Design and Performance of High Pressure Blowoff Silencers

7/25/2019 Design and Performance of High Pressure Blowoff Silencers

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C E C I L R. SPARKS

Assistant

D i rec to r ,

Depa r tmen t o f App l i ed Phys i cs ,

S o u t h w e s t

Resea rch

Ins t i tu te,

Sa n

A n ton io , Texas

D . E . U N D G R E N

Sen io r

Res ea rch Eng ineer ,

T e n ne s s e e G a s P i p e l i n e C o m p a n y ,

Ho us t o n , T e x a s

D e s i g n a n d P e r f o r m a n c e

of

H i g h - P r e s s u r e

B l o w o f f S i l e n c e r s

Through the application offl/uid dynamic and acoustic theory, the noise generation of a

high pressure blowoff can be approximated. The effects of silencer configurations can

likewise be predicted through the application of pertinent field data taken to define per

formance of the silencer components. This paper describes recent test results and their

application to improved silencer design for natural gas pipeline applications.

Introduction

U N E O F the most severe pipeline noise problems inso

far as sound intensity is concerned is that associated with high-

pressure blowoff S3 'stems. Wit hin the wide netw ork of domest ic

pipeline installa tions, b lowdown valve locations range from

areas of a lmost complete isolation to locations where residential

areas have expanded to within a few hundred feet of the blow-

d o w n v a l v e . T h e p r o b l e m s a s s o c i a t ed w i t h b l o w d o w n n o i s e

have increased s teadily as rural areas adjacent to the

right-of-

way have become heav ily popu lated . In order to avoid noise

a n n o y a n c e p r o b l e m s a r i s i n g f r o m t h e s e b l o w d o w n s , t h e i n d u s t r y

has taken many steps ranging from moving blowoff valves out

side populated areas to notifying residents well in advance of

plann ed blowd owns. In the la tt er case, residents a t d is tanc es

up to one-half mile are notif ied and residents often leave home

u n t i l t h e b l o w d o w n is co m p l e t e d . O t h e r s , p a r t i c u l a r l y t h o s e

who are unprepared for the noise , readily voice their annoyance

and objection.

I n o r d e r t o m i n i m i z e c o m m u n i t y a n n o y a n c e r e s u l t i n g f r o m

blowdown noise, major pipeline companies have turned to the

development of b lowoff s i lencers to predictably control generated

noise levels . M uc h of the work in this area was based upo n re

search performed for the American Gas Association by South

west Research Insti tute , and s i lencers were buil t and tested for

a wide range of applications within the industry .

O ne of the prim aiy a reas of concern was the devel opm ent of a

portable blowoff s i lencer whose design could be generalized to

extend i ts applicabil i ty to the wide range of p lanned blowoff

applica tions. As such, i t was desirable to obta in ma xim um noise

a t t e n u a t i o n , b u t w i t h i n t h e s i z e a n d w e i g h t l i m i t a t i o n s o f n o r

mally available f ie ld equipment to move the s i lencer from location

to location and place i t on the blowdow n valv e. T hi s pap er

presents the theory of b lowoff noise suppression, describes the

Con tribute d by the Petroleum Division and presented at the

Winter Annual Meeting, New York, N. Y. , November 29-December

3, 1970, of T H E AMERICAN SOCIETY O F MECHANICAL E N G I N E E R S .

Manuscript received at ASM E Headquarters , July 30, 1970. Paper

No . 7 0 - W A / P e t - l .

a t tenuation technique uti l ized in the design of these s i lencers ,

and presents f ie ld data as to their effectiveness.

Blowoff Noise Generation

T he noise generation mechanisms of a h igh-pressure blowoff

are characterize d by turbu lence- induc ed noise from high velocity

flow. T h e prim ary sourc e of th is noise is mixing of th e high ve-

loci t} ' g a s s t r e a m w i t h t h e a t m o s p h e r e , w h i c h i n t u r n p r o d u c e s

shear eddies or vortices a long the shear bou nda ry. T hese shear

v o r t i c e s t h e n r a d i a t e a c o u s t i c p r e s s u r e p e r t u r b a t i o n s o r n o i s e

t h r o u g h o u t t h i s m i x i n g r e g i o n .

