GB1569125A - Vortex detection - Google Patents

Description

(54) VORTEX DETECTION (71) We, IMPERIAL CHEMICAL IN DUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement. This invention relates to vortex detection.

If a liquid passes an obstruction in a conduit and the velocity of the liquid is sufficiently high, vortices may be continuously formed and detached from the obstruction forming a "street" of vortices moving with the liquid and gradually decaying as they pass downstream. The rate of formation of vortices, f, in number per second is governed by the following equation.

f = SV d where S is the Strouhal number, V is the velocity of the liquid in metres/sec and d is the diameter in metres of an obstruction past which flow takes place on one or more sides.

A determination of the rate of formation of vortices gives a means of estimating the rate of flow of liquid past the obstruction since over a wide variety of conditions S is substantially constant.

A number of procedures for determining the rate of formation of vortices have been proposed which require the introduction of detectors into the conduit. This exposes the detectors to the possibility of chemical or physical damage and involves interference with the conduit. This invention provides apparatus for vortex detection in which a detector is positioned outside the conduit.

The invention provides apparatus which comprises a cylindrical conduit for the passage of a liquid which contains an obstruction past which a liquid may flow, which obstruction is capable of forming vortices during such flow, means outside the conduit for transmitting sound waves continuously through a region downstream of the obstruction in which vortices so formed can exist, means outside the conduit to detect sound waves which have passed through the said region, the conduit having sound transmitting and receiving surfaces which are flat. and means to amplitude-or frequencydemoldulate an electrical analogue of the detected sound waves and to provide an indication of the frequency or period of the demodulated signal.

As vortices pass. the phase of the detected sound changes, which produces the equivalent of a transient frequency change and this may lead to a change in the amplitude of the detected sound due to resonance as well as to any change in amplitude due to deflection of sound towards or away from the detector due to the vortex. If the conduit has an appropriate geometry the resonance effect may be used to reinforce the amplitude effect due to deflection of the sound and to increase the sensitivity of the system.It has been found desirable that the geometry should be such that the resonance effect produces succes sive maxima of sound received at frequencies which are spaced apart by 5% of the frequency of the sound to be used, and that the frequency of the sound used should lie between those frequencies producing a successive maximum and minimum; thus the variation in the amplitude of sound received due to resonance as a single vortex passes produces a single maximum or minimum.

The geometry of our conduit comprising flat transmitting and receiving surfaces in a cylindrical conduit is verv suitable. The flat surfaces may be provided by inserts set into the wall of a cylindrical tube, the outer ends of the inserts communicating with the transmitting and detecting transducers, the inner ends being flat. The inserts may be retained in position by known means. for example welding, screwing them in or securing them with an adhesive.

The frequency of the demodulated signal corresponds to the frequency of passage of vortices through the path of the sound waves and the period of the demodulated signal indicates the time interval between the passage of successive vortices through the path of the sound waves.

The indication of the frequency or period may be an indication that the demodulated signal has passed a threshold frequency or period or may be for example an indication of the absolute frequency or period of the demodulated signal. The indication may be displayed to an operator or may be fed to apparatus adapted for example to process the information or to apparatus adapted to modify directly or indirectly the flow conditions in the conduit thereby restoring the flow conditions to a value within prescribed limits.

Obstructions in the conduits may be of any shape, for example they may be cvlindrical or in the form of wedge-shaped bars or bars of rectangular cross section. The effective diameter in the aforesaid equation may be determined empirically.

Where the obstruction is elongated and lies across the conduit it is preferred that the path of the sound waves should be parallel to its axis and directly behind it in the direction of volume flow of the liquid.

The sound waves may be passed through the conduit by known means; it is preferred that the detector of sound waves should contact the conduit and that it should be directed at right angles to the wall of the conduit and toward the means for transmitting sound waves. The means for transmitting and for detecting sound waves may be of conventional form and may be mounted in known manner.

It is preferred that the sound waves should be passed through a point downstream of the obstruction and at a distance of up to 3d from the obstruction, where d is of the same significance as in the beforementioned equation. If this is impractical. a distance of up to 6d from the obstruction should preferably be chosen.

The means for amplitude-or frequencydemodulating sound waves may be of known type.

The sound waves are preferably of a frequency in the range 0.25 to 10 megaherz and more preferably 0.5 to 5 megaherz.

One form of the invention will now be described with reference to the accompanying drawing which is a diagrammatic representation of apparatus according to the invention.

A power supply comprising an oscillator with a square wave voltage output 1 is isolated by a safety barrier from line receiver and transducer drive unit 2 which it feeds.

