GB2464283A

GB2464283A - Fluid pressure regulator

Info

Publication number: GB2464283A
Application number: GB0818400A
Authority: GB
Inventors: James Fishwick
Original assignee: BiFold Fluidpower Ltd
Current assignee: BiFold Fluidpower Ltd
Priority date: 2008-10-08
Filing date: 2008-10-08
Publication date: 2010-04-14
Anticipated expiration: 2028-10-08
Also published as: GB0818400D0; GB2464283B

Abstract

The flow of fluid to a fluid actuator is controlled using a fluid pressure regulator 20 to regulate the pressure of fluid supplied to a downstream actuator. Intermediate the regulator and the actuator there is at least one fluid flow restriction downstream of the pressure regulator and upstream of the actuator. The fluid pressure regulator 20 has an inlet 37 and an outlet 38, a flow path between which is interrupted by a valve 45 movable between an open and a closed position in order to vary the flow from the inlet to the outlet. A control chamber 42 of the regulator has a diaphragm assembly 43 responsive to pressure changes across it to regulate the position of the valve. A control force is applied to a first side of the diaphragm assembly and a fluid pressure is applied to a second side of the diaphragm assembly. The fluid pressure is derived from a location in the system downstream of the at least one flow restriction and upstream of the actuator and is fed through a fluid feed port in the housing wall of the regulator.

Description

FLUID FLOW CONTROL SYSTEM AND METHOD

The present invention relates to a fluid flow control system and associated method for controlling the flow of pneumatic or hydraulic fluid to an actuator of the kind used, for example, in a valve positioner.

In many applications it is desirable to automate the actuation of a pipeline valve via a remote control system. This is particularly necessary in harsh environments such as, for example, a petrochemical pipeline located on land or off-shore. The operation of, for example, a ball valve in such a pipeline is often effected by a fluid (e.g. pneumatic) actuator.

The actuator is typically a single or double-acting piston and cylinder assembly to which the pneumatic or hydraulic fluid is routed under the control of a positioner that determines the flow required to achieve the desired actuator position. The fluid is supplied from a source via other fluid circuit components such as, for example, a volume booster and a pressure regulator. The operation of the volume booster may be controlled by a positioner, and is designed to ensure that there is a sufficient sustained volume of fluid (e.g. air) available to the actuator such that there is a rapid response time. The regulator is generally disposed upstream of most of the circuit components and is designed to regulate the pressure of the fluid supplied to the rest of the circuit. It often includes a filter (in which case it is often referred to as a "filter regulator") that serves to remove contaminants etc. Volume boosters are generally controlled by a pneumatic pilot signal received from a control part of the valve positioner. The signal is generated in response to a command signal directing the positioner to move the valve to a desired position. The command signal may be an open-loop signal or a closed-loop feedback electrical control signal that takes into account the position of the actuator. In an alternative arrangement the booster may be controlled directly by an electrical signal that operates a solenoid valve in the booster.

A conventional regulator, or filter regulator, is disposed downstream of the source of pneumatic or hydraulic fluid. It has an inlet to which the source of unregulated fluid pressure is directly, or indirectly connected, and an outlet for supplying fluid at regulated pressure to the rest of the circuit downstream. The flow between the inlet and outlet is selectively throttled by a valve member (e.g. a plug, poppet, or needle valve or the like) whose position is controlled by movement of a flexible diaphragm.

Control means is provided on the regulator for setting the desired regulated system pressure such that air is supplied to the system at the desired pressure regardless of variations in the flow and fluid pressure upstream. A typical manifestation of the control means for setting the system pressure is an adjustable screw that acts on a compression spring that, in turn, applies a control force to a first side of the diaphragm. An internal sensing feed port provides fluid communication between the outlet and a second side diaphragm such that the outlet (i.e. downstream) fluid pressure is applied to the second side thus providing a reaction force in opposition to the control force. Any imbalance between the two forces causes movement of the diaphragm and therefore the valve member in order to regulate the flow. For example, if the downstream pressure (at the outlet) is greater than the desired regulated pressure, the force applied on the second side of the piston will exceed that of the control force thus moving the diaphragm and the valve towards the closed position so as to throttle the flow from inlet to outlet. Similarly, if the downstream (outlet) pressure drops below the desired regulated pressure then the force acting on the first side of the piston exceeds that of the reaction force so that the valve is maintained in an open position and allows flow from inlet to outlet. On this basis, as the downstream flow demand varies, the regulator automatically regulates flow.

