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Introduction : Shortened version of a venturi tube, Differs significantly from venturi as per below, Abrupt shoulder at the inlet cone, Low pressure tap at the throat, Produces about twice the dP of venturi

Application : The dall flow tube has a higher ratio of pressure developed to pressure lost than the venturi flow tube. It is more compact and is commonly used in large flow applications. The tube consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube. This section, called the inlet shoulder, is followed by the converging inlet cone and a diverging exit cone. The two cones are separated by a slot or gap between the two cones. The low pressure is measured at the slotted throat (area between the two cones). The high pressure is measured at the upstream edge of the inlet shoulder. The dall flow tube is available in medium to very large sizes. In the large sizes, the cost is normally less than that of a venturi flow tube. This type of flow tube has a pressure loss of about 5%. Flow rate and pressure drop are related as shown in Equation Below

V=K/DP

where V = volumetric flow rate K = constant derived from the mechanical parameters of the primary elements DP = differential pressure

Disadvantages :  The Dall tube is a shortened version of a Venturi meter. In both flow meters the flow rate is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is measured using diaphragm pressure transducers with digital read out. These meters have significantly lower permanent pressure losses that orifice meters and are widely used for very large flow rates where even a small decrease in loss is economically significant.

*- When it was introduced in the early 1950s, the Dall tube was hailed for its apparent low energy consumption (more about that below), but what was sacrificed was the ability to accurately and reliably predict the meter discharge coefficient. The classical venturi tubes that preceded the Dall design had discharge coefficients that were essential independent of line size and beta ratio ((throat diameter)/(inlet diameter), or d/D), therefore it was easy to determine the performance of the design, INDEPENDENT OF LINE SIZE AND BETA RATIO. The Dall design violates the venturi principle, so much more data must be collected to define the performance to the same degree. Furthermore, when high Reynolds number flow test facilities came on line in the late 1960s and early 1970s, it was found that the Dall design discharge coefficient was indeed a function of Reynolds number, The resulting data indicated errors of up to 12% on flow rate. Once that data was published, the Dall tube fell out of favor, almost universally.

One exception to the universal condemnation of the Dall tube has been the in UK and in former British protectorates. Specifications out of Hong Kong, Singapore, and Indonesia (many large petroleum engineering firms are located there) still specify the Dall tube, even with all the published data that contradicts the specified performance of +/-1%.

With respect to headloss, while the overall energy loss of the Dall tube appears to be about half that of a venturi meter with the same beta ratio, it only appears so when the energy loss is expressed in terms of percentage of differential pressure produced. The reason for this is that the high pressure tap is located right before the sharp convergence angle and the low pressure tap is located immediately after the convergence. These tap locations sense not only the local "static" pressure, but also a portion of the flowing fluid's momentum. The fluid momentum effectively increases the high pressure reading and decreases the low pressure reading. The differential is essentially "magnified" (by a factor of almost two), hence the permanent pressure loss, when expressed in terms of percentage of differential pressure produced, appears to be half that of a classical venturi tube. The actual energy consumption, however, is about the same (for meters with the same beta ratio).

It is to be noted that overall energy loss in terms of percentage of the Dall meter is about half that for a Venturi meter for a similar differential pressure loss. The reason for this is that the high pressure tap is located right before the sharp angle creating a momentum effect. This magnifies the high pressure reading. Conversely, the low pressure tap is located in the middle of the throat where the pressure is sucked down creating a lower pressure reading. The differential is seen as greater in terms of percentage.