Flow measurement that is reliable, accurate and cost effective is more important today than it ever has been. The styles and grades of flowmeter have changed and developed over the years, because the primary problem in meter specification is to determine what the measurement has to achieve.
Dispensing a volume of juice concentrate to within 2% is a different problem to realizing and alarming when the cooling water flow drops to half of the desired flow, with the system shutting down at 25%. These are both different to measuring beer, whiskey or petrol delivery in a financial transaction. The different techniques that can be used for liquid measurement will be presented in this article: it does not include every type of flowmeter that is available, just a selection of the more familiar ones.
The ownership cost is frequently an issue for end users and suppliers, but for an OEM (original equipment manufacturer) then a better phrase should be “consequential costs of use”. The cost of ownership is not just defined by its purchase price, but it can also include the cost of any breakdown within the warranty period; loss of performance resulting from measurement uncertainty; and the equipment or power cost of running both the flowmeter and the fluid pressure drop that it introduces.
The cost of use for an OEM can also include any separate wiring needed, the positioning and space taken up in the equipment, including the straight line pipe requirements, the weight and the mounting arrangement, and any separate display or output interface.
Adding all of these resultant costs to the function required will normally lead to the choice of a simple mechanical vane or paddle wheel indicator, or a mechanical totalizer. Nowadays this could include self-contained battery powered micro-electronics: for example the modern electronic gas meter. The most appropriate flowmeter for the task is the one that will perform the task actually required, to a certain level of satisfaction, and at an acceptable fully installed cost.
While flowmeters can all make use of modern technology, the basic techniques are fairly well established. Flowmeters fall into six large groups:
- Inferential devices like turbine meters, propeller meters
- Positive displacement meters, (including oval gear, nutating disc, oscillating piston)
- Differential pressure devices:(including variable area meters and mechanical flap devices)
- Velocity measuring devices such as electromagnetic and ultrasonic
- Fluidic devices, vortex meters
- Mass flow measurement meters (Coriolis, thermal)
Every one of these flowmeters has its own pros and cons: every flowmeter style within these categories was developed for a particular OEM or customer application!
Differential pressure (DP) flow measurement is the original method used, but it is not necessarily the best! It can be used for the most ordinary, but also the most critical duties. The Pitot tube can use the differential pressure created by fluid hitting the open end of a tube facing the direction of flow, while a second tapping at ninety degrees to the first measures the “static” pressure.
This can be found on an aircraft to measure the airspeed: for example several years ago a maintenance engineer left some masking tape over the static pressure tappings on the Pitot tube in the aircraft: the flight engineer then calibrated his instruments to the ground static pressure. When the aircraft took off in bad weather, the instruments gave readings which made no sense and the plane crashed. This highlights the consequences of an unsatisfactory end user installation and maintenance procedures!
An orifice plate is the most common method of using this principle. A hole in the bore of the pipe which creates an obstruction to flow: the flow rate is proportional to the square root of the pressure differential. This is a common source of one of the problems with this type of device. If the pressure sensors have a 50:1 range then the resulting flow range of the flowmeter is only 7:1, after the square root extraction.
It is often said that DP cells with orifice plates are globally the most common form of flow measurement today: but this is probably not true for OEMs. Not unless they purchase the DP cell with integral orifice complete, pressure tested, calibrated and in one unit, but then it is very likely to be too expensive. Without the integral unit the technique would involve too many parts to assemble and too many connections.
Techniques Derived from DP Principles
There are other times that OEMs would use other “DP devices”: a popular example is a V-notch weir or flume in an open channel, with a liquid level measuring device that can be an ultrasonic level mete, a float or similar. Specialist suppliers can provide flumes for such applications in the form of fiberglass molds.
Far more often with OEMs are 'Variable Area' flowmeters which are made of a movable 'float' in a widening aperture, but in this case the differential pressure can be used to balance the 'float' against a force. Typically, this is gravity as seen in a bench top glass tube laboratory VA meter, but certain meters balance a float, flap or vane, against a spring, either in a glass or metal housing, or as an insertion device: this makes them independent of orientation, but it results in a higher operational pressure drop.
The forces which balance the float are the velocity, mass and viscosity of the liquid. Simple visual devices can be capable of ±1% accuracy, but ±5% is more probable. All of these devices can have electronic 'bolt-on' analog outputs nowadays, or alarm trips, driven by a magnet in the flap or float.
