Basics of flow measurements 4

Orifice and DP type flow meters, Vortex flow, Rotameter (variable area), Turbine flow meter, Positive displacement type flow meters... We have covered the above in the articles part 1,2 & 3. But that's not all.

First we will discuss Magnetic flow meter. This is an in-line flow meter which works good for liquid which are conductive in nature to some extent. It is not good for inert gases or non conductive fluids. It works on the principle of electromagnetic induction. The magnetic flux generated by its coils is perpendicular to the axis of flow. Flow tube is lined with PVC, Teflon or some insulation material known as liner. The deviation in magnetic flux caused by the fluid velocity is translated in terms of flow. It calculates volumetric flow as the area of the flow tube is fixed. It is now widely used in many application as it requires less maintenance. Normally MAG flow meters as these are called use a four wire system. One pair for excitation power and other for the signal. Now Yokogawa has recently launched its two wire MAG flow version in Japan.

Ultrasonic flow meters are another type of in-line meters which use Ultra sonic waves to detect the flow of fluids in a pipe. Reflective or diagonal type are there based on the location of the sensors across the tube. It has good repeatability and accuracy but is more prone to noises. The transmitter generates ultra sonic signal. Receiver is either diagonally located on the other side of the pipe or in other cases reflective receiver picks up signal downstream of the pipe at 45 degree angle detect these signals. It works on the principle of Doppler's effect. Due to flow velocity, the signal strength will vary and is translated in terms of flow. It is also widely used in industries. Another version of these flow meters come as clamp-on meters which are clamped externally on a pipe or tube at specific distances to measure the flow in that pipe or tube. It is more popular to measure flow at remote areas or places where a fixed meter cannot be installed.

Both the MAG flow meters and the ultrasonic flow meters do not cause a pressure drop in the flow and have good resolution because they are using inline flow measurement. Major drawback is when replacing the Magflow meters, the line needs isolation, de-pressurizing, draining and purging if required. Ultrasonic flow meters also need full isolation if the sensor is an inline one. In case clamp-on type is used, it is easily replaceable.

As we unfold the methods of measurement of flow many more other industrial flow measuring instruments will be discussed in the series of articles on this newsletter, so next week we will be again coming back to you...Just subscribe to this newsletter INSTRUMENTATION REVISITED and receive updated version every week. Please feel free to share it if you like it. Comments, suggestions, feedback are always welcomed.

(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)