GB2197472A

GB2197472A - Ultrasonic flowmeter

Info

Publication number: GB2197472A
Application number: GB08627436A
Authority: GB
Inventors: J G O'hair; M E Nolan
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1986-11-17
Filing date: 1986-11-17
Publication date: 1988-05-18
Also published as: GB8627436D0

Abstract

Apparatus for the measurement of fluid flow within a conduit 1 including at least one pair of transducers 4, 5, each mounted in the closed end of a recess 2, 3 provided within the conduit wall and cooperating with the other member which is in an opposed but axially displaced relationship to project a beam of ultrasonic energy along a pathway between the cooperating pairs of transducers, is characterised in that said transducers are mounted at an angle alpha with respect to the axis of the conduit and in that the depth and width of the recesses are such that the angle of the beam emitted by the transducer can have an uninterruptible path width between alpha +/- beta with respect to the axis of the conduit, beta being the path deviation from a median line between the opposed axially displaced transducer pairs, where sin beta =V DIVIDED C.X DIVIDED Ssin alpha (V=mean velocity of fluid in the region between the transducers at maximum flow rate, C=velocity of sound in the fluid, S=axial distance between closed ends of recesses, X=axial distance between open ends of recesses.) <IMAGE>

Description

SPECIFICATION Fluid flow measurement This invention relates to apparatus for the measurement of fluid flows. More particularly, the invention is concerned with the arrangement of the transducers employed in ultrasonic flowmeters, especially those used for the measurement of gas flows.

The use of ultrasonic measuring techniques has found practical application first in the measurement of liquid flows and more recently in the measurement of gas flows. For example, our GB PS-2139755 describes an ultrasonic flowmeter wherein a plurality of transducer pairs are arranged to define a number of interlacing paths. Typically the number of paths is at least four and beams of ultrasonic energy are projected along the paths first in one direction and then in the other. In a preferred form of construction the transducers are mounted in the closed ends of recesses provided in the conduit wall. This arrangement of recessed transducers confers a number of advantages. Firstly, because the transducer bodies are non-intrusive there is no disturbance to the fluid flow regime. Thus anomalies which disturb the flow patterns are reduced or eliminated.Secondly, again, because the transducers are non-intrusive, the passage of pipeline inspection and maintenance vehicles within the conduit is not prohibited in the region of the metering device. Thirdly, if the transducers can be recessed sufficiently, valves can be included in the recess for isolating the transducer from the conduit interior, thereby allowing the transducers to be removed and replaced for service, without stopping the flow of fluid through the conduit.

One reason why it has only recently been possible to measure gas flows by ultrasonic methods is that many problems have been encountered associated with signal coupling between the transducers. Loss of signal strength with associated attenuation problems has been a constant source of difficulty.

We have now found that the advantages of using recessed transducers together with improved signal coupling can be achieved by a novel configuration and arrangement of the recesses or ports which hold the transducers.

In accordance with the present invention, there is provided apparatus for the measurement of fluid flow within a conduit including at least one pair of transducers, each mounted in the closed end of a recess provided within the conduit wall and co-operating with the other member which is in an opposed but axially displaced relationship, to project a beam of ultrasonic energy along a pathway between the cooperating pairs of transducers, the improvement consisting in that said transducers are mounted at an angle a with respect to the axis of the conduit and in that the depth and width of the recesses are such that the beam emitted by the transducer can have an uninterruptible path width between a+p with respect to the axis of the conduit, ss being derived from the relationship:: V X sin ss= - - sin a CS where V is the mean velocity of the fluid in the region between the transducers at maximum flow rate, C is the velocity of sound in the fluid, S is the axial distance between the closed ends of the recesses and X is the axial distance between the open ends of the recesses.

The invention will be described in greater detail by reference to the accompanying drawing which is a schematic representation of a cross-sectional view of the spool piece of the ultrasonic flowmeter.

Referring to the drawing, a pipeline spool piece 1 is provided with recessed parts 2, 3. The remote ends of the ports are closed off with transducer bodies 4, 5. The ports are disposed at an angle a with respect to the axis of the spool piece and thus the ports and transducers are axially displaced by distance X at the open end of the recess and by distance S at the closed end.

For a pulse leaving point A with velocity C (the speed of sound in the medium) at an angle 6, the component in the y direction is C sin 6 and in the X direction is C cos 6 The medium is flowing in the X direction from E to F with velocity V(y). After time t the pulse will have travelled a distance Ct sin 6 in the y direction.

Thus y=Ct sin #. For a total transit time T for the pulse to travel from A to B in the y direction, y3 will be equal to CT sin #.

The velocity of the pulse in the X direction at time t is given by the relationship Vx=C cos #+V(y) where y=ct sin 6 The distance covered in the X direction between t and dt is dx. Thus dx=Vx dt=[C cos #+V(y)] dt and hence the distance covered in the X direction between starting from A (t=o) and arriving at B (t=T) is #@T[C cos #+V(y)] dt which is equal to distance S (if the pulse is to arrive at B).

Hence S=#@T[C cos #+V(y)] dt =CT cos #+#@T V(y) dt =CT cos #+#@y3 V(y) dy/C sin # since t=y/C sin 6 Now #oy3 V(y)=#y1y2 V(y) dy since V(y)=o for o#y#y1 and for y2#y#y3 Furthermore fYy, Vy) dy=VD where D is the distance across the conduit normal to the axis ie. if the conduit is a cylindrical spool piece and V is the mean velocity of the fluid in the region between y=y, and y=y2 i.e. v=1/D #y1y2 V(y) dy Hence S=CT cos #+VD/C sin # =CT cos #+VDT/y3 =CT cos #+VTZ (where Z=X/S) Thus T=y3/C sin 6 (1) S=T[C cos #+VZ] (2) Likewise for a pulse leaving B and going to A we have T'=y3/C sin 6 (3) and S=T'[C cos #-VZ] (4) For a pulse leaving A, to arrive at B, the pulse would have to initially travel at an angle ss=#-α from the medium line AB Thus sin ss=sin (6-a) =sin 6 cos a-cos 6 sin a Using equations (1) and (2) y3 S sin α sin ss = cos α ;-[ -VZ] CT T C S sin a S sin a V - + - Z sin α CT CT C V sin ss= - Z sin α C Likewise for a pulse returning in the direction BA and angle y from the median line is decreased by γ=(α-#) V sin gamma;=-Z sin a C

Claims

1. Apparatus for the measurement of fluid flow within a conduit including at least one pair of transducers, each mounted in the closed end of a recess provided within the conduit wall and co-operating with the other member which is in an opposed but axially displaced relationship to project a beam of ultrasonic energy along a pathway between the cooperating pairs of transducers, characterised in that said transducers are mounted at an angle a with respect to the axis of the conduit and in that the depth and width of the recesses are such that the angle of the beam emitted by the transducer can have an uninterruptible path width between aifi with respect to the axis of the conduit, ss being derived from the relationship: : V X sin fi - - sin a CS where V is the mean velocity of the fluid in the region between the transducers at maximum flow rate, C is the velocity of sound in the fluid, S is the axial distance between the closed ends of the recesses and X is the axial distance between the open ends of the recesses.