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