Imagine a development engineer coming to a process plant to audit how it is operating with a view to confirming designing changes to the process.  Rather than relying on fixed plant flowmeters, which may not be where he wants to measure, he is able to measure the flow anywhere without breaking the pipeline and compromising the process.  Well now he can.  In a previous article, we discussed the uses and merits of transit time ultra-sonic flow measurement in pipelines.  We also noted that there is no restriction in the pipeline flow and that the flush mounted ultrasonnic transducers do not need cleaning or maintenance. 

The natural progression to the fixed installation ultrasonic meter is the portable transit time flowmeter.  These nifty devices are carried in a small rugged and waterproof suitcases and contain all he needs (along with his trusty laptop and gloves) to meaure flows in liquid pipelines from 25mm (1″) to 1200mm (48″) diameter.  With accuracies approaching ±1% of reading and fluid temperature ranges between -40ºC to +80ºC, this portable flowmeter is quick and easy to use.

The brillant thing about this type of portable flow meter is that the transducers can be clamped to nearly all uninsulated pipelines whether they are magnetic steel, non-magnetic uPVC or underground exposed clayware drainage systems.  This portable kit contains both 2 magnetic clamp transducer carriers and 2 straps for non-magnetic pipelines.

Calibration of such portable transit time ultrasonic flowmeters is just as simple by inputting data on the ruggedised touchpad on the meter itself.  Output is recorded in the meter body on a Micro-SD card for subsequent downloading  on to his trusty laptop or a handy PC using the supplied software on a CD.

You can therefore see that such a useful tool along with other portable tools, are making working on site shorter and more productive and without disruption.  This must be a great cost-saver for the process company in the longer term.

Dove River Weir

This article about liquid flow meters has been prompted by a real need of a community mini-hydro project team to find a cheap and reliable form of flow measurement for river flows at a flood control weir in the UK, beside which, the team want to place an 80kW twin Archimedean screw turbine electricity generator.  The intention is to inject the generated power into the national grid close by.

The team has got flow records from an Environmental Agency weir gauging station 2 miles down stream and the consultant has estimated the flow figures upstream at the weir in question.  However, for such a large community project, it is vital to monitor the water flow to validate the estimated figures at the weir adjacent to their projected turbine plant.  So what liquid flow meter equipment was available to hire that a) would be unobtrusive to avoid being vandalised or stolen, b) record and store data over a long period and c) provide a wireless connection for downloading data to a volunteer’s laptop.

The neat high-tech solution is based is a low range pressure transducer that comprises a  piezo-resistive sensor and a temperature sensor housed in a metal case that is fixed in a convenient point below the weir level just upstream in the weir pool.  The weir dimensions are known and so the flow over the weir can be calculated accurately if the depth of water over the weir is known.  A few simple depth of water over the weir measurements on set up and the unit is calibrated.

The transducer flow meter is connected by sensor wires and a vent tube to a datalogger contained in an IP66 ruggedised case which in turn is placed in a covered pit on the bank adjacent.   The long-life battery-powered datalogger/sensor is  fitted with a wifi sender to enable the data to be directly uploaded by a volunteer.   A neat solution that could continue to be employed after the generating plant is in operation.

It was another Italian, Giovanni Battista Venturi a few decades later, made the first pipe flow measurements using Bernoulli’s principle by creating what is now commonly known as a venturi tube.  It was this principle that gave rise to the first reliable process pipeline flow measuring devices that are now still very much in use today.   Indeed, this principle holds good for all fluids both compressible gases and ‘incompressible’ liquids and it is upon this principle for example, that wings keep aircraft aloft.

Two famous names and four types of liquid flow meters that are in common usage today.  The orifice plate , the nozzle, the Venturi Nozzle and Venturi Tube meters are all based on this famous differential pressure principle.  So what is the difference between them and how does one select one from another as they all can be used for both gas and liquids?

Venturi Tube Diagram

The first selection criterion is the particulate criteria.  If plate/ restriction nozzle erosion will be an issue, then perhaps the smoother profile change of the venturi nozzle or the venturi tube would be the best selection.

