15 control systems - valvesfluid.itcmp.pwr.wroc.pl/~zaik/dydaktyka/basics of automatics/15... ·...
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Faculty of Mechanical and Power Engineering
Avalveisadevice that regulatesthe flow of
substances(either gases,fluidized solids,slurries,
or liquids) byopening,closing,or partially
obstructing various passagew
ays.Valves are
technically pipe fittings, butusually are discussed
separately.
Valves are used inavariety of applications
including industrial, military,commercial,
residential, transportation. Plumbingvalves are the
mostobvious in everyday life, butmany more are
used.
Some valves are driven
bypressure only,they are
mainly usedforsafety purposes in steam engines
and domestic heating or cooking appliances.
Others are used in acontrolled way,like in Otto
cycleenginesdriven
by a camshaft,where they
playa major rolein engine cycle control
VA
LV
E
Alarge variety of valves are available and have many applications with sizes ranging
from tinytohuge.The cost of valves ranges from very cheap simple disposable
valves,in some item
stovery expensive valvesforspecialized applications.
Often
notrealized
bysome,sm
all valves are even inside some common household
item
s including liquid or gelmini-pumpdispenser spigots, spraydevices,some
rubber bulbsforpumping air, etc.,manual air pumps and some other pumps,and
laundry washers.Valves are almostasubiquitousaselectrical switches.Often
avalve
is part of some object,the valve bodyand the object made inone piece; forexam
ple,
a separatoryfunnel. Faucets, taps,and spigots are all variations of valves.
Manyfluidsystem
s such
aswater and natural gaslines in houses and other buildings
have valves. Fluidsystem
s in chem
icaland power plantsand other facilities have
numerous valvestocontrolfluidflow.
APPLIC
AT
ION
Body
The majority of the valve consists of
the valvebody,includingmostof the
exterior.The valvebodyis the vessel
or casing that holds thefluidgoing
through inside the valve.Valve bodies
aremostcommonly made of various
metals or plastics,although valve
bodies fused with glass laboratory
item
sinone piece
are also made of
glass.
PA
RT
S
Ports
Thebodyconsists of two or more openings,called portsfrom which movement occurs fromone
openingtothe next.These ports are controlled
by avalve.
Valves with two or three ports are themostcommon,while valves consisting of four or more ports
arenot asfrequently used.Extra ports that arenotneeded can
beclosed offbythe valve.
Manufacturing of valves often occurs with the intent that they
will beconnected with another
specific object.These objects can vary, butgenerally these include some type of piping, tubing,or
pump head. Insome cases, avalveportis immediately connected to a spraynozzle or container. To
makeaconnection,valves are commonly measuredbythe outer diameter the ports they connectto.
Forexample, a 1-inch valve is sized
toconnectto 1-inch outer diameter tubing.
Combined withavalve,ports have the abilitytoactasfaucets,taps,or spigots,all whileoneor more
of its remaining ports are left unconnected. Mostvalves are built with some means of connection at
the ports.This includes threads, compression fittings,glue orcementapplication(especiallyfor
plastic),flanges,or welding(formetals).
PA
RT
S
Discs and rotors
Inside the valvebody,flow through the valve may
bepartly or fully blocked
byan
object calledadisc.Although valve discs of some kinds of valves are traditionally disc-
shaped,discs can come in various shapes.Although the valvebodyremains stationary
within thefluid system
,the disc in the valve is movable so it can control flow. Around
type of disc withfluidpathway(s)inside which can
berotatedtodirect flow between
certain ports canbecalled
a rotor. Ball valvesare valves which use spherical rotors,
exceptfortheinterior fluidpassagew
ays. Plug valvesuse cylindrically-shaped or
conically-tapered rotors called plugs.Other round shapesforrotors are possible too in
rotorvalves, aslongastherotorcanbeturned inside the valvebody.Howevernotall
round or spherical dics are rotors; forexam
ple, aball check valveuses the balltoblock
reverse flow, butisnot a rotorbecause operating the valve doesnotinvolve rotation of
the ball.
