Technology goes with the flow
David Gerrard, managing director of Flow Measurement, discusses the advantages and disadvantages of an array of techniques available for open channel flow monitoringOpen channel flowmeters measure the flow of liquids that are open to the atmosphere at some point in the measurement system. This includes rivers and streams, sewers or tunnels flowing part-full (including drains), irrigation ditches, canals, stormwater channels, tides and estuaries.
The demand for measurement of sewage and trade effluent discharges has increased and is now driven by Environment Agency (EA) regulations, which require all discharges to watercourses that are in excess of 50m3/d to be measured within +/- 8% of the daily flow. The measuring installation must be provided with a certificate of conformity to BS3680, an accuracy certificate and authorised by an independent MCERTs inspector. So, clearly there is a lot of pressure to measure flow in such a way that it meets with the requirements of the EA because water companies in the UK need to have all their open channel flow measurement in excess of 50m3/d approved. That means a great deal of auditing to check each and every installation and upgrade those that fall outside the +/- 8% limit, as well as installing flowmeters into those channels which so far are not metered at all. In addition to this there is the need for the EA to measure the flow of streams and rivers for regulation, construction and flood control. It all adds up to a lot of flow measurement. There are a number of different methods to measure flow in open channels and within most techniques there are a further number of options. head measurement
Open channel flowmeters generally consist of a primary device, transducer and transmitter. The wetted primary device restricts the liquid flow stream. Under flowing conditions, this restriction causes a rise in liquid level at a location upstream and within the flowmeter. When the flow increases, the level rises higher. A transducer is mounted on or near the primary device to sense the level. The electronic transmitter uses the signal from the transducer to measure the level to determine liquid flow. Differing structures are used for head measurement in open channels, depending on the shape, flow rate and size of the channel in question. Flumes make the channel narrower and weirs and notches force the liquid over a dam-like obstruction.
A straight run of channel is usually installed upstream of the primary device to condition the flow. The channel design is critical because some open channel flowmeters require free-fall conditions, whereas others require a downstream channel.TYPES OF Flumes
Since their invention in the 1920s, flumes have provided an accurate, reliable method of measuring flow in a channel - the choice of flume depends on the application. There are a number of designs available, some named after the inventor, others which conform to BS 3680 and ISO 4359. They include:
- Parshall flumes were first introduced in the 1920s to measure the flow of irrigation water. Today, Parshall flumes are the most widely-used type of flume for permanent flow monitoring installations. They can be used in sewers and WwTWs,
- Palmer-Bowlus flumes were developed in the 1930s to measure wastewater flow,
- Trapezoidal flumes were originally developed to measure flows in irrigation channels. This flume is now used in a growing number of sanitary and industrial sewer applications for very low flow rates,
- Cutthroat flumes are used in flat gradient channels. They are so-named due to the absence of a parallel-wall throat section (as on the Parshall flume). They are commonly used in stream gauging and agricultural applications. The use of the Cutthroat flume is now expanding into sanitary and flow splitting applications,
- large flumes are cast in concrete, smaller ones are now favoured in stainless steel rather than plastics (GRP) because they are less likely to distort and the concrete backfill is more likely to adhere to the structure. There are more types, most of which are a mixture of the above and are called compound structures.
As water flows over the weir, the depth or head of water is measured. The value is entered into a discharge formula specific to the geometry of the weir. The most important use for v-notch and rectangular weirs is measuring low flow. They are also relatively inexpensive. They are not suitable for liquids with suspended solids, which would settle upstream of the weir plate. Particles can easily lodge across v-notches, which can result in quite large measurement errors. Other techniques of measurement include:
- the Manning Equation - proposed in 1889 by Robert Manning, an Irish civil engineer, and modified in the 1930s. The equation balances the gravitational acceleration of water in an inclined open channel against the surface area and bed roughness. It is true for streams and also constructed canals and ditches,
- dye dilution - a tracer, such as a radioactive solution, salt solution or fluorescent dye is added to the flow. Downstream, the tracer concentration is measured and used to calculate flow rate based on a theoretical formula. The continuity equation given by Q = V x A is area velocity measurement. V is measured by a variety of techniques, from timing a floating twig between two markers (Pooh sticks), impellors in the stream, Doppler shift, transit time ultrasonics, electromagnetic meters and, most recently, radar and cross correlation instruments. A is determined by a level measurement. This value, together with values relating to the geometry of the conduit, determines area. Current metering uses either a turbine impellor or an electromagnetic sensor, both on the end of a stem to determine the stream velocity.
