65174112-piping-flexiblility-calculation.pdf

8/20/2019 65174112-Piping-Flexiblility-Calculation.pdf

1/8

Quick heck

o

Piping Flexibility

L. C. Peng

PE

Peng Engineer ing Houston Texas

BSTR CT

One

major

requirement

in p ip ing des ign

i s

to

provide adequate

f l e x

i b i l i t y fo r absorb ing the thermal

expansion

of the pipe . However

due to

l ack

of quick method of

checking

pip ings a re of ten l a id -ou t

to

be e i the r

too stiff or too

f l ex ib l e .

In

e i t h e r case

va luab le

t ime and

mate r ia l

are wasted.

This

paper

presents

some of the

quick

methods for

checking piping

f l e x i b i l i t y . These methods include

vi sua l hand ca lcu la t ion and micro computer approaches. They are

a l l quick and easy for des igners

to

use in planning t h e i r l ayout s .

Once

the des igners

have

taken

care

of

the

f l e x i b i l i t y

problem

the

i t e r a t i v e

procedure

between

the

s t r e s s

engineers

and

the

des igners

become s imple r .

The

pro j e c t

schedule can a l s o be improved.

PIPING FLEXIBILITY

As the p ipe t empera ture

changes

from

the

i n s t a l l a t i o n cond i t ion

to

the

ope ra t ing

condi t ion t

expands

o r

c on t r a c t s .

In

the genera l

term both expansion and

c on t r a c t i on

a re

c a l l e d

thermal

expansion.

When a

pipe expands it

has

the p o t e n t i a l of gene ra t ing

enormous

force

and

s t r e s s

in the sys tem. However i f the p ip ing i s f l e x ib l e

enough

the expansion

can

be absorbed without

c re a t ing

undue force

or

s t r e s s .

Provid ing

the

proper f l e x i b i l i t y

i s

one of the major

t asks

in

the

des ign of

pip ing

sys tem.

Piping

i s

used

to

convey

a

c e r t a i n

amonut

of

f l u i d

from

one

poin t to ano the r .

It

i s

obvious

t h a t the

s ho r t e r

the

pipe

i s used

the

l e s s e r

the c a p i t a l

expend i tu re

i s

r equ i red .

The long pipe may

a lso

genera te excess ive pres su re

drop making it unsu i t ab le fo r the

proper

ope ra t ion . However

the

d i r e c t s h o r t e s t l ayovt

genera l ly

i s

not accep tab le fo r

absorb ing

the thermal

expansion.

Figure

1

shows

what wi l l

happen when

a s t r a ig h t pipe

i s d i r e c t l y


2/8

Where

=

thermal

expansion in

L

e

=

expansion

r a t e i n / i n

L

=

pipe

l en g th

in

s

=

a x i a l

s t r e s s

ps i

E

=

modulus

of

e l a s t i c i t y

psi

=

pipe

c ro s s

s e c t i o n a rea

inZ

F

=

a x i a l fo rce l bs

Figure

The

force

r eq u i red

to

squeeze

t h i s amount

i s

F

=

A S

=

E e

Take

a

6- inch

s tandard wal l carbon

s t e e l

pipe

fo r ins tance an

increase o f tempera ture from 70F

ambient to

300F opera t ing c rea t e s

an

ax ia l s t r e s s

o f 42300

p s i

and an a x i a l force o f 236000 lbs in

the

pipe .

These a re

excess ive

even though the t empera tu re

i s

only

300F.

t

i s

c l ea r t h a t

the

s t r a i g h t

l i n e d i r e c t

l ayou t

i s

not

accep tab le to most of the p ip ing

F l e x i b i l i t y has

to

be provided.

EXPANSION OOP

Pip ing

f l e x i b i l i t y a re

prov ided

in many d i f f e r e n t

ways. The

tu rns

and

o f f s e t s needed fo r running the pipe from one po in t to another

provides some f l e x i b i l i t y

by

t hemse l f .

This i n h e ren t f l e x i b i l i t y

may

or may not be s u f f i c i e n t

depending

on the ind iv idua l cases .

