Lesson 07 UG28_New2.ppt

8/20/2019 Lesson 07 UG28_New2.ppt

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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….

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Lesson 07 UG28_New2.ppt