Abstrict A coriolis-type mass flow meter is provided for use directly in
a flow line. The meter has at least two straight flow tubes that
have fluid flowing therethrough mounted with parallel axes. The
flow tubes are vibrated in a direction normal to the flow direction
and their axis by an exciting means at about 50% of their length.
Detector means are spaced equidistantly from the exciting means
(about 25% and 75% of the flow tube length) to detect the difference
in phase between the upstream and the downstream portions of the
flow tube relative to the exciting means. The phase difference between
the two detectors is proportional to the mass flow rate through
the flow tubes.
Claims What is claimed is:
1. Coriolis-type mass flow meter to be used in a flow line, comprising:
at least two straight flow tubes for having a fluid flow therethrough
having their longitudinal axes aligned in a parallel arrangement
and having their extreme ends rigidly disposed,
an exciting means disposed adjacent said tubes at about 50 percent
of the length of said flow tubes for vibrating said tubes in a direction
normal to the flow direction,
means adapted to detect the phase difference occurring between
upstream and downstream portions of the flow tubes when said flow
tubes are subjected to vibration by said exciting means and located
at equal distances from said exciting means, and
means adapted to connect the flow tubes to said flow line, wherein
said flow tubes comprise a circular arrangement of said tubes, and
wherein the vibration is rotational.
2. Coriolis-type mass flow meter to be used in a flow line, comprising:
sixteen straight flow tubes for having a fluid flow therethrough
having their longitudinal axes aligned in a parallel arrangement
and having their extreme ends rigidly disposed,
an exciting disposed adjacent said tubes at about 50 percent of
the length of said flow tubes for vibrating said tubes in a direction
normal to the flow direction,
means adapted to detect the phase difference occurring between
upstream and downstream portions of the flow tubes when said flow
tubes are subjected to vibration by said exciting means and located
at equal distances from said exciting means, and
means adapted to connect the flow tubes to said flow line.
Description BACKGROUND OF THE INVENTION
The invention relates to a Coriolis-type mass flow meter and, in
particular, to such a meter comprising at least two straight parallel
vibrating tubes.
These meters can be used, in particular, for the monitoring of
multi-phase flows in flow lines. Coriolis-type mass flow meters
comprising one straight vibrating tube are already known. These
meters are based upon the following principle: The flow to be measured
flows through a straight tube which is part of a flow line. The
ends of the tube are clamped. The tube itself is adapted to be excited
at 50 percent of its length and vibrates at or near its resonance
frequency. A mass flow through the tube causes distortion of this
forced vibration and a phase difference to occur between the upstream
and downstream part of the tube, which is proportional to mass flow.
The theoretical relations between mass flow rate and phase difference
are known to those skilled in the art and will not be explained
in detail. The length of the meter is a critical parameter: the
meter sensitivity increases proportionally to the total tube length
and in multi-phase flow a minimum length-to-inner diameter ratio
is required.
However, for practical applications the length of the meter should
be limited and therefore the diameter of the vibrating tube must
be reduced with respect to the flow line diameter. For one straight
vibrating tube, this results in too high a pressure loss in the
meter, which may cause serious measuring problems.
It is therefore an object of the invention to provide a Coriolis-type
mass flow meter of restricted length, which is suitable to be used
for monitoring multi-phase flow and can be handled easily.
SUMMARY OF THE INVENTION
The invention therefore provides a Coriolis-type mass flow meter
to be used in a flow line, said meter comprising at least two straight
parallel flow tubes for having a fluid flow therethrough, said flow
tubes being adapted to be vibrated by an exciting means at 50 percent
of the length of the flow tubes in a direction normal to the flow
direction, and means adapted to detect the phase difference occurring
between upstream and downstream parts of the flow tubes, at equal
distances from the exciting means, when subjected to vibration at
a certain frequency, and means adapted to connect the flow tubes
to the flow line.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 represents a schematic three dimensional view of an embodiment
of the invention.
FIG. 2 is a simplified block diagram of the exciting and detecting
arrangement for one representative flow tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to the drawings, FIG. 1 shows a system 1 of a bundle
of 16 parallel straight tubes 2 and a simplified illustration of
the exciting and detecting arrangement for one of these flow tubes
is shown in FIG. 2.
The system 1 is provided with flanges 3 3a at its ends. In this
way the system is adapted to be mounted into a flow line (not shown)
for example, with flanges 3 and 3a clamped or fixed within a substantially
rigid cylindrical flow line.
