Note: Descriptions are shown in the official language in which they were submitted.
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Turbine Meter With A Rotor Having Accuracy
Enhancing Rotor Blades
Backgiround of the Invention
This invention relates to turbine meters of the type used to measure the
flow of fluids by converting kinetic energy of a flowing fluid to rotation of
a
turbine, and more specifically, to rotors and rotor blades for such turbine
meters.
Turbine meters are used to measure the flow of fluids by converting
kinetic energy of the flowing fluid to rotation of a turbine. While turbine
meters
can measure both the flow of liquids and the flow of gases, the theory of
operation of gas turbine meters differs somewhat from that of liquid driven
1A meters due to the differences in the density and kinematic viscosity of the
two
fluids.
Since liquids are essentially incompressible, the density of liquids does
not vary significantly with pressure or temperature. Also, the density of
liquids
is relatively high so there is ample driving torque from liquid flow to
overcome
1a mechanical friction in the meter. Thus, small changes in retarding torques,
for
example due to increases in friction between moving parts, do not affect the
performance of liquid turbine meters. Conversely, the density of gas is
relatively low so that gas turbine meters are highly sensitive to changes in
retarding torques within the meters, especially at low pressure and low flow
2~ rates. Changes in kinernatic viscosity, however, do affect the performance
of
both gas turbine meters and liquid turbine meters.
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Referring to a gas meter by way of illustration, the total volume of gas
passing through the meter is determined by counting the number of revolutions
of a measuring rotor mounted within the meter. Gas turbine meters are known
as inferential meters because they infer how much gas has passed through by
observing something other than the displacement of gas; i.e. gas turbine
meters
infer how much has passed by measuring the speed of the rotor rotation. A gas
turbine meter is a gas velocity measuring device. The actual flow rate can be
interred from the velocity of the gas because the cross-sectional area of the
annular passage preceding the rotor is a known quantity.
"10 The driving energy to turn the rotor is the kinetic energy, or energy of
motion, of the gas being measured. The gas impinges on rotor blades mounted
on the measuring rotor and overcomes retarding forces that inhibit the rotor
from turning.
A conventional gas turbine meter typically includes an elongated,
115 cylindrical housing which forms a flow path for gas which is flowing
within a
pipeline in which the housing is mounted. An inlet flow straightener is
mounted
adjacent an inlet port in the housing to cause gas flowing from the inlet port
to
flow in an axial direction within the housing. A measuring rotor is mounted
within the housing downstream of the inlet flow straightener so as to rotate
~0 about its axis of rotation. in an axial turbine meter the axis of rotation
is also
the central axis of the cylindrical housing. The measuring rotor has an
upstream end and a downstream end with respect to the flow of gas through
the housing.
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The measuring rotor has turbine blades mounted on it at an angle with
respect to its axis of rotation to cause the rotor to rotate at a speed
approximately proportional to the velocity of the gas flowing through the
housing. Each of the rotor blades has a high pressure surface which faces
toward the flow of gas within the housing and a low pressure surface which
faces away from the flow of the gas. Each turbine blade also has a leading
edge at the upstream end of the rotor and a trailing edge at the downstream
end of the rotor.
Gas turbine meters have typically been constructed with a metal
'10 cylindrical housing having a removable measuring cartridge mounted within
it.
The measuring cartridge narmally includes at least the measuring rotor, its
rotor
bearings and a coupling for interconnecting the measuring rotor to a
mechanical
register mounted on top of the measuring cartridge. The rotor blades are
usually
mounted on a rotor cylinder, which forms the hub of the rotor.
'16 Gas Meter Accuracy
Gas turbine meters are commonly installed in pipelines used in the natural
gas industry for the measurement of the flow of large volumes of gas. The
volumes are often large so that small errors in measurement can result in
large
losses of revenue to gas transmission companies and local distribution
;20 companies. An example of the magnitude of losses which can occur was
presented in a 1992 technical publication of the Netherlands Measurement
Institute. Consider a 12-inch turbine meter operating at a pressure of 680
psig
and having a gas volume which is 69% of maximum capacity. Assuming the
cost of natural gas is $0.0037 per cubic foot, an error of only 0.2% results
in
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a Loss of revenue of $160,000 per year. Clearly it is vital to maintain the
accuracy of gas turbine meters.
