FIELD OF THE INVENTION
The present invention relates to a compact and accurate self-averaging square multi port pitot airflow measuring device to measure flow rates in secondary airflow circuits in power plants.
BACKGROUND OF THE INVENTION
Airfoils are used to measure flow rates in secondary airflow circuits in power plants. An airflow measuring device should be accurate, repeatable, and reliable to ensure efficient operation of equipment like a thermal power plant, which use air for burning fossil fuels and deriving the energy. The conventional airfoil although dominates in present scenario in boilers for airflow measurement, the following problems of it call for the development of an alternative airflow measuring device.
1. It is bulky, occupies more space and difficult for transporting and erecting.
2. Unrecoverable pressure drop is as high as 20 mmwc.
3. Duct layout constraint due to its own length and the requirement of considerable straight lengths before and after for streamlining the flow.
Hence, a new alternate airflow measuring device is required to overcome the above issues. A flow meter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a gas. While selecting a flow meter we should consider intangible factors like the familiarity of plant personnel, their experience with calibration and maintenance, spare parts available and mean time between failure histories, Cost etc. at the particular plant site. The main method followed to find the airflow rate is by measuring the average velocity or the change in kinetic energy before and after the flow measuring device. Velocity depends on the pressure head and can be arrived from Bernoulli’s relationship. Finding out pressure head is economical and this is measured by using different differential pressure sensors or transmitters. The self-averaging square pitot air flow measuring device proposed takes care of the above aspects,

reduces the problems of aerofoil, and has been tested and validated at a thermal power plant.
US4154100 describes a method for stabilizing the pressure sensed by the downstream-facing port of a pitot tube type flow meter over a broad flow range, thereby providing a stable and repeatable flow coefficient. The invention comprises localizing the areas of boundary layer separation across deflecting surfaces located upstream of said port by sharply contouring the edges thereof and directing the flowing stream there across, and preventing reattachment of said boundary layer by positioning and contouring the surfaces contouring said port downstream of said sharply contoured edges so as to continuously lie within the wake of the fluid flowing around the latter over a broad range of flow rates. The invention also encompasses the improved averaging pitot-type flow meter characterized by flow deflecting means having sharply contoured edges on both sides thereof effective to fix the location at which boundary layer separation occurs over a broad range of laminar and turbulent flow conditions, an upstream-facing impact surface shaped to direct the flowing fluid across said sharply contoured edges, and a downstream-facing surface containing a port for sensing downstream pressure so contoured and positioned relative to said sharply contoured edges as to cooperate there with in preventing reattachment of the boundary layer under varying flow conditions.
US4559836 describes an improved pitot tube type flow measuring instrument for use in pipes and other conduits characterized by a generally diamond-shaped sensor portion constituting a bluff body for splitting the flow that includes at least a large-radius leading edge in which three or more impact ports are located, a pair of planar portions diverging rearward from the leading edge to a transversely-spaced pair of much shorter radius side edges that cooperate therewith and the downstream-facing portion of the bluff body to define flow separation zones capable of stabilizing the flow coefficient over a broad range of flow conditions provided that the distance separating them is at least five times either of their radii. The invention further includes the unique method of

