A valve for controlling fluid flow

Abstract

A valve 1 for controlling flow of a fluid including a flow reducing device 20, the valve movable between a fully open position 15 and a closed position 18 and having a housing 5 including an inlet 2,

Description

Field The present invention relates to flow reducing devices in valves. More particularly, the present invention relates to high flow, low noise flow trim devices. Background A valve’s simplest function is to seal a pipe to prevent flow of fluid therein. A fluid may be a liquid or a gas. However, valves are also used to meter or modulate the flow of fluid in a pipe. This can be to reduce pressure in the fluid flowing in the pipe or simply to reduce the flow volume. There are a number of challenges that are presented when designing a valve optimised for trimming flow in a pile. The success of a design of valve can measured on how well it prevents flow when in the closed, sealed position, how much obstruction or reduction in flow the valve provides compared to a section of plain pipe when the valve is in the open position. In addition, for valves used to modulate flow or pressure, how efficient or successful the valve is at this purpose is important. Any valve may be used to modulate flow if positioned between the fully open and the closed sealed position. However, the range of movement through which a standard valve, such as a ball valve with a clean circular bore, must move to control flow is very small. In a standard ball valve, the flow increases rapidly as the valve is moved away from the closed position making precise control difficult to achieve. In addition, when flow is restricted through a valve the pressure held back by the valve can be substantial and in a valve without a flow trim device to change the form on the bore opening, a rapid and large single pressure drop results across the valve, leading to potentially damaging high fluid velocity and noise. High velocity flow accompanied by an instantaneous drop in pressure results in at least turbulent flow and likely cavitation. Cavitation can be damaging for the valve and the pipe structure. Both result in a decrease in efficiency and increase in pumping losses through the valve. A rapid pressure drop, high velocity flow and cavitation also create high levels of noise which present a health and safety issue for workers near such items forcing the use of cumbersome hearing protection. Even in a pipe where flow metering is not required if a valve is restraining high pressure the shock wave that can pass down the pipe when a valve is opened and the pressure released can be damaging. The function of a flow trim device is to make the change in flow with valve movement more progressive and / or smooth the transition between the high pressure fluid on the upstream side of the valve to the low pressure fluid on the downstream side of the valve. Most commonly, if a ball style valve is desired for high flow capacity, a modification referred to as a segmented ball valve is used to modulate flow. This style of valve includes a noncircular bore in the ball with a V or C channels machined into the bore such as those described in US3403887. This creates a more manageable change in flow as the valve is moved from the closed position. However, does not affect the rapid (single stage) pressure drop between the upstream and downstream sides of the valve. Baffle plates or fences may also be used such as described in US5180139. These are obstacles placed in the flow that include smaller apertures to restrict the velocity of the flow when the valve is used to restrict flow by partially closing the valve. Rather than a single pressure drop, each aperture now contributes a smaller pressure drop, limiting damaging high flow velocities. An advance on a fence EP0869302 discloses a shaped trim placed in the valve outlet, including a plurality of apertures to a cavity between flow passages. Similar theories are disclosed in US005988586A and US5771929A. These legacy solutions all severely compromise the efficiency (flow capacity) of the fully open free flow characteristic of the valve and in most cases address only one or other of the adjustability and pressure drop challenges of a valve. All of the prior art solutions introduce substantial blockage into the pipe. Summary of Invention Aspects and / or embodiments seek to provide a solution to the problems discussed above. According to a first aspect, there is provided a valve for controlling flow of a fluid including a flow reducing device; the valve movable between a fully open position and a closed position and having a housing including an inlet, connected to an outlet, and the valve further including an obturating portion including an aperture , defining an axis B at its centre, fluidly connecting the inlet and the outlet when the valve is moved from the closed position toward the fully open position; the inlet, outlet and aperture defining a bore through which fluid may flow and an axis A at their centre when the obturating portion is in the fully open position; the flow reducing device comprising a first tapered portion, a second tapered portion, each extending circumferentially about and axially along the bore between a wide open end, to face the other at a narrow open end, one tapered portion extending along the aperture of the obturating portion the other tapered portion extending along the bore of the inlet or outlet; each tapered portion including a circumferentially and axially extending tapered face, extending from the wide open end to the narrow open end and a radially extending end face at the narrow open end; the first tapered portion including a circumferentially extending plenum separated from the bore by the tapered face and the end face of the first tapered portion; the plenum connected to the bore by a plurality of first apertures in the end face of the first tapered portion and a plurality of second apertures in the tapered face; wherein the fluid flows through the plenum when the valve is moved from the closed position towards the fully open position. Advantageously, providing a multi-step pressure drop when flow is being controlled by the valve reducing high flow velocities and the size of pressure drops that can cause damaging high flow velocities, noise and cavitation. Further advantageously providing the aforementioned benefits whilst maintaining a clear unobstructed flow path when the valve is not controlling flow in the fully open position. Optionally, the tapered face of the first tapered portion is tangential and / or co-planar to the tapered face of the second tapered portion where they end at the narrow open end when the valve is in the fully open position. Advantageously forming a venturi providing smooth flow when in the open position. Optionally, the tapered face of the first tapered portion and the tapered face of the second tapered portion together form a venturi, for maintaining laminar flow through the valve, when the valve is in the fully open position. Optionally, the