WO2003010496A1

WO2003010496A1 - Adjustable venturi flow meter for open channel flow

Info

Publication number: WO2003010496A1
Application number: PCT/NL2002/000502
Authority: WO
Inventors: Alex Peter Van Loon
Original assignee: Enrin Environmental Research Instruments
Priority date: 2001-07-25
Filing date: 2002-07-25
Publication date: 2003-02-06
Also published as: NL1018632C2

Abstract

The invention relates to a venturi measuring passage for measuring the flow rate in a water channel, comprising a constriction incorporated in the water channel, which forms a passage for the water in the water channel, while a body is incorporated in the constriction, which determines the height of the passage, with the body in the water channel's direction of flow having a symmetrical shape.

Description

ADJUSTABLE VENTURI FLOW METER FOR OPEN CHANNEL FLOW

The invention relates to a venturi measuring passage for measuring the flow rate in a water channel, comprising a constriction incorporated in the water channel, which forms a passage for the water in the water channel. Such a venturi measuring passage is known from the manual "Debietmeten in open waterlopen " (Measuring flow rates in open water channels) by . Boiten, A. Dom- merholt and M. Soet, published January 1995; ISSN 0926- 230X. This publication discloses both an open and a dammed-up venturi flow, and on page 96 of the manual it is shown that a constriction is incorporated in the water channel.

With the aid of the prior art venturi measuring passage the flow rate is calculated from the water levels measured in front of and behind the constriction in the water channel, applying a correction coefficient. Incidentally, this manner of determining a£p>.Qw. (g^t^ is known as such and is based on Bernoulli's law.

From FR-A-2 506 010 a vent&$ifQm^ygsuring passage according to incorporated

height of the passage.

It is the object of the invention to provide an improved venturi measuring passage that has a wider meas- uring range and is able to operate more accurately.

To this end the venturi measuring passage according to the invention is characterized in that the body has a wall surface extending substantially parallel to the direction of flow in the water channel. Due to the fact that said body conditions the flow of water and defines the flow front of the passage more clearly, the flow rate can be determined more accurately. The inventor believes that the thus designed body only causes a minimal contraction of streamlines. This is the principle on which the accu- racy of the flow rate measurement is based. Another advantage is that the body can be adjusted in height. This provides the possibility of adapting the measuring range of the venturi measuring passage to the amount of water in the water channel. The body may be conveniently realised as a float to allow the adaptations referred to, to ensue even automatically. In order to facilitate the precise adjustment of the float it is desirable for the same to be provided with a ballast organ for taking on an adjustable amount of bal- last water.

In order to be able to also measure very low flow rates properly it is desirable for a threshold to be provided in the constriction and under the body.

The body and also the threshold if used, may be designed such that when passing the venturi measuring passage, the energy loss is kept to a minimum. In a particular aspect of the invention it may, however, be desirable for the body and/or the threshold in the water channel's direction of flow to have a symmetrical shape. This makes it possible to operate the venturi measuring passage in two opposite directions of flow, while providing equally precise measuring results.

For the benefit of taking precise flow rate measurements, low-flow sections are provided upstream and op- tionally downstream near the constriction in the water channel. These low-flow sections then allow measurements of the water level to be carried out in the water which is there almost still. These water level measurements can then be taken into account in the usual manner used for measuring the flow rate.

A special aspect of the invention is that the threshold and/or the body is provided with at least one pressure gauge for determining the local water pressure. In contrast with any other known method of determining the flow rate this, together with the water level in a low flow section immediately preceding the venturi measuring passage, provides a direct measure for the kinetic energy at the constriction, thereby making it possible to precisely determine the flow rate. In a further aspect of the invention the venturi measuring passage is characterized in that the same is provided with a water level gauge in the water channel, and a control system coupled with the water level gauge for adjusting the height of the body subject to the water level measured. In this way it is possible to completely automatically adjust the venturi measuring passage to the amount of water in the water channel, allowing the flow rate to be measured continuously and optimally. In order to allow the installation to work properly, it is desirable for the body to be provided with a guiding, and for this purpose it is preferably at either side of the constriction accommodated in guides. In another suitable embodiment of such guiding the body is sus- pended on a hinged jib construction that is adjustable in height. The venturi measuring passage according to the invention, and in particular the embodiment with the hinged jib construction allowing the body to be raised high without losing the guiding, is suitable for measuring the flow rate even if the water levels in the water channel are very high.

