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PII Oct-Nov 2013

Has the Magflow Meter Had Its Day? By Keith Flint – Sales & Marketing Director, Pulsar Process Measurement Ltd The well known and well-proven Magnetic flowmeter (magflow) is the default solution when the requirement is for monitoring flow in closed pipes. They are well understood, and are, for small pipe diameters at least, relatively inexpensive. Alternatives have been available, but have typically been difficult to install accurately, expensive and often unreliable. Now, though, new technology, powered by newly available acoustic signal processing techniques, is starting to make real inroads into the magflow meter’s dominant position, with lower purchase costs and a non-invasive installation that requires nothing more than a screwdriver to complete. There are many situations where reliable flow monitoring is desirable. Beyond the obvious of simply measuring flow rate, there are applications where flow throughout a process network should be monitored, checking on flow movement or warning of failures of valves, identifying when PIGs should be deployed to clean pipework, and often to check on the efficient operation of pumps. The magnetic flowmeter is a ubiquitous sight in process plants of all types across the world, providing reliable flow measurement for an enormous range of flowing liquids and slurries. Well understood, it has become the ‘go-to’ technology where flow needs to be monitored. That said, it has a number of drawbacks. As pipe diameter increases, the cost of the instrument rises dramatically and accuracy can fall. Because a magnetic flow meter is an invasive device, with a pair of electrodes in contact with the flow, corrosion and electrode coating or scaling are also potential problems. The main drawback associated with magflow meters, though, is the need to break into the pipe to install the device in the first place. Regularly, the civil engineering costs associated with the installation of the unit dwarfs the price of the magflow meter itself. In many of the applications mentioned above, a magflow meter installation would be too expensive, too cumbersome or require too much downtime, so process managers have to make do with inferred values or wait for problems to manifest themselves elsewhere! The obvious solution would be a non-invasive device, capable of being installed on the outside of the pipe and, of course, such things have been available, in the form of time-of-flight ultrasonic instruments, which typically feature a pair of sensors that need to be very precisely aligned on opposite sides of a pipe. Again, there can be significant issues in the installation of these systems, mainly because the alignment has to be extremely accurate. There are also limitations on the types of liquid flows that can be measured. Solving the conundrum… So, an ideal specification emerges; a class of device that is non-invasive, that can be retro-fitted to an existing pipe, that will work over a wide range of pipe diameters, that will be reliable and repeatable, that is easy to install and requires no special positioning or alignment to work, and that will measure across the full range of pipe materials, brand new to fifty years old. It needs to be compact and able to interface with existing plant control infrastructure. Readers who have faced these issues over time will probably be aware that there is another group of products, one which has fallen out of favour over the years, based on the Doppler Effect. Doppler is a basic physical principle, which describes how the measured frequency of a wave reflecting from an object changes depending on whether that object is moving towards or away from a sensor. The most common example is the change in pitch of an ambulance siren as it passes you in the street. Or, indeed, the police speed gun! The Doppler Effect has been used for many years to measure flow, by the detection of particles as they pass a Doppler sensor. In many cases they have been very successful, and, because the Doppler effect is a basic physical principle, it should be extremely reliable and repeatable. The issue with Doppler devices has always been one of signal processing. Analogue and earlier microprocessor-driven instruments have simply not had the appropriate algorithms to allow the unit to reliably discriminate among the mass of echoes returning from the particles in the stream. Also, to be in any way reliable, the Doppler sensor has to be in the stream, so we are back to all the issues surrounding the installation of an invasive sensor. Not inside but outside… There is, however, a new form of Doppler sensor now available, one that takes the basic physical principle and applies to it a completely new approach to analysing the signal. A non-invasive single point sensor, that simply clamps to the outside of a pipe, almost regardless of pipe material and on pipes of up to a metre diameter, with no issues around alignment or positioning, in fact installed with a single, simple screwdriver. This new class of sensor, exemplified by the Flow Pulse from Pulsar Process Measurement, works by ‘firing’ an ultrasound pulse from a high-output ceramic crystal through the pipe wall at 90º to the liquid flow, and analysing the signal that comes back from particles or bubbles in the fluid stream. The Pulsar system uses a novel signal analysis technique never before used in flow monitoring called Refracted Spread Spectrum Analysis (RSSA), which is able to consolidate the real flow information from the mass of signals coming from the particles, bubbles, turbulence, vortices and eddies within the flowing liquid. The crystal fires signals at a wide angle and RSSA analyses the returning ‘echo’ over a wide frequency range, then slices them for realtime analysis and flow rate calculation. Laminar flow is not necessary to get a reading, so there is a more flexible choice of installation position. One of the things that make these devices so attractive to process users is their versatility. The pipe material can be rigid plastic, stainless steel, mild steel or cast iron. Corrugated pipe is not a problem, and even light corrosion on the pipe does not prevent a good result. Typically, flow rates from 0.3m/s through to around 4m/s can be measured. In terms of the flows that can be measured, these are just as versatile, ranging from a slurry at the ‘thick’ end down to particle sizes as small as 100 micron and concentrations as low as 200ppm, the equivalent of hard water. The key to the success of these new non-invasive devices is repeatability in flow monitoring. In the majority of process plant applications the fundamental issue is to monitor and maintain relative flow rates, warn of impending plant issues around valves and pumps and keep a clear view of network issues. The Magflow meter still has an important place, for applications where it can be installed without disruption in an ideal, laminar flow position, and where accuracy rather than repeatability is key. For everything else, maybe the time has come to look at the very real alternatives. Pulsar Process Measurement Ltd, Malvern, Worcestershire Can be contacted on: Tel: 01684 891371 E-mail: info@pulsar-pm.com Web: www.pulsar-pm.com Process Industry Inforem r October-November 2013 17


PII Oct-Nov 2013
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