GB2539826A

GB2539826A - An auxiliary Non-return valve

Info

Publication number: GB2539826A
Application number: GB1614563.3A
Authority: GB
Inventors: Delaney Kenneth; Lorgen Alexander
Original assignee: Score (europe) Ltd
Current assignee: Score (europe) Ltd
Priority date: 2016-08-26
Filing date: 2016-08-26
Publication date: 2016-12-28
Anticipated expiration: 2036-08-26
Also published as: GB201614563D0; GB2539826B

Abstract

An auxiliary non-return valve 16 for mounting to a further non-return valve (8, Fig. 1) of a main valve (2, Fig. 1) comprises a body 58 having a through-bore 60 defining a longitudinal axis, a valve seat 66 and a fitting 70 for mounting to the further non-return valve 8. A valve stem 30 is mounted in the bore 60 and has a valve member 30a to seat on the valve seat 66 and a tip 30b, and is moveable axially to project into the further non-return valve 8 and actuate the further non-return valve. A biasing member 64 urges the valve member 30a against the valve seat 66. A seal 68 seals the auxiliary non-return valve 16 against the further non-return valve 8 and retains pressure within the bore. A stinger assembly 11 is provided to inject lubricant or sealant into the main valve 2 via the auxiliary non-return valve 16.

Description

Intellectual Property Office Application No. GII1614563.3 RTM Date:18 October 2016 The following terms are registered trade marks and should be read as such wherever they occur in this document: Duplex, Inconel Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo An auxiliary non-return valve

FIELD OF THE INVENTION

The present invention relates to an auxiliary non-return valve, to a stinger incorporating such an auxiliary non-return valve, and to a method of injecting a fluid using such a stinger.

BACKGROUND OF THE INVENTION

Valves used in pipelines and industries such as oil and gas production and chemical processing, are commonly actuated infrequently and may be subjected to extreme operating conditions, but are nevertheless expected to be reliable when used to shutoff flow in a section of pipe.

Ball valves are an example of the types of valves commonly used in the applications described above. Such valves have seal surfaces to the external face of the ball. In certain instances the seal surfaces may become worn, allowing fluid to leak under pressure into the body cavity of the valve. In other instances, in particular where a valve has not been actuated for an extended period, the valves may become seized in a closed or open position.

To overcome leakage, at least temporarily, a sealant may be injected into the valve via non-return valves mounted to the body of the ball valve for this purpose. To free a seized valve, a lubricant may be injected into the valve via the non-return valve.

Prior to injecting sealant or lubricant into the valve it is typically also necessary to relieve any pressure build-up in the body cavity of the valve, as well as to perform diagnostic tests on the valve, e.g. to ensure that these remedial measures may succeed.

However, during these maintenance operations, the fittings used to relieve pressure, commonly known as 'stingers are also often used as a means of injecting the sealant or lubricant despite not being designed or certified to contain pressure (often of the order of 690 Bar, 10,000 PSI). Further, the non-return valves fitted to the valves for this purpose are often themselves found to be defective during maintenance, due to their infrequent use and harsh operating environment.

The present invention seeks to overcome or at least mitigate the problems of the

prior art

STATEMENTS OF INVENTION

According to a first aspect of the invention there is provided an auxiliary non-return valve for mounting to a further non-return valve of a main valve; the auxiliary non-return valve comprising; a body having a through-bore defining a longitudinal axis, a valve seat and a fitting for mounting to the further non-return valve; a valve stem mounted in the bore and having a valve member arranged to seat on the valve seat and a tip and moveable axially to project into the further non-return valve and actuate the further non-return valve; a biasing member arranged to urge the valve member against the valve seat; and a seal arranged to seal the auxiliary non-return valve against the further non-return valve and retain pressure within the bore.

Advantageously, this arrangement permits the auxiliary non-return valve to attach to another non-return valve and transmit inputs received from a stinger, but optionally be left in place if the integrity of the non-return valve is compromised.

