Angular momentum mass flowmeter

An angular momentum mass flowmeter for measuring the mass flowrate of a fluid stream includes a flowmeter housing having an inlet or upstream end for receiving the fluid stream and an outlet or downstream end for discharging the fluid stream. A rotatable impeller is provided for measuring the mass flowrate. A swirl cap is disposed downstream of the impeller for imparting angular momentum to the fluid stream passing thereover and for causing the angular momentum to rotate the impeller. A control valve, disposed intermediate the impeller and the swirl cap, modifies the flow area of the fluid stream passing over the swirl cap to regulate the angular momentum of the fluid stream.

BACKGROUND OF THE INVENTION 
The present invention relates to an angular momentum mass flowmeter (AMMF) 
and more particularly to a fluid-driven AMMF. 
An angular momentum mass flowmeter (AMMF) employs a motor- or fluid-driven 
device to impart a known angular velocity (and hence angular momentum) to 
the fluid flow to be measured relative to a rotating spring-restrained 
impeller. As the fluid flow impinges on the vanes of the impeller, the 
relative angular momentum of the fluid flow produces a torque on the 
impeller which is proportional to the mass flowrate of the fluid and the 
angular speed of the impeller. This relative angular momentum sensed by 
the impeller is interpreted as an indication of the mass flow rate of the 
fluid. 
In some of the AMMF's, the angular momentum is created by an electrically 
powered device which introduces an angular fluid-flow component to the 
fluid flow, thereby instilling angular momentum. The modern AMMF's more 
frequently utilize the momentum of the incoming fluid flow to mechanically 
introduce an angular fluid-flow component thereto. Since the fluid-driven 
device does not require an electrical power source, it is generally 
preferred over the electric motor-driven device because of its lower cost 
and lower weight. The present invention is directed to an AMMF utilizing a 
fluid-driven device to generate the angular momentum. 
More particularly, in a fluid-driven device the angular momentum is 
developed by the combined functions of a flow control valve and a swirl 
generator (or swirl cap) downstream thereof. The swirl generator defines a 
series of helical grooves about its periphery in order to give the fluid 
stream passing thereby a swirl velocity (i.e., an angular momentum) as the 
fluid passes through the helical grooves. The control valve defines a 
plurality of spring fingers which restrain the fluid flow so that, at the 
lower flow rates, all of the fluid flow passes through the helical grooves 
of the swirl generator. As the fluid-flow rate is increased, however, the 
pressure drop through the helical grooves increases, thereby creating an 
outward force on the spring fingers. At some point (approximately 1,000 
pounds per hour of fluid flow), the outward force developed on the spring 
fingers is sufficient to lift the spring fingers off and away from the 
grooved surface of the swirl generator so that not all of the fluid flow 
enters the helical grooves. As flow is increased beyond this point, the 
plurality of spring fingers continue to open, thereby allowing an 
increasing portion of the fluid flow to bypass the helical grooves of the 
swirl generator. Thus the plurality of spring fingers essentially 
constitute a pressure-operated valve which uses the pressure drop across 
the swirl generator to regulate the amount of swirl imparted by the swirl 
generator to the fluid stream. 
The position of the swirl generator relative to the spring fingers and the 
tension of the spring fingers are among the variables which may be 
controlled in order to ultimately control speed and "start-up" rates. The 
position of the spring fingers relative to the swirl generator provides a 
controlled angular speed of the fluid flow which acts on the downstream 
turbine vanes and minimizes the level of pressure drop in the fluid-drive 
section of the flowmeter at the higher fluid-flow rates. In other words, 
the spring fingers act as a control valve to modify the flow area as a 
function of pressure drop in order to regulate the angular momentum of the 
fluid flow and the rate of rotation of the impeller. 