One of the problems with the above described arrangement is that, when the pressure at the outlet approaches the desired pressure, there is a tendency for the valve to creep towards the closed position so as progressively to restrict the flow from inlet to outlet. This restricts the flow rate through the regulator and, as a result, the stroke time of the actuator (i.e. the time it takes to open or close) is prolonged unnecessarily. It has been realised that the pressure drop in the fluid across the circuit components downstream of the regulator means that the pressure at the outlet of the filter regulator is generally higher than that at the actuator so the creeping movement of the valve starts to occur sooner than required. In effect, the "back-pressure" generated by the downstream components is transmitted through the sensing feed port and builds on the second side of the piston such that the valve creeps towards the closed position as described.

It is an object of the present invention to obviate or mitigate the above, and other, disadvantages. It is also an object of the present invention to provide for an improved, or alternative, fluid flow control system and/or associated method.

According to a first aspect of the present invention there is provided a fluid flow control system comprising a fluid pressure regulator for regulating the pressure of fluid supplied to a downstream fluid actuator and at least one fluid flow restricting component downstream of the pressure regulator and for location upstream of the actuator, the fluid pressure regulator comprising an inlet and an outlet interrupted by a valve movable between an open and a closed position in order to vary the flow from the inlet to the outlet, a valve operator for regulating the position of the valve and comprising a diaphragm assembly responsive to pressure changes, wherein the regulator comprises a control force applicator for applying a control force to a first side of the diaphragm assembly and a fluid feed for applying fluid pressure to a second side of the diaphragm assembly, and a feed pressure line connected between the fluid feed and a location in the system downstream of the at least one flow-restricting component and upstream of the actuator.

The provision of an external fluid feed pressure line to the fluid feed allows the fluid pressure supplied to the second side of the diaphragm assembly to be taken as a reference from a position downstream of one or more fluid restricting components downstream of the regulator outlet. This means that the "back pressure" applied at the outlet by the, or each, fluid restricting component is effectively removed from the reference pressure at the fluid feed. The pressure applied to the fluid feed is lower than that at the outlet of the regulator and means that the valve stays open longer such that the operation of the actuator is faster than in

comparison to prior art fluid control systems.

The feed pressure line may be connected to any suitable location in the system upstream of the actuator provided that location is provided with fluid pressure from upstream and that is not interruptible by a shut-off valve or exhaust valve or the like. The location should not therefore be downstream of a shut-off or exhaust valve or the like that effectively interrupts the flow to the actuator.

The regulator may comprise a housing and the fluid feed may be a port in a wall of a housing that provides fluid communication between the second side of the diaphragm assembly and the feed pressure sensing line.

The diaphragm assembly may comprise a flexible diaphragm that may be supported by a piston member, the control force applicator acting on the piston member to impart the force to the first side of the diaphragm. A pair of diaphragms may be used: one each side of the piston member. The control force applicator may be housed in a control chamber in which the diaphragm assembly is received. The flexible diaphragm separating the chamber into a first portion in which the control force applicator is housed and a second portion with which the fluid feed is in fluid communication. The fluid feed may open into the second portion of the control chamber.

The control force applicator may be a mechanical device comprising a resiliently compressible member such as, for example, a spring acting between an adjustment element and the piston member. The adjustment element may be a screw adjustable to compress the spring. Alternatively it may be a pilot pressure port for applying fluid pressure to the first side of the diaphragm assembly.

