Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
2000-02-07
2002-10-15
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
C073S861000
Reexamination Certificate
active
06463810
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the measurements of fluids flowing through conduits or pipes; and, in particular, to a pressure differential method for the determination of flow rate, fluid velocity and direction in bi-directional low-velocity flow system; and further, to a novel flowmeter in accordance with the said method suitable for the measurement of steady and transient flow, and, especially, applicable to fluid at high-temperature high-pressure conditions.
BACKGROUND OF THE INVENTION
The flow measurement is accomplished by a variety of means, depending upon quantities, flow rates, and types of fluids involved. Many industrial process flow measurements consist of a combination of two devices: a primary device placed in intimate contact with the fluid and generates a signal, and a secondary device to translate this signal into a motion or a secondary signal for indicating, recording, controlling, or totalizing the conditions of the flow. Other devices indicate or totalize the flow directly through the interaction of the flowing fluid and the measuring device placed directly or indirectly in contact with the fluid stream. A number of flow measurement methods are available; presently, commercialized metering devices can be classified by their operating principles into the following categories: (1) pressure-differential flowmeters; (2) variable-area flowmeters, (3) magnetic flowmeters, (4) turbine flowmeters, (5) oscillatory flowmeters, (6) thermal-loss flowmeters, (7) vortex flowmeters, (8) fluidic-oscillator flowmeters, (9) momentum mass flowmeters, (10) ultrasonic flowwmeters, (11) positive-displacement flowmeters, (12) open-channel flowmeters, (13) laser Doppler velocimeters, and (14) nuclear magnetic resonance flowmeters, and still some others. Such a wide diversity in flowmeter design results on the one hand from multifarious and specific use conditions, and, on the other hand, from tireless progress of science and technology which never a moment stops bringing out wonderful instruments. Nevertheless, to our knowledge, a metering device capable of measuring precisely flow rate and fluid velocity of bi-directional low-velocity flow does not as yet exit.
Conventionally, the commercialized flowmeters are so designed as to suit a flow system driven by pumps. As a consequence, they are not applicable to bi-directional flow systems. Moreover, when a metering device is of the same diameter as the pipeline system being measured, such as the case of turbine or drag disk type flowmeter, its minimum flow range is usually too high to give reliable measurement for low-velocity flow or for natural circulation. The driving force in low-velocity flow system mainly comes from the delicate balance between a very weak gravitational force and the buoyant force of the fluid itself, owing respectively to parts of the flow system assembly positioned at unequal heights, and to density gradient within the flowing stream. In such flow system, it is to be expected that, backward flow may occur when the flow is at transient state. Accordingly, it is the principal objective of this disclosure to overcome the aforesaid problems by providing a novel method and a novel device suitable for precision measurement of bi-directional low-velocity flow. To be the flow at steady or transient state, the accurate data of flow rate, fluid velocity, and direction can be obtained by the extremely sensitive pressure differential metering device of the invention, even if the fluid is at a high-temperature high-pressure condition.
For safety, economy, reliability and other factors, the low-velocity flow precision measurement technique has found many applications in industry, such as the on-line inputs proportion control in chemical continuous processes; such as the cyclic transport and storage of low-velocity fluid within the black pipeline system of a solar energy absorption plate; such as corrosive or toxic liquid wastes disposal, or industrial wastewater disposal, that makes use of a non-power transport system, siphon for instance, such as oil-well drilling and subterranean heat extraction industry where measurement of extracted fluids, including fluid mixtures of different liquid phases (e.g. water/oil) or all kinds of gas-liquid mixtures (e.g. natural gas/hydrocarbons), is massively important; such as natural circulation heat transfer system; and such as the cooling system of an advanced passive nuclear reactor, etc. As to natural circulation of high-temperature high-pressure heat transfer system, the driving force is, as a rule, by far weaker than the forced circulation of a pumping system; thereupon, when the flowing fluid is at transient state, backward flow sometimes occurs. In commercial flow measurement, this backward flow phenomenon is wholly neglected, besides its inability to adequately deal with low-velocity natural circulation. One point worthy of note is that the natural circulation is highly susceptible to flow resistance, therefore it is not advisable to reduce the size of conduit or transport pipeline in accommodation to the installation of a certain commercialized flowmeter of desirable flow range, for that will disturb flow conditions and sometimes even altogether stop the circulation flow.
Among many flow measurement devices as mentioned before, pressure-differential flowmeters embody the oldest method of measuring flowing fluids, and are still most widely in use. In this type of flowmeters, the flow rate is determined from pressure drop (or pressure difference) across a constriction (or restriction) in flow path. They operate on the principle of energy conversion between static pressure and velocity. The velocity increase resulted from a constriction in a pipe will have associated with it a decrease in static pressure. Thus, flow rate can be determined from a measurement of the pressure drop due to constriction. With static pressure-drop data, the fluid mass density and the sectional flow area at the constriction, a theoretical flow rate can be calculated according to Bernoulli's equation. However, the deviation from this theoretical value is not only possible but prevalent in practice, and this is usually attributed to viscosity change in the fluid passing through constriction, and to the geometry of flowmeter. Traditionally, such deviation is put into consideration in the flow equation by introducing a discharge coefficient which can only be determined by empirical means. More will be said on this later.
The common examples of pressure-differential flowmeters are orifice meter, flow nozzle, and Venturi meter. Orifice meter is a thin plate inserted between pipe flanges, usually having a round, concentric hole with a sharp, square upstream edge. Orifice presents large flow resistance in comparison with the other two devices. In addition, orifice may cause tangible stream turbulence, for, as the streamlines of flow field approaching a sharp-edged orifice they converge on the orifice from all directions, and so as soon as passing through the orifice they continue to converge for a short distance downstream depending on flow rate and fluid viscosity, forming as it were a ‘free jet’, which contracts to a section somewhat smaller in diameter than the orifice, after which the jet increases in size to fill the pipe. This contracted section of flow field, where the minimum cross-sectional area of said free jet is, is known as vena contracta, which is of supreme importance in the calculation of flow equation and also in the placing of pressure taps. As to flow nozzle, it consists of a bell-shaped approach section of elliptical profile attached to a cylindrical throat tangent to ellipse. Originally, the flow nozzle was designed to reduce pressure loss of fluid flowing through a traditional orifice plate; nowadays it is often used where solids are entrained in the flowing liquid, where the stream is a high-temperature high-pressure flow, and where fluid velocity is high. Only Venturi meter is suitable for low-velocity flow measurement in deed. More than a convergent inle
Allen Andre
Fuller Benjamin R.
Institute of Nuclear Energy Research (INER)
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