If you have space limitations then the selection criterion would favour an orifice plate flow meter as the shortest followed by the nozzle flow meter and the venturi nozzle and then the longest being the venturi tube meter.  If pressure losses are an issue for selection, then the venturi tube has the lowest turbulence and the orifice plate the highest with the others in between.  If you are looking for the highest accuracy at the lowest flow, then perhaps you should be thinking of another form of meter such as the electromagnetic flow meter or Coriolis meters.

You can see that the selection process is multi-faceted.  Whatever meter one uses however, modern metering comes with industry-standard signal conditioners, transmitters and display units.

First of all, do you want volumetric flow or mass flow measurement?  The difference could be quite fundamental to your process or other applications.  Most liquid flow meter types are volumetric flow but meters that measure mass flow include coriolis flow meters, vortex meters and ultrasonic meters.  This is only important for liquids (that are incompressible) when you cannot predict the composition or there is a gaseous element.  It is, self-evident however, crucial for gases.

What liquid are you proposing to measure?  Is it an electrically conductive fluid such as a water-based liquids with a conductivity of over 1 Siemens/cm²?  If so, you could use the most widely use type, the electromagnetic flowmeter or magmeter as it is sometimes known.  If the flow contains gas bubbles and you can’t eliminate them by pipework design, then this option would not be appropriate  Then, an ultrasonic flow meter or coriolis flow meter may be indicated.

If you want a low cost process flow meter, you could use one of the many  differential pressure flow meters.  The choice is quite massive and each have their pros and cons.  Nearly all meters are now available for use with process industry-standard fieldbus signal conditioners and transmitters.  Meters in this class include orifice plate in it’s many variants, venturi meter amd its shorter version, the Dall Tube meter, pitot tube meter or even using a calbrated pipe bend.  All of these rely on the Bernoulli Principle where pressure upstream of the device is compared with the flow downstream.  All involve press loss and this varies as to type.

If you really just want a flow indicator, you could go for the variable area meter also known as the Rotameter or rising ball meter.

This is a massive subject and we will return to it next time.

Hasta Mañana

Sierra Instruments' Innovasonic flow meter

In recent times, the transit-time ultrasonic flow meter has been replacing the traditional Doppler-Effect flow meters and now they are beginning to supplant electromagnetic flow meters for municipal water and waste water flows.  They are normally strap-on units to the pipeline and are thus quite unobtrusive.  This is in stark contrast to my early experiences of overseeing the construction of a large waste water treatment plant that I had designed.  I remember being surprised at the bulk (and the expense) of an 18” diameter in-line electromagnetic flow meter being installed.

Well how does it work?  A digital transit-time ultrasonic flow meter is a pair of transducer sensors are mounted at opposite sides of the acoustically-conductive pipe so that the direct path between them is at some 45 degrees to the flow direction.  A 1 MHz ultrasonic sine-wave signal is fed to each transducer sensor and is received by the sensor in the other position.  The signal flowing upstream goes slower than the one going downstream and when the two digitised sine-wave output signals are compared by the on-board transmitter software, the phase shift gives the velocity flow.

Flow measurement of liquids is essential part of all the processes involved from the inlet works where most of the general industrial waste water and domestic inflows are measured using open channel flowmeters.  These are usually arranged to measure the flows from different areas separately using concrete venturi profiles, stilling chamber upstream where the water level is measured and the flow in that channel is deduced and integrated by the local transmitter for onwards process control.   The level measurement for channel venturi flowmeters in a modern waste water treatment plant uses a number of technologies ranging from bubbler probes, capacitance probes, radar and laser sensors.

If the flow is excessive and beyond the capacity of the treatment works, the process flowmeter measurements then enable the storm  storage to come into use until the peak is past and then the flow is fed back into the main flow for treatment via a storm pump arrangement controlled by storm flow pipeline meters.  The the type of flow meter vary but electromagnetic flow meters types are common.

