GB2400676A

GB2400676A - A flow measuring apparatus including a weir with an integral nozzle

Info

Publication number: GB2400676A
Application number: GB0308777A
Authority: GB
Inventors: Bryan Frederick Davenport; Nigel James Davenport
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-16
Filing date: 2003-04-16
Publication date: 2004-10-20
Anticipated expiration: 2023-04-16
Also published as: GB2400676B; GB0308777D0

Abstract

A weir 7 is provided for use in a flow measuring apparatus 1 for measuring the rate of flow of a liquid in a channel 3. The weir 7 comprises a parallel sided plate for vertical location across the channel 3 and defines an aperture which is provided with a nozzle 10 on its downstream side. The plate may also define a second aperture 11 located above the first aperture 10, with a significantly larger cross-sectional area. The weir 7 may be used to replace a V-notch weir of an existing apparatus or alternatively form part of a flow measurement apparatus 1 comprising a pre-fabricated trough 4 with a level sensor 8 mounted in an upper region thereof on the upstream side of the weir 7.

Description

LIQUID FLOW MEASURING APPARATUS

The present invention relates to flow measuring apparatus for measuring the rate of flow of a liquid in a channel.

A V-notch weir is a conventional form of apparatus which is used to enable an accurate liquid flow measurement to be taken in an open channel or in a closed channel where a liquid flow occurs solely under the influence of gravity without any additional motive force from a pump or similar device. Such an apparatus comprises an artificial trough which is used to control the liquid level in a channel upstream of a flat, vertical plate that is provided with a V notch in its top edge through which the liquid flows. The head of liquid created by the weir determines the rate of flow. The upstream depth of liquid can therefore be measured and used to determine the rate of liquid flow. Such weirs are usually employed within the water industry so that rates of water flow can be measured for various purposes, typically for water supply purposes as such weirs are not suitable for use in the measurement of flow rates for dirty liquids with suspended solids as the solids could settle in the trough upstream of the weir or foul the V-notch and thereby adversely affect the accuracy of the apparatus.

However, even when used to measure the flow rate of clean' liquids, Vnotch weirs do have several disadvantages, in particular, when they are used for measuring very low flow rates. First, liquid tends to cling to the downstream face of the vertical plate and adversely affect the flow through the notch. Second, over a given flow range the liquid head range can be quite small and as the accuracy and repeatability of level sensors used in the apparatus are fixed, the percentage error in the readings increases at lower flow rates. - 2 -

The object of the present invention is to provide a weir and a flow measuring apparatus incorporating such a weir that overcomes or substantially mitigates the aforementioned two disadvantages.

According to a first aspect of the present invention there is provided a weir for use in a flow measuring apparatus for measuring the rate of flow of a liquid in a channel comprising a parallel sided plate for vertical location across the channel, the plate defining an aperture which is provided with a nozzle on the downstream-side of the plate.

Preferably, the nozzle defines a circular cross sectional profile.

Preferably also, the nozzle tapers along its length in a direction away from the plate.

Advantageously, the plate defines a second aperture located above the first aperture.

Preferably also, the cross-sectional area of the second aperture is greater than that of the first aperture.

Preferably also, the second aperture is also provided with a nozzle on the downstream-side of the plate.

Preferably also, the plate is made from stainless steel.

According to a second aspect of the present invention there is provided a flow measurement apparatus for measuring the rate of flow of a liquid in a channel comprising a weir in accordance with the first aspect of the present invention.

According to a third aspect of the present invention there is provided a flow measurement apparatus for measuring the rate of flow of a liquid in a channel comprising a pre- fabricated trough, a weir disposed vertically across the trough, and a level sensor mounted in an upper portion of the trough on the upstream side of the weir, the weir comprising a parallel sided plate defining an aperture which is provided with a nozzle on the downstream-side of the plate.

Preferably, the trough defines a uniform cross sectional area along its length.

Preferably also, the trough is provided with an inlet section and an outlet section, which are coaxial.

Preferably also, the level sensor comprises a means enabling readings to be taken remotely.

Preferably also, the level sensor is calibrated prior to the trough being located in situ.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic vertical cross-section through a liquid flow measuring apparatus comprising a weir in accordance with the various aspects of the present invention; Fig. 2 is a plan view of the apparatus shown in Fig. Fig. 3 is a view similar to Fig. 1 but showing a second embodiment of weir.

With reference to Figs. 1 and 2, a liquid flow measuring apparatus 1 for measuring the rate of flow of an - 4 unpressurized liquid 2 in a channel 3 comprises a trough 4 with an inlet section 5 and an outlet section 6 that are secured to or form part of the channel 3. The longitudinal axes of the inlet and outlet sections, 5 and 6, are arranged to be horizontal and at the same vertical height so that flow through the apparatus 1 is not influenced by gravity.

