Venturi Tube Design

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Venturi Tube Design Venturi Tube Design All rights reserved to thepetrostreet.com Prepared by: Muhammad SaqibJawed

Transcript of Venturi Tube Design

Page 1: Venturi Tube Design

Venturi Tube DesignVenturi Tube Design

All rights reserved to thepetrostreet.com

Prepared by: Muhammad Saqib Jawed

Page 2: Venturi Tube Design

Contents

• Introduction

• Bernoulli's Equation

• Pressure Differential Head Meters

• Venturi Flow meter

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Introduction

• Why measure Flow??

• Flow

• Types of Flow

• Velocity Profile

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Why Measure Flow

Billing Purposes

• To determine total quantity of fluid for billing purposes

• Flow meters used to ensure process is operating satisfactorily

Monitor the Process

• Flow meters used to ensure process is operating satisfactorily

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Why Measure Flow

Improve the Process

• Heat & Material balance calculations

Improve the Process

Monitor a Safety Parameter

• Flow meters used to ensure process and equipment safety

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Liquid or Gas in MotionLiquid or Gas in Motion

Flow

Liquid or Gas in MotionLiquid or Gas in Motion

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Types of Flow

Volumetric Flow Rate

Volume of fluid which passes through a given surface per unit time

Mass Flow Rate

Mass of a substance which passes through a given surface per unit time

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Velocity Profile

Laminar Flow Regime

• Molecules move straight down pipe

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Turbulent Flow Regime

Velocity Profile

• Molecules migrate throughout pipe

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Transitional Flow Regime

Velocity Profile

• Molecules exhibit both turbulent and laminar behavior

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Velocity Profile

• Many flow meters require a good velocity profile to operate accurately

• A distorted velocity profile can introduce significant errors into the measurement

of most flow metersof most flow meters

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Dynamic Pressure + Static Pressure + Weight

Bernoulli’s Equation

1/2ρV² + P + z = Constant

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Examples

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Pressure Differential Head Meters

Orifice

PDH

Primary Element Flow Nozzle

Venturi

Secondary Element ΔP Transmitter

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� To avoid pressure loss, venturi tube is

Venturi Tube

� To avoid pressure loss, venturi tube is

used

� curved, stream lined section, long and

gradually expanding down stream

section

� 60% more flow than orifice

� Fluid can flow with much higher velocity

without turbulence

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Flow through Venturi follows four paths Key

1 Conical divergent E

2 Cylindrical throat C

3 Conical convergent B

4 Entrance cylinder A

Flow Pattern

4 Entrance cylinder A

5 Connecting planes

a 7° ≤ f ≤ 15°

b Flow direction

Figure - 1

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Types of Venturi Tubes

Basis of different types are different methods of manufacturing of internal surface of

the entrance cone and the profile at intersection of cone and the throat. Three types the entrance cone and the profile at intersection of cone and the throat. Three types

of venturi tubes are

1. Cast

2. Machined

3. Rough welded sheet iron

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Venturi-“As Cast” Convergent Section

• Fabricated by casting in a sand mould, or by other methods leave a finish on the

convergent section surface similar to that produced by sand casting. The throat is convergent section surface similar to that produced by sand casting. The throat is

machined and the junctions b/w cylinders & cones are rounded

• Use for pipe dia b/w 100 & 800 mm with beta ratio b/w 0.3 and 0.75 inclusive

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Venturi - “Machined” Convergent Section

• Fabricated as previous type but with machined convergent section as the throat

and the entrance cylinder. The junctions b/w the cylinders & cones may or may and the entrance cylinder. The junctions b/w the cylinders & cones may or may

not be rounded

• Use for pipe dia b/w 50 & 250 mm with beta ratio b/w 0.4 and 0.75 inclusive

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Venturi - “Rough-Welded Sheet-Iron”

Convergent Section

• Fabricated by welding. For larger size it may not be machined but in smaller sizes

the throat is machined

• Use for pipe dia b/w 200 & 1200 mm with beta ratio b/w 0.4 and 0.70 inclusive

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Flow Measurement

εεεε

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Venturi Tube sizing

• Several standards are available for sizing of venturi Tubes like ISO, ASME, DIN

and UNI

• This presentation shall cover ISO standard i.e. Measurement of fluid flow by

means of pressure differential devices inserted in circular cross-section conduits

running full

Note: Figure 1 to be used as reference

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Entrance Cylinder, A

• The minimum cylinder length, measured from the plane containing the

intersection of the cone frustum B with the cylinder A, may vary for each type of intersection of the cone frustum B with the cylinder A, may vary for each type of

venturi tube

However, it is recommended to choose the length equal to dia “D”

• No diameter along the entrance cylinder shall differ by more than 0.4% from the

value of the mean diameter

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Conical Convergent, B

• The angle / overall length of convergent section B shall be 21°±1°and 2.7(D-d)

respectively

• Section B blended with section A by a curvature of radius R1 which depends on

venturi type

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Cylindrical Throat, C

• The length of throat C shall be equal to d±0.03d whatever the type of venturi tube

• Throat C connected to section B and to the section E by radii of curvature R2 and

