Filter Bag Bypass: Diagnose Poor Filtration After a Change

If liquid quality becomes worse immediately after a filter bag change, a finer micron rating is not automatically the answer. First check whether the liquid is passing through the intended filtration path. A sealing fault, installation problem, damaged bag or contaminated sample can look like inadequate particle retention.

This guide helps maintenance and purchasing teams collect evidence before ordering another liquid filter bag. It concerns particle filtration; removing dissolved substances or establishing microbiological safety requires a separately specified treatment process.

Start with a controlled comparison

Record the old and new bag part numbers, media, retention rating, ring design and dimensions. Note when the quality problem started. Compare inlet and outlet samples taken using the same approved method, at comparable flow, temperature and feed conditions. A photograph of cloudy liquid alone cannot establish particle size or prove a seal failure.

Record differential pressure as inlet pressure minus outlet pressure. Compare it with the clean-filter baseline at the same operating conditions. Low differential pressure can occur with a clean, correctly operating bag; it does not prove bypass. An unexpected fall accompanied by poor outlet quality deserves investigation, but the instruments, flow and upstream process must also be checked.

Separate four possible causes

Inspect the bag-to-housing seal and basket

Inspection requires a safe shutdown. Trained personnel must follow the housing manual and site isolation procedure, prevent unexpected restart, relieve pressure safely, verify zero pressure, and cool or drain hazardous contents as required before opening the vessel. Never loosen a pressurized closure.

Check the following items against the approved assembly drawing:

• Ring and seat: Is the correct ring fitted? Is it seated evenly without trapped material, debris or visible damage?
• Bag support: Does the bag fit the specified restrainer basket without folds or unsupported sections? Inspect the basket for deformation and sharp edges.
• Retention components: Are required hold-down parts present and installed as specified? A similar-looking bag is not proof of correct fit.
• Bag condition: Inspect seams, welds and media for tears or damage. Preserve the failed item and its label for the supplier.
• Housing gasket: Inspect the vessel closure seal separately. A leak-tight outer closure does not establish that the internal bag seal is working.

Eaton’s UNIBAG guidance highlights correct positioning inside the restrainer basket and the role of the sealing construction. Those principles support checking the complete assembly; they do not establish that an INCEFILTER bag fits a particular third-party housing.

Do not confuse a micron label with verified retention

Two bags marked with the same micron value may have different efficiency characteristics. Request the retention efficiency, test method, test conditions and applicable product reference. Do not assume a nominal rating represents complete removal of every particle at that size.

Also confirm compatibility for the entire assembly: media, sewing thread or welded construction, ring, housing and seals. Include the process liquid, concentration, normal and maximum temperature, cleaning chemicals and pressure conditions. A material name by itself does not qualify the finished bag for the application.

Restart with documented acceptance criteria

After correcting an identified fault, reinstall and restart only according to the equipment manufacturer’s procedure. At a stable operating point, record flow, temperature, inlet and outlet pressures, and the agreed quality measurement. Keep these values as the new baseline. Escalate persistent quality failures rather than repeatedly fitting finer bags without identifying the cause.

Set acceptance criteria before the trial: required outlet quality, permitted operating conditions, observation period and the procedure for handling a failed result. Do not use a generic pressure-change threshold in place of the limits for the actual bag and housing.

What to send with an INCEFILTER inquiry

For a replacement review, send the bag label and dimensions, close-up photographs of the ring and basket seat, housing model, liquid and temperature, flow, pressure readings and description of the quality problem. Tell us whether you need polypropylene felt bags, another medium, or help identifying the existing specification.

Contact INCEFILTER with your bag and housing details. Ask for written confirmation of the proposed configuration and trial criteria before placing a bulk order. Housing options can be reviewed in our filter housing range.

Technical reference

Eaton: UNIBAG filter bag construction and installation guidance. Manufacturer-specific performance statements apply to the named Eaton products, not to other suppliers’ products.