Know the Flow. Flowmetering Training Manual

Transcription

1 Know the Flow Flowmetering Training Manual

2 Acknowledgments ANCID and its member organisations National Program for Irrigation Research and Development Goulburn Murray Water Training Centre Manly Hydraulics Laboratory Manufacturers MACE Trimec Parametrics ABB Metering Amiad Combined Instruments/Tyco ABB Australia This training manual is a first draft of an on-going process to gather and compile information about the selection, installation, operation and maintenance of flowmeters for rural irrigation water supply for learning purposes. The manual has been developed by the Australian National Committee on Irrigation and Drainage (ANCID) with support from Land and Water Australia and remains the intellectual property of, and is copyrighted to ANCID. If you wish to use this material, please obtain permission from: ANCID PO Box 165, Tatura, VIC 3616 Phone: johnmap@g-mwater.com.au More information about Flow Metering can be found on the Know the Flow website at Alison Carmichael Naturally Resourceful Pty Ltd PO Box 355, Alstonville NSW 2477 Phone: Fax: Mobile: alison@naturallyresourceful.com.au

3 Contents Flowmetering Training Manual Introduction The need for metering Basic concepts Types of flowmeters Dethridge meter Dethridge Long meter Propeller meter - open flow Propeller meter - closed flow Paddlewheel meter Turbine meter Ultrasonic meter Electromagnetic meter Venturi and orifice flowmeters Flumes and weirs Selecting a flowmeter Installing flowmeters Operation of flowmeters Maintenance of flowmeters Testing of flowmeters...47 Appendix 1: Flowmeter selection guide...51 Appendix 2: Irrigation meter testing facilities...54 Appendix 3: Unit conversion tables...55 Appendix 4: Units of Measurement...58 Appendix 5: Australian standards relevant to flow measurement...59 Appendix 6: Meter manufacturers and distributors...62

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5 KNOW THE FLOW INTRODUCTION Introduction The Know The Flow (KTF) project was initiated in 1997 to address issues relating to the accurate measurement of the delivery of irrigation water to farms. Up until this time, most water that was metered was done so using Dethridge Wheels. The accuracy of these devices was variable and they posed a risk in terms of OH&S. Since the introduction of national COAG water reforms there has been a need for greater accuracy in water measurement. This has come about as a result of higher charges for water, increased accountability in water distribution, the need to minimise losses and in some cases, less water being available for irrigation. To obtain the accuracy levels required now, water supply authorities are replacing Dethridge wheels with other types of meters and are placing meters in previously unmetered supplies. This trend will continue through the next few years. Three major recommendations were made as a result of the original KTF project. They were to: 1. develop standard testing procedures for water meters 2. scope the development of a training program in basic hydraulics and meter installation for relevant staff of authorities. 3. create a website to centralise and communicate information about metering. This current project is designed to address the second recommendation. The KTF website was developed in 2000/2001 to meet the communication requirements expressed in the third objective. An essential component of this site was publication of user reports of meter performance in the field. Reports that were received showed that most problems with meters were due to the wrong type of meter being selected for the situation, incorrectly installed or poorly maintained. This reinforced the need for improved knowledge and skills in meter selection, installation and maintenance within the irrigation water supply sector. An effective way of improving knowledge and skills is to develop a training course that will meet the needs of the sector. Potential users of this course are irrigation manufacturers, consultants, irrigators, installers and staff members of irrigation supply authorities. There has been a high degree of on-the-job learning about metering within organisations, and the website is the first step in summarising this information and making it nationally available. It is now time to bring together the technical knowledge in more detail and present it in a way that is accessible to all who need it. Over the next few years the knowledge about metering will increase substantially and the training course will need to be structured in such a way that new knowledge can be added as it becomes available. Irrigation water supply is a rural activity and the people who will want or need to undertake training may not always be able to access group training. Therefore, it is understood that the training course will need to suit not just training in groups but be able to be used by individuals with varying degrees of support. Scope of this training manual This course is aimed at providing people who work in the rural irrigation water supply sector with information to effectively carry out flowmetering operations for customer off-take. It does not intend to cover the operation of supply channel metering, though the principles and many of the operations are similar. The target audience will be technical staff who are working with water meters, or are about to be involved in the reading, installation or maintenance of flowmetering devices. These people will have a working knowledge of hydraulics and will most likely be working in a supervisory capacity. It is not expected that all the contents of the training manual will be used during a training event. It has been developed as a resource manual for students and trainers to select from it the material they need. ANCID

