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The pitot tube is one of the simplest flow sensor, it is used in a wide range of flow measurement applications such as air speed in racing cars and Air Force fighter jets. In industrial applications, pitot tubes are used to measure air flow in pipes, ducts, and stacks, and liquid flow in pipes, weirs, and open channels. While accuracy and rangeability are relatively low, pitot tubes are simple, reliable, inexpensive, and suited for a variety of environmental conditions, including extremely high temperatures and a wide range of pressures.
The pitot tube is an inexpensive alternative to an orifice plate. Accuracy ranges from 0.5% to 5% FS, which is comparable to that of an orifice. Its flow rangeability of 3:1 (some operate at 4:1) is also similar to the capability of the orifice plate. The main difference is that, while an orifice measures the full flowstream, the pitot tube detects the flow velocity at only one point in the flowstream. An advantage of the slender pitot probe is that it can be inserted into existing and pressurized pipelines (called hot-tapping) without requiring a shutdown.
The total impact pressure (PT) is the sum of the static and kinetic pressures and is detected as the flowing stream impacts on the pitot opening. To measure impact pressure, most pitot tubes use a small, sometimes L-shaped tube, with the opening directly facing the oncoming flowstream.
Wall taps can measure static pressures at flow velocities up to 200 ft/sec. A pitot static tube (resembling an L-shaped pitot tube) can have four holes of 0.04 inches in diameter, spaced 90° apart.
Errors in detecting static pressure arise from fluid viscosity, velocity, and fluid compressibility. The key to accurate static pressure detection is to minimize the kinetic component in the pressure measurement.
If the pressure differential generated by the venturi is too low for accurate detection, the conventional pitot tube can be replaced by a pitot venturi or a double venturi sensor. This will produce a higher pressure differential.
A calibrated, clean and properly inserted single-port pitot tube can provide ±1% of full scale flow accuracy over a flow range of 3:1; and, with some loss of accuracy, it can even measure over a range of 4:1. Its advantages are low cost, no moving parts, simplicity, and the fact that it causes very little pressure loss in the flowing stream. Its main limitations include the errors resulting from velocity profile changes or from plugging of the pressure ports. Pitot tubes are generally used for flow measurements of secondary importance, where cost is a major concern, and/or when the pipe or duct diameter is large (up to 72 inches or more).
Specially designed pitot probes have been developed for use with pulsating flows. One design uses a pitot probe filled with silicone oil to transmit the process pressures to the d/p cell. At high frequency pulsating applications, the oil serves as a pulsation dampening and pressure-averaging medium.
Pitot tubes also can be used in square, rectangular or circular air ducts. Typically, the pitot tube fits through a 5/16-in diameter hole in the duct. Mounting can be by a flange or gland. The tube is usually provided with an external indicator, so that its impact port can be accurately rotated to face directly into the flow.
A pitot tube also can be used to measure water velocity in open channels, at drops, chutes, or over fall crests. At the low flow velocities typical of laminar conditions, pitot tubes are not recommended because it is difficult to find the insertion depth corresponding to the average velocity and because the pitot element produces such a small pressure differential. The use of a pitot venturi does improve on this situation by increasing the pressure differential, but cannot help the problem caused by the elongated velocity profile.
The number of impact ports, the distance between ports, and the diameter of the averaging pitot tube all can be modified to match the needs of a particular application. Sensing ports in averaging pitot tubes are often too large to allow the tube to behave as a true averaging chamber. This is because the oversized port openings are optimized not for averaging, but to prevent plugging. In some installations, purging with an inert gas is used to keep the ports clean, allowing the sensor to use smaller ports.
Averaging pitot tubes offer the same advantages and disadvantages as do single-port tubes. They are slightly more expensive and a little more accurate, especially if the flow is not fully formed. Some averaging pitot sensors can be inserted through the same opening (or hot tap) which accommodates a single-port tube.
Area-averaging pitot stations are used to measure the large flows of low pressure air in boilers, dryers, or HVAC systems. These units are available for the various standard sizes of circular or rectangular ducts (Figure 2-13) and for pipes. They are so designed that each segment of the cross-section is provided with both an impact and a static pressure port. Each set of ports is connected to its own manifold, which combines the average static and average impact pressure signals. If plugging is likely, the manifolds can be purged to keep the ports clean.
Because area-averaging pitot stations generate very small pressure differentials, it may be necessary to use low differential d/p cells with spans as low as 0-0.01 in water column.
The pitot tube is an inexpensive alternative to an orifice plate. Accuracy ranges from 0.5% to 5% FS, which is comparable to that of an orifice. Its flow rangeability of 3:1 (some operate at 4:1) is also similar to the capability of the orifice plate. The main difference is that, while an orifice measures the full flowstream, the pitot tube detects the flow velocity at only one point in the flowstream. An advantage of the slender pitot probe is that it can be inserted into existing and pressurized pipelines (called hot-tapping) without requiring a shutdown.