The fitting may comprise a Tee slot coupling.

Advantageously, this provides a convenient and secure way of coupling to a standard non-return valve buttonhead fitting.

The Tee slot coupling may comprise a cowl threadably connected to the body.

Advantageously, this enables tightening of the coupling to achieve a sealed connection.

The body may comprise an arrangement for applying torque thereto, such as first and second opposing flats on an external face thereof for the releasable attachment of a spanner, wrench or the like.

Advantageously, this enables tightening to be achieved using standard tools.

The body may comprise a butmnhead flange at an end thereof opposing the fitting.

Advantageously, this enables other devices to be coupled to the auxiliary non-return valve.

The body may further comprise an externally threaded portion.

According to a second aspect of the invention there is provided a stinger assembly comprising an auxiliary non-return valve according to the first aspect and a main stinger mounted thereto.

Advantageously, this provides a complete assembly for relieving pressure in a main valve and subsequently maintaining the seal integrity of the main valve if the non-return valve is defective.

The main stinger may be releasably mounted to an auxiliary non-return valve.

Advantageously, this enables just the auxiliary non-return valve to be left in place if the non-return valve is defective.

The stinger may be threadably mounted to an auxiliary non-return valve.

The main stinger may comprise a stinger body having a chamber therein, a stinger stem arranged within the chamber such that a tip thereof is arranged to selectively depress the valve stem.

Advantageously, this enables the main stinger to release a non-return valve via the auxiliary non-return valve The chamber may be a through-bore and the stinger stern may be threadably mounted within the through-bore.

Advantageously, this permits a simple rotational action to depress the valve stem.

The stinger stem may further comprises an input, such as a handle, arranged such that rotation of the input results in axial movement of the stinger stem.

The stinger assembly may further comprise a seal arrangement sealing between the stinger stem and stinger body.

Advantageously, this enables assembly to retain pressure such that it may be utilised for purposes beyond pressure relief.

The stinger assembly may comprise a seal arrangement that is configured to seal between the stinger and the auxiliary non-return valve so as to substantially isolate the chamber and the through-bore.

Advantageously, this further enables assembly to retain pressure such that it may be utilised for purposes beyond pressure relief.

The stinger may further comprise a monitoring port in fluid flow communication with the chamber via a valve.

Advantageously, this provides a way in which stinger can be used for additional purposes beyond pressure relief.

The valve may be a needle valve.

The stinger assembly may further comprising an injector port for the fitment of an injector.

Advantageously, this provides a further way in which stinger can be used for additional purposes beyond pressure relief.

According to a third aspect of the invention there is provided a stinger assembly comprising: a stinger body having a through-bore defining a longitudinal axis and a chamber therein; a fitting for mounting to a non-return valve of a main valve; a stinger stem arranged within the through-bore and having a tip moveable axially to project into the non-return valve and actuate the non-return valve to selectively depress a valve stem of the non-return valve; a seal arrangement sealing between the stinger stem and stinger body to substantially prevent the passage of fluid from the chamber therebetween; a seal arranged to seal the stinger assembly against the non-return valve and retain pressure within the chamber; and a monitoring port in fluid flow communication with the chamber via a valve.

Advantageously, this provides a stinger assembly that can be used for additional maintenance purposes beyond pressure relief.

The stinger assembly may further comprising an auxiliary non-return valve according to the first aspect of the invention Advantageously, this provides a stinger assembly that additionally includes an auxiliary non-return valve that can optionally be left in place to retain pressure after the stinger has been removed if the integrity of the non-return valve is compromised.

According to a fourth aspect of the invention there is provided a method of performing a maintenance operation on a main valve comprising the steps of: a) attaching a stinger according to claim 18 to a non-return valve of the main valve; b) actuating the stinger stem to depress the valve stem of the non-return valve; and c) performing the maintenance operation.