mounting the probe in two-phase or multi-phase flowing systems where one of the fluids is a liquid and another a gas that calls for tilting the axis of the sensing portion thereof at an angle to the horizontal such that pools of the liquid are trapped in one end or the other depending upon whether the gaseous constituent is hot or cold.
US6564651B1 discloses a high temperature gas flow sensing element module, using Pitot tube technology, for use within a fluid conduit consisting of a housing having an inlet, an outlet, and forming a hollow interior cross-sectional area. Individually, the gas flow sensing element modules fit easily through typical furnace access doors. Thus, in typical furnace retrofit applications, a plurality of equally sized gas flow sensing element modules are arranged adjacent one another in a manner such that the inside dimensions of the entire arrangement coincide with the internal dimensions of the plenum or duct opening into which it is inserted. Pressure averaging piping is used to provide average total and static pressure across the entire gas flow sensing element to differential pressure flow indicators and /or transmitting devices.
US2014/0130608A1 discloses an apparatus and method for sensing position according to flow velocity includes at least two pitot tubes each defining a central axis is along mutually orthogonal axes. Each of at central axis is along mutually orthogonal axes. Each of at least two pressure sensors is positioned in fluid communication with a corresponding one of the at least two pitot tubes. A controller receives outputs from the at least two pressure sensors and analyses to determine at least one of an angular and translational velocity according to the outputs. A distance travelled is then determined according to the at least one of an angular and translational velocity.
US2014/0260671A1 discloses a pressure measurement system for measuring pressure in a conduit has a bluff body extending into the conduit. The bluff body has an upstream opening and a downstream opening. An upstream pitot tube is slidably engaged within the bluff body and has an open end positioned in the upstream opening. A downstream pitot tube is slidably engaged within the bluff body and has an

open end positioned in the downstream opening. A differential pressure sensor is fluidly coupled to the upstream pitot tube and the downstream pitot tube to measure a pressure difference between the upstream pitot tube and the downstream pitot tube.
These devices found in the above prior arts are not equal to the one proposed in the current inventionfor continuous air flow measurements in a thermal power plant firing high ash containing Indian coals.
OBJECTS OF THE INVENTION
It is therefore an object of present invention is to propose a compact and accurate self-averaging square multi port pitot airflow measuring device to measure flow rates in secondary airflow circuits in power plants.
Another object of the present invention is to propose a self-averaging multi-port square pitot airflow measuring device.
A further object of the present invention is to propose a compact and accurate self-averaging square multi port pitot airflow measuring device to measure flow rates in secondary airflow circuits in power plants in which the number of total pressure and static pressure ports is optimized.
A still further object of the present invention is to propose a compact and accurate self-averaging square multi port pitot airflow measuring device to measure flow rates in secondary airflow circuits in power plants, in which the stagnation tube cut-off angle, square tube size, upstream and downstream lengths of the device is optimized.
SUMMARY OF THE INVENTION
A self-averaging square multi-port pitot airflow measuring deviceis proposed as an alternate to airfoil for secondary airflow measurement in power plants firing high ash Indian coals to overcome the issues of its bulky size, which causes issues in transportation and erection, requirement of considerable space and likely inaccuracy

due to ash erosion. This square pitot array is lesser in weight than airfoil and does not have choking of pressure tapping lines. The alternate self-averaging square multi-port pitot airflow measuring device has been developed through Computational Fluid Dynamic (CFD). In order to cover the entire cross section of flow, adequatetotal and static pressure ports equally distributed across the flow cross section have been provided in the design. The sizes of square tube and the stagnation tube cut-off angle for total pressure measurement were optimized using Computational Fluid Dynamic (CFD) w.r.t minimum permanent pressure loss and a reasonable differential pressure for flow measurement for the range of required flow conditions. The values obtained from Computational Fluid Dynamic (CFD) analysis are plotted and the optimum case is obtained. Theunrecoverable pressure drop of alternate square pitot device has 10 times lower than existing airfoil. The square pitot assembly occupies only 1/20th of the length of airfoil.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figures of the accompanying embodiments in which:
Fig. 1 is a side orthographic view of a self-averaging square multi-port pitot, according to the present invention, for use with a rectangular duct.
Fig. 2 is a front elevation view thereof;
Fig. 3 is a side elevation view thereof;
Fig. 4 is a topplan view thereof;
Fig. 5 is all view of individual’s stagnation legs of self-averaging square multi-port pitot, according to the present invention, for use with a rectangular duct.
Fig. 6 is all view of an individual’s interconnected manifold of self-averaging square multi-port pitot, according to the present invention, for use with a rectangular duct.

