Section 17 Guidance notes on metal pipes for water service

17.1.1 These guidance notes, except where it is specifically stated, apply to sea-water piping systems.

17.1.2 1In addition to the selection of suitable materials, careful attention should be given to the design details of the piping system and the workmanship in fabrication, construction and installation of the pipework in order to obtain maximum life in service.

17.2.1 Materials used in sea-water piping systems include:

• Galvanised steel.
• Steel pipes lined with rubber, plastics or stoved coatings.
• Copper.
• 90/10 copper-nickel-iron.
• 70/30 copper-nickel.
• Aluminium brass.

17.2.2 Selection of materials should be based on all of the following:

• The ability to resist general and localised corrosion, such as pitting, impingement attack and cavitation throughout all the flow velocities likely to be encountered.
• Compatibility with the other materials in the system, such as valve bodies and casings, e.g. in order to minimise bimetallic corrosion.
• The ability to resist selective corrosion, e.g. dezincification of brass, dealminification of aluminium brass and graphitisation of cast iron, the ability to resist stress corrosion and corrosion fatigue.
• The amenability to fabrication by normal practices.

17.2.3 Consideration is to be given to avoiding the formation of a galvanic couple where an electrical component in the piping system is connected to the ship’s earth.

17.3.1 Steel pipes should be protected against corrosion, and protective coatings should be applied on completion of all fabrication, i.e. bending, forming and welding of the steel pipes.

17.3.2 Welds should be free from lack of fusion and crevices. The surfaces should be dressed to remove slag and spatter and this should be done before coating. The coating should be continuous around the ends of the pipes and on the faces of flanges.

17.3.3 Galvanising the bores and flanges of steel pipes as protection against corrosion is common practice, and is recommended as the minimum protection for pipes in sea-water systems, including those for bilge and ballast service.

17.3.4 Austenitic stainless steel pipes are not recommended for salt-water services as they are prone to pitting, particularly in polluted waters.

17.3.5 Rubber lined pipes are effective against corrosion and suitable for higher water velocities. The rubber lining should be free from defects, e.g. discontinuities, pinholes, etc. and it is essential that the bonding of the rubber to the bore of the pipe and flange face is sound. Rubber linings should be applied by firms specialising in this form of protection.

17.3.6 The foregoing comments on rubber lined pipes also apply to pipes lined with plastics.

17.3.7 Stove coating of pipes as protection against corrosion should only be used where the pipes will be efficiently protected against mechanical damage.

17.4.1 Copper pipes are particularly susceptible to perforation by corrosion/erosion and should only be used for low water velocities and where there is no excessive local turbulence.

17.4.2 Aluminium brass and copper-nickel-iron alloy pipes give good service in reasonably clean sea-water. For service with polluted river or harbour waters, copper-nickel-iron alloy pipes with at least 10 per cent nickel are preferable. Alpha-brasses, i.e. those containing 70 per cent or more copper, must be inhibited effectively against dezincification by suitable additions to the composition. Alpha beta-brasses, i.e. those containing less than 70 per cent copper, should not be used for pipes and fittings.

17.4.3 New copper alloy pipes should not be exposed initially to polluted water. Clean sea-water should be used at first to allow the metals to develop protective films. If this is not available the system should be filled with inhibited town mains water.

17.5.1 Where pipes are exposed to sea-water on both external and internal surfaces, flanges should be made, preferably, of the same material. Where sea-water is confined to the bores of pipes, flanges may be of the same material or of less noble metal than that of the pipe.

17.5.2 Fixed or loose type flanges may be used. The fixed flanges should be attached to the pipes by fillet welds or by capillary silver brazing. Where welding is used, the fillet weld at the back should be a strength weld and that in the face, a seal weld.

17.5.3 Inert gas shielded arc welding is the preferred process but metal arc welding may be used on copper-nickel-iron alloy pipes.

