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A new valve concept for
high-speed gas compressors
4. The sticking effect and its resultant adhesiveforces are
normally encountered when liquids (e.g. lubeoil, water or
condensate) or other sticky contaminantsare caught between the
retainer or seat and the sealingelements.
The sticking effect is detrimental because it is coun-teractive
to normal valve motion.
All other forces mentioned earlier act in the maincontinuously
during the motion cycle onto the sealingelement. The sticking
effect tends to prevent the begin-ning of the motion until the gas
force can overcome theadhesive reaction. As soon as the adhesive
bond isbroken, the sticking effect and its force is zero, and
theincreased gas flow force results in abnormally highacceleration
of the moving valve component. The delay-ed valve action and
subsequent acceleration results in
heavier than normal impact of the valve element onto theseat. A
similar effect can be experienced in the openingmotion of the valve
where sticking of the sealing elementto the seat can occur.
Figure 1 shows a valve motion diagram with a varyingdegree of
heavy oil residue. The sticking effect has twoelements:
An adhesive counter-force which is time dependent;the shorter
the time the greater the necessary force, theeffect of which is
more critical as the RPM of thecompressor increases.
The adhesive effect can be theoretically defined andcorrective
mathematical values can be used in calcula-tions. Such values are
generally derivatives of experi-ence and empirical
measurements.
A0V016E01HZ10XV016E
For decades the HOERBIGER plate valve has beenproving its
advantages; namely, good flow characteris-tics, high reliability,
long life, simple maintenance andthe possibility to regulate the
suction valve with flowefficient unloaders.
The preceding features also typify the essentialadvantages of
the plate valves versus ring valves.Correct valve operation for a
given compression condi-tion is a key to long life of all
self-actuated sealingelements.
The motion of a free-floating valve element is de-pendent on the
following forces:
1. Gas flow force2. Weight of the moving component3. Friction
involved in the opening and closing
movement4. The sticking effect5. The spring force
1. The flow force is governed mainly by the thermo-dynamic
events inside the cylinder and the momentaryposition of the sealing
element in the overall motioncycle. Additional influence comes from
the pressurepatterns and/or pulsations of the manifold side of
thevalve. They will be discussed under the aspect of highclearance
later on in this article.
2. Gas velocities frequently found in today's com-pression
equipment are such that the weight of the
moving valve element can be neglected, especiallywhen the
sealing element is of a low weight as on to platevalves. Needless
to say, that in computer programscalculating valve motion the
weight of the moving com-ponent can easily be considered.
3. Friction forces are non-existent with frictionlessguided
HOERBIGER valves. They are present in valvedesigns with sliding
guidance but, generally, friction isso low that it can be ignored.
An exception might bewhere side forces are present which result in
higherfriction forces in the valve movement. Fig.1: Plate motion of
a suction valve plate
1 2 3
Valve
lift
closed
open
Crank angle
1 Valve in normal condition2 Valve slightly overlubricated3
Valve overlubricated
HOERBIGER CORPORATION OF AMERICA, INC.
3350 Gateway Drive, Pompano Beach, Florida 33069Phone (954) 9
74-5700 or Toll-Free (800) 327-8961 Fax (954 ) 974-0964
Internet http://www.hoerbigercorp.com E-mail
[email protected]
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5. The spring force is correctly defined by taking intoaccount,
aside from other values, the four forcesmentioned before. Modern
computer programs basedon a long history of experience assist in
the properselection of the spring force.
One can now appreciate, that it is easier to apply thecorrect
spring load to a webbed valve plate than toseveral individual
rings. Each ring, in turn, would have tobe individually and
correctly spring loaded which isdifficult simply from the point of
the prevailing geometryof these components. Frequently, it is only
possible byusing a large number of springs which makes thesevalves
service intensive and costly to maintain. A platevalve is also more
conducive to later changes or tailoringof the spring force to cope
with changes in the operatingconditions.
Many of today's valves are designed for minimumpressure loss by
varying the number and shape of theseat and guard ports, the ports
of the sealing element
and by applying the highest acceptable valve lift.
Brakehorsepower is saved and the compressor efficiency
isimproved.
Frequently, valve life is sacrificed in this effort. Ex-tremely
narrow valve rings or excessive valve lifts oftenresult in
premature valve failure. The resultant down-time and cost of repair
often defeat the desired increasein efficiency.
For considerable time, HOERBIGER has employedvarious principles
of dampening the valve motion, andthe product has been recognized
for its excellent valvelife.
A new valve concept is introduced here which com-bines optimum
flow with a new damping system; a valvedesign that can cope with
today's challenges and designrequirements.
The new HOERBIGER-CT Valve series is theanswer to the following
performance requirementswhich are expected with increased frequency
in today'smarket.
