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Discussion
and References
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Information about Wall
thickness calculation of straight pipes under external pressure:
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ASME Section VIII Div 1
paragraphs UG-28, UG-29 and UG-30
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Tables and Standards
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ASME Section VIII Div 1 paragraph UG-28, Thickness of Shells and
Tubes under External Pressure
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ASME Section VIII Div 1 paragraph UG-29, Stiffening Rings for
Cylindrical Shells Under External Pressure
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ASME Section VIII Div 1 paragraph UG-30, Attachment of
Stiffening Rings
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ASME Sec. II
D Fig. G, Geometric Chart for Components Under External or Compressive
Loadings (All Materials)
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ASME Sec. II D Fig. CS, Chart for Determining Shell Thickness of
Components Under External Pressure Developed for Carbon or Low Alloy Steel
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ASME Sec. II D Fig. HA, Chart for Determining Shell Thickness of
Components Under External Pressure Developed for Austenitic Steel
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ASME Sec. II D Fig. HT, Chart for Determining Shell Thickness of
Components Under External Pressure Developed for Quenched and Tempered Low
Alloy Steel
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ASME Sec. II
D Table TM-1 ( Modulus of Elasticity,
E )
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ASME
B36.10M-Welded and Seamless Wrought Steel Pipe
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ASME
B36.19M-Stainless Steel Pipe
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Cylinders having Do/t values ≥ 10
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Step 1.
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Assume a value for t and determine the ratios L/Do and Do/t.
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Step 2.
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Enter Figure G in Subpart 3 of Section
II, Part D at the value of L/Do determined in Step 1. For values of L/Do greater than 50, enter the chart at a value of L/Do = 50. For values of L/Do less than 0.05, enter the
chart at a value of L/Do
= 0.05.
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Step 3.
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Move horizontally to
the line for the value of Do /t
determined in Step 1.
Interpolation may be made for intermediate values of Do/t; extrapolation is not
permitted. From this point of intersection move vertically downward to
determine the value of factor A.
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Step 4.
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Using the value of A calculated in Step 3, enter the applicable
material chart in Subpart 3 of Section II, Part D for the material under
consideration. (See figures CS for Carbon and Low
Alloy Steel and HT for Ferritic Steels
with Properties Enhanced by Heat Treatment). Move
vertically to an intersection with the material/temperature line for the
design temperature (see UG-20).
Interpolation may be made between lines for intermediate temperatures. If
tabular values in Subpart 3 of Section II, Part D are used, linear
interpolation or any other rational interpolation method may be used to
determine a B value
that lies between two adjacent tabular values for a specific temperature.
Such interpolation may also be used to determine a B value at an intermediate temperature that lies between two
sets of tabular values, after first determining B values for each set of tabular values.
In cases where the value of A falls to the right of the end of the material/temperature
line, assume an intersection with the horizontal projection of the upper end
of the material/temperature line. If tabular values are used, the last
(maximum) tabulated value shall be used.
For values of A
falling to the left of the material/temperature line, see Step 7.
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Step 5.
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From the intersection
obtained in Step 4, move
horizontally to the right and read the value of
factor B.
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Step 6.
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Using this value of B, calculate the value of the
maximum allowable external working pressure Pa using the following equation:
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Pa =
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4B
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3(Do/t)
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Step 7.
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For
values of A falling to the left of the applicable material/temperature line,
the value of Pa can
be calculated using
the following equation:
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Pa =
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2AE
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3(Do/t)
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If tabular values are
used, determine B as in Step 4 and apply it to the
equation in Step 6.
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Step 8.
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Compare
the calculated value of Pa obtained in Step 6 or Step 7 with P.
If Pa is smaller than
P, select a larger value for t and
repeat the design procedure until a value of Pa is obtained that is equal to or greater than P.
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The parameters in the
wall thickness calculation of straight pipes under external pressure
equations above are:
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A =
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factor determined
from Figure G in Subpart 3 of Section II, Part D and used to enter the
applicable material chart in Subpart 3 of Section II, Part D. For the case of
cylinders having Do/t values less than 10, see (c)(2).
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B =
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factor determined
from the applicable material chart or table in Subpart 3 of Section II, Part
D (Figures CS,
HA, HT) for maximum design metal temperature [see
UG-20(c)]
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Do =
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outside diameter of
cylindrical shell course or tube.
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E =
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modulus of elasticity
of material at design temperature. For external pressure design in accordance
with this Section, the modulus of elasticity to be used shall be taken from
the applicable materials chart in
Subpart 3 of Section II, Part D. (Interpolation may be made made between
lines for intermediate temperatures.)
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L =
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the running
centerline length between any two sections stiffened in accordance with
UG-29. in. (mm).
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P =
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external design
pressure [see Note in UG-28 (f)].
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Pa =
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calculated value of
maximum allowable external working pressure for the assumed value of t, [see Note in UG-28(f)].
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t =
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minimum required
thickness of cylindrical shell or tube, or spherical shell, in. (mm)
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