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 Section 3.
PITOT TUBES
1. INTRODUCTION. Pitot tubes are used to q easure the flow of gas, steam,
water, and other liquids in ducts and pipes. A pitot tube is a device that
consists of a tube having a short , right-angled bend that is placed vertically
in a moving body of fluid with the mouth of the bend directed upstream
(figures 5-12 through 5-16). The most common applications include heating,
ventilating, and air-conditioning (HVAC) systems and low velocity drafts.
Pitot tubes are widely used as permanent and spot-check meters. Their
characteristics are well known and information is widely available. Pitot
tubes are generally less than l/2-inch in diameter and are inserted into the
flow stream at right angles to the flow. Pitot tube meters measure the
velocity of flow at only one point. If properly installed, they provide
accurate and reliable flow measurement. Pitot tubes are suited for low to
medium flow in large ducts. Due to their small size, pitot tubes cause very
little permanent pressure loss. They are excellent for monitoring purposes
because they are so portable and easily inserted and withdrawn from a flow
stream.
1.1 Operating Principle. The principle used in a pitot tube meter is that
when a flowing fluid impacts an object, its velocity drops to zero and the
pressure at impact increases. The pitot tube measures the difference between
impact (velocity) pressure and static pressure created at impact. Flow rate
is determined by comparing the two pressures and the known relationship
between pressure and flow rate.
2. METER DESIGNS. Differences in pitot tube designs are in the positioning
of static and dynamic pressure taps that are used in measuring pressures and
whether the installation is to be permanent or for temporary spot monitoring.
Some designs place the taps separately, with the static tap located at the
duct wall and the impact tap at the end of the pitot tube (Figure 5-12).
Other designs locate both taps on the tube. A single point pitot tube infers
the flow from single position readings.
Averaging pitot tubes have more than one impact hole along the leading edge of
the tube (Figure 5-13). The static tap is generally placed on the downstream
side of the tube. The tube is screw-fitted into place. The multiple impact
holes provide for an averaging of the velocity across the pipe instead of at a
single point. Cylindrical-bodied averaging pitot tube meters have proven to
be nonlinear (and nonrepeatable) over most of the flow range. Only designs
that incorporate some sort of vortex shedding body should be used. Both
permanent and portable units can be used to measure flow rates of gases, air,
and steam. The pitot tube is especially useful in odd-shaped or large ducting.
3. RECOMMENDED APPLICATIONS. Pitot tube meters are appropriate for measuring
gas and steam flow in round pipes with diameters greater than 3 inches,
especially when venturi tubes and orifice plates are too expensive to utilize
or cause too great a permanent pressure drop. A pitot tube is recommended
where developed flow is possible by assuring minimum straight lengths of pipe
5-19
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