Each gas turbine meter must be separately calibrated to determine its
accuracy after it is manufactured. Calibration is necessary because normal,
minor variations in meter components cause each gas turbine meter to register
a slightly different volumetric flow for a given volume of gas. By way of
example, from meter-to-meter blades on otherwise identical turbine measuring
rotors vary slightly in shape due to minor manufacturing inconsistencies. As a
result, each turbine measuring rotor rotates at a slightly different speed for
gas
'10 flowing at the same velocity. Similarly, separate sets of measuring rotor
bearings of the same make and model can impose slightly different frictional
forces on the rotors of separate meters on which they are mounted.
Additionally, the gas turbine meter's mechanical register, sometimes called an
index, gives a reading of gas flow volume on a set of dials. The register is
typically connected to the turbine measuring rotor through a coupling which
includes gears, magnetic couplings and other camponents which load the
turbine rotors of different gas turbine meters to a somewhat different extent.
As a result, each gas turbine meter will register its own unique flow level
for a
given volume of gas.
~ZO At the time of manufacture of a gas turbine meter, the accuracy of a
meter is proved by tasting the meter against a known standard such as a
master meter or a bell prover or a sonic nozzle. At a given temperature, a
given
gas line pressure and a given gas flow rate, the volume of gas registered by
the
meter is compared to the actual volume of gas which flowed through the meter
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as determined by the known standard. This ratio of the volume of gas
measured by a meter's mechanical register to the actual volume of gas flowing
through the meter is called the accuracy of the meter. The calibration factor
of a meter, referred to by the letter "K," expressed in terms of pulses per
unit
of volume flowing through a meter. The calibration factor "K", is the amount
by which the registered reading of the meter is divided to get a 100% accurate
volume reading. For each of a given series of line pressures at which a gas
turbine meter may operate, the K factors are determined for a range of flow
rates expected for the meter. A table of these K factors is normally provided
with each meter.
As a result of the calibration of the meter, the accuracy of the meter at
a given fine pressure can be graphed over a range of gas flow rates. See FtG.
fi
by way of example which shows a graph of two accuracy curves. The lower
of the two curves 10 is a typical accuracy curve of a gas turbine meter having
't 5 rotor blades which do not incorporate this invention. The upper curve 11,
which will be discussed more fully in the Description of the Preferred
Embodiments, is an accuracy curve of a gas turbine meter using this invention.
Along the Y-axis of F1G. 6 is a measurement of the "percent accuracy" of the
meter. Along the X-axis is a measurement of the "flow rate" of the meter in
terms of the percentage of the total flow rate capacity of the meter. The
resulting graphed curved line is the accuracy curve.
The meaning of the graph is as follows: If the amount of gas measured
by the meter is equal to the actual amount of gas that has passed, the meter
accuracy is 100%. Thus, if the meter reads 100 units of gas and 100 units
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have actually passed, the meter would be 100% accurate. If the amount of gas
measured by the meter is less than the amount of gas that actually flowed, the
meter percentage accuracy is said to be less than 100%, and there is a false
low. For instance, if a meter reads 99 units of gas when in actuality 100
units
of gas has flowed, the meter would be 99% accurate. The meter would be
undermeasuring the amaunt of gas that has actually flowed and the customer
will pay too little. If the amount of gas measured is more than the amount
that
has actually flowed, the meter's percentage accuracy is said to be over 100%,
and there is a false high. For instance, if a meter reads 102 units of gas
when
only 100 units actually flowed, the percent accuracy would be 102%. The
meter is overmeasuring the amount of gas that has actually flowed and the
customer wilt pay too much.
As indicated above, the accuracy of the volume of gas recorded on the
dials of a meter's register is checked at the time of a meter's calibration
over
a range of the meter's operating conditions. Components of the meter, such
as the gears and magnetic couplings between the measuring rotor and the
register, are often modified to attempt to get the accuracy of the meter as
consistent as possible over its expected range of gas flow rates.
If the gas meters were inaccurate by the same amount across all flow
rates, fewer problems would exist. If a meter consistently overmeasured or
undermeasured by the same amount all the time, a correction could easily be
made by using the gears or other correcting mechanisms to correct the readings
of the register. The correcting mechanism would simply shift the accuracy
curve up or down so that it would be 100% accurate all of the time.
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However, as can be seen by examining the lower curve 10 of FIG. 6, gas
turbine meters do not have the same percentage of error across all flow rates.
