Choosing fluid fittings for a cryogenic / oxygen test system: which family for which joint

Useful mental model when explaining or designing a propellant feed: every fitting has to hold and to seal, and families differ in where the seal is.

- NPT (tapered pipe): seals on jammed threads plus tape/dope. Fine warm and moderate pressure; avoid on cold, oxygen or frequently remade joints (sealant shrinks/hardens cold, debris is an oxygen ignition risk, threads deform with each remake, taper wedges thin walls). - SAE straight-thread O-ring boss (J1926, aerospace AS5202/MS33649): thread holds, O-ring in the port chamfer seals. Best for ports machined or printed into a part. For LN2/LOX ports the stock nitrile/FKM ring is the weak point: use metal boss seals or spring-energised seals and prove them cold. - 37 deg flare (JIC, aerospace AN): metal cone on metal cone, swivel nut. Standard hose end; good cold; remakeable. - Double-ferrule compression (Swagelok, Parker A-LOK, DK-Lok): best for rigid seamless stainless tube; very high pressure; do not mix brands; cut square, deburr, bottom the tube, tighten 1-1/4 turns past finger tight (1/4 to 1 in) and check with the gap gauge. - Face seal (ORFS, VCR metal gasket): most leak-tight, costly; ultra-high-purity gas or high-pressure hydraulics. - Barb + clamp, cam-lock: low-pressure water only. - CGA cylinder connections: one thread per gas (540 O2, 350 H2/CH4, 580 N2/inert) so regulators cannot be crossed.

Typical stack from facility to engine: facility hose (37 deg flare) -> flare-to-tube adapter -> stainless tube with compression tees/reducers/elbows -> compression-to-SAE connector -> SAE port. Oxygen-clean everything in O2 service and use only oxygen-compatible lubricant; keep NPT on warm instrument ends at the far end of a tube.