The oscillator is adjustable in its frequency and is tuned to the natural frequency of a transducer 3 which it feeds through unit 2 and isolating transformer 4. Transducer 3 is stuck with an adhesive to the outer surface of the wall of conduit 5 and faces a detector 6 across the pipe and down stream of an obstruction to liquid flow 7, which is a straight rectangular bar. by a distance equivalent to three times the effective diameter of obstruction 7 (this is the width of the surface of the obstruction facing the liquid flow). The line between the transducer and the detector is parallel to and in the "shadow" of obstruction 7.Detector 6 feeds an isolating transformer and drive amplifier 8 which passes a signal through the safety barrier to a phase locked loop demodulator 9 capable o demodulating either amplitude modulated or frequency modulated signals at option and also to a rectifier 10 feeding an alarm 11 adapted to give an alarm at signals below a threshold value. The signal from amplifier 8 mav also if desired be fed to a high frequency AC alarm 12 which can be set to operate at low signals. If desired a second aplifier 28 within a safety barrier may be provided.

The phase locked loop demodulator 9 feeds through an amplifier 29 and threshold detector (which cuts out noise) and monostable 13 which produces a square wave output. The output from threshold detector and monostable 13 may be processed either by passing it to a differentiator 14 feeding a rate meter 15 which is preferably adjustable to give an average rate over a time constant of for example 1-160 seconds. time constants suitablv of 25 seconds being preferred. The rate meter may feed either a meter or control circuit 17 through a drive amplifier 16 or a threshold alarm 18 producing alarms at high and/or low rates of liquid flow.

Alternatively the output of threshold detector and monostable 13 may be fed to clock/counter/store 19 which can be set to count the square wave pulses for a desired period and may feed a binary alarm system 30 or a binary coded decimal output which may be displayed in decimal form and/or a digital to analogue convertor 20 which may feed a drive amplifier 21 which in turn feeds an analogue meter 22. If desired the output from the digital to analogue converter 20 may be fed to a threshold alarm system 23 which feeds relays 24.

During operation of the device oscillator 2 is operated continuously and passes sonic waves at a frequency of 2.5 megaherz through the liquid in conduit 5 to detector 6.

The sonic waves are moldulated by vortices passing through the path of the sound and detector 6 produces a modulated signal which is passed by the drive amplifier directly or indirectly to demodulator 9 where a demodulated signal is produced and fed to threshold detector and monstable 13.

If the signal fails, i.e. if no sonic vibration is detected, alarms 11 or 12 operate. The threshold detector and monostable 13 produces a succession of pulses which are counted in the alternative systems above described.

If desired a frequency multilplier may be interposed between threshold detector and monostable 13 and the counter systems.

WHAT WE CLAIM IS: 1. Apparatus comprising a cylindrical conduit for the passage of a liquid which contains an obstruction past which a liquid may flow which obstruction is capable of forming vortices during such flow, means outside the conduit for transmitting sound waves continuously through a region downstream of the obstruction in which vortices so formed can exist, means outside the conduit to detect sound waves which have passed through the said region. the conduit having sound transmitting and receiving surfaces which are flat, and means to amplitude- or frequency-demodulate an electrical analogue of the detected sound waves and to provide an indication of the frequency or period of the demodulated signal.

2. A conduit as claimed in Claim 1 in which the flat surfaces are provided bv inserts set into the wall of the conduit, the outer ends of the inserts communicating with the transmitting and the detecting transducers, the inner ends being flat.

3. Apparatus as claimed in Claim 1 which comprises means to indicate that the demodulated signal has passed a threshold frequency or period.

4. Apparatus as claimed in Claim 1. 2 or 3 in which the obstruction is elongated and lies across the conduit and the path of the sound waves is parallel to the axis of the obstruction and directly behind it in the direction of volume flow of the liquid.

5. Apparatus as claimed in any preceding Claim in which the sound waves pass through a point downstream of the obstuction at a distance from the obstruction of up to three times the effective diameter of the obstruction.

6. Apparatus as claimed in any preceding Claim in which the sound waves are of a frequency in the range 0.25 to 1() megaherz.

7. Apparatus as claimed in any preceding Claim in which the detector of sound waves contacts the conduit and is directed at right angles to the wall of the conduit and toward the means for transmitting sound waves.

8. Apparatus for vortex detection substantially as described with reference to the drawing.

9. A method of detecting vortices using apparatus according to any preceding

Claims (1)

1. Claim.