The various aerobic and anaerobic processes to separate the solids in the form of sludge from the supernatent liquor are pumped into other parts of the process and controlled by various types of monitor and pipeline liquid flowmeters, mainly electromagnetic and venturi flowmeters.  For thicker sludge, coriolis mass flow meters are normally specified.

For industrial waste water treatment where process waste contains toxins or corrosive liquids are present, precise control is needed to enable these dangerous liquids and solids to be separated and treated to ensure they do not enter the food chain or to otherwise pollute the environment downstream.  Liquid flow measurement naturally plays a prominent part in the process control and a number of different types are used, particularly, electromagnetic meters.

Take for example, the setting up of a large construction site where the ground needs to be cleared with proper regard to the UK’s planning and environmental regulations as well as local government planning requirements.  The use of fresh and domestic water supplies are often regulated to the amount used and what is done with the polluted high-turbidity run-off from the sites into streams, rivers and aquifers below.  In some cases where the construction site has to deal with run-off for a considerable period, semi-permanent stilling ponds with open channel flow measurement are required to limit the site outflows to an agreed maximum figure.

This form of liquid flow measurement is essentially well known where rectangular or trapeizoidal weir plate is bolted to a vertical wall at the outlet chamber of a lagoon (or within a channel) and the level of the water flowing over the weir is measured and converted to a flow using Mannings Formula by uinputting the dimensions of the weir plate.  These days, there are a variety of means for measureemnt of the water level, ranging from the traditional float in a stilling chamber and no-flote electrodes, to radar and ultrasonic units.  A high accuracy of flow readings is not normally required and this fits the bill admirably.  The flow measurement is integrated by the on-board unit to give a total volume.

Metering of water supplies for construction sites is now more often than not done using a modified hydrant meter that contains a turbine meter with an integrated  digital output that is read in the manner of domestic water meters and the contractor is charged accordingly.   These turbine hydrant meters are robust and usually tamper-proof.

Typical concrete batching plant

Similarly, large heavy construction sites have their own concrete batching plant and the amount of water added to cement, aggregate and sand is critical for the resulting concrete mix to cure properly without excessive shrinkage, achieve it’s design strength and remain workable within close limits.  Batching plant liquid flow measurement is traditionally done by filling a tank to a known level and thence releasing it into the dry mix.  Modern concrete plant tend to use turbine meters controlled by computer to deliver the designated volume of water automatically.

Modern construction sites are now generally well prepared, kept reasonably tidy and extensive risk assessments are made to ensure that environmental restrictions are met.  At the end of the day, this reduces waste and is consequently good business.

To start, let us look at how they work.  If you get your garden hose, hold a loop in your hands, turn on the water and wave both hands from side to side, the loop will twist one way and then the other thus demonstrating the Coriolis force principle.  If you increase the rate of swing, the amount of twisting will not change.  This effect is applied to coriolis mass flow meters by introducing a single vibrating straight tube in the meter body between, typically, two flanges.  The frequency, usually induced electrically using piezoelectical principles, is not important for the effect to work but for efficiency, the induced vibration is set at the natural resonant frequency of the loaded tube.

Sierra ST50 Mass Flowmeter

The resulting twist of the tube is proportional to the mass flow of the liquid passing though this liquid flowmeter and this is measured by sensors and the sensor outputs are translated into mass flow signals by the integral transmitter using industry-standard protocols.  The significant feature of a coriolis mass flow meter is that the meter will not need recalibration for differing densities of the liquid being measured.  The volumetric flow rate will change with the density change of course , but the mass flow will not change.

Coriolis mass flowmeters are used for mass flow of water, acids, caustic, chemicals as well as  gases and vapors.  Flow measurement is extremely accurate with coriolis mass flowmeters, so much so that they are often used to measure high value liquids.  To realise this accuracy, manufacturer’s recommendations for installation should be followed closely, particularly in relation to the upstream and downstream pipework design with minimum straight pipe lengths introduced.  The low head losses across the meter make this type of flow meter low maintenance.