Pressure drop over Bag filter:

When a flow passes through a Bag filter, a certain pressure drop occurs. This pressure drop depends on the Bag filter media, the Bag filter housing and the flow.

An increasing pressure drop over the Bag filter indicates that the filter has to be replaced: When a filter is nearly clocked, pressure drop higher than in a new, clean Bag filter. 

This replacement pressure drop differs from system to system, and is dependent on the Bag  filter media.