obturating portion includes a sealing face and the outlet includes a valve seat for receiving said sealing face for preventing flow of fluid through the valve when the valve is in the closed position. Optionally, each tapered portion includes a leading arc comprising a first portion of the end face exposed to the bore when the valve moves from the closed position, the leading arc extending for an angle of between 40 and 180 degrees about the axis A, preferably 70-110 and most preferably 90 degrees, and the first and second apertures (31,32) are located in the leading arc. Advantageously fewer apertures in the tapered portion of the obturating portion provides improved control of the pressure reduction and allows the volume of the plenum of said tapered portion to absorb turbulence. Additionally, manufacturing is simplified. Optionally, the plurality of first apertures includes a first subset of first apertures and a second subset of first apertures wherein the first subset of first apertures are smaller in area than the second subset of first apertures and the first subset of first apertures are located radially outboard of the second subset of first apertures. Optionally, the first set of first apertures and second set of first apertures are each arranged in a circumferentially extending arc about the axis A or axis B. The lower area first subset of apertures are the first to be exposed when the valve moves from the closed position controlling the flow as the valve is opened. This is the most sensitive movement of a valve without a flow reducing device. Adventurously, the smaller area of the first subset provides greater control of any shockwave that can occur when a valve is first moved from the closed position. The subsets and the arrangement thereof also provide a more linear response between valve position and flow rate. Optionally, the first plurality of apertures in the end face each extend from a first end at the plenum to a second end at the end face, and wherein the first plurality of apertures are at an acute bore angle a to the axis A. Optionally, the angle a is 5-45 degrees. Optionally, the plurality of second apertures in the tapered face each extend from a first end at the plenum to a second end at the tapered face at an acute bore angle p to the axis A. Optionally, the angle p is 45 to 90 degrees. Angles a and p advantageously aligning the first and second apertures with the flow of fluid through the valve when restricting flow close to the closed position and / or reducing flow therethrough when the valve is in the open position. Optionally, the plenum extends circumferentially through an arc of angle greater than the first apertures or second apertures are present and preferably about the complete circumference of the bore. Advantageously providing a volume for absorbing energy of high velocity flow before releasing it into the bore. Optionally, the first plurality of apertures and the second plurality of apertures have a size of between 1mm2 and 100mm2 and preferably 1mm2 and 10mm2 Optionally, the first plurality of apertures and / or the second apertures have a rhombus profile. Optionally, the first tapered portion and the second tapered portion each include a plenum, a first plurality of apertures and a second plurality of apertures. Optionally, the first tapered portion is located in the aperture in the obturating portion and the second tapered portion is located in the inlet or outlet and the first tapered portion includes the first plurality of apertures and the second plurality of apertures located only in the leading arc and the second tapered portion includes first and second apertures through an arc greater than the leading arc and preferably about the full circumference of the second tapered portion. Optionally, the first plurality of apertures of the first tapered portion are offset from the first plurality of apertures of the second tapered portion. Optionally, the outlet has a maximum diameter D and the second tapered portion there in extends at least the distance D along axis A and preferably the distance two times D along axis A . Optionally, the valve includes a bypass flow control means wherein the obturating portion includes a lip extending circumferentially about at least a portion of the aperture at the wide open end having a profile P and the housing includes a complementary profile P’ for receiving the profile P when the valve is between the open position and the closed position. Optionally, the valve includes a plurality of third apertures extending through a wall of the aperture of the obturating portion for connecting the bore of the obturating portion with the inlet or outlet when the valve moves form the closed position preferably the plurality of third apertures are located on the opposite side of the bore to the first and second plurality of apertures. Optionally, the obturating portion includes a first device seat and the inlet or outlet includes a second device seat, and the device comprises a first insert including the first tapered portion secured in the first seat and a second insert including the second tapered portion secured in the second seat. Advantageously allowing different specifications of flow control device to be used in the same valve body for different applications. Further advantageously allowing the flow control device to be manufactured using different manufacturing techniques and / or materials to the rest of the valve. Optionally, each tapered portion includes a concave portion at or toward the wide open end and / or a convex portion at or toward the narrow open end. Providing a smooth venturi and maintaining smooth flow through the valve. Optionally, the plenum includes a flow control structure for dissipating energy therein. Optionally, the valve is a ball valve and preferably an eccentric ball valve. Optionally, the device is 3D printed. Brief Description of Drawings Specific embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a section view of a valve including a flow trim according to the current invention comprising a single plenum in an open position, according to the present invention; Figure 2 shows a schematic section view of half a valve embodiment comprising a single plenum in an open position, according to the present invention; Figure 3 shows a section view of a valve embodiment comprising a single plenum in a partially open position, according to the present invention; Figures 4A to 4E show a detail view of the point of connection between a first and second tapered portion as the valve moves from closed to open, according to an embodiment of the present invention; Figure 5 shows a section view of a valve embodiment comprising a dual plenum in an open position, according to the present invention; Figure 6A shows a side view of a conventional ball valve and the velocity of fluid flow through the ball valve. Figure 6B shows a side view of a valve embodiment according to the present invention and the velocity of fluid flow through the