What is a venturi tube?
This method, which is used to measure fluid flow, was discovered about 120 years ago by Giovanni Battista Venturi. (Turbulent) can be used.
This method is relatively inexpensive in small sizes, due to its horny shape, it allows suspended solids to move in the liquid. Therefore, it is suitable for measuring effluents and slurries. The fluid passing through the throat of the venturi pipe will return to its initial calm state very quickly, which is due to the shape of the equipment horn, which allows the flow profile to be opened slowly. .
According to calculations, approximately at a distance of 3D (D is the diameter of the pipe) after the venturi pipe, the flow returns to its initial calm state, where the flow range is large and the pressure difference is low, this method is used.
With the same pressure difference, it passes approximately 25 to 50% more flow compared to Orifis, and it can be concluded that its throughput capacity is higher than Orifis.
This method can be used up to a pipe size of about 72%, the measurement accuracy is almost good. Although it is difficult to build (especially in sizes over 20 inches) and more expensive than the Orifac.
The question is, how does this device measure flow? According to Bernoulli’s law, the higher the speed, the lower the pressure. Since the velocity at point 2 is greater than at point 1, the pressure P1 is greater than at P2
If we measure the pressure difference between these two points with the help of a transmitter, the measured value will indicate the flow rate. (5-way manifold is required for installation)
Flow can be measured by calculating the pressure difference.
The relationship of mechanical energy for these two points is:
E 1 = E 2 K 1 + U 1 = K 2 + U 2
½ MV 1 2 + MP 1 = ½ MV 2 2 + MP 2
P 1 -P 2 = ½ (V 2 2 – V 1 2 ) (*)
Q = AV A 1 .V 1 = A 2 .V 2
V 1 = V2 V 1 = () 2 V 2
(*) 2Δp = V 2 2 – () 4 V 2 2 V 2 =
Q = A 2 .V 2 Q = A 2 Q = B
The above relation shows that the flow is directly related to the square of the pressure difference between the two Venturi heads.
This relationship is the basis of flow operation and measurement in devices that calculate flow from pressure difference.
Vena Contracta : is the point where the minimum pressure and the maximum velocity of the fluid occur.
As can be seen from the figure above, in the venturi tube, the fluid pressure after leaving the measuring element has approximately the same amount of inlet pressure.
The Venture Tube creates a slight drop in pressure along the route.
Remember that when working with the pressure difference transmitter, you must select the Root Extractor mode to send the root of the measured value to the control room in the form of a signal of 4 to 20 mA.
What is the value of Δ P ?
The term ΔP is called Max Allowable Press Drop.
The maximum for a Venturi pipe is 1000 mbar , the minimum is 250 mbar , although this amount is announced by process engineers for Venture sizing.
- Venture pipes are hard to make and expensive.
- There is a possibility of choking (fluid suffocation) in it, so it is not recommended for fluids that have a high amount of sediment.
- Usually requires a lot of space for installation.
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