Doppler flowmeters transmit high-frequency sound waves into the flowing stream, these are reflected off suspended particles, air bubbles or any discontinuity that has reflective properties in the flow. Sensors detect the reflected waves and determine their outgoing and return frequencies. By processing the Doppler phase shift of these two frequency streams, the flowmeter determines the velocity of the flowing stream.
This technique has been used for some time with success in certain applications. Sewer surveys are probably the best example of where a large number of Doppler meters are used. The conditions are ideal, the media has guaranteed particles in it and the flow can range from very shallow to a few feet deep.
Short-term surveys, where speed of data gathering is critical, favour the Doppler shift method of ultrasonic velocity sensing. The sensor uses twin transducers positioned at the pipe invert by a single steel-fixing band. The Doppler shift transducer measures velocity and a pressure transducer, normally integral with the same sensor that measures depth.
Given known pipe dimensions this is then used to calculate flow rate. Data is logged and stored for downloading back at base. Many of these meters have ATEX certification and are safe for use in hazardous areas.Doppler Limitations
However, one of the drawbacks to the traditional Doppler method is that the return signal can come back from any of the reflectors anywhere in the stream. The majority of these are usually near the transducer itself.
This is fine if the water layers are not too deep and of uniform flow. The difficulty arises in the flow of large volumes of water in rivers, sewers or channels where accuracy is important and there is a need to know the average velocity across the entire cross-section of the water being measured.
Fortunately, Doppler meters have progressed and, in addition to the standard and simpler Doppler meters, Acoustic Doppler Current Profiler (ADCP) technology now exists. This is an important development for open channel flow for continuous measurement and flow surveys.VELOCITY PLOTS
The Doppler signals are 'time gated', which means they are sent out from transducers and from an accurate knowledge of the speed of sound in water they will travel a specific distance in a specific time.
Signals are sent out into 'bins' across the width of a river, stream or channel. The maximum number of bins varies between manufacturers but RDI Instruments will offer up to 128 bins for any width of channel. This means the velocity is determined at up to 128 points across the river, stream or channel if the transducer is looking sideways or, if looking down, will plot the precise velocity at any depth. For downward-looking transducers, not only is the velocity plotted from top to bottom but the bed of the channel is also plotted. There is nothing new about sonar depth measurement, most boats have it, fishermen and divers use it to find fish and divers to locate wrecks. However, ADCP also measures the velocity all the way down to the bed.
Downward-looking Doppler plotting of velocities is fine for a survey. It can be done from a boat or the transducers can be shuttled back and forth across a stream on ropes or poles. It is also a possible replacement for current meters and the obvious hazards of wadding in deep and fast-flowing water. However, a downward-looking meter does not give a continuous signal for a permanently installed device. For this, the sensors are mounted on the side of the river bank or channel and they plot the true average velocity across the channel. The ChannelMaster H-ADCP unit is installed at a datum level below the surface with an upward-looking transducer to measure the depth below the surface. The profile of the river bed or channel must be independently measured with either a staff, calculations or a downward-looking instrument, such as the Streamflow (small channels) or Rio-Grande (deep and large rivers). The discharge is then calculated from a knowledge of the bed profile or channel width and depth, for example, Q = V x A.
Nick Everard of the EA uses all the RDI ADCPs and says they have shortened survey time, made it safer for operators and have produced more accurate data. Using the same technology, MGD markets its 'starflow' Acoustic Current Flow Meter (ACFM). This transducer is fixed to the channel bed. The transducer emits a time-gated, very short pulse of ultrasonic Doppler signal along each of the four narrow acoustic beams that are at an angle to each other. Like the ADCPs, the return signals are divided into discrete bins and, as with the ADCPs, produce an independent velocity measurement in each of these bins.