There are various specific designs: such as metal bodied VA units for high pressure, using magnetically driven indicators, single alarm set point low flow switches for domestic water heaters and PTFE lined tubes for corrosive liquids. All of these tolerate any format of installation pipework, and dirt in the liquid, because the aperture just opens further if the pressure is available. Some are very cheap and easy to troubleshoot: flow range is normally between 7:1 and 10:1, and a 1% device is possible.
Turbine and Propeller Meters
The most common and easiest to understand of all the different inferential meters is a turbine meter. The axial turbine meter is simply a propeller in a pipe, but the speed of the turbine is directly proportional to the flow rate thanks to careful design, meaning that an accuracy of ±0.25% can be achieved. There are many benefits: they are fairly small and normally the same diameter as the pipe in which they are fitted, and the loss of pressure is reasonably low. High pressures and temperatures are readily accommodated due to this particular tubular construction.
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.
All turbines are sensitive to any changes in viscosity and should be manufactured and calibrated with consideration to the final application. They are only as good as their bearings and when the axial turbine becomes smaller, the bearing characteristics become more important. The energy available to overcome the bearing friction is decreased and the bearings, being smaller, are more likely to degrade.
The turbines, therefore, more suited to low flows are Pelton wheel or radial flow turbines, as the bearings can be very robust. The energy available from the liquid flow around this type of enclosed 'water wheel' is far greater than the energy available with the axial device.
The most suitable OEM applications are therefore beverage dispensing and monitoring. One disadvantage of this kind of meter is the relatively large body in comparison to the line size, reduced accuracy and greater pressure drop. However, some advantages include: a larger dynamic range, low manufacturing costs and the ability to meter very low flows, 10 mL/minute or lower.
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.
Electronic outputs can be provided either by an optical or magnetic pick off to count the wheel or turbine rotation. Pick-offs are molded into the body housing: even the electronics for a totalizer or flowrate display can be housed inside the meter body.
Normally axial or Pelton wheel devices are created in molded modern engineering plastics, which are resistant to corrosion and can be fitted with push-on fluid connectors, screw threads or hose barbs. Custom engineering of these connections can allow them to incorporate panel or bulkhead mounting arrangements to suit any application, and the electrical wiring loom and terminations can be customized in a similar fashion.
Positive Displacement Meters
There are many different types of positive displacement meter, gear, oval gear, sliding vane, nutating disc, oscillating piston, helical screw and many more. All have the same basic mode of operation in that they take a discrete volume of liquid and pass it from the inlet to the outlet without loss or slippage.
The better types can have a range of ±0.1% linearity over a wide flow range. These devices perform better at higher viscosities as the increasing fluid thickness decreases the leakage rate even further and extends the useful operating range to lower flow rates.
The most common form of this device worldwide is likely to be domestic water meters and the meters found within petrol dispensing equipment. Due to how these meters work, they often have a high pressure drop, especially with more viscous fluids, but some types like the oval gear design operate with a very low differential pressure, sometimes with only millimeters of head.
They are highly suited to measuring oils, but some models are manufactured especially for corrosive media. For instance, there is a version of an oval gear meter created from totally non-metallic plastic and ceramic components. The fluid should never contain any solid particles or stringy materials, as these could have a chance of jamming the meshing gear or other mechanisms.
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.
Larger pipe sizes can have very large bodies, and housings suitable for high-pressure use become heavy. In the smaller sizes, they are an accurate and economical metering solution.
These devices output a simple pulse, which defines the passage of a certain volume of liquid. It is, therefore, possible to easily interface them with simple counting electronics. There are several versions have integral electronic displays and transmitters, some being battery powered.
Fluidic and Vortex Flowmeters
These flowmeters use the natural oscillations that can happen as fluids move past an obstruction, such as flapping a flag on a flagpole. Detecting these oscillations is fairly difficult, especially if there is extraneous noise present in the pipeline, so they are only used on some specific flowmeter applications, and normally not by OEMs.
Ultrasonic and Electromagnetic Flowmeters
The ideal flowmeter would be a section of pipe with no intrusions and therefore no pressure drop. There are two types of meters which are commercially available that have come very close to this: ultrasonic and electromagnetic flowmeters. These flowmeters both use full pipe bores, measure the liquid velocity and are inherently bidirectional.
