The ability to measure flow in a dependable, precise and economical manner is of greater importance in today’s world than in any other era. A variety of grades and types of flowmeters have emerged over the years, simply because it is difficult to specify the aim of the measurement which determines the design of the meter. To illustrate, being able to measure out volumes of concentrated juices to within 2% of the requirement needs a different type of meter compared with the instrument that can detect and set off an alarm if the flow of cooling water falls to less than half the set level, and can shut down the system when it becomes a quarter of the set level. Neither of these setups is suited to measuring volumes of liquor or gasoline in a commercial establishment. This article is therefore aimed at describing different methods of liquid measurement, while presenting the more common ones on the market.
A frequent issue with flowmeters is their cost of ownership, for both suppliers and end users. However, with OEMs (original equipment manufacturers) this is better termed “consequential costs of use” as it covers all the following: the price of purchase, the cost of any machine failure while under warranty, any drop in performance due to unreliable measurement, and the running costs, both for the instrument as well for the drop in fluid pressure that it generates, which includes power and equipment cost. The OEM cost may also stretch to include the cost of additional wiring, the space needed for the instrument, the cost of placing it in the right position, as well as the straight line pipes needed for such positioning, the weight of the machine, the mounting, and any dedicated display or output interface.
When all these costs are added up, economics may dictate the use of a simpler flowmeter, such as an indicator based upon a mechanical vane or paddle wheel, or a mechanical totalizer. In today’s setting, such instruments may incorporate microelectronics powered by a battery, typical of current electronic gas meters. Finding the appropriate flowmeter for a specified job becomes a matter of selecting the instrument that matches the job requirements at a cost (following full installation) that falls within the client’s budget.
The basic principles of flowmetry are well known, and based upon them, there are six categories of flowmeters:
- Differential pressure devices such as mechanical flap devices and variable area meters
- Inferential devices such as propeller meters and turbine meters
- Positive displacement meters such as oscillating piston, oval gear and nutating disc
- Fluidic devices and vortex meters
- Devices that measure velocity such as electromagnetic and ultrasonic flowmeters
- Mass flow measurement meters including thermal flowmeters and Coriolis type meters
While each type has distinctive advantages and disadvantages, it must be remembered that any flowmeter, within any category, was first designed to meet the needs of a single specific OEM or client application.
The first method of measuring flow was based on differential pressure (DP) but this is not always the best. The DP technique is often useful in both routine and highly essential measurements. It uses a Pitot tube to measure the differential pressure caused by the impact of fluid on the open end of a tube that is in the same direction as the flow. Another tube tapping into the first at right angles measures the static pressure. It can be used to measure airspeed.
The importance of installing and maintaining the instrument properly at the user end is illustrated by the story of a maintenance engineer who inadvertently taped over the static pressure tube on a plane’s Pitot tube, preventing it from showing the actual pressure. The flight engineer used the ground static pressure shown by the meter to set his instruments. The poor weather surrounding the takeoff and the erroneous readouts on the instruments resulted in the crashing of the plane.
This principle is typically used with an orifice plate, namely, a hole bored into the wall of the pipe that blocks free flow. In this case, the flow rate is directly related to the square root of the pressure differential. This brings up a common problem: if the pressure sensors have a range of 50:1, this results in a flowmeter having a flow range of only 7:1 once the square root is removed.
DP cells are often the most common type of flowmeters around the world, but not for OEMs unless they come as a single unit which incorporates an integral orifice, and has been pressure tested and calibrated, in which case it is likely to be too expensive. On the other hand, a unit without this integrated design requires many separate connections to be made and components to be assembled.
Techniques Derived from DP Principles
DP devices are used by OEMs in several situations such as the archetypal weir or flume formed by a V-notch in an open channel, with a device that measures the liquid level such as a float or an ultrasonic level meter. Fiberglass molded flumes of this type can also be obtained from some suppliers who specialize in this type of device. However, OEMs are much more likely to use variable area flowmeters, comprising a float that moves within a widening aperture, which takes advantage of differential pressure to balance the float against gravity, or some other force.
Gravity is the force used in a benchtop laboratory VA meter. Other meters use a spring within a metal or glass tube to balance a float or vane, or a flap. The spring may also be used as an insertion device. The use of the spring is important in making these meters independent of orientation, but the pressure drop during operation is higher. The float is balanced by three forces, namely, mass of liquid, velocity of flow, and viscosity. With simple devices, a margin of accuracy within ±5% is common, though ±1% is achievable. These devices are currently fitted with alarm trippers, or ‘bolt-on’ electronic analog outputs which are activated by a magnet within the float or flap.
The designs vary with the application, from VA units with metal bodies to withstand high pressures, equipped with magnet-powered indicators, through tubes lined with PTFE to handle corrosive liquids, to low-flow switches that have a single set point for alarm activation intended for home water heaters. All these devices can be fitted with any form of installation piping, and tolerate some amount of dirt in the flow, because the aperture simply opens wider depending on the availability of pressure. Among these, several models are extremely inexpensive and easy to repair in the event of any trouble, with flows ranging from 7:1 to 10:1, and accuracy up to 1%.
