Self-averaging pitot tube probe for measuring fluid flow

Abstract: A self-averaging pitot tube probe has an elongated elliptically-shaped body (42) which produces laminar flow of fluid sub-boundary layers (44) around the body. The body includes upstream high pressure sensing ports (50) and a high pressure sensing passageway, and downstream low pressure sensing ports (54) and a low pressure sensing passageway. The low pressure sensing ports open through respective opposite sides of the body upstream from separation points at which the laminar fluid flow sub-boundary layers (44) separate from the body.

Description: This invention relates to a self-averaging pitot tube probe according to the preamble of claim 1, and more particularly to a self-averaging pitot tube probe and method for use with a measuring device for measuring a pressure differential to determine fluid flow rate in a conduit, wherein the probe and method provide a stable and more accurate sensing of the low pressure component of the pressure differential and, thus, a higher degree of accuracy in the measurement of the fluid flow rate. US-A-3,581,565 to P.D. Dieterich discloses a pitot tube flow measuring device comprising an upstream probe including an elongated hollow cylindrical body of circular cross section, which extends diametrically across a pipe or conduit during a flow measuring operation. The hollow cylindrical body has a plurality of longitudinally arranged high (dynamic) pressure sensing ports at an upstream side of the body. An interpolating or averaging tube is located within the hollow cylindrical body and has, at one end, an open port located centrally of the body for determining an average of the fluid flow into the body through the upstream sensing ports. The flow measuring device further includes a downstream probe of circular cross section and cylindrical construction, with a downstream facing port located centrally of the conduit in a low (static) pressure area. Both of the upstream and downstream probes are connected to a pressure differential measuring mechanism mounted externally of the conduit. Another pitot tube flow measuring device, which is intended to be an improvement over the flow measuring device disclosed in the above-mentioned Dieterich patent, is disclosed in US-A-4,154,100 to J.B. Harbaugh et al. This flow measuring device comprises a probe including an elongated hollow body having a portion of its length of triangular or diamond-shaped cross section, rather than circular. A key feature of the probe is the provision of sharply contoured edges located in transversely-spaced relationship on opposite sides of the hollow body, intermediate upstream and downstream facing surfaces. More specifically, the upstream-facing surface is contoured to divide the fluid flow and direct essentially equal parts thereof across the sharply contoured opposite side edges, which function to fix the locations at which fluid flow boundary layer separation from the body occurs over a broad range of laminar and turbulent flow conditions. The downstream-facing surface is shaped so that reattachment of the fluid to the body then is precluded and so that a low (static) pressure sensing port in the downstream-facing surface lies within the wake of the fluid flowing past the sharply contoured edges, in a low (static) pressure area. The probe also includes upstream high pressure sensing ports and an internal interpolating or averaging tube, as in the flow measuring device of the above-mentioned Dieterich patent, and further includes an internal low pressure sensing tube, one end of which defines the above-mentioned low pressure sensing port in the downstream-facing surface of the probe body. A purpose of this invention is to provide a new and improved self-averaging pitot tube probe and method for use in a measuring device for measuring a pressure differential to determine a fluid flow rate in a conduit, such as disclosed in the Dieterich and Harbaugh et al patents, wherein the probe provides a stable and more accurate sensing of the low pressure component of the pressure differential, and thus provides a higher degree of accuracy in measuring the fluid flow rate. A generic pitot tube probe is shown in Transaction of ASME, Vol. 78, No. 5, July 1956, pages 1079-1089, "Streamlined pitot-tube bar for measuring water flow in large pipe". This known pitot tube probe has a drop-shaped cross-section symmetrical to its longituinal axis. This was ascertained to be a particularly streamlined body whose performance does not become unstable. The openings for sensing the static pressure open at 40 per cent of the longitudinal axis behind the leading edge, i.e. in the front portion of the pitot tube probe. It is the object underlying the present invention to further develop a self-averaging pitot tube probe according to the preamble of claim 1 for measuring a pressure differential to determine a fluid flow rate in a conduit such that it is stable and more accurate in sensing the low pressure component of the pressure differential and thus provides a higher degree of accuracy in measuring the fluid flow rate. According to the invention, this object is achieved by the features of the characterizing portion of claim 1. According to the invention, the self-averaging pitot tube probe for use in a flow measuring device for measuring a pressure differential to determine a fluid flow rate in a conduit comprises an elongate, elliptically-shaped body insertable transversely with respect to a stream of