Measurement of the speed and flow of the fluid is one of the basic problems of fluid mechanic. For this purpose, there are mostly used indirect methods, in which the fluid flow speed influences another value that can be measured more easily. One of the simplest method is the indirect measurement of fluid speed and flow by conversion to pressure – utilization of Bernoulli equation.
Speed sensor is a meter working on principle of differential pressure measurement. At the inlet there are tappings for measurement of total pressure of the flowing fluid and at the outlet there are static pressure tappings. Differential pressure is determined by the difference of these two values. Speed sensor is mainly being used for the measurement of speed and mass flow of fluids, gases and steams. The speed sensor can be designed and supplied within nominal diameters of the pipelines from DN 20 to DN 2,000. These meters are widely used in the power industry, petrochemistry and in other industries. For proper functioning the speed sensor requires conduits running full with fluids as well as steady flow without pressure pulsations.
The advantage of this speed sensor is: low pressure loss, simple design, min. failures in operation caused by mechanical damage and, possibility of utilization of sensor in the pipelines of circular and square (rectangular) cross-sections.
The main disadvantage of the speed sensor when designing it, is that neither its calculation, nor the design are defined by standards. Another disadvantage in operation is the change of surface at the inlet due to abrasion by the flowing medium. Higher surface roughness of the sensor leads to higher measurement uncertainty. Other negatives of the operational character are: higher vulnerability to clogging of tapping slots by impurities and sediments from the flown fluid, possibility of deflection of the sensor (probe) due to pressure of the medium, or due to exposure of high temperatures in the long term (so-called temperature creep).
Speed sensors can be made of carbon, stainless steel or refractory alloyed steels and nickel alloys. These are inserted in the pipelines by weld-in pieces, to which the sensor itself is mounted by dismountable flanged joint – see figure on the right.