Charles Wemyss, Litre Meter Limited
Types of Flowmeters fall into many categories. One could use the involvement of moving parts and electronics to define this. Mechanical meters, used and invented before domestic electricity was prevalent must be Old Generation. These would include what you and I have outside our houses for the measurement of domestic water. They would also include meters in our gas supply for the measurement of our consumption of gas. The very first turbine meters credited to Woltmann in 1790 were considered for calculating the loss of energy in open canals. It would be true to say that these were used for counting or totalising flow rather than providing an instantaneous rate display or output.
From the Old to the New
Those using electricity or electronics with a moving part like a rotor are also Old Generation as turbine meters have been around for several decades, for example. The first of these were axial turbine types developed, in essence from Woltmann, in the Second World War for accurately determining the fuel consumption of military aircraft and torpedoes. The pick-up or sensor with a magnet and rotating conductor enabled the number of rotations to be counted, totalised and used for rate display.
If we define Next Generation meters as having no moving parts so that the definition encompassed Thermal, Coriolis, Ultrasonic and Electromagnetic, then there would be a modern outlook. Apart from the fact that Faraday tried making an electromagnetic meter to measure the River Thames almost 200 years ago! He only failed because his instrumentation wasn’t sophisticated enough.
The obvious question to ask is: What is Next Generation, What is Current Generation and What is Old Generation? We can be certain that Old Generation does not mean unusable. We can also be certain that Old Generation in some people’s eyes is more than adequate for various tasks. This article explores the provenance of some flowmeter technologies, what might be round the corner and how to select the best meter for each project.
Some new and not-so-new flow measurement techniques:
Coriolis, inertial force was first formulated by Gustave Coriolis in 1835 but MicroMotion didn’t release a commercial unit until 1977.
Electromagnetic, proven by Faraday but commercially produced from 1952. *Ultrasonic, from 1963.
Vortex, using the van Karmann effect of the generation of alternate vortices past a bluff body commercially from 1969, famously spotted by Leonardo da Vinci in 1504.
Thermal, hot wire anemometers were used from the early 1900s, commercially from the 70s.
Sonar, unconventional and measures turbulence since 2003.
Optical, measuring the speed and direction of individual particles using a laser beam, in research labs in the 70s and 80s but only commercially used in flare gases.
Differential Pressure like an orifice plate or Dall tube with a separate differential pressure transmitter. Also nozzles, pitots, Venturis and wedges. Still the most popular non-domestic meter type.
Positive Displacement, commercially pre 1830s for diaphragm gas meters with sheepskin diaphragms and sheet steel enclosures.
Turbine, first drawn up in 1790, commercially available post Second World War.
Variable Area, available for most of the 20th century.
Low Flow technology and the next ten years
There are various technologies that present themselves for low flow shown below. Many of the others mentioned elsewhere do not scale effectively.
Coriolis: Most manufacturers concentrate on ½” (15mm) and above. The issues of balance and producing thin wall tube to the required dimensional tolerances are hard to overcome. Smaller sizes exacerbate this.
Thermal: Microelectromechanical systems (MEMS), generally 0.01 mm to 0.1 mm in size, consist of a CMOS circuit on a thin silicon substrate. For lower flows these will replace a larger heated element and sensor. Liquids have a massively different thermal conductivity so the same device can measure at grams per hour rather than grams per minute.
Positive Displacement: Generally their purpose is to positively measure a trapped volume of fluid ? either gas or liquid. Gas versions tend to be for higher flows with the most popular one being used for domestic gas measurement. At lower flow the leakage between successive volumes is too large for effective measurement. For liquids where there is more viscosity the PD meters work well. Developments focus on some novel types and constant improvements to existing designs. There is a law of diminishing returns as the smaller the mechanical parts are, the harder they are to manufacture accurately. Also, leak paths are proportionally larger. One of the new types is the pendulum. This has one moving part with low mass and minimal friction loss, enabling it to respond to extremely low flow volume rates from 0.3 litres/hour. Unusually, this unit only works with viscosities up to 5 centiStokes. The rotary piston meter also has one moving part. In terms of flow rate, like most PDs, these prosper on viscosity. At 10 cSt a typical meter will start measuring at 0.08 l/h and when water is measured this increases to 0.4 l/hr.