Turbine and Propeller Meters
Among the family of inferential meters, turbine meters are most widely used and easy to visualize. The axial turbine meter, for instance, is simply a propeller fitted within a pipe, and designed so that the turbine rotates at a speed that is in direct relation to the rate of flow, accurate within ±0.25% in the best models. The advantages of this type are many, including their small size which is identical to the diameter of the pipe itself, and the low loss of pressure. This allows tubular construction, which is easily adaptable to high temperatures and pressures.
All turbines are affected by viscosity, and their manufacture and calibration should be sensitive to this factor in the end application. The bearings are very important in determining the quality of the meter. The smaller the axial turbine, the more important are the characteristics of the bearing, since the smaller build provides less energy to overcome the friction of the bearings which are themselves more prone to wear and tear because of the small diameter.
The Titan FT2 turbine flowmeter covers flow ranges from 0.01 to 160 LPM. With a PPS body and low inertia PVDF rotor, it operates up to 125C and 15 bar: the process fittings can be supplied in any material or specification: threads, hose barbs, flanges, fitted custom support brackets or tank connections.
The range of Pelton wheel based low flow metering sensors from Titan measure flows from 0.05 to 30 LPM. Units with built in battery powered LCD totalizers have been adapted for use in vending and drinks dispensing machines, and also to monitor beer flow totals in busy bars and clubs.
Pelton wheel or radial flow turbines are a superior choice in applications which have low flows, because they can use extremely sturdy bearings which utilize the energy from the liquid flow, and because of the enclosed design of the water wheel, this energy is much greater than that available with axial turbine flowmeters. OEM applications such as monitoring or dispensing beverages are ideal for these meters. The downside is their large size compared to the diameter of the pipeline, the higher drop in pressure and the lower accuracy. Their advantages include the low cost of production, a wider dynamic range and suitability for very low flows of 10mL/minute or even less.
The rotation of the turbine or the wheel is counted using optical or magnetic devices which provide the electronic output. These devices, called pick-offs, are built into the molded body housing which may also contain the electronic components for a display of the rate of flow or a totalizer. The corrosion-resistant molded engineering plastic of which the axial or Pelton wheel flowmeters are made can be also equipped with fluid connectors which are easily pushed on to make sound connections, or hose barbs or screw threads. These connections are custom manufactured so that they can be fitted with the appropriate bulkhead or panel mounting accessories for each application. Similar customization may be applied to the electrical wiring loom and terminations.
Positive Displacement Meters
Positive displacement meters cover a wide range, including gear and oval gear, nutating disc, helical screw, sliding vane and oscillating piston, among many others. The principle common to all of them is the passage of a specific amount of liquid from the inlet to the outlet intact, avoiding slippage or loss.
With the high-end models, a linearity of ±0.1% can be achieved over a range of flow rates. These instruments are inherently capable of handling viscous liquids better than thinner ones, because the viscosity further decreases the leakage of fluid, while enabling extremely low flow rates to be measured. These meters are employed most often in domestic water meters and petrol dispensing meters. The chief disadvantage is the high drop in pressure observed with increasing viscosity. Some modifications such as the oval gear type are designed to run on low differential pressures, even a few millimeters of head pressure with some models.
Positive displacement meters are ideal for oil measurements, but some are designed especially for corrosive fluids, such as an oval gear meter manufactured from non-metallic components such as plastic and ceramic alone. The fluids measured by these meters must be free of solid or stringy matter which could result in jamming of the internal mechanism, such as the meshing gear wheels. The larger the pipeline, the bigger the device body, and the heavier the housing becomes when the pressure is high. Small meters of this type are extremely precise and cost-effective. They produce a simple pulse output depending on the passage of a preset liquid volume. This makes it convenient to interface them with electronic counters. Many models come equipped with electronic displays and transmitters, often powered by batteries.
Fluidic and Vortex Flowmeters
Flowmeters of this type make use of the natural oscillations in fluids flowing past a blockage. This kind of oscillation is responsible for the flapping of a flag on a flagpole, by the way. It is a little complicated to detect these oscillations and more so if there is any source of external noise in the pipeline. For this reason, they are used only for certain flow measurement applications, and not by OEMs.
The basic oval gear flowmeter system is available with a transparent lid to allow visual observation of the rotating gears as an immediate flow indication. Bodies can be made from stainless steel, aluminum or PEEK. Electronic flow sensing uses Hall effect detection of the rotation of a ceramic magnet embedded in the rotor.
Similar units have been custom engineered to be small and lightweight, using aluminum housings, for use on portable medical equipment and robot arms, in the latter case to monitor hydraulic oil flows to press tools.
