FLOW & LEVEL CONTROL FEATURE


A GUIDE TO CHOOSING THE RIGHT FLOW METER


The following article from Bell Flow Systems outlines the key criteria in flow meter selection, to help ensure a successful application when the meter is installed


F


low measurement is a critical aspect of plant and process operation in many


industries. Users choosing equipment to measure the flow of liquid or gas processes must consider a wide range of factors to arrive at an optimal solution. Experience has shown there are significant differences between flow meter technologies, with each type of device having its own advantages and disadvantages. Typical flow-metering applications in the


chemical/petrochemical sector for example include: Chemical Batching; Dosing/ Blending; Custody Transfer; Leak Detection; and Safety Shutdown. Most chemical processing plants have


two primary flow measurement challenges: accuracy and cost. The goal is to correctly match the right flow meter to the right application to achieve the best performance for the lowest purchase prices and total cost of ownership. When it comes to specifying a flow meter,


the question is which technology to use, since a wide variety of meter designs are available. Each type of meter has pros and cons, and must be properly deployed to achieve optimal performance. Flow meter technologies include:


Coriolis; Differential Pressure meters; Electromagnetic meters; Positive Displacement meters, Thermal Mass meters; Turbine meters, Impeller meters; Variable Area meters; Ultrasonic meters (transit time and Doppler); Vortex meters; Oval Gear meters and Nutating Disc meters. As mentioned, meters offer different


technologies and suit different applications. For example, Coriolis flow meters directly measure fluid mass over a wide range of temperatures with a high degree of accuracy. However, they are sometimes considered too sophisticated, expensive or unwieldy for certain applications. Differential Pressure meters are versatile


instruments, which employ a proven, well- understood measuring technology that does not require moving parts in the flow stream. DP meters are not greatly affected by viscosity changes, however, they have a history of limited accuracy and turndown, as well as complex installation requirements. Electromagnetic meters will measure


virtually any conductive fluid or slurry, including process water and wastewater.


They provide low pressure drop, high accuracy, high turndown ratio, and excellent repeatability. Their propensity to foul can cause maintenance issues. Thermal mass meters carry a relatively


low purchase price, but have historically offered low-to-medium accuracy. Turbine meters incorporate a time-tested


measuring principle, and are known for high-accuracy, wide turndown and repeatable measurements. However, they require periodic recalibration and service. In a typical chemical production facility,


fluid characteristics eg. single or double phase, viscosity, turbidity, flow profile, flow range, and accuracy requirements are all important factors in determining the right flow meter for a particular task. Extra considerations such as mechanical restrictions and output-connectivity options also impact choice. For most general industrial applications,


the key factors in flow meter selection are: Process Media - The most important


difference between fluids and gases lies in their relative compressibility (i.e, gases can be compressed much more easily than liquids). Consequently, any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than in the case of a liquid. Type of Measurement - Industrial flow


measurements fall under one of two categories: mass or volumetric. Volumetric measuring devices, like variable area meters or turbine flow meters, are unable to distinguish temperature or pressure changes. Mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for the variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure. Flow Rate Information - A crucial aspect


in selection is determining whether flow rate data should be instantaneous or continuous. A typical continuous flow measurement system consists of a primary flow device, flow sensor, transmitter, flow recorder, and totalizer. Desired Accuracy - In many production


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Turbine meters incorporate a time- tested measuring principle, and are known for high-accuracy, wide turndown and repeatable measurements


processes, accurate flow measurements can be the difference between on-spec quality and wasted product. Flow meter accuracy is specified in percentage of actual reading (OR), calibrated span (CS), or full-scale (FS) units. It is normally stated at minimum, normal, and maximum flow rates. A clear understanding of these requirements is needed for acceptable performance over a meter’s full range. Application Environment - Flow meters


Vortex meters have no moving parts that are subject to wear, so regular maintenance is not necessary


Other considerations:


Companies purchasing flow meters should remember that accurate instruments cost more based on their capabilities.


All flow meters are affected to some extent by the flowing medium they are metering and by the way they are installed. Consequently, their performance in real world conditions will often be different from the reference conditions under which they are calibrated.


Flow meter users should examine long-term ownership costs.


Generally speaking, flow meters with few or no moving parts require less attention than more complex instruments.


The need to recalibrate a flow meter depends on how well the instrument fits a particular application.


can be employed under many varying conditions in a chemical process plant, which users must consider when sizing the correct instrument for the job. Fluid Characteristics - Users should be


cautious that the selected flow meter is compatible with the fluid and conditions they are working with. Installation Requirements - Planning a


flow meter installation starts with knowing the line size, pipe flow direction, material of construction, and flange-pressure rating. Complications due to equipment accessibility, valves, regulators, and available straight pipe run lengths should also be identified. Power Availability and Hazardous Areas -


Pneumatic instrumentation was once used in most hazardous area applications, since there was no power source to cause an explosion. Today’s ATEX installations normally call for intrinsically safe instruments, which are “current limited” by safety barriers to eliminate a potential spark or Explosion proof solutions which feature approved E xd enclosures. Self- powered flow meters offer an advantage in such environments or where a power source is not available. Necessary Approvals - UK firms are


obliged to comply with strict standards set by the EU, including ATEX, MID and EMC. Output/Indication - Flow meter users


must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analogue, digital, or shared.


Bell Flow Systems www.bellflowsystems.co.uk


PROCESS & CONTROL | OCTOBER 2019 11


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