4.4.6.1 Scantlings based
on internal pressure should be calculated as follows:
-
.1 The thickness and form of pressure-containing
parts of pressure vessels under internal pressure, including flanges should be
determined according to a standard acceptable to the Administration. These
calculations in all cases should be based on generally accepted pressure vessel
design theory. Openings in pressure-containing parts of pressure vessels should be
reinforced in accordance with a standard acceptable to the Administration.
-
.2 The design liquid pressure defined in 4.3.2
should be taken into account in the above calculations.
-
.3 The welded joint efficiency factor to be used in
the calculation according to 4.4.6.1.1 should be 0.95 when the inspection
and the non-destructive testing referred to in 4.10.9 are carried out. This figure may be increased up to
1.0 when account is taken of other considerations, such as the material used, type
of joints, welding procedure and type of loading. For process pressure vessels the
Administration may accept partial non-destructive examinations, but not less than
those of 4.10.9.2.2 depending on such factors as the material used,
the design temperature, the nil ductility transition temperature of the material
as fabricated, the type of joint and welding procedure, but in this case an
efficiency factor of not more than 0.85 should be adopted. For special materials,
the above-mentioned factors should be reduced depending on the specified
mechanical properties of the welded joint.
4.4.6.2 Buckling criteria
should be as follows:
-
.1 The thickness and form of pressure vessels subject
to external pressure and other loads causing compressive stresses should be to a
standard acceptable to the Administration. These calculations in all cases should
be based on generally accepted pressure vessel buckling theory and should
adequately account for the difference in theoretical and actual buckling stress as
a result of plate edge misalignment, ovality and deviation from true circular form
over a specified arc or chord length.
-
.2 The design external pressure P
eused for verifying the buckling of the pressure vessels should not be
less than that given by:
- where:
|
P
1
|
= |
setting value of vacuum relief valves. For vessels not fitted
with vacuum relief valves P
1 should be specially considered, but should not in general be
taken as less than 0.25 bar. |
|
P
2
|
= |
the set pressure of the pressure relief valves for completely
closed spaces containing pressure vessels or parts of pressure vessels;
elsewhere P
2= 0. |
|
P
3
|
= |
compressive actions in the shell due to the weight and
contraction of insulation, weight of shell, including corrosion allowance,
and other miscellaneous external pressure loads to which the pressure vessel
may be subjected. These include, but are not limited to, weight of domes,
weight of towers and piping, effect of product in the partially filled
condition, accelerations and hull deflection. In addition the local effect
of external or internal pressure or both should be taken into account. |
|
P
4
|
= |
external pressure due to head of water for pressure vessels or
part of pressure vessels on exposed decks; elsewhere P
4 = 0. |
4.4.6.3 Stress analysis in
respect of static and dynamic loads should be performed as follows:
-
.1 Pressure vessel scantlings should be determined in
accordance with 4.4.6.1 and .2.
-
.2 Calculations of the loads and stresses in way of
the supports and the shell attachment of the support should be made. Loads
referred to in 4.3 should be used, as applicable. Stresses in way of the
supports should be to a standard acceptable to the Administration. In special
cases a fatigue analysis may be required by the Administration.
-
.3 If required by the Administration, secondary
stresses and thermal stresses should be specially considered.
4.4.6.4 For pressure vessels,
the thickness calculated according to 4.4.6.1 or the thickness required by 4.4.6.2 plus
the corrosion allowance, if any, should be considered as a minimum
without any negative tolerance.
4.4.6.5 For pressure vessels,
the minimum thickness of shell and heads including corrosion allowance,
after forming, should not be less than 5 mm for carbon-manganese steels
and nickel steels, 3 mm for austenitic steels or 7 mm for aluminium
alloys.