FLUID FLOW RATE MONITOR PROBE

Abstract: The probe is of the withdrawable type and consists of an elongated cylindrical body made up of a tube 10 joined to an upper T-connector and to a lower tip portion 18. The tip portion 18 is of stepped shape and has a through-passage 34 beneath the step. A pressure tapping 36 opens at a port 38 in the wall of the through-passage 34 to sense reduced pressure at a vena contracta in the passage 34. Another tapping 40 opens at a port 42 in the step face 26 adjacent a concave face 28 which causes a pressure higher than static pressure. The total pressure difference can be three velocity heads and is communicated through tubes 20, 22 to a differential valve so that changes in flow rate in the pipe 16 affect the valve. The probe is relatively of small diameter and is especially applicable to control of the pressure in relation to the rate of flow of natural gas in distribution systems.

Description: The invention relates to probes for use in monitoring flow rate in fluids. The invention particularly, though not exclusively, relates to such probes used for monitoring the flow rate of natural gas in distribution pipes. A well-known kind of probe is the so-called Pitot-static tube. Such a tube is L-shaped and has a lower end portion extending parallel to the direction of the monitored flow. The tip of the end portion is presented towards the oncoming flow and has a first sensing port the plane of which is transverse to the monitored flow. A second sensing port, or each one of an array of second ports, is provided downstream of the first port in the wall of the end portion. The plane of the, or of each, second port is parallel to that monitored flow. The difference between the pressures at the first and second ports is proportional to the square of the velocity of the fluid at the tip. The L-shape of such a probe precludes its general use because, for practical purposes, it is required that the probe be straight so that it can be inserted or withdrawn through a gland or valve in the wall of the pipe confining the monitored flow. A probe of staight construction has been proposed in the UK patent specification No. 2012056A (Dieterich Standard Corporation). In that proposal, an elongate body has longitudinally extending edges at its lateral surfaces. At each edge a boundary layer separates from the body. A sensing port, facing downstream, is positioned at the downstream side of the body in the wake of the two streams of fluid flowing around the body past the two edges. Thus, the port is located somewhat remotely from the two edges. The edges require to be accurately and symmetrically formed on the body. The object of the invention is to provide a probe having a sensing port but which avoids the need for two such edges. A withdrawable probe for use in monitoring fluid flow rate, according to the invention, comprises an elongate cylindrical body including a tip portion which has a wall intersecting the upstream and downstream sides of the tip portion and the tip portion having at its upstream side an edge formation which extends transversely to the direction of monitored flow and which is effective to cause flow-separation of fluid from said wall downstream of said edge formation, and the tip portion having in said wall a sensing port which is parallel to said direction or inclined thereto to face downstream in relation to said direction, said sensing port being located where said flow-separation occurs adjacent said edge formation. In one form of the invention, the wall is at the extremity of the tip portion. In another form of the invention, the wall is a bounding wall of a passage extending through the tip portion. For example, the bounding wall entirely surrounds a vena contracta caused by said flow-separation, the edge formation being, for example, a circular edge. Alternatively, the wall is at the extremity of the tip portion and the passage is channel-shaped and only partly encloses the flow through the passage, the edge formation being for example semi-circular, or V-shaped or U-shaped with a flat or curved base. In one form of the invention, a second sensing port is provided which opens at the wall of the pipe or conduit confining the monitored flow. In the preferred form of the invention the second port is provided on the tip portion, the second port being parallel to the direction of the monitored flow and the second port being positioned adjacent the upstream side of the tip portion with respect to said direction. It is particularly preferred that the tip portion is of stepped shape defined by two faces, the first of which is presented towards the oncoming monitored flow and the second of which is transverse to that face, the second port opening at the second face adjacent the first face. The first face is either plane or preferably concave and part cylindrical, having an axis of curvature parallel to the longitudinal central axis of the body. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a part elevation of the probe in its operating position inserted through the wall of a gas pipe as seen in the direction of monitored flow through the pipe; Figure 2 is a vertical section on the line II-II in Figure 1; Figure 3 is an enlarged horizontal section on the line III-III in Figure 1; The probe comprises an elongate, cylindrical body made up of a stainless steel tube 10 joined by brazing at its upper end to a brass T-shaped connector body (not shown) and joined at its lower end to a tip portion 18 described below. The tube 10 extends through a coupling 12 and ball valve 14. The valve 14 is fitted at its lower end into a circular hole in the wall of a distribution pipe 16 through which natural gas flows. The lower end of the tube 10 is joined by brazing to the brass tip portion 18. Two stainless steel tubes 20, 22 extend through the tube 10 and their lower ends are received in bores in the upper end of the portion 18 and are joined to the portion by brazing. The upper ends of the tubes 20, 22 are received in bores in the T-connection body (not shown) and are joined by brazing to the body. The tip portion 18 is machined from solid rod, typically in this example of 0.5 inch (12.7 millimetre) outside diameter. Beneath the lower end of the tube 10 and beneath the lower end of the spigot 24 of the valve 14, the portion 18 is machined at the upstream side with respect to the direction of gas flow in the