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Valves & pipeline materials - Cause of corrosion and erosion
Galvanic corrosion is a major challenge for any pipes which carry sea water.
Rust is a particular corrosion problem for steel pipes exposed to contact with
sea water or moisture generally and air. Pipe runs along tank tops or on deck,
are examples of the latter. Steel pipes in these areas require external as well as
internal protection.
Sea water is an electrolyte and therefore a conductor of electricity, because
the molecules of its dissolved salts split into positive and negative ions which
are available as current carriers. Electrolytic action can result if there are
different metals or even differences in the same metal in a pipeline. Galvanic
corrosion can occur if the different metals are connected electrically and
mutually in contact with the sea water.
A corrosion cell formed between steel
and brass in contact with sea water results in wastage of the less noble steel. A
list is given in the galvanic series, in which the more noble metals are placed in
order after the less noble thus: zinc, aluminium, carbon steels, cast iron, lead—tin
alloys, lead, brass, copper, bronze, gunmetal, copper-nickel iron, monel metal.
A metal in contact with one occurring later in the series, as with steel and brass,
may corrode rapidly in sea water. Because the action is galvanic, less noble
sacrificial anodes can give protection.
Steel
Steel being subject not only to galvanic corrosion but also to rusting, appears
to be a poor material to select for sea water pipes or for installation in tank top
or deck areas. Mild steel pipes for sea water are protected by being galvanized
or rubber lined. Welding and pipe bending should be completed before
galvanizing or application of a lining, so that weld spatter and deposits from
manufacture can be removed. The mild steel, electric resistance welded (ERW)
or hot rolled pipes are galvanized by hot dipping. Inadequate protection of
steel, results if there are pinholes or discontinuities in protective linings.
Linings should always be carried over the flange faces.
Mild steel welded
fabrications, similarly lined, are also used for large ship side fittings.
Seamless mild steel is used for steam, high pressure air, feed discharges and
all oil fuel pressure piping. Its strength reduces however, at about 460°C and
above this figure, steels require small additions of alloying materials such as
molybdenum and chromium. Flanges are secured to steel pipes by fusion
welding or by screwing and expanding.
Cast iron
Cast iron has poor corrosion resistance in sea water, being especially
vulnerable to graphitization. This form of attack gradually removes the iron
from the surface in contact with sea water to leave soft, black graphite. The
weakness of ordinary grey cast iron in tension and under shock loading limits
its use to low pressure applications, and the brittle nature of ordinary grey cast
iron excludes its use for side shell fittings where failure could result in flooding
of the machinery space. Ease of casting makes the material ideal for the
production of fittings and fortunately techniques for improving strength have
been developed.
Spheroidal graphite cast iron (SG iron) and meehanite are
examples of high strength versions of the material. These are suitable for use in
ship side valves if made to specification by an approved manufacturer. SG iron
may be used for high pressure services and for steam below 461°C.
Cast iron with its high carbon content and consequent low melting
temperature is ideal for the production of fittings by casting.
Copper
Copper pipes are suitable for moderate pressures and temperatures. Flanges are
secured to copper and its alloys -by brazing or sweating.
Non-ferrous alloys
Basically, brass is an alloy of copper and zinc; bronze an alloy of copper and tin.
In both cases there may be additions of other metals and there is some
confusion of nomenclature; some high-tensile brasses are called 'bronze' and
the practice has prevailed for so long as to be accepted.
Aluminium brass and other non-ferrous pipelines, are considered very
resistant to corrosion in sea water, but concentrated galvanic corrosion can
occur if some part of the pipe system has a different make up.
A localized
corrosion cell can be set up when a fitting, such as a thermometer pocket, is of a
brass, bronze or other material which is different to the parent material. Pipe
systems are ideally of the same material throughout but non-ferrous alloys are
protected against corrosion by the deposition of iron ions so that use of iron or
steel fittings is beneficial Iron ion protection can alternatively be supplied from
sacrificial or driven iron anodes or by dosing with ferrous sulphate.
Dezincification of brasses is a particular type of corrosion that occurs in the
presence of sea water. The attack removes zinc from the alloy, leaving porous
copper which is soft. The problem is marked by a patch of copper colour in the
brass. Dezincification is inhibited in brasses which are intended for sea-water
contact by additions of a very small amount of arsenic (0.04%) or other elements.
