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Venturi Flowmeters

The differential producing flowmeter or Venturi has a long history of uses in many applications.  Due to its simplicity and dependability, the Venturi is among the most common flowmeters.  With no moving parts or abrupt flow restrictions, the Venturi can measure fluid flowrates with a minimal total pressure loss.

The principle behind the operation of the Venturi flowmeter is the Bernoulli effect.  The Venturi measures a fluid's flowrate by reducing the cross sectional flow area in the flow path and generating a pressure difference.  After the pressure difference is generated, the fluid is passed through a pressure recovery exit section where up to 80% of the differential pressure generated at the throat is recovered.  The pressure differential follows Bernoulli's Equation. 

Bernoulli's Equation:  

The Venturi Principle

In the illustration above, the fluid, either liquid or gaseous, enters the Venturi at the location with a cross-sectional area A₁, pressure P₁, and velocity v₁.  These properties form the potential and kinetic energy of the fluid at one location.  Energy is conserved in a closed system, that is, the sum of potential and kinetic energy at one location must equal the sum of the potential and kinetic energy at any another location in the system.  If potential energy decreases at one location, the kinetic energy must proportionally increase at that location.  The fluid now enters the throat of the Venturi with a new area A₂, which is smaller than A₁.  In a closed system mass can be neither created nor destroyed (law of conservation of mass, simply, what goes in, must come out), and as such, the volumetric flowrate at area A₁ must equal the volumetric flowrate at area A₂.  If the area at location A₂ is smaller than A₁, the fluid must travel faster to maintain the same volumetric flowrate.  This increase in velocity results in a decrease in pressure which follows Bernoulli's equation.  The result: by knowing the pressure and cross-sectional area at two locations, one can calculate the velocity of the fluid.  With the velocity of the fluid and its density, one can calculate the flowrate.

A Venturi requires two pressure and one temperature measurement to accurately determine flow.  The first pressure is measured at the Venturi's upstream location, P₁.  This is used for the density calculation and the high side input to the differential pressure measurement.  

The Venturi Flowmeter

The second pressure is measured at the Venturi's throat, P₂.  This is connected to the low side of the differential pressure gauge to form the DP pressure measurement.  The temperature reading is taken several pipe diameters in length upstream of the Venturi so as not to disrupt the uniform flow profile.  

With proper instrumentation and flow conditioning, a Venturi's flowrate can be reduced to about 10% of its full scale range without adding multiple transducer configurations.   This provides a 10 to 1 turn-down in a Venturi's flow range, For example:  If a Venturi is designed for a maximum flow of 50 SCFM, approximately 5 SCFM would be the lowest value readable but there are compromises.  See Instrumentation for a detailed discussion on instrumenting venturi flowmeters and how to maximize the measurement accuracy.

Applications:

Installation Requirements for  Venturi Flowmeters:

1)  10-20 diameters in length of straight pipe upstream of the Venturi

2)   5 diameters in length of straight pipe downstream of the Venturi

3)   Flow conditioning tube before the Venturi, if the flow is non-uniform or swirling

Free Flow System    This arrangement is probably the most common, allowing the Venturi to be installed at any point within the flowing system.  The location is generally selected for installation convenience and best flow conditioning.  Flow conditioning is necessary to minimize swirling and non-uniform flow profiles such as patterns created by fittings, regulators, and any other flow affecting devices. 

Pressure Reducing Flow System   A Venturi can be installed at several different locations within a pressure reducing flow system.  Each location generally has advantages and disadvantages.  In the diagram below, two locations are shown for the Venturi's location.  Location A is in the high pressure supply line to the system.  Location B is after the pressure regulator has substantially reduced the flowing pressure and density.  Because of the higher gas density at location A, the Venturi will have a greater flow range than at the lower pressure location B.  However, this greater range comes at the cost of lower signal resolution as compared with location B.  This would be similar to measuring the accuracy of a clock by timing either the minute hand or the second hand.  Both clock hands carry the same accuracy information, the timing of the clock, but it would be much easier to measure the movement of the second hand versus the movement of the minute hand over a small time period.  This is because the second hand has higher resolution, that is, it's easier to see a change in movement of the second hand versus a change in movement of the minute hand.  If a greater time range is required, then one would need to measure the minute hand.  It is the same with the Venturi's location.  Some flow systems may only operate over a relatively small range.  In that case, location B would be more advantageous than location A, and a properly sized Venturi at that location would provide the best signal resolution and accuracy.  However, if measuring the full flow range of the system is required, then location A is the proper configuration.  It is tempting to size a Venturi to measure the full air supply flow range, but, if it is only operated over a small flow range, this wide range comes at the cost of lower signal resolution and lower measurement accuracy.  Require the Venturi to measure only the range that you need.

Free Standing Inlet System    A Venturi can be installed as the inlet component to a flow system.  This arrangement is often used as a calibration standard for a CVS (Constant Volume System) flow stand in emissions testing.  Venturis used in this application are referred to as SAOs (Smooth Approach Orifice).  SAOs have several advantages over LFEs (Laminar Flow Elements), also used in this application.  SAOs are calibrated to an accuracy level which is expressed in percent of reading.  LFEs are calibrated typically to percent of full scale.  This is very advantageous at mid range and lower flow rates, the error being a percent of reading not of full scale.  As with Sonic Nozzles, Venturis (SAOs) will retain their original calibration accuracy unless they are physically damaged or abused.  The stainless steel or anodized aluminum surface will not erode through normal everyday use with clean air.