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ابزاردقيق هاي سنجش
دبي
(Flow meters)
جعفر قيصري
86ارديبهشت
Differential Pressure method
Bernoulli’s Principle
1 2
V1 V2D1 D2Dt
Assumptions:• Steady flow• Flow along a streamline
• Incompressible flow• No friction
cteg
Pmgmghmv
2
2
1
Bernoulli’s Equation
1 2
V1 V2D1 D2Dt
g
PmgmghmV
g
PmgmghmV
2
22
21
12
1 2
1
2
1
Assume: h1=h2
)(2
212
22
1 PPVV
Bernoulli’s Equation
2211 AVAV
1 2
V1 V2D1 D2Dt
]1[
22
12
212
AA
ppV
221121 AVAVmm
)(2
212
22
1 PPVV
21212
12
222ltheoretica 2
]1[pp
AA
AAVm
21212
1
actual 2
]1[
.pp
AA
ACm
t
td
Cd: Discharge Coefficient
Let = Dt /D1, then (At / A1)2 = (Dt /D1)4 = 4
21214actual 2]1[
ppCA
m t
Bernoulli’s Equation Modification
Flow Conditioners
Orifice Plate
Front view of orifice plate
Orifice Plate
74 1010 and 75.02.0For eR
75.0
5.281.2 71.91
184.00312.05959.0e
dR
C
21214actual 2]1[
ppCA
m t
The Flow Nozzle
5.0
5.053.69975.0
ed
RC
74 1010 and 75.02.0For eR
21214actual 2]1[
ppCA
m t
Venturi Tube
Discharge coefficients range (Cd) from 0.98 to 0.995 at high Reynolds numbers.
21214actual 2]1[
ppCA
m t
Flow Meter Type Head Loss Cost
D1 DtOrifice High Low
D1 DtVenturi Low High
D1 DtFlow Nozzle Intermediate intermediate
Differential Pressure Techniques Comparison
Pitot Flowmeter
0
02
)()(
2
21
2212
VPitottheIn
g
VV
g
PP
Bernoulli Equation:
2112 2
1VPP
12
1
2or
PPV
21 actual 2.. ppACm td
P1,V1Stagnation Point V2=0
1 2P2
Samples of Pitot Flowmeters
Electromagnetic Flowmeters
• The most common flowmeter, apart from the mechanical flow meters, is electromagnetic flowmeter.
• Electromagnetic flowmeters obtain the flow velocity by measuring the changes of induced voltage of the conductive fluid passing across a controlled magnetic field.
magnet
electrodes
conductive fluid
measure voltage here
Principle of Electromagnetic Flowmeter
Types of Electromagnetic Flowmeters
• Insertion Model• Inline Model
A typical magnetic flowmeter places electric coils around (inline model) or near (insertion model) the pipe of the flow to be measured and sets up a pair of electrodes across the pipe wall (inline model) or at the tip of the flowmeter (insertion model)
Insertion Electromagnetic Flowmeter
• The operation principle of insertion magnetic flowmeters
Coils to generate magnetic field
Inline Electromagnetic Flowmeters
• The operation principle of inline magnetic flowmeter
Coils to generate magnetic field
Electromagnetic Flowmeters Equations
• Applying faraday’s law to magnetic flowmeters with N turns we have:
where D is the distance between the two electrodes (the length of conductor), and V is the flow velocity.
• If we combine all fixed parameters N, B, and D into a single factor , we have
• It is clear that the voltage developed is proportional to the flow velocity
K
EV
NBVDDdt
dlNB
dt
dANBE
Electromagnetic Flowmeters
• Advantages:– Minimum obstruction in the
flow path yields minimum pressure drop
– Low maintenance cost because of no moving parts
– High linearity – Two and multi beam models
have higher accuracy than other comparably priced flowmeters
– Can be used in hazardous environments or measure
corrosive or slurry fluid flow
• Limitations:
– Requires electrical conductivity of fluid higher than 3 µS/cm in most cases
– Zero drifting at no/low flow (may be avoided by low flow cut-off; new designs improve on this issue)
• F-3000 Series electromagnetic flowmeters from Onicon Inc.
• Typical applications include monitoring cooling water, heating water, condenser water, and domestic water.
Electromagnetic Flowmeter: An example
Ultrasonic flowmeters
• Historical review– Ultrasonic flowmeters have been the center of
attention within the natural gas industry for the last decade. To date, current commercial devices have been developed and used in various industrial and medical applications
• Types of ultrasonic flowmeters:– Transit-time ultrasonic flowmeters– Doppler effect ultrasonic flowmeters
Why using ultrasonic types?
• Wide range of applications such as blood, pure water, wash water, sewage, process liquids, oils, and other light homogeneous liquids
• Clamp-on types measure flow through the pipe without any wetted parts, ensuring that corrosion and other effects from the fluid will not deteriorate the sensors.
• Clamp-on types simplify and speed up meter installation and minimize maintenance.
• Ultrasonic flowmeters may be portable.
• Measurement accuracy can be in the range of 1% of flow rate, and speed of response can be as fast as 1 s.
Transit-Time Ultrasonic Flowmeters: Basic Concept
Transit-Time Ultrasonic Flowmeters: Independent of C
Transit-Time Ultrasonic Flowmeters: Transmitter and Receiver at the One Side
Transit-Time Ultrasonic Flowmeters: Operation Principle
Transit Time Ultrasonic flowmeters:A Typical Portable Flowmeter
A Portable Ultrasonic Flowmeter with Bluetooth® Wireless from SIERRA INSTEUMENTS INC.
The Innova -Sonic™ Portable ultrasonic flowmeter is a state of the art universal transit-time flowmeter incorporating the latest developments in digital signal processing.
Sophisticated electronics coupled with powerful ultrasonic transducers deliver highly accurate flow measurement for liquids in full pipes.
Ultrasonic Flowmeters: Doppler Effect
• To use the Doppler effect to measure flow in a pipe, one transducer transmits an ultrasonic beam of ~0.5 MHz into the flow stream
• Liquid flowing through the pipe must contain sonically reflective materials such as solid particles or entrained air bubbles
Ultrasonic Flowmeters: Doppler Effect
Ultrasonic flowmeters
Measure flow from outside a pipe. The EESIFLO 3000 is ideal for "difficult liquids" that would damage regular flow meters
A typical Doppler effect flowmeter
• Limitations of Doppler effect flowmeters– Liquids to be metered must have an excess of approximately 2%
suspended solids by volume
– Liquid linear velocities must exceed 0.15 m/s– Piping material must be of a homogenous composition – Pipe wall thickness cannot be greater than 1.91 cm
Ultrasonic flowmeters
Vortex Flowmeter
• When a object (object that generates vortices) is placed in the flow path of a fluid, regular channels of vortices, called Karman vortex channels, are generated at the back of the object.
• Since the frequency of a vortex generated is linearly proportional to the flow velocity within a given range, the flow amount can be measured by counting the number of vortices.
Principle of Vortex Flowmeter
Principle of Vortex Flowmeter
Vortex Flowmeter Using Ultrasonic