PA
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PA
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Seat
The valve seatis theinteriorsurface in thebodywhich
contacts or could contact the discto form
aseal which
shouldbeleak-tight,particularly when the valve is shut
(closed).If the disc moves linearlyasthe valve is
controlled,the disc comes into contact with the seat
when the valve is shut.When the valve hasa rotor,the
seat is always in contact with therotor, butthe surface
area of contactontherotorchangesastherotoris
turned.If the disc sw
ingson ahinge, asina sw
ing
check valve,it contacts the seattoshut the valve and
stopflow. Inall the above cases,the seat remains
stationary while the disc or rotormoves.Thebodyand
the seat could both come in one piece
of solid material,
or the seat couldbe aseparatepiece
attached or fixed
to
the inside of the valvebody,dependingonthe valve
design.
Stem
The stemisa rodor similarpiece
spanning the
inside and the outside of the valve,transm
itting
motiontocontrol the internal disc orrotorfrom
outside the valve.Inside the valve,therodis joined
toor contacts the disc/rotor.Outside the valve the
stem
is attached
to a handleor another controlling
device.Between inside and outside,the stem
typically goes throughavalve bonnet if there isone.
Insome cases,the stem
and the disc canbe
combined inone piece,or the stem
and thehandle
are combined inone piece.
PA
RT
S
Stem
The motion transm
ittedbythe stem can
be alinear
push or pull motion, arotating motion,or some
combination of these. A
valve witha rotorwouldbe
controlled
byturning the stem.The valve and stem
canbethreaded such that the stem can
bescrewed
into or out of the valvebyturning it inonedirection
or the other,thus moving the disc back or forth inside
thebody. Packing is often used between the stem and
the bonnettosealfluidinside the valve inspiteof
turning of the stem.Some valves havenoexternal
control anddo notneedastem; forexample, most
check valves. Check valves are valves which allow
flow inonedirection, butblock flow in the opposite
direction.Some refertothemas one-way valves.
PA
RT
S
Stem
Valves in which the disc is between the
seat and the stem
and where the stem
moves inadirection into the valvetoshut
it are normally-seated(also called'front
seated').Valves in which the seat is
between the disc and the stem
and where
the stem
moves inadirection out of the
valvetoshut it are reverse-seated(also
called 'back seated').These termsdo not
applytovalves withnostem
nor to
valves using rotors.
PA
RT
S
Bonnet
A bonnetbasically actsas acoveronthe
valvebody.It is commonly sem
i-
permanently screw
ed into the valvebody.
During manufacture of the valve,the
internal parts were put into thebodyand
then the bonnetwasattached
tohold
everything together inside. Toaccess
internal parts ofavalve, auser would take
off the bonnet,usuallyformaintenance.
Many valvesdo nothave bonnets; for
exam
ple,plug valves usuallydo nothave
bonnets.
PA
RT
S
Spring
Many valves havea springforspring-
loading, tonormally shift the disc into
some positionbydefaultbutallow
controltoreposition the disc. Relief
valvescommonly useaspringtokeep
the valve shut, butallow excessive
pressuretoforce the valve open against
the spring-loading,
PA
RT
S
Valveballs
A valve ball is also usedforsevere
duty,high pressure,high tolerance
applications.They are typically made of
stainless steel,titanium,Stellite,
Hastelloy,brass,and nickel.They can
also
bemade of different types of
plastic,such
as ABS, PVC, PPor
PVDF.
PA
RT
S
OPER
AT
ING
CO
ND
ITIO
NS
2-way valves
2-portvalves are commonly called2-way valves.
Operating positionsforsuch valves can
beeither
shut(closed)so thatnoflow at all goes through,
fully open
formaximum flow,or sometimes
partially open
toany degree in between.Many
valves arenotdesigned
toprecisely control
intermediate degree of flow;such valves are
considered
to beeither open or shut,with maybe
qualitative descriptions in between.
2-way valves
Some valves are specially designed
to
regulate varying amounts of flow.Such
valves have been calledbyvarious nam
es
like regulating,throttling,metering,or
needle valves. Forexam
ple,needle valves
have elongated conically-tapered discs and
matching seatsforfine flow control. For
some valves,there may
be amechanismto
indicate howmuch
the valve is open, butin
many cases other indications of flow rate are
used,such
asseparate flow meters.