Advantageously, this provides method of maintaining a main valve that is safe and efficient to perform.

The stinger assembly may further comprise an auxiliary non-return valve according to the first aspect of the invention.

In step b), the stinger stem may depresses the valve stem of the auxiliary non-return valve and the valve stem of the auxiliary non-return valve may depresses the valve stem of the non-return valve.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: FIGURE 1 is a simplified cross-sectional view of an exemplary ball valve; FIGURE 2 is an isometric view of a stinger assembly according to an embodiment of the present invention; FIGURE 3 is a side view of the stinger assembly of Figure 2; and FIGURE 4 is a cross-sectional view of the stinger assembly of Figure 2 on the plane J-J of Figure 3.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 illustrates in simplified schematic form a ball valve 2 of the type that may be found in fluid pipelines, e.g. natural gas pipelines, chemical plants and other process industries. The ball valve 2 indicated generally at 2 has a housing 3 that accommodates a rotatable ball 4 having a bore 5 extending through the centre thereof.

The external face of the ball is seated against opposing seal assemblies 6 such that in the position showed in Figure 1 the fluid cannot flow through the valve, but when the ball is turned through 90 degrees fluid flows through the bore 5. In both conditions, the contact between the seal assemblies 6 and the outer surface of the ball 4 is intended to prevent the leakage of fluid into a body cavity 7 of the valve 2, and from there potentially into the surrounding atmosphere. This is clearly undesirable from safety and environmental perspectives. Although a hall valve 2 is illustrated in this example, sealing and lubrication problems may be encounter with other types of valves when used in similar circumstances and for the purposes of this application the valve which is to be maintained is referred to generically as the "main valve".

If the torque required to rotate the ball 4 between its open and closed position is too great due to the valve sticking or seizing at the interface between the seal assembly 6 and the ball it may be necessary to introduce a lubricant via a port on the housing 3 of the valve, which is fitted with one or more pressure retaining non-return valves 8. The same non-return valve 8 may also be used to inject a sealant into the body of the valve 2 in the event that leakage is occurring. This approach to valve maintenance is desirable, since the alternative would he the isolation and disassembly of the ball valve, resulting in undesirable downtime for the system incorporating the valve 2.

In order to carry out either of the two aforementioned operations, it is first necessary to release any pressure build-up in the body cavity 7 before injecting the sealant or lubricant. The non-return valves 8 are commonly of the check valve type utilising a spring-loaded ball that is biased against an annular seat in a bore of the valve. In order to depress the ball and allow pressurised fluid to by-pass it, a device commonly referred to as a stinger is attached to the one-way valve via a buttonhead flange on the outer end of the one-way vale.

Stingers currently used for this purpose are not typically designed to retain pressure -i.e. they are only intended to depress the one-way valve and allow fluid to pass through the stinger and he collected in a suitable collecting vessel. A separate pressure retaining injector should be used, that is designed for the purpose for injecting sealant or lubricant. However, in the field it is not uncommon for stingers to also be used for injection, contrary to their operating instructions.

Additionally, it is also been found that the non-return valves 8 can themselves no longer retain pressure once the stinger is removed after being in a harsh environment for an extended period of time. As such, it is also known for the stinger to be left in place in an attempt to retain pressure, despite not designed as a pressure retaining fitting itself, in order to avoid the need for more extensive repairs to be undertaken, which is itself an unsatisfactory situation.

According to an embodiment, a stinger assembly 10 is illustrated in Figures 2, 3 and 4. The stinger assembly comprises a main stinger 11 having a body 12 manufactured from a suitable high strength and corrosion resistant material such as Duplex or Inconel steel, or 316 stainless steel. A longitudinal axis X-X extends through die centre of the body 12 and defines a main longitudinal through bore 14 extending there through. The bore is stepped to define various diameters thereof and is partially threaded as described in more detailed below. In addition, in this embodiment two ports extend from the main bore 14 at an angle thereto, in this case substantially at 90' thereto, transverse to the axis X-X.

