by Jason Schoenbaechler • Sep 5, 2018
When designed and installed correctly, a venturi flow meter will provide you with years of reliable flow measurement. It doesn't matter if you're a design engineer, contractor, or plant in plant maintenance, understanding what a proper venturi flow meter installation should look like is something you should know.
We won't go in depth on all the theory and calculations here, but a basic understanding of how a venturi flow meter works will help you in the following sections. A venturi works by measuring the difference in pressure at two different locations. The pressure difference is created by reducing the diameter of the pipe causing the fluid to flow faster. The fast moving fluid has a lower pressure than the slower fluid in the larger section of the venturi. The CFD flow simulation below shows the higher fluid velocity in the throat (the small part) of the venturi in red compared to the slower moving fluid in blue in the inlet and outlet.
CFD Velocity Plot of BIF Universal Venturi Tube
• How a Venturi Flow Meter Works
• Piping Conditions That Can Affect Venturi Flow Meter Performance
• Installation Concerns
• Instrumentation connections
Almost any change in diameter can be used to create this pressure difference, take an orifice plate for example, it is simply a plate with a hole in the middle. BIF has been building and testing venturi flow meters for years to develop the ideal combination of accurate flow measurement and low head loss.
Most flow meters have to be installed a specified distance before and after piping components that will affect the fluid in the line. This give the flow some time to stabilize before the venturi does it's work. One advantage BIF venturi flow meters have is that there is usually no downstream requirement. In fact, some our rate of flow controllers have a butterfly valve integrated right into the back of the flow meter.
The most intuitive way to think about it is this; the more disruption a component causes, the further away it should be located.
Whenever possible valves should be located after the flow meter. A partially open valve can create a very uneven flow profile causing uncertainty in the venturi's measurement.
Like valves, these components can be bolted directly to the outlet of a venturi and they will not affect the flow measurement. For upstream fittings we give recommendations based on the number of diameters before the venturi.
5 diameters on a 12″ pipe line = 5 x 12″ = 60″
|Component||Static Tap Venturi||Corner Tap Venturi|
The exact requirements will vary depending on the type of venturi and beta ratio (the throat diameter divided by the inlet diameter). For more information to determine the exact requirements for your application signup for the BIF Newsletter and you will receive a free copy of the BIF flowmeter selection guide.
Now that you know where to locate a venturi relative to other components, lets look at some other concerns.
• Flow Direction
• Orientation of pressure taps
• Alignment of venturi to pipe
This may seem obvious, but we have seen several cases where a venturi was performing poorly because it was installed backwards. All differential producing flow meters will create a differential pressure with reverse flow, but the value of the pressure difference will not be the same as in normal flow condition.
All BIF venturi nameplates have a flow direction arrow. Additionally, most cast UVTs have a directional arrow formed into the body.
The orientation of the venturi pressure taps depends on the type of venturi as well as the fluid being measured.
• Pressure connections in liquid applications should be oriented to avoid air pockets.
• Pressure connection in air applications should be oriented to avoid condensation.
BIF's model 20181 cast UVT has 2 sets of pressure taps, located 180 degrees apart. It also has a vent and drain port located 90 degrees from the pressure taps.
In horizontal pipelines the pressure taps should be oriented on the horizontal centerline and the vent and drain ports should be located on the top and bottom.
In vertical pipelines the orientation of the pressure connections will not affect performance.
BIF Model 20181 Pressure Connections
Model 20182 FRP insert venturi meters have 1 high and 1 low pressure connection. These pressure connections are located between the bolt holes of the mounting flange and the angle between them will vary depending on the flange size and rating.
BIF Model 20182 Pressure Connections
|Flowing Medium||Orientation of Pipeline||Tap Orientation|
|Liquid||Horizontal||One tap on or slightly below horizontal centerline|
|Dry Gases||Horizontal||One tap on vertical centerline|
|Vapors, Moist or Dirty Gas or Steam||Horizontal||One tap on or slightly below horizontal centerline|
|Any Fluid||Vertical||Taps can be at any radial position|
When installing a venturi in a pipeline care should be taken to center the meter. If the meter is installed off center a flow disruption could be created at the joint between the pipe and venturi causing a loss in accuracy.
Properly Aligned Flange vs. Misaligned Flange
The final consideration when installing a venturi flow meter is the connection between the venturi and the differential pressure (DP) transmitter. A DP transmitter converts the differential pressure created by a venturi into an electrical signal that can be used for monitoring and controlling the flow rate.
• Piping and fittings should be made of corrosion resistant materials such as stainless steel or Plastic
• Install an isolation valve close to the high and low-pressure connections
• Piping should have a slope of at least 1″ per foot
• Avoid peaks and valleys in the instrument piping, these areas may collect dirt, silt or air pockets
• If peaks or valleys are unavoidable install a drain or blow-off valve at these points
• Securely anchor instrument piping to avoid vibration
• Instrument piping size recommendation
• 1/2” diameter for runs less than 25 feet long
• 3/4” diameter for runs greater than 25 feet long
Example of Vent Valves in Instrument Piping
• When installing a Differential Pressure Transmitter, always use a 3-valve manifold like the one shown below. The manifold makes the initial setup and routine maintenance much easier
• Install vent valves on the transmitter or manifold to easily purge air from liquid systems
Transmitter with 3-Valve Manifold and Vent Valves
We covered a lot of information in this guide, from how a venturi flow meter works through installing the meter and instrumentation. If you have any additional questions, Click here to use the BIF Sales Rep locator and contact your local representative.