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Continuous Fluid Flow Measurement

The measurement of fluid flow is arguably the single most complex type of process variable measurement in all of industrial instrumentation1. Not only is there a bewildering array of technologies one might use to measure fluid flow – each one with its own limitations and idiosyncrasies – but the very nature of the variable itself lacks a singular definition. “Flow” may refer to volumetric flow (the number of fluid volumes passing by per unit time), mass flow (the number of fluid mass units passing by per unit time), or even standardized volumetric flow (the number of gas volumes flowing, supposing different pressure and temperature values than what the actual process line operates at). Flowmeters configured to work with gas or vapor flows often are unusable on liquid flows. The dynamic properties of the fluids themselves change with flow rates. Most flow measurement technologies cannot achieve respectable measurement linearity from the maximum rated flow all the way to zero flow, no matter how well matched they might be to the process application.

Furthermore, the performance of most flowmeter technologies critically depends on proper installation. One cannot simply hang a flowmeter at any location in a piping system and expect it to function as designed. This is a constant source of friction between piping (mechanical) engineers and instrumentation (controls) engineers on large industrial projects. What might be considered excellent piping layout from the perspective of process equipment function and economy is often poor (at best) for good flow measurement, and visa-versa. In many cases the flowmeter equipment gets installed improperly and the instrument technicians have to deal with the resulting measurement problems during process unit start-up.

Even after a flowmeter has been properly selected for the process application and properly installed in the piping, problems may arise due to changes in process fluid properties (density, viscosity, conductivity), or the presence of impurities in the process fluid. Flowmeters are also subject to far more “wear and tear” than most other primary sensing elements, given the fact that a flowmeter’s sensing element(s) must lie directly in the path of potentially abrasive fluid streams. Given all these complications, it is imperative for instrumentation professionals to understand the complexities of flow measurement. What matters most is that you thoroughly understand the physical principles upon which each flowmeter depends. If the “first principles” of each technology are understood, the appropriate applications and potential problems become much easier to recognize.


Pressure-Based Flowmeters - All masses require force to accelerate (we can also think of this in terms of the mass generating a reaction force as a result of being accelerated). This is quantitatively expressed by Newton’s Second Law of Motion... Click here to continue reading...

Laminar Flowmeters - A unique form of differential pressure-based flow measurement deserves its own section in this flow measurement chapter, and that is the laminar flowmeter. Click here to read more...

Variable-Area Flowmeters - A Variable-area flowmeter is one where the fluid must pass through a restriction whose area increases with flow rate. This stands in contrast to flowmeters such as orifice plates and venture tubes where the cross-sectional area of the flow element remains fixed. Click here to read more...

Velocity-Based Flowmeters - The Law of Continuity for fluids states that the product of mass density (ρ), cross-sectional pipe area (A) and average velocity (v) must remain constant through any continuous length of pipe... Click here to continue reading...

Positive Displacement Flowmeters - A positive displacement flowmeter is a cyclic mechanism built to pass a fixed volume of fluid through with every cycle. Every cycle of the meter’s mechanism displaces a precisely defined (“positive”) quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the total fluid volume passed through the flowmeter. Many positive displacement flowmeters are rotary in nature, meaning each shaft revolution represents a certain volume of fluid has passed through the meter. Some positive displacement flowmeters use pistons, bellows, or expandable bags working on an alternating fill/dump cycle to measure off fluid quantities. Click here to read more...

Standardized Volumetric Flow - The majority of flowmeter technologies operate on the principle of interpreting fluid flow based on the velocity of the fluid. Magnetic, ultrasonic, turbine, and vortex flowmeters are prime examples, where the sensing elements (of each meter type) respond directly to fluid velocity. Translating fluid velocity into volumetric flow is quite simple... Click here to continue reading...

True Mass Flowmeters - Many traditional flowmeter technologies respond to the volumetric flow rate of the moving fluid. Velocity-based flowmeters such as magnetic, vortex, turbine, and ultrasonic generate output signals proportional to fluid velocity and nothing else. This means that if the fluid flowing through one of these flowmeter types were to suddenly become denser (while still flowing by at the same number of volumetric units per minute), the flowmeter’s response would not change at all. Click here to read more...

Weighfeeders - A completely different kind of flowmeter is the weighfeeder, used to measure the flow of solid material such as powders and grains. One of the most common weighfeeder designs consists of a conveyor belt with a section supported by rollers coupled to one or more load cells, such that a fixed length of the belt is continuously weighed... Click here to continue reading...

Change-of-Quantity Flow Measurement - Flow, by definition, is the passage of material from one location to another over time. So far this chapter has explored technologies for measuring flow rate en route from source to destination. However, a completely different method exists for measuring flow rates: measuring how much material has either departed or arrived at the terminal locations over time...Click here to read more...

Insertion Flowmeters - This section does not describe a particular type of flowmeter, but rather a design that may be implemented for several different kinds of flow measurement technologies. When the pipe carrying process fluid is large in size, it may be impractical or cost-prohibitive to install a full-diameter flowmeter to measure fluid flow rate. A practical alternative for many applications is the installation of an insertion flowmeter: a probe that may be inserted into or extracted from a pipe, to measure fluid velocity in one region of the pipe’s cross-sectional area (usually the center). Click here to read more...

Process / Instrument Suitability of Flowmeters - Every flow-measuring instrument exploits a physical principle to measure the flow rate of fluid stream. Understanding each of these principles as they apply to different flow-measurement technologies is the first and most important step in properly applying a suitable technology to the measurement of a particular process stream flow rate... Click here to continue reading...


1Analytical (chemical composition) measurement is undeniably more complex and diverse than flow measurement, but analytical measurement covers a great deal of specific measurement types. As a single process variable, flow measurement is probably the most complex.



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Comments (2)Add Comment
Temperature & Pressure Compensatation formula for Flow Measurement
written by HIRENSINH A. PARMAR, September 28, 2012

pl send with & without Temperature & Pressure Compensatation formula for Flow Measurement
written by ahmed , November 20, 2014
Flowmeters configured to work with gas or vapor flows often are unusable on liquid flows. ??? why it'sunusable on liquid flows ??? all flowmeters can measure fluid flow !!

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