The flow nozzles is a flow tube consisting of a smooth convergent section leading to a cylindrical throat area. The throat is the smallest section of the nozzle. Pressure taps are located on the upstream side of the nozzle plate and on the downstream side of the nozzle outlet. They may be in the form of an annular ring, i.e. equally spaced holes connected together which open into the pipeline, or in the form of single holes drilled into the pipeline.
As mentioned above, flow nozzles are primary elements in differential pressure flow meters. These flow meters use the primary elements as an obstruction to generate a pressure drop to calculate the flow rate. This is based on Bernoulli’s principle, according to which any obstruction placed in the path of a flowing fluid will cause the velocity of the fluid to increase and the pressure to decrease in the area of the obstruction.
As the fluid passes through the nozzle, the obstruction causes the velocity of the fluid to increase while its static pressure decreases simultaneously. At the point of maximum convergence, i.e. at the vena contracta, the velocity is at its maximum and the pressure is at its minimum. As the fluid exits the nozzles, its flow expands and the velocity reduces and the pressure rises again. This difference in pressure before and after the primary element is measured using differential pressure transmitters, also called secondary elements. Bernoulli’s equation is used to determine that the pressure differential is proportional to the square of the flow rate. The flow rate is therefore calculated as follows: