Valve balancing device

Abstract

A system comprises a pressure regulating valve 6, comprising an aperture that is opened and closed by a valve member. The valve member is acted on by a fluid pressure in a first direction. A balancer

Description

The present invention relates to a pilot valve, in particular to a device for balancing a pressure regulating valve, in particular a pilot regulator. Pilot valves may be used in fluid control systems to regulate the supply of control fluid to control valves or pressure regulators. Typically small in size and easily operable, pilot valves are able to control the operation of much larger valves, the actuation of which would otherwise require much greater force. A pilot valve may include a pressure sensing component (e.g. a piston or a diaphragm) and a poppet assembly which is moved between an open position, in which fluid may flow through the pilot valve, and a closed position, in which the flow of fluid is restricted. Pilot valves may be used to enable a number of different control strategies. This may include the control of the pressure downstream of a control valve or pressure regulator (commonly referred to as pressure reducing) and the control of the pressure upstream of a control valve (commonly referred to as pressure sustaining). In a conventional pressure reducing valve, the pressure downstream of the valve acts on the pressure sensing component of the pilot valve against the force of a loading spring. The force exerted by the loading spring on the pressure sensing component may be adjusted by compressing or decompressing the loading spring. This determines the set target pressure, or setpoint, of fluid flowing out of the pilot valve. If the downstream pressure exceeds the setpoint, the force from the downstream pressure acting on the pressure sensing component will be greater than the opposing force of the loading spring such that the pressure sensing component and poppet will be biased to restrict flow through the pilot valve. The reduction in flow through the pilot valve results in a change of the control pressure transmitted to the control valve or pressure regulator to which the pilot valve is fluidly connected. In turn, this causes the control valve or pressure regulator to close, which acts to reduce the flow through the control valve or pressure regulator and subsequently reduce the downstream pressure. Conversely, if the downstream pressure is less than the setpoint, the force of the loading spring on the pressure sensing component will be greater than the opposing force from the downstream pressure acting on the pressure sensing component, such that the pressure sensing component and poppet will be biased to increase the flow through the pilot valve. The increase in flow of the control fluid through the pilot valve results in a change of the control pressure transmitted to the control valve or pressure regulator. In turn, this causes the control valve or pressure regulator to open, which acts to increase the flow through the control or pressure regulator and subsequently increase the downstream pressure. The setpoint may be adjusted by compressing or decompressing the loading spring using an actuator, thereby respectively increasing or decreasing the biasing force acting on the pressure sensing component and poppet arrangement In order for the outlet pressure of a pressure reducing valve to be insensitive to the upstream pressure, the upstream pressure should not impart any force on the pressure sensing component of the pilot valve. Similarly, the downstream pressure should not impart any force on the pressure sensing component of a pressure sustaining pilot valve. However, this is difficult to achieve in practice owing to the mechanical link between the poppet arrangement that throttles the flow through the pilot valve, from the upstream side of the pressure regulating valve to the downstream side, and the pressure sensing component. Conventional arrangements for offsetting the imbalanced forces imparted on the pressure sensing component are unsuitable for many applications as they typically reduce the accuracy and stability of the pressure regulating valve, and are often poorly scalable for use with the actuators of small devices. The present invention seeks to provide an improved device for balancing a pressure regulating valve. Thus, when viewed from a first aspect, the present invention provides a system comprising a pressure regulating valve and a balancer device attached to the pressure regulating valve, wherein: the pressure regulating valve comprises: a housing, defining a conduit for receiving a fluid pressure, wherein the conduit comprises a valve aperture; and a valve member movably mounted on the housing so as to open and close the valve aperture, wherein a first surface of the valve member is arranged to be acted on by the fluid pressure in a first direction relative to the valve aperture; and the balancer device comprises: a casing defining: a volume; and a port for establishing fluid communication between the volume and the first surface of the valve member of the pressure regulating valve such that the fluid pressure is introduced into the volume; a balancer member reciprocally mounted on the casing, wherein the balancer member is arranged to be acted on by the fluid pressure in the volume such that the fluid pressure displaces the balancer member relative to the casing; a first repelling arrangement, for exerting a first repelling force between the balancer member and the valve member of the pressure regulating valve, so as to bias the valve member in the first direction relative to the valve aperture; a second repelling arrangement, for exerting a second repelling force between