A fluid is a substance that continually deforms (flows) under an applied shear stress.  Fluids are a subset of the phases of matter and include liquids, gases

1. Dhaneshwar Kumar Vishwakarma
Kailash Institute of Pharmacy and Management
GIDA, Gorakhpur
Flow of Fluids
VENTURI METER and PITOT TUBE
PHARMACEUTICAL ENGINEERING
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2. VENTURI METER (variable head meter)
 It consists of two tapered sections in the pipeline with gradual constriction
(throat) at its centre.
 No power loss is observed (absences of eddy).
 It is placed between long straight pipe (other fitting will not alter).
 Upstream is cone shorter than down stream.
 At point B the section of has minimum diameter.
 Point B is called the ‘throat’.
 Pressure difference at A and B
is measured by a manometer.
U
A B
. .
Throat
Manometer
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3. Working-
 When fluid stream is allowed to pass through gradual narrow constriction then
 Velocity of fluid at increased throat’ due to constriction.
 Create Pressure difference.
 Difference in pressure heads may reads from manometer.
 Equation for venturi meter
.....(1)
H
g
U
U A
B 

 2
2
2
U
A B
. .
Throat
Manometer

4. Derivation
 Bernoulli’s equation -
)
1
..(
..........
..........
2
2
2
2
B
B
B
B
A
A
A
A
P
g
U
X
w
F
P
g
U
X










5. ……….(2)
Equation (2) may be written as:
........................(3)
Since, PA – PB = P,
Principle of statics = H
 Equation (3) can be written as
.........................................(4)
)
(
2
2
2
B
A
A
B P
P
g
U
U 



P

H
g
U
U A
B 

 2
2
2
B
B
B
A
A
A P
g
U
P
g
U





2
2
2
2
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6.  Point B was chosen at the throat.
 Since there are no losses due to eddies
 Equation (4) then becomes:
....(5)
Where UB= Velocity at throat of venturi (m/s)
UA= Velocity at point A of venturi (m/s)
Value of coefficient CV = 0.98.
 If diameter of throat is 1/5th of pipe diameter or less - then UA is small
compared to U
---------------(6)
H= difference in heads
 The pressure difference P between A and B is read directly from the
manometer.
H
g
C
U
U V
A
B 

 2
2
2
H
g
C
UB 
 2
0

7. Advantage
 It can be used for both liquid and gas.
 It is very simple device
 It can be easily installed i.e. cost of installation is less.
Disadvantage- Expensive and occupy more space.
 Ratio of throat diameter to pipe diameter cannot be changed.
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8. COMPARISON BETWEEN ORIFICEMETER AND VENTURIMETER:
Orifice meter Venturimeter
1. Cheap
2. Installation is easy.
3. The power loss is more in long run.
4. They are best used for testing
purposes
5. Handle less fluid
6. Higher Reynolds number(30000)
7. Installing a new orifice plate is
simple.
8. Greater flexibility
1. Expensive
2. Installation is costly.
3. Power loss is less in long run
4. Used for permanent/online installation.
5. Handle more fluid
6. Lower Reynolds number(5000)
7. Throat cannot be change
8. Not flexible
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9. PITOT TUBE (insertion meter)
 Invented by the French engineer Henri Pitot in the early 18th century
 It consist sensing element with small constriction.
 Sensing element is placed at the centre of stream.
 One tube ‘A’ is opposite to the direction of flow of fluid.
 The second tube ‘B’ is connected parallel to direction of flow.
 Tow tubes are connected to legs of manometer

10. Working-
 When fluid stream is allowed to pass through gradual narrow constriction then
 Velocity of fluid at increased narrow constriction.
 Create Pressure difference.
 Difference in pressure heads may reads from manometer.
 The difference of liquid in two arms of the manometer is the reading.
 The tube in the ‘A’ hand
 Measure the pressure head (HA)
 Measure the velocity head
 The ‘B’ hand measures only pressure head (HB)
 Actual velocity given by
 Where Cv = coefficient of pitot tube








g
U A
2
2
Hp
2
2

 g
Cv
U

11.  Note-
 Orifice meter or venturimeter measured average velocity of fluid.
 Pitot tube measured velocity of only one point (i.e. at the center of the pipe).
Advantage –it measure the velocity at one point
Aircraft use pitot tubes to measure airspeed
Disadvantage
 It does not give the average velocity directly.
 When velocity of gases are measured the reading are extremely small.
 Eddies within the pressure tube disturb the reading

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