Service Prefilter - MPC (systems WITH boost pump)

Service Prefilter Alarm (systems WITH boost pump)


During normal operation, the feed water is filtered in two stages. First it enters a fine mesh metal sea strainer then passes through a boost pump. This protects the boost pump from damage. After passing through the boost pump the feed water enters the filter housings containing 20 and 5 micron elements. These filters remove very fine particles which could damage the Feed Pump or Clark pump, and which would shorten membrane life. Some systems will only have a 5 micron filter. Service schedules will vary widely depending on how and where the system is used. If large amounts of feed water are run through the system in a relatively short period in biologically fertile near shore waters, the prefilters will plug up, the pressure drop across the filters will rise, product production and quality will drop, and the system will sense plugged filters and give a SERVICE PREFILTERS alarm.

When the system is on the sensor should be reading a value around the 12 – 20 range, anything below 10 will trigger the alarm. The service prefilter alarm on your system can be triggered by anything causing a flow restriction/pressure drop between your feed pump and the ocean. Obviously the prefilter is one option but others worth looking into are:

  • Black sea strainer inlet check valve clogged with debris*
  • Clogged through hull
  • Sea strainer screen
  • Kinked or leaking hose
  • Boost pump weak, off, or receiving less than 24V (Boost Pump Test)

*NOTE: Older MPC models that do not freshwater flush through sea strainer will not have this check valve.


If you have a Service Prefilter alarm that persists, the best thing to do is to check out what your absolute pressure sensor is seeing in real time.


For MPC 5000 systems the Prefilter Fault is programmed via Clean Pressure, Low Vacuum Limit, and the Inlet Offset parameter.

CLEAN PRESSURE: This should be set to one whole number below the pressure seen at the inlet sensor when the system is running with a brand new pre-filter. After installing a new filter, run the machine by flipping the toggle switch on the MPC box to RUN MAN. The number on the right is adjusted to be one whole number below the number on the left, which is the inlet sensor reading. This parameter sets the “Clean” end if the pre-filter condition bar graph. Factory default is 15.

LOW VACUUM LIMIT: Set point for the maximum allowable pressure drop through the prefilter. If the inlet pressure reading drops below this point the unit will alarm “Service Prefilter” and shut down. This set point is in absolute pressure, and determines the “Replace” end of the Prefilter Condition bar graph. In most cases this parameter should be set to 10.

INLET OFFSET: This parameter calibrates the boost pressure sensor found on the intake manifold of the feed pump (marked yellow). The number on the left is the real time pressure reading and the number on the right is the offset. This should be set to 0.0, other settings are typically masking the real issue.


To view the real time reading for an MPC 5000 system run the system with the “Run Manual” toggle switch, get into the programming settings.

Our linked YouTube video goes over how to access the MPC programming. Programming From Display Video - (MPC)

Print instructions are also available at Programming From Display (MPC)


When you are in the programming navigate to the Inlet Offset screen note how the real time (left side) value is behaving (right side is the offset and should be at 0.0.)


If the numbers jump around, you probably need a new sensor. If the numbers seem steady but unreasonable, we suggest your take off the sensor and clean out the port with a wire or paper clip. You can also check to see that all connections are good at the MPC circuit board. If neither of these do the trick, you may need to disable the alarm and get a new sensor. The alarm can be disabled in the same programming menu by navigating to DISABLE PREFILTER and changing NO to YES. Check prefilters frequently if sensor is disabled and change often. Running the system without prefilter monitoring greatly increases chances of prefilter failure and greater system damage occuring.

Further pressure sensor troubleshooting can be found at: Pressure Sensor Tests


------------- If you are seeing Service Prefilter when you freshwater flush your system, see this link for help -----------------

Vane Pump Speed Adjustment - Freshwater Flush