Venturi Tube DesignVenturi Tube Design
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Prepared by: Muhammad Saqib Jawed
Contents
• Introduction
• Bernoulli's Equation
• Pressure Differential Head Meters
• Venturi Flow meter
Introduction
• Why measure Flow??
• Flow
• Types of Flow
• Velocity Profile
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
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
Liquid or Gas in MotionLiquid or Gas in Motion
Flow
Liquid or Gas in MotionLiquid or Gas in Motion
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
Velocity Profile
Laminar Flow Regime
• Molecules move straight down pipe
Turbulent Flow Regime
Velocity Profile
• Molecules migrate throughout pipe
Transitional Flow Regime
Velocity Profile
• Molecules exhibit both turbulent and laminar behavior
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
Dynamic Pressure + Static Pressure + Weight
Bernoulli’s Equation
1/2ρV² + P + z = Constant
Examples
Pressure Differential Head Meters
Orifice
PDH
Primary Element Flow Nozzle
Venturi
Secondary Element ΔP Transmitter
� 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
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
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
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
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
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
Flow Measurement
εεεε
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
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
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
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
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
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
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
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
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
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
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
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
“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
“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
“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
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%
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
Table of Expansibility [Expansion] Factor
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
Pressure Loss
Straight lengths for Installations
Symbols & Subscripts
Symbols & Subscripts
For more details & information, please contact us.
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