Flow measurement

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Flow measurement is the quantification of bulk fluid movement. It can be measured in a variety of ways.

Contents

[edit] Units of measurement

Both gas and liquid flow can be measured in volumetric or mass flow rates (such as litres per second or kg/s). These measurements can be converted between one another if the materials density is known. The density for a liquid is almost independent of the liquids conditions, however this is not the case for a gas, whose density highly depends upon pressure and temperature.

In engineering contexts, the volumetric flow rate is usually given the symbol Q and the mass flow rate the symbol \dot m.

[edit] Gas

Due to the nature of an Ideal gas or a Real gas, the volumetric gas flow rate will differ for the same mass flow rate when at differing temperatures and pressures. As such gas volumetric flow rate is sometimes measured in "standard cubic centimeters per minute" (abbreviation sccm). This unit, although not an SI unit is sometimes used due to the additional information attached to the unit symbol, which indicates the temperature and pressure of the gas. Many other similar abbreviations are also in use, for two reasons, firstly mass flow and volumetric flow can be equated at known conditions, and secondly due to the imperial system older units such as standard cubic feet per minute or per second may still be used in some countries. It is often necessary to employ standard gas relationships (such as the ideal gas law) to convert between units of mass flow and volumetric flow.

[edit] Liquid

For liquids other units used depend on the application and industry but might include gallons (U.S. liquid or imperial) per minute, liters per second, bushels per minute and, when describing river flows, acre-feet per day.

[edit] Mechanical flow meters

There are several types of mechanical flow meter

[edit] Piston Meter

Because they are used for domestic water measurement, piston meters, also known as rotary piston or semi-positive displacement meters, are the most common flow measurement devices in the UK and are used for almost all meter sizes up to and including 40 mm (1 1/2"). The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passes through the piston chamber. Through a gear mechanism and, sometimes, a magnetic drive, a needle dial and odometer type display is advanced.

[edit] Woltmann Meter

Woltman meters, commonly referred to as Helix meters are popular at larger sizes. Jet meters (single or Multi-Jet) are increasing in popularity in the UK at larger sizes and are commonplace in the EU.

[edit] Multi-jet Meter

A multi-jet meter is a velocity type meter which has an impeller which rotates horizontally on a vertical shaft. The impeller element is in a housing in which multiple inlet ports direct the fluid flow at the impeller causing it to rotate in a specific direction in proportion to the flow velocity. This meter works mechanically much like a paddle wheel meter except that the ports direct the flow at the impeller equally from several points around the circumference of the element, where a paddle wheel normally only receives flow from one offset flow stream.

[edit] Venturi Meter

Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure (using a pressure sensor) that results across the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times.

[edit] Dall Tube

The Dall tube is a shortened version of a Venturi meter with a lower pressure drop than an orifice plate. Both flow meters the flow rate of Dall tube is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is measured using diaphragm pressure transducers with digital read out. Since these meters have significantly lower permanent pressure losses than the orifice meters, the Dall tubes have widely been used for measuring the flow rate of large pipeworks.

[edit] Orifice Plate

Another simple method of measurement uses an orifice plate, which is basically a plate with a hole through it. It is placed in the flow and constricts the flow. It uses the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's principle).

[edit] Pitot tube

A Pitot tube is a pressure measuring instrument used to measure fluid flow velocity by determining the stagnation pressure. Bernoulli's equation is used to calculate the dynamic pressure and thence fluid velocity.

[edit] Multi-hole Pressure Probe

Multi-hole pressure probes (also called impact probes) extend the theory of pitot tube to more than one dimension. A typical impact probe consists of three or more holes (depending on the type of probe) on the measuring tip arranged in a specific pattern. More holes allow the instrument to measure the direction of the flow velocity in addition to its magnitude (after appropriate calibration). Three-holes arranged in a line allow the pressure probes to measure the velocity vector in two dimensions. Introduction of more holes e.g., five holes arranged in a 'plus' formation allow measurement of the three-dimensional velocity vector.

