Measuring and testing – Volume or rate of flow – By measuring thrust or drag forces
Reexamination Certificate
1999-08-25
2002-08-20
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring thrust or drag forces
Reexamination Certificate
active
06435041
ABSTRACT:
FIELD OF INVENTION
This invention relates to a device useful for measuring the flow rate of cryogenic liquids flowing through a tube. The present invention particularly relates to a device useful as a flow meter for measurement of flow rate of liquid nitrogen (LN
2
) or any other cryogenic liquid through tubes without offering any additional resistance to the flow.
BACKGROUND AND PRIOR ART DETAILS OF THE INVENTION
Large amount of liquid nitrogen is now a days routinely infused in coal mines through bore holes for control of fire. For this purpose, the cryogenic liquid from tankers is flushed to underground mines direct through bore holes or through super insulated tubes laid along bore holes. For best results, it is imperative that flow of LN
2
be measured accurately.
However, presently the following practices are being adopted for measuring flow rate of liquid nitrogen to control fire in underground mines:
Conventionally, the level different of LN
2
in a tanker is measured with the help of a meter which is suitably calibrated. This level difference serves as an indicator for measuring flow rate of inlet cryogenic liquid flushed. If there is any leakage in the tanker, flow of cryogenic liquid can not be measured accurately by this process. Further, if LN
2
or any other cryogenic liquid is flushed through two or more bore holes simultaneously flow through individual bore holes can not be measured.
The known flow meters using orifice principle, Venturi meter, Rotameter, the flow meter using vortex principle are not well suited for measurement of flow rate of cryogenic liquid because of typical cryogenic properties of the liquid. Further, they always offer some amount of resistance to the flow being measured. Thus, a number of flow meters based on different principles are available for measurement of flow rate of cryogenic liquid. Various types of flow meters along with their basic principle are discussed below:
Orifice meter
The orifice meter consists of a thin plate (1.6 mm to 3.2 mm) with a sharp-edged hole in the centre of the plate. For ensuring a symmetrical velocity distribution upstream of the meter, a length of straight line of at least ten times the tube diameter should be placed upstream of the orifice and a length of line about five times the tube diameter should be placed downstream of the meter (Barron, 1985). For liquid mass flow rate measurement the equation for the orifice meter is
m=C
d
C
a
A
0
(2g
0
&rgr;&Dgr;P)
0.5
Where
C
d
=discharge coefficient
C
0
=velocity-of approach coefficient
A
0
=area of orifice
&rgr;=fluid density
&Dgr;P=pressure drop across the orifice.
The calibration curve obtained by using water as the flowing fluid can be applied directly to the measurement of flow of liquid hydrogen, liquid nitrogen and liquid oxygen with ±percent accuracy as long as the fluid is single phase upstream of the meter.
Venturi meter
In order to welcome the problem like large frictional and turbulent energy dissipation as happened when fluid flows through the orifice, this meter is often being preferred as a flow measuring element. The Venturi meter consists of a conical reducer section and a straight throat section, followed by a more gradual enlargement to the original tube diameter. The inlet cone angle is usually 20° to 22° and the exit cone angle is about 5° to 7°. The throat diameter is of the order of one-half the tube diameter. Pressure taps are placed about one-half tube diameter upstream of the venturi entrance and at the centre of the throat section (Barron, 1985). The volumetric flow rate may be determined using the equation same as that of the Orifice meter.
Turbine flow meter
Flow rate is measured by measurement of the rotational speed of a freely spinning turbine wheel placed in the centre of the flowing stream. Each time a turbine blade passes the face of the permanent magnet which is placed in the body.
The change in the permeability of the magnetic circuit produces a voltage pulse at the output terminal. The frequency of these pulses is directly proportional to rotational speed of the turbine wheel. The pulse rate may be indicated by a Frequency meter, displayed on a cathode-ray oscilloscope screen or counted by an EPUT (events per unit time) meter.
At high Reynolds number (about 6000) the volumetric flow rate is related to rotational speed of turbine n by (Hochreiter, 1958).
V=&pgr;D
b
A
ff
·n/tan 0
b
=Kn
Where
D
b
=rotor blade tip diameter
0
b
=angle between the blade and the meter centre line
A
fF
=free-flow area through the turbine
Kent Vortex Meter:
This flow meter works on eddy-shedding principle (Ower, E & Pankhurst, R. C., 1977). The eddies are generated by a sharp-edged cylinder of rectangular cross section, which produces a better-defined eddy pattern than a circular cylinder.
Difference between an Orifice meter as compared to proposed invention:
To ensure symmetrical velocity distribution upstream of the meter, a length of straight line at least ten times the tube diameter should be maintained upstream of the orifice and a length of straight line at least five times the tube diameter be maintained downstream of the meter. This is sometimes not possible while dealing with cryogenic fluids in underground. This is used when trouble free installations is essential.
Further, in the orifice meter a few components create resistance to the flow of the liquid. However, the present device, no obstruction is introduced in the flow path therefore, it is capable of functioning and deliver accurate result without the above restrictions.
In addition, the orifice meter produces a relatively large permanent pressure drop which is undesirable when measuring flows of cryogenic fluids under saturation condition (Barron, 1985) whereas no such pressure drop exists in the proposed device.
As regards Venturi meter, it suffers from problem o cavitation (Purcell et al 1960). To avoid the problem of cavitation, it is necessary to maintain the upstream pressure high enough so that the vapor pressure is not reached at the throat of the Venturi. Whereas the present invention, the flow of liquid is not disturbed in any way, therefore, the question of cavitation does not arise.
In case of a turbine flow meter, a turbine wheel is placed in the centre of the meter body itself it may cause obstruction to the flow of liquid. The proposed system does not introduced any resistance to the flow of liquid being measured.
The second problem in case of a turbine flow meter is that it must be protected during cool down of a cryogenic fluid transfer line in which the meter is placed because severe flow oscillations and surges may destroy the meter (Steward 1965), whereas no such problem is envisaged in the proposed device.
As regards, Kent Vortex Meter, the accuracy of the instrument deteriorates at lower Reynolds number (Ower, E & Pankhurst, R. C., 1977). However, no such problem is identified in the proposed device.
Further, the pressure loss across the meter is two velocity heads in a Kent Vortex meter whereas, in the proposed device, there is not obstruction in the flow path, the pressure loss is the normal pressure loss n the pipe due to friction.
Apart from the above known devices, there are two U.S. Pat. Nos. 5,765,602 dated Jun. 16, 1998 and 3,958,443 dated May 25, 1976 relating to metering and transfer of cryogenic liquid and apparatus for providing and calibrating cryogenic flow meters respectively. These two devices do not envisage the present invention.
The main object of the present invention is to a provide a device useful for measuring the flow rate of inert cryogenic liquids flowing through a tube.
Another object of the present invention is to provide a device for measuring flow rate of inert cryogenic liquid precisely as the rate of injection as well as the total amount of LN
2
injected in an underground fire area are two very important parameters for quick control of fire and optimization of use of LN
2
.
Yet another object of the present invention is to measure the flow rate
Ahmad Ishtiaque
Bandyopadhyay Lakshmi Kanta
Bhowmick Bimal Chandra
Dutta Amal Kumar
Ray Santosh Kumar
Council of Scientific & Industrial Research
Fuller Benjamin R.
Ladas & Parry
Thompson Jewel V.
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