GB2177204A

GB2177204A - Measurement of fluid flows

Info

Publication number: GB2177204A
Application number: GB08516153A
Authority: GB
Inventors: Roger Norman
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1985-06-26
Filing date: 1985-06-26
Publication date: 1987-01-14
Also published as: GB2177204B; GB8516153D0

Abstract

Fluids, eg. domestic fuel gases may be metered using apparatus for the measurement of both high and low fluid flowrates including a conduit 11 for receiving a flow of fluid, transducer means e.g. 8, 9, 10, located within the conduit in the direct path of fluid flow and being responsive to changes in flow conditions whereby an output is produced which is representative of said changes and means for processing and expressing said output as a function of the fluid flowrate (Figs. 4a, 4b, not shown) characterised in that said transducer means is adapted to produce an output having at least two components, e.g. analogue and digital, and said processing means includes means for comparing said components. A Coanda effect meter is described, but other kinds are referred to as alternatives. <IMAGE>

Description

SPECIFICATION Measurement of fluid flows This invention relates to the measurement of liquid flows and more particularly to apparatus for the metering of gases, especially fuel gases supplied for domestic consumption.

The diaphragm type of volumetric meter has been in use for over one hundred years.

Whilst it has served the gas industry well over this time, it is, however, large and relatively expensive.

Current technology in flowmetering can provide relatively cheap and small meters, but they do suffer from certain disadvantages.

Gas flow rates,for domestic appliances, can range from as high as 3 to 6 m3hr-' when the full range of domestic gas appliances is being used at one time down to as low as 0.015m3hr-' when the only gas being consumed is that required for maintenance of pilot lights. Existing non-diaphragm type flowmeters, which satisfy the criteria for size and cost suffer from the disadvantage that they cannot measure accurately over the whole 0.015 to 6 m3hr ' range, and that they cannot measure consistently over long periods of time viz. 20 years, which is the required working life of such a meter.

Attempts have been made to rectify this deficiency. For example, according to one proposal, two metering instruments are connected in parallel. A diverter valve, operated either mannually or automatically directs the flow of gas over one meter or the other. The two meters are constructed, respectively, to accurately measure high or low flow rates.

However, meters based on analogue designs are subject to errors of drift and moreover, those designed for low flowrates are often far too sensitive to the effects of large flowrates and may be damaged by such flows or may cause excessive pressure loss at high flows.

The provision of the diverter is thus necessary to protect the low flow meter under high flow regimes. The presence of the diverter may cause aberations in flow, particularly if it leaks under low flow conditions and, being a mechanical device, is subject to wear and may require considerable maintenance over its working life.

The present invention seeks to overcome the disadvantages of the prior art by avoiding the use of a diverter valve and by effecting metering over the whole range of flows in the direct flow path of the fluid.

Accordingly, the present invention provides apparatus for the measurement of fluid flow rates which apparatus includes a conduit for receiving a flow of fluid, transducer means located within the conduit in the direct path of fluid flow and being responsive to changes in flow conditions whereby an output is produced which is a representation of said changes and means for processing and expressing said output as a function of fluid flowrate, characterised in that said transducer means is adapted to produce an output having at least two components and said processing means includes means for comparing said components.

The separate output components can be utilised to represent changes in flow conditions over respectively seperate predetermined flowrate ranges. Thus one component can be used to express high flow rates whilst the other can express low flow rates. Preferably the flow rate ranges will overlap and thus, in the overlap region, both components will be used to express the same flow rate and in this flow regime the components can be compared to provide a check on the accuracy of the transducer outputs. Transducers having an analogue output eg. a DC signal (voltage), although efficient for very low flow conditions, have a tendency for signal drift. On the other hand transducers having digital outputs eg. the frequency of an AC signal, are drift-free. Although such transducers are not accurate enough for low flow conditions, they are useful for representing high flow conditions.Thus, by comparing the AC and DC signals in medium range of flow conditions where both components can give accurate representations, it is possible to use the digital signal to check and re-calibrate (if necessary) the analogue output.

The invention will now be further described with reference to the accompanying drawings in which: Figure 1 is a schematic plan view of apparatus in accordance with the invention, Figure 2 is a plot showing the variation of voltage with flowrate for the low-flow section of the apparatus of Figure 1, where the plot is for a typical output from a thermal flow sensor based on the well known platinum resistance thermometer.

Figure 3 is a plot showing the variation of frequency with flow rate for the high-flow rate section of the apparatus of Fig 1, where the detection of the periodic oscillation of the flow is carried out by means of a typical differential pressure transducer.

Figures 4a and 4b are a flow chart for the electronic processing of the output components from each section.

Figures 5 and 6 are a plot of the variation of amplitude (Fig. 5) and frequency (Fig. 6) for a single sensor and illustrate an alternative embodiment of the invention.

Referring to Fig. 1, the flowmeter comprises a high flow metering section 1 and a low flow metering section 2.

The design and construction of the high flow section 1 is, in this embodiment based upon the known Coanda Effect. The body 1 is typically a solid black of suitable material into which rectangular channels are formed. The minimum depth of the channels is typically of the order of 4 times (4D) the diameter of the orifice of nozzle 3.

Within the body 1 is formed a chamber 11, which at the end remote from the nozzle 3 is divided into the regions 14 and 15 by splitter 12. Feedback channels 16 and 17 are provided between the middle sections of regions 14 and 15 and the front end of chamber 11.

Exiting from regions 14 and 15 are outlet channels 6 and 7 respectively.

Located within the splitter 12, but in communications with both regions 14 and 15 is a pressure sensitive sensor/transducer 8. Alternatively a sensor/transducer 9, such as a thermal sensor may be located in the chamber 11.