The conventional AMMF produces the angular momentum in the fluid flow 
upstream of the torque-sensing element (i.e., the impeller). The drawback 
of the conventional AMMF which produces the angular momentum upstream of 
the torque-sensing element is that the changing flow-passage geometry 
(which is a function of the spacing between the spring fingers of the 
control valve and the swirl generator) alters the exit-flow profile from 
the swirl generator as a function of flow rate. The exit-flow profile can 
be described as having an average radius of gyration (r) about which fluid 
flows at a nominal rate and enters the passage of the impeller. The mass 
flow rate is directly related to the square of the average radius of 
gyration (r.sup.2) of the flow exiting the impeller. As a result, the 
accuracy of the flowmeter is dependent upon the stability of the 
flow-velocity profile of flow entering the impeller. Where the 
fluid-driven device is disposed upstream of the impeller, changes in the 
velocity profile of fluid exiting the fluid-driven device influence the 
impeller located downstream thereof. 
Accordingly, it is an object of the present invention to provide an AMMF 
wherein the fluid-drive device (that is, the swirl generator) is disposed 
downstream of the torque-sensing element (that is, the impeller). 
Another object is to provide such a flowmeter wherein the variable flow 
profile associated with the fluid stream exiting the helical grooves of 
the swirl generator over the wide range of possible flow rates does not 
negatively affect the radius of gyration of the fluid flow through the 
impeller. 
A further object is to provide such a flowmeter wherein the torsion spring 
is positioned for easy accessibility for repair or replacement, and the 
skew vane and a sensing coil are easily accessible for calibration 
purposes. 
SUMMARY OF THE INVENTION 
The above and related objects and features of the present invention are 
obtained in an angular momentum mass flowmeter (AMMF) for measuring the 
mass flow rate of a fluid stream. The flowmeter comprises a housing having 
an inlet or upstream end for receiving the fluid stream and an outlet or 
downstream end for discharging the fluid stream. Rotatable measuring means 
are provided for measuring the mass flowrate, as are means disposed 
downstream of the measuring means for imparting angular momentum to the 
fluid stream passing thereover and for causing the angular momentum to 
rotate the measuring means. A control valve means is disposed intermediate 
the measuring means and the momentum imparting means for modifying the 
flow area of the fluid stream passing over the momentum imparting means to 
regulate the angular momentum of the fluid stream. 
In a preferred embodiment the flowmeter comprises a flowmeter housing for 
receiving a fluid stream, the mass flowrate of which is to be measured. A 
shaft extends axially through, and is rotatable relative to, the housing. 
A stationary flow conditioner is disposed in the housing adjacent the 
entry point of the flow stream thereinto and defines vaned passages for 
the fluid stream. A rotating impeller and shroud unit is disposed in the 
housing downstream of the flow conditioner. The shroud is secured to the 
shaft for rotation by and with the shaft as a unit, while the impeller 
defines vaned passages for the fluid stream and is resiliently linked to 
the shaft by a torsion spring for rotation by and with the shaft under 
steady state flow conditions. The torsion spring is secured at one end to 
the impeller and at an opposite end to the shaft. A stationary swirl cap 
is disposed in the housing downstream of the impeller and has a spherical 
surface defining helical grooves thereon for imparting angular momentum to 
the fluid stream passing thereover. A rotating turbine is disposed in the 
housing downstream of the swirl cap and defines vaned passages for the 
fluid stream, the turbine being rotated by the angular momentum of the 
fluid stream passing therethrough and the shaft being secured to the 
turbine for rotation by and with the turbine. A control valve means is 
disposed closely adjacent to and upstream of the swirl cap for modifying 
the flow area to regulate the angular momentum of the fluid stream leaving 
the swirl cap and hence the rate of rotation of the turbine. A first 
magnet is disposed on the impeller for rotation therewith, and a second 
magnet is disposed on the shroud for rotation therewith, the first and 
second magnets producing pulses and the lag between pulses being 
indicative of the mass flowrate of the fluid stream. Thus the fluid stream 
entering the housing passes successively through the flow conditioner 
passages, the impeller passages, over the swirl cap, and through the 
turbine passages in order to exit the housing. 