valve; Figure 7 shows a section view of a valve embodiment comprising a dual plenum in a closed position, according to the present invention; Figures 8A to 8F show a detail view of the point of connection between a first and second tapered portion as the valve moves from closed to open, according to an embodiment of the present invention; and Figures 9A to 9C shows an isometric cutaway view of the valve of figure 5 further including a third subset of the plurality of first apertures in the end face of the first tapered portion. Figure 9A shows the valve in the closed position; Figure 9B shows the valve between the open position 15 and the closed position 18; and Figure 9C shows the valve in the open position. Specific Description In overview, and without limitation, the application discloses a valve 1 with a flow reducing device 20, which may be referred to as a trim configuration or flow trim 20, that operates within a modulating control valve 1 between an open position 15 and closed position 18. The valve 1 is preferably a ball style valve 1 for modulating flow having a housing 5 with an inlet 2 connected to an outlet 6 when the valve moves from the closed position 18, together defining a bore 4 of the valve 1 having a central axis A extending along the bore 4 and a valve seat 3 for sealing the valve 1 when in the closed position 18. The valve 1 further includes an obturating portion 10 for preventing flow through the valve 1 when in the closed position 18. The obturating portion 10 having an aperture 12 extending there through defined by a wall 13 and defining a bore 14 of the obturating portion 10. The obturating portion 10 further includes a sealing face 11 Preferably, the flow reducing device 20 is bi-directional. The flow reducing device 20may comprise a first tapered portion 21 and a second tapered portion 22. One tapered portion 21,22 positioned in the bore 14 of a ball or plug style valve positioned in the inlet 2 or outlet 6 downstream of the ball 10 or obturating portion 10. The tapered portions 21, 22 extend circumferentially and axially along the bore 4 of the inlet 2 or outlet 6 and of the bore 14 of the obturating portion 10. Each tapered portion 21, 22 forming a narrowing of the bore 4,14 in which it is located having a wide open end 24 and a narrow open end 25. When in a fully open position 15, the flow reducing device 20 meet at the narrow open end 24 and together the tapered portions 21, 22 of the bore 4 of the inlet 2 or outlet 6 and the bore 14 of the obturating portion 14 together form a venturi. This is advantageous as the venturi influences the fluid to remain attached to the surface, promoting laminar flow and reducing turbulence or Eddie formations across the valve which may be caused by obstructed bores of valves including flow trims of the prior art when subject to large pressure gradients observed in high-velocity flow of compressible media. Limiting unrecovered pressure losses and improving flow efficiency / capacity. At least one of the tapered portions 21, 22 of the flow reducing device 20 comprise a plenum 30 therein connected to the associated adjacent bore 4, 14 by a first plurality of apertures 31 and second plurality of apertures 32 or flow passages that promote diffusion. When the valve moves away from the closed position 18 to a partially open configuration the fluid or media flowing through the valve 1 can pass through the apertures 31, 32 to the plenum 30 and then leave the plenum 30 through the apertures 31, 32. Thus providing a multi stage reduction in pressure allowing lower resistance for a particular reduction in flow; providing both a reduction in velocity jet coalescence and multi-stage pressure reduction, minimising aerodynamic noise. Referring to Figure 1, a valve according to the present invention will now be described. The valve 1 comprising a flow reducing device 20 is suitable for controlling flow of a fluid through a bore 4, 14. The bore 4, 14 may also be referred to as a channel or passage. In Figure 1, the valve is in the open position 15. There is provided a housing 5 having an inlet 2 and an outlet 6 connected thereto as the valve is in the open position 15, an obturating portion 10 and a flow trim device 20 or flow trim 20. The inlet 2 and outlet 6 defining a bore 4 through which fluid may flow, preferably from the inlet 2 to the outlet 6. When the valve is in a fully open position 15, or in some implementations, a partially open position, fluid can flow through the bore 4. Axis A is positioned at the centre of the bore 4 and defines the axis of fluid flow. It is depicted with long dashed lines. The valve further includes an obturating portion 10, having an aperture 12 extending there through defining a bore 14 of the obturating portion and having Axis B is positioned along its centre. Aperture 12 may also be referred to as a hole or passage through the obturating portion 10, and it may be defined by an internal wall 13 of the obturating portion 10. Axis B is depicted with a dotted line. When the valve 1 is in the fully open position 15, as in Figure 1, axis B is parallel and aligned with axis A as the inlet 2, outlet 6 and aperture 12 are in alignment. Although the inlet 2 is shown on the left of the figures and the outlet 6 is shown on the right of the figures, depicting the points at which fluid enters and exits the valve 1 respectively, it will be understood that the valve 1 is bi-directional. The terms inlet 2 and outlet 6 are therefore interchangeable depending on the direction in which fluid is flowing through the valve 1. The flow trim 20 includes a radially and axially extending first tapered portion 21 and a radially and axially extending second tapered portion 22, each of which extend circumferentially about and axially along the bore 4, 14. In Figure 1, the tapered portions 21, 22 extend around the entire circumference of the bore, however, the skilled person will understand that the tapered portions 21, 22 may extend around the same portion of the circumference as defined below. One tapered portion 21, 22 forms the bore 14 of the obturating portion 10 or the hole or passage of a ball valve, the other tapered portion 21, 22 forms the bore of the inlet 2 or outlet 6 of the valve 1. Each tapered portion 21 includes an axially and circumferentially extending tapered face 26 and a radially and circumferentially extending end face 27. In the open position 18 depicted in Figure 1, the tapered face is exposed to and defines the bore 4, 14. The first and second tapered portions 21,22 each extend from a wide open end 24 to a narrow open end 25. The first and second tapered portions 21, 22 face each other at their respective narrow open ends 25. Accordingly the end face 27 of the first and second tapered portions 21 face each other when in the open position 15 shown in figure 1. The tapered face 26 of the first tapered portion 21 and the tapered face 26 of the second tapered portion 22 are tangent or coplanar to each other where they meet they meet such that they together