Solid materials flow rate measurement

A flow gauge for measuring the mass flow rate of freely flowing material, falling through a path, said flow gauge comprising a pair of sensors spaced longitudinally along the chute, the sensors being connected to a computing means for receiving the output of each sensor, where said output is a measure of the density of the flow at the location of the sensor, the computing means utilizing the output of the sensors to provide a measure of the mass flow rate, said sensors comprising a radiation source and a radiation detector in opposed relation to the source and on the other side of the flowing material from the detector, for detecting the radiation passing through the flowing material.

This invention relates to the measurement of flow rate of materials. 
In the processing, or handling or loading of particulate materials such as 
ores and grains, it is usually necessary at some stage to measure the flow 
rate of material. Existing methods of carrying out this function involves 
the use of some type of gauge or weighing device placed over or otherwise 
used in conjunction with moving belts on which the material is being 
conveyed. There are several factors which limit the accuracy of these 
devices and they cannot be used where there is no conveyor belt or where 
the working area is very confined. 
It is an object of this invention to measure the flow rate of particulate 
material flowing under the influence of gravity from a chute feeder or 
like outlet. 
The term particulate material may refer to material composed of homogenous 
material of substantially uniform particle size and/or density or it may 
refer to heterogeneous material of differing particle size and/or density, 
or any combination of such parameters. In addition the material may be 
fluidised or in a slurry form. 
In one form the invention resides in a flow gauge for measuring the mass 
flow rate of freely flowing material, falling through a path, said flow 
gauge comprising a pair of sensors spaced longitudinally along the chute, 
the sensors being connected to a computing means for receiving the output 
of each sensor, where said output is a measure of the density of the flow 
at the location of the sensor, the computing means utilising the output of 
the sensors to provide a measure of the mass flow rate, said sensors 
comprising a radiation source and a radiation detector in opposed relation 
to the source and on the other side of the flowing material from the 
detector, for detecting the radiation passing through the flow material. 
In another form the invention resides in a flow gauge for measuring the 
mass flow rate of particulate material falling freely through a chute, 
said flow gauge comprising a pair of sensors spaced longitudinally along 
the chute and being connected to a computing means, said sensors 
comprising a radiation source and a radiation detector mounted to the 
chute in opposed relation on either side of the flow of particulate 
material, said sensing means providing an output during a no flow and a 
flow state in the chute and the computing means determines from the output 
of each sensor under flow and no flow conditions a measure of density at 
the location of the sensor, said measure of density being determined by 
##EQU1## 
("d" represents the area density of flow, "N" and "No" represents the 
sensor output under flow and no flow states and "a" represents the area of 
the cross sectional area of falling material) and wherein the measure of 
density at each sensor location is utilised to determine the velocity of 
the flow at one or the other sensor by 
##EQU2## 
(g represents the gravitational accelleration constant, s represents the 
spacing between the sensors, and d.sub.1 and d.sub.2 represents the 
measure of density obtained from the two sensors respectively) and the 
velocity is utilised to determine the mass flow rate by 
EQU R=V.times.d.times.W 
("d" represents the area density at the sensor location at which the 
velocity has been calculated, and W represents the width of the flowing 
material).