fluid flowing within the conduit. The elliptically-shaped body has a configuration which produces laminar flow of fluid sub-boundary layers around the body so that the sub-boundary layers remain attached to the body at least until the sub-boundary layers reach a downstream body portion. An upstream portion of the body includes sensing ports and an elongated passageway for sensing a high fluid pressure component of the measured pressure differential. An elongated low pressure sensing passageway is located in the body adjacent a transition from an intermediate body portion to the downstream body portion, and downstream ports open from the low pressure sensing passageway through the body for sensing a low pressure component of the measured pressure differential. More specifically, the downstream low pressure sensing ports open through each of respective opposite sides of the elliptically-shaped body along its length, and are located upstream from (ahead of) separation points at which the laminar fluid flow sub-boundary layers separate from the body. The upstream high pressure sensing ports face upstream and are arranged along an upstream longitudinal edge of the body. Further, a transverse cross section of the elliptically-shaped body has a major axis length -to- minor axis width ratio in a range on the order of 1.70 to 5.65, with a chord of each quadrant of the body cross section intersecting the cross section major axis in a range on the order of 10° to 30°. The elliptically-shaped body may be of hollow or essentially solid construction. Figure 1 is a schematic cross-sectional view illustrating the operation of a prior art probe of circular cross-section; Figure 2 is a schematic cross-sectional view illustrating the operation of a prior art probe of diamond-shaped cross-section; Figure 3 is a schematic cross-sectional view illustrating the operation of a probe in accordance with the invention, having an elliptically-shaped cross-section; Figure 4 is a cross-sectional view of a flow measuring device provided with a probe in accordance with the invention; Figure 5 is a cross-sectional view taken along the line 5-5 in Figure 4; Figure 6 is a cross-sectional view taken along the line 6-6 in Figure 4 and illustrating additional features of the invention; and Figure 7 is a cross-sectional view, similar to Figure 6, of another embodiment of the invention. Referring to Figure 1, the normal manner of operation of a prior known-type self-averaging pitot tube probe 10 of circular cross-section, is schematically illustrated. The probe 10 includes an elongate cylindrical hollow body 12 which is insertable into a stream of fluid flowing within a pipe or conduit 14, diametrically across the pipe or conduit, and thus transversely to the direction of fluid flow. The flowing fluid creates an impact or high (dynamic) pressure zone 16 at an upstream side of the probe 10, and a series of upstream high pressure sensing ports 18 (only one shown) are provided along an upstream-facing surface of the probe. In operation, as the fluid flows around an exterior surface of the probe body 12, fluid flow sub-boundary layers 20 separate from the body 12 at variable points on opposite sides of the body within arcuate separation regions, designated SR in Figure 1, depending upon flow conditions. This creates a low pressure (static) zone 24 on a downstream side of the probe 10, including variable intensity vortices 26 and an essential vacuum in an area 28 downstream from the probe. The low pressure is sensed by a downstream port 30 located, for example, in one end of a low pressure sensing tube 32 disposed within the cylindrical body 12 or externally thereof, with the interior of the body and the low pressure sensing tube being suitably connected to respective portions of a pressure differential measuring mechanism (not shown). The probe 10 of circular cross section as shown in Figure 1 has a number of disadvantages. For example, the separation of the fluid flow sub-boundary layers 20 from the probe 10 at the variable points in the separation regions SR upstream from the low pressure zone 24 produces undesirable variations in the low pressure signal sensed by the low pressure sensing port 30. The probe 10 also has a turn down ratio (T.D.R.) on the order of only 4 to 1, creating variations in the above-mentioned vacuum in the downstream area 28, which has an adverse effect upon the measured low pressure signal. (Turn down ratio is defined as the ratio of maximum flow rate to minimum flow rate maintaining the same accuracy range, i.e., a variation in accuracy of ± 1% from 31,5 l/s to 126,2 l/s (500 GPM up to 2,000 GPM) equals a T.D.R. of 4 to 1, and the same variation in accuracy of ± 1% from 31,5 l/s to 315 l/s (500 GPM up to 5,000 GPM) equals a T.D.R. of 10 to 1). The variable intensity vortices 26 also create extraneous low pressure signal amplifications by causing vibration of the probe 10 and noise or acoustic effects in the sensed signal, so that reliable repeatability of performance of the probe cannot be obtained. Figure 2 illustrates schematically another prior known-type probe 10' similar to the probe 10 shown in Figure 1, except that the probe 10' is of diamond-shaped cross-section, rather than circular cross-section. As in the case of the probe 10 shown in Figure 1, in operation, fluid flowing in a pipe or conduit 14' creates an impact or high (dynamic) pressure zone 16' at an upstream side of the probe 10', and the upstream side of a probe body 12' includes a plurality of