Orifice, Nozzle and Venturi Flow Rate Meters

The orifice, nozzle and venturi flow rate meters use the Bernoulli Equation to calculate the fluid flow rate by using the pressure difference between an obstruction in the flow.

In flow metering devices based on the Bernoulli Equation the downstream pressure after an obstruction will be lower than the upstream pressure before the obstruction. To understand orifice, nozzle and venturi meters it's therefore necessary to explore the Bernoulli Equation.

The Bernoulli Equation and Flow Meters

Assuming a horizontal flow (or neglecting a minor elevation difference between the measuring points) the Bernoulli Equation can be modified to:

p₁ + 1/2 ρ v₁² = p₂ + 1/2 ρ v₂² (1)

where

p = pressure

ρ = density

v = flow velocity

Vertical flow can be adapted by adding elevation heights h₁ and h₂ in (1).

Assuming that the velocity profiles are uniform in the upstream and downstream section the Continuity Equation gives:

q = v₁ A₁ = v₂ A₂ (2)

where

q = flow rate

A = flow area

Combining (1) and (2), assuming A₂ < A₁, gives the "ideal" equation:

q = A₂ [ 2(p₁ - p₂) / ρ(1 - (A₂ / A₁)²) ]^1/2 (3)

For a given geometry (A), the flow rate can be determined by measuring the pressure difference p₁ - p₂.

The theoretical flow rate q will in practice be smaller (2 - 40%) due to the geometrical conditions.

The ideal equation (3) can be modified with a discharge coefficient:

q = c_d A₂ [ 2(p₁ - p₂) / ρ(1 - (A₂ / A₁)²) ]^1/2 (3b)

where

c_d = discharge coefficient

The discharge coefficient c_d is a function of the jet size - or orifice opening - the

area ratio = A_vc / A₂

where

A_vc = area in "vena contracta"

Vena Contracta is the minimum jet area that appears just downstream of the restriction. The viscous effect is usually expressed in terms of the nondimensional parameter Reynolds Number Re.

Due to the Benoulli and Continuity Equation the velocity of the fluid will be at it's highest, and the pressure at it's lowest in "vena contracta". After the metering device and "vena contracta" the velocity decrease to the same level as before the obstruction. The pressure recover to a pressure level lower than the pressure before the obstruction and adds a head loss to the flow.

Equation (3) can be modified with diameters to:

q = c_d π/4 D₂² [ 2(p₁ - p₂) / ρ(1 - d⁴) ]^1/2 (4)

where

D₂ = orifice, venturi or nozzle inside diameter

D₁ = upstream and downstream pipe diameter

d = D₂ / D₁ diameter ratio

π = 3.14

Equation (4) can be modified to mass flow for fluids by simply multiplying with the density:

m = c_d π/4 D₂² ρ [ 2(p₁ - p₂) / ρ(1 - d⁴) ]^1/2 (5)

When measuring the mass flow in gases, its necessary to considerate the pressure reduction and change in density of the fluid. The formula above can be used with limitations for applications with relatively small changes in pressure and density.

The Orifice Plate

The orifice meter consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. There are in general three methods of placing the taps. The coefficient of the meter depends upon the position of taps.

• Flange location - Tap location 1 inch upstream and 1 inch downstream from face of orifice
• Vena contracta location - Tap location 1 pipe diameter (actual inside) upstream and 0.3 to 0.8 pipe diameter downstream from face of orifice
• Pipe location - Tap location 2.5 times nominal pipe diameter upstream and 8 times nominal pipe diameter downstream from face of orifice

The discharge coefficient - c_d - varies considerably with changes in area ratio and the Reynolds number. A discharge coefficient - c_d - of 0.60 may be taken as standard, but the value varies noticeably at low values of the Reynolds number.

The pressure recovery is limited for an orifice plate and the permanent pressure loss depends primarily on the area ratio. For an area ratio of 0.5, the head loss is about 70 - 75% of the orifice differential.

• The orifice meter is recommended for clean and dirty liquids and some slurry services.
• The rangeability is 4 to 1
• The pressure loss is medium
• Typical accuracy is 2-4% of full scale
• The required upstream diameter is 10 to 30
• The viscosity effect is high.
• The relative cost is low.

References

• American Society of Mechanical Engineers (ASME). 2001. Measurement of fluid flow using small bore precision orifice meters. ASME MFC-14M-2001.
• International Organization of Standards (ISO 5167-1:2003). Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.
• International Organization of Standards (ISO 5167-1) Amendment 1. 1998. Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:1991/Amd.1:1998(E).
• American Society of Mechanical Engineers (ASME). B16.36 - 1996 - Orifice Flanges

The Venturi Meter

In the venturi meter the fluid is accelerated through a converging cone of angle 15-20^o and the pressure difference between the upstream side of the cone and the throat is measured and provides the signal for the rate of flow.

