Abstrict A turbine gas flow meter (10) includes a meter body (12) including
an inlet portion (24) having an inlet body (32) mounted therein,
with an exit end (30) of the body inlet portion being defined at
an internal plenum (22) of the meter body, and a removable turbine
meter measuring module (44) including a rotor assembly (48) which
is inserted into the plenum with an inlet end (127) of the rotor
assembly and the exit end of the inlet portion defining an interface
therebetween. A closed space (114) is formed about the rotor assembly
within the plenum between an inner wall (106) of the body and outer
walls (116 118) of the rotor assembly. An axial gap (128) between
a surface (130) of the rotor assembly inlet end and a surface (132)
of the body inlet portion exit end, and/or radial notches in either
of the surfaces (130 132), provide fluid pressure communication
from the interface to the closed space. A pressure tap (138) extends
through the body into the closed space for measuring pressure within
the closed space.
Claims What is claimed is:
1. A turbine meter with a pressure measuring system, comprising:
a meter body having an entrance end and a discharge end with a
flow path defined therebetween and having a body inlet portion along
the flow path, the body inlet portion having an exit end downstream
of the meter body entrance end and directed downstream toward the
meter body discharge end;
a turbine meter measuring module including a rotor assembly having
an inlet end and an outlet end, the rotor assembly being mounted
in the meter body along the flow path downstream of the body inlet
portion with the inlet end of the rotor assembly confronting the
body inlet portion exit end to define an interface therebetween
such that the rotor assembly and an inner wall of the meter body
radially outward of the rotor assembly define a closed space about
the rotor assembly and extending downstream of the body inlet portion;
gap means defined at the interface and communicating with the closed
space for coupling pressure from the interface to the closed space;
and
a pressure tap for measuring pressure through the meter body and
opening directly into the closed space.
2. The turbine meter of claim 1 wherein the gap means comprises
a circumferentially extending axial gap between the exit end of
the body inlet portion and the inlet end of the rotor assembly.
3. The turbine meter of claim 1 wherein the exit end of the body
inlet portion includes a circumferentially extending aft-facing
surface and the inlet end of the rotor assembly includes a circumferentially
extending forward-facing surface confronting the aft-facing surface,
and the gap means comprises an axial gap between the aft-facing
surface and the forward-facing surface.
4. The turbine meter of claim 3 wherein the gap means further
comprises a plurality of radially extending notches in the forward-facing
surface of the inlet end of the rotor assembly.
5. The turbine meter of claim 3 wherein the gap means further
comprises a plurality of radially extending notches in the aft-facing
surface of the exit end of the body inlet portion.
6. The turbine meter of claim 1 wherein the exit end of the body
inlet portion includes a circumferentially extending aft-facing
surface and the inlet end of the rotor assembly includes a circumferentially
extending forward-facing surface confronting the aft-facing surface,
and the gap means comprises radially extending notches in the forward-facing
surface of the inlet end of the rotor assembly.
7. The turbine meter of claim 1 wherein the exit end of the body
inlet portion includes a circumferentially extending aft-facing
surface and the inlet end of the rotor assembly includes a circumferentially
extending forward-facing surface confronting the aft-facing surface,
and the gap means comprises radially extending notches in the aft-facing
surface of the exit end of the body inlet portion.
8. The turbine meter of claim 1 wherein the inlet end of the rotor
assembly further comprises a circumferentially extending forward-facing
surface confronting the exit end of the body inlet portion, the
forward-facing surface having a plurality of circumferentially spaced
ball plungers embedded therein, each ball plunger residing within
a recess formed in the forward-facing surface, wherein the gap means
comprises the recesses.
9. The turbine meter of claim 1 the measuring module being removable
from the meter body.
10. The turbine meter of claim 9 the body inlet portion being
an integral part of the meter body so as not to be removable with
the measuring module.
11. The turbine meter of claim 10 further including an inlet body
in the flow path and within the body inlet portion.
