20 Compressor Valves
CHAPTER20
COMPRESSOR VAL VES
Walter J.Tuymer
Hoerbiger Corporation of America,Inc.
Dr.Erich H.Machu
Consulting Mechanical Engineer
Hoerbiger Corporation of America,Inc.
20.1PURPOSE
Compressor valves are check valves that control the?ow into and out of a com-
pressor cylinder.There has to be at least one suction valve and one discharge valve
in every compression chamber.
20.2HISTORY
The modern era of compressor valves started in1897when the Austrian engineer,
Hanns Hoerbiger,designed and patented a steel plate valve intended for a low
pressure air blower in a steel mill application.Interestingly,this design employed
a frictionless guided valve plate useful for non-lubricated compressors,a feature,
which until the introduction of non metallic materials to compressor valves was
poorly achieved by most other valve designs.
In1910,Hans Mayer patented a different valve design that became known as the‘‘Feather valve.’’This design uses several?exible steel strips as sealing ele-
ments and became the standard for the Worthington Company for many years.
In1931,Ingersoll Rand patented yet another valve design that became known as the‘‘Channel valve.’’The channel valve employed several strips with a cross
section like a‘‘U’’(therefore the name‘‘channel’’),each supported by a leaf spring.
It was the standard valve for the Ingersoll Rand Company and was probably pro-
duced in higher numbers than any other compressor valve.
A valve design older than all the others was rediscovered in the late1950’s with
the event of pipeline compressor applications and the availability of high strength
plastics,in this case nylon.Thompson Industries developed this poppet valve for
20.1
20.2CHAPTER TWENTY
the Clark Brothers,a compressor manufacturer in Olean,New York.This valve
proved to be extremely ef?cient in low compression ratio applications.
20.3SURVEY OF VALVE DESIGN
All automatic compressor valves have several basic components in common:
?Valve seat
?Sealing element(s)
?Lift constraint(guard)
?Spring(s)
The?ow passages in a valve can be arranged in various different ways:
?Circular rows of ports
?Parallel series of ports
?Irregular arranged number of holes
20.3.1Valve Designs Used in Air and Gas Compressors
Ring Valve.Ring valves are probably the most commonly used valves in air as
well as gas compressors.A properly manufactured valve ring is a very simple
element with a perfectly uniform stress distribution.It therefore has a high tolerance
for impacts.From a valve designer’s point of view,rings can easily be guided and
the utilization of the available area is good(for manufacturing reasons most valves
are round).The disadvantage of individual rings is the need for each ring to be
separately spring loaded.Since the speci?c spring load for each ring cannot be
perfectly identical and because the?ow distribution across a valve is not uniform,
it is dif?cult if not impossible to make all rings move uniformly.On the other hand,
due to the possibility of individual motion of each ring,this design is more tolerant
to liquids than other designs.
Several ring valve designs which are used are discussed below.
Simple Ring Valve.This design uses plain rings guided in the valve guard.The valve springs can either be small individual coil springs,separate for each ring,or
slightly larger springs supporting two rings.Some older valve designs use one large
coil per ring.Wafer or lentoid springs are used in smaller valves.The advantage
of these springs is their low space requirement,allowing for extremely thin valve
guards and therefore low clearance volume.The disadvantage of these springs is
the limited possibility of adaptation to different operating conditions.For this rea-
son,they are mostly used in air compressors.
Damped Ring Valve.The only damping system used in ring valves is gas damp-ing.This design uses very thick valve rings guided on the full diameter in a closely
?t groove in the valve guard.In theory,when the valve opens,the gas beneath the
COMPRESSOR V ALVES
20.3
FIGURE 20.1Ring and spring set for ring
valve with one spring covering 2rings.
FIGURE 20.2Ring valve with wafer springs.
valve ring is trapped in the guard groove and has to be squeezed out through the narrow passages on either side of the valve ring.The effect of damping largely depends on production tolerances,state of wear,and/or presence of liquid or solid contamination in the gas.On valves with non-metallic valve rings and steel guards,the different coef?cient of thermal expansion makes this an almost impossible task for the designer.It follows that the application for this damping system is limited to metallic sealing elements.
Contoured Ring Valves.Some valve designs utilize a contoured valve ring to-gether with a heavily chamfered or contoured valve seat groove.This is done to achieve a lower ?ow resistance in the lift area where,with conventional designs,the gas has to make two 90?turns.On the other hand,for given valve lift and given total length of seat lands,the geometric valve port area is reduced in the proportion of sine of the angle of ?ow deviation.
