• Easy to retrofit (no rebuilding of the pipe)
• Easy to install
• Good for large nominal diameters
• Wide range of application (media, nominal diameters, process conditions)
• Minor measurement inaccuracy
• Special designs possible for special applications
• Also work in rectangular ducts and pipes

• Technical Gases
• Compressed Air
• Exhaust Air
• Fresh and Combustion Air
• Heat Transfer Fluids
• Water
• Exhaust Gas
• Steam/Heat Quantities

Basics: Averaging pitot tubes for flow measurement

• Mounting by insertion into the pipe (no flange-to-flange instrument)
• Differential pressure generation through forced flow
• Variation of the classic "pitot tube" through multiple metering orifices (so-called "averaging pitot tube")
• Design follows manufacturer guidelines, not standardized

Designs

• Averaging Pitot Tube for gas and liquids (7ME161)
• Averaging Pitot Tube for steam (7ME162)
• Averaging Pitot Tube with FASTLOK (7ME163), to remove sensor during operation without interruption of process

System design

• Compact design for dry gases and liquids without integrated temperature measurement
• Compact design for wet gases with or without integrated temperature measurement as well as for dry gases and liquids with integrated temperature measurement
• Compact design for steam with or without integrated temperature measurement
• Remote design for dry or wet gases, liquids and steam

Design of the averaging pitot tube

Similar to other differential pressure devices averaging pitot tubes create a differential pressure to measure flow. They are not specified in the general standard ISO 5167, but they follow the same technical principle. In contrast to the classic primary elements, averaging pitot tubes are not "in-line" devices but consist of a "profile" that is inserted into the side of the pipeline.

Differential pressure is created when fluid flows around the profile of the averaging pitot tube. Since the constriction of the pipeline by the profile in relation to the cross-sectional area is much smaller than, for example, with an orifice plate, the created differential pressure and the respective permanent pressure drop are smaller.

The flow comes to a complete stop at the upstream side of the averaging pitot tube creating the upstream pressure. At the downstream side a negative pressure is created by the so-called Kármán vortex street. The differential pressure (difference between upstream pressure and negative pressure) is the measurement signal and is proportional to the flow rate. This results in the following basic formula for flow measurement with averaging pitot tubes:

q_m = A • k • √(2 • ∆p • ρ)

q_m: mass flow

A: cross-sectional area of the pipe

k: device factor of the pitot tube

∆p: differential pressure

ρ: density

The k-factor is the device factor of the averaging pitot tube and is determined, among other things, by the shape of the profile of the pitot tube. Thanks to the sharp-edged shape of the profile, it remains constant over a very wide Reynolds number range and enables linear flow measurement.

The averaging pitot tube features the same number of measuring openings on the front and back. The special distribution of the measuring openings over the cross section allows geometric averaging in case of uneven flow distribution and thus an accurate measurement even with very short inlet and outlet distances. The generated upstream and downstream pressures are averaged in the respective chambers and routed to the differential pressure transmitter.

Z-Options for Cable Glands, Plugs, Labeling, Approvals, blanking plugs, flangs seals, device settings, etc. according to SITRANS P320

Further versions that are available on request:

• Weld-in sensor for high pressure steam
• Calibrated metering pipes
• FASTLOK with flange ball valve
• Etc.

For more information please see the Installation Instructions and the Instruction Manuals SITRANS P on SIOS.