Some brasses are prone to corrosion-stress cracking but this is a
phenomenon associated chiefly with brass tube which has been stressed by
expanding or by being worked in the unsoftened condition and which is also in
contact with corrosive fluids, such as sea water. Splitting can occur suddenly, or
even violently as a result of stress corrosion cracking.
Stainless steel
A different problem is presented by corrosive liquids and those that contain
hard particles and are therefore likely to cause erosion. These can cause
differing rates of wastage in conventional metal pipes or cargo tanks. With
some corrosive liquids wastage is slow enough, lasting over a period of years,
to permit the use of common metals. Expensive stainless steel is widely used
for the cargo pipes of chemical tankers intended for carriage of very corrosive
cargoes.
Erosion
Erosion of metal may be the result of abrasives or of high water speeds,
entrained air, turbulence and cavitation. The latter are often caused by
protuberances, tight bends or an abrupt change of pipe cross sectional area.
Erosion from turbulent flow and cavitation also aids corrosion (corrosion/
erosion) by removing the oxide film that assists in the protection of metal
surfaces.
The exposed metal surfaces can form galvanic corrosion cells with
adjacent areas where oxide film is still present. Erosion is reduced by limiting
speed of flow, avoiding sharp bends, changes of section and impediments to
flow such as incorrectly cut jointing or weld deposits. Speed of liquid flow
should be no greater than 1 m/s for copper; 3 m/s for galvanised steel and
aluminium brass; 3.5 m/s for 90/10 cupro-nickel: 4 m/s for 70/30 cupro-nickel.
Summarized below some of the basic procedure of machinery valves & pipeline systems :
- Valves & pipeline materials corrosion & erosion
Galvanic corrosion is a major challenge for any pipes which carry sea water. Rust is a particular corrosion problem for steel pipes exposed to contact with sea water or moisture generally and air. Pipe runs along tank tops or on deck, are examples of the latter. Steel pipes in these areas require external as well as internal protection.....
- valves-&-pipelines-strength-of-materials
The strength of materials used for pipes and fittings must be adequate for the
system pressures and possible over-pressures. Pipelines and valves, for
example, used to carry and control the flow of high temperature, high pressure
steam must obviously be made to very exacting specifications by approved
manufacturers.....
- Valves & pipelines-system cleaning & draining
It is often found, in new ships, that the bilges and bilge systems have not been
thoroughly cleaned with the result that wood, nuts, bolts, rags and other debris
are found inside valves and pipes after initial bilge pumping. These choke the
valve-chests and prevent the valves from being properly closed. They also
block strainers. ....
- Expansion arrangements
Provision must be made in pipe systems to accommodate changes in length due to change of temperature, and so prevent undue stress or distortion as pipes expand or contract.....
- Valves & cocks
Cocks and valves are designed to control or interrupt flow. This is done in cocks by rotating the plug, and in valves by lowering, raising or rotating a disc in relation to a seating surface or by controlling the movement of a ball. ...
- Butterfly valves
A butterfly valve consists basically of a disc pivoted across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted.....
- Gate valves
Unlike the globe valve, gate (or sluice) valves give full bore flow without change of direction. The valve disc known appropriately as a gate,....
- Globe valves
The globe valve has a bulbous body, housing a valve seat and screw down plug or disc arranged at right angles to the axis of the pipe....
- Relief valves
Excess pressure is eased by a relief valve . This consists of a disc held closed by a spring loaded stem. The compression on the spring can be adjusted so that the valve opens at the desired pressure. ....
- Valves traps
A steam trap is a special type of valve which prevents the passage of steam but allows condensate through. It works automatically and is used in steam heating lines to drain condensate without passing any steam. ....
- Flap valves & valve chest
Scupper pipes from accommodation spaces are fitted with non-return valves. Those scuppers from spaces below the bulkhead deck, are required to be fitted with non-return valves which can be positively closed from above the bulkhead deck or, if this is not practical, with two non-return valves.....
- Quick closingvalves
Fuel oil service and some other tanks must be fitted with valves that can be closed rapidly and remotely in the event of an emergency such as fire. Wire operated valves are commonly fitted,....
- Strainers & filters
The term strainer is sometimes used specifically for a simple device made up with a single layer of coarse gauze, a very coarse wire mesh or a drilled or perforated plate. ...
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