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7 KNOW THE FLOW THE NEED FOR METERING 1. The need for metering At the end of this section you will be able to: 1. describe the metering devices used by your organisation 2. identify the reasons they have been installed 3. discuss the potential changes to your organisations metering requirements in the future. You can t manage what you can t measure Since the introduction of national water reforms water is widely recognised as a valuable resource and is therefore being managed more intensely. There is now more interest in metering of previously unmetered water supplies and where metering was carried out there is now a demand for greater metering accuracy. To obtain the accuracy levels required, water supply authorities are replacing Dethridge wheels with other types of meters and are placing meters in previously unmetered supplies. This trend will continue through the next few years. Some of the reasons that metering is carried out include: Measurement for use. This is the most common reason for metering. Metering is used by authorities to monitor individual customer use against entitlement, to find out how much water is being extracted from the system and to bill customers for water used. As charges for water get higher, users will demand more accuracy in measurement. Measurement for distribution. Water suppliers need to know how much water is in each part of their delivery system at any time to be able to manage delivery. As there will be less water available for irrigation they will need to be even more accountable for the water they distribute. Measurement for management. Water suppliers are metering for management purposes. Strategic metering helps them to calculate the water distribution efficiency of the system and to identify and minimise water losses such as seepage and evaporation. Measurement for environmental purposes. To monitor what is actually being supplied for environmental flows in natural water courses to meet legislative and environmental requirements. In the past, when Dethridge Wheels were the most commonly used metering device, measurement of water supply and measurement of flow in the distribution system were separate activities. The future trend is for metering of distribution and supply to be integrated into a single water control system. This has been made possible by the use of electronic meter readouts linked to computerised data collection systems. Distribution and supply can be monitored at all times and water release can be controlled centrally or even automated. There is an increasingly wide range of meters available. Irrespective of which meter is used, it is essential that water meters are: installed correctly well maintained read accurately. These issues will be the focus of the rest of the course. ANCID

8 THE NEED FOR METERING KNOW THE FLOW The perfect meter? In 2000, an industry survey was conducted to find out what criteria would describe the perfect water meter. The findings of the survey were validated at a workshop. They found that any device used to measure flows from a water supply channel onto a farm should have the following characteristics and features: Provide a consistent level of accuracy commensurate with the value of the water resource being delivered. The desirable accuracy level is of the true flow throughout the required flow range. In some applications, an accuracy tolerance of up to 5% is acceptable. Operate with accuracy level over a flow range from 0.3 to 25 ML/d. Flows up to 50 ML/d may occur in some cases although use of an additional meter to record at high flow rates would be acceptable if practicable. Be able to measure flows accurately over a wide range of channel water levels. The suggested minimum head requirement is 20 mm and maximum head is at least 300 mm. Be vandal proof or, where unauthorised interference occurs, be easily detectable. Be simple to operate and read, provide flow data as both instantaneous flow rates and totalised volume with provision for remote interrogation and/or transmittal of data. Use robust technology and construction with non-intrusive mechanism so as not to be affected by blockages, fouling or poor water quality. Have moderate capital cost with low operation and maintenance costs to result in low overall life cycle costs. Spare parts and support services should be readily available. Pose minimal risk to operators and the general public. Provide minimum impedance to access along channel banks for operation and maintenance purposes. Learning activity 1. Why and how is metering carried out by your organisation? 2. How do you think metering will change in the future for your organisation? 3. Do you know of any meter that fits the criteria of the perfect meter? 8 ANCID 2002

9 KNOW THE FLOW BASIC CONCEPTS 2. Basic concepts At the end of this section you will be able to: 1. describe flow parameters 2. define terms relating to flow 3. define terms of measurement such as accuracy, precision and error. Q = AV In the context of irrigation water supply, the aim of flowmetering is to find out how much water is flowing from one point to another at any one time (flow) or to establish the flow rate which is the volume of water passing a point in a given period of time. The flow rate is calculated by using the basic formula: Q = AV Where: Q is the flow rate, or discharge rate Ais the cross-sectional area and V is the average velocity of the water. Discharge, or flow rate, is usually expressed as megalitres/day (ML/day) or litres/second (L/s). From this equation you can calculate the volume of water by multiplying Q x time. This means there are only two things that you will need to know to calculate the rate of flow: 1. The size of the inside of the pipe or channel dimensions. Larger pipes and channels will allow for a higher flow rate than smaller pipes or channels. 2. The velocity, or speed of the water. Speed can be increased by increasing the pressure or the head, (height). Pressure can be applied by forcing water through the conduit, usually by means of a pump on the systems. Pressure is measured in kilopascals (kpa) or in imperial units in pounds per square inch (psi). Head being the vertical height of the water level above a datum point measured in metres (m). Pressure is increased as the head increases between two points. Flow occurs in pipelines when there is a difference in pressure or head (height) between the two ends. Water will flow from high head to low head, and from high pressure to low pressure. The bigger the pipe cross-section, the higher the flow rate. How water flows Open channel flow: Water in an open channel will only flow if there is a downward slope of the water surface. If there is a slope then water will flow due to the effects of gravity. The greater the fall, or head, the faster the water will flow. Closed conduit flow: This is also known more descriptively as full pipe flow. If water does not completely fill the pipe the water movement is classified as open channel flow. Ground water for irrigation can only be extracted by being pumped under pressure through pipes unless it comes from artesian bores and is therefore forced out of the ground under pressure. ANCID

