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Lesson 34: Dif

f

erential

Flow Meters

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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.