Flow, level & control


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.


Choosing the right flow meter F


This essential article, from Bell Flow Systems, describes the key criteria in flow meter selection. It evaluates the most common instrument designs and offers guidance in implementing the right solution for specific applications


low measurement is a critical aspect of plant and process operation in many industries. Users choosing equipment to measure the


Coriolis Coriolis meters contain a vibrating tube in which a fluid flow causes changes in frequency, phase shift or amplitude. The sensor signal is fed into the integrally mounted pc-board. The resulting output signal is strictly proportional to the real mass flow rate, whereas thermal mass flow meters are dependent of the physical properties of the fluid. One of the most important features of


TYPICAL FLOW APPLICATIONS In modern plants and facilities, personnel need to make faster and better decisions by capturing, managing and analysing the right data at the right time. These facilities rely heavily on flow processes, and thus accurate and reliable measurement technology is vital to the efficiency and safety of their operations.


Typical flow-metering applications in the chemical/ petrochemical sector, for example, include:


Chemical batching Dosing/blending Catalyst injection Chemical recovery Custody transfer Steam flow Lube oil loading Process cooling Pressure regulation


Leak detection Fuel consumption monitoring Fiscal transfer Product load-out Reactor feed Safety shutdown Waste treatment Emissions monitoring


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.


COMMON MEASUREMENT TECHNOLOGIES Flow meters are excellent tools to measure, monitor and control the distribution of a host of process fluids. 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.


Impeller meters are frequently used in large diameter water distribution systems


Coriolis flow meters is that they directly measure fluid mass over a wide range of temperatures with a very high degree of accuracy. Their unobstructed, open flow design is suitable for viscous, non-conductive fluids that are difficult to measure with other technologies. With no internal moving parts, Coriolis meters require a minimum amount of attention once installed. However, they are sometimes considered too sophisticated, expensive or unwieldy for certain applications.


Differential Pressure Differential Pressure (DP) meters measure the pressure differential across the meter and extract the square root. They have a primary element that causes a change in kinetic energy, creating differential pressure in the pipe, and a secondary element measuring the differential pressure and providing a signal or read-out converted to the actual flow value. 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 Electromagnetic meters employ Faraday’s law of electromagnetic induction, whereby voltage is induced when a conductor moves through a magnetic field. The liquid acts as the conductor, with energised coils outside the flow tube creating the magnetic field. The produced voltage is directly proportional to the flow rate. 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. The meters have no moving parts or flow obstructions, and are relatively unaffected by viscosity, temperature and pressure when correctly specified. Nevertheless, their propensity to foul can cause maintenance issues. Electromagnetic meters tend to be heavy in larger sizes and may be prohibitively expensive for some purposes.


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Coriolis meters are suitable for viscous, non-conductive fluids


Positive Displacement Positive Displacement (PD) meters separate liquid into specific increments, and the flow rate is an accumulation of these measured increments over time. The rotational speed of a PD meter’s impeller is a function of the process flow. An internally coupled counter, either electronic or mechanical, monitors the measuring element’s rotations to provide a volumetric recording of the flow total. Positive Displacement meters are highly


accurate (especially at low flows) and have one of the largest turndown ratios. The devices are easy to maintain as they have only one or two moving parts. There is no need for straight pipe lengths as with other metering approaches. However, PD meters require clean fluids and can be large and burdensome to install.


Thermal Mass Thermal Mass meters utilise a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element, which is directly proportional to the mass flow rate. The meter’s electronics package includes the flow analyser, temperature compensator, and a signal conditioner providing a linear output directly proportional to mass flow. Thermal mass meters carry a relatively low


purchase price. They are designed to work with clean gases of known heat capacity, as well as some low-pressure gases not dense enough for Coriolis meters to measure. The main disadvantage of thermal technology is low-to-medium accuracy, although suppliers have improved the capabilities of these meters in recent years.


October 2019 Instrumentation Monthly


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