12. The turbine meter of claim 11 the inlet body being rigidly
attached to the body inlet portion so as not to be removable with
the measuring module.
13. The turbine meter of claim 9 the meter body including a removable
top plate through which to remove the measuring module.
14. The turbine meter of claim 13 the pressure tap being in the
top plate.
15. The turbine meter of claim 1 the closed space being sealed
from the rotor assembly outlet end.
16. The turbine meter of claim 1 the closed space being downstream
of the body inlet portion.
17. The turbine meter of claim 1 further including an inlet body
in the flow path and within the body inlet portion.
18. A method for measuring the pressure existing at an inlet end
of a turbine rotor assembly of a gas flow meter in a meter body
wherein the meter body has an entrance end and a discharge end with
a flow path defined therebetween, the meter body including a body
inlet portion along the flow path adjacent the entrance end of the
meter body such that an exit end of the body inlet portion is directed
downstream toward the discharge end of the meter body, the method
comprising:
mounting the rotor assembly in the meter body along the flow path
with an inlet end of the rotor assembly confronting the exit end
of the body inlet portion to define an interface therebetween and
with an inner wall of the meter body radially outward of the rotor
assembly defining a closed space about the rotor assembly;
providing gap means at the interface such that fluid pressure at
the interface is communicated to the closed space;
providing a pressure tap through the meter body and opening directly
into the closed space; and
measuring the pressure within the closed space via the pressure
tap, thereby measuring the pressure at the interface.
19. The method of claim 18 wherein providing gap means includes
providing an axial gap between the inlet end of the rotor assembly
and the exit end of the body inlet portion.
20. The method of claim 18 wherein the inlet end of the rotor
assembly includes a circumferentially extending surface confronting
the exit end of the body inlet portion, and providing gap means
includes providing radially extending notches in the surface confronting
the exit end of the body inlet portion.
21. The method of claim 18 wherein the exit end of the body inlet
portion has a circumferentially extending surface confronting the
inlet end of the rotor assembly, and providing gap means includes
providing radially extending notches in the surface confronting
the inlet end of the rotor assembly.
22. The method of claim 18 further comprising removably mounting
the rotor assembly in the meter body.
Description FIELD OF THE INVENTION
The present invention relates to turbine flow meters for measuring
the flow volume of a gaseous medium through a pipe and, more particularly,
to such a turbine flow meter including a system for accurately measuring
the static pressure existing within the meter over a wide range
of flow rates.
BACKGROUND OF THE INVENTION
Axial flow turbine meters are widely used for measuring flow of
gas by which to determine gas usage and the like. To this end, an
axial flow turbine meter includes a meter body having an inlet end
connected to a gas supply line or pipe and an outlet end which may
be connected to a gas delivery line or pipe. Inside the meter body
at the inlet end is a cone or bullet-shaped inlet body, the exit
end of which is placed adjacent to a turbine rotor assembly having
at least first rotor which rotates in response to gas flowing thereover
so as to provide a measure of the gas flow. A second rotor may be
positioned downstream of the first rotor for more accurate or calibrating
measurements. Blades on the rotor periphery cause the rotor to rotate.
To focus the flowing gas on the blades, the inlet body conditions
the flowing gas to change it from the space of the supply pipe to
the annular path.
In order to provide reliable measurements, certain correction factors
must be taken into account. In particular, the pressure and temperature
of the gas passing through the meter are usually not equal to the
standard pressure and temperature upon which gas suppliers base
their pricing structure, and therefore the actual volume of gas
indicated by the meter is corrected to standard conditions in order
to determine the total price for the gas used.