The, accuracy curve 10 is an accuracy curve of a typical prior art gas turbine
meter. This gas turbine meter is equipped with 8 inch helical rotor blades
which
have a mean helical angle of 45° through the axis of rotation of the
rotor on
which the blades are maunted. The accuracy curve 10 is not linear in that it
does not have a constant percentage of error across all flow rates of the
meter
at the temperature and pressure at which the meter was tested. A linear
accuracy curve, which is a desirable characteristic in gas meters, would be a
straight line. The accuracy curve 10 shows a non-linear distribution of the
percentage of accuracy for a gas turbine meter which is generally
representative
of most prior art gas turbine meters. The accuracy curves of these turbine
meters tend to have an undesirable "hump" at low flow rates at between 10-
20% of the maximum flow rate of a meter. Thus, the accuracy curve 10 has
a high reading of approximately 102% at point 12 and the readings above and
below this flow rate decrease appreciably. Readings below about 5% of the
meter capacity measured at atmospheric pressure, become unreliable because
gas tends to slip past the clearance between the rotor blades and the walls of
the housing in which the rotor is installed.
The accuracy measurement of the turbine meter at a particular line
pressure, represented by the accuracy curve 10 in FIG. 6, fiends to fall
fairly
rapidly as the flow rate of the meter increases. At point 14 on the accuracy
curve 10, which is about 28% flow rate capacity of the meter, the percentage
of accuracy of the meter has decreased to about 100.7%; at point 16,
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approximately 50% of the flow rate capacity, the percent of accuracy of the
meter is 100%; while at point 18, approximately 100% flow rate capacity of
the meter, the percent accuracy is about 99.3%. Between about 5% and
100% of the flaw rate capacity of this meter the accuracy ranges from a high
of 102% to a low of about 99.3%. Thus, the linearity of this meter, that is
the
difference between the highest percent of accuracy of the meter and its lowest
percent of accuracy over the meter's operating range, is about 2.7%.
Only a single set of gears and/or couplings can be installed at one time
between the measuring rotor and the dials of a meter's register. Thus, the
register can only be calibrated to be 100% accurate at one flow rate, called
the
change gear rate, which is usually about 50% or 60% of the maximum flow
rate of the meter. At other flow rates significant inaccuracies must sometimes
be tolerated. As shown by accuracy curve 10, because gas meter accuracy
curves are not linear, at some flow rates turbine meters typically
undermeasure
the amount of gas that has flowed, white at other flow rates they tend to
overmeasure the amount of gas that has flowed. The non-linearity of the
accuracies of these meters over the range of expected flow rates is difficult
to
compensate for while calibrating the meter and can result in an undesirable
range of inaccuracies for the meter.
Summary
This invention relates to a turbine meter which has an elongated housing
that provides a path for the flow of fluid to be measured by the meter. The
housing has an upstream end into which the fluid flows and a downstream end
out of which the fluid flows. A rotor having an axis of rotation is mounted
within the housing.
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A plurality of rotor blades is mounted on the rotor at an angle with
respect to the axis of rotation of the rotor so that each rotor blade has a
high
pressure surface which faces toward the upstream end of the housing. Contact
of the fluid on the high pressure surface causes the rotor to rotate as the
fluid
flows. Each of the rotor blades also has a trailing edge closest to the
downstream end of the housing. In accordance with this invention, an
extension is located at approximately the trailing edge and on the high
pressure
surface of at least one and preferably a plurality of the rotor blades. The
extension has a length and forms an angle with respect to the high pressure
surface of a rotor blade which causes the percentage of error across a chosen
operating range of the meter to become more linear and within predetermined
limits.
In the preferred embodiment of this invention the extension is placed at
the trailing edge on the high pressure surface across the width of each of the
'I5 rotor blades. However, the benefits of this invention can be obtained by
placing the extension somewhat away from the trailing edge of the high
pressure surface of the rotor blades andlor on fewer than all the rotor blades
of a rotor. Additionally, the extension can extend across the total width of
each rotor blade on which it is included, or the extension can extend across a
ZO portion of the width of a rotor blade if desired. However, the percent of
error
across a chosen operating range of a meter will become less linear as
extensions on the high pressure side of the meter's turbine blades ace moved
away from the trailing edge or are placed on fewer rotor blades or extend
across less than the total width of the rotor blades.