The piston member may have a bore in which part of valve member is receivable in a sealing relationship. The bore may be defined in a spigot formed on the piston member, the spigot being designed to secure an internal edge of the diaphragm. The spigot may have an annular undercut for this purpose. A peripheral edge of the diaphragm may be secured between parts of a housing of the regulator.

The valve member may comprise a valve stem, one end of which is received in said bore in the piston member. The other end of which may define a valve element that is designed to seal against a valve seat defined in the regulator when the valve is in the closed position. The valve seat may be defined by an internal part of a housing of the regulator. The valve element may take any suitable form including, for example, a plug. The valve element may be movable towards and away from the valve seat by virtue of reciprocal movement of the valve stem so that the flow between the inlet and outlet is varied. A biasing member may act on the valve member to bias it towards the closed position in a direction countering the force applied by the control force applicator.

The regulator may further comprise an integral filter which may be upstream of the valve and downstream of the inlet.

The fluid flow restricting component may be disposed upstream of an exhaust valve, the feed pressure line being connected between the fluid feed and a location in the system downstream of the at least one flow-restricting component and upstream of the exhaust valve.

The fluid flow restricting component may be disposed upstream of a shut-off valve, the feed pressure line being cormected between the fluid feed and a location in the system downstream of the at least one flow-restricting component and upstream of the shut-off valve.

The actuator may be a linear actuator such as, for example, a single-acting or double-acting piston and cylinder arrangement. The actuator is preferably hydraulic or pneumatic.

The control system may further comprise a volume booster, which may include an exhaust valve, in which case the location to which the fluid feed is connected is upstream of the volume booster. The operation of the volume booster may be controllable by a valve positioner that receives control signals for directing the movement of the actuator.

According to a second aspect of the present invention there is provided an actuator system for controlling the position of a piston within a cylinder comprising a source of pressurised fluid and a fluid flow control system as defined above.

According to a third aspect of the present invention there is provided a fluid pressure regulator fluid pressure regulator for regulating the pressure of fluid supplied to a downstream actuator, the regulator comprising a housing defining an inlet and an outlet interrupted by a valve movable between an open and a closed position in order to vary the flow from the inlet to the outlet, a valve operator for regulating the position of the valve and comprising a diaphragm assembly responsive to pressure changes, wherein the regulator comprises a control force applicator for applying a control force to a first side of the diaphragm assembly received in the housing and a fluid feed for applying fluid pressure to a second side of the diaphragm assembly, the fluid feed penetrating a wall of the housing and being connectable to an external feed pressure line.

According to a fourth aspect of the present invention there is provided a method for controlling the fluid flow to a fluid actuator comprising using a fluid pressure regulator to regulate the pressure of fluid supplied to a downstream actuator via at least one fluid flow restriction downstream of the pressure regulator and upstream of the actuator, the fluid pressure regulator comprising an inlet and an outlet interrupted by a valve movable between an open and a closed position in order to vary the flow from the inlet to the outlet, and using a diaphragm assembly responsive to pressure changes to regulate the position of the valve by applying a control force to a first side of the diaphragm assembly and applying a fluid pressure to a second side of the diaphragm assembly, the fluid pressure being derived from a location in the system downstream of the at least one flow restriction and upstream of the actuator.

Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a first embodiment of a fluid control system in accordance with the present invention; Figure 2 is a longitudinal cross section through a filter regulator of the system of figure 1; Figure 3 is a schematic block diagram of a second embodiment of a fluid control system in accordance with the present invention; Referring now to figure 1 of the drawings, a valve 10 is operable between open and closed positions by a pneumatically-operated actuator 12, such as, in this case, a piston 13 disposed in a cylinder 14. In the embodiment shown the piston 13 is a single-acting type with a spring 15 biasing it in a direction against a force applied by compressed air introduced into the cylinder 14 to the left of the piston. It will be appreciated that in an alternative embodiment of the present invention a double-acting piston cylinder arrangement may be used with compressed air being selectively introduced or exhausted from the cylinder 14 on each side of the piston 13. The valve 10 is, in this exemplary embodiment, a ball valve that is rotationally disposed within a valve body and has an actuation stem 16 that is rotatable by the actuator 12 via a suitable mechanical link 17.