17.5.4 Mild steel flanges may be attached by argon arc welding to copper-nickel-iron pipes and give satisfactory service, provided that no part of the steel is exposed to the sea-water.

17.5.5 Where silver brazing is used, strength should be obtained by means of the bond in a capillary space over the whole area of the mating surfaces. A fillet braze at the back of the flange or at the face is undesirable. The alloy used for silver brazing should contain not less than 49 per cent silver.

17.5.6 The use of a copper-zinc brazing alloy is not permitted.

17.6.1 Water velocities should be carefully assessed at the design stage and the materials of pipes, valves, etc. selected to suit the conditions.

17.6.2 The water velocity in copper pipes should not exceed 1 m/s.

17.6.3 The water velocity in the pipes of the materials below should normally be not less than about 1 m/s in order to avoid fouling and subsequent pitting, but should not be greater than the following:

• Galvanised steel 3,0 m/s
• Aluminium brass 3,0 m/s
• 90/10 copper-nickel-iron 3,5 m/s
• 70/30 copper-nickel 5,0 m/s

17.7.1 Attention should be given to ensuring streamlined flow and reducing entrained air in the system to a minimum. Abrupt changes in the direction of flow, protrusions into the bores of pipes and other restrictions of flow should be avoided. Branches in continuous flow lines should be set at a shallow angle to the main pipe, and the junction should be smooth. The following points should be observed:

1. Short stiff bends are to be avoided, see Vol 2, Pt 7, Ch 1, 17.7 Fabrication and installation 17.7.4.

2. Pipe runs downstream of turbulence raising components such as reducing valves and orifices are to be straight and as long as practicable.

3. Changes in pipe bore dimensions are to have a shallow taper transition.

17.7.2 Pipe bores should be smooth and clean.

17.7.3 Jointing should be flush with the bore surfaces of pipes and misalignment of adjacent flange faces should be reduced to a minimum.

17.7.4 Pipe bends should be of as large a radius as possible (in general, the radius of curvature at centreline is to be not less than three times the pipe outside diameter) and the bore surfaces should be smooth and free from puckering at these positions. Any carbonaceous films or deposits formed on the bore surfaces during the bending processes should be carefully removed. Organic substances are not recommended for the filling of pipes for bending purposes.

17.7.5 The position of supports should be given special consideration in order to minimise vibration and ensure that excessive bending moments are not imposed on the pipes.

17.7.6 Systems should not be left idle for long periods, especially where the water is polluted.

17.7.7 Strainers should be provided at the inlet to seawater systems.

17.7.8 Where pipes and associated fittings are required to be thermally insulated after installation on board, the piping should be arranged to permit efficient application of insulating materials.

17.7.9 Non-ferrous piping should not be arranged within bilge wells or spaces. Where this is not possible, the piping should be suitably treated to avoid galvanic action in the bilge spaces.

17.8.1 Mild steel or copper pipes are normally satisfactory for service in fresh water applications. Hot fresh water, however, may promote corrosion in mild steel pipes unless the hardness and pH of the water are controlled.

17.8.2 Water with a slight salt content should not be left stagnant for long periods in mild steel pipes. Low salinity and the limited supply of oxygen in such conditions promote the formation of black iron oxide, and this may give rise to severe pitting. Where stagnant conditions are unavoidable, steel pipes should be galvanised, or pipes of suitable non-ferrous material used.

17.8.3 Copper alloy pipes should be treated to remove any carbonaceous films or deposits before the tubes are put into service.

17.8.4 Brass fittings and flanges in contact with water should be made of an alpha-brass effectively inhibited against dezincification by suitable additions to the composition.

17.8.5 Aluminium brass has been widely used as material for heat exchanger and condenser tubes, but its use in ‘once through’ systems is not recommended since, under certain conditions, it is prone to pitting and cracking.

17.8.6 Piping systems for potable water are to be designed to avoid dead ends and other configurations that would lead to stagnant conditions.