This produces a linear distribution of velocity values at different levels along the length of each beam, allowing the velocity profile throughout the entire flow area to be established. The ADFM overcomes the limitations of poor velocity profiles by plotting the distribution of velocities throughout the entire flow area. It can produce accurate results in almost any location, including in conditions of high-turbulence such as near flumes, weirs, overflows and also in large-diameter pipelines or channels where there is often a significant variation in flow velocities throughout the cross-section.
Nick Martin pioneered the work, now carried on by Tony Curling, both at Thames Water, of permanent metering in London's sewer networks. As with all Doppler meters, whether standard, ADCP or ADFM, there must be a minimum solids loading. This is naturally occurring in all but the purest of rivers and streams, in most final effluents and, of course, in sewers and inflows. Suppliers of ADFM claim it operates in potable water that is clear and clean.CROSS CORRELATION
This technique has been around in the laboratory for the last 25 years but only now are we seeing some practical applications. It can be used to measure the speed of fluids by tagging identifiable noise. Imagine taking a photograph of a passing cloud and then, a short time later, a second photo of the same cloud.
By comparing the two cloud photos it will be clear they are the same cloud, even if there are some minor changes in shape and from the time difference it is easy to calculate the speed of the cloud. That is cross correlation. Where cross correlation is used in a flowmeter, the pattern of a transmitted ultrasonic signal and the reflected signal are compared. If it is clear the reflected echo is from the same particle as the original echo a fraction of a second earlier, then we can calculate the velocity of the fluid.
Both pictures are stored as received echoes in memory. The same sensor also measures the height of the fluid. Applications are everywhere, both in open channel and closed pipes, particularly in sewer flows where the profiles are uneven.Electromagnetic
When a conductor moves through a magnetic field, it generates a voltage proportional to its velocity. In 1850, Faraday attempted to measure the flow of the River Thames using the earth's magnetic field with electrodes hanging from London Bridge.
Indeed, it was the first open channel electromagnetic flowmeter, although it did not produce a result because the instrumentation was too crude to pick up the tiny mV signals generated by the water flow, but it has been shown to work with today's sensitive electronics. There is no reason to limit the electromagnetic flowmeter to a circular tube for full-bore metering.
Advanced Flow Technology (AFTCo) has taken the magnetic meter, spilt it open and unrolled it flat for use as a meter in open channels. The meter is known as ChannelMag and measures the average velocity of water flowing over it. By ensuring most of the channel width is taken up with the flat meter, the average flow and hence the accuracy is guaranteed given good hydraulic conditions. An ultrasonic sensor measures the height. However, ChannelMag also has another important advantage in that it operates using pulsed AC, energising current to the coils. The result is high field strength with an excellent signal-to-noise ratio, hence the meter never needs cleaning irrespective of the build-up of grease, fat, algae or calcium carbonate deposits on the electrodes. It also means the strong magnetic field will reach every part of flowing media in the cross-section of the channel - effectively turning the channel into a rectangular full-bore meter.
ChannelMag is calibrated in a large tow tank, as well as against a full-bore mag meter traceable to NIST - the US National Standard Laboratory at Alden Massachusetts. ChannelMag is also traceable to NEL, which is approved by the National Accreditation of Measurement and Sampling (NAMAS) through a cross-checking exercise of all major flow rigs worldwide. Accuracy is 2% of reading at site conditions if installed according to specification and +/- 4% for a relaxed accuracy.
In order to transfer the exact calibration from the flow rig to the measuring site, enhancement plates are fitted to the channel walls at exactly the same width as the calibration width. The enhancement plates also complete the circuit for the magnetic field.
Their use need not be limited to channels but can also be fitted to large, circular non-full conduits by placing them on the bottom of the conduit or partially up the sides if the conduit is very large. ChannelMag was installed at an installation at Esholt WwTW near Leeds, despite the fact there was a perfectly good and operative flume just ahead of the ChannelMag.
The reason it was installed was that if a blockage occurred downstream of the meter, the level would rise with no corresponding flow. However, the level transducer would report rising flow rates and hence over treatment of the WwTW. Andy Webb and Rob Bainbridge of Yorkshire Water report there is an exceptionally good correlation between the ChannelMag and the flume under normal flow conditions.
Transit time flowmeters have pairs of transducers, which generate sound pulses travelling at an angle to the flow. Sound impulses move faster when travelling with the flow stream and decelerate when travelling against it. By measuring the transit time of the sound pulses, the flowmeter calculates velocity.
Transit time ultrasonics is an excellent, accurate and well-proven method of flow measurement for both closed pipes and open channels. Transit time ultrasonic flowmeters for open channels normally utilise multiple parallel paths, which can determine exactly the upstream and downstream travel time between two transducers at a number of levels across the channel or river.
This takes into account the various flow profiles at different levels in open channels. In combination with level measurement, it is possible to achieve an excellent accuracy even under difficult inlet conditions. One of the largest suppliers of multi-path ultrasonic flowmeters is Accusonic Technologies, which has installations worldwide. There have been 20 ultrasonic flowmeters at the Hoover damn since 1976 and hundreds more around the world.
Applications are for penstock leak detection systems, cavitation monitoring systems, chemical treatment, chlorine treatment, discharge compliance monitoring, large sewers and CSO flow monitoring, as well as large channels and river flows. One can expect 1-5% accuracy in any channel, dependent on the number of paths and the hydraulic conditions. French manufacturer Ultraflux, via UK agent Flowline, has installed a multi-path ultrasonic open channel flowmeter at the river Erewash gauging station. Two posts were installed in the river with three pairs of transducers mounted on each post.
The entire width of the river does not need to be monitored, provided the cross-section and bed profile of the river are known at the slice where the measurement is being taken. The downside of ultrasonic flow measurement are the air bubbles in the water, which block the ultrasonic signals, as well as weeds and other obstructions.
All area x velocity flowmeters have one thing in common - they all need to be submerged to operate. Inevitably this means draining down the channel or at least wetting the sensors. The great beauty of radar is that it is installed above the flow and it is truly non-contact. Radar flowmeters determine flow velocity in much the same way police radar guns can measure the speed of an automobile. Marsh McBirney has pioneered this technology with Flo-Dar, which transmits a digital Doppler radar beam that interacts with the fluid and reflects back signals at a different frequency than it was transmitted at.
These reflected signals are compared with the transmitted frequency. The resulting frequency shift provides a measure of the velocity and the direction of the flow. Level is determined by an ultrasonic pulse echo from the same unit. Radar-based open channel flowmeters require some degree of surface roughness so the transmitted signal will be returned to the radar receiver.
A mirror-like surface will cause the transmitted signal to completely escape, however, in nearly all real-life situations, there are adequate reflections for a reliable measurement. Disadvantages are that the site must be carefully chosen because the flow requires to be uniform without surface swirl or standing waves caused by unusual flow conditions. The area seen by the beam will be much like a car headlight - an oval patch of radar light. Anything outside this beam or more than a few inches below the surface of the beam will not be measured. This is fine for small channel widths, such as drains, the inlet to WwTWs, final water or trade effluents, but less suited to rivers. Like all flow measurement applications, choosing the right meter for the job is crucial. The foregoing will, I hope, help with a better understanding of open channel flow measurement and might make choosing a meter easier or, if now spoilt for choice, more difficult
Acknowledgements to: Fluidsenors for information on flumes and weirs. Hydro-Logic for data on RDI ADCP Instruments. Flow Measurement for data on Advanced Flow. Flowline Manufacturing for data on Marsh McBirney and Ultraflux. IETG for data on Accusonic Instruments and ADS Doppler meters. Nivus UK for data on Nivus Cross Correlation meters. On-Site for information on MGD ADFM Instruments.