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Transcript
Magnetic vs Mechanical Flow Meters What are the main differences between the two flow meters? The primary difference between a mechanical meter and a magnetic meter is that a mechanical meter has moving parts and a mag meter doesn’t. Typical examples of mechanical meters are target meters, where liquid hits a spring loaded plate, and turbine meters, where liquid flowing over blades causes rotation. Positive displacement meters involve a set amount of material moving through chambers and being converted to flow rates. A magnetic (or inductive) flow meter works by having a conductive liquid flow through a lined flow tube and of the flow passing through a magnetic field generated as part of the flow tube design causing a voltage to be created. This voltage increases or decreases based on velocity. So, as long as the material meets the minimum conductivity level, there are no moving parts. One of the biggest disadvantages is that mechanical parts wear. As these parts wear, there are a variety of effects that the wear can have on your process. When parts wear, they do not work as well as they did when they were first designed or installed Magnetic flow meters, on the other hand, do not require as much maintenance as mechanical meters do although any flow measurement device should be checked intermittently. Magnetic (or inductive) flow meters are suitable for the measurement of conductive liquids, typically liquids with conductivity of 5 microsiemen/cm or greater. These flow meters are also suitable for liquids with either suspended solids or slurries. This makes them ideal for water-based products and most acids and bases as well as slurries like the ones you find in wastewater treatment, mining, and pulp and paper applications. Inductive flow meters are also very flexible when it comes to viscosity, but they are not suited for hydrocarbon-based products, gases or steam. With hydrocarbon-based products, there is just not enough conductivity present for a flow meter with this technology to work. Obviously mechanical meters are not subject to conductivity restrictions. In most cases, there is an upper limit for the viscosity tied to the ability of the meter to physically operate and/or pressure drop across the mechanical aspects of the meter to allow a highly viscous liquid to even flow. Mechanical meters are also not suitable for slurries or liquids with suspended solids as these can cause either wear and/or build-up on the meter, and will require higher levels of maintenance or replacement of the device entirely. Magmeters are accurate to 0.2% of flow rate with a 5 diameter upstream and a 3 diameter downstream of straight run. Some mechanical technologies, such as PD/Oval Gear, have no requirements for straight run. A Positive Displacement flow meter offers an interesting compromise as it has only one moving part. They’re simple with both high resolution open collector and reed switch outputs as standard. As each piston rotation passes a known liquid volume, the inherent repeatability of the Multipulse positive displacement flowmeter makes it particularly suited to batching and dispensing duties. Commonly metered liquids range from non-conductive low viscosity solvents through to extremely viscous lubricants, chemicals and food bases. Application flexibility is further enhanced as meter performance is independent of flow profile eliminating the restrictive need for straight pipe runs required with most alternate metering technologies. Telephone: 01892 664499 Fax: 01892 663690 www: http://www.pvl.co.uk/ e-mail: [email protected] So, what type of flow meter is best for your application? There are no “universal” flow meters which are suitable for all applications. To select the proper technology for an application requires writing a flow specification which covers the use of the meter. There are usually trade-offs with each meter type, so knowing the critical specifications will be important. What Gas or Liquid will be measured? • • • • • • • • • Minimum and maximum flow rates. What are the accuracy requirements? The fluid temperature and viscosity. Fluid compatibility with the materials of construction The maximum pressure at the location. What pressure drop is allowable? Will the meter be mounted in a hazardous location? Is the fluid flow continuous or intermittent? What type of output signal or readout do is required? This list can be used to eliminate the flowmeter technologies that do not apply (Turbine flowmeters don’t work for viscous fluids. Coriolis meters don’t respond fast enough for injection flow, etc.). Then a comparison of the remaining technologies is available for selection for the correct flowmeter. Accurate meters are priced based on their capabilities. It is better to locate the type of meter which fits the application before trading features for cost savings. Closely evaluate extreme conditions, such as low flow rates, high pressure or temperature or the need to measure over a wide operating range. If these conditions are important, do not consider lower priced alternatives that would be applied outside of their capabilities. While all measurement technologies have their place, we encourage you to always weigh the cost of purchase with the overall cost of ownership whenever selecting a flow measurement technology for your application. For further information contact: David Almond, Head of Sales and Marketing, PVL Ltd, Unit 9 Lexden Lodge Industrial Estate, Jarvis Brook, Crowborough, East Sussex, TN6 2NQ Telephone: 01892 664499 Fax: 01892 663690 www: http://www.pvl.co.uk/ e-mail: [email protected] Blog: http://blog.pvl.co.uk/ Twitter: http://twitter.com/PVLLimited LinkedIn: http://linkd.in/PVL_Ltd YouTube: http://bit.ly/YouTube_PVL_Ltd