Ultrasonic and Electromagnetic Flowmeters
An ideal flowmeter is one which offers no obstruction to the flow of liquid through its pipe section, resulting in the absence of any pressure drop. There are two commercially available flowmeters which almost achieve this aim: electromagnetic and ultrasonic types. These use full-bore pipes to measure liquid velocity and allow undisturbed flow in both directions. The electromagnetic type have a high ratio of maximum to minimum controllable flow, and a full array of pipe diameters, as well as a low pressure drop and ongoing reduction in power consumption which is being further lowered with advancing electronic and magnetic technology. These meters are suited to measuring slurries, sewage and paper pulp. Careful material selection can allow the measuring of corrosive materials. The only requirement is that the liquid being measured be electrically conductive, with the minimum conductivity limit being reduced continuously. They are on the higher side as regards cost, but this is also falling. The expense has limited their use to slurries and other similar liquids which are hard to measure. They gauge the velocity of liquid flow as an average using the flow profile, but some amount of disturbance during the flow profiling is acceptable without undue impact on the accuracy of measurement.
Ultrasonic flowmeters mainly use two transducers (or more) to transmit ultrasound pulses angled to move both with and against the path of fluid flow. The time difference between these two pulses is the effective flow rate. When the pipe is very large, a multi-beam ultrasonic unit must be used. They can be bought as clamp-on instruments in a comprehensive range of sizes. When the pipeline is small, custom designs are made to produce multiple reflections and so increase the path length, or to produce a flow path parallel to the axis of the pipe as in the domestic gas meter. These devices measure the flow of clean liquids much better than slurries, and are predicted to grow in usage the most of all flowmeters over the next decade.
Ultrasonic multi-path devices are chosen for a more accurate averaged flow profile, across the pipe, and to compensate for skew flow and so prevent gross inaccuracy in measurement.
The earliest ultrasonic meters were developed around 1978 and included a single transducer which could be clamped on. The signals reflected from suspended particles in the flow showed a shift from the transmitted frequency, in accordance with the Doppler effect. If the pipe in which the fluid flowed was free of vibration, and the transducer was properly secured to the pipe by bonding or clamping, the flow could be measured with reasonable accuracy. In case of flow stoppage or flow falling below a critical limit, an alarm would be set off. This type of flowmeter was useful to measure the flow of slurry or sewage at reasonable speeds. However, its low cost and clamp-on design sometimes worked against it because it was used inappropriately, causing it to fail. This led to an undeservedly low reputation. They are still ideal for some applications which require meters to measure sounds made by particle impact or flow noise.
Most mass flowmeters are based upon the Coriolis principle, namely, that the acceleration of a liquid along a curved path causes a force of reaction at right angles to the acceleration. The measurement of the resultant movement or force is carried out in a mass flowmeter. The difference between this and other flowmeters is that the latter measure velocity or volume, often making use of electronics to make the conversion from velocity to volume flow provided the dimensions of the flowmeter are available. Mass flowmeters, on the other hand, measure mass flow rather than calculate it, as well as independently measuring the fluid density, and use these readings to calculate volume flow. Their advantage is their immense accuracy, up to ±0.1% in homogeneous fluids, and possibly 0.01% with proper calibration. They do not impede flow beyond the pipe bend, and even this is avoided in some models which use a straight tube. Their striking disadvantage is their high cost, though this is being brought down. Also, some of them are highly error-prone in conditions under which two-phase flow occurs, such as with vapor and liquid flows are combined. While such situations lead to increased error in all flowmeters, mass flowmeters are more prone to suffer from them because of the way they are engineered for incredible accuracy.
Thermal mass flow systems which were once used to control low mass flows for gas are now being rediscovered for liquids as well. They are based on measuring the power consumed in sustaining an increase in temperature within the liquid flowing in a bypass capillary, typically in a side flow pattern which moves around an orifice on the main pipe. This type of meter usually incorporates a flow controller like a valve, so that a preset flow rate fixed by an electronic input is maintained.
Recommendations for Flowmeters According to the Task
The chart presented here summarizes different flowmeters by their best-suited application type.
The Atrato Ultrasonic Flowmeter
Once a flowmeter is bought, it is essential to install it properly to get the best performance out of it. Except for positive displacement meters and the smaller Pelton wheel meters, the configuration of the upstream and downstream pipelines is an important factor in the efficiency of operation. If two bends at right angles to each other are present in a pipe within a turbine meter, for instance, the measurement may stop at some rates of flow because the liquid moves in the same angled pattern as the blades of the turbine, thus slipping through them without impeding their movement. On the other hand, the liquid sometimes swirls directly opposite to the direction of movement of the blades, and this causes the meter to register too high rates of flow. All kinds of obstructions and interferences, including bends, valves, regulators, tees, and pumps, may disrupt liquid flow in this way. Thus every manufacturer specifies suitable meters for each type of application, since using other meters can negatively impact the performance.
In spite of laying the pipelines correctly, poor wiring can still ruin the efficiency of the meter. Some essential precautions include screening signal wires, directing them away from, and providing shielding from, all sources of interference, such as the mains, inverters, loads with high inductivity, solenoid valves, relays and switchboards. If the right conditions are provided and specified protocol is followed for electrical wiring, such problems can be mitigated to a very large extent.
This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.
For more information on this source, please visit Titan Enterprises Ltd.