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FLOW MEASUREMENT DEVICES

FLOW MEASUREMENT DEVICES. VENTURIMETER. VENTURIMETER. INTRODUCTION. In this meter the fluid is accelerated on expense of static pressure by its passage through a converging cone of angle 15-20 degree.

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FLOW MEASUREMENT DEVICES

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  1. FLOW MEASUREMENT DEVICES

  2. VENTURIMETER

  3. VENTURIMETER

  4. INTRODUCTION • In this meter the fluid is accelerated on expense of static pressure by its passage through a converging cone of angle 15-20 degree. • The pressure difference between the upstream end of the converging cone and the throat (minimum area section) is measured. • Throat length = throat diameter • The fluid is then retarded in a cone of smaller angle 5-7 degree in which large proportion of kinetic energy is converted back to pressure energy.

  5. VENTURIMETER

  6. PRINCIPLE USED Bernoulli's Principle same as orifice meter P↓ = V ↑

  7. DISADVANTAGES • Highly expensive in installation & replacement • Space requirement is more

  8. ADVANTAGES • High pressure recovery • Less possibility of becoming clogged due to smooth surface • Suited for large water flow • High coefficient of discharge

  9. VENTURIMETER SECTIONS • CONVERGING CONICAL SECTION • THROAT • DIVERGING SECTION

  10. CONVERGING CONICAL SECTION • Angle of converging conical section is about 21 ± 2 degree. • Velocity ↑ and static pressure ↓

  11. THROAT • Cylindrical section of Minimum Area • At this section V = MAX and P = MIN • Throat diameter = 0.25 – 0.5 of inlet diameter

  12. DIVERGING SECTION • Change of stream area back to the entrance area • Kinetic energy is converted into pressure rise in this section • Angle of diverging section is about 5-7 degree • Angle is kept less to having least tendency of flow separation from boundary of this section • With small angle length and cost ↑

  13. INTERESTING REASONING QUES. Why mercury is preferred as manometric fluid? • Good conductor of heat • Shining quality provide clear visibility • Mercury does not evaporate easily • Mercury's freezing point is much lower(-38.72 deg C) • More sensitive than water to the changes in the atmospheric pressure • Mercury is relatively denser than water • Does not stick on the walls of the tube of manometer, resulting in accurate measurements • Large temperature expansion coefficient

  14. NUMERICAL PROBLEMS • In a 100 mm diameter horizontal pipe, a venturimeter of 0.5 contraction ratio has been fitted. The head of water on the meter when there is no flow is 3 m (gauge). Find the rate of flow for which the throat pressure will be 2 m of water absolute. Discharge coefficient for the meter is 0.97.

  15. NUMERICAL PROBLEMS • A venturimeter is to be fitted in a pipe of 20 cm diameter where pressure head is 7.6 m of flowing fluid and the maximum flow is 8100 litres per minute. Find the least diameter of the throat to ensure that the pressure head does not become negative. Presume Cd = 0.96 What is the quantity of liquid flowing through it when a differential manometer shows a steady deflection of 20 cm of mercury? (relative density of mercury is 13.6)

  16. NUMERICAL PROBLEMS • A 30 cm x 15 cm venturimeter is provided in a vertical pipeline carrying oil of specific gravity 0.9, the flow being upwards. The difference in elevations of the throat section and entrance section of the venturimeter is 30 cm. the differential U-tube mercury manometer shows a gauge deflection of 25 cm. calculate (i) the discharge of the oil, and (ii) the pressure difference b/w the entrance & throat section. Take the discharge coefficient as 0.98 and the specific gravity of mercury as 13.6.

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