Introduction to Water Engineering

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Slide 1 Introduction to Water Engineering This is a walkthrough of the flow measurement prac Practical 2 Flow measurement Dr James Ward Lecturer School of Natural and Built Environments Slide 2

Please note COMMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been produced and communicated to you by or on behalf of the University of South Australia pursuant to Part

1 Slide 1 Introduction to Water Engineering This is a walkthrough of the flow measurement prac Practical 2 Flow measurement Dr James Ward Lecturer School of Natural and Built Environments Slide 2 Copyright Notice Do not remove this notice. Please note COMMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been produced and communicated to you by or on behalf of the University of South Australia pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act. Do not remove this notice. Slide 3 Basic layout So the basic layout of our flow measurement rig s like this. The water comes in here And shoots along this pipe It goes around these bends and the first device it passes through is a thing called a bend meter. You can read more about the individual flow measurement devices in the online materials. Anyway, it keeps on going and hits the Venturi meter, Then the Pitot tube 1

Slide 4 Basic layout Orifice plate: high head loss device; not appropriate for high flows Pitot tube: good for high flows (doesn t register much head difference at low flows) After the Pitot tube It

So we only use this one for low flows and in the experiment we actually run the flow around the orifice for the higher flows, using a bit of bypass plumbing.

Slide 5 Basic layout V-notch weir is quite accurate at low & high flows When it gets to the end of the pipe, the water empties Into this short section of open channel, And it finally flows out of the

2 Slide 4 Basic layout Orifice plate: high head loss device; not appropriate for high flows Pitot tube: good for high flows (doesn t register much head difference at low flows) After the Pitot tube It goes through an orifice plate The thing with an orifice plate is it s really, really intrusive to the flow so it creates a big pressure difference compared to other devices. So we only use this one for low flows and in the experiment we actually run the flow around the orifice for the higher flows, using a bit of bypass plumbing. Interestingly the reverse is the case for the Pitot tube, which doesn t register much at low flows but is really good at high flows since it doesn t interrupt the flow too much. Slide 5 Basic layout V-notch weir is quite accurate at low & high flows When it gets to the end of the pipe, the water empties Into this short section of open channel, And it finally flows out of the V-notch weir at the end. So we get to compare a bunch of pipe flow measurement devices with this open channel flow measuring device. Slide 6 The point of the prac Calibrate the coefficient of discharge for each measurement device How? Determine the head-discharge relationship Measure flow rate accurately Measure head difference The whole point of the prac s to calibrate the coefficient of discharge for each device. How do we do this? Well, we have to work out the headdischarge relationship experimentally and use the experimental data to flesh out an equation derived theoretically. To get the head-discharge relationship we need to accurately measure the flow rate using a device we trust, which means we know its accuracy And then measure the head difference on each device 2

Slide 7 Measuring head You might have noticed the little blue tubes everywhere on the earlier photos Each one of these tubes is connected to the main pipe at what s called a tapping point And they

Each blue tube has a tap on the manifold, so we can turn on just the tube we want, to get pressure from a particular point in the pipe.

Alternatively we could connect a Bourdon gauge to the manifold if we wanted to.

3 Slide 7 Measuring head You might have noticed the little blue tubes everywhere on the earlier photos Each one of these tubes is connected to the main pipe at what s called a tapping point And they relay the pressure at that point in the pipe to a pair of manifolds. Each blue tube has a tap on the manifold, so we can turn on just the tube we want, to get pressure from a particular point in the pipe. Then the manifolds each have a single tapping point Which relays the pressure out to the manometer where we actually take the measurement. Alternatively we could connect a Bourdon gauge to the manifold if we wanted to. Slide 8 Measuring head So those lines from the two manifolds Come into the main manometer at the bottom here And that gives us a differential pressure reading By reading the difference in elevation of the two water levels in the manometer, We get the head difference. Now assuming we ve switched on the right taps at the manifold, we should have, for instance, the pressure measured upstream and in the throat of the Venturi meter, or in the case of the Pitot tube we should have the taps on to show us the static head and the stagnation pressure. In any case assuming we ve selected the right tapping point to measure, we should have a head difference to correspond to the recorded flow rate. 3

Slide 9 V-notch weir Being an open channel device, the V- notch is different from the others. In this case we want to measure the height of water above the bottom of the crest.

Assuming we know where the bottom of the weir is with respect to the channel wall, we can just use a ruler to work out the depth. Slide 10 Independent variable Flow rate, Q (L/s) 3.8 6.75 5.13 1.86 2.

4 Slide 9 V-notch weir Being an open channel device, the V- notch is different from the others. In this case we want to measure the height of water above the bottom of the crest. The way we measure this is to stick a tapping point in the side of the channel near the weir, and run a tube up the side to use as a manometer. Assuming we know where the bottom of the weir is with respect to the channel wall, we can just use a ruler to work out the depth. Slide 10 Independent variable Flow rate, Q (L/s) Measured variable Meter Differential Pressure, h (m) Bend Venturi Pitot Tube V-notch The results of the prac are straightforward to assemble You ve got the independent variable, which is flow rate in this case we just close and open a valve to get a range of different flow rates, and record them from the trusted meter Then the dependent variable, the one we re measuring, is the head term measured either using the differential manometer or the manometer on the side of the V-notch weir. Slide 11 So we just chuck the results in Flow rate, Q Meter Differential Pressure, h (m) (L/s) Bend Venturi Pitot Tube V-notch

5 Q (m 3 /s) Introduction to Water Engineering Slide 12 Flow rate, Q Meter Differential Pressure, h (m) (L/s) Bend Venturi Pitot Tube V-notch And eventually we ve got a pretty good bunch of data. As you can see, like I said before, the Pitot tube really doesn t do a lot of good at these low flow values I mean you couldn t really have much faith in a head difference of 2 millimetres given that the measurement was only accurate to a millimetre and even then there s a fairly significant opportunity for human error. Slide 13 Low flow: Meter Differential Flow rate, Q (L/s) Pressure, h (m) Orifice For really low flows we let the water pass through the orifice and get some extra results. We don t let the higher flows through this one though, or the pressure buildup might blow up the rig. Slide Interpretation y = x This is the headdischarge relationship Q Cd A 2gH H (m) Now the approach to interpreting your results is explained in detail in the online materials, and it s also a lot like one of the examples in lecture 5. Basically once you ve got the head and flow data, you can plot it all up And it should be comparable to what you d expect from the discharge equation for each device. This one happens to be the discharge equation for a bend meter. I don t actually recommend you plot a power type trendline like this, so this is just to demonstrate the basics here. Anyway, assuming you can get some sort of power relationship out of your data analysis software or your spreadsheet or whatever, you can relate it to the known form of the equation where y represents Q, X represents H, 5

6 And the power should represent the power in your discharge equation. In this case it s a square root so it should be 0.5, but the trendline we ve got from the spreadsheet s actually come out with a power of Obviously different flow measuring devices have different equations and some of them have different powers of H, like the V-notch weir which has H to the 2.5. So you should see your data lining up at least roughly in the same sort of arrangement. Then the coefficient you get should be equal to all the constants in the discharge equation, which means you should be able to rearrange all this to get the coefficient of discharge. And that s this prac, in a nutshell. 6

Flow Meter Calibration

Flow Meter Calibration fluid mechanics H4 A compact unit that compares and shows the accuracy, losses and use of fundamental flow meters Key Features Unique quick-change flow meter adaptors and pressure connections Four tube