There are significant differences between flow meter technologies, with each type of device having its own advantages and disadvantages.
With a growing focus on corporate responsibility and sustainability, the monitoring of greenhouse gas (GHG) emissions is critical. Firms must ensure regulatory compliance while protecting assets, personnel and the environment. Another crucial concern is custody transfer, with increasing energy costs driving the need for improved fiscal metering of high-value products.
Common Flow Applications
In modern industrial plants, personnel need to make faster and better decisions by capturing, managing and analyzing data. These facilities rely heavily on flow processes, and thus accurate and reliable measurement technology is vital. Most plants have two 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 price and total cost of ownership.
Popular Measurement Devices
Flow meters are excellent tools to measure, monitor and control the distribution of a host of fluids. The question is which technology to use.
Coriolis. Coriolis meters have 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, while thermal mass flow meters depend on the physical properties of the fluid.
Coriolis flow meters directly measure fluid mass over a wide range of temperatures with a 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 minimum attention once installed. However, they are sometimes considered too sophisticated, expensive or unwieldy (see Image 1).
Differential pressure (DP). 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 DP in the pipe, and a secondary element measuring the differential pressure and providing a signal or read-out converted to the actual flow value. DP meters employ a proven, well-understood measuring technology that does not require moving parts in the flow stream and 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 use Faraday’s law of electromagnetic induction, whereby voltage is induced when a conductor moves through a magnetic field. The liquid is the conductor, with energized 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. 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, they have a propensity to foul, tend to be heavy in larger sizes and may be prohibitively expensive (see Image 2).
Positive displacement (PD). 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 monitors the measuring element’s rotations to provide a volumetric recording of the flow total. PD meters are highly accurate and have one of the largest turndown ratios. They are easy to maintain since they have only one or two moving parts. Systems do not need straight pipe lengths, as they do with other metering approaches. However, PD meters require clean fluids and can be large and difficult to install.
Thermal mass. Thermal mass meters use 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 analyzer, temperature compensator and a signal conditioner providing a linear output directly proportional to mass flow.
Thermal mass meters have a relatively low purchase price and 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.