Preferably the control valve means comprises a pressure-operated valve 
means responsive to the flowrate of the fluid stream therethrough to 
regulate the amount of angular momentum imparted by the swirl cap to the 
fluid stream passing thereover. More particularly the valve means 
comprises a plurality of spring fingers disposed upstream of the swirl cap 
and biased inwardly towards the swirl cap. The spring fingers retreat 
outwardly from the swirl cap in response to an increased flowrate of the 
fluid stream to lower the angular momentum imparted by the swirl cap to 
the fluid stream passing thereover. Optionally the disposition of the 
plurality of spring fingers is a variable distance upstream of the swirl 
cap. 
The novel design of the flowmeter affords several advantages. The torsion 
spring is disposed substantially intermediate the flow conditioner and the 
impeller and shroud unit for easy access. The skew vane is disposed in the 
housing intermediate the flow conditioner and the impeller and shroud unit 
for modifying the angular momentum of the flow stream, the skew vane being 
disposed in longitudinal alignment with one of the vaned passages of the 
flow conditioner and being tiltable from outside the housing relative to 
the flow direction of the fluid stream. The flowmeter additionally 
includes sensing coils for detecting the movements of the first and second 
magnets, the coils being disposed on the housing coaxially with the shaft 
and at fixed radial distances from the impeller and shroud unit. Threaded 
engagement means are provided for arcuately circumferentially displacing 
one of the sensing coils relative to the other for calibration purposes 
without varying the radial distance thereof from the impeller and shroud 
unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, and in particular to FIG. 1 thereof for an 
overview, therein illustrated is an angular mass momentum flowmeter 
according to the present invention, generally designated by reference 
numeral 10. The main and outlet housings have been removed to reveal 
details of internal construction. 
The fluid flow to be measured enters into the flowmeter 10 through the 
inlet flow conditioner, generally designated 20. After being conditioned 
by the inlet flow conditioner 20, the flow passes through the vaned 
passages of an impeller 70 and a shroud 60 disposed about the impeller 70. 
While the shroud 60 is linked with a rotatable shaft 30 for rotation 
therewith as a unit, the impeller 70 is linked to the rotatable shaft 30 
by a torsion spring 76 so that the impeller 70 rotates with the shaft 30 
under steady-state conditions but lags behind the shaft 30 (and shroud 60) 
as a result of the presence of the fluid flow passing through the passages 
of the impeller 70. The degree of lag increases with increasing flow rates 
and decreases with decreasing flow rates. A magnet 110 is fixedly disposed 
in the shroud 60, and a magnet 112 (not shown in FIG. 1) is fixedly 
disposed on the impeller 70, so that the lag in rotation between the 
impeller 70 and the shroud 60 can be determined by appropriately 
positioning sensors 114, 116 (e.g., electrical sensing coils). 
After passage intermediate the impeller 70 and shroud 60, the fluid flow 
passes through a strut and spring-finger assembly generally designated 80. 
The spring fingers 90 of the assembly 80 channel the fluid flow onto a 
swirl cap 100 defining on its outer surface a plurality of helical grooves 
102 for imparting angular velocity or swirl to the fluid passing 
therethrough. The assembly 80 acts as a valve since the spring fingers 90 
which funnel the fluid flow from the assembly 80 onto the swirl cap 100 
tend to separate and open as the flow (and hence the back-pressure) of the 
fluid flow increases, so that less of the fluid flow is channeled into the 
helical grooves 102 and thus less swirl is imparted to the fluid flow. 
The fluid flow from the swirl cap 100 passes through and rotates the 
turbine 46 and then exits the flowmeter 10. Turbine 46 is secured to the 
rotatable shaft 30 so that rotation of the turbine 46 causes a like 
rotation of the shaft 30. As earlier noted, the shroud 60 rotates with the 
shaft 30, and the impeller 70 rotates with the shaft 30 under the 
steady-state conditions. 