Fig. 7 is all view of a total pressure instant taping system of self-averaging square multi-port pitot, according to the present invention, for use with a rectangular duct.
Fig. 8 is all view of an static pressure interconnected manifold and instant taping system of self-averaging square multi-port pitot, according to the present invention, for use with a rectangular duct.
Fig. 9 is a pneumatic flow schematic of a self-averaging square multi-port pitot airflow measuring device and its purging systemfor the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION
As shown in Fig. 1, a self-averaging square multi-port pitot airflow measuring device, generally noted as, according to one embodiment of the present invention is disclosed forduct of a non-circular cross sectional inlet (2)and an outlet (3)in fluid communication mounted within a generally rigid flow passage (hot air duct) (1). In its preferred embodiment, the housing is made with the same vertical internal dimensions as the fluid conduit in which it is to be utilized, and as shown in this embodiment a rectangular housing is provided. Anaverage total pressure tap (23)and anaverage static pressure tap (24)are provided protruding outward from the housing, and will be described in greater detail below.
As shown more clearly in Fig. 2, Fig. 3 and also in Fig. 4, total pressure sensing ports (14, 15, 16 and 17) are affixed such that inclined opening facing the flow and traversing the interior cross sectional area of hot air duct (1) for sensing the total pressure of fluid flowing into the fluid flow hot air duct. Also, a static pressure sensing ports (25)areaffixed such that the square opening is parallel to the flow and traversing the interior cross sectional area of the flow element for sensing the static pressure within the hot air duct (1). These ports will be more clearly described below. This arrangement allows for Pitot tube flow principles to be utilized, sensing the averaged total pressure

of the flowing gas with the total pressure sensing Pitot tubes and the averaged static pressure within the conduit with the static pressure sensing Pitot tube.
As shown best in Fig. 2, Fig. 3 and Fig. 4, the total pressure ports(14, 15, 16 and 17) and the static pressure ports(25) are not placed randomly within the hot air duct (1).Rather, they are meticulously placed in a traversing manner. The total number and location of sensing ports are positioned in accordance with the following guidelines.
The dimensions of the square multi-port pitot array depend on the size of the duct and has been taken as the following optimum parameter as per the performance evaluation conducted in Computational Fluid Dynamic (CFD) simulation:
a) Optimised square tube of size (S) can be used according toboth vertical (W) and horizontal (H) dimensions of the duct.
b) Optimum cut off angle of (α) isused for stagnation pressure measuring ports.
In Fig. 1, Fig. 2, Fig. 3 and Fig. 4show the parameters of duct vertical dimension (W) and duct horizontal dimension (H). The square pitot average pressure taps (23 and 24) are located at the mid-point on top of the hot air duct (1). The self-averaged square multi-port pitot is symmetrical to the axis of the Pitot head. The length of the stagnation legs (4, 5, 6, 7, 8 and 9) from the mid-point of total pressure ports (15 and 16) is geometrically calculated as a function of vertical dimension W, square pipe size S and the angle of cutting(α) w.r.t to vertical.
As shown in Fig. 2 and Fig. 3, optimised numbers of total pressure sensing ports(14, 15, 16 and 17) of stagnation leg (4, 5, 6, 7, 8 and 9) are arranged in fluid communication and anchored to an interconnected manifold (18, 19 and 20). Fig.1, Fig. 2 and Fig. 5 show the location of interface ports in stagnation legs for interconnected manifolds.In Fig. 5,Fig. 6 and Fig. 7,the first view is normal to the flow and right hand side view for Fig. 1 and Fig. 2 and other 3 figures are 90° clockwise rotational views of thereof. 6 types of different location of interface ports are in stagnation legs. These are

