I. Overview

Pitot tube flowmeter is a device to measure flow rate by differential pressure. It is developed on the basis of pitot tube velocity measurement principle. The flow rate of liquids, gases and vapors can be measured. Because there are no moving parts, almost no pressure loss, easy installation and maintenance, very low operating cost, favored by users.

The sensor is composed of a detecting rod, a pressure port and a guide rod, which traverses the interior of the pipeline perpendicular to the pipe shaft. A plurality of pressure measuring holes are arranged on the flow surface of the measuring rod to measure the average value of total pressure. There are static pressure measuring holes on its back and side flow surface, which are respectively drawn from the total pressure guide pipe and static pressure guide pipe. Static pressure of flow tube wall can also be used instead of static pressure of sensor back surface.

The uniform klystron flow transmitter is a flowmeter composed of sensor, pressure attachment and differential pressure transmitter, pressure transmitter and flow integrator.

The pitot tube flow sensor is equipped with differential pressure transmitter, pressure transmitter, temperature transmitter and flow integrator produced by our company to form various types of pitot tube flowmeters. It can also use differential pressure transmitter, pressure transmitter, temperature transmitter and flow integrator produced by any manufacturer to form various types of uniform pitot flowmeters.

Two, the working principle

Pitot tube flowmeter is developed based on the principle of pitot tube velocity measurement. It determines the flow rate by the product of the average velocity of the pipeline and the effective cross-sectional area of the pipeline.

The flow velocity distribution in the general pipeline is not uniform. If it is a fully developed fluid, the velocity distribution is exponential. For accurate measurement, the whole circle section is divided into a plurality of semicircles and a plurality of half rings with equal area of multiple units. The detection rod of the sensor is composed of a hollow metal tube, and multiple pairs of total pressure holes are drilled on the oncoming surface. They are located in the center of each unit area, respectively reflecting the velocity of each unit area. Since the total pressure holes are connected, the total pressure value of each point in the detection rod is averaged, and the total pressure extraction pipe is led to the high pressure joint and sent to the positive pressure chamber of the sensor. When the sensor is correctly installed on the process pipeline with a long enough straight pipe section, there should be no vortex on the flow section, and the static pressure of the whole section can be considered as a constant. A detection hole is provided on the back or side of the sensor, representing the static pressure of the whole section. The static pressure outlet pipe is led to the negative pressure chamber of the sensor through the low pressure joint. The square of the differential pressure in the positive and negative pressure chambers is proportional to the average velocity of the flow section, and the differential pressure is proportional to the flow. On the basis of this relation, the flow calculation formula of uniform pitot flowmeter can be derived from Bernoulli equation and continuity equation

Qv=α·ε· (π / 4) ·D2· (2∆P / ρ1) 0.5

Qm=α·ε· (π / 4) ·D2· (2∆P·ρ1) 0.5

Qv: volume flow rate Qm: mass flow rate

α : sensor structure coefficient

△P: Differential pressure value ε : fluid expansion coefficient

ρ : density ε : fluid expansion coefficient

For the incompressible fluid ε=1, for the compressible body ε<1, if the formula D, △P, ρ1 are used SI units, the unit of QV is M3 / S, Qm unit for ㎏ / S.

The flow coefficient α and expansibility coefficient ε of the sensor are determined by the standard device and indicated in the certificate when leaving the factory.