OPER
AT
ING
CO
ND
ITIO
NS
2-way valves
In some plants withfluidsystem
s,some2-way valves can
bedesignated
as
normally shut or normally open during regular operation.Exam
ples of
normally shut valves are sam
pling valves,which are only opened whilea
sample is taken.Exam
ples of normally open valves are isolation valves,
which are usually only shut when there isa problemwithaunit orasection
ofa fluid system
such
as a leak.Then,isolation valve(s)are shut inorder to
isolate theproblemfrom the rest of thesystem
.
OPER
AT
ING
CO
ND
ITIO
NS
2-way valves
Although many2-way valves are made in which the flow can
goin
either direction between the two ports,when
avalve is placed intoa
certain application,flow is often expectedto gofromonecertainport on
the upstream
side of the valve, tothe otherport onthe downstream
side.
Pressure regulators are variations of valves in which flow is controlled
to
produce
acertain downstream
pressure,if possible.They are often used
tocontrol flow of gas froma gascylinder. Aback-pressureregulatorisa
variation ofavalve in which flow is controlled
tomaintainacertain
upstream
pressure,if possible.
OPER
AT
ING
CO
ND
ITIO
NS
3-way valves
3-way valves have three ports. 3-way valves are commonly made such
that flow coming in atone portcanbedirectedtoeither the secondport
inoneposition or the thirdportin another position or in an intermediate
position so all flow is stopped.Often such
3-way valves are ball orrotor
valves.Many faucets are made so that incoming cold and hot water can
beregulated in varying degreestogive outcoming water atadesired
temperature.Other kinds of3-portvalves can
bedesigned
forother
possible flow-directing schem
es and positions; forexam
ple,see Ball
valve.
Invalves having more than 3ports,even more flow-directing schem
es
are possible. Such valves are often
rotorvalves or ball valves.Slider
valves have been used also.
OPER
AT
ING
CO
ND
ITIO
NS
CO
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Many valves are controlled manually witha handle
attached
tothe valve stem
.If thehandleis turned
a
quarter ofafull turn(90°)between operating
positions,the valve is calledaquarter-turn valve.
Butterfly valves, ball valves,and plug valves are
often quarter-turn valves.Valves can alsobe
controlled
bydevices called actuatorsattached
tothe
stem
.They can
beelectromechanical actuators such
asan electricmotor or solenoid,pneumatic actuators
which are controlled
by air pressure,or hydraulic
actuators which are controlled
bythe pressureofa
liquid such
asoil or water.Actuators can beusedfor
the purposes of automatic control such
as in washing
machine cycles,remote control such
asthe use ofa
centralized control room,or because manual control
is too difficult; forexam
ple,the valve is huge.
Pneumatic actuators and hydraulic actuators need pressurized airor liquid
linestosupply the actuator:an
inletline and an outlet line. Pilotvalvesare
valves which are usedtocontrol other valves. Pilotvalves in the actuator
lines control the supply of air or liquid goingtothe actuators.
The fill valve ina commodewatertankisaliquid level-actuated valve.
When
ahigh water level is reached, amechanism shuts the valve which
fills thetank.
Insome valve designs,the pressure of the flowfluiditself or pressure
difference of the flowfluidbetween the ports automatically controls flow
through the valve. Inan open valve, fluidflows inadirection from higher
pressuretolower pressure.
CO
NT
RO
L
Valves are typically rated
formaximum tem
peratureand pressurebythe
manufacturer.The wetted materials inavalve are usually identified also.
Some valves rated at very high pressures are available.When
a designer,
engineer,or user decidestouseavalveforan application,he/she should
ensure the rated maximum tem
perature and pressure are never exceeded
and that the wetted materials are compatible with thefluidthe valve
interioris exposedto.
Somefluid system
designs,especially in chem
ical or power plants,are
schem
atically represented in piping and instrumentation diagrams. Insuch
diagrams,different types of valves are representedbycertain symbols.
Valves in good condition shouldbeleak-free.However,valves may
eventually wear out from use and developa leak,either between the inside
and outside of the valve or,when the valve is shut to stopflow,between
the disc and the seat. Aparticle trapped between the seat and disc could
also cause such leakage.