All these valves use non-metallic valve ring materials.Since these materials have a different thermal expansion coef?cient than the valve seat material,there may be a leakage problem either in the cold or hot condition of the valve.This problem is reduced by a certain ?exibility of the valve ring which allows it to ‘‘roll into’’the valve seat groove.
20.4CHAPTER TWENTY
FIGURE20.4Contoured ring
valve(courtesy Cook Manly).
COMPRESSOR V ALVES
20.5
FIGURE 20.5Double damped valve.
FIGURE 20.6Velocity diagram of mass damped
valve.
plate travels the ?rst portion of the lift alone and then collides with the damping plates.During this impact,linear momentum is conserved,but kinetic energy is destroyed.In other words,the energy required to accelerate the mass of the damp-ing plates results in a reduction of the velocity of the valve plate itself and con-sequently reduces the ?nal impact velocity against the valve guard.As an extra bonus,the valve plate tends to be leveled when contacting the damping plates.Channel Valve.By numbers,this valve design is by far the most widely used compressor valve in the Western hemisphere.It uses a number of straight sealing elements with the U shaped cross section,therefore the name channel valve .Each individual channel is spring loaded by a leaf spring and guided on both ends of
20.6CHAPTER
TWENTY
FIGURE 20.7Channel valve (courtesy of
Dresser Rand ).
FIGURE 20.8Feather valve
(courtesy of Dresser Rand ).
the channel by a comb like guide.This valve design is very ef?cient in low to medium pressure,low to medium speed compressors.
Feather Valve.This valve design is intriguing for its apparent simplicity.It uses a leaf type sealing element which is allowed to bow into a machined recess in the guard.The leaf is therefore also its own spring.Feather valves are no longer used in new compressors and their application was limited to low and medium pressure compressors with clean gas service.
Poppet Valve.Poppet valves use rather large (approx.7/8inch diameter)holes in the valve seat and for each of these holes there is a mushroom shaped sealing element,called a poppet.Each poppet has its own valve spring.The original pop-pets were made of bronze,which due to its high mass,was practically useless.The advent of Nylon made this design the valve of choice for low compression ratio,low speed compressors.Valve lifts of up to 3/8inch were commonly used and in pipeline service this valve’s ef?ciency was unequaled.
COMPRESSOR V ALVES20.7
FIGURE20.9Poppet valve(courtesy
of Dresser Rand).
When PEEK(Polyetheretherketone,1a polymer)became available,poppet valves were also successfully used in higher compression ratios.With reduced valve lifts necessary for these applications,the ef?ciency of the standard poppet valve should be checked against other valve designs.
For high speed compressors,a variation of the common poppet valve was de-veloped.This design utilizes a much smaller poppet—called Minipoppet—and therefore a smaller valve seat hole.Lower valve lifts without reducing?ow area are possible with this design.
Double Deck Valve.In conventional cylinder designs,there is limited space avail-able for compressor valves.If the valve area achieved is insuf?cient and added clearance volume can be tolerated,double deck valves can be used.Double deck valves can be of different basic valve designs such as ring-,plate-,poppet-or feather valves.In all but the feather valve design,a second valve is positioned upside down above the?rst valve.The two valves are separated by a spacer and held together by means of a sleeve.The gas?ow to the bottom deck is straight from the cylinder;the?ow to the top deck is through the sleeve and then reversed by means of a cover.The combined valve area of a double deck valve is approx-imately40%larger than that of the same size single deck valve.
Deck-and-One-Half Valve.Deck-and-one-half valves are valves where a full deck valve is positioned above a ring type bottom valve and the?ow through both valves is in the same direction.These valves provide an even larger valve area than double deck valves,but require a very deep valve pocket since the cylinder porting has to be above the top deck valve.A cone type spacer is normally used to separate the two valve decks and provides?ow access to the top deck.The total valve area of a deck-and-one-half valve can be60to65%larger then the same size single deck valve.
20.8CHAPTER TWENTY
FIGURE20.10Double deck poppet
valve.
Concentric Valves.Concentric valves are valves where a suction and a discharge
valve are arranged concentrically either within one body,or as two individual
valves.Either the suction or the discharge valve can be arranged in the center,the
other valve is then positioned around it.These valves are normally used on single
acting cylinders and are positioned on the face of the cylinder.The original objec-
tive was to make maximum use of available cover area with minimum clearance
volume.This is needed in compressors with very high compression ratios as seen
in refrigeration compressors and small air compressors.Small displacement high
pressure cylinders also use this valve design.
