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Valves and pipelines - Material strength, installation & color coding
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.
Various and often varying pressures and temperatures pose problems.
Temperatures of about 450 deg C can cause recrystallization and creep in iron and
steels. Very low temperatures as with liquefied natural gas, can result in brittle
failure. Varying temperatures give problems with stress due to expansion and
contraction.
The term fittings covers valves, cocks, branch and bulkhead pieces, reducers,
strainers and filters, separators and expansion pieces, in short, everything in a
system which is not a pipe. Couplings and unions are used only in small bore
pipes.
Cast iron and gunmetal fittings are used freely in small sizes at moderate
pressures. Large fittings, those for high pressure and temperature and for oil
fuel under pressure, are cast or fusion welded (fabricated) mild steel or SG iron.
For temperatures above 460°C they are usually of 0.5% molybdenum steel.
The addition of 0.5% molybdenum, inhibits recrystallization and therefore the
resulting creep.
The safety and reliability of critical
individual fittings is ensured by a requirement that they are made to
specification by an approved manufacturer. Materials are tested, welds are
inspected, major fittings are tested and marked, systems are pressure tested by
or in the presence of a representative of the appropriate authority. Every effort
is made to ensure safety and reliability.
Replacement components for pipe
systems must be of the same standard and obtained if necessary, from an
approved maker. Some accidents have been the result of replacement valves
and other components being of inferior quality.
Pipe installation
Vibration is the frequent cause of eventual pipe failure but supports and clips to
prevent this problem must permit free expansion and contraction. A pipe
which has to be twisted or bowed when being connected, has inbuilt stress
which can lead to ultimate failure. Pipes should be accurately made (particularly
replacement sections) and installed with simple supports before being
permanently clipped.
If pumps are designed so that the driving motor or
turbine is mounted upon an extension to the pump casing proper, the tendency
for mal-alignment, due to pipeline stresses, is practically eliminated.
Nevertheless, it is essential that the pipe systems and heavy valve chests, are
separately supported and stayed during installation, the flanged connection to
the pumps being the last to be coupled after the faces are correctly aligned. This
can contribute materially to the life of the unit.
Horizontal pumps should be laid down on suitable chocks, accurately fitted
to ensure that the couplings, with their bolts removed, are in correct alignment
and with their faces parallel. This alignment should be checked after tightening
the holding down bolts and again after the pipes are coupled and preferably full
of liquid.
Colour coding
It is usual to identify pipes by a colour code for the individual system or by
bands of paint at intervals on pipes of a common colour. There are standard
codes but individuals or companies may prefer variations. Frequently pipes are
incorrectly coloured. Before working on or using a pipe system, it should be
traced and verified.
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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