subject: Measurement of Flow of Fluids [print this page] Measurement of Flow of Fluids Measurement of Flow of Fluids
In modern industrial technology, pipelines are largely used for conveying a wide variety of gases liquids and even solids. Surely, it is important to be able to measure the rate of flow of materials conveyed through pipelines i.e., the quantity that passes in the unit of time (ft.3/sec., m3/min., etc) Flow measuring devices are of various kinds. The present article is concerned only with so-called volumetric meters. Such meters are used more particularly for the measurement of the gas and water, respectively.
Both gas and liquid flow can be measured in volumetric or mass flow rates such as litres per second or kg/s. These measurements can be converted between one another if the materials density is known. The density for a liquid is almost independent of the liquids conditions, however this is not the case for a gas, whose density highly depends upon pressure and temperature.
Flow-measuring devices of the direct type for liquids function on either of two principles: (1) a measuring chamber of known capacity is repeatedly filled and emptied the rotating measuring element displaces a known quantity of liquid in performing each revolution. These displacement-type meters, though differing in design and technical features, all operate on the same principle, which will here be described more particularly with reference to the oval-runner meter.
It consists of two rotating elements of oval cross-sectional shape which mesh with each other. They are enclosed within a cylindrical casing which forms the measuring chamber and is provided with an inlet and an outlet. Fig.6 shows the oval rotating elements in four successive positions in the course of one revolution, during which each crescent-shaped space at the top and bottom of the measuring chamber is twice filled.
The total volume of liquid that is passed through the measuring chamber from the inlet to the outlet during each revolution of the oval elements is equal to 4 Fs h, where Fs is the cross-sectional area of each crescent-shaped space and h is the transverse dimension of the measuring chamber. The power for driving the oval elements is supplied by the liquid flow itself.
The pressure difference Dp across the meter acts upon the major and minor projected areas f and F of the lower oval element, which areas are thus subjected to the resultant forces PF and Pf respectively. Since PF is larger than Pf and moreover has a larger lever arm with respect to the center of rotation of the oval element, the latter is thus subjected to a torque (turning moment) which causes it to rotate. The upper oval element, when in the position is subjected to a torque of zero magnitude, since the resultant forces acting on each side of the center balance each other.