the casing and the balancer member so as to oppose the displacement of the balancer member by the fluid pressure; wherein the first repelling force and the second repelling force are proportional to the displacement of the balancer member by the fluid pressure. When viewed from a second aspect, the present invention provides a device for attaching to and balancing a pressure regulating valve, the device comprising: a casing defining: a volume; and a port for introducing a fluid pressure into the volume and for establishing fluid communication between the volume and the pressure regulating valve when the device is attached to the pressure regulating valve; a balancer member reciprocally mounted on the casing, wherein the balancer member is arranged to be acted on by the fluid pressure in the volume such that the fluid pressure displaces the balancer member relative to the casing; a first repelling arrangement, for exerting a first repelling force between the balancer member and the pressure regulating valve when the device is attached to the pressure regulating valve; a second repelling arrangement, for exerting a second repelling force between the casing and the balancer member so as to oppose the displacement of the balancer member by the fluid pressure; wherein the first repelling force and the second repelling force are proportional to the displacement of the balancer member by the fluid pressure. Thus, the present invention provides a balancer device that may be used to balance a valve member of a pressure regulating valve that is exposed to a fluid pressure that acts on the valve member in a first direction. The balancer device may be used to reduce the effect of the fluid pressure on the valve member. The first repelling arrangement is provided for exerting a first repelling force on the valve member of a pressure regulating valve, in the direction in which the fluid pressure acts on the valve member (i.e. the first direction). The first repelling force is proportional to the displacement of the balancer member within the volume of the balancer device, and the balancer member is arranged, when the balancer device is attached to the pressure reducing valve, to be displaced by the same fluid pressure that also acts on the valve member. As the displacement of the balancer member depends on this fluid pressure, and as the first repelling force is proportional to the displacement of the balancer member by the fluid pressure, the first repelling force varies with fluctuations in the fluid pressure. Thus, when the fluid pressure acting on the valve member of the pressure regulating valve in the first direction is relatively high, the first repelling force acting to bias the valve member in the first direction is relatively low. Conversely, when the fluid pressure acting on the valve member in the first direction is relatively low, the first repelling force acting to bias the valve member in the first direction is relatively high. This means that the net force acting in the valve member in the first direction (i.e. the sum of the fluid pressure force and the first repelling force) may remain substantially constant. It will be appreciated therefore that the device of the present invention may allow a pressure regulating valve to be balanced. The balancer member is preferably distinct from the valve member of the pressure regulating valve. Preferably the balancer member is not fixedly attached to the valve member of the pressure regulating valve. Preferably the balancer member is movably mounted relative to the valve member. This may help to avoid the frictional forces resulting from the movement of the balancer member restricting the movement of the valve member off the pressure regulating valve, which can lead to instability in the valve. Thus, the balancer member may be mechanically decoupled from the valve member. In particular, small movements in the valve member that may be required for accurate pressure regulation may be decoupled. In some embodiments, the balancer device is replaceably attachable to the pressure regulating valve. This may allow the balancer device to be attached to different pressure regulating valves to allow the valves to be balanced. Preferably, the balancer device is threadedly attachable to the pressure regulating valve. The balancer device may be arranged to be screwed into an aperture defined in the housing of the pressure regulating valve. In some embodiments, the balancer device comprises a threaded boss. Preferably the housing of the pressure regulating valve defines a threaded aperture. Preferably the threaded aperture corresponds to the threaded boss. Preferably the threaded boss is arranged to cooperate with the threaded aperture when the balancer device is attached to the pressure regulating valve. In some embodiments, the port extends through the threaded boss. Preferably the balancer device is arranged such that, when the balancer device is attached to the pressure regulating valve, the balancer member and the valve member are coaxial. The first repelling arrangement (e.g. the first biasing member) may be coaxial with the balancer member and / or the valve member. The balancer member may comprise a diaphragm. The balancer member may comprise a bellows. Preferably the balancer member comprises a piston. This may allow the balancer member to move with a greater stroke than that which may be achieved using a diaphragm or a bellows, which may improve the precision of the balancer device. The balancer member may be mounted on the casing in any suitable way such that the balancer member is arranged to be acted on by the fluid pressure in the volume such that the fluid pressure displaces the balancer member relative to the