OT
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CO
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ER
AT
ION
S
CH
OSEN
TY
PES O
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ALV
ES
Ball valve
Butterfly valve
Check valve
Cock (tap)
Flow control valve
Gate valve
Globe valve
Pressure reducing valve
Pressure regulator
Thermostatic valve
Aball valve(like the butterfly valve, oneofafamily of valves called
quarter turn valves)isa valvethat opensbyturninga handleattached
to a
ballinside the valve.The ball hasa hole,orport,through the middle so
that when theportis in line with both ends of the valve,flowwilloccur.
When the valve is closed,theholeis perpendiculartothe ends of the
valve,and flow is blocked.Thehandleposition lets you"see"the valve's
position.
TY
PES O
F V
ALV
ES
Ball v
alv
e
Ball valves are durable and usually worktoachieve perfect shutoff even after
years of disuse.They are therefore an excellent choiceforshutoff
applications(and are often preferred
to globe valvesand gate valvesforthis
purpose).They
do notoffer the fine control that may
benecessary in
throttling applicationsbutare sometimes usedforthis purpose.
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ALV
ES
Ball v
alv
e
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ALV
ES
Ball v
alv
eThebodyof ball valves may
bemade ofmetal,ceramic,or plastic.The ball
may
be chromeplatedtomake it more durable.
There are four generalbodystyles of ball valves: single body,splitbody, top
entry,and welded.
There are four general types of ball valves:fullport, standard port,reduced
port,andv port.
•Afullportball valve has an oversized ball so that theholein the ball is
thesamesize
asthe pipelineresulting in lower frictionloss.Flow is
unrestricted, butthe valve is larger.
•A standard portball valve is usuallylessexpensive, buthasasm
aller ball
andacorrespondingly smaller port.Flow through this valve isonepipe
size smaller than the valve's pipe size resulting in slightly restricted flow.
•Inreducedportball valves,flow through the valve isonepipe
sizes sm
aller than the valve's pipe size resulting in restricted
flow.
•A v portball valve has eithera 'v'shaped ball ora 'v'shaped
seat.This allows the orifice
to beopened and closed inamore
controlled manner withaclosertolinear flow characteristic.
When the valve is in the closed position and opening is
commenced the sm
all end of the'v'is opened first allowing
stable flow control during this stage.This type of design requires
agenerally more robust construction duetohigher velocities of
the fluids,which would quickly dam
age a standardvalve.
•A trunnionball valve hasamechanical means of anchoring the
ball at thetopand the bottom,this design is usually applied
on
larger and higher pressure valves(say
4inch and above600 psi
and above)
TY
PES O
F V
ALV
ES
Ball v
alv
e
Manually operated ball valves can
beclosed quickly and thus there isa
danger of water ham
mer.Some ball valves are equipped with an actuatorthat
may
bepneumatically ormotor (electric)operated.These valves can
beused
eitherfor on/off or flow control. A
pneumatic flow control valve is also
equipped witha positionerwhich transforms the control signal into actuator
position and valve opening accordingly.
TY
PES O
F V
ALV
ES
Ball v
alv
e
schem
atic3way ball valve-L-shaped
ball right, T-shaped left
Straight Through,Two
WayORThree Way Ball
Valves
Three-way ball valves havea L-orT-
shaped
holethrough the middle.The
different combination of flow are
shown in the picture.Multiportball
valves with4or more ways are also
commercially available,theinletway
often being orthogonaltothe plane of
the outlets.
TY
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F V
ALV
ES
Ball v
alv
e
Forspecial applications,such
as
driving air powered motors from
forwardtoreversebyrotatinga single
lever operated
4way ball valve.This
valve has twoL-shaped ports in the
ball thatdo notinterconnect,
sometimes referredto asan
"x" port.
TY
PES O
F V
ALV
ES
Ball v
alv
e
Ball Valves in sizes upto 2inch generally come insingle
piece,two or threepiece
designs. One piece
ball valves
are almost always reduced bore,are relatively
inexpensive and generally are throw-away.Twopiece
ball valves are generally slightly reduced(orstandard)
bore,they can
beeither throw-away or repairable.The3
piece
design allowsforthe center part of the valve
containing the ball,stem
&seatsto beeasily rem
oved
from the pipeline.This facilitates efficient cleaning of
deposited sediments,replacement of seats and gland
packings,polishing out of sm
all scratchesonthe ball,all
this without removing the pipes from the valvebody.
The design concept ofathreepiece
valve isforitto be
repairable.
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ALV
ES
Ball v
alv
e
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ALV
ES
Butt
erf
ly v
alv
e
Abutterfly valve isatype of flow control device,
typically usedtoregulatea fluidflowing througha
section of pipe.The valve is similar in operationto a
ball valve. A
flat circular plate is positioned in the
center of the pipe.The plate hasa rodthrough it
connectedto a handle onthe outside of the valve.
Rotating thehandleturns the plate either parallel or
perpendiculartothe flow.Unlikea ball valve,the plate
is always present within the flow,thereforeapressure
dropis always induced in the flow regardless of valve
position.
There are three types of butterfly valve:
1.Resilient butterfly valve which hasa
flexible rubber seat.Working pressure up
to 1.6 megapascals(M
Pa)/232 pounds
persquare inch
(PSI)
2.High performance butterfly valve
which is usually double eccentric in
design.Working pressure upto 5.0
MPa/725 PSI
3.Tricentric butterfly valve which is
usually withmetalseated design.
Working pressure upto 10.0MPa/1450
PSI
TY
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F V
ALV
ES
Butt
erf
ly v
alv
e
Butterfly valves are widely used in water distribution and wastewater processing
(notrecommended, asthe debris may block the operation of the disc).Butterfly
valves can come in twobodytypes,affecting installation and maintenance:
lugged or wafer.Waferstylevalves are more common.They are typically
installed between two flanges using bolts or studs and nuts.Lugstylevalves are
also installed between two flangesbutwithaseparatesetof boltsforeach flange.
The lugstylesetup makes it possibletoremoveoneside of the piping while the
other rem
ains intact.
An additional application is found within the exhaustsystem
of automobiles. By
incorporatingabutterfly valve in the exhaustsystem
,it is possibletocontrol the
backpressure and noise output from the mufflerand catalytic converter.When in
aclosed position,the valve increases the am
ount of back pressure produced and
suppresses noise.The angle of valve canbecontrolled inavariety of ways,
including manual control,vacuum control, aswellasbeing tired directlytothe
throttle.
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F V
ALV
ES
Butt
erf
ly v
alv
e
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ALV
ES
Check v
alv
e
Aclosed ball check valve
An open ball check valve
Acheck valve isa mechanical device, a valve,that
normally allows fluid(liquidor gas) to flowthrough
it in onlyonedirection.Check valves are two-port
valves,meaning they have two openings in the
body, one for fluid toenter and the otherfor fluid to
leave.There are various types of check valves used
inawide variety of applications.Check valves are
often part of common household items.Although
they are available inawide range of sizes and costs,
many check valves are very small,simple,and/or
cheap.Check valves work automatically andmost
arenotcontrolled
by a personor any external
control;accordingly, most do nothave any valve
handleor stem
.The bodies(external shells)ofmost
check valves are made of plastic ormetal.
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F V
ALV
ES
Check v
alv
e
An important concept in check valves is the
cracking pressure which is theminimum
upstream
pressureat which the valvewill
operate.Typically the check valve is designed
forand can thereforebespecifiedfor aspecific
cracking pressure.
Heart valvesare essentiallyinletand outlet
check valvesforthe heartventricles,since the
ventricles actas a pump.
An artificial heart valvemay
be
usedtosurgically replace
a
patient's dam
aged valve.
ABall check valve isacheck valve in which
the disc,the movable parttoblock the flow,is
aspherical ball. Inmany ball check valves,the
ball is spring-loaded
tostay shut, butalso many
do nothaveaspring inside.Theinterior
surface of the seats of ball check valves are
more orlessconically-tapered
toguide the ball
into the seat and/orform apositive seal when
stopping reverse flow.
Aclosed ball check valve.
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ALV
ES
Check v
alv
e
Ball check valves are often very small,simple,and
cheap(although some are expensive).They are
commonly used in liquid or gelmini-pump
dispenser spigots, spraydevices,some rubber bulbs
forpumping air, etc.,manual air pumps and some
other pumps,and refillable dispensing syringes.
Although the balls aremostoften made ofmetal,
they can
bemade of other materials,or in some
specialized cases out of artificial ruby.High pressure
HPLCpumpsand similar applications commonly
use smallinletand outlet ball check valves with
balls made of artificial ruby and seats made of
artificial sapphire,bothforhardness and chem
ical
resistance.
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PES O
F V
ALV
ES
Check v
alv
e
After prolonged use,such check valves can eventually wear out or the seat
can developacrack,requiring replacement.Therefore,such valves are
madeto bereplaceable,sometimes placed inasm
all plasticbodytightly-
fitted insidea metal fittingwhich can withstand high pressure and which is
screwed into thepumphead.
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F V
ALV
ES
Check v
alv
e
There are similar check valves where the disc is
not aball, butsome other shape.Ball check
valves shouldnot beconfused with ball valves,
which isadifferent type of valve in whichaball
actsas acontrollablerotor to stopor direct
flow.
There are check valves where the pressureon
the upstream
side mustbegreater than the
pressureonthe downstream
sideby acertain
amount,the pressure differential, forthe check
valvetoopen allowing flow.
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F V
ALV
ES
Check v
alv
e
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ALV
ES
Tap (
cock)
Cock,colloquialterm for asm
all valve
ora stopcock.
Tap isa valveforcontrolling the release ofa
liquidor gas. Inthe British Isles and normally in
the Commonwealth the word is usedforany
everyday type of valve,particularly the fittings
that control water supplyto bathtubsand sinks. In
theU.S.the usage is sometimes more specialised,
with theterm "tap"restricted
touses such
as beer
tapsand the word faucet being usedforwater
outlets;however some Americans use"tap"in the
broader senseaswell.
Indoor Tap
-commonly found in
the bathroom/laundry and/or
kitchen.ThisGerman
faucet isa
single-handle,double-spout tap
(onespoutforhot, onespoutfor
cold); most modernNorth
American faucets havea single
spout shared
byhot and cold
water supplies allowing warm
flows.
Mostwater and gas taps have adjustable flow.
Turning the knob or working the lever sets the flow
ratebyadjusting the size of an opening in the valve
assembly,giving riseto choked flowthrough the
narrow opening in the valve.The choked flow rate is
independentof the viscosityor temperatureof the
fluidor gas in the pipe,and depends only weaklyon
the supply pressure,so that flow rate is stable ata
given setting.At intermediate flow settings the
pressure at the valve restrictiondropsnearlyto zero
from the venturi effect;in water taps,this causes the
waterto boilmomentarily at room tem
perature asit
passes through the restriction.
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ALV
ES
Tap (
cock)
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ALV
ES
Tap (
cock)
Bubbles of cool water vaporform
and
collapse at the restriction,causing the
familiar hissing sound.At very low flow
settings,the viscosity of the water
becomes important and the pressuredrop
(and hissing noise)vanish;at full flow
settings,parasitic drag in the pipes
becomes important and the water again
becomes quiet.
Onereason thatmostbeer taps arenot
designed
foradjustable flow is that the
beer itself is dam
aged
bythe pressuredrop
inachoked-flow valve: holding abeer tap
partially open causes the beertofoam
vigorously,ruining the pour.
Mostolder taps useasoft rubberor neoprenewasher
which is screwed down ontoavalve seat inorder to stop
the flow.This is called
a "globe valve"in engineering
and,while it givesaleak-proof seal and good fine
adjustment of flow,both the rubber washer and the valve
seat are subjecttowear(andforthe seat, corrosion)over
time,leadingtoleakage(see photo).The washer can
be
replaced and the valve seat resurfaced
(at leastafew
times), butglobe valves are never maintenance-free.
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ES
Tap (
cock)
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ALV
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Tap (
cock)
Also,the tortuousS-shaped path the water is forced
to
follow offersasignificant obstructiontothe flow. For
high pressure domestic water system
s this doesnot
matter, but forlow pressure system
s where flowrate is
important,such
as ashower fed
by astoragetank, a
"stoptap"or,in engineering terms, a "gate valve"is
preferred.
Gate valves usea metaldisc thesamediameterasthe
pipe which is screwed intoplace
perpendicularlytothe
flow,cutting it off.There isnoresistance
toflow when
the tap is fully open, butthis type of tap rarely givesa
perfect seal when closed. IntheUKthis type of tap
normally hasawheel-shaped
handlerather than
acrutch
or capstan
handle.
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Flo
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Aflow control valveregulates the flow or pressure ofa fluid.