Compressed air for operating the actuator 12 is delivered from a source 18 (such as a compressor) via a pneumatic control circuit 19. Immediately downstream of the source 18 the circuit 19 has a filter-regulator 20, that serves to regulate the flow and to filter out contaminants, and two spring check valves 21. From there the compressed air is supplied along a main flow line 22 to the actuator 12 through a flow control device 23 of any kind that affords some flow restriction and a two position, three port, manually-operated valve 24.

The valve 24 is normally disposed, as depicted in figure 1, so that flow is exhausted out of the cylinder 14 of actuator 12. When the valve position is changed manual operation air flows through the main flow line 22 to the actuator 12. It is to be understood that the valve 24 may be take any appropriate form and not necessarily the one depicted. For example, it may be any kind of three or five port valve and may be pilot-operated. Alternatively it may take the form of a volume booster operating under the control of directional control valves and a positioner as illustrated in figure 3 below.

Instead of having an internal sensing feed port as in the prior art (and as depicted in figure 1 by dotted line 30 for comparison purposes) the filter regulator has a side port 31 in the regulator housing that is connected to the main flow line 22 at a location between the valve 24 and the actuator 12 by an external sensing feed line 32. The side port 31 is in fluid communication with the second side of the diaphragm as before and the operation is generally the same as described in the prior art. With this configuration of filter regulator 20 the reference pressure applied to the sensing feed port 31 is taken from a point further downstream in the circuit so that the pressure drop across the check valves 21 and the flow control device 23 is taken into account. The result is that the valve in the filter regulator 20 stays open longer and the stroke time for the actuator 12, and therefore the opening time of the valve, is significantly reduced. The arrangement allows the use of smaller circuit components at reduced cost. For example, it is possible to use Y4" flow diameter components in place of V2" components.

The filter regulator 20 is shown in more detail in figure 2, and comprises a three-part body with a central body section 36 defining an inlet port 37 and an opposite outlet port 38, an upper body section 39 defining a control chamber 42 for a control spring 40 and a lower body section 41 in the form of a cap for the filter and a drain. The control chamber 42 extends slightly into the central body section 36 and receives a diaphragm assembly 43.

The central and lower body sections 36, 41 define between them a main fluid flow path that extends between the inlet and outlet ports 37, 38 and which is interrupted by an intervening valve chamber 44 defined in the lower body section 41. The regulator body houses a valve stem assembly 45 that is disposed for reciprocal movement through a central vertical passage 46 in the central body section 36. The assembly comprises an elongate stem 47 having a first end 47a that terminates in a plug 49 in the valve chamber 44 and a second end 47b that extends into the diaphragm chamber 42. The plug 49 defines an annular shoulder that supports a sealing ring 51 and which is designed to seal against a valve seat 52 defined by an annular surface on the central body section 36. In operation, axial movement of the valve stem assembly 45 in the vertical passage 46 determines the size of the clearance between the sealing ring 51 and the valve seat 52 and therefore the flow between the inlet and outlet ports 37, 38.

The lower body section 41 is in the form of a substantially cylindrical housing with a bottom wall 55 penetrated by a drain port 56 and housing a cup-shaped filter 57 that is disposed around a lower part of the stem assembly 45 so to divide the interior of the lower body section into an annular outer chamber 58 in direct fluid communication with the inlet port and the valve chamber 44. The filter 57 is constructed from a rigid porous material such as, for example, a sintered stainless steel and serves to remove particulate or liquid/moisture contaminants from the operating fluid. A filter support ring 59 fixed to the central body section 36 supports the filter 57 at its upper edge.

The filter 57 supports a cylindrical guide 60 in which the plug 49 of the valve stem assembly 45 is slidably guided in reciprocal movement. The valve stem assembly 45 is biased upwardly so that the plug shoulder 50 is, by default, sealed against the valve seat 52 by means of a compression spring 61 disposed between the end of the plug 49 and a bottom of the guide 60.

The bottom of the guide 60 has an externally threaded spigot 62 that projects on the opposite side to the spring 61 and extends through an aperture in the bottom wall 55 of the filter 57 where it is secured by a nut 63 so as to retain the filter 57 and an associated baffle 64 in position.

In operation, fluid flows into the filter regulator 20 via the inlet port 37, into the lower body section 41 around the outer chamber 59, through the filter 57 and to the valve chamber 44. Any moisture or oil etc. that is filtered out from the fluid collects on the baffle 64 and flows downwardly along it past an annular clearance between the baffle edge and the inside surface of the body section 41 to the drain port 56. If the valve stem assembly 45 has been moved to an open position such that the plug 49 is lifted off the valve seat 52 against the biasing force of the compression spring 61 then the fluid is able to flow through the valve chamber 44 along part of the vertical passage 46 to the outlet port 38.

The position of the valve stem assembly 45, and therefore the relative position of the plug 49 in the valve chamber 44, is controlled by the diaphragm assembly 43 and its relative position in the control chamber 42. The diaphragm assembly 43 comprises a generally cylindrical support member 70 that operates in the manner of a piston. It has a first major surface 70a with an upstanding guide tube 71 for receipt of a first end of the pressure control spring 40 and an opposite second major surface 70b with a depending spigot 72 having an annular undercut 73 in which an internal edge of a flexible diaphragm 74 is secured. The outer edge of the diaphragm 74 is secured at the interface of the central and upper body sections 36, 39. The second major surface 70b effectively supports the diaphragm 74. The spigot 72 is penetrated by central stepped bore 75 in which the second end 47b of the valve stem 47 is received. An 0-ring seal 76 is provided between the two to prevent leakage of fluid. It will be appreciated that the spring 61 at the opposite end of the valve stem assembly biases the end 47b into a sealing relationship with the seal 76.

Inside the control chamber 42, a second end of the pressure control spring 40 supports an end plate 77 on which a pressure adjusting screw 78 acts. The screw 78 is engaged in a threaded bore of the upper body section 39 and extends into the control chamber 42 where it abuts the end plate 77. It is manually adjustable such that turning the screw in one direction increases the force applied to the end plate 77 and therefore the spring 40 and the first surface 70a of the diaphragm support 70. This force acts in opposition to a force applied to the second surface 70b of the support 70 via the diaphragm 74 by fluid pressure in the control chamber below the diaphragm 74.

The diaphragm 74 effectively divides the control chamber 42 into upper and lower chamber portions. If the force applied by the fluid acting on the underside of the diaphragm 74 (i.e. in the lower chamber portion) and the second major surface 70b of the diaphragm support 70 exceeds the force applied by the control spring 40 then the support 70 moves upwardly so as to compress the spring 40. The valve stem assembly 45 simultaneously moves upwardly, under the biasing influence of spring 61, towards the position shown in figure 2 in which the plug 49 is seated on the valve seat 52 and interrupts the flow between the inlet and outlet ports 37, 38. If, on the other hand, the force applied by the screw 78 and spring 40 to the first major surface 70a of the support 70 exceeds that applied by the fluid pressure on the underside of the diaphragm membrane 74 (i.e. the pressure in the lower portion of the control chamber 42) and the second major surface 70b then the support moves downwardly against the second end 47b of the valve stem assembly 45 and serves to lift the plug from the valve seat 52 so as to enable fluid to flow from the inlet to the outlet port through the valve chamber 44. It will be appreciated from this that the imbalance of forces applied to the diaphragm assembly determines the length of travel of the valve stem 45 and therefore the size of the opening between the valve plug 49 and the valve seat 42. This in turn determines the degree of throttling applied to the flow of fluid between the inlet and outlet ports 37, 39.

In this maimer it will be understood that the position of the pressure adjusting screw 76 effectively determines the pressure of the fluid supplied at the outlet port 38 and therefore to the downstream part of the circuit.