casing. Preferably the balancer member is mounted in the volume defined by the casing of the balancer device. The balancer member may divide the volume into two subvolumes. Preferably the first repelling arrangement is arranged within the first subvolume. Preferably the second repelling arrangement is arranged within the second sub-volume. In some embodiments, the first sub-volume and the second subvolume are arranged to receive the fluid pressure. The first direction, in which the valve member is acted on by the fluid pressure, is preferably towards the valve aperture of the pressure regulating valve. The balancer member is preferably arranged to be displaced by the fluid pressure in a second direction. The second direction is preferably opposite to the first direction (e.g. away from the valve aperture of the pressure regulating valve). The fluid pressure may be the pressure of fluid upstream of the pressure regulating valve. The fluid pressure may be the pressure of fluid downstream of the pressure regulating valve. In some embodiments, the first repelling arrangement comprises a (first) biasing member. The second repelling arrangement preferably comprises a (second) biasing member. The first and / or the second biasing member preferably comprises a (respective) spring. The first and / or the second spring are preferably compression springs. Preferably the stiffness of the second spring is greater than the stiffness of the first spring. The stiffness of the second spring is preferably between 1.2 and 5 times greater than the stiffness of the first spring, e.g. between 1.75 and 4 times greater, e.g. between 2 and 3 times greater. The first biasing member is preferably arranged between the balancer member and the valve member. The second biasing member is preferably arranged between the casing and the balancer member. The first biasing member and / or the second biasing member is preferably arranged to act directly on the balancer member (i.e. rather than via an intermediate component). The first biasing member is preferably arranged to act directly on the valve member (i.e. rather than via an intermediate component). The second biasing member is preferably arranged to act directly on the casing (i.e. rather than via an intermediate component). Preferably the first biasing member and the second biasing member are arranged concentrically. The first biasing member and the second biasing member are preferably nested. Preferably the first biasing member (e.g. the first spring) is arranged concentrically within (i.e. nested within) the second biasing member (e.g. the second spring). Preferably the diameter of the second biasing member is greater than the diameter of the first biasing member. Preferably the second biasing member is arranged to contact the balancer member radially outwards of the first biasing member. Nesting the first biasing member and the second biasing member may reduce the total axial space taken up by the first and second biasing members, without requiring the length of the biasing members themselves to be reduced. Indeed, for a given total axial space, longer biasing members may be used, which may help to increase the mechanical decoupling between the balancer member and the valve member. In some embodiments, the first repelling arrangement is arranged to exert the first repelling force on a first surface of the balancer member. The second repelling arrangement is preferably arranged to exert the second repelling force on the second surface of the balancer member. The second surface of the balancer member is preferably opposite the first surface of the balancer member. Preferably the first surface of the balancer member (e.g. together with the casing) at least partially defines the first sub-volume. In some embodiments, the valve member of the pressure regulating valve comprises a second surface. The second surface may be opposite the first surface of the valve member. The second surface of the balancer member (e.g. together with the casing) may at least partially define the second sub-volume. In some embodiments, the balancer member comprises a protruding portion extending through the second sub-volume. The balancer member may comprise a flange. The balancer member may be mounted on the casing by the flange. Preferably, the second repelling arrangement (e.g. the second biasing member) extends between the flange and the casing. The casing of the balancer device may define an aperture through which a distal end of the protruding portion extends. Preferably the aperture is arranged coaxially with the port for introducing the fluid pressure into the volume and / or with the balancer member. Preferably the second surface of the balancer member is arranged at the distal end of the protruding portion. The second repelling arrangement (e.g. the second biasing member) may surround the protruding portion. Preferably the balancer device comprises a seal arranged between the protruding portion and the casing defining the aperture. Preferably the seal is arranged to seal the second sub-volume (e.g. from atmosphere). In some embodiments, the balancer member comprises a recess. The recess may be defined by the first surface of the balancer member. Preferably the balancer member comprises a central recess. The recess is preferably cylindrical. Preferably the first repelling arrangement (e.g. the first biasing member) is arranged (at least partially) within the first repelling arrangement. This may help to keep the first repelling arrangement in line with the balancer member and / or the valve member of the pressure regulating valve. The recess may be arranged coaxially with the protruding portion of the balancer member. In some embodiments, the second surface of