Addi t iona l f l e x i b i l i t y can be provided

by

adding expansion loops

or

expansion

j o i n t s .

In the s t r a i g h t l i n e example discussed

above

the s t r e s s can be reduced by a loop i n s t a l l e d as

shown in

Figure

2

or by an

expansion j o i n t as

shown

in

Figure

3.

The

idea in

Figure 2

i s

to

provide

some

pipe

perpend icu la r

to the

d i rec t ion

of

expansion.

In

t h i s

way when the p ipe

expands it bends

the loop l eg f i r s t before t ransmi

t t ing any

load

to the

anchor .

The

longer the loop

l eg

the l e s se r the

force wi l l

be c rea ted . The fo rce


3/8

other reasons , engineers tend to favor pip ing loops over expansion

jo in t s . However, expansion

jo in t s can

be used e f f e c t i v e ly

in many

appl ica t ions when they are proper ly designed. One

of

the major

requirements

in the design

of expansion j o in t system

i s to i n s t a l l

su f f i c i en t

r e s t r a in t s

fo r

maintaining

the s t a b i l i t y .

This

a r t i c l e

deals

mainly

the

loop

approach.

THE

CRITICAL

P TH

In designing

a

plan t the p ip ing

i s genera l ly

routed or

l a id -ou t

by

the

pip ing

des igners

then checked by

the

s t r e s s

engineers

as

shown

in f igure 4.

Piping

Designer

l fnexperienced)

Layout

Piping-1

Not

Flex ib le

Layout

Piping-2

Feel

ad

No

Space

evise

Both

Pipings

St re ss

Engineer

Piping

Designer

Experienced)

Check

St re s s

Too

Flex ib le

No Revision

I f Works

Layout

Piping-1

Layout

Piping-2

Feel

Good

Waste

Space

Waste

MS:terial


4/8

There i s a marked

d i f fe rence

in the layou t done

by

the exper ienced

and the inexper ienced des igne r s . The exper i enced des igners

know

the importance

of

the

f l e x i b i l i t y .

However they

t end

to provide

too much

f l e x i b i l i t y in co n t r a s t to the

i nexper ienced

ones

who

tend to provide little f l e x i b i l i t y . In e i t h e r case the resulL i s

an over pr iced pro jec t .

The layout done

by

an inexper ienced des igner i s

normally too

s t i f f

because

the des igner

does

not know how o r

too

t imid

to add loops

or o f f se t s .

I f

a

piping system

i s

too

s t i f f

the s t r e s s

engineer

wil l

almost

ce r t a in

to

f ind

it out . The s t r e s s

eng inee r

wi l l send

the des ign

with

recommended loops back to the des igner fo r

rev is ion .

At

t h i s

t ime the

des igner

have made some more layouts

in

the

same a rea making

the rev i s ion

very

d i f f i c u l t y .

On

the

o ther

hand a

layout

done by an exper ienced des igner o f ten con ta ins

the

loops

which

are excess ive

or not needed. The

excess ive

loops

are normally maintained without r e v i s i o n

becuase it

i s

a common

prc t i ce

not to

change

something

which works. The exper ienced one

might

have

saved the

manhour

needed

for

the rev i s ion . The cos t of

the excess ive

loops

can be p r o h ib i t i v e .

The

cos t

of

the

p r o j ec t

can

be

reduced

su b s t a n t i a l l y

i f

~ h

r igh t

amount

of

f l e x i b i l i t y i s bu i l t in the p ip ing

a t

the i n i t i a l l ayou t

s tage . This

requi res

some

quick

methods which can be used br the

designers to check the p ip ing f l e x i b i l i t y .

VISU L CHECK

The

visua l

check

i s

the

f i r s t

impor tant examinat ion

on

anyth ing

we

do. I f

the

des ign

looks s t range

then most

l i k e l y

something i s

wrong

with it

By now

we a t l e a s t know

t h a t

we can not run

a

pip ing s t r a i g h t from

one

poin t to

ano ther .

