Long pipe runs which convey fluids between items of plant such as pressure vessels are usually made up from short lengths of pipe welded together, however the pipes are not welded permanently into the vessels but rather attached in a manner which permits easy separation. One of the simplest demountable attachments is the screwed connection ( a) though these are suitable only for relatively low pressures, due to the difficulty of making good a leaking thread.
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The higher the fluid pressure and temperature, the more robust must be the flanges and bolting to contain them. AS 2129 Flanges and Bolting for Pipes, Valves and Fittings contains tables of dimensions for flanges and other pipe fittings. Each Table ( D, E, F . . . T ) lists for the various nominal pipe sizes the minimum dimensions which are suitable for a certain limiting combination of fluid pressure and temperature. Thus for steel in the range -50 to 232 oC, the maximum pressure pFT permitted by each flange Table is :
| Table | D | E | F | H | J | K | R | S | T |
| pFT (MPa) | 0.69 | 1.38 | 2.07 | 3.45 | 4.83 | 6.20 | 8.27 | 12.4 | 19.3 |
These limits are halved linearly from 232 to 427 oC and drop to zero at 532 oC as sketched. So, when steel flanges and fittings have to be selected for a design pressure p and temperature T, their size is dictated by a Table for which :
( xiii) pFT ≥ p . maximum [ 1 , 390/( 622 - T ) , 210/( 532 - T ) ] ; T ( oC) < 532
A small bore pipe is often demountably attached to a vessel by means of a flange butt-welded to the pipe and mating with a pad which is welded into or formed on the vessel wall, ( c) above. Clearly the pad dimensions must match the flange. Studs are screwed into holes tapped in the pad, since through- bolts like those of ( b) would allow leakage.
Acceptable pad forms are defined in AS 1210. Pipe wall thicknesses, if required, may be found from ( 1) using design stresses for pipe materials cited in AS 1210, in conjunction with manufacturers' lists or AS 1835.
Openings are required to monitor the condition of the vessel's interior if subject to corrosion, and may be necessary also for manufacture. The size and disposition of the opening(s) depend upon the duty and size of the vessel - in a small vessel a single handhole or a flanged- in inspection opening may be adequate whereas large vessels require elliptical manholes, often with reinforcement / seating rings, though heads may be flanged inwards (reverse knuckle) to provide a seating surface. The minor axis of an elliptical opening in a cylindrical shell should lie parallel to the longitudinal axis of the shell.
The opening is sealed usually by an internal door, a gasket and one or two bridges and studs. The door is ellliptical to permit its removal, if necessary for remachining a damaged gasket seating surface.
The studs provide the initial sealing force, ie. the initial seating pressure on the gasket face before the fluid is pressurised. When the fluid pressure later rises, the door tends to be self- sealing as the pressure load on the door increases the gasket contact pressure. The load on the studs therefore decreases, however the Code specifies that the door must withstand simultaneously bending by both fluid pressure and maximum possible stud (or bolt) tightening. The flat door calculation thickness t is thus given by :-
( xiv) ( C1. fluid pressure . door area + C2. bolt stress . bolt area ) / t2 ≤ S ; C1, C2 constant
The door is equipped with a locating spigot to aid its engagement when closing. If the door is heavy then provision must be made for supporting it during opening or closing - any such support must not interfere with even take-up of the gasket, nor must it hinder easy access to the vessel. The designer of the door support must visualise the door's detailed operation.
The choice of gasket material depends upon the vessel duty - fluid, temperature and pressure - and the flanges' surface finish and rigidity. The stiffer the gasket, the greater must be the initial seating force and hence door thickness.
Allowable stresses for studding materials are quoted in AS 1210, which stipulates that the core areas and not the effective stress areas should be used in stress calculations :
| Screw size (mm) | M8 | M10 | M12 | M16 | M20 | M24 | M30 | M36 |
| Core area (mm2) | 32.8 | 52.3 | 76.2 | 144 | 225 | 324 | 519 | 759 |
A horizontal pressure vessel (length L, diameter D mm) is commonly mounted on two saddle supports - more would result in static indeterminacy and difficulty in predicting the load distribution in the event of foundation settlement. Each support should extend at least 120o around and approximately √( 30D ) along the vessel [ BS 5500 ] in order to transmit the reaction gradually into the shell wall. One support is attached to the vessel to prevent axial movement, the other is not attached but merely supports the vessel's weight, thus permitting free longitudinal expansion of the vessel when thermal strains occur.
The safety of any artefact must be verified under all possible circumstances - not just in normal duty but also during manufacture, erection, test, aberrant service and so on. Under hydrostatic test for example, a pressure vessel is subjected to a superposition of loads - internal pressure plus bending due to the distributed weight w of shell and water charge. A simply supported beam model of the vessel indicates that the supports are optimally located at 0.207L from the ends, corresponding to bending moment magnitudes of 0.0214wL2 at both the centre and at the supports - but the model neglects possible distortion which may occur when concentrated loads are applied to the relatively thin shell of a pressure vessel. To lessen this possibility, supports should be situated within D/4 of the ends to take advantage of the stiffening afforded by the heads, although this location will lead to bending stresses larger than those arising from the optimum location.
Having examined the theory and various details of pressure vessels, we are now in a position to design simple internally pressurised vessels to AS1210.
Copyright 1999-2005 Douglas Wright,
doug@mech.uwa.edu.au