pipe 16 indicated by the arrow to form stepped shape defined by a downwardly presented horizontal face 26, extending parallel to the flow direction. Beneath the face 26 the machining of the portion 18 leaves a concave face 28 further defining the stepped shape. As shown in Figure 3 the centre of curvature of the face 28 is at 30, the face 28 thus being part of the inner surface of a notional hollow right cylinder having a vertical central axis of symmetry at 30 and having a radius of 12mm, for example. The machining leaves a plane vertical face at 32 beneath the lower end of the face 28. The face 32 extends downwardly to the lower extremity of the portion 18. A circular horizontal hole is drilled through the portion 18 to form a through-passage 34 with a sharp leading circular edge 35 defining an entrance at the face 32 and a sharp-edged exit at the cylindrical downstream surface of the portion 18. It is preferred, as shown, for the central axis of the passage 34 to coincide with the central axis 37 of the pipe 16. The passage 34 thus extends parallel to the direction of monitored flow in the pipe 16. Typically in this example, the diameter of the passage 34 is 6mm. Vertical pressure tappings drilled through the portion 18. The first tapping 36 opens at its upper end into the tube 20 and at its lower end opens at a sensing port 38 in the upper part of the wall of the through-passage 34. The sharp leading edge 35 of the passage 34 causes flow-separation of the gas away from the wall of the passage 34 and a vena contracta is formed within the passage 34. The port 38 is positioned so as to be at the flow-separation around the vena contracta. The port 38 is preferably positioned as shown in Figure 3 slightly nearer to the face 32 than to the convex downstream side of the portion 18 so that the pressure sensed at the port 38 is as low as possible. The optimum positioning of the port 38 and the optimum size of the through-passage 34 cannot be precisely stipulated but can be readily checked experimentally. The second tapping 40 opens at its upper end into the tube 22 and at its lower end at a sensing port 42 in the face 26. The port 42 is closely adjacent the upper end of the concave face 28. The T-connection (not shown) is connected by couplings to small diameter tubing leading to a differential pilot valve, for example a pilot valve of high gain type forming part of a system by which pressure in the pipe 16 can be related to the flow through the pipe. The probe in such a case is located downstream of a governor valve the setting of which is controlled byoperation of the high gain pilot valve. The probe can be connected in other systems (not shown) to a different component, such as a manometer gauge or other device. Operation The pressure at the port 38 is less than the static pressure of the gas in the pipe 16. It has been found in the case of the example of construction described above that the pressure at the port 38 is less than the static pressure by an amount equal to twice the dynamic pressure, owing to the effect of the flow-separation of the gas away from the wall of the passage 34 around the vena contracta. The concave face 28 is effective to produce a stagnation zone immediately upstream of the face 28 and at the port 42 in which zone the pressure is higher than the static pressure by an amount equal to the dynamic pressure. Thus, the difference between the pressures at the two ports 38,42 is equal to three times the dynamic pressure, alternatively referred to as the velocity head. Changes in the mean rate of flow of gas through the pipe 16 are readily monitored since the pressure sensed at the port 38 or 42 is dependent on the square of the velocity of the gas which approaches the port 38 or which passes the port 42, respectively. The convex form of the downstream side of the portion 18 facilitates flow to maintain high velocity past the probe and through the hole 34. The probe described above with reference to the drawings has the following advantages: (a) both of the ports 38, 42 are parallel to the direction of the monitored flow in the pipe 16 so that the risk of clogging of the ports by particulate matter, such as dust, in the gas flow is eliminated or greatly minimised; (b) the pressure difference is greater, for comparable conditions, than the difference available at the ports of a Pitot-static tube or available from elbow tappings; (c) the diameter of the probe is relatively smaller than the diameter of modified Pitot-type probes of withdrawable type incorporating Venturis and the probe diameter is not required to be increased as the diameter of the pipe 16 confining the monitored flow increases. Only a relatively small ball-valve or other fitting is required to mount the probe in the pipe wall and the probe construction is relatively simple. In particular, only pressure tappings extending longitudinally of the probe are required; (d) the probe gives sufficient pressure difference to enable it to be substituted for an orifice plate in control systems. This achieves major gains with regard to greatly simplified installation and elimination of the major part of the pressure loss and flow disturbances casued by the orifice plate. The probe is effective over a useful range of rates of monitored flow, (e) the probe can be installed in or withdrawn from a pipe while the pipe is conveying the gas flow at full pressure by means of a suitable valve assembly 12, 14, which greatly enhances the range of applicability and effectiveness of the probe. In a modified form of probe (not shown) the face 28 is plane instead of concave. Some reduction of the pressure differences available from the probe will probably result. The curved face is preferred also because the pressure difference is unaffected by slight errors in angular orientation of the probe about its central longitudinal axis, which may arise during fitting to the pipe 16. In another modified form of probe (not shown) the port 42, the tapping 40 and the tube 22 are dispensed with or not used. Instead, the static pressure of the gas flow is sensed by a port provided by a tapping in the wall of the pipe 16, eg upstream of the probe. In that case the pressure difference available between the tapping port and the port 38 would be reduced to a value equal to some two times the dynamic pressure. However, the other advantages of the probe would remain unaffected. The size of the