The embodiment as shown in the drawings comprises two gamma radiation 
sources 11 of Cs.sup.137 which are fixed to the side wall of the chute 13. 
Suitable radiation shielding is provided around the source 11 other than 
the opening which is directed in line with the diametric axis of the chute 
and is provided with a shutter (not shown) to selectively permit the 
escape of the radiation. Suitable collimation means 15 is associated with 
the aperture such as to reduce any scatter radiation from the source such 
that any radiation emitted therefrom is substantially in line with the 
diametric axis of the chute. The detectors 17 are disposed on the 
diametrically opposed side of the chute 13 from the source 11 such as to 
receive the peak radiation from the opposed source 11 and each detector is 
set to detect the characteristic energy radiation of the Cs.sup.137 source 
(i.e. 0.662MeV). The emission face of each collimator 15 is associated 
with an air outlet 19 which is in communication with a source of 
compressed air. The outlet is located such as to prevent material flowing 
past the emission face of the collimator from coming in contact with the 
face and adhering thereto. Similar compressed air outlets 21 are 
associated with each detector to maintain the collection face of each 
detector substantially clear of deposits. 
In locating the sources and detectors to the chute it is necessary that the 
spacing between the upper and lower set be precise. It is also necessary 
that the orientation of each source and its associated detector be precise 
in order that the detector be located to receive the maximum intensity of 
the radiation beam. 
The gamma radiation beam passing through the curtain of ore passing through 
the chute is attenuated by the ore. The ratio between the transmission of 
the gamma radiation through the chute with and without the ore flowing 
gives a transmission factor 
##EQU3## 
(N and No represent the count at the detector when ore is flowing and when 
ore is not flowing respectively for identical count periods). The area 
density of the ore curtain flowing through the chute can be calculated 
from the transmission factor by 
##EQU4## 
(a is the mass absorbtion coefficient of the ore) 
Knowing the area density of the ore curtain and the width of the curtain 
the velocity of the ore flow must be determined in order to calculate the 
actual ore flow through the chute. 
In order to determine the velocity the two sets of nuclear sources and 
detectors described above are used to determine the area density of the 
ore curtain at their respective locations through the transmission factor 
T. On its passage through the chute the ore is accelerated and thus the 
velocity of the ore between the nuclear source and detector set is 
increased and while the mass flow past each source detector set is the 
same, the ore densities measured are different. Thus from the basic law of 
linear motion 
EQU V.sub.2.sup.2 =V.sub.1.sup.2 +2gs 
(g is the gravitational acceleration) it may be established that the 
velocity at the upper source and detector set can be calculated from 
##EQU5## 
(s is the vertical spacing between the upper and the lower source and 
detector set). The mass flow rate can then be calculated as 
EQU R=d.sub.1 V.sub.1 W 
(W is the curtain width) 
As well as providing the instantaneous flow rate the gauge also can 
incorporate a counter which can accumulate the total tonnage which passes 
the source detector sets during a given period. 
In practice there will be a slight variation in the value of No because of 
diurnal and seasonal temperature variations which will cause the detector 
to drift slightly. There will also be a slight reduction in the value of 
No due to gradual decay of the radioactive source. These variations are 
allowed for by taking measurements of No at every shut down of the ore 
supply through the chute so that any trends which are present in the 
drifting of the zero count rate can be detected and the trend can be 
projected into the next run to ensure that the most accurate value 
possible is used for No. Provision may be made to cause a temporary 
shutdown during a run in the event of the current value of No being sensed 
to be unreliable in order to facilitate the determination of a reliable 
value of No. 
The nuclear source and detector sets are associated with a programmed 
computor which receives the count from the detectors and carries out the 
required calculation. The gauge incorporates a flow sensor to detect a 
flow of ore through the chute together with sensors to determine whether 
the shutters of the sources are closed or open. 
The programme of the computor of the gauge is such that during a period of 
there being no flow of ore the sensors provide a periodical No count which 
is used to update and maintain a current value of the zero transmission 
factor of the chute when no ore is flowing through the chute. In the event 
of the ore flowing through the chute a start up signal from a suitable 
sensor initiates the programme into its calculation mode. The count N 
which is obtained with the ore flowing through the chute is initially 
submitted to a series of tests to determine whether or not the value 
received for N is realistic or not. 
The tests on N basically comprise comparing the value obtained with the 
value of No to determine whether the value of N is larger or smaller than 
predetermined limits in which the anticipated value of N can be expected 
to fall. A count in excess of the upper limit would be an indication that 
the ore is in fact not flowing and that the initial start up signal was a 
false alarm. In such an event the programme would return to the No 
counting mode and await a fresh start up signal as well as providing a 
visual signal at the flow gauge read out of the false alarm. In the event 
of the value for N falling below an anticipated value such would be an 
indication that in fact the shutters of the sources are closed. In such an 
event the programme may initiate the necessary action to open the shutters 
and/or indicate at the gauge readout that a fault exists at the shutters. 
A further test may be applied to the incoming value of N to determine any 
drift in the value of N which may indicate some drifting of the detector 
due to temperature variations or like effects. If such drifting is 
detected as being possible, the programme initiates a shut down of the ore 
flow in order that the value of No may be rechecked and once a value has 
been established may reinitiate the ore flow. Alternatively the programme 
may only indicate that the value of No has possibly become unreliable 
without causing a shutdown of the ore flow. 
It has also been found desirable to subject the value of N and No to a 
filter in order to overcome the error resulting from the periodic 
fluctuation of the value of N and No due to the utilisation of a 
radioactive source. The filters are of an automatically programmable form 
which comprises mathematical equivalents of conventional electronic 
filters but having the specific advantages that their parameters adjust to 
follow the rate of flow of ore, thereby maximising the accuracy of flow 
measurement and the accuracy of total tonnage measurement. 
It should be appreciated that while the embodiment has been described in 
relation to one particular source of gamma radiation the invention need 
not be restricted to that source or indeed to the use of gamma radiation. 
The radiation of the sensors may comprise, any suitable form of 
electro-magnetic radiation, radioactive source radiation such as alpha or 
beta radiation or X-radiation, or ultrasonic radiation.