Method and corresponding sensors for measuring mass flow rate

Details Method and corresponding sensors for measuring mass flow rate Method and corresponding sensors for measuring mass flow rate

1999-04-01

2002-03-26

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

Reexamination Certificate

active

06360614

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method based on the Coriolis principle and to sensors for measuring the mass flow rate of a fluid flowing through a pre-existing, permanently installed pipe, or through a single measuring tube to be inserted into a pipe, on the Coriolis principle.
BACKGROUND OF THE INVENTION
Present-day mass flow sensors of mass flowmeters are manufactured as measuring instruments which are installed by the final customer into a pre-existing pipe in situ.
In the case of one type of ultrasonic flowmeters, i.e., in the case of flowmeters based on a different physical principle of measurement, it has been customary for a long time to secure ultrasonic transmitters and sensors on the external surface of a permanently installed pipe; such devices are commonly referred to as “clamp-on ultrasonic flow sensors”.
It is desirable to apply the clamp-on design principle to Coriolis mass flow sensors and flowmeters, so that the mass flow rate of a fluid flowing through a permanently installed pipe can be measured on the Coriolis principle.
U.S. Pat. No. 5,321,991 discloses a Coriolis effect mass flowmeter with a corresponding sensor which is formed by means of an existing, permanently installed pipe through which a fluid flows at least temporarily, the sensor being characterized in that
the pipe is fixed to a support at two points spaced a predetermined distance L apart for defining a measuring length forming a pipe section,
approximately in the middle of one half of the pipe section, a driver is mounted
which excites the pipe section at a frequency f in a second mode of vibration in a first plane containing an axis of the pipe section,
either a single motion sensor is mounted in the middle of the pipe section
or a first and a second motion sensor are mounted at a distance from each other near the middle of the pipe section,
with evaluation electronics deriving a signal representative of the mass flow rate from the amplitude of the single sensor signal provided by the motion sensor or from the amplitude of the sensor signals provided by the two motion sensors, respectively.
Since it evaluates exclusively the amplitude(s) of the sensor signal(s), the prior-art assembly requires a further sensor which is mounted at one of the fixing points to suppress disturbances originating from the pipe and thus achieve sufficient measurement accuracy.
SUMMARY OF THE INVENTION
It is therefore a general object of the invention to improve and refine the clamp-on design principle of Coriolis mass flowmeters in such a way that optimum accuracy is achieved. This general object includes, firstly, that not the amplitudes of the sensor signals are evaluated, secondly, that two spaced-apart sensors are provided, and thirdly, that the measuring length or the length of the vibrating pipe section is precisely predefined. This means that a section of the pipe has to be configured and defined so that it can serve and act as a measuring length.
Another object is to apply the principle underlying the invention for pre-existing and permanently installed pipes to conventional installation Coriolis mass flow sensors, i.e., to make this principle usable in a separately manufactured device which is to be installed into a pipe as a finished mass flow sensor.
The following variants of the invention serve to attain these objects.
A first variant of the method according to the invention provides a method based on the Coriolis principle for measuring the mass flow rate of fluids one of which flows at least temporarily through a pre-existing, permanently installed pipe or through a single measuring tube to be inserted into a pipe, said method comprising the steps of:
fixing a first and a second isolating body having identical masses to the outside of the pipe or the measuring tube at a predetermined distance L from each other to define a measuring length forming a pipe or tube section, each of the identical masses being at least five times as great as the mass of the pipe or tube section;
attaching in the middle of the pipe or tube section a vibration exciter
which excites the pipe or tube section in a third mode of vibration, in a first plane containing an axis of the pipe or tube section, at a frequency f which, if the pipe or tube section is filled with one of the fluids, lies between approximately 500 Hz and 1000 Hz;
said distance L being calculated by the following formula:
L
=5.5·2
½
·(2
&pgr;f
)
−½
·{E
(
r
4
a
−r
4
i
)/(d
M
+d
F
)}
−¼
where
r
a
is the outside diameter of the pipe or tube section,
r
i
is the inside diameter of the pipe or tube section,
E is the modulus of elasticity of the material of the pipe or tube section,
d
M
is the product of the density of the material of the pipe or tube section and the cross-sectional area of the wall of the pipe or tube section, and
d
F
is the product of the mean density of the fluids and the cross-sectional area of the lumen of the pipe or tube section;
each of said isolating bodies
having a first axis lying in the first plane, a second axis identical with the axis of the pipe or tube section, and a third axis perpendicular to the first and second axes, and
having an areal moment of inertia about the first axis which is at least one order of magnitude less than its areal moment of inertia about the third axis;
fixing a first acceleration sensor and a second acceleration sensor to the pipe or tube section at respective positions where, if the pipe or tube section is excited in the third mode of vibration, a deflection of the pipe or tube section caused by a disturbance originating from the pipe has a first zero and a second zero, respectively;
determining a phase difference between a first sensor signal provided by the first acceleration sensor and a second sensor signal provided by the second acceleration sensor; and
deriving therefrom a signal proportional to the mass flow rate.
A second variant of the method according the invention provides a method based on the Coriolis principle for measuring the mass flow rate of fluids one of which flows at least temporarily through a pre-existing, permanently installed pipe or through a single measuring tube to be inserted into a pipe, said method comprising the steps of:
fixing a first and a second isolating body having identical masses to the outside of the pipe or the measuring tube at a predetermined distance L from each other, each of the identical masses being at least five times as great as the mass of the pipe or tube section;
attaching in the middle of the pipe or tube section a vibration exciter
which excites the pipe or tube section in a third mode of vibration, in a first plane containing an axis of the pipe or tube section, at a frequency f which, if the pipe or tube section is filled with one of the fluids, lies between approximately 500 Hz and 1000 Hz;
said distance L being calculated by the following formula:
L
=5.5·2
½
·(2
&pgr;f
)
−½
·{E
(
r
4
a
−r
4
i
)/(d
M
+d
F
)}
−¼
where
r
a
is the outside diameter of the pipe or tube section,
r
i
is the inside diameter of the pipe or tube section,
E is the modulus of elasticity of the material of the pipe or tube section,
d
M
is the product of the density of the material of the pipe or tube section and the cross-sectional area of the wall of the pipe or tube section, and
d
F
is the product of the mean density of the fluids and the cross-sectional area of the lumen of the pipe or tube section;
each of said isolating bodies
having a first axis lying in the first plane, a second axis perpendicular thereto and identical with the axis of the pipe or tube section, and a third axis perpendicular to the second axis, and
having an areal moment of inertia about the first axis which is at least one order of magnitude less than its areal moment of inertia about the third axis;
fixing to the first isolating body an inlet-side first sensor support
having a longitudinal axis extending parallel to the axis of the pipe section or the measu

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