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| Operating
Principle |
Detecting element
of metal variable area flowmeter consists of a tapered tube
extending from bottom to top and a float inbuilt in tapered
measuring tube vertically. Working principle is shown as
Fig. 1: When fluid flows upward into annular space between
tapered tube and float, the float will move upward along
the tube due to lifting force formed by differential pressure
of up and down end of float. When the flowrate of fluid
increases, the float will displace much more; on the contrary,
when the flowrate of fluid decreases, the displacement of
float will reduce correspondingly, too. Therefore, the position
of float is decided by the flowrate of fluid, as well as
the flowrate determines annular area between the maximum
outside diameter of float and inner wall of tapered tube.
When fluid keeps a stable flowrate, the float locates in
a dynamic balance state, the annular area between float
and tapered tube also keeps constant. At this moment, there
are three forces acted on the float: downward weight W,
upward floating force F and fluid dynamic force P, and these
forces locate in balance. According to Bernoulli equation
of hydrodynamics, force equilibrium equation and fluid continuity
principle, we can calculate the average instantaneous flowrate
flowing into the annular area, so the float flowmeter is
also called variable area flowmeter.
A high-performance permanent magnet steel is embedded inside
float. By this way, the magnetic field will be produced
around the float. When flowrate of fluid tends to stable
and float locates in dynamic balance state, the surrounding
magnetic field distribution reaches stable, too.
By means of the flowrate indicator mechanically connected
with tapered tube, the magnetic signal of the float will
be transmitted in non-contact form. In another word, indicator
is able to detect and process with the flowrate in the mode
of magnetic transducer, and finally shows the flowrate by
pointer on indicator scale board, or intelligently process
to display instantaneous and total flow in LCD indicator
and outputs a standard 4-20mA current signal.
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| Basic formula of volumetric flowmeter
Q is: |
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| If the float is hollow body, Q is: |
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Thereinto:
α-------flowrate factor, determined by float shape
ε---------gas expansion parameter; ε= 1 for liquid
＠F-----annular flowing area, m2
g------local gravity acceleration, m/s2
Vf-----float volume, m3
ρf-----density of float material, kg/m3
ρ-----density of measured fluid; for gas, density of gas
upstream of float, kg/m3
Ff-------cross sectional area of float at maximum, m2
Gf------ float weight, kg
The relationship between the annular area and lifting height
of the float is shown as below, when structure design is
confirmed, d and β are constants. Formula contains quadratic
term of h, usually, this nonlinear term can not be neglected.
Only when the tapered angle is little, it can be considered
as linear approximately.
Thereinto
d-----------------Maximum diameter of the float, m
h-----------------Lifting height of float (starting
from ID of tapered tube =Dmax of float), m
β---------------- Tapered angle of the tapered tube
a、b------------Constant
According to process mode of measuring
signal, metal variable area flowmeter can be divided
into:
a. Local display: Rotate follow-up magnet in the local
indicator by the magnetic coupling with magnet inbuilt
in the float to drive the pointer to indicate the
flowrate on scale directly (Figure as below).
b. Intelligent remote display:
Rotate follow-up magnet in the intelligent indicator
by the magnetic coupling with magnet inbuilt
in the float to drive the sensing magnet and
pointer, at same time the change of the magnetic
field is converted to electrical signal by magnet
transducer, and after process by A/D converter、digital
filter、micro-processor processing、D/A converter,
to LCD display instantaneous and total flowrate
(Figure as below).
Intelligent
Type |
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