GB2142725A

GB2142725A - Fluid flow meter

Info

Publication number: GB2142725A
Application number: GB08316837A
Authority: GB
Inventors: Laurence Sidney James Hamblett
Original assignee: United Gas Industries Ltd
Current assignee: United Gas Industries Ltd
Priority date: 1983-06-21
Filing date: 1983-06-21
Publication date: 1985-01-23
Also published as: GB8316837D0

Abstract

A Karman vortex shedding flow meter for use with natural gas includes a Venturi. A bluff body (14) is located at the throat of the Venturi so causing vortices to spin off. An ultrasonic or optical detection device (15, 17) detects the vortices and the rate of gas flow is computed therefrom. The vortex shedding principle is only reliable at Reynolds numbers of 10,000 or more. By using the Venturi, the Reynolds number of the flow is temporarily increased, for instance by 30% or more, so allowing the low flow rates to be measured accurately. <IMAGE>

Description

SPECIFICATION Fluid meter This invention relates to fluid flow meters operating on the Karman vortex shedding principle, in which a bluff body obstruction is placed in the flow path, forming vortices which travel downstream. Above a minimum velocity the frequency of these vortices is directly related to fluid flow rate and may be used as the basis for fluid flow measurement.

The problem with which the present invention is concerned is that the vortices are only directly proportional to flow rate at the Reynolds numbers greater than 10,000. Thus at low flow rates the meters based on this principle do not measure accurately or even at all.

The Reynolds number defines the characteristic of the flow and is proportional to velocity and density and inversely proportional to vis cQsity.

The invention provides a flow meter operating on the Karman vortex.shedding principle comprising a Venturi through which the flow is'directed, a bluff body located at or near the throat of the Venturi, and sensing means for detecting the presence of vortices downstream of the bluff body. The effect of passing the flow through the Venturi is to increase the gas velocity temporarily, so as to increase the Reynolds number. By this means the minimum flow rate which can be measured accurately is substantially reduced. Increases in Reynolds number of 30% or even more can be achieved.

Preferably the sensing means are sonic (either ultrasonic or subsonic) but may alternatively be optical, using a light beam.

A specific embodiment of the invention is shown in the accompanying drawing, which is a side section through a vortex meter, not showing the associated electronic circuitry and indices.

The meter comprises a cylindrical pipe (11) with end attachments for securing it in-line in a gas flow to be measured. Internally the pipe is formed into a Venturi having a short converging section (12) on the inlet end, a throat section and a larger diverging section (13) at the outlet end. The internal surfaces of the Venturi. are made very smooth to reduce overall pressure loss to a minimum. As is well known, the effect of such a Venturi is to increase the velocity of flow at the throat, while decreasing the pressure. At the end of the diverging section the flow velocity and the pressure regain most of their original values but with some losses due to friction and eddy loss.

At the throat of the Venturi, i.e. the location of highest gas velocity, a bluff body (14) comprising a bar of triangular section is mounted across the stream. The base of the triangular section faces upstream, the apex downstream, so causing vortices to be formed, spin off and travel downstream in the gas stream, the rate at which the vortices are formed being proportional to the velocity of the gas stream. Alternatively, the apex may face upstream. Other configurations of bluff body, e.g. circular section bars, may also be used.

Also at the throat of the Venturi, downstream of the bluff body, is mounted an ultrasonic vortex detection device comprising transmitter (1 5) which forms an ultrasonic beam which travels along path (16) across the throat to be detected by receiver (17). Receiver (17) contains piezo-electric transducers (not shown) which are sensitive to ultrasonic vibrations. When the gas stream contains vortices, these disturb the regular pattern of the oscillations of the ultrasonic beam and these irregularities are electronically detected by circuitry (not shown) which also calculates the flow rate of the gas from them.

The maximum internal diameter of the flow path through the meter is 25 mm and the minimum, at the throat, is 9.9 mm. In use it is found that the pressure loss, end to end, in the meter is only of the order of 0.45 inches water gauge for natural gas, with a volumetric flow of 210 cubic feet per hour, dropping to 0.1 5 inches water gauge at a flow rate of 100 cubic feet per hour.

The meter shown when measuring a flow rate of 210 cubic feet per hour has a Reynolds number of 5000 at the inlet, which is increased to 1 2000 at the throat, thus a flow which was unsuitable for measurement by the Karman vortex principle is rendered suitable temporarily and then returned to its original state with only acceptably small pressure losses. It will be noted that it is essential that the bluff body be placed at or near the throat of the Venturi since it is only in this area that the maximum Reynolds number is achieved.

The maximum Reynolds number achieved is a function of the velocity at the throat, so the number can be increased by a greater factor by reducing further the throat diameter.

1. A flow meter operating on the Karman vortex shedding principle, comprising a Venturi through which the flow is directed, a bluff body located at or near the throat of the Venturi and sensing means for detecting the presence of vorticeS downstream of the bluff body.

