Differential Pressure Flowmeter The basic principle of nearly all differential pressure flowmeters is that if a restriction is placed in a pipeline, then the pressure drop across this re
Trang 1The following sections will consider the most popular types of flowmeter from each of the eight main categories in Table 19.3 For information on other flowmeters and those in the miscellaneous group see one of the many textbooks on flow measurement such as [3–6]
Differential Pressure Flowmeter
The basic principle of nearly all differential pressure flowmeters is that if a restriction is placed in a pipeline, then the pressure drop across this restriction is related to the volumetric flowrate of fluid flowing through the pipe
The orifice plate is the simplest and cheapest type of differential pressure flowmeter It is simply a plate with a hole of specified size and position cut in it, which can then be clamped between flanges in
a pipeline (Fig 19.53) The volumetric flowrate of fluid Q in the pipeline is given by Eq (19.66):
(19.66)
where p1 and p2 are the pressures on each side of the orifice plate, ρ is the density of the fluid upstream
of the orifice plate, d is the diameter of the hole in the orifice plate, and β is the diameter ratio d/D where D is the upstream internal pipe diameter The two empirically determined correction factors are
C the discharge coefficient, and ε the expansibility factor C is affected by changes in the diameter ratio,
Reynolds number, pipe roughness, the sharpness of the leading edge of the orifice, and the points at which the differential pressure across the plate are measured However, for a fixed geometry it has been
shown that C is only dependent on the Reynolds number and so this coefficient can be determined for
a particular application ε is used to account for the compressibility of the fluid being monitored Both
C and ε can be determined from equations and tables in a number of internationally recognized
TABLE 19.3 Main Categories of Closed Conduit Flowmeter Type 1—differential pressure flowmeters
Sharp edged orifice plate, chord orifice plate, eccentric orifice plate, Venturi, nozzle, Pitot tube, elbow, wedge, V-cone, Dall tube, Elliot-Nathan flow tube, Epiflo
Type 2—variable area flowmeters Rotameter, orifice and tapered plug, cylinder and piston, target, variable aperture
Type 3—positive displacement flowmeters Sliding vane, tri-rotor, bi-rotor, piston, oval gear, nutating-disc, roots, CVM, diaphragm, wet gas
Type 4—turbine flowmeters Axial turbine, dual-rotor axial turbine, cylindrical rotor, impeller, Pelton wheel, Hoverflo, propeller
Type 5—oscillatory flowmeters Vortex shedding, swirlmeter, fluidic Type 6—electromagnetic flowmeters
AC magnetic, pulsed DC magnetic, insertion Type 7—ultrasonic flowmeters
Doppler, single path transit-time, multi-path transit-time, cross-correlation, drift
Type 8—mass flowmeters Coriolis, thermal Type 9—miscellaneous flowmeters Laser anemometer, hot-wire anemometers, tracer dilution, nuclear magnetic resonance
1 – b4
- e p
4
- d2 2 p ( 1– p2)
r
-=
Trang 2The following sections will consider the most popular types of flowmeter from each of the eight main categories in Table 19.3 For information on other flowmeters and those in the miscellaneous group see one of the many textbooks on flow measurement such as [3–6]
Differential Pressure Flowmeter
The basic principle of nearly all differential pressure flowmeters is that if a restriction is placed in a pipeline, then the pressure drop across this restriction is related to the volumetric flowrate of fluid flowing through the pipe
The orifice plate is the simplest and cheapest type of differential pressure flowmeter It is simply a plate with a hole of specified size and position cut in it, which can then be clamped between flanges in
a pipeline (Fig 19.53) The volumetric flowrate of fluid Q in the pipeline is given by Eq (19.66):
(19.66)
where p1 and p2 are the pressures on each side of the orifice plate, ρ is the density of the fluid upstream
of the orifice plate, d is the diameter of the hole in the orifice plate, and β is the diameter ratio d/D where D is the upstream internal pipe diameter The two empirically determined correction factors are
C the discharge coefficient, and ε the expansibility factor C is affected by changes in the diameter ratio,
Reynolds number, pipe roughness, the sharpness of the leading edge of the orifice, and the points at which the differential pressure across the plate are measured However, for a fixed geometry it has been
shown that C is only dependent on the Reynolds number and so this coefficient can be determined for
