Flow Nozzle Introduction, working Principle, operation, design, advantages, disadvantages, limitations, used and applications

 Flow Nozzle

What is Flow Nozzle ?

Flow Nozzle has a smooth elliptical inlet leading to a throat section with a sharp outlet. Restriction in the fluid flow causes a pressure drop, which relates to the flow rate by applying Bernoulli’s equation. The smooth inlet of the Flow Nozzle results in a higher coefficient of discharge than most other differential meters. This higher efficiency means greater flow capacity when compared to most other differential meters of the same size.

How it works

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. When a gas/liquid accelerate through a nozzle, the velocity increase and the pressure and density decrease. The maximum velocity is achieved at the throat. After the pressure difference has been generated in the differential pressure flow meter. Flow rate can be calculated.

Flow Nozzle Principle

When a flow nozzle is placed in a pipe carrying whose rate of flow is to be measured, the flow nozzle causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure becomes a measure of flow rate.

Description of Flow Nozzle

The main parts of flow nozzle arrangement used to measure flow rate are as follows

  • A flow nozzle which is held between flanges of pipe carrying the fluid whose flow rate is being measured. The flow nozzle’s area is minimum at its throat.
  • Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (u-tube manometer, differential pressure gauge etc.,).

Operation of flow Nozzle

  • The fluid whose flow rate is to be measured enters the nozzle smoothly to the section called throat where the area is minimum.
  • Before entering the nozzle, the fluid pressure in the pipe is p1. As the fluid enters the nozzle,the fluid converges and due to this its pressure keeps on reducing until it reaches the minimum cross section area called throat. This minimum pressure p2 at the throat of the nozzle is maintained in the fluid for a small length after being discharged in the down stream also.
  • The differential pressure sensor attached between points 1 and 2 records the pressure difference (p1-p2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated.

Design of Flow Nozzle 

  • Nominal size: ≥ 2"
  • β-ratio: 0.20 ... 0.80
  • Accuracy: ≤ ±0.5 % of full scale flow rate
  • Repeatability: 0.1 % of flow rate

Advantages of flow Nozzle

  • Installation is easy and is cheaper when compared to venturi meter
  • It is very compact
  • Has high coefficient of discharge.

Disadvantages of flow Nozzle

  • Pressure recovery is low
  • Maintenance is high
  • Installation is difficult when compared to orifice flow meter.


  • It has lower pressure recovery
  • The fluid must be clean for measurement accuracy
  • Only for limited ranges of pressure and temperature
  • Require a good maintenance schedule and are high repair and maintenance meters
  • Need to be replaced after specific time
  • It is expensive as compared to orifice and is relatively difficult to install properly.


  • Water suppliers
  • Petrochemical industries
  • Oil industries
  • Natural gas industries
  • LPG suppliers
  • Refineries
  • Power industries

Applications of Flow Nozzle

  • It is used to measure flow rates of the liquid discharged into the atmosphere.
  • It is usually used in situation where suspended solids have the property of settling.
  • Is widely used for high pressure and temperature steam flows.


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    Mass flow meter


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