Pitot tube (annubar principle)

A pitot tube based on the Annubar principle (also Annubar, Ellison Annubar, S-Type Pitot tube or integrating Pitot tube ) is a special form of Pitot tube that is used in plant construction and process engineering for flow measurement.

Pitot tube in pipeline

principle

These pitot tubes are used to measure the flow of gaseous or liquid fluids such as air, natural gas, steam, water etc. in pipes and channels. Pitot tubes have several openings (mostly bores) in and against the direction of flow. A dynamic overpressure (the so-called back pressure ) arises at the openings against the direction of flow, and a dynamic negative pressure arises at the openings in the direction of flow . The pressures present at the differential pressure openings are averaged inside the dynamic pressure probe and measured outside the dynamic pressure probe. The differential pressure measured outside of the pitot tube is the difference between the dynamic overpressure and the dynamic negative pressure: ${\displaystyle p_{dyn1}}$ ${\displaystyle p_{dyn2}}$ ${\displaystyle dp}$

${\displaystyle {\begin{aligned}dp&=p_{1}-p_{2}\\p_{1}&=p_{stat}+p_{dyn1}\\p_{2}&=p_{stat}+p_{dyn2}\\dp&=p_{dyn1}-p_{dyn2}\end{aligned}}}$

• ${\displaystyle dp}$ - differential pressure
• ${\displaystyle p_{1}}$ - total pressure 1
• ${\displaystyle p_{2}}$ - total pressure 2
• ${\displaystyle p_{dyn1}}$ - dynamic overpressure
• ${\displaystyle p_{dyn2}}$ - dynamic vacuum

Thanks to the averaging function of this type of pitot tube, the pitot tube can correct disturbed flow profiles, such as those present in pipeline construction behind fixtures or diversions, better and record the flow more precisely than is the case with a single-point measurement.

Calculation bases

The flow calculation according to the dynamic pressure principle is derived from the energy conservation law . At the flow point (stagnation point) of the dynamic pressure probe, the flow is slowed down and converts its kinetic energy (speed) into potential energy (pressure).

The flow rate of the fluid can be calculated from the measured differential pressure:

${\displaystyle q_{m}=K\epsilon {\frac {\pi }{4}}d_{i}^{2}{\sqrt {2dp\rho }}}$

Here are:

• ${\displaystyle q_{m}}$ - mass flow (or mass flow)
• ${\displaystyle K}$ - dimensionless calibration constant of the pitot tube (K number)
• ${\displaystyle \epsilon }$ - expansion number
• ${\displaystyle d_{i}}$ - Inside diameter of the pipeline
• ${\displaystyle dp}$ - differential pressure
• ${\displaystyle \rho }$ - density of the fluid

The dimensionless calibration constant K is determined by the various manufacturers for their pitot tubes and communicated to the user. A typical value is between 0.62 and 0.68.

The expansion number corrects the change in density of the fluid caused by the pressure loss at the pitot tube. For incompressible fluids (liquids) is ; for compressible fluids it becomes smaller than 1, but typically remains in the range . The density of the fluid is the density immediately in front of the pitot tube. ${\displaystyle \epsilon }$ ${\displaystyle \epsilon =1}$ ${\displaystyle \epsilon }$ ${\displaystyle 0{,}97<\epsilon <1}$ ${\displaystyle \rho }$

Designs

Pitot tubes based on the annubar principle differ in terms of measurement profile and connection design.

The measuring profile of a pitot tube is the part in the pipeline that is in flow. The connection part with which the pitot tube is installed in the pipeline and measuring devices such as differential pressure transducers, pressure or temperature transducers is visible from the outside.

Applications / limits of use

Pitot tubes based on the annubar principle are mainly used in process engineering systems, for example in power plants, in chemical and petrochemical plants, in breweries, incineration plants and sewage treatment plants. Since pitot tubes have dead volumes, they are usually not used for food or other sensitive products, but primarily for auxiliary media such as steam, compressed air, heating water, thermal oil, flue gas, etc.

The simpler installation and the low pressure loss (or energy loss) are advantageous compared to classic dp measuring methods such as orifices, venturis or nozzles.

Classic primary elements, on the other hand, have the advantage of being standardized and standardized internationally (ISO 5167). The use of pitot tubes is inexpensive, especially with large pipelines, since the price of pitot tubes develops approximately linearly with the diameter, whereas that of classic primary elements becomes square or cubic.

literature

• Measurement technology on machines and systems , Heinz Stetter (Ed.), BG Teubner, Stuttgart 1992
• Willy Heusing: More efficiency through pitot tubes . In: BWK . tape 59 , no. 6 , 2007, p. 24 ( systec-controls.de [PDF; 560 kB ]).
• Reinhold Kuchenmeister: Instead of diaphragm, venturi or vortex . In: CITplus . No. 1–2 , 2014, pp. 28–31 ( systec-controls.de [PDF; 832 kB ]).
• Calculation bases for Iso primary elements and pitot tubes. (PDF; 127 kB) systec-controls.de 
• ISO 10780
• ISO 5167

Individual evidence

1. ↑ Calculation of the speed for S-Type Pitot tubes, 2017, Airfl ow Lufttechnik GmbH (PDF file; 4 pages)