Originally Posted by
David Gladwin
1 bar is 14.7 psi, 760 mm of Hg or 29.92 ins Hg, so - .95 bar is- 13.9 psi etc, almost as close as it is possible to get to a total vacuum, ie - 1 bar. So either your maths or mine are wrong ! Perhaps I have never understood pressures or vacuums!
(just looked it up. -.95 bar is 95% vacuum or -13.97 psi or 95% of 760 mm of Hg or of 29.92 inches of Hg. ) Can't change physics !
David, this is an AIR rating. Has nothing to do with the hydraulic force a pump's impeller delivers. The pumps used in our jets are pushing a hydraulic column at working pressures of 40psi and up (3Bar+). Fuel is incompressible so the same force can potentially apply on the suction side of the pump if there is flow restriction. This could occur momentarily because of a Lin restriction (clogged UAT filter, for instance) or a change in the commanded pump voltage from the throttle. This suction force can be enough to collapse the tubing just enough that the barbs inside the fitting no longer grip and then air gets pulled. Again, this is hydraulic force and has nothing to do with air vacuum. Apples and oranges.
I am not suggesting the suction side forces are this high in a properly flowing fuel system, but it can easily approach a 1bar air rating in a large turbine and sequential plumbing under certain conditions. After the pump, fine. Before the pump, not so good. Again, why introduce any more risk than one has to? The only routine disconnect needed anyway should be at the turbine.