Theory of Operation
Pitot tubes were invented by Henri Pitot in 1732 to measure the flowing liquid or air velocity. Basically a differential pressure (d/p) flowmeter, a pitot tube measures two pressures: the static and the total impact pressure. 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 locationThe total impact pressure (PT) is the sum of the static and kinetic pressures and is detected as the flowing stream impacts on the pitot opening. To measure impact pressure, most pitot tubes use a small, sometimes L-shaped tube, with the opening directly facing the oncoming flowstream.
Static Pressure Measurement
In jacketed (dual-walled) pitot-tube designs, the impact pressure port faces forward into the flow, while pitot static tube port do not, but are, instead, spaced around the outer tube. Both pressure signals (PT and P) are routed by tubing to a d/p indicator or transmitter. In industrial applications, the static pressure (P) can be measured in three ways: 1) through taps in the pipe wall; 2) by static pitot probes inserted in the process stream; or 3) by small openings located on the pitot tube itself or on a separate aerodynamic element.Wall taps can measure static pressures at flow velocities up to 200 ft/sec. A pitot static tube (resembling an L-shaped pitot tube) can have four holes of 0.04 inches in diameter, spaced 90° apart.
Errors in detecting static pressure arise from fluid viscosity, velocity, and fluid compressibility. The key to accurate static pressure detection is to minimize the kinetic component in the pressure measurement.
Single-Port Pitot Tubes
A single-port pitot tube can measure the flow velocity at only a single point in the cross-section of a flowing stream . The probe must be inserted to a point in the flowing stream where the flow velocity is the average of the velocities across the cross-section, and its impact port must face directly into the fluid flow.If the pressure differential generated by the venturi is too low for accurate detection, the conventional pitot tube can be replaced by a pitot venturi or a double venturi sensor. This will produce a higher pressure differential.
A calibrated, clean and properly inserted single-port pitot tube can provide ±1% of full scale flow accuracy over a flow range of 3:1; and, with some loss of accuracy, it can even measure over a range of 4:1. Its advantages are low cost, no moving parts, simplicity, and the fact that it causes very little pressure loss in the flowing stream. Its main limitations include the errors resulting from velocity profile changes or from plugging of the pressure ports. Pitot tubes are generally used for flow measurements of secondary importance, where cost is a major concern, and/or when the pipe or duct diameter is large (up to 72 inches or more).
Specially designed pitot probes have been developed for use with pulsating flows. One design uses a pitot probe filled with silicone oil to transmit the process pressures to the d/p cell. At high frequency pulsating applications, the oil serves as a pulsation dampening and pressure-averaging medium.
Pitot tubes also can be used in square, rectangular or circular air ducts. Typically, the pitot tube fits through a 5/16-in diameter hole in the duct. Mounting can be by a flange or gland. The tube is usually provided with an external indicator, so that its impact port can be accurately rotated to face directly into the flow.
A pitot tube also can be used to measure water velocity in open channels, at drops, chutes, or over fall crests. At the low flow velocities typical of laminar conditions, pitot tubes are not recommended because it is difficult to find the insertion depth corresponding to the average velocity and because the pitot element produces such a small pressure differential. The use of a pitot venturi does improve on this situation by increasing the pressure differential, but cannot help the problem caused by the elongated velocity profile.
Averaging Pitot Tubes
Averaging pitot tubes been introduced to overcome the problem of finding the average velocity point. An averaging pitot tube is provided with multiple impact and static pressure ports and is designed to extend across the entire diameter of the pipe. The pressures detected by all the impact (and separately by all the static) pressure ports are combined and the square root of their difference is measured as an indication of the average flow in the pipe (Figure 2-12).
The number of impact ports, the distance between ports, and the diameter of the averaging pitot tube all can be modified to match the needs of a particular application. Sensing ports in averaging pitot tubes are often too large to allow the tube to behave as a true averaging chamber. This is because the oversized port openings are optimized not for averaging, but to prevent plugging. In some installations, purging with an inert gas is used to keep the ports clean, allowing the sensor to use smaller ports.
Averaging pitot tubes offer the same advantages and disadvantages as do single-port tubes. They are slightly more expensive and a little more accurate, especially if the flow is not fully formed. Some averaging pitot sensors can be inserted through the same opening (or hot tap) which accommodates a single-port tube.
Area Averaging
Because area-averaging pitot stations generate very small pressure differentials, it may be necessary to use low differential d/p cells with spans as low as 0-0.01 in water column.
Installation
The velocity profile of the flowing stream inside the pipe is affected by the Reynolds number of the flowing fluid, pipe surface roughness, and by upstream disturbances, such as valves, elbows, and other fittings. Pitot tubes should be used only if the minimum Reynolds number exceeds 20,000 and if either a straight run of about 25 diameters can be provided upstream to the pitot tube or if straightening vanes can be installed.Vibration Damage
Natural frequency resonant vibrations can cause pitot tube failure.
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