FLOW MEASUREMENT

(ULTRASONIC FLOW MEASUREMENT)

Name: Pritesh ParikhClass: 5th , BDept: I.C

Flow Through A Pipe

Real WorldIdealized

Pipe Velocity Profile

VV

Flow Profile Correlation

Velocity profile is a predictable function of Reynolds number. Fluids with the same Reynolds number will have similar velocity profiles.

SteamWaterHeavy Crude

V

Characterization of Fluid Flow

Types of Flow RE < 2100

RE >3000

Transitional

Turbulent

Laminar

Volumetric Flowrate (Q)

Volume = Area x LengthVolume Flow = Area x

Velocity

VA

Q

Mass Flowrate (m)

Where m = Mass FlowQ = Volume Flow = Fluid Density

= AV

Mass = Volume x Density

Mass Flow m = Qp

VA

m

Q=VA

Flowmeter Performance

Accuracy Repeatability Linearity Rangeability

Types of Ultrasonic Flow Meters

Doppler processTransit time difference process

Doppler flowmeters

Doppler flow meters operate similarly to the radar speed traps used on the road. An emitter sends ultrasonic waves at frequency f1 (approx. 1 - 5 MHz) at angle a into the flowing product. The ultrasonic waves strike particles moving through the sound field at velocity Vp. The wavelength of the emitted wave at frequency f1 amounts to: λ1= C/f1

Due to its rate of motion Vp, the particle moving away from the emitter

‘sees’ the wavelength: λp= (C-Vp COS α ) / f1

In turn, the receiver now ‘sees’ the reflected frequency out of line because the reflecting particle is moving further away all the time, and the wavelength changes

as follows: λp= (C- 2Vp COS α ) / f1

For Vp « c we obtain: This difference in frequency is therefore a linear measure of the rate of motion o

PRINCIPLE

The fluid velocity can be expressed asv = c (f2 - f 1 ) / 2 f1 cosΦ         (1)wheref2 = received frequencyf1 = transmission frequencyv = fluid flow velocityΦ = the relative angle between the transmitted ultrasonic beam and the fluid flowc = the velocity of sound in the fluid

Doppler meters may be used where other meters don't work. This might be liquid slurries, aerated liquids or liquids with some small or large amount of suspended solids. The advantages can be summarized to :Obstruct less flowCan be installed outside the pipes(clamp on version)Low flow cut offCorrosion resistantRelative low power consumption

Advantages of Doppler flowmeter

•Needs a sufficient number of reflecting particles in the medium on a continuous basis.

•The particles must be large enough to provide sufficiently good reflections (> ./4). •The sound velocity of the particulate material must be distinctly different from that of the liquid. • The sound velocity of the medium is directly included in the measurement result. • The particle velocity often differs noticeably from the velocity of the liquid. •Usually, the ultrasonic field extends only into the peripheral flow. That is why indication is heavily dependent on the flow profile. • The velocity needs to be far higher than the critical velocity at which particles settle. •Very long unimpeded inlet runs (20 x D) are needed to allow conclusions to be drawn from the flow rate.

Disadvantages of Doppler flowmeter

1. Medical applications (measurement of the blood flow)

Absolute accuracy is not a requirement. Only good dynamic performance is required which reproduces blood pulsation in the veins and arteries in great detail for diagnosis (similar to an ECG). The sensor pad is simply applied to the skin (with coupling gel) avoiding bleeding.

2. Measurement of the flow of slurries (e.g. iron ore)

By its very nature, particle concentration is high. The sound velocity of the particles also differs sufficiently from that of the carrier medium. The flowmeter is often used to provide a signal when the velocity drops below the critical level. The penetration depth of the ultrasonic beam, which depends on the concentration, and also the flow profile tend to cause considerable errors of measurement.

Application of Doppler flow meter

Transit time difference flowmeter

To diagonally canoe across a river with the current flow takes less time than against the current flow. The stronger the current,then faster the crossing than against it.

This difference between the travel times with and against the current is directly proportional to the flow velocity of the river.

This effect is exploited by ultrasonic flowmeters to determine flow velocity and flow rate. Electro-Acoustic converters(piezo’s) emit and receive short electronic pulses through the product flowing in the tube

V = L/t ; (where L= D/sinα )

V = {D(Tba – Tab )}/ {sin(2α)(Tba .Tab)}

Q= V*A = [D3π (Tba - Tab)] / [4 sin(2α) . (Tab T ba )]

Placement Methods

** Z-method also called direct method

Use reflect mode whenever possible, however for short pipe run and plastic pipes use direct method For reflect method, the spacing between transducers is to be adjusted according to pipe wall thickness and hence procedure becomes complex.

BENEFITS WITH ULTRASONIC FLOW METERS•Obstruction less flow•Pressure drop equal to an equivalent length of straight pipe•Unaffected by changes in temperature, density or viscosity•Bi-directional flow capability•Corrosion-resistant•Accuracy about 1% of flow rate•Relative low power consumption

PRECAUTIONS to be taken The pipe wal should be cleaned and be rust free.Pipe is full.Fluid media conduct sonic energy.Avoid installing transducer anywhere near a downstream of throttling valve , mixing tank, etc.TWO TYPES OF TRANSITION FLOWMETERS

Clamp On methodIn-Line method

NOTE: For Inline methodThe more the no of measuring paths= Higher the

accuracy at varying “Reynolds number”

CLAMP ON TYPE

IN-LINE TYPE

Limitations with Ultrasonic FlowmetersThe operating principle for the ultrasonic flowmeter requires reliability high frequency sound transmitted across the pipe. Liquid slurries with excess solids or with entrained gases may block the ultrasonic pulses.Ultrasonic flowmeters are not recommended for primary sludge, mixed liquor, aerobically digested sludge, dissolved air flotation thickened sludge and its liquid phase, septic sludge and activated carbon sludge.Liquids with entrained gases cannot be measured reliably.

Thank You