Achieving optimal performance from small bore flowmeters (less than 12.5mm / ½-inch) has very different challenges to those faced when using larger diameter meters. As the flows are lower, less energy is available in the liquid to convert to mechanical movement. The liquid is often running in the laminar flow region and viscosity and boundary layer effects become very important. Generally speaking the smaller the flowmeter the harder it is to make an accurate inline measuring device but the easier it is for the user to check the performance.
Our informative guide (see below) discusses the main flowmeter technologies and provides an unbiased comparison between each based upon 5 critical operating parameters (cost, linearity, operating range, minimum flow and repeatability) using an easy-to-understand pentagonal graphic illustration of their relative performance in these aspects.
- Ultrasonic Flowmeter
Ultrasonic flowmeters often appear to offer the ideal solution to low flows. They do not interfere with the flow, can measure most types of liquid, do not require liquid conductivity and have good turndowns.
Commercial ultrasonic flow devices from Titan Enterprises will meter flows down to below mL/minute with excellent accuracy. Some low flow versions already exist but with limited performance due to the sensor inefficiencies and limitations in the signal processing.
Clamp-on meters are rarely used in pipe sizes below 1“. The existing (below 1”) devices are also affected by the fluid viscosities, liquid velocity profiles and have quite small operating ranges; the latest technology will negate these problems and offers a more commercial solution with turndowns approaching 500:1 at ±1.5% of reading over the whole range – all at competitive prices.
This makes these devices Reynolds numbers independent and can therefore operate from laminar flow right up to turbulent flow; in other terms, accurately measuring liquids ranging from water to high viscosity oils. Being through flow devices they can also be tolerant to impurities in the system which would cause havoc to meters with moving parts.
- Variable Area Flowmeters
Probably the commonest small bore flow indicator is the variable area flowmeter or “rotameter” as seen in industry, many laboratories and hospitals. This is a simple plastic or glass tube with a shaped float or ball that rises up a tube, whose cross-sectional area changes up its length, hence the name variable area meter.
The flow of the fluid “lifts” the float and its height in the tube is an indication of flow rate. Some systems use springs to counter the fluid forces and this makes the devices less dependent on gravity and they can be mounted in any orientation. The displacement can also be sensed remotely which is essential with opaque fluids.
- Turbine Flowmeters
Turbine flowmeters – small traditional axial turbines are very rare as these “propeller” type meters usually rely on turbulent flow, fully developed velocity profiles and consistent, very low friction bearings. They are also extremely sensitive to changes on the surfaces of the turbine and are only used in specialised applications.
Several alternatives are available where the turbine has plain blades completely in line with the flow unlike the “angled” blades in a traditional turbine. An upstream fixed helical screw rotates the fluid and this spinning fluid rotates the turbine. Because these devices are usually moulded, tight tolerances can be maintained and the entire meter can be made from thermo-plastics.
The rotors are very light and the forces imparted by the fluid “spinner” are somewhat larger than can be usually extracted from a normal axial turbine. These give better range ability and overall performance however the bearings are still a crucial component being small and often plastic they are subject to wear and contamination from small particles. Temperature changes affect not only the fluid properties but the internal geometry of the meters themselves and this family of devices are not very suitable for applications with wide temperature ranges or fluids which change characteristics with temperature. Ideally they should be calibrated on a fluid at the same viscosity and temperature as the product being metered.
- Pelton Wheel (Radial Flow Turbine) Flowmeters
So called Pelton wheel or radial flow turbines act in a similar way to an old water wheel but within an enclosed chamber. Many domestic water meters around the world work in this way. The power available from this arrangement is considerably more than the propeller type meters and they are therefore capable of much lower flows. However, great care must be taken to ensure good linearity over the flow range and, in addition, the pressure drop is quite high.
Pelton Wheel turbines are viscosity sensitive and will give erroneous readings if the fluid properties change significantly. Bearings can be stronger but this is necessary due to the higher forces involved. Very low friction devices of this type are used for the smallest flows but rarely have long service lives.
- Positive Displacement Flowmeters
Some manufacturers make positive displacement (PD) meters for small bore pipes including the domestic water meter. There are many differing types of PD meters and generally these are very successful in small sizes providing that the fluid is slippery and viscous. As for metering water, only a handful of suppliers make meters suitable for water as its lubricating properties are very poor.
Other low viscosity liquids such as solvents or the myriad of water based solutions also present problems. Modern low friction plastics have helped, but these meters do insist on clean liquids for effective performance and even the small particles that sometimes get into domestic water systems can stop this type of device.
Most oil meters fall into the Positive Displacement category as the properties of the product and the meter measuring it are totally compatible.
- Electromagnetic Flowmeters
Electromagnetic meters use Faraday’s law of electromagnetic induction– a conductor moving in a magnetic field will generate an emf at 90° to the magnetic field, the amplitude of the emf being proportional to the velocity of the conductor.
The fluid is the conductor and therefore the Electromagnetic meter is limited to fluids that can conduct electricity, however modern meters can measure flows in liquids with quite low conductivities.
In practice an alternating magnetic field is placed across the pipe with electrodes on the centre line between the two field generators. As the fluid flows through this variable magnetic field an alternating voltage is induced at the probes, which enables the flow-induced emf to be distinguished from electrochemical potentials on the electrodes.
These meters are now made in quite small sizes down to 3mm bores. They are tolerant of impurities and can measure below 0.3M/S i.e. below 0.12 litres per minute.
- Thermal Flowmeters
Thermal meters inject energy into the system and are therefore capable of detecting extremely low flows down to micro-litres per minute. A small thermal element puts heat into the fluid and the dispersal of this energy is registered and converted into a flow reading.
Response time is slow for these extremely low flow devices and the accuracy is not up to the standard of other products mentioned in this article. The slightly larger devices perform better with improved linearity and response times. Ideally these meters should be calibrated for the liquid being metered at the operating temperature.
- Fluidic Flowmeter Devices
There are a number of meters that use the physical properties of moving liquids, these are called fluidic devices and include such things as vortex shedding meters, fluidic oscillators and laminar flow elements.
The vortex shedding meters require quite high Reynolds numbers and are not often used for very low flows or in small pipes.
Laminar flow devices are inherently linear as the pressure drop across an element is directly proportional to flow rate – providing the Reynolds number of the fluid is kept below the turbulent region. Small flows are attainable with these devices and ranges of up to 50:1 are possible with the correct pressure measurement system.
- Coriolis Flowmeters
The Coriolis flowmeter gives a direct reading of mass flow and can meter down to low rates. It uses the fact that when a fluid is in motion any change in direction will produce a reaction in the system. This reaction is proportional to the mass of the fluid being accelerated.
With the Coriolis, there is no obstruction in the bore (although the flow path can be very contorted) but these meters can be very accurate, with linearity’s quoted down to ±0.1%. Coriolis type meters are also usually very expensive.
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