top interface port in left bottom row corner stagnation leg (4), top interface port in middle bottom row corner stagnation leg (5), top interface port in right bottom row corner stagnation leg (6), bottom interface port in left top row corner stagnation leg (7), bottom interface port in middle top row corner stagnation leg (8), bottom interface port in right toprow corner stagnation leg (9). The top interface ports (10) are located geometrically calculated as a function of vertical dimension W, square pipe size S and the angle of cutting(α) w.r.t to vertical from the centre point of the top total pressure port (16) in bottom row stagnation legs (7, 8 and 9) and bottom interface ports(10) are located at a distance geometrically calculated as a function of vertical dimension W, square pipe size S and the angle of cutting(α) w.r.t to vertical from the centre point of the bottom total pressure ports (15) in top row of stagnation legs (4, 5 and 6). In addition,left top (4) and bottom (7) row corner stagnation legs are having interface ports on right hand side of the stagnation legs and the top (5) and bottom (8) row middle stagnation legs are having two side of interface ports (10) on stagnation legs. As shown in Fig. 6, each stagnation leg is affixed to the interconnected manifoldbya conventional manner like welding. Each total pressure has a plurality of sensing ports oriented so as to face directly toward the inlet, thereby providing unrestricted fluid communication between the impacting fluid flowing into the hot air duct (1), through the total pressure ports (14, 15, 16 and 17), and to the interconnected manifold (18, 19 and 20). The interconnected manifold (18, 19 and 20) thereby consolidates this combined pressure and communicates it to the instant total pressure tap (12)and further consolidates the combined pressure of instant total pressure (12) and send it to the average total pressure tap (23) as shown in Fig. 7. Threenumbers of static pressure sensing ports (25)are also arranged in fluid communication with average static pressure tap (24). In an alternate embodiment as shown in Fig. 8, a plurality of static pressure sensing tubes (26) may also be arranged in a manner similar to that of the instant total pressure taping line (21), incorporating an interconnected static pressure manifold (27 and 28). The static pressure sensing port (25) has also a plurality of sensing ports

oriented so as to face perpendicular to the gas flow through the fluid flow element module. This pressure is communicated through the static pressure port (25) to the static pressure taps (29). In terms of contrast, classic Pitot tubes consist of a concentric double tube, the inside tube having a port facing into the flowing stream for sensing total pressure and the outside tube having radially aligned holes for sensing static pressure.
This distance between every stagnation leg centre can be called as the pitch and the pitch is geometrically calculated using thehorizontal dimension(H)of the duct. The distance between the intersect of interconnected manifold and the stagnation legs is geometrically calculated using the pitch. The distance between duct top plate, in which the average total pressure tap is located and interface of the interconnected manifold with a length that is geometrically calculated using vertical dimension(W) of the duct. Three such tubes are connected from the head Pitot to the interconnected manifold. The two remaining static pressure measuring tubes are made in such a way that the openings to measure static pressure are at the same level.
As shown in Fig. 2, Fig. 3 and Fig. 4, average total pressure tap (23)provide a connection to the total pressure port (14, 15, 16 and 17) through an instant total pressure tap (12) and aninter connected manifold (18, 19 and 20) and average static pressure tap (24)provide a connection to the static pressure ports (25). These connections allow for the use of a differential pressure instrument for computing average flow rate and/or transmitting a flow rate signal as shown in Fig. 9.
Once the self-averaged square multi-port pitot airflow measuring device is installed, instrument piping is installed in order to average the total (23) and static pressure (24) readings from the individual fluid flow stagnation and static legs. The instrument piping is connected to a differential pressure instrument for indicating flow rate and/or transmitting a flow rate signal.

Generally, ash-clogging problem may occur during operation in the pressure tapping lines offlow measuring deviceused in high ash content Indian coal fired steam generator. Due to bigger openings, the ash clogging may not occur in this square pitot device. However, considering trouble free operation, ash purging system is proposed to clean the device.The schematic diagram of purging control valve system is shown in Fig. 9. Both the ball valves of compressed air purging line are in closed position (33 and 34) and both the ball valves of instrumentation pipeline (31 and 32) are in open position when the instrument is used for measurement. During purging, both the valves of instrumentation pipeline (31 and 32) are closed to isolate the DP transmitter and both the ball valves (33 and 34) of compressed air purging line are opened.
The square pitot assembly occupies only 1/20th of the length of the present airfoil for 210 MW capacity boiler. Hence, this device is more suitable for plants with layout constraints. Due to low blockage ratio,the unrecoverable pressure drop is10 times lower than that for airfoil.

