Pipes and Unions
Core concepts
Rigid fluid lines use aluminium alloy tubing (3003 or 5052-O for low pressure, 6061-T6 for higher pressure), corrosion-resistant steel (304, 321) or titanium where temperature or pressure demands it. End fittings are either flared or flareless. The aviation flare is 37 degrees, formed with a proper flaring tool and checked for cracks, eccentricity and correct diameter. Flareless fittings use a ferrule that bites into the tube when the nut is torqued; once swaged the ferrule must not be re-used.
Flexible hoses range from low-pressure rubber to PTFE-lined, braided high-pressure assemblies, with defined minimum bend radii, a shelf life and a date of manufacture. Route lines with slack for expansion, clipped at approved intervals, clear of hot areas and structure, and never used to pull two components into alignment.
Traps in questions
A 45-degree automotive flare is not interchangeable with a 37-degree AN flare. A cracked or distorted flare must be cut off and re-made, never straightened. Do not reuse flareless ferrules or olive-type fittings. Aluminium tubing is not for high-pressure hydraulic systems. Hoses must not be twisted during installation - twist shortens life - and must not be bent tighter than the stated radius.
Rule of thumb
Keep bends to at least three tube diameters radius with no wrinkling or flattening, and support the line so it carries no weight and no vibration-induced stress.
Worked example
A leaking flare on a 3/8 in aluminium line is found to be oval. The correct action is to cut back past the damaged section, re-anneal if required, re-flare to 37 degrees, inspect, and pressure-test - not to tighten further.