lesson 07 ug28_new2.ppt
TRANSCRIPT
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API Preparatory ClassAPI Preparatory Class
Lesson 7Lesson 7
External Pressure CalculationsExternal Pressure Calculations
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(a) Rules for the design of shells and tubes under external
pressure given in this Division are limited to cylindrical
shells, with or without stiffening rings, tubes, and
spherical shells…..
(b) The symbols defined below are used in the procedures of
this paragraph
A = factor determined from !ig. " in #ubpart $ of #ection
%%, &art D and used to enter the applicable material chart
in #ubpart $ of #ection %%, &art D……
B ' factor determined from the applicable material chart
in #ubpart $ of #ection %%, &art D for maximum design
metal temperature, psi .
" *+ Thicness of #hells and Tubesnder -xternal &ressure
Page 24 Section VIII
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Do ' outside diameter of cylindrical shell course or tube,
in.
E ' ot on exam.
L ' total length, in. (mm), of a tube between tubesheets,
or design length of a vessel section between lines of
support (see !ig. "/*+.0). 1 line of support is
(0) a circumferential line on a head…. ot on exam2
(*) a stiffening ring……. ot on exam2($) a 3acet closure ……. ot on exam2
(4) a cone/to/cylinder ot on exam2
P ' external design pressure, psi
" *+
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P a ' calculated value of maximum allowable external
woring pressure for the assumed value of t, psi
R o ' outside radius of spherical shell, in.t ' minimum re5uired thicness of cylindrical shell or
tube, or spherical shell, in.
t s ' nominal thicness of cylindrical shell or tube, in.
6eginning with "/*+(c) there are step by step
instructions for woring these problems. 7e will go
through these steps one at a time.
" *+
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(c) Cylindrical Shells and Tubes. The re5uired minimumthicness of a cylindrical shell or tube under external
pressure, either seamless or with longitudinal butt 3oints,
shall be…….
0. Cylinders having Do /t values > or = !"
#tep 0 1ssume a value for t and determine the ratios
L/Do and Do /t.
8 9ou do not assume a value for thicness (t) on the exam, it
will be given in the stated problem for the external pressureshell or tube calculation. 1s will the (Do) Diameter :utside
and the (;) ;ength in other words all that is needed to solve
the problem will be provided.
;ooing at an example, we can start learning this process.
" *+
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0. Cylinders having Do /t values > or = !"#tep 0 1ssume a value for t and determine the ratios
L/Do and Do /t.
-xample The cylinder has corroded to a wall thicness of
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#tep * -nter !ig. " in #ubpart $ of #ection %%, &art at thevalue of L/Do determined in #tep 0. !or values of L/Do
greater than =
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#tep $ ove horiEontally to the line for the value Do /tdetermined in #tep 0.... 7hich in our case was 0B, but
we will round this to *< since these problems are not
meant to be extremely precise. #o now we have.
" *+
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#tep $. @ontinued.
!rom this point of intersection move vertically downward to
determine the value of factor A.
7hich gives us the following?
" *+
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#tep 4
sing the value of A calculated in #tep $, enter the
applicable material chart in #ubpart $ of #ection %%, &art D
for the material under consideration. ove vertically to an
intersection with the materialAtemperature line for the design
temperature see "/*
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!actor 1 ' .
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!actor 6 is 0*,
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1ll that wor 3ust to find !actor 6 is and plug it into thesimple formula below.
" *+
)
t
D3(
4B =Pa
o
1s regards the final answers to these problems, because of
the difficulty of being precise with the !ig. " there will always
be some difference from one person to the next in the
determination of !actor 1. This is allowed for on the exam by
listing choices of answers that are in a range of FA/ =G. %n our
previous problem the answer was +
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To #ummariEe "/*+
-xternal calculations depart significantly from internal
calculations simply because under external pressure the
vessel is being crushed. %nternal pressure wants to tear the
vessel apart.
6ecause of the crushing or bucling load, the Length the
#utside Dia$eter and the Thic%ness of the vessel are
important. -xternal pressure problems are based on the
thicness of the shell to the outside diameter ratios. There
are two types of external pressure calculations, the type we
will use is when the :.D to (Do) thicness ratio (t) is greaterthan 10 and the other type, not on the test, is when it is less
than 10 .
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To #ummariEe "/*+
%n order to solve these types of problems two charts will bere5uired. The first chart !ig. " is used to find a value called
&actor A and then &actor A is used to find a &actor B in the
second material specific chart. The value of &actor B found
is the number needed to solve the problem using theformula given in paragraph "/*+ (c)(0) step I. 1s stated in
the 1&% =0< 6ody of Jnowledge, these charts will be
provided in the exam body, %! an external calculation is
given on the examination.
:ne more problem. !ind the allowed external pressure on
an existing vessel of a nown thicness with a DoAt ratio K
0
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"/*+
&roblem 1 vessel is operating under an external pressure,
the operating temperature is =
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" *+
!rom "/*+ (c)Cylindrical Shells and Tubes. There5uired minimum thicness of a shell or a tube under
external pressure, either seamless or with longitudinal
butt 3oints, shall be determined by the following
procedure.
(0) @ylinders having a
Testing to see if this paragraph applies 10values
t
Do
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" *+
Step & :ur value of Do is 4< inches and ; is H< inches. 7e
will use these to determine the ratio of
32= 1.25
40=
t
Do
1.75=40
70 =
o D
L
Step 2 -nter the &actor A chart at the value of 0.H=determined above.
Step ' Then move across horiEontally to the curve DoAt '
$*. Then down from this point to find the value of &actor A
which is '!!()
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" *+
Step 4. sing our value of Factor A calculated in #tep $,
enter the Factor ( (@#/*) chart on the bottom. Then
vertically to the material temperature line given in the stated
problem (in our case =
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" *+
#tep = Then across to find the value of &actor B. 7e find
that &actor B is approximately *!!! .
#tep I sing this value of &actor B, calculate the value of
the maximum allowable external pressure P a using the
following formula
)t
D3(
4B =Pa
o
psi541.66=96
52,000 =3(32)
4x13,000=P a
Anser !ange" 5&4 $ 5#% psi
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Class )ui*
+,-2%
1 vessel under external pressure has been found to a
thicness of 0.0*$ O. The vessel is +P/*O long and operates at
a temperature of $
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(0) @ylinders having
" *+
10valuest
Do
Testing to see if this paragraph applies
48.08= 1.123
54
=t
Do
#tep 0 :ur value of Do is =4 inches and ; is B+ inches.
7e will use these to determine the ratio of
1.81=54
98 =
o D
L
#tep * -nter the &actor A chart at the value of 0.+
determined above.
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" *+
Step '. Then move across horiEontally to the curve at
approxiately DoAt ' 4+. Then down from this point to
find the value of Factor A which is approxiately .0022
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#tep 4 sing our value of &actor A calculated in #tep $,enter the &actor B (@#/*) chart on the bottom, then
vertically up to the material temperature line given in the
stated problem (in our case $
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#tep I sing this value of &actor B, calculate the value ofthe maximum allowable external pressure Pa using the
following formula
" *+
)tD3(
4B =Pa
o
psi416.66=144
60,000 =
3(48)
4x15,000=P a
Anser !ange" '/5 - 4'7 psi
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;ater….