hole 34 was found by experiment in order to give satisfactory results. The same size of probe is applicable to pipes up to at least 12 inches (300mm) inside diameter. In modifications, the size of the hole may be different from that specified above, subject to the requirement that a flow-separation occurs, in use, giving a pressure low enough to be useful. In further modifications (not shown) one of the tubes 20, 22 is dispensed with, the respective sensing port communicating with the T-connection via the interior of the tube 10. In a further modification (not shown) the tip portion 18 is stepped as described above but the face 28 extends right to the extremity of the tip portion 18 or nearly to the extremity and the step shape is positioned between the extremity and the through-passage, which typically, for example, is a circular hole similar to the hole 34 but extending through the full diameter of the portion 18. In such a case the face 28 conveniently has an axis of curvature offset from the central plane 50 shown in Figure 3 so that the tapping and the port (corresponding to the port 42) are also offset so that the tapping avoids the through-passage. In another modification (not shown) the through-passage has a pressure-recovery section at its downstream end. Such a modification enhances the velocity of flow through the passage by reducing the effective resistance presented by the passage. This can increase the total pressure differential available from the probe up to some six times the dynamic pressure. For example, the through-passage in such a case is a circular hole having a section at its downstream end which is conically divergent in the direction of fluid flow. In other modifications (not shown) the through-passage is formed at the extremity of the tip portion 18, for example by machining a channel in the end wall of the tip portion, extending lengthwise in the direction of the monitored flow. Typically, the end wall is a plane wall normal to the length of the body and the transverse cross-section of the channel is semi-circular, or V-shaped or U-shaped, with a flat or curved base. The leading edge of the channel (with respect to the flow direction) is such as to cause flow-separation of the gas away from the wall or walls of the channel, ie the leading edge is sufficiently sharp, for example, to produce the necessary flow-separation. A partial vena contracta is formed in the channel. The sensing port corresponding to the port 38 is positioned at or adjacent the position of maximum pressure depression caused by the flow-separation. In all forms of the through-passage only a relatively small fraction of the total monitored flow passes through the passage. In yet another modification (not shown) the tip portion 18 is devoid of any through-passage and the leading edge of the end wall of the portion causes flow-separation from the end wall. A sensing port in the wall is positioned at the flow separation and at or adjacent the position of maximum pressure depression caused by the flow-separation. The end wall is either normal to the length of the body so that the port is parallel to the general direction of monitored flow or the wall is inclined to that direction so that the port is inclined and faces downstream in relation to that general direction. The probe is applicable to monitoring flow of gases other than natural gas and to monitoring flows of liquids and fluid mixtures, including steam, for example.

Claims: 1. A withdrawable probe for use in monitoring fluid flow rate comprising an elongate cylindrical body including a tip portion which has a wall intersecting the upstream and downstream sides of the tip portion and the tip portion having at its upstream side an edge formation which extends transversely to the direction of monitored flow and which is effective to cause flow-separation of fluid from said wall downstream of said edge formation, and the tip portion having in said wall a sensing port which is parallel to said direction or inclined thereto to face downstream in relation to said direction, said sensing port being located where said flow-separation occurs adjacent said edge formation. 2. A probe according to claim 1, in which the wall is at the extremity of the tip portion. 3. A probe according to claim 1, in which the wall is a bounding wall of a passage extending through the tip portion. 4. A probe according to claim 3, in which the bounding wall entirely surrounds a vena contracta caused by said flow-separation, the edge formation being a circular edge. 5. A probe according to claim 3, in which the wall is at the extremity of the tip portion and the through-passage is channel-shaped and only partly encloses the flow through the passage, the edge formation being semi-circular, or V-shaped or U-shaped with a flat or curved base. 6. A probe according to claim 4, in which the through-passage has a section adjacent its downstream end which is divergent towards the downstream end and is effective to recover fluid pressure. 7. A probe according to any preceding claim, in which a second sensing port is provided on the tip portion and is parallel to said direction, the port being positioned adjacent the upstream side of the tip portion with respect to said direction. 8. A probe according to claim 7, in which the tip portion is of stepped shape defined by two faces the first of which is presented towards the oncoming monitored flow and the second of which is transverse to that face, the second port opening at the second face adjacent the first face. 9. A probe according to claim 8, ad dependent on claim 4, in which the through-passage is intermediate said first face and the extremity of the tip portion. 10. A probe according to claim 8, as dependent on claim 4, in which said first face is intermediate the extremity of the tip portion and the through-passage. 11. A probe according to claim 8, 9 or 10, in which the first face is of concave, part-cylindrical shape, the axis of curvature of the first face being parallel to the central longitudinal axis of the body. 12. A probe according to claim 8, 9 or 10, in which the first face is plane. 13. A probe according to claim 1, substantially as herein described with reference to the accompanying drawings.

Inventors: CRAIGEN, JOSEPH GORDON, DIXON, BRYAN FRANCIS
Applicants: BRITISH GAS CORPORATION
Application year: 1986
Main classification: G01F1
IPC codes: G01F1, F16L41