Control valves normally respondto signalsgenerated
by
independentdevices such
as flow metersor temperature gauges.
Control valves are normally fitted with actuatorsand
positioners. Pneumatically-actuated globe valves are widely
usedforcontrol purposes in many industries,although quarter-
turn types such
as (modified) balland butterfly valvesare also
used.
Control valves can also work with hydraulic actuators(also knownas
hydraulic pilots).These types of valves are also knownasAutomatic
Control Valves.The hydraulic actuatorswillrespondtochanges of
pressure or flow andwillopen/close the valve.Automatic Control
Valvesdo notrequire an external power source,meaning that thefluid
pressure is enoughtoopen and close the valve.Automatic control
valves include:pressure reducing valves,flow control valves,back-
pressure sustaining valves,altitude valves,and reliefvalves.An
altitude valve controls the level ofa tank.The altitude valve will
remain open while thetankisnotfull and itwillclose when the tanks
reaches its maximum level.The opening and closing of the valve
requiresnoexternal power source(electric,pneumatic,or man power),
it is done automatically,hence its nam
e.
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Gate
valv
e
22 mm Gate Valveon
domestic hot water pipe.
Gate Valves are proneto
corrosion.Here the shaft has
broken.
AGate Valve,or Sluice Valve, asit is
sometimes known,isa valvethat opensby
lifting around or rectangular gate/wedge out
of the path of the fluid.The distinct feature of
agate valve is the sealing surfaces between
the gate and seats are planar.The gate faces
canform awedge shape or they can
be
parallel.Gate valves are sometimes usedfor
regulating flow, butmany arenotsuited
for
that purpose,having been designed
to befully
opened or closed.When fully open,the
typical gate valve hasnoobstruction in the
flow path,resulting in very low frictionloss.
AGlobe valve isadeviceforregulating flow ina pipeline,
consisting ofamovable disk-typeelem
entandastationaryring
seat inagenerally sphericalbody.
Globe Valves are nam
edfortheir sphericalbodyshape with the
two halves of thebodybeing separated
byan internal baffle. This
has an opening that formsaseat onto whichamovable plug can
be
screwed intoclose(or shut)the valve. Inglobe valves,the plug is
connectedto astem
which is operated
byscrew action in manual
valves.Typically,automated valves use sliding stems.Globe
valves haveasm
ooth stem rather than threaded
and are opened
and closedbyan actuatorassembly.When aglobe valve is
manually operated,the stem
is turned
by ahandwheel.
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Although globe valves in thepasthad the spherical bodies
which gave them
their nam
e,manymodernglobe valvesdo
nothavemuch
ofaspherical shape.However,theterm
globe valve is still often usedforvalves that have such an
internal mechanism. In plumbing,valves with such
a
mechanism are also often calledstopvalves since they don't
have the global appearance, buttheterm stopvalve may
refertovalves which are usedto stopflow even when they
have other mechanisms or designs.
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Globe valves are usedforapplications requiring throttling
and frequent operation. Forexam
ple,globe valves or
valves withasimilar mechanism may
beusedassampling
valves,which are normally shut except when liquid
samples are being taken.Since the baffle restricts flow,
they'renotrecommended where full,unobstructed flow is
required.
Globe valves are typically two-port valves,although three
portvalves are also produced.Ports are openings in the
body for fluidflowing in or out.The two ports may
be
oriented straight across from each otheronthebody,or
oriented at an angle such
as a 90°angle.Globe valves with
ports at such an angle are called angle globe valves.
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Asafety valve isa valvemechanismforthe
automatic release ofagas froma boiler,
pressure vessel,or other system
when the
pressure or temperature exceeds preset limits.
They are often calledbymore specific nam
es
such
as pressurereliefvalves, T&Pvalves,or
temperature and pressurereliefvalves.
