Pitot tubes are another type of differential pressure flowmeters. They are named after Henri Pitot who came with this invention in the year 1732. Pitot tubes are basically used to detect flow velocity of fluids. Pitot tubes have the potential to measure two pressures at the same time i.e. impact (dynamic) and static. “The static pressure is the operating pressure in the pipe, duct, or the environment, upstream to the Pitot tube. It is measured at right angles to the flow direction, preferably in a low turbulence location”.1
In a Pitot-static tube, the kinetic energy of the flowing fluid is transformed into potential energy for measurement of fluid flow velocity. A typical pitot tube schematic is shown in figure below:
Major advantages of pitot tubes include:
- Cost effective measurement
- No moving parts
- Simple to use and install
- Low pressure drop
- A Pitot tube can be employed either as a permanently fixed flow sensor or as a portable monitoring device which supplies data on a periodic basis.
- Natural frequency resonant vibrations can result in the failure of a pitot tube.
Averaging Pitot Tube
With the advent of averaging pitot tubes, the difficulty of determining the average velocity point has almost been sorted out. An averaging pitot tube is designed with manifold impact and static pressure ports. Besides, it is constructed in such a manner that it extends across the whole pipe diameter. Averaging pitot tubes cost somewhat extra and offer better accuracy as compared to single port pitot tubes, particularly in cases where the flow is not entirely formed.
Pitot Tube v/s Orifice
A pitot tube is considered to be an economical alternative to an orifice plate. Accuracy of a pitot tube varies from 0.5% to 5% FS, which is almost equivalent to that of an orifice meter. The flow rangeability of a pitot tube which is 3:1 (in a few cases it is 4:1) is also analogous to the competence of an orifice plate. An orifice meter used to measure the full flowstream whereas the pitot tube generally detects the liquid flow velocity at only a single point in the cross-section of a flowstream. This is the major point of distinction between the two types of flowmeters. An extra advantage offered by a slender designed pitot tube is that it can be inserted into existing and pressurized pipelines (usually referred to as hot-tapping) exclusive of any shutdown.
Pitot tube is employed in a variety of flow measurement applications like air speed in racing cars and Air Force fighter jets. In industries, pitot tubes are invariably put into use for measurement of
- Air flow in pipes, ducts, and stacks, and
- Liquid flow in pipes, weirs, and open channels
Since pitot tubes are trouble-free and offer consistent performance, they are appropriate for a range of environmental conditions such as in exceptionally high temperatures and an extensive range of pressures. Pitot tubes ought to be used in applications where the minimum Reynolds number is more than 20,000 and in cases where a straight run of approximately 25 diameters can be provided upstream to the pitot tube. Besides, in areas where straightening vanes are installed, pitot tubes can find their use.