The system is further provided with exciting means adapted to vibrate
the system of tubes at 50 percent of their lengths. The exciting
means are not represented for the sake of clarity, but are arranged
to impart transverse or normal vibration to the tubes.
The system is further provided with distortion detecting means
(not represented for the sake of clarity). These means are provided
at detecting points 5 and 5a at 25 percent and 75 percent of the
tube lengths respectively
In this embodiment the tubes have lengths of 0.9 meter; each tube
has an inner diameter of 20 mm and a wall thickness of 0.5 mm.
Since, in the figure, the tubes 2 are fixed together at the exciting
point 4 and detecting points 5 5a respectively, they can be considered
as one single tube. The operation of the system is as follows: The
system or bundle of tubes is mounted in the flow line in any way
suitable for the purpose. The space B--B.sup.1 between the tubes
2 is closed by any means suitable for the purpose (not shown for
reasons of clarity). A fluid is flowing through all separate parallel
tubes 2 of the system 1 by any means suitable for the purpose, for
example, a pump (not shown). The mounting of the system in the flow
line is carried out in such a way, that the mounted system has clamped
ends (not shown).
The system is excited at 50 percent of its length to give it a
vibration in a direction normal to the flow direction. The flow
direction is represented by the arrows A. Only a few arrows A have
been represented for reasons of clarity. When the fluid flows through
the tube-system 1 the fluid particles are forced to follow the
transversal vibration of the tube walls.
In the first half of the tube, i.e., the upstream part, the amplitude
of the normal or transversal vibration of each fluid particle will
increase, whereas in the second half (i.e. the downstream part)
of the tube it will decrease. The interaction between fluid and
tube wall yields a force acting on the tube wall, which is known
as the "Coriolic force". The term "Coriolic force"
is known to those skilled in the art and will not be explained in
further detail. In the first half of the tube the Coriolis force
tends to slow down the vibration of the tube, since energy has to
be stored in the vibration of the fluid, but in the second half
of the tube the Coriolic force tends to stimulate the vibration:
the energy is released again from the fluid.
The result is that the flow of a fluid through the tubes yields
a S-shaped distortion of the tubes, the amplitude of which is proportional
to the mass flow rate. Since the distortion occurs in a system vibrating
at a certain frequency, the phase of the oscillation of the various
points of the tubes is no longer equal. The upstream part of the
tubes has a phase lag, whereas the downstream part leads in in phase.
This consequence of the distortion allows the distortion to be measured
in a simple way: the oscillating motions of the upstream and downstream
parts of the tubes are detected by mounting suitable detectors,
for example accelerometers, on the tubes, at about 25 percent and
75 percent of the tube lengths respectively. The phase difference
or time lag between these two (sinusoidal) signals is now proportional
to the mass flow through the tube. A variety of methods for measuring
this phase difference is known to those skilled in the art and will
not be described in detail.
It will be appreciated that the system of tubes may comprise any
number of parallel-straight tubes suitable for the purpose, provided
that this number is at least two. In advantageous embodiments of
the invention the number of tubes is 3-24.
It will further be appreciated that tubes of any length and diameter
suitable for the purpose can be used. In an advantageous embodiment
of the invention the length-to-inner diameter ratio is larger than
50. The advantage of the use of tubes having a large length-to-inner
diameter ratio is the increases sensitivity of the apparatus.
It will also be appreciated that several ways of arranging the
tubes of the system and modes of vibrating the tubes are possible:
for example, a bundle of tubes or a horizontal row of tubes, which
are vibrated in a vertical direction or a system of tubes arranged
in a circle with rotational vibration.
In case of two parallel tubes, the tubes may be vibrating in antiphase.
The advantage of the above two ways of vibration is the balance
of motion. In the above mentioned embodiments each individual tube
always performs a linear vibration. However, it will be appreciated
that a circular translation of the tube or bundle of tubes may be
carried out, for example, by combining two linear vibrations--one
moving horizontally, the other vertically, and 90.degree. out of
phase. The latter mode of vibration may be advantageous in multiphase
flow, if the residence time of the individual fluid particles in
the tube might not cover a sufficiently large number of periods
of the vibration to ensure proper measurement.
It will be appreciated that the tubes may consist of any material
suitable for the purpose.
It will further be appreciated that any suitable exciting means
and any suitable detecting means may be used.
Further, the tubes need not be necessarily fixed at the exciting
and detecting points. Various modifications of the invention will
become apparent to those skilled in the art from the foregoing description
and accompanying drawing, and are intended to fall within the scope
of the appended claims. |