Reed Valve.The domain of the reed valve encompasses small refrigeration and
air compressors.A reed valve normally consists of a single seat plate with valve
reeds positioned on both sides of the seat plate.The reeds themselves are made of
thin strip steel and can have almost any shape.The reed is held on one end and
covers a port with the other end.The valve opens when the differential pressure
starts to bend the reed away from the seat plate.This elastic deformation of the
reed also acts as the valve spring and no other springs are used.
There are reed valve designs using two seat plates with a spacer(normally a gasket)and the valve reeds positioned between the seat plates.The advantage of
this design is a positive stop for the valve reed when the valve is open;the dis-
advantage is its high clearance volume and a higher cost compared to a single seat
plate design.Due to the low mass of the moving element—the valve reed—this
design can be used at very high compressor speeds.Reed valves are commonly
used in compressors up to3600rpm and have been successfully tested at speeds
up to7000rpm.
COMPRESSOR V ALVES20.9 20.4THEORY
20.4.1Principle of operation
The essential part of every compressor valve is the sealing element.It is usually
spring loaded and allowed to move between two stops,the valve seat and the valve
guard.When resting against the valve seat the valve is closed,when against the
valve guard,it is considered to be fully opened.The distance the sealing element
can travel between these two stops is called the valve lift.
The sealing element is made to move by the action of a gas force and a spring force rather than mechanically.The forces of gravity or inertia forces due to ma-
chine vibration are usually small and can be neglected.Other forces are undesired
like the viscous sticking force due to the presence of liquids,but sometimes have
to be taken into account9.
When the valve is opened,the?owing gas will exert a dynamic gas drag force on the valve plate which is of the order of magnitude of the pressure drop of the
gas?ow across the valve multiplied by valve plate area.When closed,the valve
has to be able to support the full static pressure difference between suction and
discharge pressures,which can be very high.The?exural strength required to resist
this pressure difference is supplied by the valve seat.
Special attention has to be given to the motion of the sealing element during the opening and closing events.In fact,in most cases,it is the impact of the sealing
element against one of its stops that causes the valve to fail.Small impact velocities
are a condition necessary to assure long valve life.Other reasons for premature
valve failure may be wear,erosion and clogging due to solid or liquid contaminants,
poor maintenance or simply mechanical overload.
20.4.2Relationship between Valve Design and Compressor
Ef?ciencies and Work
The compressor ef?ciencies of interest are
?The energy ef?ciency or isentropic ef?ciency?is,comparing work requirements
of the real machine and ideal machine operating under identical conditions of
nominal suction pressure and temperature,nominal discharge pressure and gas
composition
?The volumetric ef?ciency?vol or better ef?ciency of delivery?,comparing stroke
volume with the volume of gas at nominal suction conditions delivered per cycle.
The amount of work required by a compression cycle is the amount of energy
required by one working chamber(called head end or crank end depending on the
location with respect to the crankshaft)to take a mass of gas M from the suction
line,compressing it inside the cylinder from suction to discharge pressure,then
delivering M to the discharge line,and re-expanding the gas remaining in the
cylinder to suction pressure.
20.10CHAPTER TWENTY ?The isentropic part W is which is theoretically required by the ideal machine of same size working without losses under the same external conditions,i.e.with identical gas composition,suction and discharge pressures,suction temperature.In the real machine however,the gas inside the cylinder at the beginning of the compression stroke will be at a temperature T 1,s generally higher (for reasons explained below and given in more detail in 12)than T 1,consequently already the isentropic work W is to be expended per unit mass of gas will be increased in the proportion of T 1,s /T 1.
?And an additional ?W covering the losses occurring in the real machine,corre-sponding to the work losses due to pressure drops in valves and valve pockets,repeated expenditure of work for recompressing gas that has leaked back during a previous cycle,heat exchange ‘‘in the wrong moment’’causing the process to be polytropic,mechanical friction between moving parts like piston rings and packings,etc.The term work loss is in fact a misnomer and in apparent con?ict with the principle of conservation of energy or the ?rst law of thermodynamics.What really happens is that energy of a high value (mechanical or electrical energy coming from the driver)is converted into useless energy,useless in the sense of the second law of thermodynamics,i.e.heat energy at a low temperature level whose presence is even a nuisance.
