WO2003089883A1

WO2003089883A1 - Multi-point averaging flow meter

Info

Publication number: WO2003089883A1
Application number: PCT/US2003/000651
Authority: WO
Inventors: Raymond Kee Heun Kim
Original assignee: Seojin Instech Co., Ltd.
Priority date: 2002-01-10
Filing date: 2003-01-09
Publication date: 2003-10-30
Also published as: AU2003239115A1; KR20030061108A; CN1615429A; KR100433719B1

Abstract

An averaging Pitot tube flow meter (20) having a plurality of circumferentially spaced pitot heads (2) extending radially inward from an inner surface of an annular cylindrical body (1). Each pitot head includes one or more stagnation pressure holes (3) and a static pressure hole (4). A stagnation pressure header connects the stagnation pressure holes for providing an average stagnation pressure and a static pressure header connects the static pressure holes for providing an average static pressure.

Description

MULTI-POINT AVERAGING FLOW METER BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The present invention relates in general to a class of flow meters known as "Averaging Pitot Tube (APT) Flow meters" that is used to measure rates of flow of liquids and gases, and more particularly, to a new and useful flow sensing element for improving the measurement accuracy and structural strength that are generally available with APT flow meters.

Definition of Terms

[0002] Pitot Tube: Thin and long concentric metallic tube with a 90- degree bend, which is equipped with holes to sense stagnation pressure and static pressure (See Figure 1).

[0003] Averaging Pitot Tube (APT): A mechanical assembly consisting of 1) one straight double-chambered metallic tube which is equipped with multiple holes designed to sense stagnation and static pressures, and 2) hardware for connecting the pressures to a pressure transducer (See Figure

2).

[0004] Averaging Pitot Tube (APT) Flow Meter: When an APT is installed on a fluid-carrying pipe through interfacing hardware, it becomes an Averaging Pitot Tube Flow Meter (See Figure 3).

Description of the Related Art

Basic Principle of Pitot Tube as a Velocimeter

[0005] APT flow meter is an application of a tool known today as the

Pitot Tube that was first used by Henry Pitot around 1730 for the purpose of measuring the velocity of water in a river in France. Since then, Pitot Tubes have been used to measure gas and liquid velocities such as, air speeds on airplanes and in large ducts of air conditioning systems, flue gas velocities in power plants, etc. [0006] A Pitot Tube consists of a pair of thin stainless steel concentric tubes that are bent at right angle, as depicted in Figure 1. The inner tube is open to face the direction of flow, and there are usually two or more small holes drilled around the outer tube so that the static pressure on the outer surface can be transmitted to the annular space between the inner and outer tubes. The tip of the Tube senses the stagnation pressure. These pressures are transmitted to a pressure sensing instrument, such as a manometer or an electronic pressure transducer, which measures the difference between the two pressures. It is well established in elementary fluid mechanics that this difference represents the dynamic pressure:

Stagnation Pressure - Static Pressure = Δ P = Dynamic Pressure which is defined as:

Δ P =p V²/2g where p : density of the fluid

V: velocity of the fluid g: gravitational constant Thus,

- ^■

Therefore, the local velocity of a flowing fluid can be calculated by knowing the density of the fluid and measuring the differential pressure across a Pitot tube.

Applications of Pitot Tube as a Flow Meter

[0007] Pitot tubes have also been used, as a whole flow meter, to measure the rate of flowing fluid within pipes and ducts. A Pitot tube is commonly used to measure local fluid velocities at the various points from one wall to the opposite wall by traversing the tube. Traversing in two or more di- rections are usually required to cover a large cross-sectional area. Measuring the velocities at a number of points is necessary because fluid velocities differ from point to point within any one cross-sectional area of a pipe or a duct. Fluid velocities are generally higher in the middle and lower toward the wall, but they often are also a function of azimuthal angle. A standard Pitot tube and the traversing scheme established in ASME PTC 11, Test Code for Fans, 1946, depicted in Figure 4, shows how a two-directional multi-point averaging method is used. In this example, normalized radial locations of the 20 measurement points are specified. An average velocity for the entire cross- sectional area is established by taking the average of 20 differential pressure measurements. The total flow rate for the entire pipe or duct is the product of the average velocity and the cross-sectional area of the pipe.

Averaging Pitot Tube Flow meter

[0008] Measuring many local velocities by traversing in two or more directions is a cumbersome and time-consuming process. The realization that the method cannot conveniently be adapted to general industrial environment led to the development of the Averaging Pitot Tube (APT) flow meter. An APT consists of two hydraulically separated inner tubes: a stagnation pressure plenum and a static pressure plenum. The APT flow meter illustrated in Figure 3 is equivalent to a single-direction traversing scheme with six Pitot tube measurement locations. The advantage of this device over traversing method is that the APT is a stationary hardware installed across a pipe that does not require traversing, and the stagnation pressure is averaged hydraulically by combining the six individual stagnation pressures in a plenum. The average static pressure is formed similarly. Thus, the differential pressure representing the dynamic pressure for the entire cross-sectional area can be measured by connecting the pressures from these two plenums to a manometer or a differential pressure transducer. APT flow meters for 3 or 4-inch diameter pipes typically have four or six pairs of sensing holes while larger diameter pipes may have eight or more pairs in order to cover relatively larger cross-sectional areas. Improvements Needed for APT Flow Meter Method for Obtaining Average Velocity

[0009] The basic design of the APT flow meter dictates that all pressure sensing holes on the averaging Pitot tube line up in a diametrical direction. This uni-directionality of the tube is advantageous if the apparent axis of the velocity distribution coincides with that of the averaging Pitot tube as illustrated in Figure 5. This fixed directionality of the APT, however, is also a disadvantage if the axis of the distribution does not line up with that of the tube, which is the case in most pipe and duct flows (See Figure 6). The average velocities obtained in these circumstances contain relatively large errors resulting from inadequate representation of the velocity distributions. Because the shape of any velocity distribution at any one location in a pipe section depends primarily on the upstream flow conditions, which can change with increasing and decreasing flow rates, it is practically impossible to preselect the best direction for insertion of an APT. Therefore, the probability of an APT lining up with any velocity distribution is far smaller than when it does not. This observation leads to the conclusion that most APT-based flow measurements are less than optimum, and there is room for improvement. [0010] An ideal solution for obtaining average velocity distributions under all circumstances may require 12 or more sensors, each with three or more pairs of pressure-sensing holes, so that any velocity field can be adequately covered.

[0011] However, such scheme would have a very high flow blockage, and induce an unacceptably high permanent pressure drop, which is one of the reasons why such a scheme has never been proposed as a possible commercial solution.

Problems in Structural Design and Manufacturing

[0012] Pressure sensing part of an APT consists of two hydraulically separate chambers: one for sensing stagnation pressure, and the other for static pressure. For this reason, an APT is usually made up of two or three tubes welded together, and the thermal distortion induced during welding must be corrected afterwards through such processes as heat treating and mechanical straightening. These operations add to the already expensive basic cost of forming and machining of the tubes.

[0013] An APT must be structurally strong enough to withstand drag and lift-induced vibrational forces. This requirement generally pushed designers to make APTs to be thicker than what would be needed just for sensing the pressures. In other words, structural strength of the tube is a point of concern in the conventional APT flow meter. Therefore, it would be desirable to eliminate this concern from the flow meter by design.

Installation Process

[0014] Installation of the conventional APT usually involves opening a hole in the pipe, and welding the factory-fabricated APT and Support Nozzle onto the pipe (Figure 3) while making sure that yaw, roll and pitch angles of the installed tube are within specified limits. APTs for larger diameter pipes generally require bottom supports also, in which case another hole must be drilled into the pipe and a weldolet welded onto the pipe. This installation is a time-consuming and costly operation that would benefit from simplification or elimination.

[0015] ^N The foregoing illustrates limitations known to exist in present averaging pitot tube flow meters. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

SUMMARY OF THE INVENTION

[0016] In one aspect of the present invention, this is accomplished by providing a pitot tube flow sensor comprising: an annular cylindrical body, the annular cylindrical body having two internal pressure manifolds therein; and a plurality of circumferentially spaced sensor bodies extending radially inward from an inner surface of the annular cylindrical body, each sensor having at least one stagnation pressure sensing hole therein and having a static pressure sensing hole therein, the stagnation pressure sensing holes being in fluid communication with one internal pressure manifold, the static pressure sensing holes being in fluid communication with the other internal pressure manifold.

[0017] In another aspect of the present invention, this is accomplished by providing a pitot tube flow sensor comprising: a plurality of circumferentially spaced sensor bodies extending radially inward only part way towards a common point, each sensor body having a plurality of radially spaced stagnation pressure sensing holes therein and having a static pressure sensing hole therein, the static pressure sensing hole being axially spaced from the stagnation pressure holes, the stagnation pressure sensing holes being in fluid communication with one another, the static pressure sensing holes being in fluid communication with one another.

[0018] The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0019] FIG. 1 is a schematic representation of a prior art Pitot Tube;

FIG. 2 is a schematic representation of a prior art Averaging Pitot Tube;

FIG. 3 is a schematic representation of a prior art Averaging Pitot Tube installed within a pipe;

FIG. 4 is a diagram of a method for measuring average flow velocity by traversing a duct with a Pitot tube;

FIG. 5 is an illustration showing the axis of a hypothetical skewed velocity distribution coincident with the axis of an Averaging Pitot Tube; FIG. 6 is an illustration showing the axis of a hypothetical skewed velocity distribution transverse with the axis of an Averaging Pitot Tube;

FIG. 7A is a schematic representation of the interconnecting passages of the Multi-Point Averaging Flow Meter shown in FIG. 7B;

FIG. 7B is a perspective view of a Multi-Point Averaging Flow Meter according to the present invention;

FIG. 8 is an end view of the Multi-Point Averaging Flow Meter shown in FIG. 7B;

FIG. 9 is a second end view of the Multi-Point Averaging Flow Meter shown in FIG. 7B;

FIG. 9A is a cross-sectional view taken on line 9A-9A of FIG. 9; and

FIG. 10 is a side view of the Multi-Point Averaging Flow Meter shown installed between two flanges.

DETAILED DESCRIPTION

[0020] The present invention, Multi-Point Averaging (MPA) Flow Meter

20, is a means to remove the measurement inaccuracies inherent within the conventional Averaging Pitot Tube flow meters while reducing permanent pressure drop, manufacturing cost and installation cost. The general arrangement of a typical 4-inch MPA Flow Meter 20 is illustrated in Figures 7A and 7B. The MPA Flow Meter 20 has an annular cylindrical body 1. There are six sensors or pitot heads 2, each of which is equipped with two stagnation pressure sensing holes 3 and one static pressure sensing hole 4. Stagnation pressures are sampled at 12 points. These stagnation pressures are connected to the common stagnation pressure output chamber 5 through a network of internal passages 6 machined in the meter body 1 as illustrated in the diagram shown with Figure 7A. Similarly, six static pressures 4 are connected to the common static pressure output chamber 7. The pressures in these two output chambers 5 and 7 are averaged stagnation pressure and averaged static pressure, respectively, the difference of which is the averaged dynamic pressure. A manometer or an electronic pressure transducer 8 is connected to these two output chambers 5 and 7 to measure the averaged dynamic pressure.

More Accurate Average Velocity

[0021] Compared with the one-dimensionality of the pressure sensing holes of the conventional APT Flow Meters, two-dimensional placement of stagnation and static pressure sensing holes 3, 4 of MPA Flow Meter 20 makes it more responsive to irregularity or azimuthal orientation of velocity distributions. Arrangement of the pressure sensing holes 3, 4 in an MPA Flow Meter 20 is analogous to adding eight more pressure sensing holes to a conventional APT Flow Meter (shown in dotted lines in Figure 8 at 30). Therefore, the flow rates measured with an MPA Flow Meter will inherently be more accurate in any 2-dimensionally non-uniform flow field compared to that measured with a conventional APT Flow Meter.

Lower Pressure Drop

[0022] Referring to Figures 7B and 8, each sensor 2 of the MPA Flow

Meter is thin plate-shaped and arranged within the body 1 in such a way that they create very low drag and lift forces, resulting in much lower permanent pressure drop. One of the contributing factors for the lower pressure drop is the absence of flow-blocking hardware from the central area of the pipe where velocities are the highest, and pressure drop is proportional to the square of velocity.

Better Structural Design

[0023] Moreover, natural frequencies of the sensors are sufficiently removed by design from vortex shedding frequencies and seismic vibration frequencies, and users of MPA Flow Meters do not have to be concerned for the possibility of structural damage due to mechanical vibration of the sensors.

Lower Manufacturing Cost [0024] Chief manufacturing process used for shaping of the MPA sensors is a very efficient, numerically controlled, wire-cutting method, a result of which is significantly reduced manufacturing cost.

Simpler Installation

[0025] Body 1 of the MPA Flow Meter is a thick wafer, and it can be installed between two flanges 10 of any piping system as illustrated in Figure 10. This is a much simpler and time-saving installation method compared to the conventional ATP that requires drilling of one or two holes into the pipe and welding a pre-assembled nozzle squarely into the hole.

