Filter collapse pressure, pressure drop and bypass valves
A filter has to keep catching dirt as it loads without breaking up. Three numbers decide whether it does: pressure drop, collapse pressure and the bypass setting.
Pressure drop across a filter
Pressure drop (ΔP) is the difference between inlet and outlet pressure. It starts low with a clean element and rises as dirt builds up. Cold, thick oil and higher flow also raise it. ISO 3968 sets out how to measure pressure drop against flow for a hydraulic filter.
Collapse and burst pressure
Collapse pressure is the differential pressure an element withstands before the media pack or core deforms. ISO 2941 is the test that verifies it for hydraulic elements. A spin-on can also has a burst rating for the housing. Collapse pressure must sit well above the bypass valve setting, so the valve opens before the element can fail.
What the bypass valve does
When the differential pressure reaches the valve setting, for example on a cold start or with an overdue element, the bypass valve opens and oil flows around the media. The machine keeps lubrication and flow, but that oil is not filtered.
The valve sits either in the spin-on filter's base or in the filter head. A replacement must match the original bypass setting. A higher setting puts more load on the element; a lower one sends unfiltered oil round more often.
Why elements collapse
Common causes are running far past the change interval, cold starts with thick oil, an element without a bypass fitted where the system expects one, and pressure surges. A collapsed element releases the dirt it has collected into the system downstream.
Using clogging indicators
Many hydraulic return and pressure filters carry a differential pressure indicator. Change the element when it trips, rather than letting the system run in bypass. Engine oil spin-on filters often also carry an anti-drain-back valve that keeps oil in the filter when the engine stops.
Related products
We supply replacement hydraulic elements matched to the original's dimensions.
Our spin-on oil filters are listed with height, diameter and thread for each part number.
Unsure which bypass setting your machine needs? Ask us with the original part number.