It is desirable for a gas flow meter to measure flow volume with
a high degree of accuracy, since flow volume errors of as little
as a half a percent, occurring over a significant time period, can
result in a substantial revenue error in the total gas usage. The
gas supplier accordingly may significantly overcharge or undercharge
the user. It will also be appreciated that a gas meter of a given
design may be placed in a variety of types of installations whose
usage rates may vary substantially from each other, and furthermore,
there frequently is a significant variation in usage rates even
for a particular installation as demand rises and falls in accordance
with need. Thus, the goal for the designer of a gas flow meter is
to design a meter which has a high degree of accuracy and repeatability
over a wide range of flow rates.
In order to achieve accurate measurement of gas flow, it is standard
practice to calibrate each meter by comparing the meter's indicated
flow volume to a known flow volume and repeating this test over
a range of flow rates so as to develop a "calibration curve"
representing the meter's flow volume error in percent. One calibration
method commonly used is to place the meter to be calibrated in series
with a highly accurate flow measurement device such as a "flow
prover" in a test line and to flow gas through the test line.
At a given test point, the actual flow volumes of the test meter
and the flow prover are acquired and each is corrected to standard
conditions (e.g., 1 bar pressure and 15 degrees C.) based on measurements
of the pressure and temperature of the gas entering the test meter
and the pressure and temperature of the gas entering the flow prover.
This test is repeated over a range of flow rates to arrive at the
test meter's calibration curve. Using this curve, for any indicated
flow rate, an accurate flow volume can be determined.
Thus, it will be appreciated that an accurate measurement of gas
flow volume is highly dependent on accurate measurement of the pressure
of the gas entering the meter, both at the time of meter calibration
and at the time of usage. For instance, a pressure measurement error
of one inch of water can translate into a 0.25 percent error in
a meter's calibration curve.
During field use, one of the factors affecting the accuracy of
a meter's calibration is variation in the configuration of the environment
in which the meter is installed, from one installation to another.
In one known turbine meter, the turbine rotor assembly and the inlet
body form a complete or self-contained unit or module which is removable
as a unit from the meter body for maintenance or calibration. The
combined inlet body and rotor assembly are contained within a cylindrical
sub-housing that is inserted axially into the meter body through
the inlet end thereof. An annular pressure space is formed between
the sub-housing and meter body radially outwardly of the inlet body.
The pressure space is sealed from the rest of the meter so as to
be confined over the inlet body and one or more apertures are formed
in the sub-housing over the inlet body by which to permit pressure
communication from the flow path over the inlet body to the pressure
space. A sensor coupled to the pressure space may be used to obtain
a readout of the pressure therein.
The plurality of apertures through the sub-housing require manufacturing
time for machining, deburring, inspection, and testing. Moreover,
in order to create the pressure space, the module must include the
sub-housing. Thus, replacement of a bad rotor assembly results in
disconnecting the gas line(s) from the meter body to remove the
module and necessitates removal of the entire module, along with
the inlet body, the latter being a component that is not prone to
failure. The result is a less efficient and more costly system to
build and maintain.
In other turbine gas meters, the inlet body is not part of the
removable measuring module, but rather remains with the meter body,
since it is a stationary part which is durable and seldom requires
maintenance or replacement. For instance, in a prior meter design
by the assignee of the present application, a measuring module including
a separate turbine rotor assembly is removable from and insertable
into an internal plenum within the body and adjacent to an exit
end of the inlet body through a lateral opening in the body. The
measuring module includes a main rotor which is carried within a
generally cylindrical main rotor carrier and a sensing rotor which
is carried within a generally cylindrical sensing rotor carrier,
the two rotor carriers being connected to each other and to a top
plate which covers the lateral opening in the body and supports
a mechanical counter mechanism as well as connections for various
sensors. To remove the module, the top plate is disconnected from
the body, and the entire top plate and rotor assembly is removed,
without having to disconnect the body from the gas supply line.
The body includes an inlet body in the form of a nose cone with
flow straightening vanes, but these remain with the body when the
measuring module is removed.