2. A flow meter as claimed in claim 1, wherein the Venturi is designed to increase the Reynolds number by 30% or more at the throat of the Venturi.

3. A flow meter as claimed in claim 1 or claim 2, wherein the diameter of the Venturi is approximately 25 mm at its maximum and approximately 9.9 mm at the throat.

4. A flow meter as claimed in any of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Fluid meter This invention relates to fluid flow meters operating on the Karman vortex shedding principle, in which a bluff body obstruction is placed in the flow path, forming vortices which travel downstream. Above a minimum velocity the frequency of these vortices is directly related to fluid flow rate and may be used as the basis for fluid flow measurement. The problem with which the present invention is concerned is that the vortices are only directly proportional to flow rate at the Reynolds numbers greater than 10,000. Thus at low flow rates the meters based on this principle do not measure accurately or even at all. The Reynolds number defines the characteristic of the flow and is proportional to velocity and density and inversely proportional to vis cQsity. The invention provides a flow meter operating on the Karman vortex.shedding principle comprising a Venturi through which the flow is'directed, a bluff body located at or near the throat of the Venturi, and sensing means for detecting the presence of vortices downstream of the bluff body. The effect of passing the flow through the Venturi is to increase the gas velocity temporarily, so as to increase the Reynolds number. By this means the minimum flow rate which can be measured accurately is substantially reduced. Increases in Reynolds number of 30% or even more can be achieved. Preferably the sensing means are sonic (either ultrasonic or subsonic) but may alternatively be optical, using a light beam. A specific embodiment of the invention is shown in the accompanying drawing, which is a side section through a vortex meter, not showing the associated electronic circuitry and indices. The meter comprises a cylindrical pipe (11) with end attachments for securing it in-line in a gas flow to be measured. Internally the pipe is formed into a Venturi having a short converging section (12) on the inlet end, a throat section and a larger diverging section (13) at the outlet end. The internal surfaces of the Venturi. are made very smooth to reduce overall pressure loss to a minimum. As is well known, the effect of such a Venturi is to increase the velocity of flow at the throat, while decreasing the pressure. At the end of the diverging section the flow velocity and the pressure regain most of their original values but with some losses due to friction and eddy loss. At the throat of the Venturi, i.e. the location of highest gas velocity, a bluff body (14) comprising a bar of triangular section is mounted across the stream. The base of the triangular section faces upstream, the apex downstream, so causing vortices to be formed, spin off and travel downstream in the gas stream, the rate at which the vortices are formed being proportional to the velocity of the gas stream. Alternatively, the apex may face upstream. Other configurations of bluff body, e.g. circular section bars, may also be used. Also at the throat of the Venturi, downstream of the bluff body, is mounted an ultrasonic vortex detection device comprising transmitter (1 5) which forms an ultrasonic beam which travels along path (16) across the throat to be detected by receiver (17). Receiver (17) contains piezo-electric transducers (not shown) which are sensitive to ultrasonic vibrations. When the gas stream contains vortices, these disturb the regular pattern of the oscillations of the ultrasonic beam and these irregularities are electronically detected by circuitry (not shown) which also calculates the flow rate of the gas from them. The maximum internal diameter of the flow path through the meter is 25 mm and the minimum, at the throat, is 9.9 mm. In use it is found that the pressure loss, end to end, in the meter is only of the order of 0.45 inches water gauge for natural gas, with a volumetric flow of 210 cubic feet per hour, dropping to 0.1 5 inches water gauge at a flow rate of 100 cubic feet per hour. The meter shown when measuring a flow rate of 210 cubic feet per hour has a Reynolds number of 5000 at the inlet, which is increased to 1 2000 at the throat, thus a flow which was unsuitable for measurement by the Karman vortex principle is rendered suitable temporarily and then returned to its original state with only acceptably small pressure losses. It will be noted that it is essential that the bluff body be placed at or near the throat of the Venturi since it is only in this area that the maximum Reynolds number is achieved. The maximum Reynolds number achieved is a function of the velocity at the throat, so the number can be increased by a greater factor by reducing further the throat diameter. CLAIMS

4. A flow meter as claimed in any of claims 1 to 3, wherein said sensing means is an ultrasonic vortex detection device having piezo-electric transducers.

5. A flow meter as claimed in any of claims 1 to 4, comprising also electronic circuitry which calculates the flow rate from the sensed vortices.

6. A flow meter as claimed in any of claims 1 to 5, wherein said bluff body is a bar of triangular section mounted across the flow path.

7. A flow meter as claimed in any of claims 1 to 6, comprising a gas meter for measuring the flow of natural gas, wherein at a flow rate of 210 cubic feet per hour a Reynolds number of 5000 obtains at the inlet to the Venturi and wherein the Venturi is designed to increase the Reynolds number to 12000 at the throat.

8. A flow meter substantially as described hereinbefore with reference to the accompanying drawing.