T o demonstrate effects , consider a s imple blowdown S3^stem

consisting of an open pipe discharging directly into the a tmo

spher e. In th is case, h igh velocity f low with in the r iser p iping

g e n e r a t e s s e v e r e i n t e r n a l t u r b u l e n c e ; h o w e v e r , t h i s i s n o t w h e r e

the major portion of the observed noise comes from. While th is

i n t e r n a l t u r b u l e n c e , p a r t i c u l a r l y t h a t g e n e r a t e d b y p a s s a g e

through the constric ting valve, generates dipole vortex noise , the

major noise is generated outside the piping

itself;

i.e., in the mix

ing region of the high velocity je t as i t shears with the a tm o

sphere . Un der these conditions, shown graphic ally in F ig . 1 , th e

high velocity f low of the je t shears with the bounding air , pro

d u c i n g v i o l e n t e d d i e s w h i c h a r e t h e n c o n v e c t e d d o w n s t r e a m w i t h

the je t . As the eddy is convec ted dow nstre am, i ts k inet ic energy

is converted to potentia l (pressuve) energy with the typical four-

l o b e p r o p a g a t i o n p a t t e r n o f q u a d r a p o l e s o u r c e s a s s h o w n i n t h e

figure.

Within this mechanism, therefore, the actual source of noise

is the entire mixing region of the je t ra ther than the open end

of the pipe

itself.

As such, the source extend s from the exha ust

of the pipe to a d is tance of up to 25 pipe dia downstream.

T ypically , the high frequency portion of the noise is generated

in the extremely high shear area near the pipe exhaust, whereas

the low frequencies are produced in the relatively low shear

but large eddy sections in the downstream portion of the mixing

region.

Noise Intensity and Frequency Content

T he definit ion of to tal acoustic energy generated by a je t ex-

Journal

of

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MA Y

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Copyright 1971 by ASME

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hau st has been defined largely by obser vatio n. T he se observa

tions are suffic ient, however, to perm it a definit ion of the de

pend ency of gene rated noise on the controll ing f luid para me ters .

T h e source of acoustic energy in a f low st rea m is , of course, t he

kinetic energy of the s tream which may be defined as (U

2

/-

2)pUA,

w h e r e (V-/2) is the kinetic energy per unit mass and

pU A

is the mass f low rat e of the s t rea m. T h e effic iency of

conversion of th is k inetic energy f lux to sound power has been

show n to be prop ortio nal to the f ifth powe r of Mach. num ber

CM

6

) a n d ( w h e n t h e j e t i s d i s c h a r g i n g i n t o t h e a t m o s p h e r e )

to the ratio of (p/po), wher e p is the flowing gas density a nd

po is am bien t a ir dens ity . T h us the tot al acoustic power of a

high velocity je t d ischarging into the a tm osph ere follows the

e q u a t i o n

T a b l e 1 C o m p a r i s o n o f p r e d i c t e d a n d F ie l d t e s t n o i s e r e d u c t i o n

w

p

2

U

s

d

2

Poc

5

(1)

C o n v e r t i n g t h i s a c o u s t i c p o w e r e x p r e s s i o n t o m o r e c o n v e n t i o n a l

acoustic terms, we get for the sound power level of the je t the

following expression:

p*U*A

PW L

= 10 log

Id ~ dB re

l O

1 2

w a t t s ( 2 )

Poc

or for sound pressure level

p*U

a

A

SP L = 10 log Kt dB re 0 . 0 0 0 2 d y n e / c m

2

(3)

Poc

6

W h e r e

K

2

v a r i e s w i t h d i s t a n c e f r o m t h e m e a s u r e m e n t p o i n t t o

t h eblowoff these may be taken from T able 1 .

