Section
11 Appendix - Guidance notes on metal pipes for water services
11.1 General
11.1.1 These
guidance notes, except where it is specifically stated, apply to sea-water
piping systems.
11.1.2 In
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.
11.2 Materials
11.2.1 Materials
used in sea-water piping systems include:
- Galvanised steel.
- Austenitic and duplex stainless steel, see also
Pt 5, Ch 12, 11.3 Steel pipes 11.3.4.
- Steel pipes lined with rubber, plastics or stoved coatings.
- Copper.
- 90/10 copper-nickel-iron.
- 70/30 copper-nickel.
- Aluminium brass.
- Bronze.
11.2.2 Selection
of materials should be based on:
- 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, dealuminification of aluminium brass and graphitisation
of cast iron;
- the ability to resist stress corrosion and corrosion fatigue;
and
- the amenability to fabrication by normal practices.
11.3 Steel pipes
11.3.1 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.
11.3.2 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.
11.3.3 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.
11.3.4 Austenitic and duplex stainless steel pipes are not recommended for
salt-water services in polluted waters or where stagnant conditions exist as they are
prone to pitting. Austenitic stainless steel pipes grades 316L or 317L and duplex
stainless steels grades S31803 or S32750 may give satisfactory service in water
circulating systems for clean flowing seawater.
11.3.5 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.
11.3.6 The
foregoing comments on rubber lined pipes also apply to pipes lined
with plastics.
11.3.7 Stove
coating of pipes as protection against corrosion should only be used
where the pipes will be efficiently protected against mechanical damage.
11.4 Copper and copper alloy pipes
11.4.1 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.
11.4.2 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.
11.4.3 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.
11.5 Flanges
11.5.1 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, see also
Pt 5, Ch 12, 2.3 Pipe joints - General.
11.5.2 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.
11.5.3 Inert
gas shielded arc welding is the preferred process but metal arc welding
may be used on copper-nickel-iron alloy pipes.
11.5.4 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.
11.5.5 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.
11.5.6 The
use of a copper-zinc brazing alloy is not permitted.
11.6 Water velocity
11.6.1 Water
velocities should be carefully assessed at the design stage and the
materials of pipes, valves, etc. selected to suit the conditions.
11.6.2 The
water velocity in copper pipes should not exceed 1 m/s.
11.6.3 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:
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3,0
m/s
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3,0
m/s
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3,5
m/s
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5,0
m/s
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11.7 Fabrication and installation
11.7.1 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.
11.7.2 Pipe
bores should be smooth and clean.
11.7.3 Jointing
should be flush with the bore surfaces of pipes and misalignment of
adjacent flange faces should be reduced to a minimum.
11.7.4 Pipe
bends should be of as large a radius as possible, 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.
11.7.5 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.
11.7.6 Systems
should not be left idle for long periods, especially where the water
is polluted.
11.7.7 Strainers
should be provided at the inlet to sea-water systems.
11.8 Metal pipes for fresh water services
11.8.1 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.
11.8.2 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.
11.8.3 Copper
alloy pipes should be treated to remove any carbonaceous films or
deposits before the tubes are put into service.
11.8.4 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.
11.8.5 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.
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