Flow measurement , (Instrumentación industrial - Industrial instrumentation) medición de caudal.
Flow Measurement Instruments.
Flow measurements are normally indirect measurements using differential pressures to measure the flow rate. Flow measurements can be divided into the following groups: flow rate , total flow , and mass flow. The choice of the measuring device will depend on the required accuracy and fluid characteristics (gas , liquid , suspended particulates , temperature , viscosity , and so on.)
Instrumentos de medida de caudal. Las medidas de caudal son normalmente medidas indirectas que utilizan presiones diferenciales para medir el caudal. Las mediciones de caudal se pueden dividir en los siguientes grupos: caudal volumétrico o también tasa de flujo , caudal total y caudal másico. La elección del dispositivo de medición dependerá de la precisión requerida y las características del fluido (gas , líquido , partículas en suspensión , temperatura , viscosidad , etc.).
Measurement of flow
Measurement of flow is a methodology applied in any procedure involving the movement of substances from one location to another (such as the bulk transfer of oil from a road tanker to a storage tank in a garage). It serves the purpose of quantifying the amount of material supplied or managing and regulating a specific flow rate. In numerous processes, the efficiency of the facility hinges on the precise measurement and control of flow. Well-designed systems for measuring flow are harmonious with the substance or process they are assessing. Additionally, they must possess the capability to deliver the level of accuracy and consistency that aligns with the given application.
It is commonly asserted that an "ideal" flowmeter should be non-intrusive, cost-effective, exhibit absolute accuracy, infinite repeatability, and operate indefinitely without the need for maintenance. Regrettably, such a device has not been realized, despite claims by some manufacturers. Nevertheless, in recent years, substantial enhancements have been made to existing systems, and innovative products employing novel techniques are consistently entering the market. The realization of the so-called "ideal" flowmeter may be closer than anticipated, and potential users must now, more than ever, be well-versed in the available systems.
Basic principles of flow measurement
flow measurement necessitate brief exploration before delving into the functionality of the diverse measurement systems accessible. Flow can be gauged either as a volumetric quantity or an instantaneous velocity (typically translated into a flow rate). The interconnectedness of these measurements is evident in Figure 1.
Figure 1 . Flow-time graph. - |
When the flow rate is documented over a duration, the quantity corresponds to the area beneath the curve (shaded region). Many instruments can automatically establish this, a process known as integration. The integrator of an instrument may execute this either electrically or mechanically.
Streamlined and turbulent flow
Flow characterized by streamlined and turbulent behavior can be effectively illustrated through examples. Reynolds extensively researched this phenomenon, and Figure 2 elucidates the concept of streamlined flow, also known as laminar flow.
Figure 2 Reynolds’s experiment. -
In this scenario, a thin filament of colored liquid is introduced into a flowing water quantity within a smooth glass tube. All fluid particles follow paths parallel to the tube walls, causing the colored liquid to move in a straight line, resembling a tube within a tube. However, this state is dependent on velocity and viscosity. As velocity increases, a critical velocity is reached, leading to the dispersion and mixing of the colored liquid with the carrier liquid, marking the transition to turbulent flow. In summary, below the critical velocity, flow is streamlined or laminar, and above the critical value, it is termed turbulent, a common occurrence in practice. Reynolds expressed this data dimensionlessly through the formula:
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where Re is the Reynolds number, D is the diameter of the installation throat, v is velocity, ρ is the density of the fluid, and μ is the absolute viscosity. Laminar flow is expected if the Reynolds number is less than 2000 and turbulent if it is greater than 4000 ( Re < 2000, turbulent if Re > 4000), and the critical zone lies in between. Systems with the same Reynolds number and geometric similarity are considered to have dynamic similarity.
Flow profile
The velocity profile across the diameter of a pipe, influenced by various factors, defines the flow profile. For laminar flow, the profile is parabolic, with the center's velocity about twice the mean velocity. In turbulent flow, after a sufficient straight pipe run, the flow profile becomes fully developed. The concept of "fully developed flow" is crucial for well-designed flow measurement systems. In fully developed flow, the center's velocity is only about 1.2 times the mean velocity, providing the most accurate, repeatable, and linear flow measurement.
Energy of a Fluid in Motion
Let's explore the ways in which energy is represented in a moving fluid. This will aid in understanding the utilization of the Reynolds number in universal flow equations. The fundamental types of energy associated with a moving fluid are:
(a) Potential energy or potential head.
(b) Kinetic energy.
(c) Pressure energy.
(d) Heat energy.
Potential Energy This energy exists within the fluid due to its position or elevation concerning a fixed level. For instance, if 1 m³ of liquid with a density of p1 kg/m³ possesses a mass of p1 kg, it would necessitate a force of 9.81×p1×N to uphold it at a location where the gravitational constant g is 9.81 m/s². Therefore, if it rests at a height of z meters above a reference plane, it would possess 9.81×p1×z joules of energy due to its elevation.
Kinetic Energy
This energy resides within a fluid owing to its motion. If 1 m³ of fluid with a density of p1 kg/m³ and a velocity of V1 m/s possesses a kinetic energy of (1)/(2)p1×V1 2 joules.
Pressure Energy
A fluid contains this energy due to its pressure. For instance, a fluid with a volume of v1m³ and a pressure of p1 N/m² would possess a pressure energy of p1×v1 joules.
Internal Energy
The fluid also contains energy due to its temperature, known as heat energy. When there's resistance to flow, such as friction, other forms of internal energy convert into heat energy.
Total Energy
The total energy E of a fluid is calculated by the equation:
Total energy (E) = potential energy + kinetic energy + pressure energy + internal energy
Total energy (E) = potential energy + kinetic energy + pressure energy + internal energy
E= P.E. + K.E. + PR.E. + I.E
Medición de flujo o caudal
La medición de flujo es una técnica utilizada en cualquier proceso que requiera el transporte de un material de un punto a otro (por ejemplo, suministro a granel de aceite desde un camión cisterna hasta un tanque de almacenamiento en un garaje). Puede emplearse para cuantificar un cargo por el material suministrado o para mantener y controlar una tasa específica de flujo. En muchos procesos, la eficiencia de la planta dependerá de la capacidad para medir y controlar el flujo con precisión. Los sistemas de medición de flujo diseñados adecuadamente son compatibles con el proceso o el material que están midiendo. También deben ser capaces de proporcionar la precisión y repetibilidad más apropiadas para la aplicación.
A menudo se dice que "el medidor de flujo ideal debería ser no intrusivo, económico, tener una precisión absoluta, repetibilidad infinita y funcionar siempre sin necesidad de mantenimiento". Desafortunadamente, tal dispositivo aún no existe, aunque algunos fabricantes puedan afirmar lo contrario. Sin embargo, en los últimos años se han realizado muchas mejoras a los sistemas establecidos, y constantemente se están introduciendo nuevos productos que utilizan técnicas novedosas en el mercado. El "medidor de flujo ideal" puede no estar tan lejos de la realidad, y ahora más que nunca los usuarios potenciales deben estar completamente conscientes de los sistemas a su disposición.
Los principios básicos de la medición de flujo requieren una breve exploración antes de analizar el funcionamiento de los diversos tipos de sistemas de medición disponibles. El flujo se puede medir como una cantidad volumétrica o como una velocidad instantánea (normalmente traducida a una tasa de flujo). Puedes observar la interdependencia de estas medidas en la Figura 1.(arriba) |