10 BASIC CONCEPTS KNOW THE FLOW The equation Q = AV assumes that the flow rate of water in a pipe is uniform across the cross-section though this is rarely the case. Flowmeters are designed to measure accurately only under laminar flow conditions. Figure Partially full pipe on the left is classified as open channel flow. Full pipe on right is closed conduit flow (Panametrics) Turbulent flow: Turbulent flow is when the water swirls in the pipe or channel or by obstructions in the flow stream. It can also be caused by a meter. For example, when a mechanical meter is running too fast it can cause turbulence. Obstructions can include weeds, incorrectly placed gaskets protruding into the flow, shells, build up of iron oxide in fittings and valves. Established flow: Sometimes erroneously called laminar flow. After turbulence distorts the velocity profile it takes a long length of straight pipe before the profile becomes established again. Baker in An Introductory Guide to Flow Measurement (MEP, 1989) says it needs at least 60 diameters to obtain established flow and some engineers say from experience that 100 diameters is needed when there is a good swirl set up. The 5 to 10 diameters recommended by many manufacturers assumes that there is no swirl in the pipe. Established flow is where the water is flowing through a pipe or channel in a straight line, without turbulence. Most water meters are designed to only measure accurately in established flow conditions. Figure Velocity profile in horizontal pipe for established flow (Panametrics) Because of the change in flow across the cross-section of a pipe, some meters can give an inaccurate reading if placed too near the pipe wall or in turbulence. This is especially the case with propeller meters. Therefore it is important to know if the flow meter you are using needs to be positioned in a certain way. Figure Turbulent flow conditions caused by an obstruction on the pipe wall (Trimec) Velocity profiles can be allowed for in the metering calculations if they are consistent and this occurs when the pipe is anywhere between horizontal and vertical. Downward flows in pipes have a more uneven profile due to gravity. 10 ANCID 2002

11 KNOW THE FLOW BASIC CONCEPTS Figure Difference in flow profiles in a vertical pipe for upwards flow and downwards flow. (Panametrics) It is important where a flowmeter is positioned in a pipe or channel and how it is positioned. To get the most accurate reading possible, a meter needs to be able to average the readings across the entire cross-section of the pipe or channel but only a few types of meters can do this. The most common alternative is to read the flow in the centre of the pipe, not near the pipe wall, so as to measure flow in near-laminar conditions. For this reason, it would not be wise to try and measure flow in a vertical pipe with water flowing down the pipe. Figure Ways that turbulence can develop due to bends, valves and obstructions (Trimec) ANCID

12 BASIC CONCEPTS KNOW THE FLOW Figure Ways to minimise the effects of turbulence using reducers, straight lengths and good placement of meter. (Trimec) Principles of water measurement In mathematics, you can count and you can measure and each activity has varying degrees of accuracy. Counting: Counting can provide an exact result. For example, you can count the number of apples in a box. Measurement: Measurement is inexact; it is at best an approximation and will always have a degree of error. For example, you can count the exact number of apples in a consignment of boxes without error but this can be time-consuming. An alternative is to count the number of apples in a single container and then count the number of containers and multiply the total number of apples by the number of containers. This will give you a close estimate of the number of apples within an acceptable level of error. This is a measurement. 12 ANCID 2002

13 KNOW THE FLOW BASIC CONCEPTS Accuracy and error: Accuracy of measurement relates to the quality of the result. For water meters it is the degree to which a meter confirms to a standard or true value. As discussed in Chapter 1, there are many reasons why you are now required to measure water with a greater degree of accuracy than in the past. This means that there is an expectation that equipment will record much lower flow rates than before. For example, a flood irrigation block was in the past typically irrigated at the rate of 52L/s, whereas after conversion to drip, the irrigation rate is around 13L/s or roughly a quarter of what it was before. The accuracy of scales in food stores are tested regularly with known or Standard weights. Accuracy is usually discussed in terms of deviation from the standard. Field conditions can influence the accuracy of a meter. It is important that meters are installed correctly so that meters operating under field conditions have an acceptable level of accuracy. For example, you expect greater accuracy in a meter which measures small amounts of medicine than one that meters bulk water. Accuracy is reported in percentages of error, for example, a manufacturer will claim that a meter will be accurate to within ±2%, that is, it can have up to 2% error. This meter is deemed accurate if it reads anywhere between 2% below or 2% above the correct reading. The level of accuracy that is acceptable depends on the situation. Manufacturers test their meters in what is called fully developed flow conditions therefore achieving laminar flow. In these conditions they can claim accuracies of ±2%. Similar accuracies are found when meters are tested in laboratories. In the field, the meters are often operating in a non-perfect environment. Most operators are happy if their meter is operating within 5% in a field situation. Repeatability. Accuracy is different to repeatability, which relates to the quality of measuring process. Repeatability is the degree of consistency or uniformity of a result. A measurement can be precise, or repeatable, without being accurate as shown in the figure below. In this case, the application of some systematic adjustment (aim lower and further left) would result in better accuracy. Meters are often precise and then calibrated for accuracy in this way. The following examples of an archery target illustrate the difference between accuracy and repeatability. Figure From left to right, showing repeatability without accuracy, accuracy with a moderate degree of repeatability, and accuracy with a high degree of repeatability ( Flow totalisation Water is sold and measured in terms of total volume taken from the supply over a given period of time. Many meters can provide two sets of readings. The first is a measurement of the current flow, the actual flow rate at the time. This can be an instantaneous reading which is a single reading taken or an integrated reading in which a number of readings are taken in a very short time and then the results are averaged and the display shows only the averaged figure. ANCID

