Sorting Out Filters – Monitoring Filter Health

Filters do not last forever.  In the course of doing its job, a filter becomes progressively more resistant to the flow of liquid.  As this happens, flow is reduced and, as we’ve mentioned before, this process may even be accelerated by the fact that the partially blocked filter retains smaller and smaller particles.  The price of replacing filter cartridges, bags or whatever media is used is usually not huge but does add up over time.  Finding the right scheme to monitor and service filters is today’s topic.

Periodic replacement is a surprisingly popular mode of maintenance.  Filter cartridges or bags are replaced with a frequency ranging from every few hours to every few months based on an established history of how long a filter will function in the application.  Periodic replacement is attractive especially in situations in which changing a filter requires partial or total draining of a tank or system.  The filter change is often done as a matter of course whenever the cleaning solution is dumped and replaced and is considered part of the preventive maintenance schedule.

Probably the worst filter management (but also surprisingly common) is replacing the filter only when the system won’t produce clean parts or when it is obvious through observation that there is no flow returning to the tank from the filter loop.  The big risk, of course, is that a pump may be destroyed by overheating as it pumps against the “dead” head presented by the clogged filter.

As a filter becomes more resistant to flow there are changes in pressures and flow rates in the filter loop that, if monitored, can indicate when a filter change is necessary.  Three common monitoring scenarios are described below.

Back Pressure to the pump

This method is appropriate when a pump is supplying a single filter with an outlet that is either unrestricted or presents an unchanging restriction (spray nozzles for example).  The pump provides a constant source of pressure based on the flow.  A visual or electronic pressure gauge installed on the inlet line to the filter indicates the back pressure.  The back pressure with a new filter as well as a filter that is known to be approaching the end of its useful life is established by experimentation.  Once the pressure indicating the filter’s end of life is attained, it is time to service the filter.  The risk using this method is that the pressure measuring device can’t determine if the source if elevated pressure due to a restriction is because the filter is clogged or because there is some other restriction in the outlet (clogged spray nozzles for example).  If a filter replacement does not result in the pump back pressure returning to the “new filter” reading, further investigation for the cause of the back pressure is indicated.

Pressure Differential Across the Filter

In this case, pressure monitors are placed to measure pressures at the inlet and outlet of the filter.  The pressure differential is an indicator of the filter’s condition.  Although this method measures the restriction of ONLY the filter, differences in flow caused by other changing restrictions down the line does have an impact on the differential reading.  When the pump supplying the filter is off, the pressure differential is, of course, zero no matter what condition the filter is in.  If the filter’s outlet is restricted, the pressure differential is reduced.  In fact, if the outlet of the filter is totally restricted, the pressure differential will again be zero just as it is when the pump is off.  In fact, this method is most viable if used in combination with the pump back pressure scheme described above.  IE the back pressure with the pump operating should be below a certain threshold with the differential across the filter within a specified range.

Flow Through the Filter

Finally, monitoring flow through the filter can be an indicator of filter health.  As the filter reaches the end of its useful life, the flow rate will diminish and, when the filter is totally clogged, cease completely.  This method is probably most useful when the process is mainly dependant on maintaining sufficient flow.

In summary, any of the above monitoring methods can be used effectively depending on the application.  Each, however, has its weak points and conditions that need to be met to make the observations meaningful.  Process technicians and operators should be aware of how a particular filter monitoring scheme works and what limitations it has.

– FJF –