the valve member is arranged to be acted on by a regulated pressure. The regulated pressure is preferably the pressure that the pressure regulating valve is configured to regulate. The regulated pressure may be the pressure of fluid upstream or downstream of the pressure regulating valve. In some embodiments, the fluid pressure (i.e. the pressure that acts on the first surface of the valve member) is the pressure of fluid upstream of the pressure regulating valve and the regulated pressure (e.g the pressure that acts on the second surface of the valve member) is the pressure of fluid downstream of the pressure regulating valve. In some embodiments, the fluid pressure is the pressure of fluid downstream of the pressure regulating valve and the regulated pressure is the pressure of fluid downstream of the pressure regulating valve. The force on the valve member of the pressure regulating valve that is intended to be balanced by the balancer device may be caused by the pressure differential between the fluid pressure and the regulated pressure. This pressure differential may be generated across the valve aperture. Preferably the first surface of the balancer member is arranged to be exposed to the fluid pressure. The second surface of the balancer member may be arranged to be exposed to the regulated pressure. The balancer device may comprise a regulated-pressure feed for establishing fluid communication between the second surface of the balancer member and the regulated pressure. However, the Applicant has appreciated that the regulated pressure is substantially constant during use of a pressure regulating valve, meaning that the pressure differential across the valve member of the pressure regulating valve may be defined with reference to the fluid pressure (to be balanced) and atmospheric pressure (which, like the regulated pressure, can also be assumed to be substantially constant). Thus, preferably the second surface of the balancer member is arranged to be exposed to atmospheric pressure. Such an arrangement may remove the need for a regulated-pressure feed, which can simplify the manufacture of the balancer device substantially. In some embodiments, the casing of the balancer device comprises an aperture for establishing fluid communication with the atmosphere and the (e.g. second surface of the) balancer member. The aperture preferably extends to the volume from an external surface of the casing. The aperture may be the aperture through which the protruding portion of the balancer member extends, as described above. In some embodiments, the area of the first surface of the balancer member that is exposed to the fluid pressure is greater than the area of the valve aperture. This means that the force provided by the fluid pressure on the balancer member is greater than the force provided by the fluid pressure on the valve member, which may help to improve the precision of the balancer device. The pressure differential across the valve aperture may result in a net force being applied to the valve member. The magnitude of the force may be equal to the differential pressure multiplied by the area of the valve aperture. The balancer device is preferably arranged to offset the force that is applied to the valve member by the differential pressure across the valve aperture. In some embodiments, the balancer device comprises a bearing strip arranged between the balancer member and the casing. This may reduce friction between the balancer member and the casing, which may help to improve the response of the balancer device. The bearing strip may seal the first sub-volume from the second sub-volume. Preferably, the bearing strip does not prevent fluid communication between the first sub-volume and the second sub-volume. Preferably the pressure regulating valve comprises a pilot valve. The conduit of the pressure regulating valve may comprise an inlet port. The inlet port may be arranged in fluid communication with (e.g. an upstream side or a downstream side of) a control valve for controlling a flow of fluid through a conduit. The inlet port may be connected to the control valve via an orifice. The orifice may be arranged so as to provide a pressure differential across the orifice when fluid is flowing through the orifice. The fluid flow path preferably further comprises an outlet port. The outlet port may be arranged to be in fluid communication with (e.g. a downstream side or an upstream side) of a control valve. Preferably the inlet port is fluidly connected to the outlet port via the valve aperture. The valve aperture is preferably defined by the housing of the pressure regulating valve. The housing may define a valve seat for the valve member. The valve seat preferably defines the valve aperture. In some embodiments, the pressure regulating valve comprises a sensing component. The sensing component is preferably configured to be exposed to the regulated pressure. In some embodiments, the sensing component comprises a diaphragm. The sensing component may comprise a piston. The valve member of the pressure regulating valve is preferably arranged to be moved by the sensing component. In some embodiments, the sensing component is arranged to move the valve member so as to open the valve aperture. The pressure regulating valve may comprise an intermediary component (e.g. a pin or a shaft) arranged between the sensing component and the valve member. The intermediary component may be fixedly attached to the valve member (e.g. and not fixedly attached to the sensing component). The intermediary component may extend from the sensing component to the valve member through the valve aperture. Preferably the pressure regulating valve comprises a biasing arrangement. The biasing arrangement is preferably arranged to exert a biasing force on the sensing component. The biasing arrangement is preferably arranged to set a set-point for the pressure regulating valve (e.g. the pilot valve). The biasing arrangement preferably comprises a spring. In some embodiments, the biasing arrangement comprises a fluid (e.g. gas) loading arrangement. The biasing force may be transmitted to the valve member (e.g. via the intermediary component). Preferably the pressure regulating valve comprises an actuator for adjusting the set-point of the pressure regulating valve. The actuator may comprise a screw. The biasing arrangement (e.g. the spring) may be arranged between the actuator and the sensing component. Thus, adjustment of the set-point of the pressure regulating valve may comprise moving the actuator towards or away from the sensing component (e.g. so as to compress or decompress the spring or a compressed fluid). Certain preferred embodiments for the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic of a control rail configuration for a pilot operated control valve; Figure 2 shows a cross-sectional view of the pilot valve of Figure 1, including a balancer device in accordance with an embodiment of the invention. Figure 3 shows a configuration of the balancer device of Figure 2 when the pressure input to the pilot valve is low; Figure 4 shows a configuration of the balancer device of Figure 2 when the pressure input to the pilot valve is high; Figure 5 shows a graph of the forces applied to the control valve of Figure 1 by the pilot inlet pressure and the balancer device of Figure 2; and Figures 6a and 6b show graphs of the downstream pressure against flow rate for the control valve of Figure 1 for different values of the pilot inlet pressure. Figure 1 shows a schematic of a control rail configuration 2 for a pilot operated control valve 4. The control rail 2 comprises a pilot valve 6 (i.e. a pressure regulating valve) and a fixed orifice 8. The fixed orifice 8 is arranged to receive fluid at a first pressure p} from an upstream side 10 of the control valve 4. The pilot valve 6 is arranged to receive fluid from an output of the fixed orifice 8 at a second pressure p2, referred to herein as the pilot inlet pressure p2. The output of the fixed orifice 8 is also provided to a control volume (not shown) of the control valve 4 to actuate the valve member (not shown) of the control valve 4. The output of the pilot valve 6 is connected to a downstream side 12 of the control valve 4 at a downstream pressure p3, referred to herein as the downstream pressure p3. Figure 2 shows a cross-sectional view of the pilot valve 6 of Figure 1. The other components of the control rail 2 of Figure 1 are shown schematically. The pilot valve 6 comprises a housing 7 defining an inlet port 14, connected to the upstream side 10 of the control valve 4 via the fixed orifice 8, and an outlet port 16 connected to the downstream side 12 of the control valve 4. The inlet port 14 is fluidly connected to the outlet port 16 via an aperture 30 defined in a valve seat 32 of the pilot valve 6. The pilot valve 6 also comprises a balancer device 34 in accordance with an embodiment of the present invention, as described in more detail below. The pilot valve 6 further comprises an actuator 18 that includes an adjustment bolt 20 arranged at the end of a loading spring 22 such that, when the bolt 20 is rotated, the loading spring 22 is compressed or decompressed. The compression of the loading spring 22 exerts a biasing force on a pilot diaphragm 24 and poppet 26 of the pilot valve 6 which may be adjusted in order to determine the desired setpoint. The biasing force is transferred from the diaphragm 24 to the poppet 26 via a pin 28 that is fixedly connected to the poppet 26. The pin 28 extends perpendicularly through the aperture 30 in the valve seat 32 of the pilot valve 6. The poppet 26 is arranged to engage with the valve seat 32, thereby substantially preventing flow of fluid through the aperture 30 from the inlet port 14 to the outlet port 16, in a closed configuration of the pilot valve 6. In an open configuration of the pilot valve 6, the poppet 26 is moved out of engagement with the valve seat 32 such that flow through the aperture 30 from the inlet port 14 to the outlet port 16 is permitted. The poppet 26 is moved between the open configuration and the closed configuration according to the difference between the force of the loading spring 22 acting on the upper side of the diaphragm 24 and the opposing force of the downstream pressure p3 acting on the underside of the diaphragm 24. When the force provided by the downstream pressure p3 is less than the force of the loading spring 22, the diaphragm 24 and the connected pin 28 and poppet 26 move downwards so that the poppet 26 is moved away from the seat 32, into the open configuration. When the force provided by the downstream pressure p3 is greater than the force of the loading spring 22, the diaphragm 24, pin 28, and poppet 26 are moved upwards so that the poppet 26 is moved towards the seat, towards the closed configuration. When the pilot valve 6 is in the closed configuration, fluid cannot flow from the upstream side 10 of the control valve 4 to the downstream side 12 of the control valve 4 via the fixed orifice 8 and the pilot valve 6. As there is no flow of fluid through the fixed orifice 8, the pressure p2 input to the pilot valve 6 and to the control volume 4a of the control valve 4 equalises to the pressure pt upstream of the control valve 4, i.e. p} = p2. This means that there is no fluid pressure differential across the valve member 4b of the control valve 4. The control valve 4 comprises a biasing member (not shown) that acts on the valve member 4b of the control valve 4 to close the control valve 4. As there is no fluid pressure differential to oppose the force of the biasing member, the control valve 4 closes. When the pilot valve 6 is in the open configuration, fluid is permitted to flow through the fixed orifice 8 and the pilot valve 6 from the upstream side 10 of the control valve 4 to the downstream side 12 of the control valve 4. As a result, there is a pressure drop across the fixed orifice 8, i.e. p2 <Pi- Thus, there is a fluid pressure differential across the valve member 4b of the control valve 4 that acts to open the control valve 4, against the force of the biasing member. Figures 3 and 4 show cross-sectional views of the balancer device 34 of the pilot valve 6 shown in Figure 2. Figure 3 shows a configuration of the balancer 34 when the pressure input to the pilot valve 6 is low, whereas Figure 4 shows a configuration of the balancer 34 when the pressure input to the pilot valve 6 is high. The balancer device 34 comprises a threaded boss 44 mounted within a correspondingly threaded aperture 46 defined in the housing of the pilot valve 6. The aperture 46 extends from an exterior of the housing 7 into the inlet port 14 of the pilot valve 6, adjacent the poppet 26. The balancer 34 comprises a casing 36 defining a cylindrical inner volume 37 in which a pilot balancer piston 38 is reciprocally movable. The inner volume 37 comprises a first end 37a, proximal to the poppet 26 of the pilot valve 6, and a second end 37b, distal to the poppet 26. The inner volume 37 is in fluid communication with the inlet port 14 of the pilot valve 6 via a bore 36a extending through the boss 44 from the first end 37a of the inner volume 37. The second end 37b of the inner volume comprises an aperture 41 that is open to the atmosphere. The pilot balancer piston 38 comprises a first end 39 and a second end 43, wherein the first end 39 is adjacent the first end 37a of the inner volume 37 and the second end 43 is opposite the first end 39. The second end 43 extends through the aperture 41 and is exposed to atmospheric pressure. A seal 45 is arranged within the aperture 41, between the second end 43 of the pilot balancer piston 38 and the casing 36 defining the aperture. The pilot balancer piston 38 comprises a circumferentially extending flange 38a at the first end 39 of the pilot balancer piston 38. The flange 38a comprises a bearing strip 38c arranged between the flange 38a and the inner surface of the casing 36 defining the inner volume 37, which allows the pilot balancer piston 38 to move within the inner volume 37 more easily. The bearing strip 38c is loose and, as such, does not form a seal between the flange 38a and the inner surface of the casing 36. Therefore, the first end 37a and the second end 37b of the inner volume 37 are in fluid communication with each other. Additionally, a leak path 37c is defined through the pilot balancer piston 38, which facilitates the fluid communication between the first end 37a and the second end 37b of the inner volume 37. A pilot balancer spring 40 is arranged between the flange 38a and the second end 37b of the inner volume 37 and acts to bias the pilot balancer piston 38 towards the first end 37a of the inner volume 37. The pilot balancer piston 38 further comprises a central cylindrical recess 38b, extending into the pilot balancer piston 38 from the first end 39 of the pilot balancer piston 38. A pilot actuator spring 42 is arranged between the poppet 26 of the pilot valve 6 and the pilot balancer piston 38. The pilot actuator spring 42 extends into the central recess 38b of the pilot balancer piston 38 such that the pilot balancer piston 38, the pilot actuator spring 42 and the pilot balancer spring 40 are concentrically arranged within the casing 36 of the balancer 34. Thus, the pilot actuator spring 42 is nested within the pilot balancer spring 40, which allows the total axial space occupied by the pilot actuator spring 42 and the pilot balancer spring 40 to be reduced. The provision of a recess 38b that receives the pilot actuator spring 42 allows a longer pilot actuator spring 42 to be used, which beneficially increases the mechanical decoupling between the pilot balancer piston 38 and the poppet 26 assembly that is described in more detail below. Operation of the pilot balancer piston 38 will now be described, with reference to Figures 2, 3 and 4. As discussed above, the downstream pressure p3 (i.e. the pressure downstream of the control valve 4) is exposed to the underside of the diaphragm 24, against the force of the loading spring 22. If the downstream pressure p3 changes with respect to the setpoint applied by the loading spring 22, the pilot valve 6 responds to the change by moving the poppet 26 towards or away from the seat 32 in the manner described above. However, fluid at the pilot inlet pressure p2 is provided via the inlet port 14 of the pilot valve 6 and applies an additional force to the underside of the diaphragm 24 (i.e., in addition to the force exerted on the underside of the diaphragm 24 by the downstream pressure p3. For optimal accuracy, the diaphragm 24 of the pilot valve 6 should not be sensitive to changes in the pilot inlet pressure p2. The pilot balancer 34 of the present invention is provided to offset the unwanted additional force exerted on the diaphragm 24 by the pilot inlet pressure p2 via the poppet 26. In the embodiment shown in Figures 2-4, the net force FT acting on the poppet 26 in the direction of the seat 32 is given by: FT = Fpa+FP2, (1) where FPA is the force exerted on the poppet 26 by the pilot actuator spring 42 and FP2 is the force exerted on the poppet 26 by the pilot inlet pressure