Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
1998-05-28
2001-03-20
Raevis, Robert (Department: 2856)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
Reexamination Certificate
active
06202494
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process and apparatus for measuring density and mass flow in respect to disperse systems with a gas as fluid phase.
KNOWN PROCESSES
Instruments for measuring mass flow in liquids and gases on the basis of a determination of the transit time of ultrasonic pulses are manufactured by, for example, Panametrics Ltd.
The book
Sensors
by Göpel W, Hesse J, Zemel J N, Volume 7 (Mechanical Sensors), published in 1993 by V C H Weinheim, ISBN 3-527-26773-5, contains a compilation of processes for measuring the most diverse physical magnitudes, inter alia also subject to the use of (ultra)sonic techniques. With a view to measuring density in multi-component gases, attention is drawn to the possibility of calculating the density of the gas from the measured speed of sound.
With a view to measuring flow rate and mass flow, in water and other liquids for example, processes are described in which ultrasonic pulses are transmitted obliquely through a pipe having the liquid flowing through it. In a special application for refineries etc, streams of leaking gas in gas flows are identified by the change in molecular mass.
(Smalling J W, Braswell L D, Lynnworth L C, Wallace D R, Proc. 39th Annual Symp. on Instrumentation for the Process Industries, ISA (1984) 27-38, and Smalling J W, Braswell L D, Lynnworth L C and U.S. Pat. No. 4,596,133 (1986)).
SUMMARY OF THE INVENTION
The present invention provides a process and device for measuring density and mass flow in respect to disperse systems with a gas as fluid phase, said device being characterized in that the speed of propagation of sound waves (sonic speed) is measured in the flowing disperse system both in the direction of flow and contrary to the direction of flow.
The sound waves may be used in the form of continuous sound or in the form of sonic pulses. Prior to further describing the present invention, some additional physical background discussion is provided.
1. Physical Background
1.1. Measuring the Suspension Density
In pure gases the speed of sound depends exclusively on the density and the compressibility of the medium. For instance, in air the following holds:
c
L
=
1
K
L
·
P
L
(
1
)
with c
L
=speed of sound in air [m/s], K
L
=compressibility of air [m
2
/N] and P
L
=density of air [kg/m
3
]. In disperse systems the speed of sound is generally strongly dependent on the wavelength of the sound and on the size of the dispersed particles. However, if the particles are very much smaller than the wavelength, with respect to the propagation of sound the suspension behaves like a homogeneous system and Equation (1) applies accordingly. Mean values of the two phases then have to be used for the density and compressibility, i.e.
C
susp
=
1
K
·
P
(
2
)
K
=(1−
C
V
)·
K
L
+C
V
·K
P
(3)
P
=(1−
C
V
)·
P
L
+C
V
·P
P
(4)
C
V
: volumetric concentration [m
3
/m
3
]
L: air: P: particles
If the speed of sound in such a suspension is measured, then from Equations (2) to (4) the volumetric concentration of the particles can be derived by calculation. In turn, the bulk density or transport density can easily be determined from this result.
FIG. 1
shows the sonic speed as calculated by Equation (2) plotted against the bulk density. For still higher values of the bulk density the sonic speed would again rise. If the density is able to assume such large values the unambiguous correspondence of a measured value of c to a bulk density is not possible. In the case of light bulk materials such as precipitated silicas (e.g. AEROSIL® precipitated silicas of Degussa Aktiengesellschaft) or carbon black, as a matter of principle the monotonously falling region of the curve applies.
Of course, a check has to be made as to whether the particles are sufficiently small in relation to the wavelength. The sound wave represents a vibration of the air which is transmitted more or less intensely to the particles. The stated assumption of a homogeneous medium then applies if the particles are fully able to conform to the vibration of the air—i.e. if their oscillation amplitude is exactly the same as that of the air. For the ratio of the two amplitudes, Skudrzyk in his book
Grundlagen der Akustik
(Springer Verlag 1954) states a relationship which can be solved for the sound frequency. If, arbitrarily, an amplitude ratio=0.99 is required, a sound frequency of f=2.5 MHz for a particle size of 35 nm AEROSIL® material results in this case.
One objective of the present invention is to make use of Eqn. (2) for a range of particle sizes that is as broad as possible and the invention therefore provides for measuring at frequencies that lie far below this limit, to be specific preferably in the range 20-2,000 Hz, in particular 105 to 2,000 Hz. In this frequency range the attenuation of the sound is also very slight, so that a particularly precise measurement appears to be possible through the choice of a very long measuring length.
However, macroscopic inhomogeneities in the suspension, such as large agglomerates for example, may constitute a problem. Such regions of relatively high bulk density are then no longer much smaller than the wavelength and they result in a frequency dependency of the speed of sound. But the errors arising as a result can be compensated by empirical calibration.
Another criterion to be taken into account, in accordance with the invention, in the choice of the measuring frequency is the diameter of the pipeline on which the measurement is to be carried out.
If an embodiment of the process is chosen in which the propagation of the sound is measured lengthwise in relation to the pipe axis then wave portions radiated obliquely may reach the receiver as a result of multiple reflection on the pipe wall, be superimposed with the waves transmitted directly and falsify the result.
In a preferred embodiment of the invention it is therefore stipulated that the frequency of the sound be chosen so as to result in a wavelength that is at least equal to one half of the diameter of the pipe. For such waves and longer waves, propagation is only possible in the direction of the pipe axis and the error is avoided.
On the other hand it has to be taken into consideration that the material oscillating with the sound wave experiences friction on the pipe wall, said friction being dependent on the condition of the pipe surface. This friction also results in a change in the speed of sound propagation which cannot be calculated in advance and which may also change in time-dependent manner, for example as a result of corrosion. The influence of the wall makes itself felt above all when the wavelength becomes clearly longer than the diameter of the pipe.
Therefore, in a preferred embodiment of the present invention, the wavelength should be between one half and 10 times the diameter of the pipe.
1.2 Measuring the Flow Velocity and the Mass Flow
If a sound wave is propagated in a flowing suspension in the same direction as the flow (‘with the flow’), a static observer sees a sonic speed that is increased by the flow velocity. In the case of propagation contrary to the flow the measurable value is correspondingly smaller. With a view to simultaneous measurement of suspension density and flow rate the two sonic speeds have to be measured with the direction of flow and contrary to the direction of flow as simultaneously as possible. The techniques for measuring the sonic speed are elucidated in more detail in the following section; as a rule it is the measurement of the time T needed by the wave in order to traverse the known distance L. Given the sonic speed c of the motionless suspension and the flow rate v, the distance-time law for propagation with the flow is
c
+
v
=
L
T
with
(
5
)
and for propagation contrary to the flow it is
c
-
v
=
L
T
contr
(
6
)
From these two equations the conditional equations follow for the two magnitudes being sought:
v
=
L
2
[
1
T
with
-
Khatchikian Peter
Riebel Ulrich
Degussa-Huls Aktiengesellschaft
Raevis Robert
Smith , Gambrell & Russell, LLP
LandOfFree
Process and apparatus for measuring density and mass flow does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process and apparatus for measuring density and mass flow, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process and apparatus for measuring density and mass flow will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2447967