A venturi tube is considered to be the most accurate flow-sensing element only if it is accurately calibrated. This tube basically consists of
- a converging conical inlet
- a cylindrical throat, and
- a diverging recovery cone
A typical Venturi tube is shown in the figure below:
- Pressure recovery for venturi tubes is a lot better as compared to the orifice plates.
- Venturi tubes are appropriate for clean, dirty and viscous liquid and few slurry services as well.
- Rangeability of a venturi tube varies from 4 to 1.
- In venturi tubes, pressure loss is low and viscosity effect is high.
- Usually, accuracy of a venturi tube is 1% of its full range
- Essential upstream pipe length of a venturi tube is 5 to 20 diameters
- Venturi tubes exist in sizes up to 72″.
- A venturi tube can pass 25 to 50% extra flow than that of an orifice plate keeping the same drop in pressure.
- Since, the early cost of a venturi tube is quite high, it is mainly engaged on larger flows and in more complicated or challenging flow applications.
- Unlike orifice plates, venturi tubes are almost insensitive to velocity profile effects and consequently call for less straight pipe run than an orifice.
- Venturi tubes are resistant to corrosion, erosion, and internal scale build up, owing to their contoured nature, pooled with the self- scouring action of the flow through the tube.
- Use of venturi tubes involves a lot of savings in installation and operating and maintenance costs.
- Venturi tube flowmeters are frequently used in applications involving higher TurnDown Rates or lower pressure drops, particularly in areas where orifice plates fails to perform.
- By means of right instrumentation and flow calibration, the flowrate of a venturi tube can be brought down to about 10% of its full scale range with correct accuracy which in turn results in a TurnDown Rate of 10:1.
Following are the key benefits offered by venturi tubes:
- They can handle large flow volumes at low pressure drops.
- Venturi tube flowmeters involve no moving parts.
- They can be mounted in large diameter pipes via flanged, welded or threaded-end fittings.
- They can be used with nearly all liquids, as well as those containing extreme solids content.
- Venturi tubes involve no projections into the fluid and no sharp corners. Also there are no rapid changes in contour.
Major disadvantages of venturi tubes include:
- Primary cost for installation and usage of venturi tubes proves to be very high.
- Also there can be trouble in installation and inspection of venturis.
Flow tubes are similar to venturi tubes to some extent. The major difference between the two tubes is that unlike venturi tubes, flow tubes do not contain the entrance cone. Flow tubes usually have a tapered throat, while the exit is extended and smooth. The performance of a Flow tube is exaggerated by its calibration to a great extent.