Referring now to FIGS. 2 and 3 in particular, therein illustrated is the 
complete angular momentum mass flowmeter 10 according to the present 
invention. The main housing 12 of the flowmeter 10 defines an inlet 14 
through which the fluid flow to be measured enters, as indicated by the 
arrows 16 traveling from left to right in FIG. 3. Disposed along the 
longitudinal axis of the housing 12 is a stationary (non-rotatable) inlet 
flow conditioner, generally designated 20, defining radial vanes 22 and 
vaned passages 24 through which the fluid flow passes so as to eliminate 
any non-longitudinal component thereof. 
A rotatable shaft 30 is disposed along the longitudinal axis of the housing 
12. The shaft front end is mounted in the inlet flow conditioner 20 (not 
shown in FIG. 2) by means of a ball bearing 32, a washer 34 and a locknut 
36, and the shaft rear end is mounted in an outlet housing 40 defining a 
fluid-flow outlet 42 through which the fluid flow exits from the flowmeter 
10, as indicated by arrows 44. More particularly, the rear end of the 
rotatable shaft 30 is disposed in a turbine 46 and is secured thereto by 
means of a washer 34 and a locknut 36. Throughout the length of the 
rotatable shaft 30 (which operatively includes various spacers 30' to 
enable axial play), additional support is provided by various flanged ball 
bearings 32 surrounded by shim washers 35. 
A hollow, cylindrical shroud 60 and an impeller of smaller diameter, 
generally designated 70, are concentrically mounted on the rotatable shaft 
30. The impeller 70, like the inlet conditioner 20, defines a plurality of 
radial vanes 72 and vaned passages 74 through which the fluid flow passes. 
Whereas the shroud 60 is directly connected to the rotatable shaft 30 at 
62 for rotation therewith as a unit, the impeller 70 is connected to the 
shaft 30 by a torsion spring 76, which has its inner end connected to the 
rotatable shaft 30 for rotation therewith as a unit by a hub 79 and its 
outer end attached to the impeller 70 for rotation therewith as a unit by 
a spring clamp 77 (see FIG. 2). Accordingly, under steady-state 
conditions, the impeller 70 rotates with the rotatable shaft 30. The 
presence of the fluid stream passing through the vaned passages 74 in the 
impeller 70 causes the rotation of the impeller 70 to lag behind the 
rotation of the rotatable shaft 30 and shroud 60, the degree of lag 
increasing with increasing flow rates and decreasing with decreasing flow 
rates. 
It will be appreciated that, contrary to conventional design which places 
the torsion spring 76 in a "nested" disposition between two rotating 
components (i.e., the impeller 70 and the shroud 60), the design of the 
present invention locates the torsion spring 76 on the inlet or front ends 
of the impeller 70 and the shroud 60, thereby to simplify access to the 
torsion spring 76, as necessary when the torsion spring 76 must be 
replaced or its length adjusted. Instead of the conventional extensive 
disassembly required for access to the torsion spring 76, essentially only 
removal of the outlet housing 40 followed by removal of the shaft 30 and 
the elements attached thereto is required in order to enable access to the 
torsion spring 76. 
The skew vane 52 is a single vane disposed along the length of the 
flowmeter intermediate the impeller 70 and the inlet flow conditioner 20 
and along the circumference of the flowmeter between a pair of the 
flow-conditioner vanes 22. The position of the vane 52 relative to the 
direction of fluid flow determines whether the fluid torque which will 
measured by the impeller 70 is increased or decreased. The vane position 
can be adjusted to correct for errors initiated by surface imperfections 
of the vanes of the inlet flow conditioner 20 or the impeller 70, which 
might otherwise result in a false indication of the flow rate. The effect 
of the skew vane 52 is proportional to the square of the mass flow rate 
through the impeller 70. Access to the vane is enabled by loosening a 
removable cover 54 which is secured over a port 55 penetrating the main 
housing 12. The cover is removably fastened to the housing 12 by means of 
a pair of screws 56. A radial elastomeric seal 58 is deployed in port 55 
to preclude fluid leakage therethrough. The port 55 permits the skew vane 
52 to be appropriately oriented, for example, via a hexagonal socket or 
screwdriver slot. 