Preferably, as shown in Fig 2, the inlet and outlet sections and 6 are coaxial. A weir 7, as described below, is located in the trough 4 and a level sensor 8 is mounted in an upper portion of the trough 4 on the upstream side of the weir 7.

The channel 3 may comprise an open channel or be a closed channel, as shown in Fig. 1. Fig. 1 actually shows a surcharged flow wherein upstream of the trough 4, the liquid 2 completely fills the closed channel 3 as opposed to running along the bottom of the channel 3. The flow measuring apparatus can still be used to measure the rate of such flows provided that the surcharge has occurred naturally and that the trough 4, which comprises a pressure recovery means, is sufficiently large to cope with the quantity of liquid and allows a natural head of pressure to develop upstream of the weir 7. However, it is expected that under most normal conditions, the flow will not be surcharged and will only part-fill the channel 3.

The weir 7 comprises a parallel sided plate that is disposed vertically across the trough 4. The plate is preferably made from stainless steel so that it is rigid and will not rust in use. However, any suitable material could be used to manufacture the plate, the requirements being that it is rigid and has smooth sides to which liquid will not fling.

Liquid is not intended to flow over the top of the plate, except in exceptional circumstances when no rate of flow can be measured. Instead an aperture is provided close to the top edge 9 of the plate to which is attached a nozzle 10 on the downstream-side of the weir 7. The aperture is circular in - 5 - shape and the nozzle 10 also defines a circular crosssectional profile, which may taper along its length in a direction away from the weir 7.

As in a conventional flow measurement apparatus using a weir arrangement, the mean velocity of the liquid flowing through the apparatus 1 is solely dependent on the head created in the trough 4 immediately upstream of the weir 7.

It is thus only necessary to measure a single variable, namely the depth of the head created. To this end, the level sensor 8 is located in an upper portion of the trough 4, preferably by being attached to a bracket (not shown) in a housing that is mounted above and that communicates with the interior of the trough 4. In a closed channel 3, the housing may comprise a pipe that can be accessed, for example, via a manhole which is normally closed by a cover. Alternatively and preferably in many applications, the level sensor 8 comprises a transducer enabling readings to be taken remotely, for example by the provision of a radio link, or similar, to external apparatus. Once installed, therefore, the apparatus never needs to be accessed directly except for maintenance purposes. Conveniently, the manhole can be made suitable for this purpose but in all cases it is important to ensure that the sensor 8 is not disturbed otherwise the apparatus may require recalibrating.

It will be appreciated that the height of the aperture and its crosssectional area will determine the head that builds up in the trough 4 on the upstream side of the weir 7.

There is in fact, a square root law relationship between the head and the rate of liquid flow Hence, dependent on the expected level of liquid flow within the channel 3, these factors can be varied for any given apparatus 1 in order to increase the size of head and therefore lower the percentage error in the readings taken by the level sensor at lower flow rates. In particular, the cross-sectional area of the - 6 aperture and nozzle can be small if low flow rates are a common feature of the installation in order that the comparative size of head is large and therefore that the accuracy of the readings taken is not reduced. Also, use of the nozzle 10 prevents liquid from clinging to the downstream side of the weir 7 and affecting the accuracy of the readings.

In cases where the flow may vary significantly, the weir can be modified as shown in Fig. 3. Here the weir 7 comprises a first aperture with a nozzle 10 on its downstream-side, as in Figs. 1 and 2, but is also provided with a second aperture 11 above the first that may also be provided with a second nozzle 12. Preferably, the cross-sectional areas of the first aperture is significantly smaller than that of the second aperture 11 in order that the accuracy of the apparatus is not decreased when the liquid flow rate is low. For example, the diameter of the lower aperture may in the region of 15 mm whereas the aperture 11 may have a diameter of around 50 mm.

Also, the second aperture 11 is preferably spaced a predetermined distance above the first aperture. Both of these factors mean that at low liquid flow rates, a reasonable head can be built up above the first aperture before liquid is able to flow through the second apertures.

This enables readings of the head to be taken so that an accurate calculation of liquid flow rate can be made, even when the rate is low.

The second aperture 11 is provided to cater for higher flows and the provision of the nozzle 12 is not a requirement because at high flow rates, the accuracy of the readings are not unduly affected by liquid clinging to the downstream face of the vertical plate. However, it can be advantageous to provide the nozzle 12 as it tends to discourage debris from lodging around the aperture 11. In both of the embodiments, if the nozzle 10 or the nozzle 12 become blocked, it will - 7 - usually be obvious from the readings taken as the liquid will flow over the top edge 9 of the weir, it being unlikely that either nozzle 10 or 12 will be ever only partially blocked.

It is, however, often the case that the V-notch in a conventional arrangement can become partially blocked with debris that significantly affects the accuracy of the readings taken, without it being obvious to anyone not on site that the liquid is not flowing correctly through the weir.

Also, in both embodiments, at very high flow rates, the liquid is still able to flow over the top edge 9 of the weir 7 as a safety overflow, when required.

