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The pressure-drop in a filter system is the pressure difference before and after the filter system.
Pressure-drop occurs due to transfer valves, pipe bends, filter media selection and the loading of the filter element.
Pressure drop curves in seal gas coalescing application show three stages:
Start-up (clean) pressure-drop
When a new filter element is installed, the element is clean and dry, the filter media pores are not blocked with particles or liquid droplets, and gas flows freely; most of the pressure-drop is caused by the filter system.
The start-up pressure-drop (clean and dry filter element) of the total system is described in the API 692 as filter sizing parameter.