Safety valves were first usedonsteam boilers
during the industrial revolution.Early boilers
without them
were pronetoaccidental
explosionwhen theoperatorallowed the
pressuretobecome too high,either
deliberately or through incompetence
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Pre
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reducin
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r)
Oxygen Safety Valve
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Pre
ssure
reducin
g valv
e (pressure
regulato
r)
Function and design
The earliest and simplest safety valve used
aweighttohold the pressure of the steam,
butthese were easily tam
pered with or
accidentally released. Onthe Stockton and
Darlington Railway,the safety valve
tended
to gooff when the enginehit a
bump in the track. Abetter valve useda
springtocontain the steam pressure, but
these(based
onSalter spring balances)
could stillbescrewed downtoincrease the
pressure beyond design limits. In 1856
John Ram
sbottom inventedatamper-proof
spring safety valve which becam
e
universalonrailways.
Proportional-Safety Valve
Function and design
Safety valves also evolved
toprotect equipment such
as pressure vesselsand
heat exchangers.The two general types of protection encountered in
industry are thermal protection and flow protection.
Thermalreliefvalves are generally characterizedbythe relatively small size
of the safety valve necessarytoprovide protection from thermal expansion
pressure increases in liquid-packed vessels. As mostliquids are considered
fairly incompressible,it takesarelatively small am
ount offluiddischarged
through thereliefvalvetoprovide an adequate level of protection.
Flow protection is characterized bysafety valves that are considerably larger
than those mounted in thermal protection.They are generally sized foruse in
situations where significant quantities of gas or high volumes of liquid must
bequickly discharged inorder toprotect the integrity of the vessel or
pipeline.
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Pre
ssure
reducin
g valv
e (pressure
regulato
r)
Water heaters
They are required
on water heaters,
where they prevent disaster in certain
configurations in the eventa thermostat
should fail.There are still occasional,
spectacular failures of older water
heaters that lack this equipment.Houses
canbelevelledbythe force of the blast.
Pressure and temperature
safety valveon a
water heater
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Pre
ssure
regula
tor
Apressureregulatorisavalve that automatically cuts off the flow ofaliquid or
gas atacertain pressure,usuallyforthe purpose of preventing dam
ageto
plumbing.
Pressure regulators are often used at the main entrance of waterto abuilding.
They are also used at the waterinletof recreational vehicles.They allow sources
of waterto beused which come atahigher pressure than can
betolerated.
Pressure regulators don't simplystopflow when the source pressure is too high.
Rather,they reacttothe pressureontheir output side,and close when the
pressure in the plumbing reaches the designated level.Should the pressure come
down (forexam
ple,if someone weretoopen
afaucet),the valve then opens and
allows flow until the plumbing pressure goes back up(such
aswhen the faucet is
closed).
AThermostatic M
ixing Valve(TMV)isa valvethat blends hot water(stored at
temperatures high enoughtokill bacteria)with cold watertoensure constant,safe
outlet tem
peratures preventing scalding.
The storage of water at high tem
perature rem
ovesonepossible breeding ground
for Legionella;the use ofa thermostatrather than
astatic mixing valve provides
increased safety against scalding,and increased user comfort because the hot-
water tem
perature rem
ains constant.
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Therm
ost
ati
c
Many TMVs useawax thermostatforregulation.They also shut-off rapidly
in the event ofahot or cold supply failuretoprevent scalding or thermal
shock.
It is increasingly common practice around the worldtoregulate the storage
water tem
peraturetoabove60°C,andtocirculate or distribute water ata
temperaturelessthan
50°C.Water above these temperatures can cause life
threatening scald injuries.Many countries,states,or municipalities now
require that the temperature of all bath water in new
build and extensively
refurbished domestic propertiesbecontrolled
to amaximum of48°C.
Installing Thermostatic M
ixing Valves (TMVs)can ensure that water is
delivered at the required tem
perature thereby reducing the risk of scalding
accidents;it also makes the hot water supply last longer than
onethat is
maintained atalower tem
perature.
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ost
ati
c
VA
LV
E S
IZIN
G
Flo
w c
oeff
icie
nt
kv,
cv
Pressure drop
Resistance coefficient
Density of fluid
velocity
Vk
p
pv
vo vo
.
=∆
∆ρ
ρ
VA
LV
E S
IZIN
G
Flo
w c
oeff
icie
nt
kv,
cv
Seat area
Orifice equation
metric valve flow coefficient
Uw
ag
a.
W s
kry
pcie
jest
błą
d w
tym
mie
jscu
VA
LV
E S
IZIN
G
Flo
w c
oeff
icie
nt
kv,
cv
liquid
gas
steam
saturated
steam