upstream high pressure sensing ports 18' (only one shown) in an upstream leading edge of the body. The diamond-shaped probe body 12' of Figure 2 differs from the circular-shaped probe body 12 of Figure 1 in having a pair of opposite sharply contoured edges which define fluid flow separation points, designated SP, at which sub-boundary layers 20' of the flowing fluid are separated abruptly from opposite sides of the body. This creates a low (static) pressure zone 24' on a downstream side of the probe 10' and, as a result of the large "bluff" probe body 12' obstructing the flow stream, produces high intensity vortices 26' and a vacuum in a downstream area 28'. As in the probe 10 shown in Figure 1, the low pressure in the low pressure zone 24' is sensed by a downstream port 30' located in one end of an elongated low pressure sensing tube 32', with the interior of the probe body 12' and the low pressure sensing tube 32' being suitably connected to respective portions of a pressure differential measuring mechanism (not shown). As in the case of the probe 10 of Figure 1, since in the probe 10' of Figure 2 the separation of the fluid flow sub-boundary layers 20' occurs upstream of the low pressure sensing port 30', the probe 10' of Figure 2 has the same disadvantages as the probe 10 of Figure 1. Actually, since the large "bluff" body 12' of the probe 10' produces the high intensity downstream vortices 26', and the diamond-shaped cross section of the probe body 12' produces a relatively high drag coefficent (hence the high intensity downstream vortices 26') and a high pressure loss differential, errors in the measured pressure differential can be even more pronounced. Referring to Figure 3, which illustrates the manner of operation of a probe 40 in accordance with the subject invention, it is seen that a primary feature of the invention is that the probe has a body 42 of an elliptically-shaped cross section, which produes a laminar flow of fluid sub-boundary layers 44 around the body so that the sub-boundary layers remain attached to an outer surface of the body throughout a major portion of the fluid's travel around the probe. Further, a low (static) pressure component of the pressure differential being measured is sensed upstream from the separation of the fluid flow sub-boundary layers 44 from the probe body 42 where the pressure is uniform and more consistent as compared to downstream. As a result, a stable and more accurate sensing of the low (static) pressure is obtained, thus producing a higher accuracy in the measurement of the fluid flow rate as compared to the probe designs shown in Figures 1 and 2. More specifically, as in the probe designs shown in Figures 1 and 2, in the probe 40 in accordance with the invention, the flowing fluid in a pipe or conduit 46 creates an impact or high (dynamic) pressure zone 48, having a turbulent, variable velocity distribution as illustrated by the curve 49 in Figure 4, at the upstream side of the probe. An upstream portion 42u of the probe body 42 also is provided with a series of sensing ports 50 (only one shown in Figure 3) opening into the interior of the probe body. However, in contrast to the probe designs shown in Figures 1 and 2, in the probe 40 in accordance with the invention, fluid flow around the probe body 42 is essentially laminar and the sub-boundary layers 44 of the fluid flow tend to separate smoothly from the probe body in separation regions (designated SR in Figure 3) adjacent a downstream portion 42d of the probe, creating a low pressure zone 52 at the downstream side of the probe with a turn down ratio of approximately 10 to 1, which eliminates the creation of any downstream vortices or vacuum effects. Further, in contrast to the probe designs shown in Figures 1 and 2, in the probe 40 in accordance with the invention, the sensing of a low (static) pressure component is accomplished in a laminar flow area upstream from the separation regions SR at points of transition PT between an intermediate portion 42i of the probe body 42 and the downstream body portion 42d, by low pressure sensing ports 54 located at opposite sides of the probe body. In this regard, with reference to Figures 4 and 5, the elliptically-shaped probe 40 in accordance with the invention is, in operation, disposed diametrically across the pipe or conduit 46 as shown in these Figures, with the high (dynamic) pressure sensing ports 50 in the upstream edge of the probe body 42 facing upstream. The low (static) pressure sensing ports 54 are formed, in part, in opposite sides of an elongated low pressure sensing tube 56 disposed within the probe body 42, with the sensing ports opening outwardly through opposite side walls of the body. The interior of the probe body 42 defines a passageway 42p and, at an upper end as viewed in Figures 4 and 5, is in fluid communication with a high (dynamic) pressure component sensing device 58 of a known type of pressure differential measuring mechanism 60, the details of which are not shown. Similarly, the low pressure sensing tube 56 extends upwardly (as viewed in Figures 4 and 5) within the probe body 42 and is in fluid communication with a low (static) pressure sensing device 62 of the pressure differential measuring mechanism 60. The probe 40 also is adjustably mounted on a wall of the pipe or conduit 46 by a suitable mounting mechanism 64 of a screw-threaded clamping type in a suitable manner. Figure 6 illustrates additional features of the subject invention by which smooth laminar flow of