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This invention does not reside in any single feature of the turbine meter
and rotor disclosed above and in the Description of the Preferred Embodiment
and claimed below. Rather, this invention is distinguished from the prior art
by
its particular combination of features of the turbine meter and rotors
disclosed.
Important features of this invention have been discussed in the Detailed
Description of the Preferred Embodiments of this invention which are shown
and described below to illustrate the best mode contemplated to date of
carrying out this invention.
Those skilled in the art will realize that this invention is capable of
embodiments which are different from those shown, and the details of the
structure of the turbine meters and rotors can be changed in various manners
without departing from the scope of this invention. Accordingly, the drawings
and description are to be regarded as illustrative in nature and are not to
restrict
the scope of this inventian. Thus, the claims are to be regarded as including
such equivalent turbine meters and rotors as do not depart from the spirit and
scope of this invention.
Brief Description of the Drawings
FIG. 1 is a sectional, side view of a gas turbine meter which can
incorporate the features of this invention;
~0 FIG. 2 is an exploded perspective view of a gas turbine meter which can
incorporate the features df this invention;
FIG. 3 is a perspective view, partially cut away, of a turbine meter
measuring rotor incorporating this invention;
FIG. 4 is a partial top view of the measuring rotor showing one of its
~5 turbine blades that incorporates this invention;
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AUG-21-2000 MON 06:23 PM MacDONALD ILLIG FAX N0, 18144544647 P. 05/20
P~TIU~ 9 9 / 13 ~ g 2
T1U599l13792
rl~~~ . !.. 1 (!'~ n
WO'~9166294
,~ (~: , ~1
FIG. 5 shows an enlarged detail of a portion of the turbine blade shown
in FIG. 4 which includes this invention;
F1G. 5a shows a perspective view of a portion of a measuring rotor
having turbine blades which include this invention;
FIG. 5b shows a perspective view of a turbine blade which includes
another embodiment of this invention;
FIG. 5c shows a perspective view of a turbine blade which includes still
another embodiment of this invention;
FIG. 6 is a graph of two accuracy curves of a gas turbine meter, one of
which is for a meter having a rotor which includes this invention and the
other
of which is for the same meter having a rotor which does not include this
invention;
FIG. 7 is a graph of two accuracy curves of a gas turbine meter, one of
which is for a meter having a rotor which includes this invention and the
other
of which is for the sanne meter having a rotor which does not include this
invention.
Description of Preferred Embodiment
Referring to the drawings, identical reference numbers and letters
designate the same or corresponding parts throughout the several figures
shown.
FIG. 1 and FIG. 2 show the basic components of a gas turbine meter m
which a rotor incorporating the features of this invention can be installed. A
gas turbine meter 20 comprises a metal housing 22 having an inlet port 24 and
an autlet port 26. The housing 22 is normally constructed of rnetat such as
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aluminum and is designed to withstand the pressures to which it is expected to
be submitted when installed in a gas transmission or distribution line.
The gas turbine meter 20 includes an inlet flow straightening assembly
28 adjacent the inlet port 24. The inlet flow straightening assembly 28 has a
number of vanes 29 forming passageways that minimize flow disturbances
produced in a pipeline in which the gas turbine meter 20 is installed. The
flow
straightening assembly 28 is not a feature of this invention. Those skilled in
the
art can use any assembly of this type which is convenient to them.
A measuring cartridge 30 is normally mounted within a central cavity of
the elongated housing 22. As seen most clearly in FIG. 2, the measuring
cartridge 30 is fastened to a top plate 32 with screws 33 tone of which is
shown in FIG. 1 ), and a register 34 mounted on the top plate 32. Bolts 36
(shown in FIG. 1 ) fasten the top plate 32 to the housing 22.
A measuring rotor 38 is mounted on the upstream end of the measuring
cartridge 30. The measuring rotor 38 includes a number of radially extending
turbine blades 40 which, as will be explained below, are mounted at an angle
with respect to a central rotor shaft forming an axis of rotation 42 of the
rotor
38. The turbine blades 40 cause the rotor to rotate in one direction at a
speed
approximately proportional with the velocity of gas flowing through the
housing
22 and passing the inlet flow straightening assembly 28.