In the following the invention will be further elucidated with reference to the description of an exemplary embodiment and with reference to the appended draw- ing.

The drawing shows in

- Fig. 1 a first embodiment of the venturi measuring passage according to the invention; and in

- Figs. 2a, b and c a cross-sectional view of several variants of the venturi measuring passage according to the invention; and in

- Figs. 3a and 3b a top view of the venturi measuring passage according to the invention in symmetrical and asymmetrical form, respectively; and in - Fig. 3c the application of several venturi measuring passages according to the invention in a very wide water channel; and in - Figs. 4a and 4b a cross-sectional view of the venturi measuring passage according to the invention with several gauging devices.

Identical reference numbers in the figures refer to similar parts.

Referring first to Fig. 1, which shows the venturi measuring passage 1 according to the invention. This venturi measuring passage 1 is formed as a structure provided in the water channel 2, forming a constriction caused by abutments 3 and 4. These abutments 3 and 4 form a passage 5 for the water in the water channel 2. According to the invention a body 6 (in the illustrated case connected to a jib construction 13) coupled with a guiding system is provided in the constriction 5 formed by the abutments 3 and 4, which determines the height of the passage 5. This body 6 is preferably adjustable in height. This height adjustment can be easily realised by embodying the body 6 such that it is also a float. Such a float 6 will automatically adapt to the water level in the water channel 2. The float 6 may also be provided with a ballast organ (not shown) , for taking in an adjustable amount of ballast water. This provides another manner of adjusting the float 6. It is essential to the invention (as can be clearly seen in the Figs. 2a to 2c, which will be dis- cussed later) that viewed in the direction of flow in the water channel, the body 6 has a longitudinal profile that is formed by a wall surface 6' extending substantially parallel to this direction of flow.

Fig. 1 further shows that in the constriction 5 and below the body 6 a threshold 7 may be placed on the bottom of the water channel 2. The shape of the other side of the threshold 7 is usually a mirror image of the shape of the lower side of the float body 6.

The figures 2a to c show various possible embodi- ments of the body 6 and the threshold 7. Under some conditions the threshold 7 may be omitted, as will be explained below.

Fig. 2a shows the venturi measuring passage 1 designed with a body 6 and threshold 7 having an asy metri- cal longitudinal profile. This profile is intended for measuring the flow rate in water channels 2 in which the direction of flow is always the same, namely as indicated by arrow A. When measuring the flow rate with a venturi measuring passage 1 embodied like this, there is relatively little energy loss.

Fig. 2b shows the venturi measuring passage 1 having a symmetrical longitudinal profile suitable for measuring the flow rate in two opposite directions as in- dicated by arrow B. Basically this form of the venturi measuring passage 1 causes more eddying and turbulence in the water of the water channel 2, causing a greater energy loss than in the embodiment according to Fig 2a.

The venturi measuring passage 1 shown in Fig. 2c is embodied without threshold and is suitable for rivers or canals which already have a sufficiently high flow rate of themselves. This embodiment without threshold is also suitable if the water channel 2 moves much sediment. Incidentally, the constriction caused by the abutments 3, 4 at the venturi measuring passage produces an increased flow, so that there is little probability of sedimentation in this area. The venturi measuring passage according to the invention does not require a long straight open supply canal to be incorporated in the water channel 2. However, it is useful to broaden the water surface at the inlet side of the venturi measuring passage in order to create low flow sections in the water channel. These make it possible for the water level at those locations to be measured accurately. This will be explained in more detail with ref- erence to the Figs. 3a and 3b.

Fig. 3a shows the application of a symmetrical venturi measuring passage 1 suitable for measuring the flow rate in two directions. In that case it is preferred for so-called stilling wells 8 and 9 to be provided in the abutments 3, 4, in which the water level can be measured. On the basis of the measuring results thus obtained, both the flow direction and the depletion of energy over the venturi measuring passage 1 can be determined. Fig. 3b shows an asymmetrical venturi measuring passage 1, which in accordance with the embodiment of Fig. 3a is provided with two upstream stilling wells 8. This embodiment is suitable if the direction of flow through the venturi measuring passage 1 is always one-directional. If desired, it is possible here also to provide two stilling wells 9 downstream near the constriction of the venturi measuring passage 1, as shown in the figure. In view of the fact that these downstream stilling wells 9 are lo- cated close to the flow in the water channel 2, the determination of the water level is less accurate.

