Bypass Filtration

Canonical Definition

A supplementary filtration circuit in which a portion of the total system flow (typically 5–20%) is diverted from the main circuit through a high-efficiency fine filter and returned to the reservoir, operating in parallel with the full-flow filter system. Also called kidney-loop or off-line filtration. Bypass filters operate at low flow and low differential pressure, enabling very fine filtration (β₃(c) ≥ 1,000 or finer) without the flow restrictions that would impair normal circuit operation. Over time, bypass filtration progressively polishes the total fluid volume to the target cleanliness code, even removing particles smaller than the full-flow filter's rating.

Engineering Context

Kidney-loop bypass filtration is the primary means of achieving and maintaining ISO 15/13/10 or tighter cleanliness codes in servo and proportional valve systems. A dedicated bypass filtration unit with a 3 µm absolute filter can reach ISO 16/14/11 in a contaminated system within 2–4 hours of continuous operation. Bypass filters should run continuously — not only when contamination alarms activate.

Applicable Standards

Related Technologies

Protection Systems

Knowledge Center Articles

Related Terms