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7/29/2019 Valve Sealing
1/1
Lesson 34: Dif
f
erential
Flow Meters
Lesson Index
Civ240 Frontpage
Subjects & Courses
Homepage -
Lesson 33 Print Version
W
hat Every Engineer Needs To Know
About V
alve Sealin
gV
al
ve fu
n
cti
ons an
d ch
aract
eri
stics are t
wo of t
h
e importan
t
con
siderat
i
on
s i
n determi
n
i
n
g
seal
i
ng performan
ce
By B.J. Milleville
Rockwell InternationalPittsburgh
In
a di
scussi
on
of val
ve seal
i
n
g, i
t is ben
efi
ci
al
t
o gi
ve considerati
on
to th
e mechan
i
sm of cl
osu
re an
d how it may affect
t
h
e seali
n
g fun
ct
i
on
. Th
ere are si
g
n
ifi
cant d
i
fferen
ces among the seal
i
n
g
acti
on
s of vari
ou
s
t
ypes of val
ves.
Th
e fact th
at th
ere are wi
d
el
y differi
ng
ki
n
ds of val
ves refl
ect
s the rel
ati
ve si
mpl
i
cit
y of t
h
e basi
c val
ve fun
ct
i
on
s. On
e fu
n
cti
on, of cou
rse, i
s to serve as an
i
ntermedi
ate pi
ece of pi
pe wh
en
th
e val
ve i
s open to
permi
t fl
ui
d
to pass wi
t
h
mi
ni
mu
m resi
stan
ce. A
n
ot
h
er fu
n
ct
i
on
i
s to serve as a st
opper, or t
h
e equi
val
en
t
of a bl
i
n
d
fl
an
g
e, i
n
t
h
e cl
osed posi
ti
on
, i
sol
at
i
ng
one si
de of t
h
e val
ve from th
e ot
h
er to preven
t
th
e
fl
ow of fl
ui
d.
To judg
e th
e meri
t
s of an
y val
ve, i
t i
s usu
al
l
y desi
rabl
e to eval
u
ate three ch
aracteri
sti
cs:
1. H
ow wel
l
d
oes it imi
t
at
e t
h
e pi
ece of pi
pe i
n
t
h
e open
posi
ti
on?
2. H
ow wel
l
d
oes it imi
t
at
e t
h
e bl
i
n
d
fl
an
ge i
n
t
h
e cl
osed posi
ti
on
?
3. H
ow easil
y can
i
t
be ch
ang
ed from on
e posi
t
i
on
t
o th
e oth
er?
Th
ou
g
h there are man
y types of val
ves, the ways th
ey provi
de th
e shutoff an
d seal
i
n
g
fu
ncti
on
are l
i
mi
ted i
n
pri
n
ci
pl
e. A
closu
re el
emen
t someh
ow mu
st be moved from a posi
ti
on that
h
as mi
n
i
mu
m effect on
fl
ow to a positi
on
th
at
compl
et
el
y obst
ru
cts t
h
e fl
ow.
In
t
h
e condui
t type of g
ate val
ve (Fi
g. 1), t
h
e g
ate si
mpl
y sl
i
des i
n
to and ou
t
of th
e cl
osed posi
t
i
on
. Th
i
s acti
on
i
s si
mi
l
ar t
o the act
i
on
of a tapered pl
u
g val
ve i
n
t
h
at t
h
e bl
in
d si
de of th
e pl
u
g sl
i
des i
n
t
o an
d
out
of positi
on
(See Fi
g
. 2) Th
e di
fferen
ce i
s that
the sl
i
di
ng
i
n
t
h
e tapered plu
g
val
ve i
s ach
i
eved by the plu
g
's rot
at
i
n
g
i
n pl
ace, an
d
i
n
th
e g
at
e val
ve t
h
ere i
s strai
gh
t
l
i
near mot
i
on
i
n
to and ou
t
of posi
ti
on
.
Th
e ball
val
ve (Fi
g
. 3) i
s al
so si
mi
l
ar, rot
at
i
ng
as the pl
u
g
d
oes. The bal
l
sh
ape of th
e pl
u
g
, h
owever, si
mpl
ifi
es th
e g
eometry of t
h
e seal
between
t
h
e body an
d
th
e bal
l
an
d therefore faci
l
i
tat
es the u
se of
speci
all
y d
esi
gn
ed seal
in
g
el
emen
ts.
Butterfl
y val
ves (Fi
g
. 4) al
so move t
h
e cl
osu
re el
emen
t i
nto and ou
t
of t
h
e cl
osed posi
ti
on
by rotati
on
, bu
t t
h
e el
emen
t i
s di
sc-shaped. Instead of open
i
n
g
a fu
l
l
y d
efi
n
ed port t
o th
e fl
ow, as i
n the case of bal
l
an
d
pl
u
g
val
ves, t
h
e d
i
sc's rotation si
mpl
y t
u
rns i
t paral
l
el
t
o the fl
ow st
ream.
A
n
oth
er wi
del
y u
sed t
ype of g
ate val
ve h
as a smal
l
er, si
mpl
er closu
re el
emen
t
(See Fi
g. 5). H
ere, a wedg
i
n
g
act
i
on
bet
ween the g
at
e and t
h
e body seats provi
des some of th
e seal
i
ng
force.