Fig. 3c shows the application of several adjacent venturi measuring passages 1 in a very wide water channel

2. In this application use is made of buttresses 10 placed in the water channel 2, which together with the abutments

3, 4 create constrictions in the water channel 2. Between the buttresses 10 or between the abutments 3, 4 and the respective buttress 10, bodies 6 are provided that determine the respective passages 5. The stilling wells 8 that are desirable for measuring the flow rate, may be incorporated in the abutments 3, 4 and the buttresses 10. According to the invention, the water pressure at the height of the venturi measuring passage 1 is (partly) determined with the aid of the pressure gauges incorporated in the threshold 7 and/or the body 6.

Fig. 4a shows the application in which pressure gauges 11 are incorporated in the body 6, while Fig. 4b shows the application of pressure gauges 12 in the threshold 7. The application shown in Fig. 4b of pressure gauges 12 in the threshold 7 has the advantage that the threshold 7 is static, which simplifies the placing and the use of the pressure gauges 12 incorporated therein. One possible drawback is that sediment may accumulate in the openings giving access to the pressure gauges 12, especially if there, is movement of sand in the water channel 2. This is also a real risk if the water in the water channel 2 is frequently still. These problems just mentioned may be avoided if the application shown in Fig. 4a of pressure gauges 11 in the float 6 is used. However, the drawback of this application is that the pressure gauges 11 are incorporated in a moving body which complicates reading the pressure value as well as interpreting the same, as after all the adjusted height of the float 6 has to be taken into account.

As general advantage of the venturi measuring passage 1 according to the invention may be mentioned that the same is fish-friendly because it forms no hindrance for the migratory behaviour exhibited by fish. Another advantage of the venturi measuring passage 1 according to the invention is that apart from measuring the flow rate in the water channel 2, it also makes it possible to con- trol the water level downstream or upstream. However, the price to be paid for this control function is usually the fishes' inability to pass when the resulting flow velocity in the passage 5 reaches too high a value, for example, more than one meter per second or in the alternative situation, when the float 6 is lowered all the way onto the threshold 7 so that the passage 5 is completely closed off.

Claims

1. A venturi measuring passage for measuring the flow rate in a water channel, comprising a constriction incorporated in the water channel, which forms a passage for the water in the water channel, while a body is incor- porated in the constriction, which determines the height of the passage, characterized in that the body has a wall surface extending substantially parallel to the direction of flow in the water channel.

2. A venturi measuring passage according to claim 1, characterized in that the body can be adjusted in height.

3. A venturi measuring passage according to claim 1 or 2, characterized in that the body is a float.

4. A venturi measuring passage according to claims 2 and 3, characterized in that the float is provided with a ballast organ for taking in an adjustable amount of ballast water.

5. A venturi measuring passage according to one of the preceding claims, characterized in that in the con- striction and under the body a threshold is provided on the bottom of the water channel.

6. A venturi measuring passage according to one of the claims 1-5, characterized in that the threshold and/or the body in the water channel's direction of flow have a symmetrical shape.

7. A venturi measuring passage according to one of the claims 1-6, characterized in that low-flow sections are provided upstream and/or downstream near the constriction in the water channel.

8. A venturi measuring passage according to one of the preceding claims, characterized in that the body and/or the threshold is provided with at least one pressure gauge for determining the local water pressure.

9. A venturi measuring passage according to one of the claims 2-10, characterized in that the same is provided with a water level gauge for determining the water level in the water channel, and a control system coupled with the water level gauge for adjusting the height of the body subject to the water level measured.

10. A venturi measuring passage according to one of the preceding claims, characterized in that the body is provided with a guiding.

11. A venturi measuring passage according to claim 10, characterized in that the body is at either side of the constriction accommodated in guides.

12. A venturi measuring passage according to claim 10, characterized in that the body is suspended on a hinged jib construction that is adjustable in height.