Patent GB-1493140-A (GB)
Title: Tubing
IPC: F16L9/14
Filed: 1974-12-19
Abstract: 1493140 Tubing N LAING 19 Dec 1974 [19 Dec 1973] 54921/74 Heading F2P A tube for the transport of fluids under pressure, especially hot fluids, has a wall consisting of several laminae of which one is built up of filament material, characterized in that the innermost lamina is a thin-walled metal tube to convey the fluid, the metal tube being surrounded by said one lamina which consists of a winding of mineral filaments which resists loop stresses imposed in the tube by the pressure of the fluid. In the embodiment of Fig. 1 a hollow laminar cylinder 2 surrounds tube 1. The filaments 3 and 4 of the cylinder constitute an inner laminae and have a helical angle 5 between them. The angle 5 diminishes progressively in the other laminae lying radially move outward finally becoming almost zero so that outer filaments 6 are practically circular windings. The hollow cylinder so formed can be pushed over the inside tube 1, or it can be directly wound on the tube 1. The lengths of one turn at the different laminae is the same so that they are all equally loaded when the tube 1 has fluid under pressure therein. A helically wound band 7 comprises highly porous insulating material, e.g. finely ground volcanic slag, china clay or alumina powder with an elastic, binding agent. A tube 8 is formed by welding adjacent edges of a cylindrical helical winding of thin sheet metal. The space between tubes 1 and 8 can be evacuated along conduits 9 formed by chamfers on band 7. They communicate with pipe-lines which lead to the outside. Band 7 is of a material having a substantially smaller pore size than assembly 2 so that, at the corresponding pressure, the Knudgen effect can be made to prevail only in the insulating layer, thus the temperature gradient between the inside and outside is almost entirely localized in this layer. In an alternative embodiment a further metal tube can be positioned between layers 2 and 7. Axially parallel filaments 10 extend along the length of the tube and are clamped at both ends. They carry axial tensile loads and may be glass fibres embedded in rubber or in a synthetic resin. A protective tube 11 is coated at 12 for further protection. Evacuation of the space between tubes 1 and 8 takes place during the first heating of the tube. During heating, the binding agent in band 7 evaporates and the gases so released are extracted until the Knudsen effect can be implemented. In the embodiment of Fig. 2 the inner thinwalled tube 20 is of a helical welded construction. Around the tube are inner laminae 23, 23<SP>1</SP>, then came laminae 24, 24<SP>1</SP> and then laminae 25, 25<SP>1</SP>. Insulating layer 29 consists of brick-shaped insulating blocks inserted in imbricate arrangement in outside tube 31. Grooves 32 allow gas extraction to take place. Tube 31 has a bitumen coating 36. The included angle between the helical layers, viz. 23<SP>11</SP>, 24<SP>11</SP>and 25<SP>11</SP> are different such that the inner laminae carry a larger part of the axial stresses and the outer laminae carry a larger part of the hoop stresses. Preferably the included angle for the intermediate layer is 54 degrees. If the laminar layers are to contribute to the insulation then it is preferable to insert between the layers a powder of an inorganic substance. The gas in the insulating layer after heating can be either a light or a heavy gas. The heavy gas is preferable if the pore size can be maintained small enough in which case, water, nitrogen, carbon dioxide or sulphur dioxide are suitable. If there is a steep temperature drop across the filament laminae then this must be taken into account to ensure that the loading in the different layers is the same. This may be achieved through different pre-tensioning of the layers or to select the material of the filaments as a function of the radius of winding so that the thermal expansion coefficient of the outside is greater than that of the inside lamina. To interlock the filaments of adjacent laminae, not only friction locking can be used but binding agents may be applied in the manner of adhesives. For temperatures up to 350‹ C. polyamides are suitable whilst above this temperature a binding agent having a mineral basis, e.g. water-glass is best. In another method extra filaments are orientated at right angles to the main filament direction of a material with a melting point below the melting point of the main tension-loaded filaments. As the hollow cylinder is made it is heated once so that the extra filaments fuse and the fusible material concentrates by capillary forces in the contact regions of adjacent crossing filaments and permanently bonds them.