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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-
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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
10 F T . S I L E N C E R W I T H
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
6 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
i . 3 ' . i
n
1 4
10 F T . S I L E N C E R W I T H
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
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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.
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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
T r a n s a c t i o n s
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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
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-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
\
\
\
_
__
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 ^
^c;
,y
I T H 6 F
I N
T . S I L I
P L A C i
^ ,
NCER
\
\
\
\
>
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
700 /
M
A
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8 " - 6 0 0 * R.F .W .N .
F L A N G E
3 " A D J U S T A B L E
P I P E L E G
F i g .
10 Detai ls of portable high pressure si lencer
130
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31.5 63 125 25 0 500 1000 20 00 40 00 8000 16000 AL L
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FREQUE NCY IN CYCL ES PER SECOND
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
J o u r n a l
o f
E n g i n e e r i n g f o r I n d u s t r y
M AY 1 9 7 1 / 701
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\
\
w
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1
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i
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0 5 10 15 20 2 5 3 0
I N C H E S
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