In line ultrasonic: What happens when the pipes reduce in size and the type where a sensor is clamped on the outside of the pipe is no longer applicable? The sensors are mounted inside the pipe usually contrived in the shape of a U so that the ultrasound is passed between sensors at the base of the U. By knowing the diameter of the tube and the velocity between sensors, the volumetric flow can be calculated. Liquid flow rates down to 2 ml/min can be measured.
So if it’s not the methods of measurement we use that define Next Generation what is it? Perhaps: intelligence? The rise of smart meters i.e. those with digital communications and with the ability to self-verify are undoubtedly modern but were defined decades ago and have been in use for many years.
Wireless communication is similarly up-to the- minute. HART digital communication has been around since the mid-1980s when it was developed by Rosemount Inc. for a range of measuring instruments, not just flowmeters. The HART foundation was formed in 1993 and the wireless version came along in 2007. So quite modern but Next?
So, is it the flowmeters that inhabit university laboratories and the R&D departments of flowmeter manufacturers that constitute Next Generation? Can we speculate what a cutting edge meter might look like in ten years’ time?
No Moving Parts
It would be fair to say that this Future meter would have no moving parts. This improves the chances of long term use as it would not suffer from mechanical degradation either planned or unplanned. It would ideally be non-invasive i.e. it would fit on the outside of a pipe and nothing would actually breakthrough the pipe wall. Currently, just ultrasonic meters match this criterion so let’s say that’s less than likely and the meter will therefore be non-intrusive. The sensor will break through the pipe wall but won’t impede the flow or perhaps just negligibly. What will the sensor measure, what techniques will it employ? That’s the $64,000 question. A single sensor is less likely as there will have to be a reference point for comparison.
Probably two sensors set apart, then, monitoring a property of the fluid. The clever part will be the intelligence of the signal processing; looking for perturbations in the signal amplitude and comparing it to the next sensor. Dumping thousands of comparisons for the sake of a few, locking onto patterns and pumping out high strength signals. In fact, the real hurdles will be firstly customer acceptance and secondly, electronic component obsolescence. Will the customer accept this meter and will it continue to find the small perturbations in property? Can he see it in action? Does he get a sense of goodness in the signal, in the rejection rate? What if the pipe vibrates, if the temperature ramps up, if the ‘property’ disappears? Then we find out that metering and measuring is about confidence, experience rather than Next Generation.
Bringing the Oil and Gas Industry Up-to-Date
The Oil & Gas industry is relatively conservative, relatively slow moving. The prevalence of HART and 4-20 mA signals decades after their introduction speaks volumes. Wireless, Bluetooth and fancy bus protocols are only just now making significant in-roads offshore. The creep of domestic innovation exemplified by the rise of the smart phone encourages instrument designers to bring their act up-to-date. Most instrumentation can only be compared with the most basic mobile phone. There is an inherent expectation that the modern user will have something easy-to-use, colourful and dangerously (?) customisable. The smartphone has many different uses of course beyond that of making calls. Arguably, it’s an instrument display in its own right. The logical conclusion is that the meter ‘display’ will be with the operator the whole time, in his/her hand. The obsolescence of components that bugs the subsea side of the industry is irrelevant in the actual instrument as this is replaced by the mobile phone and it’s ‘app’.
That still leaves the problem of the fast-moving world of miniature components for the clever parts – that will always be a thorn in the side of designers. Just as with most technologies, we’re not trying to design something to last for 30 years; the likelihood is that it will be overtaken by a new Next Generation device in ten years and then again in twenty years. All we can hope for is that the unit is still working in ten and twenty years and only needs replacing in thirty.
To select the best flowmeter for each application it is not just a question of looking up the first flowmeter you thought of on Google. Nor is it asking the engineer on the next desk or even consulting the internal specification guides issued by your employers. And it certainly shouldn’t be by selecting the cutting-edge meter of the day. It should be by consulting a flowmeter specialist – a specialist that has a wide range of solutions, not just one that is shoehorned into every application. Ideally, an independent specialist who can give unbiased advice and who will, if necessary, recommend an external solution.
Looking to the Future
In conclusion, the Next Generation of flowmeters is already operating, they’re already proven and they’re probably on the specification lists. Most applications can be met, more than adequately, by existing techniques. But the manufacturers aren’t standing still. They’re continually leveraging current technology with creeping demands. It’s more evolutionary than revolutionary but we’re all getting there – safely, economically and technically.