14 BASIC CONCEPTS KNOW THE FLOW The second reading is where the meter keeps adding up, or totalising, the water that flows through it, just like our domestic water meters. The amount we use during the billing cycle is calculated by subtracting the previous reading from the current reading. Current reading 30 March 02 Previous reading 31 Dec 02 Usage for Jan, Feb and Mar 2,965 ML 1,556 ML 1,409 ML Learning activity For the supply system you work in: 1. Do you have both open channel and closed conduit flow? Where? 2. Would any of your piped systems be classified as open channel flow? 3. Are they being treated as such? If so, what is the implication of this? 4. What situations in your delivery system could result in turbulent flow or swirl? 5. What are the expected field accuracies for meters used in your system? Further reading Buckner, Dr Ben, The nature of measurement: Part II, Mistakes and Errors Water Measurement Manual, US Department of the Interior, Bureau of Reclamation (This is the online version of the US Water Measurement Manual. Each chapter is available as a PDF file.) Baker, R.C., 1989, An introductory guide to flow measurement, MEP Ltd, London Kay, M, 1998, Practical Hydraulics. E&FN Spon, London 14 ANCID 2002

15 KNOW THE FLOW TYPES OF FLOWMETERS 3. Types of flowmeters At the end of this section you will be able to identify common types of meters currently used in Australia and: 1. describe how they work 2. outline their likely applications and uses 3. list their advantages and disadvantages 4. examples of brands and models. This manual provides an overview of flowmeters for both open channel and full pipe systems. Each meter has its own unique (but related) hydraulic characteristics, that are used by the flow measurement device to determine the flow. In the previous chapter we learned that flow rate is directly proportional to the velocity of the water and the cross-sectional area of the conduit. The velocity is related to the pressure or head in the system at the point of measurement. Flow measurement devices do not measure flow directly. Instead, some measure the velocity of the flow and others measure changes in head or pressure. This information is then used to calculate flow. These meters are known as inferential type meters. Common types of flowmeters are listed in the table below: Meters that measure velocity are: Meters Mechanical meters Electromagnetic meters Ultrasonic meters Subtypes Propeller meters, closed type Propeller meters, open type Paddlewheel meters Turbine meters Positive displacement meters Doppler meters Transit time meters Alternative names Propeller actuated or PA meter Helical rotor PD meters Magmeters Acoustic meters Meters that measure pressure or head are: Meters Venturi meters Orifice meters Ultrasonic meters in conjunction with calibrated weirs and flumes. Subtypes Velocity head Alternative names ANCID

16 TYPES OF FLOWMETERS KNOW THE FLOW The rest of Section 3 addresses each type as regards to how they work, their application, specifications, and advantages or disadvantages, building on the information available on the Know the Flow website: Learning activity After considering the following meter types see if you can answer the following: 1. What meters (brands and models) are used in your organisation? 2. Why do you think they were selected? 3. Why does the Dethridge Wheel need to be replaced? 4. What do you think will be the meter type most likely to be used in the future? Why? 16 ANCID 2002

17 KNOW THE FLOW DETHRIDGE METER 3.1 Dethridge meter Dethridge meters (also known as Dethridge wheels) have been installed in Australian irrigation systems for over 60 years and there are approximately 60,000 still in use today. They are cheap, reasonably accurate and easy to use and so have become an industry benchmark for flow measurement. Individual customers may have more than one meter, depending on the size of their property and how the irrigation supply to it is set up. Figure Dethridge meter ( How it works The Dethridge meter consists of a circular drum to which vanes are attached and which revolves in a concrete emplacement. The wheel is turned by water pressure on the vanes and in turning displaces a fixed quantity of water between each pair of vanes. The meter consists of a cylindrical metal drum fitted with 8 vanes around the circumference mounted on a horizontal axle in an open concrete flume emplacement. Water flow causes the wheel to rotate in the emplacement and a counting device records the number of wheel revolutions and thus a direct measure of the volume of water passing. The concrete emplacement and wheel are constructed to close dimensional tolerances. Flow rates are easy to estimate in the field as the number of revolutions of the wheel each minute could be counted and multiplied by a factor to get an approximated flow rate in ML/day. For example, in some areas of Queensland a factor of 1.2 is applied to large meter outlets and 0.5 to small meter outlets. Figure Cross-section of Dethridge meter (Sunwater) ANCID