form a venturi for promoting smooth unobstructed flow of fluid when the valve is in the open position 15. In Figure 1, the first tapered portion 21 is located in the obturating portion 10 and the second tapered portion 22 is located in the outlet 6. The first tapered portion 21 comprises a circumferentially extending plenum 30 defined and separated from the bore 14 of the obturating portion 10 by the tapered face 26 and end face 27 of the first tapered portion 21. The plenum 30 may also be described as a plenum chamber 30. The plenum 30 comprises a cavity or chamber through which fluid can flow. There are provided a plurality of first apertures 31 in the end face 27 of the first tapered portion 21 extending between the plenum 30 and the bore 4, 14, and a plurality of second apertures 32 in the tapered face 26 of the first tapered portion 21 extending between the bore 14 and the plenum 30. The plurality of second apertures 32 in the tapered face 26 are arranged near the narrow open end 25. These apertures 31, 32 allow the flow of fluid through the first tapered portion 21 via the plenum 30 as the valve moves from the closed position 18 toward an open position. Advantageously, when the valve 1 in the open position 18 depicted in Figure 1, the apertures do not disrupt the flow of fluid and therefore facilitates a high flow capacity through the valve 1. The second tapered portion 22 does not include a plenum in the embodiment of figure 1. The skilled person will understand that the first tapered portion 21 could be located at the outlet 6 and the second tapered portion 22 in the obturating portion 10. In some implementations, at least one aperture of the first plurality of apertures 31 and / or the second plurality of apertures 32 may have a rhombus profile. In other words, the cross-section of each aperture is a rhombus. This allows the apertures to be more reliably manufactured when using metal additive manufacturing methods. In some implementations, there may be provided a plurality of third apertures 33 in the tapered face 26 of the obturating portion 10 extending through the wall 13 of the bore 14. These may be arranged near the wide open end 24 for connecting the inlet 2 with the bore 14 of the obturating portion when the valve 1 is moved from the closed position 18. Preferably the plurality of third apertures 33 are located on the opposite side of the bore 14 to the first and second plurality of apertures 31, 32. Advantageously, the provision of a plurality of third apertures facilitates fluid flow when the valve is in a partially open position, as there is less fluid constriction at the wide open end 24 as the valve moves from the closed position 18. Referring now to Figure 2, a side view of a valve embodiment according to some aspects of the present invention will be described. Figure 2 depicts a single plenum arrangement in a fully open position 15, as in Figure 1. The aperture 12 can be seen running through the centre of obturating portion 10, with dotted axis B defining the centre, and in alignment with dashed axis A that runs through the centre of the bore 4. The plurality of first apertures 31 of the end face 27, and the plurality of second apertures 32 in the tapered face 26 are shown in Figure 2. Each aperture of the plurality of first apertures 31 preferably extend from a first end 31a at the plenum 30 to a second end 31b at the end face 27. In some implementations, they may extend from first end 31a to second end 31b at an acute bore angle of a to the axis A. Angle a is shown as a dashed line wedge on a single aperture of the plurality of apertures 31 depicted in Figure 2. Angle a is preferably in the range of 5 to 45 degrees, and most preferably 30 deg. Each aperture of the plurality of second apertures 32 preferably extend from a first end 32a at the plenum 30 to a second end 32b at the tapered face 26. In some implementations, they may extend from first end 32a to second end 32b at an acute angle of p to the axis A. Angle P is shown as a dashed line wedge on a single aperture of the plurality of apertures 32 depicted in Figure 2. Angle p is preferably in the range of 45 to 90, and most preferably 75. Angling the apertures in these ranges may advantageously improve fluid flow through the plenum when the obturating portion 10 is at an angle to the direction of flow or to Axis A; in particular as the valve is moving from the closed position 18. The angle of the apertures reduces the direction change required by the fluid, reducing resistance when the valve in partial open position, while discouraging fluid flow through the flow reducing device 20 when the valve 1 is in the fully open position 15. As such, the flow reducing device 20 can more precisely control fluid flow when in a partially open position, while also having the highest possible flow capacity when open 15. It is particularly advantageous that, in the open position 15, the first and second pluralities of apertures 31, 32 have little or no interaction with the flow of fluid. Therefore, the bore 4, 14 is a venturi to smooth flow of fluid as it flows through the bore 4, 14 in the open position 15, as well as facilitating a smoother pressure drop as the valve moves from a closed to an open position. The valve 1 therefore maintains an unobstructed high flow capacity whilst providing a broad controllable range of flow reduction, which allows for flow to be controlled over a greater range. The valve could also be smaller to achieve the same flow capacity, and therefore would be cheaper to manufacture. In some implementations, the tapered face 26 of the tapered portions 21, 22 may be flat i.e., at a continuous angle relative to axis B throughout the taper. Preferably, one or more of the tapered portions 21, 22 may be curved. In some preferable implementations, each tapered portion includes a concave portion at or toward the wide open end 24 and / or a convex portion at or towards the narrow open end 25. These concave and convex portions are depicted in Figure 2. Advantageously, this forms a smooth, curved venturi tube, which optimises fluid flow. It will be understood that the convex and concave curved sections in figure 2 are exaggerated and in particular the tapered face 26 of the outlet 6 preferably has an angle of taper y that is less than 10 degrees to the axis A. The valve 1 is a ball valve. The obturating portion 10 may comprise an approximate ball shape with an aperture 12 therethrough that pivots around at least one axis of movement, to allow the valve 1 to move between an open position 15 and a closed position 18. Advantageously, ball valves are durable and low maintenance, as well as having a high flow capacity. When in the open position 15, the hole through the obturating portion 10 allows fluid to flow from the inlet 2, through the aperture 12, to the outlet 6. The tapered portions 21,22 form a venturi for maintaining laminar flow through the valve 1 wen in the open position 15. When in the closed position 18 (for example, when the ball valve 1 is turned 