The Torbar is employed for flow measurement of liquids, gases, or steam in circular, square, or rectangular section ducts for large flow rates. The Torbar is an insertion type multi-port self-averaging primary sensor for flow measurement.
TORBAR is a set of Pitot tubes mounted on a bar across the pipeline with no moving parts.
An averaging Pitot tube is a technology, while TORBAR is a manufacturing brand name. There are several brands available in the market with VERABAR, ANNUBAR, etc.
Averaging Pitot tube can be employed when the average velocity of the flow profile, rather than the velocity in a specific point in the cross-section is desired.
Averaging Pitot Tubes Principle
It measures the differential pressure between the static pressure tap and the tap of full pressure of a stream. Thus such magnitude of differential pressure is directly proportional to the square of the flow rate.
The TORBAR is designed in such a way that the full pressure chamber opening is facing against the stream and the tip of the tube features a conical aerodynamic profile as shown in the figure below.
The static pressure tap opening is made on the cylindrical plane of the tube, it is perpendicular to the direction of the stream of flow. Having a set of tubes across the pipe allows good averaging of the velocity profile.
TORBAR is a dual-chamber tube with multiple sensing ports on both chambers.
An averaging pitot tube measures flow by averaging the pressure at various ports also the length of a probe.
A typical flow meter comprises low-pressure and high-pressure chambers with multiple ports drilled into each.
The ports in one chamber face upstream and the port in another face downstream. The probes insertion into the flow stream creates dynamic (impact pressure) and static pressure.
The upstream ports and downstream of the probe collect the sample and average the dynamic pressure, static pressure respectively. A readout device or transmitter measures the differential pressure between the chambers.
This flow element when inserted into the flow stream produces differential pressure. That is upstream and downstream, this differential pressure producing device which when integrated with differential pressure transmitter gives the flow signal.
Multiplying the velocity of fluid by the pipe’s cross-sectional area and a flow coefficient gives the volumetric flow rate. Manufacturers mention the flow coefficients while supplying the tube, which varies with tube diameters and pipeline diameter.
Installation and Location
Upstream and downstream straight run pipe runs are recommended as disturbance due to valves, T’s, and bends affect the performance of the measurement. Manufacturer data sheets provide installation recommendations.
The below figure illustrates the vertical and horizontal pipeline installation recommendations for gas, liquid, and steam flow service.
- Averaging pitot tubes can be installed by the hot tap method or wet tap method without the need for a process shutdown.
- Improved accuracy.
- Flexible design.
- Wide range of processes and their conditions from high vacuum to high pressure.
- The installation is a low-cost solution for measurement in large diameter pipes or ducts.
- Can be installed easily, with low-pressure loss, robust construction, and negligible wear.
- Fluids should be clean, care need to be taken regarding the acceptable size of solid particles within a range as specified by the supplier.
- Better accuracy only with high-velocity fluids.
- Helpful for mapping flow profile in large-sized ducts, channels.
Author: R.Jagan Mohan Rao
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