In this post I want to share with you application and selection and flow measurement. In my career as process engineer, this is a very important topic since we always need to consider why we select this type of flow meter and not the other one.
Key Process in Selection of Flow Measurement
At least there are two key processes in selection of flow measurements. First, select meters that meet measurement requirements and are available in the needed size and materials. Then, evaluate cost, delivery, performance, and other factors to choose the best option.
Next, focus on key performance requirements like maximum allowable error (as a percentage of reading or full scale) and metering range. Use these limits to determine the needed rangeability—the ratio between maximum and minimum flow within acceptable error—and select appropriate flow sensor types.
Flow Measurement Application
Table below shows flow measurement applications of several types of flow sensors.
Overview of Each Flowmeters
Magnetic Flowmeter
Magnetic flowmeters provide noninvasive measurements by adding external electrodes and magnets to existing pipes. They measure both forward and reverse flows, and are unaffected by viscosity, density, and other disturbances. These linear devices are suitable for a wide range of applications and respond quickly to flow changes. Recent advancements have made them more economical, precise, and compact.
Magnetic flowmeters operate based on Faraday’s law of electromagnetic induction, which states that a moving conductor in a magnetic field generates a voltage proportional to their relative velocities. This principle, also used in electric generators, enables flow measurement because many liquids conduct electricity. Although Faraday’s early attempt to measure river flow failed due to insensitive instruments, modern technology now allows successful application of his theory in magnetic flowmeters.
Coriolis Mass Flowmeter (CMF)Coriolis mass flowmeters (CMFs) measure true mass flow directly, unlike other meters that only measure volumetric flow. Their high accuracy and wide range make them popular in industry. Commercial CMFs come in various designs—single-tube, dual-tube, bent-tube, and straight-tube—and use different materials like stainless steel, Hastelloy, titanium, zirconium, tantalum, and lined tubes, enabling use with many liquids and gases. CMFs are widely used in food and beverage, chemical, pharmaceutical, and increasingly, oil and gas industries.
Orifice
Head-type flowmeters belong to a group of instruments used to measure fluid flow, such as orifice plates, venturi tubes, weirs, and flumes. These devices alter either the velocity or direction of the flow to generate a measurable difference in pressure, known as “pressure head,” within the fluid.
Head metering is one of the oldest methods used to measure flow. In the 18th century, Bernoulli explained the fundamental connection between pressure head and velocity head, and Venturi introduced the flow tube named after him. More recently, advancements have been made such as enhanced primary elements, improved data accuracy, more flexible testing and calibration tools, and superior differential-pressure sensors, among others.
Pitot Tubes
Henri de Pitot invented the pitot tube in 1732 to measure fluid velocities. Pitot tubes measure flow at a single point, with multiported probes averaging several points or area-averaging across pipes. Their benefits include low cost, minimal pressure loss, and the ability to be installed while under pressure. However, they have limited accuracy and range, and are only suitable for clean fluids unless purged.
Ultrasonic Flowmeter
Ultrasonic meters are ideal for measuring highly corrosive liquids. They come in Doppler and transit-time versions.
Doppler meters work by sending an ultrasonic pulse into the pipe, which is reflected by particles like air or dirt. Often used in clamp-on designs that attach outside existing pipes, they measure flow velocity at a small area where the beam enters the stream. If this velocity does not represent the entire pipe, accuracy suffers. The main advantage is low, size-independent cost; however, these meters are not suitable for clean fluids or gases.
Transit-time ultrasonic flowmeters, commonly used in water treatment and chemical plants, operate by sending ultrasonic beams both with and against the flow; the flow rate is determined by the difference in transit times. While more expensive than Doppler meters, they provide higher accuracy but are suitable only for clean fluids. They do not obstruct flow, resulting in low pressure drop, but require long, straight pipe runs (about 15 diameters) due to their sensitivity to piping configuration.
References:
Liptak, Bela G. Process Measurement and Analysis Volume I. 2003. CRC Press