DESCRIPTION OF THE PREFERRED EMBODIMENT [0026] Referring to the drawings in which like reference characters designate like or corresponding parts throughout the several views, in particular, Figure 6 illustrates a typical arrangement of the prior art for measuring flow rates by averaging a number of stagnation pressures and separately a number of static pressures distributed along a straight Pitot Tube installed across the diameter of pipes. As was pointed out in the foregoing sections, wile this arrangement is practical it falls somewhat short of the best possible solution for accurate measurement of flow rates. The reason for this is in the fact that the single inserted Pitot Tube is inherently inadequate to monitor the non- uniform pressures across the entire cross-sectional area of the pipe. [0027] Next, an end view of the present invention is illustrated in Figure

8, which shows six sensors 2 and twelve stagnation pressure sensing holes 3. Static pressure sensing holes 4 are shown in Figure 7B. It is clear from the illustrations that the present invention can provide better coverage of the flow field within the cross-sectional area. Internal to the body 1 , connecting passages for stagnation pressures and static pressures are schematically illustrated in Figure 7A and shown in FIG 9A.

Claims

Having described the invention, what is claimed is:

1. A pitot tube flow sensor comprising: an annular cylindrical body, the annular cylindrical body having two internal pressure manifolds therein; and a plurality of circumferentially spaced sensor bodies extending radially inward from an inner surface of the annular cylindrical body, each sensor having at least one stagnation pressure sensing hole therein and having a static pressure sensing hole therein, the stagnation pressure sensing holes being in fluid communication with one internal pressure manifold, the static pressure sensing holes being in fluid communication with the other internal pressure manifold.

2. The pitot tube flow sensor according to claim 1 , wherein each sensor body has a plurality of stagnation pressure sensing holes therein, the plurality of pressure sensing holes being radially spaced apart.

3. The pitot tube flow sensor according to claim 1 , wherein each sensor body is spaced a predetermined circumferential distance from adjacent sensor bodies.

4. The pitot tube flow sensor according to claim 1 , wherein each sensor body extends axially.

5. The pitot tube flow sensor according to claim 4, wherein the at least one stagnation pressure sensing hole is positioned in an upstream surface of the sensor body and the static pressure sensing hole is positioned axially downstream from the at least one stagnation pressure sensing hole.

6. The pitot tube flow sensor according to claim 5, wherein the static pressure sensing hole is positioned in a radially inward facing surface of the sensor body.

7. The pitot tube flow sensor according to claim 1 , wherein the number of sensor bodies is six.

8. The pitot tube flow sensor according to claim 1 , wherein each sensor body extends radially inward less than the radius of the inner surface of the annular cylindrical body. ^

9. The pitot tube flow sensor according to claim 1 , wherein each sensor body has a rectangular shape with a circumferentially extending thickness, a first edge extending axially, a second edge extending radially, a third edge extending radially and being axially spaced from the second edge more than the sensor body thickness.

10. The pitot tube flow sensor according to claim 9, wherein the at least one stagnation pressure sensing hole is positioned in the second edge of the sensor body and the static pressure sensing hole is positioned in the first edge of the sensor body proximate the third edge of the sensor body.

11. The pitot tube flow sensor according to claim 1 , wherein the annular cylindrical body has two pressure ports therein, one pressure port being in fluid communication with one internal pressure manifold, the other pressure port being in fluid communication with the other internal pressure manifold.

12. A pitot tube flow sensor comprising: a plurality of circumferentially spaced sensor bodies extending radially inward only part way towards a common point, each sensor body having a plurality of radially spaced stagnation pressure sensing holes therein and having a static pressure sensing hole therein, the static pressure sensing hole being axially spaced from the stagnation pressure holes, the stagnation pressure sensing holes being in fluid communication with one another, the static pressure sensing holes being in fluid communication with one another.

13. A flow sensor comprising: an annular cylindrical body, the annular cylindrical body having two internal pressure manifolds therein and having two pressure ports therein, one pressure port being in fluid communication with one internal pressure manifold, the other pressure port being in fluid communication with the other internal pressure manifold; and a plurality of circumferentially spaced pitot heads attached to the inner surface of the annular cylindrical body, each pitot head having a plurality of radially spaced sensing holes therein and a static pressure sensing hole therein, the stagnation pressure sensing holes being -jn fluid communication with one internal pressure manifold, the static pressure sensing holes being in fluid communication with the other internal pressure manifold.

14. The flow sensor according to claim 13, wherein each pitot head extends radially inward less, than the radius of the inner surface of the annular cylindrical body.

15. The flow sensor according to claim 13, wherein the stagnation pressure sensing holes in a pitot head lie in a first plane, the static sensing hole in the pitot head lies in a second plane, the first plane being perpendicular to the second plane.

16. A flow sensor comprising: a plurality of separate circumferentially spaced sensors, each sensor having a plurality of radially spaced stagnation pressure sensing holes therein and a static pressure sensing hole therein, the static pressure sensing hole being axially spaced from the plurality of stagnation pressure sensing holes, the stagnation pressure sensing holes being in fluid communication with one another, the static pressure sensing holes being in fluid communication with one another.