Since the days of James Watt some 200years ago,it is common practice to measure total work exchange between a piston and the gas inside a cylinder as the area of the PV -diagram or indicator diagram.The ratio between the work required by the ideal machine and the work required by the real machine is called energy ef?ciency or isentropic ef?ciency
100?W is ??[%]is T 1,s W ??W is T 1
V olumetric ef?ciency ?vol is de?ned geometrically and can be taken from an in-dicator diagram.
1/?Z P 12??100?s ?1[%]
????vol Z P 21where
s
total cylinder clearance [%]Z 1,Z 2real gas compressibility at nominal suction and discharge conditions P 1,P 2nominal suction and discharge pressures ?exponent of an isentropic temperature change in the range between P 1and
P 2,with an ideal gas,?C p /C v
The ef?ciency of delivery ?is equal to volumetric ef?ciency ?vol minus a sum of volume losses
COMPRESSOR V ALVES20.11
????(volume losses)[%]
vol
While the in?uence of the valve on?
vol is limited to its contribution to clearance
volume s,the volume losses responsible for the difference between?
vol and the
ef?ciency of delivery?depend to a great part on design and state of wear of the valves,in particular:
?Temperature corrections to account for the fact that the gas inside the cylinder at the beginning of the compression stroke will be at a higher temperature than
T 1and hence lighter.This heating results from the conversion of the part of?W
corresponding to the work loss in suction valves and pockets into heat dissipated
in the gas during the intake cycle.In addition,there can be heat exchange between
the gas and the walls of the suction plenum,of the suction valve and of the
cylinder.
?V olume losses due to leakage in valves,piston rings and packings.With the
exception of packings,the impact of leakage is twofold:it reduces the amount
of gas and increases the temperature.
?V olume losses due to excessive valve throttling
?V olume losses due to late valve closure
?Pulsations can cause compression to start from a lower pressure than p1thus
producing a volume loss.
An analytical treatment of this problem area based on differential equations is given in Ref.11,based on simple algebraic equations in Refs.12and14.It follows
that both,isentropic ef?ciency volume losses due to leakage in valves,piston rings
and packings.Except for leaking packings,the impact of leakage and ef?ciency of
delivery are affected by valve losses,which therefore have to be minimized.On
the other hand,valve life should be as long as possible.
Unfortunately,both postulates are contradictory:For a given valve design,good ef?ciencies mean small?W and therefore small pressure drops in the valves,hence
large?ow areas and a high valve lift.On the other hand,a long lasting valve means
small impact velocities and hence a small lift.It is up to the skill and the experience
of the valve engineer to?nd the best possible compromise by properly selecting
valve design,lift,springs and materials.
20.4.3Motion of Sealing Element
Under ideal conditions the opening motion of the valve sealing element is initiated
by the pressure differential across the valve overcoming the pre-load of the valve
springs.The acceleration of the element is a function of the gas drag force and an
elastic force provided by the valve springs.
During the opening period of the valve its instantaneously available valve area starts at zero and gradually increases to its full?ow area.At this time however the
piston moves with a certain velocity.Depending on which changes faster,the vol-
20.12CHAPTER TWENTY
COMPRESSOR V ALVES20.13
20.14CHAPTER TWENTY
COMPRESSOR V ALVES 20.15only be found by sophisticated calculations of mechanical strength,usually by using ?nite element methods.
Lift Area.Lift area equals the total length of seat lands multiplied by the valve lift.The seat lands area is where the sealing element seals against the valve seat.With increasing total length of the seat lands,the risk of valve leakage also in-creases.
Guard Area.Its de?nition is analogous to seat area.
Required Geometric Valve Lift Area.In the past,valves have been sized for an application primarily by using the valve velocity as the main criterion.Guidelines for acceptable velocities have been established by a number of companies.The valve velocity certainly has an in?uence in sizing a valve,but in itself is not suf?cient.In terms of losses the important criteria are the pressure drop across the valve and the assurance of full pressure recovery inside the cylinder at piston dead center,i.e.allow the cylinder pressure to recover fully to the line pressure at the end of the stroke.An indicator—the q —value was introduced 11representing the adequacy of a valve.