2

42500

6780

2130

1070

528

339

213

152

107

53.7

27.2

Ft from Source

0

25

50

75

100

125

150

175

200

300

400

W i t h r e g a r d t o f r e q u e n c y c o n t e n t , e x p e r i m e n t a t i o n h a s a l s o

shown t ha t a typica l spectral d is tr ib ution for a je t is wide ba nd

i n n a t u r e , b u t w i t h a s p e c t r a l p e a k c o r r e s p o n d i n g t o a S t r o u h a l

fd

n u m b e r (N,) of from 0.17 to 0.2 (N , = -r, w h e r e / = f r e q u en c y

i n H z , d = pipe dia , and U = e x h a u s t v e l o c i t y ) . O n e i t h e r s id e

of th is Strouhal peak, a decay of 3 dB per octave is observed.

F r o m t h i s an a l y s i s , s e v e r a l o b s e r v a t i o n s b e c a m e a p p a r e n t f o r

r e d u c i n g t u r b u l e n t m i x i n g n o i s e :

1 Re duce je t velocity . A reduct ion of 50 perc ent in je t

velocity will result in a 24 dB reduction in quadrapole generated

noise.

2 Com plete th e mixing process in a confined volume a nd

p r e v e n t i t s d i r e c t r a d i a t i o n i n t o t h e a t m o s p h e r e .

T a b l e 1 C o m p a r i s o n o f p r e d i c t e d a n d f i e l d t e s t n o i s e r e d u c t i o n

D E S C R I P T I O N D E T A I L P R E D I C T E D F I E L D T E S T

10 F T . S I L E N C E R W I T H

O R A N G E P E E L D I F F U S E R


E L L I P T I C A L W E L D C A P

D I F F U S E R

17

2 5

17

18


O R A N G E P E E L D I F F U S E R

i . 3 ' . i

n

1 4


H E M I S P H E R I C A L W E L D

C A P D I F F U S E R

17 2 3

H E M I S P H E R I C A L W E L D

C A P D I F F U S E R O N LY

10

696 / MA Y 19 7

T r a n s a c t i o n s

of

t h e A S M E



3/8

H I G H S H E A R R E G I O N ,

S E V E R E H IG H F R E Q U E N C Y M I X I N G

M I X I N G L E N G T H ' S

5 - 2 0 D I A M E T E R S

x

V /

^ S j ( V ^ S H E A R Q U A D R A P OL E

/ / \ - s

' P O T E N T I A L C O N E ' "

( U N M I X E D )

Q U A D R A P O L E

P R O P A G A T I O N

P A T T E R N

L A R G E S C A L E

LOW FREQUENCY

T U R B U L E N C E

Fig. 1 B lowofF je t no ise and tu rbu lence s t ruc tu re

3 T r e a t t h e e x h a u s t s e c t i on f r o m t h i s w i t h a b s o r b i n g m a t e r i a l

to a ttenuate mixing noise before i t is d ischarged into the a t

m o s p h e r e .

4 Red uce je t exha ust s ize by using, for exam ple, mu ltiple

small-dia holes. T hi s does two thin gs: reduces the leng th of the

mixing region ( thereby permitt ing completion of the mixing

process in a shorter length), and shifts the spectral peak to a

mu ch higher portion of the frequency spe ctrum . Hig h fre

quency noise is easier to absorb.

Details of treatments following these suggestions will appear

in a subsequent section of the text.

Blowoff Silencing

Since most b lowoff noise predominates in the high frequency

portion of the spectrum, a logical approach to s i lencing i t is

through the use of an absorbing section or l ined duct as is some

time s used to suppr ess regulat or noise . How ever, since the mix

ing region extends up to 25 pipe dia downstream, absorb

ing material should be applied along the entire mixing region,

a n d a r a t h e r c u m b e r s o m e d e s ig n w o u l d r e s u lt . O n t h e o t h e r

hand, if this jet is broken up into a series of smaller jets, or is

otherwise a ltered to produce full mixing in a shorter region, the

design and size requirements for an effective sound absorber are

considerably reduced . An effective s ilencer mig ht then consist

of a jet diffuser at the inlet of the silencer in a relatively short

section, followed by an absorbing section immediately down

stream to further a t tenuate the noise of the inlet je t and that

r e g e n e r a t e d by th e diffuser. In essence, this lat ter sectio n of

the s i lencer is a sound stream absorber for the more s tabil ized,

lower velocity f low, and acoustical material is thereby more

effectively uti l ized t han i t would be wit hout a d iffuser ( i .e ., unde r

conditions of full inlet nozzle flow).