WE CLAIM :
1. A compact and accurate self-averaging square multi port pitot airflow
measuring device to measure flow rates in secondary airflow circuits in power
plants, the device being disposable within a hot air duct having a fluid conduit
with a non-circular cross-sectional area, the hot air duct further comprising a
duct housing having an inlet, an outlet, and forming a hollow and non-circular
interior cross sectional area, wherein location and dimensional arrangement is
formed of a plurality of static ports optimally located across the flow cross
section, the device comprising :
a) multiple pressure sensing ports array affixed within said housing traversing the interior cross sectional area of a plurality of stagnation legs of the device, for sensing the total pressure of fluid flowing into said device;
b) a static pressure sensing Pitot tube affixed Within said housing and traversing the interior cross sectional area of said device for sensing the static pressure within said device; and
c) exterior piping and instrument taps for fluid communication with said total pressure sensing ports array and said static pressure sensing ports, respectively to a differential pressure instrument for indicating flow rate including transmitting differential pressure signals.
2. The device as claimed in claim 1, wherein said a total pressure sensing ports
array comprises:
a) a total pressure interconnected manifold in fluid communication with the device;

b) said plurality of linearly elongated stagnation legs having a first end and a second end in fluid communication with said total pressure interconnected manifold;
c) a plurality of sensing ports penetrating-each of said stagnation legs and optimized cut-off angle of the legs and directed to face directly toward said inlet, thereby providing free fluid communication between the impacting fluid flowing into said flow self-averaged total pressure tap through the stagnation leg and said total pressure interconnected manifold; and
d) a high temperature tubing connection for affixing said interconnected manifold to said housing, said high temperature tubing connection comprising a welding.
3. The device as claimed in claim 1, wherein dimensions of the square pitot
array depend on the size of the hot air duct and configured based on the
following relationship :
a. optimised square tube of size (S) can be used according to both
vertical (W) and horizontal (H) dimensions of the duct,
b. optimum cut off angle of (α)isused for stagnation pressure measuring
ports.
c. the square pitot assembly occupying less than 1/20th of the length of
airfoil, and
d. the unrecoverable pressure drop is 10 times lower than that of airfoil.
4. The device as claimed in any of the preceding claims, wherein said exterior
instrument taps are provided to communicate total and static pressure across
individual gas flow sensing elements of the device to differential pressure
flow indicators and transmitting devices.

5. The device as claimed in claim 1, Wherein said static pressure sensing ports
comprise:
a. a linearly elongated pressure sensing tube having a first end and a second
end in fluid communication with said static pressure interconnected manifold;
b. a plurality of sensing ports penetrating said pressure sensing of stagnation
legs and directed to face perpendicular to the hollow path across said gas
flow sensing stagnation leg; and
c. a high temperature tubing connection for affixing said first end of said static
pressure sensing leg to said static interconnected manifold, said high
temperature tubing connection comprising a welding.
6. The device as claimed in claim 1, wherein said total pressure sensing ports array comprises a plurality of individual stagnation legs arranged across said interior cross sectional area such that traversing patterns formed by the Pitot tubes are placed such that the location of sensing ports are at optimised locations.
7. The device as claimed in claim 1 comprising pressure averaging piping used to provide average total and static pressure across said housing sub-assemblies to differential pressure flow indicators or transmitting devices, said pressure averaging piping comprising:
a. instrumentation piping used to connect said total pressure interconnected
manifolds and said static pressure ports on said gas flow sensing static legs;
b. instrumentation piping that mechanically averaging the total and static
pressure across said gas flow sensing element; and
c. Instrumentation taps that provide said averaged total and static pressures to
a differential pressure indicators and transmitting devices.

8. The device as claimed in claim 7, wherein said instrumentation pipeline comprising:
a. instrumentation piping used to connect said total pressure manifolds and said
static pressure Pitot tubes on said gas flow sensing element modules
combination;
b. instrumentation piping that mechanically averages the total and static
pressure across said gas flow sensing element combination; and
c. instrumentation taps that provide said averaged total and static pressures to
a differential pressure indicators and/or transmitting devices.