The weir 7 can be fitted in place of a V-notch weir in any existing flow measuring apparatus to obtain the aforementioned advantages. However, it will be appreciated that the whole apparatus 1 could be prefabricated for installation in a new or pre-existing channel arrangement.

The trough 4 and its inlet and outlet sections 5 and 6 can be manufactured by being cast from a mouldable material such as plastics or concrete or be fabricated from metal pieces which are joined together. The joints may be formed using clamping fasteners and sealed but are preferably welded. The weir 7 can be fitted using any conventional method during manufacture of the trough 4.

While the apparatus 1 could be manufactured totally in situ, preferably the apparatus 1 is prefabricated and calibrated prior to being installed. The dimensions of the apparatus 1, in particular the cross-sectional area of the trough 4 and its inlet and outlet sections 4 and 5 can be predetermined in accordance with the dimensions of the channel 3 in which the apparatus 1 is to be installed.

Accurate measurements must be made of the interior - 8 dimensions of the trough 4 after fabrication for use in calibration of the apparatus 1.

Also, after fabrication of the trough 4, the level sensor 8 is mounted in place and the distance between the operative face of the sensor and the opposing bottom surface of the trough 4 accurately measured and recorded. This distance measurement is also used as part of the calibration data for the apparatus.

The apparatus 1 is now ready for shipment and location in the channel where it is to be used. It is expected that the apparatus 1 could be used either to replace a portion of an existing channel 3 or to fit inside an existing channel 3 which is modified to ensure all flow occurs through the apparatus 1. It will also be appreciated that the apparatus can be built-in to form a portion of a newly constructed channel.

Once the apparatus 1 has been located in the correct position on site with the longitudinal axis of the inlet and outlet portions 5 and 6 horizontal, the apparatus 1 should be held rigidly in place, for example by being bedded in a construction material such as concrete. To assist in the retention of the longitudinal axis 6 in a horizontal position, the apparatus 1 may be provided with adjustable feet (not shown) or a base frame (not shown) which enables the position of the apparatus 1 to be finely adjusted during positioning in its location. Such feet can comprise simple screw mounted feet so that the level of the apparatus 1 can be altered to that required.

The detailed measurements taken after fabrication and prior to installation of the apparatus 1 are used to produce accurate calibration data for the apparatus 1 without any need for a further visit to its location after installation. - 9 -

Hence, the calibration data or precalibrated instrumentation can be provided along with the assembly 1 as a complete package. However, the apparatus 1 can be independently calibrated using known volumetric or gravimetric techniques Thus the invention provides a flow measuring apparatus which substantially overcomes the problems described above.

It can be prefabricated in a factory or workshop and may be shipped to site with full calibration data because when fitted with reasonable care the dimensions and tolerances of the apparatus will not vary. -

Claims

1. A weir for use in a flow measuring apparatus for measuring the rate of flow of a liquid in a channel comprising a parallel sided plate for vertical location across the channel, the plate defining an aperture which is provided with a nozzle on the downstream-side of the plate.

2. A weir as claimed in Claim 1, wherein the nozzle defines a circular cross-sectional profile.

3. A weir as claimed in Claim 1 or Claim 2, wherein the nozzle tapers along its length in a direction away from the plate.

A weir as claimed in any one of Claims 1 to 3, wherein the plate defines a second aperture located above the first aperture.

5. A weir as claimed in Claim 4, wherein the cross- sectional area of the second aperture is greater than that of the first aperture.

6. A weir as claimed Claim 4 or Claim 5, wherein the second aperture is also provided with a nozzle on the downstream-side of the plate.

7. A weir as claimed in any one of Claims 1 to 6, wherein the plate is made from stainless steel.

8. A weir for use in a flow measuring apparatus for measuring the rate of flow of a liquid in a channel substantially as described herein with reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings. - 11

9. A flow measurement apparatus for measuring the rate of flow of a liquid in a channel comprising a weir as claimed in any of Claims 1 to 8.

10. A flow measurement apparatus for measuring the rate of flow of a liquid in a channel comprising a pre fabricated trough, a weir disposed vertically across the trough, and a level sensor mounted in an upper portion of the trough on the upstream side of the weir, the weir comprising a parallel sided plate defining an aperture which is provided with a nozzle on the downstream-side of the plate.

11. An apparatus as claimed in Claim 9 or Claim 10, wherein the trough defines a uniform cross-sectional area along its length.

12. An apparatus as claimed in any of Claims 9 to 11, wherein the trough is provided with an inlet section and an outlet section, which are coaxial.

An apparatus as claimed in any of Claims 9 to 12, wherein the level sensor comprises a means enabling readings to be taken remotely.

14. An apparatus as claimed in any of Claims 9 to 13, wherein the level sensor is calibrated prior to the trough being located in si tu.

15. A flow measurement apparatus for measuring the rate of flow of a liquid in a channel substantially as described herein with reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings.