3.8 averaging pitot tube

The pitot tube calculates the velocity by measuring the stagnant pressure or the pressure formed by the fluid velocity. The pressure at the impact pressure is called total pressure or stagnation pressure. If the pressure at the low static pressure tap is considered to be the static pressure of the pipe, the differential pressure is referred to as the dynamic pressure of the fluid at that point. Invented by Henry de pito and first used in 1784, this form is mainly used for pitot tube. The curved modern pitot tube used to measure flow is the averaging pitot tube or apt. The purpose of apt is to measure the flow on the pipe by measuring the dynamic pressure on the pipe diameter and averaging it.

Pitot tube only measures point velocity. Unless the velocity profile is known or the flow is considered to be fully developed, a single velocity measurement cannot represent the average velocity required to accurately calculate the flow. Therefore, the pitot tube can be moved horizontally, that is, the pitot tube can be moved in the radial direction of the tube during sampling. The average flow rate can be calculated from these sample values. The sampling location in the pipeline helps to obtain the average flow rate.

Apt provides a faster way to obtain the average velocity. The Rosemount 485 averaging pitot tube is shown. The main advantages of apt on traditional section flowmeter such as nozzle or orifice plate are as follows:

Apt can be installed through the pipe joint, so the welding operation is less and the cost is lower;

Under the condition of pipeline pressure, "opening with pressure" or apt can be installed;

The permanent pressure loss of apt is much less than that of typical cross-section flowmeter.

Compared with single point pitot tube, the following are the main differences of apt:

Installed on the pipeline plane, the front of the pitot tube is arranged with a notch or hole, and the velocity profile can be determined by sampling. This is equivalent to "continuous pitot tube displacement".

The formation pressure of the fluid staying (or stagnating) in front of each notch or hole reflects the flow velocity at this point in the flow field. In addition, the opening at the front of the pitot tube must be perpendicular to the flow velocity vector to obtain the appropriate detention pressure.

If properly designed, the pressure sensed at the top of the apt anterior chamber is the average retention pressure of the sampling notch or hole.

The back chamber measures the pressure or suction pressure at the back of the pitot tube. For the actual fluid in turbulent state, the fluid is separated from the pitot tube, so the pressure is less than the static pressure of the tube. This is advantageous because the differential pressure signal is higher than that obtained by the standard pitot tube. However, if the apt is not properly designed, the bottom pressure at all operating flows is unpredictable.

Although the pressure sensed at the top of the back chamber is the average suction pressure, in most cases, the pressure behind the pitot tube is almost the same as the pressure on the diameter of the tube due to the separation of the flowing fluid and the surface of the apt tube along the tube length.

Schematic diagram of vortex separation of Rosemount Annubar sensor. The upstream surface of the Rosemount Annubar T-shape design is flat, thus forming a fixed separation point, which can improve performance and stabilize low-pressure measurement in a large flow range compared with other apt sensor designs.

The sensor shape design of the averaging pitot tube varies greatly from manufacturer to manufacturer. Sensor shape can have a significant impact on performance. Generally, the performance of warhead shape, circular, fan-shaped or elliptical sensor shape in a specific flow range is poor, especially when the Reynolds number is low, because the differential pressure signal strength is weak when there is no fixed separation point.

The theoretical formula is based on the following assumptions:

No viscous effect

No heat transfer

Incompressible fluid

Correct the average pitot tube flow coefficient (k) under the following assumptions:

Negligible viscous effect

Negligible heat transfer

Pressure tap at ideal location

Correction of the assumed gas expansion coefficient for incompressible fluids.

3.9 precautions

The flow computer usually uses the variable of differential pressure flow device or other measuring points to calculate the flow. Configure a flow computer to calculate flow based on fluid properties and installation details, such as line size and process variables for each pressure and temperature measurement or multiparameter transmitter such as Rosemount 4088.

Another option is to use a multiparameter transmitter capable of calculating flow, the Rosemount 3051smv. Rosemount engineering assistant is a PC software program for configuring Rosemount multivariable Gamma Mass flow output of the device. In addition to the ability to configure and calibrate the unit, engineering assistant can also configure the mass flow equation inside the transmitter. This software makes the creation of compensation flow equation simpler than that of manual creation of flow equation in control system. This is because the configuration of the flow equation is completed in the engineering assistant, and the flow calculation is performed by the transmitter. Users only need to input basic flowmeter and process information to configure the transmitter's fully compensated mass or energy flow.

Engineering assistant can be used as a stand-alone window program, or as snap-on for AMS. The snap-on version runs within AMS, while the standalone version can run without AMS installed.

The common mistake in the differential pressure flow device is to perform twice square root calculation or calculate the differential pressure square root in the flow equation of the transmitter and control system. Whether it is control system or transmitter, only one square can be obtained