HEAVY MESH WIRE

-JET SHIELD SLOTTED

l/V'W.T. PIPE

Since noise from a blowoff is proportional to the e ight power

of velocity , i t is important in designing an effective s i lencer to

keep the discharge velocity of the s i lencer as low as possible .

In many high-pressure applications i t is possible to have sonic

flow not only in the valve constric tion but a lso in the s i lencer dis

charge, and in such cases the benefit derived is only by vir tue

of reduce d f lowing density . T herefore, to assure ad equ ate

silencer performance i t is necessary to calculate the f low char

acteris t ics of the s i lencer before calculations can be made of i ts

acoustic al perform ance. Since these calculations are complex

a n d a r e c o v e r e d i n p r e v i o u s l i t e r a t u r e [ 1 , 2 ] ,

1

they will not b e

t r e a t e d h e r e . D e s i g n c h a r t s p r e s e n t e d l a t e r h a v e t h e c a l c u l a

t ions a lready made for nat ura l gas. I t is wort hwh ile to discuss

what happens in regard to in ternal f low in these s i lencers , how

ever, if just to emphasize the importance of ta i loring s i lencer de

signs to the job at hand.

F low in th e s i lencer show n in F ig . 2 is charact erized b y a series

of expansion and mixing processes as f low is passed through the

discon tinuou sly diverging piping. F or a g iven gas composit ion,

this in ternal f low may be defined on the basis of p ipeline (source)

pressure and f low areas, as follows:

1 At extrem ely low pipeline pressures, f low velocity a t th e

silencer in let (F i) will be subsonic, and exha ust velocity (F

2

)

will be even lower. T hi s condition is of tr ivial imp orta nce in

most industria l applications where noise is a problem.

2 As ups tre am press ure is increased abov e a cri t ical level,

sonic f low will be experienced at the inlet constr ic tion, and no

higher velocity will be achieved in mo st practic al designs. T his

first critical line pressure (p

cl

) may be defined as follows:

Pel

= P a tm ( 2 / f c + l ) f c / ( l - f c )

j l b / f t

, 2

(4)

w h e r e

p

alm

i s t h e a t m o s p h e r i c p r e s s ur e , l b / f t

2

. F o r n a t u r a l g a s

w i t h k 1.3, this p

el

is app roxim ately 1 .85 t imes atmosp heric

pressure.

Although inlet velocity will not increase as l ine pressure is

raised, in let mass f low will increase in direct proportion to pressure

(neglecting supercompressibil i ty) because of increased f lowing

density . If Ai is the minimum inlet f low area in square feet

(valve area X flow coeffic ient) , th is mass f low may be calculated

from the following:

m = 8.02

Avpo

RT

0

\k + lj\k + 1 /

2 / ( A - - i r

lb/sec (5)

Under continuing l ine pressure increases, the exhaust, velocity

( F

2

) from the s i lencer will increase because of the increased mass

Fig. 2 Example of silencer design showing important flow dimension s

1

Numbers in brackets designate References at end of paper.

Journal

of

Engineer ing

fo r

Indus t ry

MA Y

1

9 7 / 697



4/8

P I P E L I N E P R E S S U R E , p . p s i c i

F i g .