The rotor 38 is mounted on the rotor shaft and within a hub 44 of the
measuring chamber 30. The rotor 38 is coupled within the hub 44 to a
coupling device 46 which in turn transfers the rotation of the rotor 38
through
a series of gears and couplings 48 to the register 34. At the register 34, the
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rotation of the rotor 38 is converted to the rotation of a series of dials
which
indicate the volume of gas that has flowed through the meter 20.
The meter 20 further includes a hub assembly 49 comprising a hub 50
which is attached to the downstream side of the measuring chamber 30. The
5 hub 50 has a series of vanes 52 for conditioning the gas as it flows toward
the
outlet port 26 and from the meter 20.
This structure of the meter 20 is shown by way of example only. It is
not intended to limit the structure and type of meter in which this invention
can
be used.
Referring now to FIG. 3, the rotor 38 has a hub 54 on which the rotor
blades 40 are mounted. The rotor 38 has an upstream end 56 which is shown
in FIG. 1 mounted adjacent the downstream end of the flow straightening
assembly 28. The rotor 38 also has a downstream end 58.
Each of the rotor blades 40 is mounted at an angle with respect to the
axis of rotation of the rotor 38. The turbine blades 40 have a helical twist
which reduces flow interaction between the blades and thus improves the
performance of the gas turbine meter 20. The angle of the turbine blades 40
and the helical twist are more clearly illustrated in FIG. 4. For turbine
blades
having a 45° angle, a helical twist has a lead of 10.542 inches and a
mean
helix angle of 45°.
The capacity of the rotor, and thus the capacity of the gas turbine meter
20, can be changed by changing the angle of the rotor blades 40. By way of
example, the capacity of the rotor 38 can be increased by changing the rotor
blade angle to 30°. Additionally, turbine blades can be used which do
not have
a helical twist.
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Each turbine blade 40 has a high pressure surface 60 which faces toward
the flow of gas when the rotor 38 is mounted within the pausing 22 of the
meter 20. It is the gas pressure on the high pressure surfaces 60 that causes
the rotor 38 to rotate within the housing 22 of the meter 20. The rotor blades
40 each have a low pressure surface 62 which faces away from the flow of gas
through the housing 22.
As shown in both FIG. 3 and FIG. 4, each of the turbine blades 40 has
a leading edge 64 at the upstream end of the rotor 38 and a trailing edge 66
at
the downstream end of the rotor 38. The circled portion 5-5 of the trailing
edge
66 shown in FIG. 4 is enlarged and shown more clearly in FIG. 5 and is shown
in a perspective view in FIG. 5a. In accordance with this invention, an
extension 68 is located at approximately the trailing edge 66 and on the high
pressure surface 60 of at least one and preferably a plurality of the rotor
blades
40 of the rotor 38, In the preferred embodiment of this invention, an
extension
68 is located along the total width of the trailing edge 66 on the high
pressure
surface of each of the rotor blades 40. The extension should have a length and
should form an angle with respect to the high pressure surface 60 which causes
the accuracy curve of the meter to become more linear. Thus, the extension
68 causes the percentage error across the chosen operating range of the meter
20 to be within predetermined limits of error. These limits of error are
determined for each meter by testing it when it is being designed. The angle
of the extension 68 with respect to the high pressure surface 60 of a turbine
blade is normally 90° plus the angle ~ the blade forms with respect to
the axis
of the meter. FIG. 5 shows this angle to be 135° for a 45°
blade. This angle
would normally be 120° for turbine blades of a 30° meter.
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Referring to FIG. 6,, accuracy curve 11 shows the accuracy of the 8 inch
gas meter with a 45° rotor, which was discussed above and shown in
FIGS. 5
and 5a, after extension 68 was added to the trailing edge 66 of the high
pressure side 60 of each of the turbine blades 40 of this meter. This meter
had
the same gears and couplings installed between the rotor 38 and the dials of
the register 34 as were used when the accuracy at the paints on the accuracy
curve 10 was measured.
Accuracy curve 11 shows that adding the extension 68 increased the
rotor speed at all flow rates of the meter. However, the extension 68
increased
the rotor speed more at higher flow rates of the meter than it did at lower
flow
rates of the meter. Thus, at point 12' of accuracy curve 11, which is
approximately 10% of the flow rate capacity of the meter, the accuracy is
about 104.5%; at point 14' which is approximately 25% of the flow rate
capacity of the meter the accuracy is about 104.1 %; at point 16' which is
'! 5 approximately 70% of the flow rate capacity of the meter the accuracy is
about
104.4%; and at point 18' which is about 100% of the flow rate capacity of the
meter, the accuracy is about 104.6%. The linearity of this meter is about 0.5%
using the trailing edge extensions.
