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ABB self-averaging flowmeter offers versatile, cost effective solution for gas, liquid and steam measurement

Posted: Thursday 20th October 2016

ABB takes classical Pitot tube design one step further to offer new levels of performance and versatility.

ABB's FPD350 Torbar self-averaging pitot tube offers a low cost, easy to install solution for measurement of gases, liquids and steam. With no moving parts and featuring an innovative sensor design that helps minimise pressure loss, the Torbar provides an economical, low maintenance solution for a wide variety of applications including flue gases, compressed air, water, sea water and liquids.

The Torbar can be installed in circular, square or rectangular section ducts from 15 to 8,000mm (0.5 to 315in) in diameter. All Torbar meters feature a four piece design, comprising an outer impact tube, internal averaging tube, low pressure chamber and head unit with high pressure and low pressure impulse connections.

A key feature of the Torbar is its dual averaging design, which enables it to offer improved accuracy, especially in non-ideal conditions. A pitot tube measures flow by sensing the difference between the impact pressure of the flow and the static pressure. Unlike a conventional single-point pitot tube, the Torbar has multiple impact-sensing ports across the pipe diameter and produces an averaged differential pressure signal proportional to the flow rate.

The outer impact tube has a number of pressure sensing holes facing upstream that are positioned at equal annular points in accordance with a log-linear distribution. The 'total pressures' developed at each upstream hole by the impact of the flowing medium are firstly averaged within the outer impact tube before being averaged again within the internal averaging tube.

This feature helps to address the limitations of conventional averaging pitot designs, where the upstream sensing hole nearest to the take-off point has a greater effect on the measured reading than the other holes, resulting in lower tolerance of non-uniform flow profiles. In the Torbar, the various pressures from the multiple ports are collected into a common manifold in the internal averaging tube. The pressure is taken from the neutral position of the manifold to produce the high pressure signal for the DP flow calculation.

The low pressure component is generated from a single sensing hole located on the downstream side of the outer impact tube, measuring static pressure.

For bi-directional flow measurement, the Torbar can be supplied with the same number of downstream ports as upstream.

Another key feature of the Torbar is its innovative sensor design. The use of profiled flat edges on the side of the device helps to create a fixed separation point of the flow stream, producing a stable pressure area where the static pressure can be reliably measured. By maintaining a more constant flow co-efficient at high velocities, the use of the flattened edge design produces a typical turndown of 10:1, enabling flow to be measured across a very wide range.

The compact design of the Torbar makes it suitable for use in locations where space is limited or where obstacles are present which could impair the performance of other flowmeter types. Requiring fewer straight pipe lengths than many other flowmeter types, Torbar meters can overcome this challenge. They can achieve similar accuracy to other primary flow elements, particularly if they are in the same plane of a disturbance such as a pipe bend or elbow.

With a narrow profile, the Torbar presents a much smaller restriction to flow than other types of devices such as orifice plates. Torbar meters also offer a greatly reduced pressure drop, enabling substantial savings on energy costs to be achieved.

As an insertion device, the Torbar is simple to install. Available with threaded or flanged fittings, they can be quickly fitted through the wall of a pipe or duct, reducing the time and cost of installation. Their optional retractable design also means that removing them for checking or maintenance is equally straightforward. They can be inserted or removed while a system is still under pressure, further reducing the disruption associated with maintenance.

To help users ensure that their Torbar will be suitable for their application and will be protected against potential damage, every Torbar calculation includes a check of the resonant frequency band, as well as of the maximum allowable DP.

Suitable for applications up to 1,200oC, the Torbar is available in a variety of wetted materials. As well as a standard 316 stainless steel option, users can choose from many grades of stainless steel (including Duplex and Super Duplex alloys) and other exotic alloys (such as Alloy 276, 400, 600, 800 etc).

The inherent benefits of Torbar can also be further extended to applications requiring volumetric or mass flow by fitting an optional integral DP or Multivariable transmitter and RTD element.




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September 2017

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