18 DETHRIDGE METER KNOW THE FLOW The Dethridge meter forms the water inlet to the customer s property and incorporates a sliding gate. When the gate is opened - assuming the channel is full!! - water will begin to flow through the meter onto the property, where it will be diverted by the customer to meet the needs of the land being watered. On the small meter the wheel is 1215 mm or 4 feet in diameter. A revolution counter is attached to a vane of the wheel and geared to record flows in ML and hundredths of a ML onto a readable counter. This counter is read regularly and forms the basis for charging the customers for usage when appropriate. The basic design and dimensions remained unchanged for over 80 years although there have been many modifications and improvements including the use of more durable materials and the addition of peripheral equipment such as control gates and counting mechanisms. Older wheels were made from mild steel coated with a coal tar or bituminous paint. Galvanised mild steel is the normal material used now although aluminium, various grades of stainless steel and plastics have also been used to improve durability. Axle bearings were originally red gum timber and have been mostly replaced by sealed ball bearings to retain accurate tolerances. The standard Dethridge meter is available in several sizes. Large meters will measure flow ranges between 3.5 to 12 ML/day while small meters measure flow range from 1.5 to 6 ML/day. Application and uses Dethridge meters are used for metering farm offtakes from open channels and have also been used off pipelines. The meter has proved to be satisfactory under a range of field conditions due to its relative simplicity, low cost, accuracy and robustness compared with other meters of similar capability. Advantages and Benefits Dethridge meters are suitable for wide range of irrigation applications. Reasonably accurate provided that clearances and settings are correct and channels are operated at correct levels. Relatively easy to use and the direct displacement method is easily understood by operators and farmers. Low capital cost means it is economical in comparison with many other meters of similar capacity. No power source required. Robust so can resist forces from impact by debris. Correct operation and flow rate can be ascertained from a distance. Security features make unauthorised water use difficult but easy to detect if attempted. Low head requirement - up to 75 mm. Disadvantages Reduced measurement accuracy when channel levels fluctuate significantly or flow rate is outside the range 3 to 12 ML/day. Excessive wear and/or bearing failure or incorrect setting can contribute to inaccuracy. Damage to wheels and vanes can occur at high flow rates. Corrosion of steel components can be significant in moderately saline conditions. Safety hazard. The large mass, manually operated gates and exposed rotating vanes may pose a hazard to operators, farmers and public. Reduced access. Wheel can create a barrier to access along the channel unless an access pipe or culvert is also installed. Channel leakage. Yabbies can burrow under structure and cause leakage if there is insufficient cut-off provision. Brands and models Dethridge meters are usually engineered by local manufacturing firms in irrigation regions. 18 ANCID 2002

19 KNOW THE FLOW DETHRIDGE LONG METER 3.2 Dethridge Long meter The Dethridge Long (DL) meter was developed during the 1980s and adopted for general use about So far about 200 have been installed. The meter can be constructed as a new installation or by modification of a standard meter emplacement. How it works The key dimensions and basic configuration of DL meter are as for a large standard Dethridge meter although the upstream approach section is longer. The new meter has only six vanes attached to the drum and carefully shaped to minimise splash and flow restrictions. The emplacement is also redesigned. It can be constructed from conventional reinforced concrete or light weight fibre reinforced concrete. The latter material provides the high dimensional accuracy needed to maintain high levels of measurement accuracy. Wheels and vanes are constructed from galvanised mild steel. Figure Dethridge Long meter showing the elongated vanes (Sunwater) Figure Dethridge Long meter in its emplacement (Sunwater) Application and uses The flow range for acceptable accuracy is 2 to 20 ML/day. A Dethridge Long meter is used in favour of a standard Dethridge meter when: the maximum flow to be metered is higher than 12 ML/day where there is a large amount of very level land, head losses need to be minimised to maintain good flow conditions and measurement accuracy. ANCID

20 DETHRIDGE LONG METER KNOW THE FLOW Advantages and Benefits The DL meter has the same advantages and benefits of the standard Dethridge meter and the following additional ones: Meter accuracy (<± 2%) over the wider flow rate range of 2 to 20 ML/day. Capital cost is about 30% above that of the standard large meter. However for flow rates greater than 12 ML/day, which would require two standard meters, the total metering cost is therefore generally reduced. Decreased splash and wash reduces leakage and erosion around emplacement. Improved irrigation of large, very flat properties. Meter operation and accuracy is less affected by fluctuating water levels. Very low head is required to operate the meter. Disadvantages The DL meter was designed specifically to overcome reported disadvantages of the standard Dethridge meter. Experience to date shows that the DL meter overcomes most of these and no significant disadvantages have been identified. The potential safety hazard associated with wheel rotation remains and might be greater due to increased rotation speed. Some DL wheels have experienced premature failure at high flow rates although these were due mainly to manufacturing defects rather than an inherent design fault. Higher cost than Dethridge meter Brands and models Dethridge Long meters are usually engineered by local manufacturing firms in irrigation regions. 20 ANCID 2002

21 KNOW THE FLOW PROPELLER METER OPEN FLOW 3.3 Propeller meter - open flow How it works The open flow propeller meter consists of a plastic propeller and extended spindle shaft which is mounted on the downstream end of a pipe culvert with the propeller projecting inside the pipe with its axis located at the centre of, and parallel to, the flow. The culvert pipe must always flow full of water. The rate of propeller rotation provides a measure of flow rate from which flow volume can be derived and recorded. There is little head loss through the meter. Figure Open flowmeter (ABB Metering) Application and uses These meters can be used to measure water delivered from the supply channel to a farm distribution system. A number of these meters have been installed in NSW schemes over the past two years as an alternative to Dethridge meters. Experience to date is reported to be generally good with satisfactory levels of accuracy. Advantages and Benefits Reasonably accurate measurement provided the meter is correctly installed, calibrated and maintained. Operates satisfactorily in turbid water. Supplier claims long life and low maintenance for working parts. Recording mechanism can display flow rate and totalised volume. Can operate in fluctuating water levels, provided that installation ensures full pipe flow at all times. Disadvantages Very difficult to detect malfunction or unauthorised interference to meter while it is operating. Culvert pipe and propeller can easily be obstructed by debris and weeds. The meter cost is low but total installation cost may be greater than for a Dethridge meter depending on cost of constructing the culvert. Brands and models ABB Metering R2000 ANCID