Thus, the 8 inch meter represented by FIG. 6 had an improvement in
linearity from about 2.7% to about 0.5% between its 5% and 100% flow rate
capacities as a result of adding extension 68 along the entire trailing edge
of the
high pressure surfaces of the turbine blades of the meter. Additionally, the
accuracy shifted about 4.4% at the change gear rate of 50% of the capacity
of the meter. The gear ratios of the gears between the rotor 38 and the
register 34 of this meter would be changed prior to the installation of this
meter
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in a gas line so as to move the accuracy curve down about 4.4% at 50% of the
flow rate capacity of this meter.
FlG. 7 shows the effects of adding the extensions 68 on the trailing edge
66 of the high pressure surfaces 60 of the turbine blades 40 of a 4 inch gas
meter having a 30° rotor. Without the use of these extensions, the
accuracy
measurement of this 4 inch turbine meter also tends to fall fairly rapidly as
the
flow rate of this meter increases. The accuracy curve 70 shows that the
percent of accuracy of this meter is about 101.5% at around an 8% flow rate
capacity shown at point 72 of the curve. The percent of accuracy decreases
110 to about 99.2% accuracy at 60% flow rate capacity shown at point 74 of the
accuracy curve 70. In between these extremes, at 15% flow rate capacity
shown at point 76, the accuracy is about 101.2%; at about 20% flow rate
capacity as shown at point 78 the accuracy is about 104%; at about 25% flow
rate capacity at point 80 the accuracy is about 101 %; and at about 50% flow
115 rate capacity at point 82 the accuracy curve 70 shows about 99.4%
accuracy.
As shown by the accuracy curve 84 in FIG. 7, adding extension 68 to the
trailing edge 66 of the high pressure surface 60 of each of the rotor blades
causes the rotor blades of the 4 inch meter with a 30° rotor raise the
accuracy
curve by about 6.5% at a flow rate of 25% capacity and by about 7.4% at the
20 change gear flow rate of 50% capacity. The accuracy ranges from about
106.2% at about 8% flow rate capacity at point 86 to a high of about 106.9%
at about 60% flow rate capacity at point 88 and 106.8% at point 90 which is
about 100% flow rate capacity. Accuracy curve 84 also shows that the
accuracy is about 106.5% at point 92 which is about 25% flow rate capacity
25 and at point 94 which is about 60% flow rate capacity. Thus, the linearity
of
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the 4 inch 30° rotor was improved from 2.2%, as shown on accuracy curve
70,
to about 0.8%, as shown by accuracy curve 84, as a result of adding
extensions to the trailing edge of the high pressure surfaces of the turbine
blades. If this 4 inch meter were to be installed in a gas line with the
extension
68 on its rotor blades, the gears between the rotor and the dials of the
register
of the meter would be changed to cause accuracy curve 84 to be centered on
100% accuracy.
The turbine blades of fluid meters are sometimes compared to air foils
which convert the motion of fluid into a rofiating force which is normal to
the
flow of the fluid. As a result of tests conducted following the discovery
which
led to this invention, it is believed that the extensions 68 located at
approximately the trailing edges 66 of the high pressure surfaces 60 of the
turbine blades can be compared to trailing edge flaps which have sometimes
been added to air foils such as airplane wings. Adding trailing edge flaps
'! 5 increases the lift of air foils as the speed of the fluid passing the air
foils
increases. The change in lift is non linear, increasing as the speed of the
fluid
increases relative to the air foil itself. As a result, applying this theory
to fluid
meters, the accuracy of the meter to which extensions are added to the
trailing
edges of rotor blades is increased proportionately less at the lower flow
rates
than it is at the higher flow rates of the meter.