The pressure of the fluid acting in the lower portion of the control chamber 42 acts on the lower side of the diaphragm 74 and the second surface 70b of the diaphragm support 70 and is provided through a feed port 79 defined in the wall 80 of the central body section 36. It comprises a tapped port 81 to which the sensing feed line 32 can be connected and a right angle passage 82 that opens into the control chamber 42. This is in contrast to prior art regulators in which the sensing port extends between the outlet port and the control chamber.

By using an external sensing feed line 32 (figure 1) the reference pressure for the control chamber 42 can be taken from a suitable point in the downstream circuit rather than at the outlet port 38 of the filter regulator 20.

The embodiment illustrated in figure 3 is a more sophisticated control system in which the compressed air is supplied along the main flow line 22 to a volume booster 85 that is operable by pneumatic control signals from a positioner 86 to ensure that there is a sufficient sustained volumetric flow rate in the main flow line 22 to pressurise the cylinder 14 so that the stroke speed of the actuator 12 is sufficiently high in a first direction. It is can also be operable to permit rapid exhaust of the air from cylinder 14 in the opposite direction so that the stroke speed in the reverse direction is also sufficiently rapid. Control signals are delivered to the booster 85 along a pilot line 87 from the positioner 86.

The booster 85 has an operating fluid inlet and outlet 87, 88 that supply the compressed air along the main flow line 22 to the actuator 12, an exhaust line 89 and a pilot port 90 for receipt of the pilot fluid (e.g. air) from the positioner 86. The inlet, outlet and exhaust port are configured to allow air to flow at relatively high volumetric rates in comparison to the pilot port.

The pneumatic control signal is generated by the positioner 86 in order to move the actuator 12 to a desired position to operate the valve 10. A position sensor 91 associated with the actuator 12 may sense the position of the piston 13 within the cylinder 14 and generate an electrical signal representative of that position, which signal is fed back to the positioner 86.

In this manner the positioner 86 may operate with closed-loop feedback control to determine the required pressure of the pneumatic control signal. The positioner 86 may be microprocessor based and may incorporate transducers that convert electrical signals (e.g. current) to pneumatic pressure for this purpose.

A pair of directional control valves 92, 93 in the pilot line are selectively operable to allow the pneumatic control signals to flow from the positioner 86 to the volume booster 85.

A first of the valves is a two-way solenoid-operated valve 92 that is normally closed and is selectively opened under the control of the positioner 86. The second valve 93, immediately downstream of the first, is a two-way, manually-operated, emergency shut down valve that is normally open.

The above-described embodiments can each be used in conjunction with the stacking directional control valves described in our co-pending International Patent Application No. PCT/GB2007/004555 which may be used to control the supply of the pilot pressure signal to the pilot inlet.

It will be appreciated that the device can be provided in a range of sizes to suit all applications and that all components including seals can be designed to operate at extreme temperatures such as those found in arctic conditions.

It is will be appreciated by one of ordinary skill in the art that the invention has been described by way of example only, and that the invention itself is defined by the claims.

Numerous modifications and variations may be made to the exemplary design described above without departing from the scope of the invention as defined in the claims. For example, although the device is described above in relation to controlling an actuator in a valve positioner it can be used in any application where the pressure of a large flow volume is to be regulated. Furthermore, it is to be appreciated that the filter need not be an integral part of the regulator but may be provided in the circuit as a separate component.

The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as "preferable", "preferably", "preferred" or "more preferred" in the description suggest that a feature so described may be desirable, it may nevertheless not be necessary and embodiments lacking such a feature may be contemplated as within the scope of the invention as defined in the appended claims. In relation to the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used to preface a feature there is no intention to limit the claim to only one such feature unless specifically stated to the contrary in the claim. When the language "at least a portion" andlor "a portion" is used the item can include a portion andlor the entire item unless specifically stated to the contrary.