90 degrees) the aperture 12 is no longer in alignment with the inlet 2 and the outlet 6 and flow is stopped. The obturating portion 10 includes a sealing face 11 and the outlet 6 includes a valve seat 3 for receiving the sealing face 11 and preventing flow. Even more preferably, the valve 1 may be an eccentric ball valve in which there is an eccentric cam. In other words, the centre of rotation of the ball or obturating portion 10 is ‘off centre’. As with a ball valve, eccentric ball valves are particularly durable with the advantage of lower torque requirements. Further, an eccentric ball valve provides improved flow control. At the narrow open end 25, the first tapered portion 21 and the second tapered portion 22 face each other. The first tapered portion 21 extends along the aperture of the obturating portion 10, and the other, second tapered portion 22 extends along the bore of the inlet 2 or outlet 6. In other words, the first tapered portion 21 extends along axis B, whereas the second tapered portion 22 extends along axis A. Preferably, the tapered face 26 of the first tapered portion 21 is tangential to the tapered face 26 of the second tapered portion 21 where they end at the narrow open end, when the valve is in the fully open position 15 depicted in Figure 1F. In this context, tangential means that a tangent at the outermost edge of the tapered face 26 of the first tapered portion is parallel with, or preferably aligned with, a tangent at the outermost edge of the tapered face 26 of the second tapered portion 22. In some preferably implementations, the end face 26 of the first tapered portion 21 may be In contact with the end face 26 of the second tapered portion 22, such that the tapered faces form a continuous surface when the valve 1 is in a fully open position 15. In other implementations, there may be a space between the tapered portions 21, 22 through which fluid can flow. It is further preferable that the tapered face 26 of the first tapered portion 21 and the tapered face 26 of the second tapered portion 22 together form a venturi 40 for maintaining laminar flow through the valve 1. In a narrow or constricted section of a channel, such as the narrow open end 25, there is a reduction in fluid pressure resulting from the increase in velocity of a moving fluid through that constricted section. Preferably the apertures have a size of between 1 to 10 mm nominal diameter or a cross sectional area of between 1mm2 and 100mm2 and preferably 1mm2 and 10mm2. Although Figure 2 depicts the plurality of first apertures 31 and the plurality of second apertures 32 as being the same size, in some implementations the diameter of the apertures may vary. The first plurality of apertures 31 may comprise a first subset 31c and a second subset 31 d of the plurality of first apertures 31. The first subset 31c may have a smaller cross-sectional area than the second subset 31 d of the plurality of first apertures 31 and preferably each aperture of the first subset 31c has a smaller cross-sectional area than each aperture of the second subset 31 d. Optionally, a third subset 31 e of the plurality of first apertures 31 may be included. Each aperture of the second subset 31 d of the plurality of first apertures 31 may have a smaller cross-sectional area than each aperture of a third subset 31 e of the plurality of first apertures 31. The first subset 31c of the plurality of first apertures 31 may be located radially outboard of the second subset 31d, which in turn may be located radially outboard of the third subset 31e and visible in figure 9B. In other words, apertures further away from axis B of the obturating portion 10 may have a smaller area than those that are radially inward and closer to axis B. In some implementations, there may be provided further subsets of the plurality of first apertures 31 with decreasing cross-sectional area as they extend radially outwards. This means that the first apertures that are exposed when the valve 1 moves from the closed position 18 have a lower cross sectional area providing a low flow rate through the plenum 30 and controlled flow reduction. Referring now to Figure 3, the valve of figure 1 can be seen moved from the closed position 18 in a partially open position. Axis B is at an acute angle to axis A, which means that the flow of fluid is constricted as there is only a small opening at both the wide end 24 of the first tapered portion 21 (upstream) and the narrow end 25 of the first tapered portion 21 (downstream). This constriction can lead to high velocity jets due to a rapid pressure drop between the upstream and downstream sides of the valve i.e., from the inlet 2 to the outlet 6. The obturating portion 10 may include a sealing face 11, and the outlet 6 may include a valve seat 3. As the valve 1 moves to the closed position 18, the valve seat 3 may receive said sealing face 11. This prevents the flow of fluid through the valve 1 when the valve 1 is in the closed position 18. The valve seat 3 may comprise deformable material suitable for forming a friction seal around inlet 2. In addition to flow through the aperture 12 in the obturating portion 10 and the flow reducing device 20 between the inlet 2 and the outlet 6. There are additional flow paths around the obturating portion 10 between the obturating portion 10 and the housing 5. This bypass flow path can be uncontrolled and a source of high velocity turbulent flow. In order to control such bypass flow the valve 1 includes a bypass flow control means 60 located at the end of the aperture 12 distal to the end face 27 of the obturating portion 10. The bypass flow control means 60 includes a lip 19 extending circumferentially about at least a portion of the aperture 12 at the wide open end 24 having a profile P. The lip 19 interfaces with a complementary profile P’ of the housing 5 for preventing or substantially reducing uncontrolled bypass flow when the valve 1 is between the open position 15 and the closed position 18. There is minimal clearance between the lip 19 and the housing 5 through the movement of the valve from the closed position 18 toward the open position 15. Referring now to Figures 4A to 4E, the flow of fluid through the flow reducing device 20 of a single plenum valve 1 according to the present invention will be described. Figures 4A to 4E show a close up view of the flow reducing device 20 of the valve of figure 1 in various positions moving from the closed position 18. In each figure arrows F show the flow paths available to the fluid. Figure 4A depicts a single plenum valve in a closed position 18. In the closed position 18, the flow of fluid is blocked by the obturating portion 10. The axis B that runs through the centre of aperture 12 is at its maximum angle to axis A and may be substantially perpendicular to axis A that runs through the centre of the bore i.e., from the inlet 2 to the outlet 6. As there is no alignment between any portion of the aperture 12 and both the inlet 2 and the outlet 6, fluid cannot flow through