Section 4 Carbon steels

4.1.1 The minimum thickness of steel pressure pipes is to be in accordance with an acceptable pressure piping code or standard except that in no case is it to be less than shown in Table 4.4.1 Minimum thickness for steel pipes.

4.1.2 For pipes passing through tanks, where the thickness has been determined in accordance with Pt 4, Ch 4, 4.1 General 4.1.1, an additional corrosion allowance is to be added to take account of external corrosion; the addition will depend on the external medium and the value is to be in accordance with Table 4.4.2 Values of corrosion allowance for steel pipes.

4.1.3 Where the pipes are efficiently protected against corrosion, the corrosion allowance may be reduced by not more than 50 per cent.

4.1.4 Steel stub pipes between the shell plating and the sea valve are to be of short rigid construction, adequately supported and of substantial thickness.

4.2.1 Joints in steel pipelines may be made by:

1. Screwed on, or welded on, bolted flanges.

2. Butt welds between pipes or between pipes and valve chests.

3. Socket welded joints.

4. Threaded sleeve joints (parallel thread) (see also Pt 4, Ch 4, 4.5 Threaded sleeve joints (parallel thread)).

5. Special types of approved joints that have been shown to be suitable for the design conditions (see also Pt 4, Ch 4, 4.4 Screwed fittings).

4.2.2 Where pipes are joined by welding, a suitable number of flanged joints are to be provided at suitable positions to facilitate installation and removal for maintenance.

4.2.3 For welded pipes protected against corrosion, the corrosion protection is to be applied after welding or made good in way of the weld damaged area.

4.2.4 Where it is not possible to make good the corrosion protection of the weld damaged area, the pipe is to be considered to have no corrosion protection.

4.2.5 Where backing rings are used for welding pipes, the effect of the flow obstruction of the backing ring and erosion/crevice corrosion of the backing ring is to be taken into account.

4.3.1 The dimensions and material of flanges and bolting, and the pressure-temperature rating of bolted flanges in pressure pipelines is to be in accordance with a recognized National or other established Standard.

4.3.2 The types of welded-on flanges are to be suitable for the pressure, temperature and service for which the pipes are intended.

4.3.3 Welded-on flanges are not to be a tight fit on the pipes. The maximum clearance between the bore of the flange and the outside diameter of the pipe is to be 3 mm at any point, and the sum of diametrically opposite clearances is not to exceed 5 mm.

4.3.4 Where butt welds are employed in the attachment of necked flanges in pipe-to-pipe joints or in the construction of branch pieces, the adjacent pieces are to be matched at the bores. This may be effected by drifting, roller expanding or machining, provided that the pipe wall is not reduced below the design thickness. If the parts to be joined differ in wall thickness, the thicker wall is to be gradually tapered to the thickness of the thinner at the butt joint. The welding necks of valve chests are to be sufficiently long to ensure that the valves are not distorted as the result of welding and subsequent heat treatment of the joints.

4.3.5 Where backing rings are used with a necked flange they are to fit closely to the bore of the pipe and are to be removed after welding. The rings are to be made of the same material as the pipes or of mild steel having a sulphur content not greater than 0,05 per cent.

4.3.6 Branches may be attached to pressure pipes by means of welding, provided that the pipe is reinforced at the branch by a compensating plate or collar or other approved means, or alternatively that the thickness of pipe and branch is increased to maintain the strength of the pipe. These requirements also apply to fabricated branch pieces.

4.4.1 Screwed fittings, including compression fittings, may be used in piping systems not exceeding 41 mm outside diameter. Where the fittings are not in accordance with an acceptable standard then LR may require the fittings to be subjected to special tests to demonstrate their suitability.

4.5.1 Threaded sleeve joints in accordance with a recognized National or other established standard may be used in Class III piping systems. They are not to be used in piping systems conveying flammable liquids.

4.6.1 Socket weld joints may be used in Class I and Class II piping systems with carbon steel pipes up to 50 mm nominal diameter, and in Class III systems for any pipe diameter. Socket weld fittings are to be of forged steel and the material is to be compatible with the associated piping. Such joints are not to be used where fatigue, severe erosion or crevice corrosion is expected to occur.

4.6.2 The thickness of the socket weld fittings is to meet the requirements of Pt 4, Ch 4, 4.1 General 4.1.1 but is to be not less than 1,25 times the nominal thickness of the pipe or tube. The diametrical clearance between the outside diameter of the pipe and the bore of the fitting is not to exceed 0,8 mm, and a gap of approximately 1,5 mm is to be provided between the end of the pipe and the bottom of the socket.

4.6.3 The leg lengths of the fillet weld connecting the pipe to the socket weld fitting are to be such that the throat dimensions of the weld are not less than the nominal thickness of the pipe or tube.

4.7.1 Welded sleeve joints may be used in Class III systems only, subject to the restrictions and general dimensional requirements stated in Pt 4, Ch 4, 4.6 Socket weld joints for socket weld joints.

4.7.2 The pipe ends are to be located in the centre of the sleeve with a 1,5 to 2,0 mm gap.