# Venturi tube flow rate calculator

## calculation of flow rate in Venturi tube

Venturi tube is used as flow meter in piping system and where Venturi effect is required. Due to change in diameter from Venturi flow meter inlet to Venturi tube throat mean velocity is changing while weight flow rate remains constant.

As total pressure remains mainly constant in Venturi tube, part of static pressure is transformed into dynamic pressure from inlet to throat and pressure drop is created. This is called Venturi effect. Measuring static pressure on the inlet and on the Venturi meter throat - pressure drop, with this calculator, flow rate can be calculated, based on the energy conservation law and Bernoulli theorem.

For calculation of flow rate through Venturi tube in this calculator coefficient of discharge is calculated, as well as values of Reynolds number and mean velocity on the inlet and the throat of Venturi tube.

Calculator can also be used for Venturi effect analysis which is exploited in many appliances like Venturi flow meters, Venturi vacuum pump, gas burners, wine aerator, aspirators, grills etc. Limiting factor for Venturi effect is when flow reaches chocked flow and mean velocity is close to local speed of sound.

Flow through Venturi tube calculator can be used for both liquids and gases. Calculator is applicable for subsonic flow and single phase fluid. Gas is considered as compressible and ideal. For gas flow, pressure ratio p2/p1 must be equal or greater than 0.75. Calculator is not for pulsating flow.

Calculation of gas flow with higher pressure drop and flow close or equal to choked flow conditions gas discharge calculator should be used.

Limits of use for Venturi tube with an "as cast" convergent section:

• 100 mm < D < 800 mm
• 0.3 < β < 0.75
• 2x105 < ReD < 2x106

Limits of use for Venturi tube with a rough welded sheet iron convergent section:

• 200 mm < D < 1200 mm
• 0.4 < β < 0.7
• 2x105 < ReD < 2x106

Limits of use for Venturi tube with machined convergent section:

• 50 mm < D < 250 mm
• 0.4 < β < 0.75
• 2x105 < ReD < 1x106

Flow rate through Venturi tube is calculated using:

where is:

• q - flow rate
• C - flow coefficient
• A - cross section area
• Δ p - pressure drop
• ρ - density
• gn - acceleration of gravity

The values of hL and Δ p are measured differential static head or pressure before and after the Venturi tube. Values for coefficient of discharge or flow coefficient (C or Cd) can be obtained based on applicable standards like ISO 5167 or similar ASME standards. Bellow are flow coefficients from ISO 5167:

• Venturi tubes with "as cast" convergent section Cd=0.984
• Venturi tubes with a machined convergent section Cd=0.995
• Venturi tubes with a rough-welded sheet-iron convergent section Cd=0.985

Relation between flow coefficient and discharge coefficient (C and Cd) is given bellow:

where is:

• C - flow coefficient
• Cd - coefficient of discharge
• β - diameter ratio d1/d2

Related calculators for flow rate measurement with pressure differential devices:
Orifice plate calculator
Nozzle and Venturi nozzle calculator
Prandtl probe calculator