Referring to FIG. 5, both the length of the extension 68 and the angle
between extension 68 and the high pressure surface 60 of the rotor blade on
which the extension 68 is mounted controls the amount of lift provided by the
extension 68. Both the length of the extension 68 and the angle between the
extension 68 and the high pressure surface are chosen empirically. In the
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preferred embodiment of this invention, the junction between the high pressure
surface 60 and the extension 68 is a gradual surface which can be formed as
an a.rc having a particular radius "R." The extension 68 has a length "x" as
measured from the point where the extension leaves the high pressure surface
60 to the end of the extensiori 68. The extension 68 used for the 8 inch
45°
gas meter represented by FIG. 6 had a length of 0.015" and formed an angle
between the radically lower surface of the extension 68 and the high pressure
surface 60 of the rotor blade of 135°, that is 90° plus the
45° angle the turbine
blades form with the axis of rotation of the measuring rotor. The thickness
"T"
of the extension 68 is chosen to prevent foreign particles in the flow of gas
from damaging or removing the flap from the trailing edge of the rotor blade
on
which it is mounted. A thickness of 0.015" was chosen for the 8 inch
45°
rotor. A 0.016" radius R between the lower surface of the extension 68 and
the high pressure surface 60 of the rator blade gradually blended these
surfaces
together.
The 4 inch, 30° gas meter represented by FIG. 7 has a trailing edge
flap
that was 0.016" long with a 135° angle between the radially lower
surface of
the extension 68 and the high pressure surface of the rotor blade. It also had
a thickness of 0.016".
Any angle between the radially inward surface of the extension 68 and
the high pressure surface 60 can be chosen and tested for its effect on the
accuracy of the meter. However, machining difficulties make it more expedient
to limit the choice of angle to 90° plus the blade angle of turbine
blades of the
meter.
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WO 99166294 PCT/US99/13792
The optimum length x of the extension is chosen empirically for each size
and type of turbine meter. The accuracy of the meter is tested for each length
x of the extension 6$ which is chosen to predetermine the linearity of the
meter
using an extension of that length on its turbine blades. The length of the
extensions resulting in the smallest linearity of the meter is chosen for the
rotor
blades.
As a result, a formula has been developed to predict, for a given length
x of the extension shown in FIG. 5, the approximate change in the "K" factor
of the meter that can be expected by adding the extension 68 to the trailing
70 edge of the turbine blades of the meter. This is helpful in determining the
approximate change in gearing required to cause the accuracy curve of the
meter to be approximately 100% at its change gear flow rate.
The formula is:
K2 (Ok 1 ~2
_ = [2- _ + - sinOkl
K~ n n
The angle Ok can be obtained in terms of the width of the rotor W and the
length of the trailing edge flap x from the geometry.
2
Uk = cos '' [ 9 - 1
X
1 + _sin[3cos(3
W
As seen in FIG. 4, "W" is the width of the rotor on which the turbine blades
have been mounted, and f3 is the angle of the turbine blades with respect to
the
axis of the meter.
The benefits of this invention can be obtained by adding the extension
68 to the trailing edge of the high pressure surface of fewer than all of the
rotor
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CA 02334218 2000-12-04
WO 99166294 PCTIUS99/I3792
blades of a meter. It can also be obtained by adding the extension over a
portion of the width of the trailing edge of the rotor blades to which the
extension 68 is added. FIG. 5b shows an extension 68b which extends across
the middle third of the trailing edge 66 of the turbine blade 40. FIG. 5c
shows
extensions fi8c and 68d at either end of the trailing edge 66 of the turbine
blade 40. Those skilled in the art will recognize that the lift attained from
adding the extension will be less with fewer rotor blades having the
extension.
it will also be less if the extensions do not extend across the total width of
the
rotor blades on which it has been added. Here again, the number of rotor
blades on which the extension 68 is included and/or a portion of the width of
the rotor blades over which the extension 68 is located are chosen empirically
as a particular rotor is developed. While it is preferabie to place the
extension
at the trailing edge of the rotor blades on which it is included, the
extension 68
can be placed radially somewhat inwardly of the trailing edge along the high
pressure surface of the rotor blades and still obtain some of the benefits of
this
invention. However, the lift obtained and the benefits resulting will be less
as
the extension 68 is moved farther from the trailing edge of the rotor blades.
Those skilled in the art wilt recognize that this invention has been
explained with regard to the details and arrangements of the illustrated
embodiment to explain the nature of this invention. Many modifications can be
made to this invention by those skilled in the art without departing from its
spirit and scope. Thus, the claims are intended to be interpreted to cover
such
equivalent rotary meters which do not depart from the spirit and scope of this
invention.
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