Typical dimensions for the body 1 would be: Chamber 1 l-Length-1OD Width -2.5D to 6D Region 14, 15-Width-3D Feedback Channel 16, 17-Width-2.5D Outlet Channel 6,7-Width-1.5D The principle upon which the apparatus works is based upon a jet of fluid issuing from nozzle 3 oscillates from side to side, first impinging on wall 4 and thence upon wall 5. The rate of oscillation is proportional to the flow rate over a wide flow range. However, at flow rates below about 0.15m3hr ', the correlation between rate of oscillation and flow rate falls off significantly and another metering device is required for low flow ranges.The Coanda-Type meter will accurately measure flowrates in the range of from 0.15 to 6m3hr ' and has the further advantages of cheapness, lack of moving parts, self-cleaning (due to oscillating flows) and the digital nature of the output (which is frequency).

The sensing of the fluid oscillation may be effected by measuring the oscillating differential pressure 8 between outlet channels 6 and 7 or by sensing the periodic passage of the jet across a thermal sensor 9.

The measure of low flows is effected by locating a thermal meter 10 such as hot wire, hot film or mass flow type meter in the low flow metering section 2. Changes in flow will be expressed as changes in voltage of the DC output from 10.

In an alternative embodiment (not shown), the low flow metering section can be dispensed with and the thermal sensor 9 in section 1 used to measure low flows as well. In this case above 0.15m3hr 'the amplitude of the AC signal output is used to express the low flow output whereas the frequency of the AC signal is used to express the high flow output. Below 0.15m3hr-', the only output will be a D.C. voltage (Figs. 5 and 6).

Since drift in the sensor will affect similarly the two analogue outputs (D.C. voltage below 0.15m3hr-' and A.C. amplitude above 0.15m3hr-1) then the error in the D.C. voltage can be deduced and, if necessary, corrected for, by comparing the A.C. amplitude with the frequency output whenever flow is above 1 5m3hr-1 In domestic operations flow rates of between 0.15 and 1.5m3hr 1 are common and under these conditions an electronic processor will receive reliable signals from both the high and low flow meters. These signals can be compared and since the signal from high flow meter section can be assumed to be drift-free, any drift in the low flowmeter can be detected and automatically corrected over an indefinite period.

Fig. 4 illustrates the rationale for the comparison of the analogue and digital signals. In the chart Q, is the low flow rate meter output, QH is the high flow rate meter output, Q1 and Q2 are arbitory flow rates in the overlap range eg. 0.15 and 1.5 m3hr 1 (see Figs. 2, 3, 5 & BR< 6) and a and b are functions eg. polynomials or taken from look-up tables for calculations of flow.

In the alternative embodiment (Figs. 5 and 6), the equations for calculating flow from the low flow output are, for example, Q,=alV,2+a2V,+a3 for QL Q1 and QL=C1VAC2+C2VAC+C3 for Q, Q1 The rescaling of the coefficients would then be: : a1 (NEW)=al (OLD) . (QH/Q,) a2 (NEW)=a2 (OLD) (QH/QL) a3 (NEW)=a3 (OLD) . (QH/QL) c, (NEW)=c1 (OLD) . (QH/Q,) c2 (NEW)=c2 (OLD) . (QH/QL) c3 (NEW)=c3 (OLD) . (QH/QL) Although the invention has been particularly described with reference to fluidic and thermal meters other embodiments could use vortex, rotary positive displacement, turbine or ultrasonic meters for measuring high flow rates and laminer, mass-flow or rotameter devices for the low flow section. The sensing device in the meters could utilise ultrasonic sensing, differential pressure, force or heat-transfer technology.

Claims

1. Apparatus for the measurement of fluid flowrates including a conduit for receiving a flow of fluid, transducer means located within the conduit in the direct path of fluid flow and being responsive to changes in flow conditions whereby an output is produced which is representative of said changes and means for processing and expressing said output as a function of the fluid flowrate characterised in that said transducer means is adapted to produce an output having at least two components, and said processing means includes means for comprising said components.

2. Apparatus as claimed in claim 1 wherein a first ouput component expresses changes in flowrate conditions over a first predetermined range, a second output expresses changes in flowrate conditions over a second predetermined range which overlaps said first range and said comparing means compares components in the overlapping range.

3. Apparatus as claimed in Claim 2 wherein said processing means includes means for re-calibrating or correcting said second output, after comparing said second output with said first output.

4. Apparatus as claimed in any of claims 1, 2 or 3 wherein a first output component is a digital signal.

5. Apparatus as claimed in claim 4 wherein said component is the frequency of an AC signal.

6. Apparatus as claimed in any one of claims 1 to 4 wherein a second output component is an analog signal.

7. Apparatus as claimed in claim 6 wherein said component is a voltage.

8. Apparatus as claimed in any one of the preceeding claims wherein said output components are respectively derived from separate transducer means.

9. Apparatus as claimed in any one of claims 1 to 7 wherein said output components are derived from a single transducer means.

10. Apparatus as claimed in any one of claims 1 to 8 wherein said transducer means are serially located within the conduit.

11. Apparatus as claimed in any one of claims 3 to 7 wherein said first digital output component is utilized to express a higher flow rate than said second analogue output component.

12. Apparatus as claimed in claim 10 wherein said first digital otuput component is utilized to express a flow rate of from 0.15 to 6.0m3hr 1 and said second analogue output component is utilized to express a flow rate of less than 1.Sm3hr

13. Apparatus for the measurement of fluid flowrates according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.