With the stand-alone rotor assembly, calibration of the measuring
module is performed independent of the inlet body and flow straightening
vanes. Further, the measuring module can be removed from the body
without disconnecting the body from the gas supply line. However,
because the inlet body may have a substantial effect on the flow
conditions, such as the static pressure, at the inlet to the measuring
module, it is necessary to accurately measure and account for the
static pressure during calibration and field use of the measuring
module. To this end, pressure measured in the inlet flowpath above
the inlet body is not a reliable or accurate source for calibration
of the module since the module and inlet body are independent. Thus
if a measuring module is calibrated in a test setup, the module
may not be accurately calibrated for field use where the inlet body
and other aspect of the meter body in the field may vary slightly
from the test set-up unit. For example, the location of the sensor
may vary from meter to meter or the size and spacing of the components
may vary slightly from meter to meter. As a consequence, there is
a marked risk of degraded repeatability and interchangeability of
measuring modules from one meter body to another.
To overcome problems associated with measuring pressure over the
inlet body, pressure measurements have been focused on the rotor
turbine module assembly itself. To this end, pressure is measured
at the inlet to the main rotor via a pressure tap machined through
the main rotor carrier just upstream of the main rotor blades. The
pressure tap is connected by flexible tubing to a fitting that extends
through the top plate. A pressure sensor may be connected to the
fitting. Thus, the pressure tap location is consistent from meter
to meter and moves with the module. By measuring pressure close
to the main rotor within the rotor assembly and making the pressure
measurement system part of the removable measuring module, the same
calibration curve can be used for the measuring module even when
it is placed in different bodies.
This pressure measurement system has been found to be satisfactory
up to gas flow velocities of about 110 feet per second. However,
for reasons that are not known, it has been discovered that the
accuracy of the pressure measured within the rotor assembly ahead
of the main rotor begins to deteriorate at higher flow rates. For
example, at 160 feet per second flow velocity, the meter calibration
curve determined by using this pressure measurement is about 0.5
percent in error. Additionally, it has been found that pressure
measurements taken over the inlet body exhibit inconsistencies with
different measuring module/body combinations.
Accordingly, there has been a need for a pressure measurement system
for a turbine flow meter module in which pressure can be accurately
measured for a wide range of flow rates and which permits interchanging
the meter in various meter bodies without having to recalibrate
the meter each time it is placed in a new meter body.
SUMMARY OF THE INVENTION
The drawbacks noted above in connection with prior pressure measurement
systems for a turbine flow meter are overcome in accordance with
the principles of the present invention, by eliminating the apertures
in the module housing and the pressure tube, and instead forming
a closed space about the module that communicates to the front of
the main rotor via a gap system formed between the module housing
and the inlet portion of the meter body with a tap directly into
the closed space over the module for measuring the pressure therein.
By thus measuring pressure at the interface of the rotor module
and the inlet portion of the body, it has been discovered that the
pressure measurements are as accurate below 110 feet per second
of gas flow rate as with prior techniques, and yet surprisingly
achieve comparable accuracy at flow velocities well in excess of
110 feet per second. Such results are achieved without replacing
the inlet body or matching up the module to the same inlet body
for lab calibration and field use.
Advantageously, the gap system comprises an axial gap between the
inlet end of the rotor assembly and the exit end of the body inlet
portion. Additionally or alternatively, the gap system may comprise
radial notches in the inlet end of the rotor assembly extending
from the flowpath to the closed space. The radial notches are less
susceptible to clogging by debris in the gas than the axial gap,
so that fluid communication between the flowpath and the closed
space is maintained over a longer period of use.
By virtue of the foregoing, there is thus provided a gas flow meter
including a measuring module having a pressure measurement system
that is integral to and calibrated along with the module, in which
pressure may be accurately measured over a wide range of flow rates
and in various meter bodies, thereby facilitating interchangeability,
accuracy, and repeatability of the meter.
The above and other objects and advantages of the present invention
shall be made apparent from the accompanying drawings and the description
thereof.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawings, which are incorporated in and constitute
a part of this specification, illustrate an embodiment of the invention
and, together with a general description of the invention given
above, and the detailed description of the embodiment given below,
serve to explain the principles of the invention.
FIG. 1 is a perspective view of a turbine gas meter in accordance
with the principles of the invention, partially cut away to show
internal details of the meter;
FIG. 2 is a view, partly in cross-section, of the turbine meter
of FIG. 1 showing the axial gap when the measuring module is installed
in the meter body;
FIG. 3 is a front view of the measuring module removed from the
body, showing the radial notches in the main rotor carrier and the
spring-loaded ball plungers for positioning the rotor assembly with
respect to the body inlet portion; and
FIG. 4 is a cross-sectional view taken along line 4--4 of FIG.
3 showing a radial notch and ball plunger in further detail.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
With reference to FIG. 1 a turbine meter 10 in accordance with
the principles of the present invention is shown. The meter 10 includes
a body 12 having an entrance end 14 and a discharge end 16. In FIG.
1 the body 12 has been partially cut away to show internal details
of the meter 10. The body 12 includes a flange 18 at the entrance
end 14 which is adapted to be connected to a similar flange on a
gas supply line (not shown), and similarly includes a flange 20
at the discharge end 16 which is adapted to be connected to a flange
on a delivery line (not shown) which may lead to a device or pipeline
to be supplied with gas.
With reference to FIGS. 1 and 2 the body 12 includes an internal
plenum 22 between the entrance end 14 and the discharge end 16.
The body has an inlet portion 24 which defines a gas flowpath 26
into the plenum 22. The inlet portion 24 has an inlet end 28 adjacent
the entrance end 14 of the body 12 and an exit end 30 defined at
the plenum 22.
The body further includes an inlet body 32 mounted within the inlet
portion 24. The inlet body 32 comprises a nose cone having a closed
end 34 adjacent the inlet end 28 of the body inlet portion 24 and
an open end 36 defined at the exit end 30 of the body inlet portion
24. The inner wall 38 of the body inlet portion 24 and an outer
wall 40 of the inlet body 32 define the annular gas flowpath 26.
The inlet body 32 has a plurality of circumferentially spaced flow
straightening vanes 42 extending outwardly from the outer wall 40.
The vanes 42 engage the inner wall 38 of the inlet portion 24 so
as to position the inlet body 32 within the inlet portion 24.
The meter 10 includes a turbine meter measuring module 44 which
is insertable into the body 12 through a lateral opening 46 in the
body 12. The measuring module 44 includes a rotor assembly 48. The
details of the rotor assembly 48 itself are not necessary to an
understanding of the principles of the present invention. It is
sufficient to note that the rotor assembly 48 may be a dual turbine
rotor assembly as shown, or alternatively may be a single rotor
assembly. In the dual turbine assembly illustrated in the drawings,
the rotor assembly 48 includes a main rotor 50 having a plurality
of main rotor blades 52 mounted about a main rotor disc 54 which
is supported on a shaft 56 rotatably journalled within a main rotor
hub 58. The main rotor hub 58 is mounted concentrically within a
main rotor carrier 60 which is generally cylindrical and is connected
to the main rotor hub 58 by generally radial struts 62 in conventional
manner. An annular flowpath 64 is thus defined between an inner
surface 66 of the main rotor carrier 60 and an outer surface 68
of the main rotor hub 58 and an outer surface 70 of the main rotor
disc 54 the main rotor blades 52 being disposed within this annular
flowpath 64.
The rotor assembly 48 further includes a sensing rotor 72 located
in close proximity downstream of the main rotor 50. The sensing
rotor 72 includes a sensing rotor disc 74 about which are mounted
a plurality of sensing rotor blades 75. The sensing rotor disc 74
is supported on a shaft (not shown) rotatably journalled in a sensing
rotor hub 78. The hub 78 is mounted concentrically within a sensing
rotor carrier 80 and connected thereto by generally radial struts
82. Thus, between an inner wall 84 of the sensing rotor carrier
80 and an outer wall 86 of the sensing rotor disc 74 and an outer
wall 88 of the sensing rotor hub 78 an annular gas flowpath 90
is defined for the sensing rotor 72. The annular gas flowpath 90
of the sensing rotor is a continuation of the annular gas flowpath
64 of the main rotor 50 which is a continuation of the annular
gas flowpath 26 of the body inlet portion 24. Gas exiting the annular
flowpath 90 of the sensing rotor 72 discharges into a generally
open region 92 in the body 12 and from there exits the body through
the discharge end 16 thereof.
The measuring module 44 further includes a top plate 94 which is
connected to the rotor assembly 48 via a hanger bolt 96 mounted
on the sensing rotor carrier 80 and a hanger bracket 98 mounted
on the top plate 94 and which is engaged by the hanger bolt 96
and a quick-connect adapter plate 100 connected to the main rotor
carrier 60 which has openings (not shown) for engaging lugs (not
shown) which project from the top plate 94. The top plate 94 serves
to close the lateral opening 46 in the body 12 when the rotor assembly
48 is inserted into the body 12 and also supports a mechanical
counter mechanism 104 which in known manner is connected via a mechanical
output 103 and gear train 102 through a magnetic coupling 101
and through internal shafts and gearing (not shown) to the main
rotor 50. An electronic pulse output (not shown) is supplied from
a pulse generator (not shown) mounted on the main rotor shaft 56
to a gas flow computer (not shown) which computes the volume of
gas which has passed through the meter, corrected to standard conditions
of pressure and temperature. The top plate 94 also supports connectors
(not shown) for outputs from other devices such as magnetic pickup
devices (not shown) or optical counters (not shown). The body 12
includes raised mounting shoulders 105 framing the lateral opening
46 in facing relationship with an inner surface 106 of the top plate
94. To effect a sealing of the top plate 94 to the body 12 a seal
108 is disposed between the outer surfaces 110 of the mounting shoulders
105 and the inner surface 106 of the top plate 94. The top plate
94 is secured to the body 12 by fasteners 112 which extend through
the top plate 94 into the mounting shoulders 105.
A closed space 114 exists between the inner surface 106 of the
top plate 94 and the outer surfaces 116 and 118 of the rotor carriers
60 and 80 respectively. The closed space 114 accommodates the hanger
bolt 96 and bracket 98 as well as various leads 120 (FIG. 1) for
the electronic pulse output (not shown). The closed space 114 is
sealed from the gas flowpath at the exit end 121 of the rotor assembly
48 by a seal ring 122 disposed between an aft-facing (i.e., facing
in the downstream direction) surface 124 of the sensing rotor carrier
80 and a forward-facing (facing in the upstream direction) surface
126 of the body 12 at the downstream edge of the lateral opening
46. However, at the inlet end 127 of the rotor assembly 48 there
is an axial gap 128 between a forward-facing surface 130 of the
main rotor carrier 60 and an aft-facing surface 132 of the body
inlet portion 24 at the lateral opening 46. Accordingly, there is
fluid communication via the axial gap 128 from the entrance to the
main rotor gas flowpath 64 at the exit end 30 of the body inlet
portion 24 into the closed space 114. In particular, because the
closed space 114 is a stagnation region (i.e., there is no gas flow
through the space) and the axial gap 128 opens into the flowpath
64 in a direction generally perpendicular to the direction of gas
flow, the pressure within the closed space 114 will be responsive
to the static pressure existing in the flowpath 64 at the inlet
end 127 of the rotor assembly 48. The axial gap 128 extends around
substantially the entire circumferences of the surfaces 130 and
132 and advantageously has an axial width (measured in the gas
flow direction) of about 0.01 inch to about 0.03 inch.