[edit] Paddle wheel

The paddle wheel translates the mechanical action of paddles rotating in the liquid flow around an axle into a user-readable rate of flow (gpm, lpm, etc.). The paddle tends to be inserted into the flow.

[edit] Pelton wheel

The Pelton wheel turbine (better described as a radial turbine) translates the mechanical action of the Pelton wheel rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The Pelton wheel tends to have all the flow traveling around it with the inlet flow focussed on the blades by a jet. The original Pelton wheels were used for the generation of power and consisted of a radial flow turbine with "reaction cups" which not only move with the force of the water on the face but return the flow in opposite direction using this change of fluid direction to further increase the efficiency of the turbine.

[edit] Optical Flow Meters

Optical flow meters use light to determine flow rate. Small particles which accompany natural and industrial gases pass through two laser beams focused in a pipe by illuminating optics. Laser light is scattered when a particle crosses the first beam. The detecting optics collects scattered light on a photodetector, which then generates a pulse signal. If the same particle crosses the second beam, the detecting optics collect scattered light on a second photodetector, which converts the incoming light into a second electrical pulse. By measuring the time interval between these pulses, the gas velocity is calculated as V=D/T where D is the distance between the laser beams and T is the time interval.

Laser-based optical flow meters measure the actual speed of particles, a property which is not dependent on thermal conductivity of gases, variations in gas flow or composition of gases. The different operating principle enables optical laser technology to deliver highly accurate flow data, even in challenging environments which may include high temperature, low flow rates, high pressure, high humidity, pipe vibration and acoustic noise.

Optical flow meters are very stable with no moving parts and deliver a highly repeatable measurement over the life of the product. Because distance between the two laser sheets does not change, optical flow meters do not require periodic calibration after its initial commissioning. Optical flow meters require only one installation point, instead of the two installation points typically required by other types of meters. A single installation point is simpler, requires less maintenance and is less prone to errors.

Optical flow meters are capable of measuring flow from 0.1m/s to faster than 100m/s (1000:1 turn down ratio) and have been demonstrated to be effective for the measurement of flare gases, a major global contributor to the emissions associated with climate change.[1]

[edit] Turbine flow meter

The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow traveling around it.

The turbine wheel is set in the part of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. when a steady rotation speed has been reached, the speed is proportional to fluid velocity.

[edit] Open Channel Flow Measurement

[edit] Level to Flow

The level of the water is measured at a designated point behind a hydraulic structure (a weir or flume) using various means (bubbler, ultrasonic, float, and differential pressure are common methods). This depth is converted to a flow rate according to a theoretical formula of the form Q=KHX where Q is the flow rate, K is a constant, H is the water level and X is an exponent which varies with the device used, or it is converted according to empirically derived level/flow data points (a 'flow curve'). The flow rate can then integrated over time into volumetric flow.

[edit] Area/Velocity

The cross-sectional area of the flow is calculated from a depth measurement and the average velocity of the flow is measured directly (doppler and propeller methods are common). Velocity times the cross-sectional area yields a flow rate which can be integrated into volumetric flow.

[edit] Dye Testing

A known amount of dye per unit time is added to a flow stream. After complete mixing, the concentration of the dye is measured. The dilution rate of the dye equals the flow rate.

[edit] Thermal mass flow meters

Thermal mass flow meters generally use combinations of heated elements and temperature sensors to measure the mass flow of a fluid. The fluid temperature is also measured and compensated for. They provide a direct mass flow readout, and do not need any additional pressure temperature compensation over their specified range.

Technological progress allows today to manufacture thermal mass flow meters on a microscopic scale as MEMS sensors, these flow devices can be used to measure flow rates in the range of nano litres or micro litres per minute.

Thermal mass flow meters are used for compressed air, nitrogen, helium, argon, oxygen, natural gas. In fact, most gases can be measured as long as they are fairly clean and non-corrosive.