This a l so app l i e s to

the s i t u a t i o n

when

the re a r e two or more l i n e s tops i n s t a l l e d a t a

s t r a i g h t

header

as shown in Figure 5. The l i ne s top or ax ia l s top

ac ts

d i r e c t l y

aga ins t the expans ion

of

the pipe .

When two

axia l

s tops

i n s t a l l e d on

the

same

s t r a i g h t

l eg

the

thermal expans ion of

the pipe loca ted between

the

s tops has no place to r e l i ev e .


5/8

the

d i r e c t measure

of

the f l e x i b i l i t y There fore the key

i s

to

l oca t e

the

a v a i l a b i l i t y of

the

perpend icu la r

l eg

and to determine

i f

the l eng th o f the

l eg

i s

s u f f i c i e n t .

The r equ i r ed

l eg

l eng th

can

be es t ima ted

by

t he r u l e

of thumb equa t ion

1)

der ived by

the

guided

c a n t i l e v e r

approach,

fo r

s t e e l

pipes .

= 5.5 F ll

where ,

l = l eg l eng th

r e q u i r e d f t

D

=

pipe o u t s i d e diamete r in

b = expans ion to

be absorbed in

To usc

Equat ion

1) e f f i c i e n t l y t he

expans ion

r a t e of the p ipe

has

to

be

remembered. Table shows t he expans ion r a t e s of carbon and

s t a i n l e s s

s t e e l

pipes

a t s e v e ra l opera t ing

t empera tu res . The

r a t e

a t othe r

t empera tu re

can be e8 t imated

by

propor t ion . y combining

Equat ion

1

and Table 1, t he des igner

can

es t ima te t he

l eg

l eng th

r equ i red withou t

needing a

p e n c i l . For

i n s t a n c e an 80

f ee t

long

6- inch carbon s t e e l pipe opera t ing

a t

600F expands about 4

inches

which

requ i re s a 30

f e e t

l eg

to

absorb

it

It

shou ld

be

noted

t h a t

an

expans ion loop i s c ons i de r e d a s two

l e g s

with each l eg absorbs

one h a l f

of

the

t o t a l

expansion.

Table 1

Expans ion Rate

i n /100 f t pipe

Temp,F 70

300

500 800

1000

Carbon St e e l

1 .

82

3 .62 6 .7 8 .9

St a i n l e s s

S t e e l 2. 61

5.01 8.8

11.5

H ND

CALCULATION

There are

s e v e ra l

s imp l i f i ed

c a l c u l a t i o n s

can

be performed quick ly

with

hand.

The

most popu la r one

i s t he

so

c a l l e d guided

c a n t i l e v e r

approach.

The method

can

be

exp la ined

us ing t he L-bend given in

6 When


6/8

po in t s

B

and

C

w i l l move

to

B'

and

C' r e spec t ive ly due

to

thermal

expans ion . The end po i n t C

moves

dx

and dy r e spec t ive ly

in X- and

Y- d i r e c t i o n s , but

no

i n t e rn a l force o r s t r e s s wi l l

be

genera ted .

However

1

in

t he

a c t u a l

case t he ends

of

t he

pip ing

a re alwa.,vs

cons t ra ined as shown in Figure 6 b ) . This i s

equ i va l en t

in

moving

the

f r ee

expanded

end

C'

back

to

t he

o r ig in a l

po i n t

C

fo rc ing the

poin t B to move

to

B". The dx

i s

t he expans ion from l eg AB and

dy from

l eg

CB

The

defo rmat ion o f each

l eg

can be

assumed

to

fo l low t he guided c a n t i l e v e r

shape .

This

i s

c ons e rva t ive because

the

end ro t a t i o n

i s

ignored . The force and

s t r e s s

of

each

l eg

can

now

be es t ima ted by t he guided c a n t i l e v e r formula . The leg

AB

i s

a guided

c a n t i l e v e r

s u b j e c t

to dy

d i sp lacemen t

and

l eg

CB

a guided

c a n t i l e v e r

sub j ec t to dx d i sp lacemen t r e spec t ive ly .