the fluid in the conduit 46 and around the probe 40 is achieved, to obtain beneficial results. A transverse cross section of the probe body 42 has a major axis length 66 and a minor axis width 68, with the major axis length-to-minor axis width ratio being in a range on the other of 1.70 to 5.65. Further, a chord 70 of each quadrant of the cross section of the elliptically-shaped probe body 42, as illustrated by the upper left quadrant in Figure 6, intersects the major axis of the body cross section at an angle in a range on the order of 10° to 30°. More specifically, for a chord intersection angle of 10°, the ratio is in a range on the order of 5.55 to 5.65, and for a quadrant intersection angle of 30°, the ratio is in a range on the order of 1.70 to 1.80. Figure 7 illustrates an alternate embodiment of the invention in which a probe 40' has an elliptically-shaped probe body 42' which is in the form of a solid rod. The solid rod body 42' has high pressure sensing ports 50', an elongated high pressure sensing passageway 42p', low pressure sensing ports 54' and an elongated low pressure sensing passageway 56', formed in the solid rod in a suitable manner, such as by drilling. In other respects, the probe 40' functions in the same manner as the probe 40 shown in Figures 3-6. In summary, a new and improved self-averaging pitot tube probe for use in a measuring device for measuring a pressure differential to determine fluid flow rate in a pipe or conduit, has been disclosed. The probe 40, for example, includes an elongate elliptically-shaped body 42 which is transversely insertable into a stream of the fluid flowing within the pipe or conduit 46, and has a configuration which produces a laminar flow of the fluid sub-boundary layers 44 around the body so that the fluid flow sub-boundary layers remain attached to the body for a major portion of the flow around the body. Further, the sensing of the low pressure component of the measured pressure differential is accomplished by the low pressure sensing ports 54 upstream from the sub-boundary layer separation regions SR in the laminar flow area. In addition, the creation of downstream turbulence in the form of fluid vortices, and/or the creation of downstream vacuum conditions are avoided. As a net result, more stable, uniform and accurate fluid flow rate measurements are obtained. A self-averaging pitot tube probe has an elongate elliptically-shaped body which produces laminar flow of fluid sub-boundary layers around the body. The body includes upstream high pressure sensing ports and a high pressure sensing passageway, and downstream low pressure sensing ports and a low pressure sensing passageway. The low pressure sensing ports open through respective opposite sides of the body upstream from separation points at which the laminar fluid flow sub-boundary layers separate from the body.

Claims: A self-averaging pitot tube probe (40, 40') for use in a device for measuring a pressure differential to determine the fluid flow rate in a conduit (46), comprising an elongate body (42, 42') insertable transversely with respect to a stream of fluid flowing within the conduit (46), the transverse cross section of said elongate body (42, 42') continuously increasing from an upstream portion (42u) to an intermediate portion (42i) and continuously decreasing from said intermediate portion to a downstream portion (42d), a high pressure port means (50, 50') in the upstream portion (42u) for sensing a high fluid pressure component of the measured pressure differential; an elongate high pressure sensing passageway (42p, 42p') in said elongate body (42, 42') in fluid communication with said upstream high pressure sensing port means (50, 50'); a low pressure port means (54, 54') for sensing a low fluid pressure component of the measured pressured differential, an elongate low pressure sensing passageway (56, 56') in said body in fluid communication with said low pressure sensing port means (54,54'); characterized in that the transverse cross-section of said elongate body (42, 42') has an elliptical shape having a major axis length to minor axis width ratio within a range in the order of 1.70 to 5.65, wherein said low pressure port means (54, 54') opens to a portion which is located downstream of said intermediate portion (42i) and upstream of said downstream portion (42d) such that separation points of the fluid flow are downstream of said low pressure port means (54, 54'). A self-averaging pitot tube probe according to claim 1, characterized in that said low pressure port means (54, 54') open through respective opposite sides of said elongate body (42, 42'). A self-averaging pitot tube probe according to any of claims 1 or 2, characterized in that said low pressure port means (54, 54') comprises a plurality of ports arranged along the length of said elongate body (42, 42'). A self-averaging pitot tube probe according to any of claims 1 or 3, characterized by a chord of each quadrant of the cross section of said elongate body (42, 42') intersecting the cross section major axis at an angle within a range in the order of 10 to 30 degrees. A self-averaging pitot tube probe according to any of claims 1 or 4 characterized in that said high pressure port means (50, 50') includes ports which face upstream and are arranged along the upstream longitudinal edge of said body elongate (42, 42').

Inventors: TAHA SAMI H, TAHA, SAMI H.
Applicants: PRESO IND, PRESO INDUSTRIES
Application year: 1988
Main classification: G01F1
IPC codes: G01F1