3 D i f f e r e n c e i n o p e n s t a c k n o i s e a n d t h a t r e g e n e r a t e d a t s i le n c e r e x h a u s t b y tu r b u l e n t

m i x i n g o f t h e d i s c h a r g e g a s

a:

u

m

3 5

a

0.9

a=o.8

t 3 0

15 Q

oa

o

8

3 a:

a: CQ

o n

J Q

1-'

2 5

2 0

^

/

^

j

--T.L

/ '

= i

4 . 2 6

T

a

'

4

d 2

a =0 .7

S I L E N C E R

L E N G T H T O D I A M E T E R R A T I O f - f )

x

d 2 '

F i g . 4 T r a n s m i s s i o n l o s s d u e t o d i f f u s e r a n d a b s o r b i n g s e c t i o n f o r h ig h - p r e s s u r e c y l i n d r i c a l

s i l e n c e r a s s h o w n i n F i g . 1

f low, and pressure in the s i lencer body will remain vir tually a t

a t m o s p h e r i c .

3 Wh en a second cri t ical l ine pressure

(pa)

is reached, m ass

flow will be increased to the point that sonic f low (Mach 1) will

be experienced at the s i lencer exhaust as well as a t the s i lencer

inlet . T his second cri t ical pressu re is a function of both gas

characteris t ics and s i lencer diameter ra tio , and can be expressed

as follows:

Pel

= P a t m

) (

k +

1

k/(i-k)

(6)

Upon definition of silencer flow, it is then possible to use

a c o u s t i c t h e o r y t o p r e d i c t t h e n o i se a t t e n u a t i o n a c h i e v e d . I n

such a s i lencer, there are two major noise source s: the diffuser

and the s i lencer outle t . Diffuser noise is severa l dB below open

stack noise and even this is reduced by passage through the ab

sorbing section. T h e s ilencer out le t is a source of regen erated

noise and can be reduced only by controll ing exit velocit ies . F or

a n o p t i m u m s i l e n c e r d e s i g n , t h a t n o i s e t r a n s m i t t e d t h r o u g h t h e

silencer should just equal that regenerated at the outle t .

If the power regenerated at the s i lencer exhaust is severe, i t is

app are nt tha t the s i lencer will afford but l i t t le benefit . S imi

larly , there is a practical l imit as to the absorption loss which is

useful for any given ou tle t regeneration l evel. F or exa mp le, if

diffuser noise amounts to 150 dB and regenerated noise is 140

d B ,

t h e r e i s l i t t l e v a l u e i n p r o v i d i n g m u c h m o r e t h a n a b o u t 1 0

dB of absorption within the s i lencer.

In order to design a s i lencer such as show n in F ig . 2 for a

specified total noise reduction under a g iven set of operating

conditions, i t is necessary to predict both the regeneration level

at the s i lencer outle t , and the a ttenuation of incident noise ex

perienced by the action of the diffuser and absorbing sections.

Graphical techniques for designing s i lencers for predictable per

formance characteris t ics are given in the following paragraphs.

L ight hil l 's equ atio n (equation (3)) provide s a techniqu e for

calculating the regenerated noise level a t the s i lencer, or more

c o n v e n i e n t l y , t h e r e g e n e r a t e d d r o p

SN ,

which we define as the

dB difference between open stack (no s i lencer) and s i lencer

r e g e n e r a t e d l e v e l. T h i s 8N can be obta ined direc tly from t he

design cha rt in F i g . 3 [1 , 2] .

Similarly , to f ind the reduction of open stack noise afforded

698 / MAY 19 7


o f

t h e A S M E



5/8

m

UJ

2 : 0 0 2 : 3 0

IN HOU R S

Fig. 5 Approx ima te b lowd ow n t ime fo r 30 -i n -OD p ipe l in e w i th two 8 -

in .

b lowo f f va lve s

by the diffuser and absorbing section, defined as

AN ,

we go

dire ctly t o Fi g. 4 [1, 2] . Since thes e two valu es, F F

W l

; N C E R -

O R A N G

D I F F U

r H O U T

>

'*

E

P E E

>E R

/

*-*

L

\

E L L I

. WE L

v '.