Temperature at the sensors varies depending upon the mass flow
Temperature at the sensors varies depending upon the mass flow


[edit] Vortex flowmeters

Another method of flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder, alternatively from each side of the bluff body. This vortex trail is called the Von Kármán vortex street after von Karman's 1912 mathematical description of the phenomenon. The frequency at which these vortices alternate sides is essentially proportional to the flow rate of the fluid. Inside, atop, or downstream of the shedder bar is a sensor for measuting the frequency of the vortex shedding. This sensor is often a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created. Since the frequency of such a voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross sectional area of the flow meter. The frequency is measured and the flow rate is calculated by the flowmeter electronics.

With f= SV/L where,

  • f = the frequency of the vortices
  • L = the characteristic length of the bluff body
  • V = the velocity of the flow over the bluff body
  • S = Strouhal number, which is essentially a constant for a given body shape within its operating limits

[edit] Electromagnetic, ultrasonic and coriolis flow meters

Modern innovations in the measurement of flow rate incorporate electronic devices that can correct for varying pressure and temperature (i.e. density) conditions, non-linearities, and for the characteristics of the fluid.

[edit] Magnetic flow meters

Industrial magnetic flowmeter
Industrial magnetic flowmeter

The most common flow meter apart from the mechanical flow meters, is the magnetic flow meter, commonly referred to as a "mag meter" or an "electromag". A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined non magnetic steel tube.

[edit] Ultrasonic (Doppler, Transit Time) flow meters

Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in and against flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times tup and tdown and the distance between receiving and transmitting transducers L and the inclination angle α one can write the equations:

v = \frac{L}{{2\;\sin \left( \alpha \right)}}\;\frac{{t_{up} - t_{down} }}{{t_{up} \;t_{down} }} and c = \frac{L}{2}\;\frac{{t_{up} + t_{down} }}{{t_{up} \;t_{down} }}

where v is the average velocity of the fluid along the sound path and c is the speed of sound.

Schematic view of a flow sensor.
Schematic view of a flow sensor.

Measurement of the doppler shift resulting in reflecting an ultrasonic beam off the flowing fluid is another recent innovation made possible by electronics. By passing an ultrasonic beam through the tissues, bouncing it off of a reflective plate then reversing the direction of the beam and repeating the measurement the volume of blood flow can be estimated. The speed of transmission is affected by the movement of blood in the vessel and by comparing the time taken to complete the cycle upstream versus downstream the flow of blood through the vessel can be measured. The difference between the two speeds is a measure of true volume flow. A wide-beam sensor can also be used to measure flow independent of the cross-sectional area of the blood vessel.

For the Doppler principal to work in a flowmeter it is mandatory that the flow stream contains sonically reflective materials, such as solid particles or entrained air bubbles.

[edit] Coriolis flow meters

Using the Coriolis effect that causes a laterally vibrating tube to distort, a direct measurement of mass flow can be obtained in a coriolis flow meter. Furthermore a direct measure of the density of the fluid is obtained. Coriolis measurement can be very accurate irrespective of the type of gas or liquid that is measured; the same measurement tube can be used for hydrogen gas and peanut butter without recalibration.

[edit] Laser doppler flow measurement

Laser-doppler flow meter.
Laser-doppler flow meter.

Blood flow can be measured through the use of a monochromatic laser diode. The laser probe is inserted into a tissue and turned on, where the light scatters and a small portion is reflected back to the probe. The signal is then processed to calculate flow within the tissues. There are limitations to the use of a laser doppler probe; flow within a tissue is dependent on volume illuminated, which is often assumed rather than measured and varies with the optical properties of the tissue. In addition, variations in the type and placement of the probe within identical tissues and individuals result in variations in reading. The laser doppler has the advantage of sampling a small volume of tissue, allowing for great precision, but does not necessarily represent the flow within an entire organ. The flow meter is more useful for relative rather than absolute measurements.

[edit] See also

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