This positioning of the skew vane 52 represents an improvement over the 
conventional skew vane design wherein the skew vane is disposed within the 
inlet flow conditioner 20 and in line with a vane 20 thereof. The 
longitudinal disposition of the skew vane 52 intermediate the flow 
conditioner 20 and the impeller 70 according to the present invention 
allows the skew vane 52 a greater adjustment range since it is not 
restricted within vaned passages 24 of the inlet flow conditioner 20. The 
circumferential disposition of the skew vane 52 between two adjacent 
flow-conditioner vanes 22 according to the present invention makes the 
skew vane 52 less sensitive to misalignment than the conventionally 
positioned skew vane which must theoretically be in perfect alignment with 
a flow-conditioner vane 22 in order to avoid secondary flow effects which 
might result from the skew vane becoming misaligned and having the effect 
of staggering the flow-conditioner vane 22. 
Disposed downstream of the impeller 70/shroud 60 assembly is a strut and 
spring-finger assembly generally designated 80. The assembly 80 is 
non-rotatably disposed about the shaft 30 by means of struts 82 and a 
sleeve 84, the position of the assembly 80 along the longitudinal axis of 
the rotatable shaft 30 optionally being adjustable. The downstream end of 
assembly 80 defines a plurality of spring fingers 90 which funnel the 
fluid flow from the assembly 80 onto a swirl cap or swirl generator, 
generally designated 100. 
The swirl cap 100 is non-rotatably disposed in the outlet housing 40 and 
defines a plurality of helical grooves 102 machined about the outer 
spherical surface thereof, the helical grooves 102 imparting an angular 
velocity or swirl to the fluid passing therethrough. The degree of swirl 
or angular velocity imparted to the fluid flow by the helical grooves 102 
of the swirl cap 100 will depend upon the longitudinal closeness of the 
assembly 80 and the radial closeness of the spring fingers 90. As the 
fluid flow increases, the pressure on the spring fingers 90 tends to force 
them radially outwardly so that less of the fluid flow is channeled 
through the helical grooves 102 and thus less swirl is imparted to the 
fluid flow, and vice versa. Thus, the spring fingers 90 of assembly 80 
work as control valves to modify the flow area (that is, the flow profile) 
as a function of pressure drop across the swirl cap 100, thereby to 
regulate the angular momentum and the rate of rotation of the sensing 
element 70. Similarly, the longitudinal position of the assembly 80 
relative to the swirl cap 100 may be manually adjusted, with increasing 
longitudinal separation between the two decreasing the effect of the 
helical grooves 102 of swirl cap 100 in imparting swirl to the fluid flow, 
and vice versa. 
Downstream of the swirl cap 100, the turbine 46 with its radial vanes 110 
and vaned passages 112 receives the fluid flow exiting the swirl cap 100. 
The fluid flow has an angular velocity (as a result of passage over swirl 
cap 100) and thus imparts rotation to the turbine 46. The vanes 110 of the 
turbine 46 may be straight or contoured, as desired for particular 
applications. The rotatable shaft 30 is secured to the turbine 46 for 
rotation therewith as a unit, so that the rotation of the turbine 46 in 
response to the fluid flow results in a rotation of the rotatable shaft 
30. The rotation of turbine 46 is in turn imparted via shaft 30 to shroud 
60 and impeller 70. Thus no electrical power input is required to provide 
rotation of the impeller 70 and shroud 60 of the flowmeter of the present 
invention, all necessary mechanical power for rotation of the shaft 30 and 
related components being obtained from the momentum of the fluid flow 
through the turbine 46. 