22 PROPELLER METER CLOSED FLOW KNOW THE FLOW 3.4 Propeller meter - closed flow Also known as propeller activated, or PA meters. How it works The meter consists of a metal or plastic propeller mounted inside a pipe section with its rotation axis set parallel to the water flow. As water flows past the propeller it causes it to turn. The faster the water is flowing, the faster the propeller spins. This provides a measure of flow velocity from which volumetric flow can be calculated for a given pipe cross section. Meters are produced in a range of standard sizes with calibrations determined by the manufacturers from laboratory testing. Figure ABB R2000 casing and display (ABB Metering) Figure ABB R2000 showing propeller inside the casing (ABB Metering) Application and uses Closed flow propeller meters are usually configured as an in-line meter in a closed pipe system. It is also used where water is pumped from an open channel or natural water course to irrigate land situated above the level of the water supply carrier. In the latter case the meter is located in the pipework either on the suction or delivery side of the pump. For accuracy, the meter must be carefully located clear of pipe bends or fittings and configured so that the pipe flows full at the meter. The computed water flow is normally displayed as a progressive volume. 22 ANCID 2002

23 KNOW THE FLOW PROPELLER METER CLOSED FLOW Advantages and Benefits Reasonably accurate means of measurement provided the meter is correctly installed and maintained. Can be installed to suit many different irrigation layouts. Operates satisfactorily in turbid water. Clear flow passages allow flushing of suspended solids. Tolerant of moderate levels of sand and silt. Can be installed in horizontal or inclined pipelines without loss of accuracy. Disadvantages Very difficult to detect malfunction or unauthorised interference to meter while operating. Propeller can be fouled or stopped by floating debris, weeds or other obstruction. Older type propellers were susceptible to abrasion or mineral build up. Brands and models Amiad ABB Metering R2000 inline meter as shown in Figure Tempress Water Specialties ANCID

24 PADDLEWHEEL METER KNOW THE FLOW 3.5 Paddlewheel meter A vertically orientated impeller is rotated by the velocity of water passing through the bore of the meter, which is translated into a volumetric reading logged on the six digit accumulator of a sealed register. An adjustable calibration device is utilised to calibrate the mechanism, being pre-set and security sealed, when tested after assembly in the manufacturers test facility. The meters are available in various sizes and are to be full of water during times of measurement. Figure Amiad IRT meter, paddlewheel can be seen at the top of the bore Application and use The paddle wheel meter can be used for gravity channel off-takes, pressurised and pumped systems or bore water applications. Due to the large free passage though the meter it is well suited to poor quality water with a high content of impurities and is often used in drainage systems. In pumped systems the meter can be installed in the suction or discharge pipework. Advantages and Benefits Reliable and accurate means of measurement providing the meter is correctly installed. Operates satisfactorily in turbid water. Suited to many different irrigation layouts. Low head loss characteristics. In-line maintenance with simple efficient mechanism. Same spare parts components throughout existing range. Various register options, (totaliser and flowrate, totaliser and reeds with output, etc). Can be upgraded for future automatic reading. Disadvantages Difficult to detect malfunction or unauthorised interference to meter while operating. 24 ANCID 2002

25 KNOW THE FLOW PADDLEWHEEL METER Brands and models Amiad IRT (inline meter) as shown above in Figure Trimec Dual Pulse RMC Figure Trimec Dual Pulse insertion meter Figure RMC Saddle type showing paddlewheel ANCID

26 TURBINE METER KNOW THE FLOW 3.6 Turbine meter Turbine flowmeters consist of a bladed turbine rotor installed in a flow tube. The rotor is suspended on its axis in the direction of flow through the tube. The turbine flowmeter is a transducer, which means it senses the momentum of the flowing stream. The bladed rotor rotates on its axis in proportion to the rate of the flow through the tube. As the water strikes the front edge of the rotor blades, a low-pressure area is produced between the upstream cone and the rotor hub. The blades of the turbine rotor will tend to travel toward this low pressure area as a result of this pressure differential across the blades. The pressure differential (or pressure drop) constitutes the energy expended to produce movement of the rotor. The initial tendency of the rotor is to travel downstream in the form of axial thrust. But since the rotor is restrained from excessive downstream movement, the only resulting movement is rotation. Turbine flowmeters, when first introduced, were used mainly by the aircraft industry in small sizes. Turbine flowmeters are now used on many applications. In recent years, turbine flowmeters have been competing successfully with positive displacement flowmeters in many applications due to the economy of installation, low maintenance costs, weight, size and high flow rates per comparable connection size. Increased expertise with electronics such as linearisation is permitting turbine flowmeters to be used more widely. This information was sourced from: Brands and models Amiad ABB Metering Tyco/Combined Instruments Figure Tyco/Combined Instruments. Meinecke turbine meter WPD model 26 ANCID 2002