\

\

> T I C A L

I C A P

DIFFU:

ER

> v

k \

\

V

31.5 63 125 250 500 1000 200 0 40 00 800 0 16000 AL L

PASS

FR EQU EN C Y IN C YC LES PER SEC ON D

Fig. 6 No i s e l e ve l s c ompa r in g we ld c a p d i ff u s e r a nd a s pe c i a l f a b r ic a t e d o ra n ge pe e l

d i f fuse r

Journal

o f

Engineer ing

fo r

Industry

MAY 1 97 1 / 699



6/8

-0

D

in

z

D

O

10

130

i a o

n o

100

9 0

7 0

B L O W - C F F W I T H O U T

S I L E N C E R . .

J v

S' S

/

t

D 1 F F U S E R

\ \

\

O N L Y

\

\

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_

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31.5

1 25 2 5 0 5 0 0 1 0 00 2 0 0 0 4 0 0 0 8 0 0 0 1 6 0 0 0 A L L

P A S S

F R E Q U E N C Y I N C Y C L E S P E R S E C O N D

Fig . 8 N o i s e l e v e l s s h o w i n g p e r f o r m a n c e o f d i f f u s e r a l o n e

z

O

12 0

110

W 1 0 0

a:

D

/)

en

9 0

8 0

7 0

y

/

B L O W -

S I L

) F F W

: N C E R -

T H O U T

^ ^ f c ^

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I T H 6 F

I N

T . S I L I

P L A C i

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NCER

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31.5

6 3 1 2 5 2 5 0 5 0 0 1 0 0 0 2 0 0 0 4 0 0 0 8 0 0 0 1 6 0 0 0 A L L

P A S S

F R E Q U E N C Y IN C Y C L E S P E R S E C O N D

Fig . 9 N o i s e l e v e l s s h o w i n g p e r f o r m a n c e o f 6 - f t s i l e n c e r

Field Tests

A series of f ie ld tests were conducted in order to optimize

silencer design for the particular p ipeline conditions under which

it was to be used. Prim e requ irem ents were lowest possible

weight and compactness as well as applicabil i ty to a wide range

of l ine pressur es. Of part icula r in terest in the tests , conduct ed

under typical f ie ld conditions, were predictabil i ty of noise re

duction, optimizing transmission loss and regeneration loss,

shell wall th ickness and diffuser design for maximum noise re

ductio n wit hou t flow choking. Ano ther goal of these tests was

a decision on the amount of noise a ttenuation actually needed.

T h i s a m o u n t w o u l d v a r y a t e a c h v a l v e l o c a t i o n d u e t o m a s k i n g

noise, nearness of residents , e tc . Res ults of these tests are

s h o w n b e g i n n in g w i t h F i g . 6. A s u m m a r y o f m e a s u r e d a n d p r e

dicted noise reductions for these tests are given in T able 1 to

gether with i l lustrations of basic configurations used.

Most previous s i lencer models fabricated according to the de

sign techniques outl ined herein uti l ized a perforated capped pipe

diffuser and f ie ld results show a 7 to 9 dB reduction from the

diffuser

itself.

Du rin g the pre sent f ie ld test s thre e oth er dif-

fusers were tr ied; v iz ., (a) an orang e pee l welded diffuser

w h i c h i n a p p e a r a n c e w a s b e t w e e n a h e m i s p h e r e a n d a c o n e ,

(b) an ordinary20-in . e l l ip tical weld cap, and (c) an 18-in . hemi

spherical weld cap. Diffusers are i l lustra ted in F ig . 7 . Wi th

this type of d iffuser the need for the s teel shield covering the

bottom section of the f iberglass absorbing material was not

needed, there by effecting a reduct ion in s i lencer weig ht. F ig . 6

shows a direct comparison of the e ll ip tical weld cap and orange

peel diffuser. Th e weld cap silencer gives a red uct ion of 18

dB comp ared to a theor etical drop of 17 dB . T h e orange peel

diffuser, however, showed a to tal reduction of 25 dB; i t is as

sume d tha t th is is due to a mo re diffusing hole pa tte rn . L at er

tests using a hemispherical d iffuser show its performance to be

very near tha t of the orange peel. In a ll cases the tota l num ber,

s ize, and spacing of the holes were held constant.

In one test the hemispherical d iffuser was tested with the

silencer shell and abso rbing section rem oved . A redu ction of

10 dB was achieved as shown in F ig . 8. How ever, a t 1000 cps,

1 7 d B r e d u c t i o n w a s a c c o m p l i s h e d .

Also during these tests , shell wall th icknesses of 0 .375 in . and

0.560 in . were evaluated, with no measurable difference in noise

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10 Detai ls of portable high pressure si lencer

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F i g .

11 Noise levels show ing performance of si lencer fabricated to specif ications

level. While these da ta do not quantitatively define noise

transmission loss of the shell, they do show that the

3

/s-in.

thickness is sufficient. Available lab tests show tha t for steel

pipe, transmission loss (TL) can be approximated from:

TL = 17 log (tf) -. 6, dB

where

t

= thickness, in.

/ = center frequency of band for which TL is defined.

F ig. 9 shows data for a silencer of similar design except th at

total length was reduced from 10 ft to 6 ft. Under field condi

tions, measured noise reduction was 14 dB, compared to a pre

dicted 14 dB . While agreement with theory is good, reduction

afforded by the 6-ft model is substantially below the 10-ft version

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F i g .

12 Veloci ty prof i le taken at si lencer exi t

for the oper ating conditions enco unter ed. I t should be noted,

however, that if l ine pressure were 900 psi , the 6-ft model would

perform as well as the 10-ft mod el. T his again emphasizes th e

importance of designing for the specific application involved, or

for a portable s i lencer, designing for the extreme application.

Based on results of these tests , design of the s i lencer was chosen

as shown in Pig. 10, and 14 were fabricate d for use throug hou t

the pipeline system for pressures up to 750 psi and valve s izes

up to 8 in . (42 percent opening).

A further series of tests were conducted on one of these to de

fine i ts noise and f low characteris t ics . Noise test dat a are given

i n F i g . 1 1 , s h o w i n g a t o t a l o v e r a l l a t t e n u a t i o n o f 2 3 d B . F l o w

p a r a m e t e r s a r e s h o w n s u p e r i m p o s e d o n F i g . 1 0 a n d m e a s u r e d

velocity profile is show n in F ig . 12. I t seems appa ren t tha t re

generated level could be dropped further by changing diffuser

hole pattern or hole orientation to better equalize f low profile

across the cross section.

Conclusions

Major conclusions from the s tudies are as follows:

1 T h e s i lencer designs show n perform effectively in redu cing

pipeline blowoff noise , as substantia l noise reductions can be

achieved for blowoff with up to at least 8-in. 42 percent opening-

valves a t 750 psi with a portable s i lencer model.

2 F or most p ipeline applicatio ns, a wall of th icknes s of

3

/s in.

is suffic ient for transmission losses up to 20 dB.

3 T h e t h e o r y p r e s e n te d is g e n e r a l l y a d e q u a t e fo r a r b i t r a r y

diffuser design. In these s tudies, a hem ispherica l d iffuser affords

a d d i t i o n a l a t t e n u a t i o n .

4 Diffusers sho uld have suffic ient holes such tha t the 42

perc ent opening valv e is the minim um flow area. Diffuser area

in these tests was equivalent to one 5 .31-in . hole compared to a

5.21-in . equivalent d ia for the valve.

R e f e r e n c e s

1 Dam ewood, Glenn, Sparks, Cecil R., et al. , Blowoff Noise

Suppression and Regulator Valve Noise Generation, Noise Abate

ment at Gas Pipeline Installations, Vol. Il l, American Gas Association,

Catalog No. 39/PR, Nov. 1961.

2 Sparks, Cecil R., Design of High-P ressure Blowoff Silencers,

JASA, Vol. 34, No . 5, Ma y 1962.

702 / MAY 19 7

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t h e A S M E

Design and Performance of High Pressure Blowoff Silencers

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