In order to generate an electrical output signal indicative of the mass 
flow rate, the shroud 60 (which rotates as a unit with the shaft 30) is 
provided with a permanent magnet 110, and the impeller 70 (which rotates 
with the shaft 30 under steady-state conditions) is provided with a 
separate permanent magnet 112. Disposed on the outer surface of the main 
housing 12 transverse to the axis of shaft 30 are two separate pick-off or 
sensing coils 114, 116, each disposed about a magnetically-permeable core 
117: a start coil 114 which is radially disposed relative to the shroud 
magnet 110, and a stop coil 116 which is radially disposed relative to the 
impeller magnet 112. As the magnets 110, 112 rotate with the shaft 30 
relative to the coils 114, 116, respectively, the magnets 110, 112 produce 
sinusoidal pulse wave forms in their respective radially aligned coils 
114, 116. The interaction of the shroud magnet 110 and its associated 
start coil 114 produces a "start" electrical pulse each time the magnet 
110 passes the magnetically-permeable core of the coil 114, just as the 
interaction of the impeller magnet 112 with its associated stop coil 116 
produces a "stop" electrical pulse each time the magnet 112 passes the 
magnetically-permeable core of the coil 116. The time elapsed between the 
"start" and "stop" pulses is directly related to the mass rate of fluid 
flow--that is, the lag of the impeller 70 relative to the shroud 60. 
It will be appreciated that a movement of either of the two sensing coils 
114, 116 with respect to the other will result in a change in timing for 
the "zero" flow position. Accordingly, the present invention provides a 
precise method for adjusting the angular position of the stop coil 116 
relative to the start coil 114 such that the adjustment cannot be 
accidentally lost during the life of the unit. Referring now to FIG. 4 in 
particular, two screws 120 attach the stop coil 116 to the main housing 
12. When the two screws securing the stop coil 116 are loosened, the stop 
coil 116 is permitted limited circumferential movement about the axis of 
the housing 12. A pin 122 is secured to and protrudes from the stop coil 
116 and is disposed within an arcuate slot 124 in the housing 12. Two 
screws 126 are threaded into the housing 12 and bear against the pin 122 
from opposite directions, thereby to enable movement of the pin 122 in 
either direction along an arc centered about the housing axis. To provide 
for the desired zero-timing adjustment, the screws 126 may be adjusted to 
appropriately fix the pin 122, and thus the disposition of the stop coil 
116 once the mounting screws 120 have been loosened. Once the desired 
adjustment is made using adjustment screws 126, the mounting screws 120 
are re-tightened, after which the adjustment screws 126 may be loosened or 
even removed as they are no longer required since the mounting screws 120 
now fix the position of the stop coil 116 relative to the housing 12. The 
aforementioned mechanical control provides a simple mechanism which 
enables a more accurate and sensitive adjustment than in the conventional 
flowmeter. If preferred, the start coil 114 may be made adjustable rather 
than the stop coil 116. As an option, two shoulder screws can take the 
place of screws 126 and pin 122, to provide the functions of securement 
and maintenance of a constant arc. 
In the conventional flowmeter, the zero adjustment is provided by loosening 
lock-down screws to enable the operator to relocate one or both of the 
signal sensing coils 114, 116 to either delay or advance the signal 
occurrence. However, the degree of control by which this adjustment is 
made is dependent upon the direct control and sensitivity which the 
operator can apply by hand. On the other hand, the present invention 
provides a mechanical control for relative relocation of the sensing coils 
114, 116 which reduces the dependency on operator skill. Further, the 
adjustment mechanism maintains the signal magnet 110, 112 to signal coil 
114, 116 airgap throughout the adjustment range, thereby maintaining a 
constant signal amplitude. 
Now that the preferred embodiments of the present invention have been 
described in detail, various modifications and improvements thereon will 
become readily apparent to those skilled in the art. Accordingly, the 
spirit and scope of the present invention is to be construed broadly and 
limited only by the appended claims, and not by the foregoing 
specification.