Flow Measurement free study notes for Diploma / BTech.

1. Introduction

Flow measurement is the process of determining the quantity of fluid (liquid or gas) passing through a cross-section per unit time.

Types of Flow Rate

• Volumetric Flow Rate (Q)

$$Q = A \times V$$Q=A×V

• Mass Flow Rate (\dot{m})

$$\dot{m} = \rho Q$$m˙=ρQ

Where:

• $A$A = Cross-sectional area
• $V$V = Velocity
• $\rho$ρ = Density

2. Classification of Flow Measurement Methods

(a) Direct Methods

• Measure flow by collecting fluid over time
• Example: Measuring tank

(b) Indirect Methods

• Based on physical principles:
  • Pressure difference
  • Velocity measurement
  • Area variation

3. Measurement Using Differential Pressure Devices

These devices work on Bernoulli’s principle.

(A) Venturimeter

7

Construction

• Converging section
• Throat
• Diverging section

Working Principle

• Fluid velocity increases at throat
• Pressure decreases
• Difference in pressure is used to calculate discharge

Discharge Equation

$$Q = C_d A_2 \sqrt{\frac{2(P_1 – P_2)}{\rho (1 – (A_2/A_1)^2)}}$$Q=Cd​A2​ρ(1−(A2​/A1​)2)2(P1​−P2​)​​

Advantages

• High accuracy
• Low energy loss

Disadvantages

• Expensive
• Bulky

(B) Orifice Meter

6

Construction

• Flat plate with sharp-edged hole

Working

• Flow constriction creates pressure drop
• Used to measure flow rate

Advantages

• Simple and cheap
• Easy installation

Disadvantages

• High energy loss
• Less accurate than venturimeter

(C) Flow Nozzle

6

Features

• Smooth converging section
• No diverging part

Applications

• Steam flow measurement
• High-velocity fluids

4. Velocity-Based Flow Measurement

(A) Pitot Tube

7

Principle

Based on conversion of kinetic energy into pressure energy.

Velocity Equation

$$V = \sqrt{\frac{2(P_t – P_s)}{\rho}}$$V=ρ2(Pt​−Ps​)​​

Where:

• $P_t$Pt​ = Total pressure
• $P_s$Ps​ = Static pressure

Applications

• Aircraft speed measurement
• Pipe flow velocity

5. Variable Area Flow Meter

Rotameter

6

Working

• Fluid flows upward through tapered tube
• Float rises until equilibrium is reached
• Height of float indicates flow rate

Advantages

• Simple
• No external power needed

Disadvantages

• Only for vertical installation

6. Positive Displacement Flow Meters

• Measure actual volume of fluid passing through
• Examples:
  • Gear meter
  • Piston meter

Features

• High accuracy
• Used in oil and fuel measurement

7. Modern Flow Measurement Devices

(A) Electromagnetic Flow Meter

• Based on Faraday’s law of electromagnetic induction
• Used for conductive fluids

(B) Ultrasonic Flow Meter

• Uses sound waves
• Types:
  • Transit-time
  • Doppler

(C) Turbine Flow Meter

• Fluid rotates turbine blades
• Speed proportional to flow rate

8. Selection Criteria for Flow Meters

• Type of fluid (liquid/gas)
• Accuracy required
• Cost
• Pressure loss allowed
• Maintenance requirements

9. Applications of Flow Measurement

• Water supply systems
• Oil and gas industries
• Power plants
• Chemical processing
• HVAC systems

10. Key Points Summary

• Flow measurement is essential for control and monitoring
• Venturimeter is most accurate among DP devices
• Orifice meter is most economical
• Rotameter is simple and widely used
• Modern meters provide high precision and automation

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