2bore ???2?stroke ?RPM 4v ?m n ?f ?12
e where
v m valve velocity ft/min bore
cylinder bore inches stroke
piston stroke inches n
number of suction (discharge)valves per cylinder end f e lift area for one suction (discharge)valve in sq.inches ? 3.14159
22??f 100s e q ???v ????SV
m 8p ?144?3600?g s where
?s suction gas density lb/cu.ft p s suction pressure psia ?equivalent area sq.inches q SV relative pressure drop %of suction line pressure g acceleration due to gravity ?32ft/s 2
q SV is a dimensionless parameter used in the differential equations describing pres-sure and temperature evolution inside the cylinder.11For normal ?ow through a
20.16CHAPTER TWENTY
COMPRESSOR V ALVES 20.17where
A lift area
1A area between elements
be A seat area s k ?.851k ?.66
be k ?1
s From the suction ?ange of a cylinder to the compression chamber,the gas passes not only through the valve but also through internal cylinder passages,the valve cage and from the valve through the valve port.Every one of these passages po-tentially contributes to the total losses incurred which are normally called total ventilation losses.The ratio of total ventilation losses over valve losses can be quanti?ed using a pocket loss factor.8
20.4.6Valve Springs
The function of valve springs is to close the valve in time.The objective is that all valves should be closed when the compressor piston reaches dead center.Since the valve sealing element has to travel the whole valve lift from the fully opened to the fully closed position,the closing motion has to start before dead center and therefore against the gas ?ow.If the valve springs are too weak,the valve will close late (after the piston passes the dead center position)and gas is allowed to ?ow back through the valve causing the valve sealing element to slam closed.On the other hand if the valve springs are too heavy,the valve will close early.Since the piston continues to travel toward dead center,the pressure differential between cylinder and line will increase and the valve will reopen.An extreme case of too heavy springs leads to valve ?utter.Tendency to valve ?utter increases if the spring load considerably exceeds the values given in the two formulas below.Small pres-sure drops together with high valve lifts require springs with low spring rates and thin wires lacking sturdiness.In addition,these springs have to be long and have a tendency to buckle.
p ?.4?q ?p f SV s
for a suction valve and
?
p d p ?.4?q ?p ???f DV s p s for a discharge valve,provided q DV is calculated as if it was a suction valve,https://www.360docs.net/doc/1113307309.html,ing v m ,?e and ?of the discharge valve,but ?s and p s .
Both above formulas are approximate and valid for pressures up to about 500
[psia].For other applications,it is indicated to verify spring selection by means of motion study.Properly selected valve springs have practically no effect on the valve opening.
20.18CHAPTER TWENTY
20.4.7Theoretical Valve Motion
All the formulas shown above have been found empirically and are valid only for
a typical range of parameters.They can be used to conventionally size valves,lifts
and springs for one and only one given operating condition.However if parameters
are outside typical ranges or if operating conditions are variable,these methods
fail.They cannot tell us whether a valve sized and tuned for one condition can
safely be used for another condition as well.To answer such a question,the inter-
action between valve and compressor has to be modeled.The result is a system of
at least three ordinary differential equations:one for the changes of valve lift;one
for cylinder pressure;and one for cylinder temperature.These equations are too
complex to be integrated analytically and have to be solved numerically in steps
by one of the well known algorithms(Runge-Kutta,predictor-corrector and simi-
lar),necessitating a considerable amount of computation.
In its simplest form,this approach was published almost half a century ago.1 Since then,particularly since the availability of powerful computers,the models
have been extended.For lack of space only some typical improvements shall be
mentioned here,for example including gas pressure pulsations,5considering gas
inertia in the valve ports,4,7,10the so called gas spring effect,10gas leakages,11heat
exchange,6,11,12higher modes of a valve reed,3more degrees of freedom of the
valve plate’s motion to predict the higher impact velocities resulting from cocking
of the valve plate,13etc.All this is to show that the problem is in fact rather
complex.
When interpreting computed valve motions one should therefore always bear in mind that
?The models currently in use are far from being physically comprehensive,and
real motions may look more or less different.
?The coef?cients of?ow resistance and drag force and their variation with valve
lift as used in the model may be different from those in the real valve.
?External data like pressures,temperatures,gas analysis,pulsation patterns may
vary.
?The gas may be contaminated,liquid may be present.
?Numerical algorithms are always subject to local loss of precision,particularly
in the vicinity of piston dead center positions and when valve pressure drops are
small(usually with q-values?2%).
The results of these motion studies depend therefore to a large degree on their
interpretation.As general rules the following can be used:
?A perfect looking valve motion is usually not as good as it looks,since the
absence of rebounds on the seat is often due to late closure and therefore dan-
gerous.
?A normal valve opening with no or little rebound on the guard is desirable.
Although computed rebounds do not necessarily represent reality,they largely
COMPRESSOR V ALVES20.19 depend on the way the impact is modeled and the arbitrary choice of coef?cients
of https://www.360docs.net/doc/1113307309.html,ually they are inoffensive.