p2. The pilot actuator spring 42 acts on the poppet 26 to bias the poppet 26 towards the seat 32. The force FPA that the pilot actuator spring 42 exerts on the poppet 26 depends on the compression of the pilot actuator spring 42 between the poppet 26 and the pilot balancer piston 38. The pilot balancer 34 is arranged such that the pilot inlet pressure p2 also acts on the pilot balancer piston 38 to move the pilot balancer piston 38 away from the valve seat 32 and the poppet 26. The force exerted on the pilot balancer piston 38 by the pilot inlet pressure p2 is opposed by the force of the pilot balancer spring 40. In a steady state, the force FPB exerted by the pilot balancer spring 40 on the pilot balancer piston 38 is equal to the sum of: the force FP2 exerted on the pilot balancer piston 38 by the pilot inlet pressure p2 and the force FPA exerted on the pilot balancer piston 38 by the pilot actuator spring 42: Fpb = FP2 + Fpa (2) When the pilot inlet pressure p2 reduces, the forces acting on the pilot balancer piston 38 become unbalanced and the pilot balancer spring 40 force FPB overcomes the combined forces FP2 + FPA provided by the pilot inlet pressure p2 and the pilot actuator spring 42. This causes the pilot balancer piston 38 to move towards the poppet 26 and the seat 32, as shown in Figure 3, thereby compressing the pilot actuator spring 42 and, consequently, increasing the force FPA exerted by the pilot actuator spring 42 until the forces acting on the pilot balancer piston 38 are balanced once again. Conversely, when the pilot inlet pressure p2 increases such that the forces acting on the pilot balancer piston 38 become unbalanced, the combined forces FP2 + FPA of provided by the pilot inlet pressure p2 and the pilot actuator spring 42 overcome the force FPB of the pilot balancer spring 40 such that the pilot balancer piston 38 is moved away from the poppet 26 and the seat 32, as shown in Figure 4. This causes the pilot actuator spring 42 to expand, thereby reducing the force FPA exerted by the pilot actuator spring 42 until the forces acting on the pilot balancer piston 38 are balanced once again. Figure 5 shows a graph of the net force FT applied to the poppet 26, as well as the component forces provided by the pilot inlet pressure p2 and the pilot actuator spring 42, as the pilot inlet pressure p2 increases. As can be seen, when the pilot inlet pressure p2 is low, the force exerted by the pilot actuator spring 42 on the poppet 26 is high. This corresponds to the arrangement of the pilot balancer 34 shown in figure 3. As the pilot inlet pressure p2 increases, the force FPA exerted by the pilot actuator spring 42 on the poppet 26 decreases so that the net force FT remains constant. The force FPA exerted by the pilot actuator spring 42 is at a minimum when the pilot inlet pressure p2 causes the pilot actuator spring 42 to be fully extended, as shown in Figure 4. As the pilot balancer piston 38 is not directly attached to the poppet 26, pin 28 and diaphragm 24 arrangement of the pilot valve 6, the frictional forces resulting from the movement of the pilot balancer piston 38 do not restrict the movement of the poppet 26, pin 28 and diaphragm 24. In conventional devices, these frictional forces can lead to instability in the pilot valve 6, and in particular can cause hysteresis in downstream pressure p3. Instead, in the pilot balancer 34 of the present invention, the frictional forces are decoupled from the actuator via the pilot actuator spring 42. Thus, the pilot balancer 34 compensates for fluctuations in the pilot inlet pressure p2 without causing instability. The pressure differential across the pilot balancer piston 38 is provided by the pilot inlet pressure p2, acting on the first end 39 of the pilot balancer piston 38, and atmospheric pressure, acting on the second end 43 of the pilot balancer piston 38 via the aperture 41. Although, in some embodiments, the second end 43 of the pilot balancer piston 38 may be exposed to the downstream pressure p3 (e.g. via a pressure feed extending from the second end 43 of the pilot balancer piston 38 to the downstream side of the control valve 4), the Applicant has identified that this is often not necessary. In practice, as the pressure p3 downstream of the control valve 4 is essentially constant, atmospheric pressure (which is also substantially constant) can be used instead to provide a differential across the pilot balancer piston 38. This removes the need for an additional pressure feed to be provided from the downstream side of the control valve 4 to the second end 43 of the pilot balancer piston 43, which can substantially reduce manufacturing complexity and cost. Figure 6a shows a graph of the downstream pressure p3, i.e. the pressure downstream of the control valve 4, against flow rate for different values of the pilot inlet pressure p2 when the pilot balancer 34 shown in Figures 2-4 is not attached to the pilot valve 6. Figure 6b shows a graph of the downstream pressure p3 against flow rate for different values of the pilot inlet pressure p2 when the pilot balancer 34 shown in Figures 2-4 is attached to the pilot valve 6. As can be seen in Figures 6a and 6b, without the pilot balancer 34 present, the setpoint of the pilot valve 6 changes with changing inlet pressure. By contrast, with the pilot balancer 34 attached to the pilot valve 6, the setpoint of the pilot valve 6 is substantially independent of the pilot inlet pressure p2, particularly at low flow rates. Furthermore, without the pilot balancer 34, the downstream pressure control characteristic flattens out as the flow rate increases and will eventually invert. This can cause stability issues as it increases the gain of the control system.