From

t he

ba s i c

beam

theory ,

t he

moment and d i sp lacemen t r e l a t i o n

o f

a

guided c a n t i l e v e r i s

'1

::

For

t h i n

I::: tr3 t

s =

where,

6 E I

LZ

6

F =

2 M

121

L

wal l

p i pes ,

Equat ion

and S:::M/(ltri t ,

(2)

can

be

f u r t h e r

reduced.

By

t he

above equa t ion

becomes

s

=

E

=

r

=

'

=

L

=

l

=

D

=

the rmal

modulus

E D L:

48 .t

expans ion

s t r e s s ,

o f

e l a s t i c i t y ,

ps i

ps i

mean

r ad i us o f

t he

p i pe ,

in

t o t a l expans ion to

be

absorbed ,

l eng th

of

the

l e g pe rpe nd ic u l a r

l e ng th

in f e e t un i t , f t

ou t s ide

d iamete r

o f

t he

p ipe , in

13

in

to

in

us ing

Equat ion (3) i s

a

conven ien t formula fo r t he quick

es t i ma t i on

of

the expans ion s t r e s s . By

p r e - s e t t i n g

E=29.0x10 ps i and

8::20000

ps i , Equat ion (3) becomes Equa t ion (1) used

in

f ind ing the

l eg

l ength requi red for s t e e l p i pes .

The

othe r

formula can be used for the

quick

check i s t he one

given

in

AKSI B31

Pip ing Codes. The Code uses Equat ion 4 l as

a

measure

of adequa te

f l e x i b i l i t y , sub jec t s

t o o t he r requi rements of

the

Code.


7/8

Equat ion (4) has to be used with

gre a t

ca re , because the

same

e x t r a

l eng th of pipe can have very d i f e r e n t e f f e c t s depending on the ways

the

pipe

i s

l a id -ou t .

~ o r m l l y more

f l e x i b i l i t y wi l l

be

achieved if

the pipe i s placed f a r the r away

from

the e l a s t i c a

or

geometr ica l

c e n t e r .

For

i ns t ance

with

the

same

ex t ra

leng th

of

pip ing ,

when

it

i s

l a id -ou t as shown in

Figure

7

(a)

it has much

h ighe r f l e x i b i l i t y

than when it i s

l a i d -ou t

as in

Figure

7 (b) . Designers of t en

have

the misconcept ion about the

amount

of f l e x i b i l i t y

can

be provided

by the z ig-zag arrangement . Due to

the

ex t ra elbows placed in

the

layout , one tends to th ink

t h a t

a dd i t i ona l


should

have

been

c rea ted . Unfor tunate ly ,

the a dd i t i ona l f l e x i b i l i t y

from

the

elbows i s not enough to

compensate

the

los s

of f l e x i b i l i t y ue to

the

placement

of

pipe

toward

the geome t r ica l

c e n t e r .

4

G


8/8

TEA51 3LGioOP

LOC TION S

Figure

Once i t i s dete rmined t h a t an expans ion loop i s r e qu i r e d

t he

loop

can be placed a t one

o f

the f e a s i b l e

l oc a t i ons

before the a rea i s

conges ted by

o the r

l ayou t s . This a l so saves

t he

i t e r a t i v e process

between

t he

p ip ing

des igners and t he

s t r e s s

enginee rs .

CONCLUSION

The t r ad i t iona l piping

design

procedure depends heavi ly

on

the

s t r e s s engineer to check p i p i ng f l e x i b i l i t y . With the a v a i l a b i l i t y

o f

quick

methods

in

checking

the


the

des i gne r

can

now

layout

the

pipe

to

provide

the proper f l ex ib i l i t y a t the very

beginning This subs tan t ia l ly reduces the

number

of i t e ra t ions

r equ i red between t he p ip ing des ign er

and

the

s t r e s s

engineer .

The

cos t of

t he

p lan t can be reduced

by the

s h o r t e r

schedule

and l e s s

manpower

requ i red .

65174112-piping-flexiblility-calculation.pdf

Documents