With reference to FIG. 3 the location of the main rotor carrier's
forward-facing surface 130 is established relative to the aft-facing
surface 132 of the body inlet portion 24 by a plurality of circumferentially
spaced spring-loaded ball plungers 134 secured within the forward-facing
surface 130 of the main rotor carrier 60. The ball plungers 134
engage the aft-facing surface 132 of the body inlet portion 24 to
locate the rotor assembly 48 in a consistent manner with respect
to the body inlet portion 24. Although FIG. 4 illustrates the ball
plungers 134 as having housings which are press fit in the carrier
80 the housings may instead be threadably inserted in the carrier
80.
Because the axial gap 128 is prone to clogging by debris contained
within the gas stream, and further to meet the requirements of ISO
9951 section 6.6.3.2 for reference pressure tap sizing, the meter
is provided with a plurality of radially extending recesses or notches
136 in the forward-facing surface 130 of the main rotor carrier
60. With reference to FIGS. 3 and 4 the radial notches 136 advantageously
surround each ball plunger 134 i.e., each ball plunger 134 is located
within one of the notches 136 although additional notches not having
ball plungers could also be employed. Each radial notch 136 advantageously
is cut into the forward-facing surface 130 to a depth (measured
in the axial direction) of from about 1/32 inch to about 1/8 inch,
and has a width (measured in the circumferential direction) of from
about 5/16 inch to about 9/16 inch. The radial notches 136 are less
susceptible than the axial gap 128 to clogging by debris, helping
to maintain proper fluid pressure communication between the gas
flowpath 64 and the closed space 114 for a longer period of use.
Advantageously, the radial notches 136 are used in addition to the
axial gap 128 although the notches 136 could be used without the
axial gap 128 or the axial gap 128 could be used without the notches
136. Furthermore, it will be appreciated that although the radial
notches 136 are shown as being formed in the forward-facing surface
130 of the main rotor carrier 60 the notches could alternatively
be formed in the aft-facing surface 132 of the body inlet portion
24.
The top plate 94 includes a pressure tap 138 machined through the
plate from the outer surface 140 of the top plate 94 to the closed
space 114. The top plate 94 is equipped with a fitting 142 for connecting
a pressure sensor (not shown) of suitable range and accuracy so
that the pressure within the closed space 114 may be measured. Thus,
it will be appreciated that the pressure measuring system is part
of the removable measuring module 44 and that the location of the
pressure measurement is fixed with respect to the main rotor 50
regardless of the body in which the module is installed. This feature
facilitates a high degree of repeatability of pressure measurement
as well as interchangeability of meters from body to body.
It will also be appreciated that this pressure measuring system
is relatively inexpensive to implement, as it does not require providing
a measuring insert which includes a cylindrical inlet member and
an inlet body as well as a rotor assembly, inserting the insert
axially into a tube member so that an annular space is created between
the inlet member and the inner wall of the tube member, sealing
the annular space, and machining and finishing multiple holes through
the tube member into the annular space for fluid pressure communication
between the inlet flowpath and the annular space, as in prior turbine
meters. Instead, the measuring module 44 includes a rotor assembly
48 separate from the inlet body 32 and with no inlet member, the
module 44 being positioned in the body 12 such that the axial gap
128 exists, thereby establishing pressure communication between
the main rotor flowpath 64 and the closed space 114.
In accordance with the principles of the invention, measuring the
pressure in the closed space 114 instead of through a pressure
tap in the main rotor carrier 60 has been found to yield a meter
calibration curve which is essentially flat (i.e., meter flow error
in percent is essentially constant) over a wide range of flow velocity.
While the present invention has been illustrated by a description
of various embodiments and while these embodiments have been described
in considerable detail, it is not the intention of the applicants
to restrict or in any way limit the scope of the appended claims
to such detail. Additional advantages and modifications will readily
appear to those skilled in the art. The invention in its broader
aspects is therefore not limited to the specific details, representative
apparatus and method, and illustrative example shown and described.
Accordingly, departures may be made from such details without departing
from the spirit or scope of applicant's general inventive concept.
|