Electromagnetic meters have reduced pressure loss, good rangeability, have a wide range of pipe sizes available, and power requirements are continually being lowered as magnetic and electronic techniques are improved. They will handle slurries, sewage and paper pulp. Carefully selecting the materials will mean that the “electrodes” can handle very aggressive materials. A necessary requirement is for the flowing liquid to be electrically conductive, but the lower limits on this are constantly being lowered.
They are still often expensive, despite prices currently falling, and would only be used on difficult liquids like slurries. Electromagnetic meters measure the velocity of liquid flow, averaged across the flow profile: they can tolerate some flow profile disturbance and still retain enough accuracy.
For the main type of ultrasonic flowmeters, two or more transducers that fire a pulse of ultrasound at an angle both with and against the flow; the flow rate is effectively the difference in time between the signals. Multi-beam units are used when pipes are very large and they are available as clamp-on devices for most sizes. In small lines, special designs are used to increase the path length by several reflections, or the flow path is modified to run along the pipe axis, such as in the domestic gas meter.
This type of meter is more suitable for cleaner liquids, rather than slurries, and is currently predicted to show the most growth in the next decade. The multi-path units improve the flow profile averaging, across the pipe, and also reduce inaccuracies by correcting for skew flow.
Ultrasonic meters were introduced in around 1978 and originally used one clamp-on transducer, and detected reflected signals returned by particles or suspended solids in the flow, which shifted the transmitted frequency in accordance to the Doppler effect. These units in a vibration free pipe, with a firmly clamped or bonded transducer, could achieve a consistent flow indication or flow failure alarm, particularly for when monitoring sewage or slurry flows with a reasonable velocity.
Unfortunately, when the first clamp-on reasonably priced meter system was introduced, the Doppler was often applied to applications that were not suitable, which gave a poor reputation. Some suitable applications still exist today, alongside meters which measure flow noise or particle impact sounds.
Coriolis meters are most common mass flowmeter, working based on the fact that when a fluid is accelerated in a curve there is a reaction force at ninety degrees to the acceleration. If the movement or resultant force is measurable, then the result is a mass flowmeter.
This meter is different to all of the previously mentioned, as they have all been volumetric or velocity measurement devices, which frequently use an electronic package to convert velocity into volume flow, given the dimensions of the flowmeter. Coriolis systems instead measure mass flow directly, separately measure the fluid density, meaning they can then determine volume flow.
They are highly accurate: with homogenous fluids accuracies of ±0.1% often quoted, and calibrations being discussed that can achieve 0.01%. The bend in the pipe does often act as a restriction, but straight tube models are available.
These meters are very costly: but thankfully prices are currently falling. There are some versions which are highly sensitive to errors induced by two-phase flow conditions, such as vapor included with the liquid. Although this is true for all types of flowmeter, these errors can have serious implications for mass flowmeters that are designed to achieve the highest accuracy possible.
Thermal mass flow systems commonly used for controlling low gas mass flows are now being introduced for liquids. The technique uses a measure of the power required to maintain a temperature increase in the flowing liquid down a bypass capillary, usually in a side flow path routed around an orifice in the main line. It is typically used with an integral flow controller such as a valve, to maintain a fixed flow dictated by an electronic input. Flowmeter recommendations by duty required.
The Atrato Ultrasonic Flowmeter
After purchase of a flowmeter it can be very easy to hinder its performance with a poor installation. Upstream and downstream pipework configuration can have an immense effect on the meter performance, with the exception of the positive displacement meters and small Pelton wheels.
Two bends at ninety degrees to each other can sometimes stop a turbine meter at certain flow rates, due to the liquid swirling at the same angle as the turbine blades and slipping past. Alternatively, if the liquid swirls in the opposite direction then the meter can over register. Bends, regulators, valves, pumps, tees and almost anything else that could be introduced into the pipework will disturb the flow of liquid. For ideal installations, manufacturers specify the meter and any variation from this will negate the performance characteristics.
Even with perfect pipework, many applications are compromised due to inadequate electrical installation. Signal wires should be screened where possible, and also routed away and shielded from mains supplies, solenoid valves, relays, inverters, highly inductive loads and switching apparatus, as these can all modify the signals. These problems can be reduced dramatically with correct signal conditioning and protocol.
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.