The burst or collapse pressure rating of a safety screen is a critical but sometimes overlooked safety screen design consideration. The Lee Company defines burst as a rupture in the outward direction, while collapse is the first sign of permanent deformation in the inward direction. Although the safety screen may not rupture, The Lee Company takes a very conservative view and considers any sign of deformation a failure. If a safety screen becomes partially or fully clogged, and it is not strong enough to withstand the differential pressure, the results of this failure could become catastrophic. When a safety screen ruptures, a sudden, heavy concentration of dirt flushes downstream, possibly with fragments of the screen itself, most likely disabling sensitive downstream components. This is a "hard" failure as opposed to a "soft" failure in which the flow through a Lee HI-BAR safety screen is gradually reduced (possibly leading to zero flow). A contamination induced malfunction caused by a ruptured safety screen is also unpredictable and can occur without warning since safety screens are not equipped with ΔP indicators or bypass relief valves.
However infrequent, safety screen failure does happen and therefore The Lee Company recommends at minimum, that the burst or collapse pressure rating of a second tier safety screen equal full system pressure. Selecting a safety screen with a burst and collapse pressure greater than system pressure is conservative design practice. The military specification MIL-H-5440H, "Hydraulic Systems, Aircraft, Design and Installation Requirements For" makes the following recommendation for second tier screens (Paragraph 188.8.131.52): "It is recommended that second tier screens, as defined by AIR4057 should, when fully clogged, withstand a proof pressure test without collapse, rupture, or permanent deformation." Many of today's military and commercial aircraft manufacturers are even specifying second tier safety screen burst and collapse pressure requirements equal to the proof pressure of the system. The Lee Company's HI-BAR safety screen product line is designed to meet these requirements with the rated burst and collapse pressures exceeding 7500 psid in both flow directions!
Third tier screens (mounted directly on components) typically have lower burst and collapse ratings.