Claims

1. A system comprising a pressure regulating valve and a balancer device attached to the pressure regulating valve, wherein:the pressure regulating valve comprises:a housing, defining a conduit for receiving a fluid pressure, wherein the conduit comprises a valve aperture; anda valve member movably mounted on the housing so as to open and close the valve aperture, wherein a first surface of the valve member is arranged to be acted on by the fluid pressure in a first direction relative to the valve aperture;andthe balancer device comprises:a casing defining:a volume; anda port for establishing fluid communication between the volume and the first surface of the valve member of the pressure regulating valve such that the fluid pressure is introduced into the volume;a balancer member reciprocally mounted on the casing, wherein the balancer member is arranged to be acted on by the fluid pressure in the volume such that the fluid pressure displaces the balancer member relative to the casing;a first repelling arrangement, for exerting a first repelling force between the balancer member and the valve member of the pressure regulating valve, so as to bias the valve member in the first direction relative to the valve aperture;a second repelling arrangement, for exerting a second repelling force between the casing and the balancer member so as to oppose the displacement of the balancer member by the fluid pressure;wherein the first repelling force and the second repelling force are proportional to the displacement of the balancer member by the fluid pressure.

2. The system as claimed in claim 1, wherein the balancer device is replaceably attachable to the pressure regulating valve.

3. The system as claimed in claim 2, wherein the balancer device is threadedly attachable to the pressure regulating device.

4. The system as claimed in claim 3,wherein the balancer device comprises a threaded boss for cooperating with a corresponding threaded aperture defined by the housing of the pressure regulating valve; andwherein the port extends through the threaded boss.

5. The system as claimed in any one of the preceding claims, wherein the balancer member comprises a piston.

6. The system as claimed in any one of the preceding claims, wherein the balancer member is mounted within the volume.

7. The system as claimed in any one of the preceding claims,wherein the balancer member is arranged to be displaced by the fluid pressure in a second direction; andwherein the second direction is opposite to the first direction.

8. The system as claimed in any one of the preceding claims, wherein the first repelling arrangement comprises a first spring.

9. The system as claimed in claim 8, wherein the first spring is arranged between the balancer member and the valve member.

10. The system as claimed in any one of the preceding claims, wherein the second repelling arrangement comprises a second spring.

11. The system as claimed in claim 10, wherein the second spring is arranged between the casing and the balancer member.

12. The system as claimed in any one of the preceding claims, wherein the first repelling arrangement comprises a first spring and the second repelling arrangement comprises a second spring.

13. The system as claimed in claim 12, wherein the stiffness of the second spring is between 1.2 and 5 times greater than the stiffness of the first spring.

14. The system as claimed in claim 12 or 13, wherein the first spring is nested within the second spring.

15. The system as claimed in any one of the preceding claims, wherein the first repelling arrangement is arranged to exert the first repelling force on a first surface of the balancer member; andwherein the second repelling arrangement is arranged to exert the second repelling force on a second surface of the balancer member, opposite the first surface.

16. The system as claimed in claim 15, wherein the first surface of the balancer member is arranged to be exposed to the fluid pressure; andwherein the second surface of the balancer member is arranged to be exposed to atmospheric pressure.

17. The system as claimed in any one of the preceding claims, wherein:a first surface of the balancer member is arranged to be exposed to the fluid pressure; andthe area of the first surface of the balancer member is greater than the area of the valve aperture.

18. The system as claimed in any one of the preceding claims, wherein the pressure regulating valve comprises a pilot valve.

19. A device for attaching to and balancing a pressure regulating valve, the device comprising:a casing defining:a volume; anda port for introducing a fluid pressure into the volume and for establishing fluid communication between the volume and the pressureregulating valve when the device is attached to the pressure regulating valve;a balancer member reciprocally mounted on the casing, wherein the balancer member is arranged to be acted on by the fluid pressure in the volume5 such that the fluid pressure displaces the balancer member relative to the casing;a first repelling arrangement, for exerting a first repelling force between the balancer member and the pressure regulating valve when the device is attached to the pressure regulating valve;a second repelling arrangement, for exerting a second repelling force10 between the casing and the balancer member so as to oppose the displacement of the balancer member by the fluid pressure;wherein the first repelling force and the second repelling force are proportional to the displacement of the balancer member by the fluid pressure.

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