27 KNOW THE FLOW ULTRASONIC METER 3.7 Ultrasonic meter Also commonly referred to as acoustic meters. Ultrasonic meters are in widespread use for urban water and wastewater systems and many industrial applications. For irrigation they are used in pumping stations and fully pressurised systems, but only to a limited extent in channel systems to date. A number of trial installations have been set up by irrigation authorities in recent years. This is a proven new technology that has potential for greater use in irrigation applications. Further trials are desirable, particularly of meters combining both velocity and depth measurements for measurement in open water surfaces and partially full conduits. Ultrasonic meters use transducers or sensors to measure water velocity in full pipe applications and convert this to flow rate for a given conduit cross section. Transducers can be fixed on the outside of the pipe and be known as non-wetted types. They can also be inserted into the pipe and consequently these are known as wetted. How many transducers are used depends on the brand of meter. Some use only one or one set of transducers to give a reading on a single path. Others use multiple transducers to read on more than one path. Generally, multiple paths will provide greater accuracy. There are two methods used to calculate this velocity - Transit Time and Doppler. Transit time. The Transit Time Method calculates velocity from differences in time for an impulse to pass between two transducers located on opposite sides of the pipe according to flow direction. The velocity of sound pulses in the direction of flow is compared to the velocity of sound pulses opposite to the direction of flow to determine mean velocity and therefore flow rate. Transit time meters are also known as acoustic meters. The meter generally consists of a section of pipe with transducers and is located on or outside the pipe circumference so that there are no obstructions or moving parts to impede the flow. These meters are intended to flow full and are produced in a range of standard sizes and flow capacities. The ultrasonic principle can also be used to measure flow in a part full pipe or open channel with a free surface. This is more complex and requires additional numbers of transducers and sound paths together with a means of water level measurement to determine an accurate flow profile for various water depths. The transducers may be wetted or non-wetted. Wetted transducers are placed inside the structure whereas non-wetted transducers transmit the acoustic pulses through all or part of the channel s containment structure. Figure Schematic of a transit time meter showing a single path, single traverse, wetted transducer. (Panametrics) Doppler. The Doppler Method calculates the velocity by bouncing sound pulses out into the water mass and reading the pulses that are returned after reflecting from moving particles within the water mass such as air bubbles. This is similar to how radar works. Meters using the Doppler method generally consist of a sensor that is installed within an existing pipe or structure so the sensor is wetted. There is no need to install new pipe sections or concrete structures unless there is a need for straight lengths to straighten the flow. There are various ways to mount the ANCID

28 ULTRASONIC METER KNOW THE FLOW sensors depending on the application. Some may be installed through one inch or two inch BSP fittings welded or clamped onto the external face of the pipe and others by strapping them inside a pipe or structure. Ultrasonic Doppler meters are capable of measuring flow in full pipe, partial pipe, pumped or gravity fed pipes. In situations where full pipe cannot be achieved, the ultrasonic Doppler meters can have an additional sensor installed to measure the depth of flow. By measuring the depth within a conduit it is possible to then calculate the cross-sectional area and therefore the flow rate. Depth transducers may be ultrasonic, pressure or bubbler type. The most commonly used in conjunction with ultrasonic Doppler meters are the pressure transducers due to their high reliability. Doppler flowmeter performance is highly dependent on physical properties such as the liquid s sonic conductivity, particle density, and flow profile. Likewise, nonuniformity of particle distribution in the pipe cross section results in a computed mean velocity that is incorrectly weighted. Therefore, the meter accuracy is sensitive to velocity profile variations and to distribution of acoustic reflectors in the measurement section. Unlike other acoustic flowmeters, Doppler meters are affected by changes in the liquid s sonic velocity. Figure Schematic of a Doppler meter showing the reflected path from a non-wetted transducer. (Mace) Acoustic flowmeters are often used with weirs, flumes and gates to measure flow. The water flow is restricted by the weir, flume or gate and the upstream meter is mounted in a way so it can read the changes in flow height (or head) through the restriction. Figure MACE Agriflow (Doppler) and Flo-pro transmitter and logger 28 ANCID 2002

29 KNOW THE FLOW ULTRASONIC METER Advantages and Benefits High degree of accuracy (<±1%) and consistent over full flow range when installed and calibrated properly. Robust with only minimal routine maintenance required. Can be fitted with telemetry equipment to transmit data to a remote location. Capable of measuring bi-directional flow. Simple to install. Same meter can be used in a wide range of pipe sizes. Disadvantages Repairs require skilled technician and specialised equipment. Power supply required (Solar panel with battery back up is generally suitable if mains power is not available). Electronic components vulnerable to lightning damage. Figure Panametrics Transit time strap on portable flowmeter Brands and models MACE Agriflow (Doppler) and Flo-pro transmitter and logger Panametrics Transit time strap on portable flowmeter Quadrina Probeflo (see Figure 8.1) ANCID

30 ELECTROMAGNETIC METER KNOW THE FLOW 3.8 Electromagnetic meter An electromagnetic meter consists of a section of pipe with a magnetic field across it and electrodes to detect electrical voltage changes. Under the laws of induction, when a conductive fluid passes along the pipe an electrical voltage is created in the fluid, which is proportional to the fluid velocity. Electrodes in the probe detect the voltages generated by the flowing water. Measurement of the voltage is then converted to velocity from which the flow rate can be derived for a given pipe section. This type of meter is produced in a range of standard sizes and flow capacities. The flowing water acts like as moving electrical conductor passing through a magnetic field to produce a voltage that is proportional to discharge. Figure (Left) Tyco/Combined Instruments Emflux I600 inline meter Figure (Right) ABB Aquaprobe insertion meter Application and Usage Comes in two types - insertion and inline. Electromagnetic meters are used widely in urban and wastewater systems and in industrial applications where a high degree of accuracy is required. They have been widely used in Australian irrigation over the past 3 years. They could be used in similar configurations to ultrasonic meters. 30 ANCID 2002

