Section
3 Assessment
3.1 Design and construction
3.1.1 All
piping systems are to be designed and constructed for their intended
service and working conditions.
3.1.2 Materials sensitive to heat, such as aluminium and plastics, are not to be
used in Mobility systems or Ship Type systems necessary for the safe operation of the
ship, or for containing flammable liquids or sea water where leakage or failure could
result in fire or in flooding of a watertight compartment.
3.1.3 The strength and construction of pipes, components and fittings is to
preclude loss of Mobility and/or Ship Type systems, escape of flammable liquid and
flooding.
3.1.4 The
selection of pipe connections in piping systems is to recognise the
boundary fluids, pressure and temperature conditions and location.
3.1.5 Pipe
connections in accordance with national or other approved standards
will be accepted where the standards are appropriate to the piping
system.
3.1.6 Pipe
thicknesses greater than the minimum required by this Chapter may
be necessary where the likelihood of erosion cannot be avoided and/or
where there is a likelihood of corrosion exceeding the nominal allowances
specified in the Rule requirements. Pipe thicknesses may also need
to be increased where the Owner has specified requirements for system
life, see
Vol 2, Pt 7, Ch 1, 2.1 Documentation required for design review 2.1.2.
3.2 Design symbols
3.2.1 The
symbols used in this Chapter are defined as follows:
|
a
|
= |
percentage
negative manufacturing tolerance on thickness |
|
c
|
= |
corrosion
allowance, in mm |
|
p
t
|
= |
hydraulic test pressure, in bar |
|
t
|
= |
the
minimum thickness of a straight pipe, in mm, including corrosion allowance
and negative tolerance, where applicable |
|
t
b
|
= |
the minimum thickness of a straight pipe to be used for a pipe
bend, in mm, including bending allowance, corrosion allowance and
negative tolerance, where applicable |
|
R
|
= |
radius
of curvature of a pipe bend at the centre line of the pipe, in mm |
|
σ |
= |
maximum
permissible design stress, in N/mm2.
|
3.2.2 The
outside diameter, D, is subject to manufacturing tolerances,
but these are not to be used in the evaluation of formulae.
3.2.3 The
inside diameter, d, is not to be confused with nominal
size, which is an accepted designation associated with outside diameters
of standard rolling sizes.
3.2.4 The
weld efficiency factor, e, is to be taken as 1 for seamless
and electric resistance and induction welded steel pipes. Where other
methods of pipe manufacture are proposed, the value of e will
be specially considered.
3.3 Design pressure
3.3.1 The
design pressure, p, is the maximum permissible working
pressure and is to be not less than the highest set pressure of the
safety valve or relief valve.
3.3.2 In water
tube boiler installations, the design pressure for steam piping between
the boiler and integral superheater outlet is to be taken as the design
pressure of the boiler, i.e. not less than the highest set pressure
of any safety valve on the boiler drum. For piping leading from the
superheater outlet, the design pressure is to be taken as the highest
set pressure of the superheater safety valves.
3.3.3 The
design pressure of feed piping and other piping on the discharge from
pumps is to be taken as the pump pressure at full rated speed against
a shut valve. Where a safety valve or other protective device is fitted
to restrict the pressure to a lower value than the shut valve load,
the design pressure is to be the highest set pressure of the device.
3.3.4 For
pipes containing heated oil under pressure (temperature 60°C and
above and pressure 0,18 bar and above), the design pressure is to
be taken as not less than 14 bar.
3.4 Design temperature
3.4.1 The
design temperature is to be taken as the maximum temperature of the
internal fluid, but in no case is to be less than 50°C.
3.4.2 In the
case of pipes for superheated steam, the temperature is to be taken
as the designed operating steam temperature for the pipeline, provided
that the temperature at the superheater outlet is closely controlled.
Where temperature fluctuations exceeding 15°C above the designed
temperature are to be expected in normal service, the steam temperature
to be used for determining the allowable stress is to be increased
by the amount of this excess.
3.5 Use of alternative design codes
3.5.2 Where
alternative design codes are used, they are to be stated together
with any assumptions made.
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