gauge on just the feed side. This pressure will go up as the filter starts to clog. While this won’t provide a full DP reading, it may provide enough information to determine when it is time to change the filter.

In systems with multiple housings, a single pressure gauge will not suffice. Placing a pressure gauge on the upstream side of the first filter will provide information on the total pressure drop through all of your filters required to maintain flow. But since all filters in the train are running at the same flow rate, how can you tell which filter (or filters) is causing the reduction?

The easiest solution is to install a pressure gauge on the inlet side of each filter housing and consider an additional gauge on the outlet side of the final filter. This will provide DP readings for each filter and pinpoint which one(s) is clogging and may need to be replaced.

Single Filter Housing System Example

Example 1A shows a single filter housing with no pressure gauges. The only indication of filter fouling will be a reduction in flow rate or having to increase pumping pressure to keep up.

Example 1B shows a single filter housing with a pressure gauge on the upstream side. Depending on the downstream plumbing or what the filter feeds into, this may not provide a true DP, but it will provide a tool to monitor the filter. As the filter starts to clog, the pressure will increase.

Example 1C shows a single filter housing with pressure gauges on both the upstream and downstream sides. This will provide an accurate DP measurement and can also provide important diagnostic information should the clog be in the downstream plumbing and not the filter. It can be aggravating to shut down the process to change the filter due to the high upstream pressure, only to find it wasn’t the filter after all!

Multi Filter Housing Example

Example 2A shows multiple housings with pressure gauges on the upstream side of each. Example 2B shows multiple housings with pressure gauges on the upstream side of each and a final pressure gauge on the downstream side of the last housing. These arrangements not only give an early warning of filters starting to clog but can pinpoint which filter is clogging.

Multiple housing with no pressure gauges will not provide any early indications of filter clogging and do not give any information as to which filter might be the culprit.

Note: DP for the first housing will be the difference between the upstream gauge of housing 1 and the upstream gauge of housing 2. The same will be true for subsequent housings in the filter train. DP for the last housing will be the difference between the upstream gauge of that housing and the final downstream gauge.

Understanding the Differential Pressure Between the Two Filters

If the first filter is clogged, the DP on that housing should show an increase, while subsequent housings may not change or even decrease if the flow is slowing down.

If the second filter is clogged, the upstream pressure on the first housing will increase, but so will the upstream pressure on the second housing. Thus, the DP of the first housing will be relatively stable while the DP of the second housing is increasing. This provides a more comprehensive monitoring of filter performance and allows changeout of just the filters that are fouling. It also provides data to show which filter type might need to be changed in the event you want them all to clog at about the same rate so a complete changeout can occur simultaneously.

Typically, it is recommended to consider changing the filters when the DP exceeds 30 psi.

Predicting Filter Performance

Another question often asked is how long will my filter last? This needs to take into consideration factors such as flow rate, target batch size, fluid particle load, fluid batch-to-batch consistency, and others. Often, this will require testing with the actual fluid being processed.

For an existing system, filter performance can be monitored over time to predict required filter change intervals and potential changes in the filters being used to enhance filter life. This can somewhat limit the opportunities for ultimate optimization as this may require different housings, additional housings, etc., which require capital investment and potential downtime for the process.

For a new filter system, proper testing with the actual fluid can allow determination of what type of filters are needed to meet filtration goals and the filter size (filtration area) required to meet flow rate and batch size requirements.

The primary tests used for this analysis are Pmax (monitoring DP vs time at a fixed flow rate) and Vmax (monitoring flow rate vs time at a fixed pressure). These tests are performed on filter disks of known area, and the data is then used to extrapolate what is needed to meet customer requirements. This may also require analysis of the filtrate from these tests to ensure it meets customer specifications.

Testing & Analysis Services

CPF’s Technical Team is experienced, equipped, and available to help you design a new filtration system or help optimize an existing one. As each facility, fluid, and filter system design is different, our experts tailor filtration solutions that work for your operation. Learn more about Critical Process Technical Services or contact customerservice@criticalprocess.com.