R3 respectively and vary for each type of venturi

• No diameter along the throat shall differ by more than 0.1% from the value of the

mean diameter

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Conical Divergent, E

• Section E will be conical having angle, φ b/w 7° and 15°. Recommended chosen

angle is in b/w of 7° and 8°. Its smallest diameter shall not be less than the throat

diameter

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General

• Venturi called “truncated” when outlet dia of section E is less than the D and “not

truncated” when outlet dia is equal to D truncated” when outlet dia is equal to D

• The portion E may be truncated by 35% of its length without significantly

modifying pressure loss of device or its discharge co-efficient

• The roughness criterion, Ra shall always be less than 10-4d

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Characteristics of “as cast”

• The minimum section A length shall be equal to smaller of the following two

values:

– D or – D or

– 0.25D + 250 mm

• R1 shall be 1.375D±0.275D

• R2 shall be 3.625D ±0.125d

• Length of section C shall not be less than dl3. Furthermore, length of cylindrical

part b/w end of R2 & plane of pressure tapping, as well as length of cylindrical part

b/w plane of throat pressure tapping & beginning of the joining curvature R3, shall

no be less than dl6 no be less than dl6

• R3 shall lie b/w 5d & 15d. However, value closer to 10d is recommended

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Characteristics of “machined”

• The minimum section A length shall be equal to D

• R / R & R shall be less than 0.25D / 0.25d and 0.25d respectively. Preferably • R1 / R2 & R3 shall be less than 0.25D / 0.25d and 0.25d respectively. Preferably

equal to zero

• Length of cylindrical throat b/w end of R2 & the plane of throat pressure tapping

shall no be less than 0.25d

• Length of cylindrical throat b/w throat pressure tapping and beginning of R3 shall

no be less than 0.3dno be less than 0.3d

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Characteristics of “Rough Welded Sheet-

Iron”

• The minimum section A length shall be equal to D

• Being the welded one there will be no joining curvatures b/w cylinder A and

section B and so on

• Roughness criterion shall be 5x10-4D

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Pressure Tappings

• The u/s & throat tapping's shall be made under piezometer rings or a “triple-T”

arrangementsarrangements

• Tapping dia shall be in b/w of 4-10 mm if d ≥ 33.3 mm

• Tapping dia shall be in b/w of 0.1d-0.13d for throat and 0.1d-0.1D for upstream if

d < 33.3 mm

• Tapping shall be cylindrical over a length at least 2.5 times the internal tapping

dia, measured from inner pipeline wall

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Pressure Tapings Spacing

• The spacing b/w upstream pressure tapping on entrance cylinder & plane of

intersection b/w entrance cylinder A & convergent section B shall be, for:

As castAs cast

0.5D±0.25D for 100 mm < D < 150 mm

Machined and rough welded sheet-iron:

0.5D±0.05D

• For all types of venturi, spacing b/w plane containing the axes of the points of

break-through of throat pressure tapping & intersection of section B and C shall be

0.5d±0.02d

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Discharge Co-efficients “C”

Simultaneous use of extreme values for D, β, ReD

shall be avoided which in turn

increase the uncertainties as the effects of ReD, RalD and β on C are not yet

sufficiently known

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“C” of “As cast”

“As cast” can only be used when

100 mm ≤ D ≤ 800 mm100 mm ≤ D ≤ 800 mm

0.3 ≤ β ≤ 0.75

2 x105 ≤ ReD

≤ 2 x 106

Under these conditions value of C is 0.984

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“C” of “Machined”

“Machined” can only be used when

50 mm ≤ D ≤ 250 mm50 mm ≤ D ≤ 250 mm

0.4 ≤ β ≤ 0.75

2 x105 ≤ ReD

≤ 1 x 106

Under these conditions value of C is 0.995

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“C” of “Rough Welded Sheet-Iron”

“rough welded sheet iron Machined” can only be used when

200 mm ≤ D ≤ 1200 mm200 mm ≤ D ≤ 1200 mm

0.4 ≤ β ≤ 0.7

2 x105 ≤ ReD

≤ 2 x 106

Under these conditions value of C is 0.985

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Uncertainty of “C”

The relative uncertainty of C is given below:

For “As cast” is 0.7%For “As cast” is 0.7%

For “machined" is 1.0%

For “rough welded iron-sheet” is 1.5%

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Expansibility [Expansion] Factor, ε

“ε” results are only known for air, steam and natural gas but formula could be used for

which isentropic exponent is known

Equation is applicable only for the values of D, β, ReD

defined earlier and if p2/p1≥0.075

εεεε

Equation is applicable only for the values of D, β, ReD

defined earlier and if p2/p1≥0.075

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Table of Expansibility [Expansion] Factor

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Pressure Loss

• Pressure loss caused by venturi tube is determined prior & subsequent installation

of venturi in a pipe through which there is given flow

• Tapping locations with & without venturi in a pipe, Figure 2 to be followed

Where

Δp΄ & Δp΄΄ are difference in pressure prior to and a�er venturi installation

respectively

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Pressure Loss

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Straight lengths for Installations

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Symbols & Subscripts

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Symbols & Subscripts

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For more details & information, please contact us.

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