The fluid slows down in a cone with smaller angle (5-7^o) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no "vena contracta". The flow area is at minimum at the throat.

High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are available.

A discharge coefficient - c_d - of 0.975 may be taken as standard, but the value varies noticeably at low values of the Reynolds number.

The pressure recovery is much better for the venturi meter than for the orifice plate.

• The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services.
• The rangeability is 4 to 1.
• Pressure loss is low.
• Typical accuracy is 1% of full range.
• Required upstream pipe length 5 to 20 diameters.
• Viscosity effect is high
• Relative cost is medium

References

• International Organization of Standards - ISO 5167-1:2003 Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.
• American Society of Mechanical Engineers ASME FED 01-Jan-1971. Fluid Meters Their Theory And Application- Sixth Edition

The Nozzle

Nozzles used for determining fluid's flowrate through pipes can be in three different types:

• The ISA 1932 nozzle - developed in 1932 by the International Organization for Standardization or ISO. The ISA 1932 nozzle is common outside USA.
• The long radius nozzle is a variation of the ISA 1932 nozzle.
• The venturi nozzle is a hybrid having a convergent section similar to the ISA 1932 nozzle and a divergent section similar to a venturi tube flowmeter.

• The flow nozzle is recommended for both clean and dirty liquids
• The rangeability is 4 to 1
• The relative pressure loss is medium
• Typical accuracy is 1-2% of full range
• Required upstream pipe length is 10 to 30 diameters
• The viscosity effect high
• The relative is medium

References

• American Society of Mechanical Engineers ASME FED 01-Jan-1971. Fluid Meters Their Theory And Application- Sixth Edition
• International Organization of Standards - ISO 5167-1:2003 Measurement of fluid flow by means of pressure differential devices, Part 1: Orifice plates, nozzles, and Venturi tubes inserted in circular cross-section conduits running full. Reference number: ISO 5167-1:2003.

Example - Kerosene Flow Through a Venturi Meter

The pressure difference dp = p₁ - p₂ between upstream and downstream is 100 kPa (1 10⁵ N/m²). The specific gravity of kerosene is 0.82.

Upstream diameter is 0.1 m and downstream diameter is 0.06 m.

Density of kerosene can be calculated as:

ρ = 0.82 1000 (kg/m³)= 820 (kg/m³)

• Density, Specific Weight and Specific Gravity - An introduction and definition of density, specific weight and specific gravity. Formulas with examples.

Upstream and downstream area can be calculated as:

A₁ = 3.14 (0,1 (m)/2)² = 0.00785 (m²)

A₂ = 3.14 (0,06 (m)/2)² = 0.002826 (m²)

Theoretical flow can be calculated from (3):

q = A₂ [ 2(p₁ - p₂) / ρ(1 - (A₂/A₁)²) ]^1/2

q = 0.002826 [ 2 1x10⁵ (N/m²) / 820 (kg/m³)(1 - ( 0.002826 (m²)/0.00785 (m²) )²) ]^1/2

    = 0.047 (m³/s)

For a pressure difference of 1 kPa (0,01x10⁵ N/m²) - the theoretical flow can be calculated:

q = 0.002826 [ 2 0.01x10⁵ (N/m²) / 820 (kg/m³)(1 - ( 0.002826 (m²)/0.00785 (m²) )²) ]^1/2

    = 0.0047 (m³/s)

The mass flow can be calculated as:

m = q ρ

    = 0.0047 (m³/s) 820 (kg/m³)

    = 3.85 (kg/s)

Flow Rate and Change in Pressure Difference

Note! - The flow rate varies with the square root of the pressure difference.

From the example above:

• a tenfold increase in the flow rate requires a one hundredfold increase in the pressure difference!

Transmitters and Control System

The nonlinear relationship have impact on the pressure transmitters operating range and requires that the electronic pressure transmitters have the capability to linearizing the signal before transmitting it to the control system.

Accuracy

Due to the non linearity the turn down rate is limited. The accuracy strongly increases in the lower part of the operating range.

• More about Flow Meters as Orifices, Venturi meters, and Nozzles
• Fluid Mechanics
• The Bernoulli Equation
• The Continuity Equation
• TurnDown Ratio and Flow Measurement Devices - An introduction to Turn Down Ratio and flow measurement accuracy.

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