a particular application ε is used to account for the compressibility of the fluid being monitored Both
C and ε can be determined from equations and tables in a number of internationally recognized
TABLE 19.3 Main Categories of Closed Conduit Flowmeter Type 1—differential pressure flowmeters
Sharp edged orifice plate, chord orifice plate, eccentric orifice plate, Venturi, nozzle, Pitot tube, elbow, wedge, V-cone, Dall tube, Elliot-Nathan flow tube, Epiflo
Type 2—variable area flowmeters Rotameter, orifice and tapered plug, cylinder and piston, target, variable aperture
Type 3—positive displacement flowmeters Sliding vane, tri-rotor, bi-rotor, piston, oval gear, nutating-disc, roots, CVM, diaphragm, wet gas
Type 4—turbine flowmeters Axial turbine, dual-rotor axial turbine, cylindrical rotor, impeller, Pelton wheel, Hoverflo, propeller
Type 5—oscillatory flowmeters Vortex shedding, swirlmeter, fluidic Type 6—electromagnetic flowmeters
AC magnetic, pulsed DC magnetic, insertion Type 7—ultrasonic flowmeters
Doppler, single path transit-time, multi-path transit-time, cross-correlation, drift
Type 8—mass flowmeters Coriolis, thermal Type 9—miscellaneous flowmeters Laser anemometer, hot-wire anemometers, tracer dilution, nuclear magnetic resonance
1 – b4
- e p
4
- d2 2 p ( 1– p2)
r
-=
Trang 3
Noncontact Ranging Sensors
Sensors that measure the actual distance to a target of interest with no direct physical contact are referred
to as noncontact ranging sensors There are at least seven different types of ranging techniques employed
in various implementations of such distance measuring devices (Everett et al., 1992):
• Triangulation
• Time of flight (pulsed)
• Phase-shift measurement (CW)
• Frequency modulation (CW)
• Interferometry
• Swept focus
• Return signal intensity Noncontact ranging sensors can be broadly classified as either active (radiating some form of energy into the field of regard) or passive (relying on energy emitted by the various objects in the scene under surveillance) The commonly used terms radar (radio direction and ranging), sonar (sound navigation and ranging), and lidar (light direction and ranging) refer to active methodologies that can be based on any of several of the above ranging techniques For example, radar is usually implemented using time-of-flight, phase-shift measurement, or frequency modulation Sonar typically is based on time-of-flight ranging, since the speed of sound is slow enough to be easily measured with fairly inexpensive electronics
Lidar generally refers to laser-based schemes using time-of-flight or phase-shift measurement
For any such active (reflective) sensors, effective detection range is dependent not only on emitted power levels, but also the following target characteristics:
• Cross-sectional area—determines how much of the emitted energy strikes the target
• Reflectivity—determines how much of the incident energy is reflected versus absorbed or passed through
• Directivity—determines how the reflected energy is redistributed (i.e., scattered versus focused) Many noncontact sensors operate based on the physics of wave propagation A wave is emitted at a reference point, and the range is determined by measuring either the propagation time from reference to target, or the decrease of intensity as the wave travels to the target and returns to the reference Propagation time is measured using time-of-flight or frequency modulation methods
Ranging by Time-of-Flight (TOF)
Time-of-flight (TOF) is illustrated in Figs 19.61 and 19.62 A gated wave (a burst of a few cycles) is emitted, bounced back from the target, and detected at the receiver located near the emitter The emitter and receiver may physically be both one sensor The receiver may also be mounted on the target The TOF is the time elapsed from the beginning of the burst to the beginning of the return signal The distance is defined as d= c ⋅ TOF/2 when emitter and receiver are at the same location, or d=c ⋅ TOF when the receiver is attached to the target The accuracy is usually 1/4 of the wavelength when detecting the return signal, as its magnitude reaches a threshold limit Gain is automatically increased with distance to maintain accuracy Accuracy may be improved by detecting the maximum amplitude,
as shown in Fig.19.63 This makes detecting the time of arrival of the wave less dependent on the amplitude
of the signal Ultrasonic, RF, or optical energy sources are typically employed; the relevant parameters
FIGURE 19.61 A wave is emitted and bounced from a target object The distance d is determined from the speed of travel of the wave, c, and the time-of-flight, TOF
as d= ( 1/2) · c · TOF.
Emitter/Receiver Target
d
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