the valve 1. The sealing face 11 is in contact or frictional engagement with the valve seat 3 to ensure a secure seal in this closed position 18 in order to prevent flow. As the valve 1 moves from the closed position 18 toward the open position 15, the valve 1 may enter a partially open position where there is some fluid communication between the inlet 2, aperture 12 and outlet 6. This is shown in Figures 4B, 4C and 4D, which in order each depict the valve 1 in a more open position than the last. When valve 1 moves from the closed position 18 depicted in Figure 4A, a first portion of the end face 27 is exposed to the bore 4. This exposes the first plurality of apertures 31 on the end face 27 to the bore 4. Not shown in the close up are the plurality of third apertures 33 distal the end face 27 allowing fluid to flow from the inlet 2 through the plurality of third apertures 33 into the bore 14 of the obturating portion 10 and subsequently fluid is able to flow from the bore 14 through the second plurality of apertures 32 on the tapered face 26, through the cavity of the plenum 30, and then through the first subset 31c of the first plurality of apertures 31 on the end face 27. In combination with the plurality of third apertures 33 the valvel provides a three step pressure reduction. This fluid flow reduces the pressure differential between the inlet 2 and the outlet 6, reducing the incidence of damaging velocity jets. The first point on the end face 27 to be exposed to the bore 4 on opening is the point for reference for defining a leading arc 28 of the end face 27. Said first point being a midpoint of the leading arc 28. Preferably, the leading arc 28 extends for an angle of between 40 and 180 degrees about the axis A. Even more preferably, it extends for an angle of 70 to 110 degrees and most preferably 90 degrees. The skilled person would understand that the ideal size of the leading arc depends on a number of factors, including, for example, the maximum speed of fluid, the angle of the tapered portions, the diameter of the narrow open end of the aperture. The plenum 30 may extend circumferentially, and in some implementations, may extend through an arc greater than that of the first apertures 31 or second apertures 32. In other words, a portion of the plenum 30 may be absent apertures on either the tapered face 26 and / or the end face 27. In some implementations, the plenum 30 may extend about the complete circumference of the bore 4. Preferably, the first plurality of apertures 31 and second plurality of apertures 32 of the first tapered portion 21 may be located only on the leading arc 28. In an embodiment where both the first tapered portion 21 and second tapered portion 22 have a plenum 30, a plurality of first apertures 31 and a plurality of second apertures 32 such as that shown in figure 5; the first tapered portion 21 is located in the obturating portion 10 and the second tapered portion 22 is located in the outlet 6 or inlet 2. The first plurality of apertures 31 and second plurality of apertures 32 of the second tapered portion 22 may be located through an arc greater than the leading arc 28. Optionally, the plurality of first apertures 31 and or plurality of second apertures 32 of the second tapered portion 22 may be located about the full circumference of the second tapered portion 22, which may extend around the complete circumference of the bore 4. As is discussed above, the cross-sectional area of each aperture of the plurality of first and second apertures 31, 32 may vary. Preferably, the size of the plurality of first apertures 31 in the end face 27 may vary such that the cross-sectional area of the first subset 31c of radially outer apertures is less than the cross-sectional area of the second subset 31 d of radially inner apertures. Therefore, the subset of the plurality of first apertures that are first exposed to the bore as the valve 1 moves from a closed position 18 to an open position 15, for example in Figure 4B, are smaller and allow less fluid to flow. Radially inward apertures that are subsequently exposed to the bore 4 as the valve opens, for example in Figures 4C and 4D, may be larger. This allows the rate of fluid flow to be controlled more accurately when first moving the valve 1 from the fully closed position 18, and reduces the chance of a damaging shock wave on opening the valve whilst continuing to reduce the negative effects of a pressure differential. Referring now to Figure 5, a second embodiment according to some aspects of the present invention will now be described. Figure 5 depicts a variation of the valve of figure 1 wherein the flow reducing device 20 includes a second plenum 30 and a plurality of first and second apertures 31,32 in the second tapered portion 22. Both plenums 30 may provide a cavity through which fluid can flow. Both the first tapered portion 21 and the second tapered portion 22 include a first plurality of apertures 31 and a second plurality of apertures 32. In a preferable implementation as shown in figure 5, the first tapered portion 21 is located in the obturating portion 10, and the second tapered portion 22 is located in the inlet 2 or the outlet 6. Most preferably, the second tapered portion 22 is located in the outlet 6. As described elsewhere in the present application, the plurality of first and second apertures 31, 32 of the first tapered portion 21 may be located only in the leading arc 28, and the plurality of first and second apertures 31, 32 of the second tapered portion 22 may be located through an angle greater than that of the leading arc 28. Even more preferably, they may be located about the full circumference of the second tapered portion 22. Each aperture of the plurality of first apertures 31 of the second tapered portion 22 preferably extends from a first end 31a at the plenum 30 to a second end 31b at the end face 27. The apertures may be substantially perpendicular to their respective face, or may be angles. In some implementations, they may extend from first end 31a to second end 31b at an acute bore angle of a to the axis A. Likewise, each aperture of the plurality of second apertures 32 of the second tapered portion 22 preferably extend from a first end 32a at the plenum 30 to a second end 32b at the tapered face 26. In some implementations, they may extend from first end 32a to second end 32b at an acute bore angle of p to the axis A. Angles a and p for the apertures of the second tapered portion 22 may be different from angles a and p for the apertures of the first tapered portion 21. The first plurality of apertures 31 of the first tapered portion 21 may be offset circumferentially and / or radially from the first plurality of apertures 31 of the second tapered portion 22 in order to provide a greater barrier to flow when the radial faces are aligned as the valve 1 moves from the closed position as shown in figures 8B, 8C, 8D. Preferably, they may be offset by at up to 10 degrees about axis A. For differing flow characteristics the first plurality of apertures 31 of the first tapered portion 21 may be circumferentially and / or radially aligned. For quicker and cheaper manufacturing, it is advantageous that apertures be provided in the first tapered portion 21 only where they will be exposed to the bore when the valve 1 is in a partially open position i.e., in the leading arc 28. The flow reducing device may be provided as a first insert 53. The first insert 53 may comprise the first tapered portion 21 secured in a first device seat 51. The first insert may be positioned in the aperture 12 in the obturating portion 10. Likewise, there may be provided a second insert 54. The second insert 54 may comprise the second tapered portion 22 secured in the second device seat 52. The second insert may be positioned in the inlet 2 or the outlet 6. As discussed with reference to Figure 2, the plurality of first and second apertures 31, 32 may vary in size. Specifically, the plurality of first apertures 31 in the end face 27 of the second tapered portion 22 may vary in cross-sectional area such that the radially outer first subset of apertures 31c have a lower cross-sectional area than the cross-sectional area of the radially inner subset of apertures 31 d. Referring now to Figures 6A and 6B, Figure 6A shows a simulation of flow velocity for a valve without a flow trim of the current invention fitted. Figure 6B shows a simulation of flow velocity for a valve with a flow reducing device 20 according to the current invention fitted. Each shows the valve 1 in a partially open configuration such that the flow of fluid there through is substantially trimmed. These figures demonstrate the improvement in flow velocity as the valve 1 moves from an open position 15 to a closed position 18. In both Figure 6A and 6B, the valve has been turned to 40 degree angle relative to axis A. In Figure 6A, a conventional ball valve is depicted having the same bore profile at the inlet, the outlet and the aperture 12 through the Obturating portion 10 but no other features of the flow control device 20 of the current invention. There are velocity jets that result from the large pressure differential as the fluid passes through a narrow gap between the edge of the channel through the ball and through the bore 4. Fluid flows from the inlet 2 upstream to the outlet 6 downstream, and there is a rapid pressure drop between the upstream and downstream sides of the valve. These velocity jets can be seen in Figure 6A depicted by the darkest sections of fluid representing the highest velocity. The velocity may be up to 79.5 m / s. Figure 6B depicts a valve according to embodiment of the present invention. There is provided a first tapered portion 21 and a second tapered portion 22. Each tapered portion has a plenum 30, and includes an end face 27 and a tapered face 26. The plurality of first apertures 31 and the plurality of second apertures 32 in the first tapered portion 21 (in end face 27 and tapered face 26 respectively) facilitate the flow of fluid, and thus do not cause a rapid pressure drop between the upstream and downstream sides of the valve, as in Figure 6A. Therefore, velocity jets such as those in Figure 6A are avoided in Figure 6B. This is advantageous as cavitation, which can be damaging to both the valve and the pipe structure, can be avoided, reducing noise. It also is advantageous in that it is more efficient, and there are fewer pumping losses through the valve. Figure 6A also shows high velocity fluid flow around the outer face of the obturating portion 10. This may be referred to as bypass flow. Fluid can be seen flowing between the obturating portion 10 and the housing 5 resulting in a high velocity area at the bottom of the outlet 6. Figure 6B also shows the remaining bypass flow taking a low velocity path through the plurality of first apertures 31, the plenum 30 and plurality of second apertures 32 of the second tapered portion 22. This is possible due to the plurality of first apertures 31 extending around the full circumference of the end face 27 of the second tapered portion 22. Optionally, there may be provided a bypass control mechanism 60, discussed above with reference to figure 3, for reducing or preventing the flow of fluid around the obturating portion 10. Referring now to Figure 7, which shows the valve 1 of figure 5 in a closed position 18. Figure 7 shows the valve 1 in a closed position 18, as the sealing face 11 of the obturating portion 10 is in contact with a valve seat 3. There is no overlap between the aperture 12 and the inlet 2 or the outlet 6. As such, there is no path for fluid to take through the bore 4 and no fluid communication between the inlet 2 and the outlet 6. Referring now to Figures 8A to 8F, the process of opening a dual plenum valve according to some aspects of the present invention will be described. Figures 8A to 8F show a close up view of the flow reducing device 20 of the valve of figure 1 in various positions moving from the closed position 18. In each figure arrows F show the flow paths available to the fluid. Figure 8A depicts a dual plenum valve in a closed position 18, as in Figure 7. The axis B that runs through the centre of aperture 12 is at its maximum angle to and may be substantially perpendicular to axis A that runs through the centre of the bore i.e., from the inlet 2 to the outlet 6. As there is no alignment between any portion of the aperture 12 and both the inlet 2 and the outlet 6, fluid cannot flow through the valve 1. As the valve 1 moves from the closed position 18 toward the open position 15, the valve 1 is in a partially open position. This is depicted in Figures 8B, 8C, 8D and 8E, which in order each depict the valve 11 in a more open position than the last. When valve 1 moves from the closed position 18 depicted in Figure 8A, a first portion of the end face 27 of the first tapered portion will be exposed to a first portion of the end face 27 of the second tapered portion 22. Specifically, as shown in figure 8B this exposes the first plurality of apertures 31 on the end face 27 of the first tapered portion 21 to the first subset 31c of the plurality of first apertures 31 on the end face 27 of the second tapered portion 22. Therefore, fluid is able to flow through the plurality of third apertures 33, through the bore 14 of the obturating portion 10, through the second plurality of apertures 32 on the tapered face 26, of the first tapered portion 21, through the plenum 30 and then through the first subset 31c of the first plurality of apertures 31 on the end face 27 of the first tapered portion 21; into the first subset 31c of the first plurality of apertures 31 on the end face 27 of the second tapered portion 22, to the plenum 30 and through the plurality of second apertures 32 of the second tapered portion 22 and to the outlet 6. The exposed first portion may be referred to as a leading arc 28 of the first tapered portion 21. Accordingly a five stage pressure reduction is provided when the flow is controlled. Preferably the