A double wall tube assembly comprising: an inner tube extending in an axial direction between two opposed ends of the inner tube; a sleeve fitting (104a; 104b; 204) mounted to one end of the inner tube; an outer tube outboard of the inner tube and extending in the axial direction between two opposed ends of the outer tube; and a collar fitting (108a; 108b) mounted to one end of the outer tube wherein the collar fitting is outboard of the sleeve fitting; wherein the inner tube is a first inner tube, wherein the sleeve fitting is a first sleeve fitting, wherein the outer tube is a first outer tube, and wherein the collar fitting is a first collar fitting and further comprising: a second inner tube with a second sleeve fitting mounted to an end thereof, with a second outer tube outboard of the second inner tube with a second collar fitting mounted to an end of the second outer tube outboard of the second sleeve fitting; and a seal assembly sealingly engaged between the first and second collar fittings and sealingly engaged between the first and second sleeve fittings, wherein the seal assembly is sealingly engaged with the collar and sleeve fittings with axial seals (124, 126) to form a junction of two tube assemblies; characterised in that the seal assembly includes passages for passage of fluid between the axial seals (124, 126); in that at least one peripheral passage is defined in each sleeve fitting; and in that alignment of peripheral passages in the sleeve fittings with passages of the seal assembly permits flow through the junction, in an outer passage between inner and outer tubes (102, 106) of the two tube assemblies. an inner tube extending in an axial direction between two opposed ends of the inner tube; a sleeve fitting (104a; 104b; 204) mounted to one end of the inner tube; an outer tube outboard of the inner tube and extending in the axial direction between two opposed ends of the outer tube; and a collar fitting (108a; 108b) mounted to one end of the outer tube wherein the collar fitting is outboard of the sleeve fitting; wherein the inner tube is a first inner tube, wherein the sleeve fitting is a first sleeve fitting, wherein the outer tube is a first outer tube, and wherein the collar fitting is a first collar fitting and further comprising: a second inner tube with a second sleeve fitting mounted to an end thereof, with a second outer tube outboard of the second inner tube with a second collar fitting mounted to an end of the second outer tube outboard of the second sleeve fitting; and a seal assembly sealingly engaged between the first and second collar fittings and sealingly engaged between the first and second sleeve fittings, wherein the seal assembly is sealingly engaged with the collar and sleeve fittings with axial seals (124, 126) to form a junction of two tube assemblies; characterised in that the seal assembly includes passages for passage of fluid between the axial seals (124, 126); in that at least one peripheral passage is defined in each sleeve fitting; and in that alignment of peripheral passages in the sleeve fittings with passages of the seal assembly permits flow through the junction, in an outer passage between inner and outer tubes (102, 106) of the two tube assemblies. a second inner tube with a second sleeve fitting mounted to an end thereof, with a second outer tube outboard of the second inner tube with a second collar fitting mounted to an end of the second outer tube outboard of the second sleeve fitting; and a seal assembly sealingly engaged between the first and second collar fittings and sealingly engaged between the first and second sleeve fittings, wherein the seal assembly is sealingly engaged with the collar and sleeve fittings with axial seals (124, 126) to form a junction of two tube assemblies; characterised in that the seal assembly includes passages for passage of fluid between the axial seals (124, 126); in that at least one peripheral passage is defined in each sleeve fitting; and in that alignment of peripheral passages in the sleeve fittings with passages of the seal assembly permits flow through the junction, in an outer passage between inner and outer tubes (102, 106) of the two tube assemblies.