Pitot-tube traverse

When the veloeity pressure is more than 5% of the pressure rise, it should be determined by a pitot-tube traverse of two stations. For each station, the traverse consists of 10 readings at positions representing equal areas of the pipe cross section, as shown in Figure 20-2. The average velocity pressure Py is given by... 

This is seen to be similar in form to Eq. (10.57), and if the velocity at point 1 could be determined in some way, the value of the jet velocity could be computed directly. If Q were measured, then Vt = Q/Au or V1 might be determined by a pitot-tube traverse. But even if Vj is known... 

If the volume or mass flow rates are required in a round or rectangular ducts then one must take measurements at various locations across the duct. It is important that the Pitot traverse be conducted at least 8 duct diameters downstream or 2 diameters upstream of an obstruction such as fans, elbows, dampers, expansions, and so on. Figure 4.28 shows the locations for a Pitot tube traverse in a round or rectangular duct, based on centroids of equal area, for determining the volume or mass flow rate [28]. 

Pitot tube traverse locations for round and rectangular ducts based on centroids of equal areas. (Adapted from Flow Kinetics LLC, "Using a Pitot Static Tube for Velocity and Flow Rate Measurement.," http //www.flowmeterdirectory.com/flowmeter artc/flowmeter artc 02111201.html)... 

Pitot-Tube Traverse for Flow Rate Measurement. In a pitot tube traverse of a... 

Once these traverse points have been determined, velocity measurements are made to determine gas flow. The stack-gas velocity is usually determined by means of a pitot tube and differential-pressure gauge. When velocities are very low (less than 3 m/s [10 ft/s]) and when great accuracy is not required, an anemometer may be used. For gases moving in small pipes at relatively high velocities or pressures, orifice-disk meters or venturi meters may be used. These are valuable as continuous or permanent measuring devices. 

Pitot-static traverse The set positions of a Prandtl tube m a duct run required to provide a statistically valid set of readings, A series of measurements of the torn 1 and static pressure taken across an area of a duct to determine the air veloc-it> at that point. The sampling distance should be at least 7.5 times the diameter of the duct away from any disturbances of air flow. 

Traversing The process of moving across a grid line in a duct or on a hood with a Pitot tube in order to determine the velocity or pressure distribution. 

I he flowrate of air at 298 K in a 0.3 m diameter duct is measured widi a pilot tube which is used (o traverse the cross-section. Readings of the differentia) pressure recorded on a water manometer are taken with the pitot tube at ten different positions in the cross-section. These positions are so chosen a to be the midpoints of ten concentric annuli each of the same cross-sectional area. The readings are ... 

The flowrate of air in a 305 mm diameter duct is measured with a pitot tube which is used to traverse the cross-section. Readings of the differential pressure recorded on a water manometer are taken with the pitot... 

Although Pitot tubes are inexpensive and have negligible permanent head losses they are not widely used. They are highly sensitive to fouling, their required alignment is critical and they cannot measure volumetric flow rate Q or mean velocity u. The latter can be calculated from a single measurement only if the velocity distribution is known this can be found if the Pitot tube can be traversed across the flow. 

The Prosser was calibrated by measuring the air flows using a laminar flow meter (1% accuracy) for the odorous sample and a pitot tube with a micromanometer for the fan-blown air (3). The pitot pressures were converted to air velocities (4) and hence, from the cross sectional area of the tube, to volumetric flow rates. Since flow near the tube wall was slower than the centre, the tube was traversed by the pitot head and the average value calculated. A rotameter was also tried but it induced a back-pressure of 250 N/m2 and, as the manufacturer states that the maximum permissible back-pressure is 60 N/m for calibration to be accurate, its use was not pursued. 

The coefficient of velocity may be determined by a velocity traverse of the jet with a fine pitot tube in order to obtain the mean velocity. This is subject to some slight error, as it is impossible to measure the velocity at the outer edge of the jet. The velocity may also be computed approximately from the coordinates of the trajectory. The ideal velocity is computed by the Bernoulli theorem. 

Henri de Pitot invented the Pitot tube in 1732. It is a small, open-ended tube that is inserted into the process pipe with its open end facing into the flow. The differential between the total pressure on this open impact port and the static pipeline pressure is measured as an indication of the flow. The Pitot tubes provide a low-cost measurement with negligible pressure loss and can also be inserted into the process pipes while the system is under pressure (wet- or hot-tapping). They are also used for temporary measurements and for the determination of velocity profiles by traversing pipes and ducts. 

The principle of operation of a pitot tube is discussed in Section 6.3.1. It should be emphasised that the pitot tube measures the point velocity of a flowing fluid and not the average velocity so that in order to find the average velocity, a traverse across the duct is necessary. Treatment of typical results is illustrated in Problem 6.16. The point velocity is given by u = (2gh) where h is the difference of head expressed in terms of the flowing fluid. 

Determine the pitot tube locations in an 8-ft inner diameter (ID) circular pipe for a 16-point traverse. 

Sample and velocity traverses for stationary sources with small stacks or ducts Determination of stack gas velocity and volumetric flow rate (Type S pitot tube)... 

Pitot static tube, developed by the National Physical Laboratory, is recommended in BS 1042 Part 2.1 for measuring the flow of air in ducts by the traverse method (Figures 11.5a and b). The Pitot tube consists of two concentric tubes with the end turned through 90°, so that after insertion through the duct wall, the tip can face the air stream. The nose senses the total pressure and the ring of holes in the side senses the static pressure, with the inlets terminating in tails that are connected to the opposite sides of a sensitive manometer (Figure 11.6). The... 

Pitot tube An instrumentused to measure the velocity ofaflowingfluidby measuring the difference between the impact pressure and static pressure in the fluid. The device normally consists of two concentric tubes arranged in parallel one with a face directed towards the flow to measure the impact pressure, the other face perpendicular to the flow to measure the static pressure. By taking a number of readings at various points in the cross section of a pipe or duct, known as a Pitot traverse, the overall rate of flow can be determined. As with all flow measurement devices, Pitot tubes should ideally be located away from disturbances such as bends. The device was devised by Italian-born French engineer Henri de Pitot (1695-1771). 

Figure 11.5 Location of velocity measurements in ducts, a. Log linear rule for posifions of fraverse points for a Pitot survey on three different diameters of a circular ducf b. Log Tchebycheff Rule for position of measuring points and traverse lines for Pifof survey of a recfangular ducf. Note Minimum ratio of ducf diameter to Pitot static tube diameter is 32 BS 1042 Part 2.1. Source Reprinted from Daly BB, Woods Practical Guide to Fan Engineering, Woods of Colchester, 3rd ed., 231, 1992. By kind permission of Flakt Woods Ltd 2004.

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