first subset 31c have a smaller cross-sectional area than a second subset 31 d of the plurality of first apertures 31. The second subset 31 d may be arranged radially inward of the first subset 31c. In other words, the second subset 31d of apertures are located closer to the axis A when the valve 1 is in an open position 15. Figure 8B depicts an arrangement in which the first subset 31c of the plurality of first apertures 31 of the first tapered portion 21 are in alignment with the first subset 31 c of the plurality of first apertures 31 of the second tapered portion 22. It is particularly advantageous to provide the first or second plurality of apertures 31,32 about the full circumference of the second tapered portion 22 as it controls the flow of fluid around the obturating portion of the valve through to the bore. Bypass flow may be further reduced by the bypass flow control means 60 discussed above. Now considering Figure 8C the end face 27 of each of the first and second tapered portion 21, 22 is aligned. Accordingly, the fluid can now flow through all of the plurality of primary apertures 31 in the first tapered portion 21 and through all of the plurality of primary apertures 31 in the second tapered portion 22 not only a subset of the primary apertures. The remainder of the flow path is as for figure 8B. Thus, whilst flow has increased there is still a five stage pressure reduction provided by the flow reducing device 20. Referring to figure 8D the valve 1 has opened further. Now a portion of the end face 27 of both first and second tapered portions 21,22 are exposed to the bore. Thus there are three possible flow paths, two passing through only one plenum 30 of either the first or second tapered portion 21, 22 and a third passing through both plenums as follows. In this configuration a portion of the flow will pass from the plenum 30 of the first tapered portion 31 through the first subset of apertures 31c of the plurality of first apertures 31 to the second subset 31 d of the plurality of first apertures of the second tapered portion 22 and into the plenum 30 thereof. There may optionally be provided further subsets of apertures radially inward of the third subset, with subsequently increasing cross-sectional area. Referring now to Figure 8E, fluid may flow through the plenum 30 and the plurality of first and second apertures 31, 32 as well as through the bore. The apertures 31, 32 facilitate the flow of fluid to decrease the pressure differential and avoid velocity jets in the bore 4. Finally, Figure 8F depicts an open position 15 of the valve 1. In this position, the apertures 31, 32 do not affect the flow of fluid. Rather, fluid is able to flow through the bore 4 via a venture 40 that is formed at the narrow open end 25 when the first tapered portion 21 and the second tapered portion 22 meet. It should be noted that in the trimming positions where flow is controlled shown in figures 8B, 8C, 8D, 8E it is likely that around the leading arc a plurality of different path options will be available due to the opposing arcs of the end face 27 of the first and second tapered portions 21, 22. The commentary of figures 4 and figures 8 deals with the interaction of flow at the cross section plane. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. Figures 9A, 9B and 9C show cut away isometric views of the valve of figure 5 in various positions looking from the direction of the outlet 6. The closest quarter of the housing 5 has been cut away to expose the obturating portion 10 and associated first tapered portion 21 therein. Figure 9a shows the valve in the closed position. The sealing face 11 can be seen engaged with the valve seat 3 for preventing flow through the valve. In figure 9B the valve is between the closed position 18 and the open position 15. It can be seen that the plurality of first apertures 31 and / or the plurality of second apertures 32 of the first tapered portion 21 in the obturating portion 10 do not extend about the full circumference of the end face 27 or the tapered face 26 respectively. The plurality of first apertures 31 and / or the plurality of second apertures 32 can be seen extending about a portion or an arc of the circumference of the end face 27 or the tapered face 26 respectively. This is defined above as the leading arc 28. The leading arc 28 is bisected by the first point of the end face 27 that is exposed to the bore 4 when the valve 1 moves from the closed position 18. In the valve shown in figure 9 the first point sits on a plane perpendicular to the axis of rotation of the obturating portion 10 and the axis A that thus also bisects the leading arc 28. The plurality of first apertures 31 and the plurality of second apertures 32 in the second tapered portion 22 can be seen extending further than the angle of the leading arc 28. Indeed in the valve of figure 9 it can be seen that the apertures 31, 32 and the plenum 30 extend about the full circumference of the second tapered portion 22. A first subset 31c, second subset 31 d and a third subset 31 e of the plurality of first apertures 31 can be seen on the end face 27 of the first tapered portion 21. The first subset 31c are radially outer most and have the smallest cross sectional area of the subsets 31c, 31 d, 31 e. The second subset 31 d has a greater cross sectional area than the first subset 31c and is located radially inboard of the first subset 31c. The optional third subset 31 e included in the figures 9 have a greater cross sectional area than the second subset 31 d and are located radially inboard of the second subset 31 d. Preferably, each of the first apertures 31 of the second subset 31 d have a greater cross sectional area than each of the first apertures 31 of the first subset 31c. Preferably, if a third subset 31 e is included each of the first apertures 31 of the second subset 31 d have a greater cross sectional area than each of the first apertures 31 of the second subset 31c. Each subset 31c, 31 d, 31 e of the plurality of first apertures 31 may be arranged along a circumferentially extending arc about the axis A or axis B as appropriate. These arcs may be concentric and / or extend to a complete circle as shown on the second tapered portion 22. Figure 9C shows the valve in the open position. The tapered faces 26 of the first and second tapered portions 21, 22 can be seen to form a venturi having a near continuous flow path for providing smooth unobstructed flowthrough the valve 1 when in the open position. For example, although the valve 1 has been described with reference to a ball valve in a cylindrical bore, it would be understood that in another implementation, the obturating portion 10 could be replaced with any suitable valve component. Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. Any feature in one aspect may be applied to other aspects, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects can be implemented and / or supplied and / or used independently.

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