Claims

CLAIMS1. A fluid flow control system comprising a fluid pressure regulator for regulating the pressure of fluid supplied to a downstream actuator and at least one fluid flow restricting component downstream of the pressure regulator and for location upstream of the actuator, the fluid pressure regulator comprising an inlet and an outlet and a flow path therebetween interruptible by a valve movable between an open and a closed position in order to vary the flow along the flow path from the inlet to the outlet, a valve operator for regulating the position of the valve and comprising a diaphragm assembly responsive to pressure changes, wherein the regulator comprises a control force applicator for applying a control force to a first side of the diaphragm assembly and a fluid feed for applying fluid pressure to a second side of the diaphragm assembly, and a feed pressure sensing line connected between the fluid feed and a location in the system downstream of the at least one flow-restricting component and upstream of the actuator.
2. A fluid flow control system according to claim 1, wherein the pressure regulator comprises a housing and the fluid feed is a port in a wall of the housing.
3. A fluid flow control system according to claim 2, wherein the fluid feed port provides fluid communication between the second side of the diaphragm assembly and the feed pressure sensing line.
4. A fluid flow control system according to any preceding claim, wherein the diaphragm assembly comprises at least one flexible diaphragm that is supported by a piston member.
5. A fluid flow control system according to claim 4, wherein the control force applicator acts on the piston member to impart the force to the first side of the diaphragm.
6. A fluid flow control system according to claim 4 or 5, wherein the control force applicator is housed in a control chamber in which the diaphragm assembly is received.
7. A fluid flow control system according to claim 6, wherein the flexible diaphragm separates the control chamber into a first portion in which the control force applicator is housed and a second portion with which the fluid feed is in fluid communication.
8. A fluid flow control system according to any preceding claim, wherein the control force actuator is a resiliently compressible member acting between an adjustment element the diaphragm assembly
9. A fluid flow control system according to claim 4, 5 or 6, wherein the piston member has a bore in which part of valve member is receivable in a sealing relationship.
10. A fluid flow control system according to claim 9, wherein the valve member has a valve stem, one end of which is received in the bore in the piston member.
11. A fluid flow control system according to claim 19, wherein the other end of the valve stem defines a valve element that is designed to seal against a valve seat defined in the regulator when the valve is in the closed position.
12. A fluid flow control system according to any preceding claim, wherein the regulator further comprises an integral filter.
13. A fluid flow control system according to any preceding claim, wherein the fluid flow restricting component is disposed upstream of an exhaust valve or a shut-off valve, the feed pressure line being connected between the fluid feed and a location in the system downstream of the at least one flow-restricting component and upstream of the exhaust valve or shut-off valve.
14. An actuator system for controlling the position of a piston within a cylinder comprising a source of pressurised fluid and a fluid flow control system according to any preceding claim.
15. A fluid pressure regulator for regulating the pressure of fluid supplied to a downstream actuator, the regulator comprising a housing defining an inlet and an outlet interrupted by a valve movable between an open and a closed position in order to vary the flow from the inlet to the outlet, a valve operator for regulating the position of the valve and comprising a diaphragm assembly responsive to pressure changes, wherein the regulator comprises a control force applicator for applying a control force to a first side of the diaphragm assembly received in the housing and a fluid feed for applying fluid pressure to a second side of the diaphragm assembly, the fluid feed penetrating a wall of the housing and being connectable to an external feed pressure sensing line.
16. A method for controlling the fluid flow to a fluid actuator comprising using a fluid pressure regulator to regulate the pressure of fluid supplied to a downstream actuator via at least one fluid flow restriction downstream of the pressure regulator and upstream of the actuator, the fluid pressure regulator comprising an inlet and an outlet interrupted by a valve movable between an open and a closed position in order to vary the flow from the inlet to the outlet, and using a diaphragm assembly responsive to pressure changes to regulate the position of the valve by applying a control force to a first side of the diaphragm assembly and applying a fluid pressure to a second side of the diaphragm assembly, the fluid pressure being derived from a location in the system downstream of the at least one flow restriction and upstream of the actuator.