?The valve should stay open without?utter during most of the piston stroke.
?An early valve closing(before dead center)is preferred.
?Ignore rebounds which start shortly before the piston reaches dead center at the
end of valve closure.
?The model computes impact velocities.A sealing element fails due to excessive
local stresses in the material.One of the problems,therefore,is to convert impact
velocities into stresses,and set allowable limits for them.These limits depend on
the valve design,material choice,quality of production and also on the compu-
tational model used and the experience gained with it so far.
Some hints may be taken from the theory of one-dimensional wave propagation in an elastic medium.With this assumption,the pressure amplitude?p(?change
in pressure?stress?)is related to?u(?velocity amplitude?impact velocity)
by?p???c??u,where?is the density and c the velocity of sound in the
c??E/?.
sealing element material,given by Thus,knowledge of very basic ma-
terial properties such as density?,modulus of elasticity E and allowable stress?
is suf?cient to estimate allowable impact velocities?u.This method?nds about
1100ft/min for valve plate steel,and2200to3900ft/min for PEEK depending
on temperature.With all the imperfections of theoretical valve motion studies ex-
plained above and doubts as to the applicability of linear wave propagation theory,
it seems indicated not to exceed one-third of these values(‘‘factor of safety?
factor of ignorance’’).A more sophisticated approach,still using the theory of wave
propagation in elastic media,but considering the properties of the seat as well,can
be found in Ref.16,laboratory results in Ref.17.
The proper interpretation of a theoretical valve motion study is a matter of experience.People with little or no experience should consult with an expert before
drawing wrong conclusions.
20.5VALVE MATERIALS
For most applications,normal carbon or alloy steels are used for the valve body.
The determining factor is the necessary valve seat strength to withstand the differ-
ential pressure the valve is subjected to.For corrosive gases,martensitic or austen-
itic stainless steels are used and some highly corrosive applications require Monel,
Hastelloy or similar materials.For aggressive gases such as hydrogen sul?de,sep-
arate speci?cations18have been established and should be followed.
20.5.1Valve Sealing Elements
In the past,most valve sealing elements have been manufactured from heat treated
martensitic stainless steel like AISI410or420.For more corrosive gases,precip-
20.20CHAPTER TWENTY
itation hardening steels(17-7PH)or Inconel X750were used.These materials are
still in use for high temperature and high pressure applications or when?nger
unloaders are used and the valve plate de?ection in unloaded condition is high.
Today’s materials of choice are injection molded composite materials with Nylon 6/6(30%glass-?lled)being the best choice providing the operating temperature
is within the limit of this material.The maximum allowable operating temperature
for Nylon6/6varies from manufacturer to manufacturer,but should be approxi-
mately250?F or less,depending on the operating pressure.For higher temperature,
PEEK(Polyetheretherketone)is used.The temperature limit for this material again
varies depending on the manufacturer and is approximately450?F.Constantly,new
materials are developed especially by the aerospace industry,which are then used
for compressor valve parts.
20.5.2Valve Springs
The most widely used compressor valve spring material is a precipitation hardening
stainless steel of type17-7PH.For highly corrosive application nickel alloys such
as Inconel X750or Nimonic90are used,with some special applications using a
multiphase material type MP35N or Elgiloy.
20.6VALVE LIFE
In ideal conditions,a properly designed valve has almost in?nite life.Unfortunately,
this condition is never found.Contamination in the gas in the form of foreign
particles or liquids,including oil,corrosion as well as poorly designed or applied
valves with inferior dynamic behavior result in?nite life of compressor valves.
?Foreign Particles:Depending on the materials used for the valve components and
the size and substance of these particles,they do more or less damage.The
important thing to recognize is that they do damage.Very small particles normally
pass through any valve design,but they cause abrasion on the valve seat and the
sealing element,eventually leading to valve failure.
Somewhat larger but still small particles can cause breakage of the sealing element when metallic sealing elements are used but are normally embedded into
the material on non-metallic sealing elements.
No valve will survive if large,hard parts are passed through it.
?Liquid in the gas is also detrimental to valve life.Valve designs with individual
sealing elements possibly made of non-metallic materials tolerate liquids better
than valves with one solid sealing element.Liquids drops impact on the sealing
element causing a deformation on a single element valve which can result in
instant breakage.In single element valves,the one element struck by the liquid
drop can move independently not resulting in any damage.
The effect of‘‘oil sticktion’’was explained above in‘‘Motion of Sealing El-ement.’’