31 KNOW THE FLOW ELECTROMAGNETIC METER Advantages and Benefits High degree of accuracy (<±0.5%) and consistent over full flow range when calibrated correctly. Wide flow range. No obstructions to flow. Robust with only minimal routine maintenance. No moving parts. Can be buried. Disadvantages Relatively high cost. (Indicative costs for meter and sensor only range from $2,500 to $7,000 for 3.5 to 20 ML/d units. Power and installation costs could double these amounts). Power supply required (solar panels with battery back up suitable if mains power not available). Electronic components vulnerable to lightning damage. Repairs require skilled technician and specialised equipment. Brands and models Amiad ABB Australia Aquaprobe (insertion type) ABB Australia Magmaster Tyco/Combined Instruments Emflux Irriflow 12D, EM2020 or I600 Figure Tyco/Combined Instruments Emflux Irriflow 12D - for use in Dethridge wheel emplacements Figure Tyco/Combined Instruments Em2020 Figure ABB Australia Aquaprobe Figure ABB Australia Magmaster ANCID

32 OTHER METERS KNOW THE FLOW 3.9 Venturi and orifice flowmeters These are more uncommon types of meters, rarely used for irrigation. Venturi Tube - This differential pressure element actually forces the flow into a smaller diameter section of pipe, then measures the pressure differences between the unrestricted flow and the restricted flow. This element can be used for very accurate measurements if calibrated correctly Flumes and weirs In Australia, flumes and weirs are more commonly used to measure flow in supply channels. A weir is a small holding wall and the height of the water is measured as it flows over the wall, or through a cut-out in the wall. For example, a v-notch weir is one with a v-shaped notch cut out of the wall and the height of the water is measured as it falls through the notch. A flume is a narrowing of a channel. Changes in height were made with measuring sticks but are now more commonly being measured with ultrasonic meters. Rubicon s FlumeGate is an example of the latter system. It combines precision metering technology as an integral component of future total channel control systems and on-farm management solutions. The process is currently being deployed and field-tested. The FlumeGate is an automated meter outlet to directly replace the Dethridge Meter and integrates ultrasonic flow measurement technology and wireless connectivity within the gate design. The gate automatically starts and stops, adjusting dynamically to head fluctuations, which ensures delivery of water at a constant flow rate to the farm from the channel network. As well as providing metering, the Flumegate allows customers to order water with greater precision by phone and by Web Access with lead times as low as 4 hours. This means that the new FlumeGate technology will provide near pipeline performance from open channel systems. 32 ANCID 2002

33 KNOW THE FLOW SELECTING A FLOWMETER 4. Selecting a flowmeter At the end of this chapter you will be able to: 1. identify the parameters that need be considered when selecting a flowmeter. Note: The aim of this chapter is not to provide you with the skills and knowledge to make a meter selection. The decision about what meters to purchase will always be made at a management level of your organisation. Instead, this chapter aims to provide you with an overview of the parameters that are considered when selecting a meter so you can recognise why a meter has been chosen for a particular situation. Purpose The first set of decision parameters are about purpose. Why do you need a flowmeter? Is it for accounting purposes - to measure the amount of water supplied to the landholder so they can be charged accordingly? Is it for management purposes - to know how much water is going where and when? Each purpose will have different requirements for selection parameters such as flow environment, data accuracy and cost. Flow environment There are a number of parameters to consider when deciding on what type of flowmeter to choose for a particular job. Some parameters relate to the flow environment. Flow. Are you metering in full pipe, open channel or partially full pipe? As seen from Chapter 3, there are meters that can operate in all three situations and some such as the propeller meter, closed flow that can only be used in full pipe. Water source. The source could be a river, surface water, groundwater, open channel or pressurised pipe. The source will have a bearing on water quality with surface water and river water carrying weeds, shells grit and foreign material while some groundwater can cause iron oxide and iron bacteria buildup on the internal surface of meters and pipes. The source will have a bearing on the range of flow rates and head. Head. How much head do you have? Do water levels change much during a season. How much does it differ throughout the year? What is the minimum head a meter needs to work? Do you need to minimise headloss? Flow range. What is the flow range throughout the year and what are the fluctuations in flow? The operating range for a particular meter can be found in the makers specifications. Most meters have a minimum flow below which they cannot provide an accurate reading. Where the flow rate is near the maximum range for the size of meter, it may be best to choose a larger meter or even a different meter all together to ensure velocities are not too high. Remember that if you choose a larger meter, you may lose accuracy at the lower end of the flow range. Meters continually operated in the high flow range wear out and fail much quicker than meters that operate in the middle of their flow range. Turndown ratio. This is the ratio between the maximum and minimum flow range for which a meter can be trusted. Flow conditions. Most meters operate best in established flow conditions. These flow conditions are most likely to be found in long straight sections of flow with no restrictions. Many manufacturers specify installing minimum lengths of straight pipe or channel before and after the meter. If this is not possible they recommend engineering the inside of the conduit to straighten the flow. Access to power. When selecting meters for remote locations you will need to consider if they can run accurately on solar power or batteries or even need power at all. ANCID