Production • Processing • Handling
● A wax layer build-up on the inner wall of the spool piece reduces the flow area, which increases the meter’s K-factor. Since flow area A is equal to the pipe radius squared (A = πR2
) even a relatively
small build-up can cause a large measurement error. For example, a 0.1 mm build-up on the interior diameter of a six-inch meter will result in a 0.27 per cent measurement error. For a 20-inch meter, the error is proportionally smaller, about 0.08 per cent. Regular in-situ proving of the flow meter will correct for misreading due to such wax build-up.
Proving recommendations In-situ proving at regular intervals is recommended to maintain optimum measurement accuracy. Ultrasonic meters, as previously stated, are like
turbine meters in that they infer the volumetric throughput by measuring the velocity over the flow area. For low viscosity products the velocity profile is flat and the flow velocity is nearly constant over the flow area, except for a region near the pipe wall (Fig. 2b). Terefore, the average stream velocity can be measured at any point except near the pipe wall. As the viscosity increases and/or the flow decreases, the flow profile becomes parabolic (Fig. 2a). Maximum velocity is at the center of the pipe and the velocity decreases gradually to zero at the pipe wall. To determine the average stream velocity for this type of profile, the stream velocity is measured at several selective points and the velocities are integrated with an algorithm to determine the average velocity. Te relationship between velocity and viscosity is defined by Reynolds Number which is the ratio of flow rate to the meter size and the viscosity {Re No ≈ (flow rate)/(meter size x viscosity)}
Fig. 2a. High viscosity.
w w w.wir elinet ech .com Fig. 2b. Low viscosity. Circle 49 or ✔ at
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