The operating conditions of compressors are vari-able; RPM,
pressures and gas composition fluctu-ate and valves cannot always
be tailored to thesevariances.The presence of contaminants in the
gas stream hasincreased. Gases contain liquids, elements of
lubri-
cation and even solids.Abnormally high clearance volumes are at
timesprovided to allow reduced output even at low com-pression
ratios.
These high clearances often cause valve failures,especially in
discharge valves, an effect whichbecomes obvious when pulsations in
the valvepockets are taken into account.Figure 2 demonstrates such
a condition where thevalve motion of a discharge valve is
calculated bymeans of a computer for 3 different clearances(10 %,
20 %, 50 %) without consideration of pulsa-tions (above) and with
consideration of a pressure
pulsation of 2.5 % (below) in the valve chamber.The influence of
the larger clearance volume onclosing is substantial.
Figure 3 shows the basic design features of the CT valvewhich
are described in more detail as follows:
For reasons mentioned before the plate valve de-sign was
selected.A valve plate made from a high-temperature, high-strength
polymer seals against the ports of the seat.A thin, steel cushion
plate identical in port configu-ration to the non-metallic valve
plate rests on thevalve plate. The support plate can be flat or
slightlydished in its free state. Both the valve plate andsupport
plate cover a wider port on the outer perime-
ter of the valve.Coil springs are located only on this outer
port andpre-load the plate evenly on the outer circumfer-ence.The
inner sector of the valve plate and support platecomes in contact
with a wafer-shaped cushion platewhich is positioned against the
retainer.
Fig.3: Design of HOERBIGER-CT valves
Fig.2: Diagrams of discharge valve lift at differentclearance
volumes
270 300 330 270360 300 330 360 330300 360
0,0
0,2
0,4
0,6
0,8
1,0
20 % 50 %
270 300 330 270360 300 330 360 330300 360
0,0
0,2
0,4
0,6
0,8
1,0
20 % 50 %
Without pressure pulsation in valve chamber
Valve
lift
Crank angle
10 % Clearance volume
With pressure pulsation ( 2,5 %) in valve chamber
Valvelift
Crank angle
10 % Clearance volume
2
1
3
4
5
1 Valve seat2 Non-metallic
valve plate3 Steel plate4 Coil springs5 Wafer plate
Discharge
Suction
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Fig.5: Typical assembly sequence for CT-Valve
This valve concept combines several advantages:
Positioning the coil springs on the outer perimeterstabilizes
the motion and reduces wobbling of the valveplate.
Concentration of opening and closing impacts at apoint on the
outer edge of the sealing element is sub-stantially reduced.
Thermoplastic valve plates are generally able to holdup under
high, localized stresses. They are known toabsorb foreign particles
effectively. They tend to be lesstolerant to bending stresses,
however, where steelgenerally outperforms non-metallic plates.
The steel support plate enhances the non-metallicplate's
capability to absorb bending stresses. Bending iscaused by the
dominant forces acting on the valve plateat two points offset from
one another.
Purposely employing a balanced elasticity of thenon-metallic
valve plate and its steel support plate can
create an effect that brings on a phased motion of theinner
versus the outer perimeter of the two plates.
Figure 4 shows the time phase motion during theclosing cycle of
the valve.
Similarly, it stands to reason that the flexibility of theplates
is also advantageous to cope with the stickingeffect caused from
liquids or oil.
It is easier to separate an elastic component from aflat and
rigid surface than to do the same with two rigidparts. A minimum on
elastic bend is adequate since theoil film that causes the bond is
generally very thin.
The wafer plate resting against the retainer cushionsthe opening
impact of the inner sector of the valve plateagainst the retainer.
It also substantially improves theseparation of the plates upon
their closing motion. Thesticking effect is greatly reduced since
the cross-sectionof the wafer plate is smaller than the width of
the valveplate and support plate, and the waviness further
mini-mizes the bond.
The outer ring being wider than all the inner rings
provides extra strength and resistance against impactsfrequently
found in this area.
Summary:
The valve concept demonstrated here using a non-
metallic sealing element and a steel support plate pro-vides
more elasticity than a solid steel plate of equalthickness. It has
a greatly reduced tendency to wobble.It protects the sealing
element from heavy impacts andsupports the non-metallic valve plate
in bending stres-ses. The sticking effect is virtually eliminated
throughfeatures designed into the valve.
This new valve design provides the optimum an-swers to today's
requirements in high-speed compres-sion equipment. Initially, tests
were performed to sub-stantiate the design objectives, and these
tests werelimited to in-house installations. Over the last two
years,controlled tests were performed in real life conditions
in
the field which further confirm that the merits of the
newHOERBIGER-design are very much evident.
Fig.4: Plate motion measured on a HOERBIGER-CTdischarge
valve
1
22
1Valve
lift
closed
open
Crank angle
1 Valve plate periphery(spring loaded)
2 Valve plate center
TDC