In this embodiment, an auxiliary non-return valve 16 is releasably attached to the body 12 in co-axial alignment (herewith, and discussed in more detail below.

Considering the body 12 in more detail, in order to selectively release pressure from a non-return valve of a main valve e.g. the ball valve 2, a main pressure relief rod is threadably mounted within the bore 14 via a threaded connection 20 part way down the length of the bore. A portion of the rod 18 extends out of the body 12 at the outer, opposing end to the one-way valve connection and is provided with a suitable handle 22, e.g. the T-handle depicted, in order that a user may apply sufficient torque to the rod 18 that is converted by the threaded connection 20 into axial movement of the rod.

A suitable sealing system is also provided at the upper end of the body 12 in order to inhibit the passage of fluid past the rod 18. In this embodiment, a bonnet gland 24 of suitable known construction is mounted to the body 12 by bolts 25 such that pressure may be retained within a cavity 15 in the body 12 at least partially defined by the bore 14.

A lower end of the pressure release relief rod 18 proximate the auxiliary non-return valve 16 is provided with a relatively thin stem 26 which is intended to move along the axis X-X in response to rotation of the handle 22, and whose tip 28 is intended to actuate a valve member 30 of the auxiliary non-return valve 16, as discussed in more detail below.

A lower end of 14a of the bore 14 is of an enlarged diameter and has at least a partial internal thread 32 at its lower end and an annular sealing face 34 at its upper end. The sealing face 34 is provided with a suitable 0-ring-type seal 36, e.g. a PTFE type seal so as to retain pressure within the chamber 15.

The first port provided in the body is a monitoring port 38, which comprises an enlarged threaded end 38a for the fitment of suitable pipework to instrumentation (not shown) and/or a suitable collection vessel (not shown) for pressurised fluid that is relieved from the body cavity 7 of the ball valve 2. In this embodiment, in order to control the flow of fluid from the chamber 15 defined by the axial bore 14, a valve, such as a needle valve 40 separates an upstream section 38b of the monitoring port from the enlarged opening 38a. Again, the needle valve 40 is pressure retaining and comprises a sealing gland 42 mounted to the body 12 by fasteners such as bolts 44 and includes a threaded needle 46 that is arranged to seal against a complimentary seat 38e of the port 38. As such, the monitoring instrumentation may be isolated from the chamber 15 as desired.

In some embodiments, a second injector port 50 is also provided in the body 12 and has an injector fitting 52 threadably mounted to an enlarged end 50a of the injector port 50. The injector fitting comprises a central through-bore 54 and a buttonhead flange in order to mount a standard injector apparatus (not shown) thereto. In other embodiments, the injector port 50 may be omitted and the injection, if required, may be carried out via the monitoring port 38 either using a separate manifold or by replacing the monitoring instrumentation with, for example, the equivalent of the injector fitting 52 in the enlarged end 38a. Additionally, it will be appreciated that the injector fitting 52 may be replaced by a threaded blanking plate or any other suitable type of fitting that may engage with the thread 50a, depending upon desired usage.

The auxiliary non-return valve 16 includes a body 58, manufactured from suitable materials similar to the body 12 of the main stinger 11, having a through-bore 60 arranged to be co-axial in use with the bore 14 of the main stinger 11 and having the valve stem 30 mounted therein via a support 62. A biasing member 64 is arranged to urge the valve member 30 towards a valve seat 66 formed by a reduction in the diameter of the through-bore 60 towards the upper end of the auxiliary non-return valve 16. In this embodiment, the biasing member is a helical spring arranged concentrically with the valve stem 30 to act between the support 62 and an enlarged valve member 30a of the valve stem. The remainder of the valve stem comprises an elongate rod 30b which extends a greater distance towards the bottom of the auxiliary non-return valve than is usual in standard non-return valves. Consequently, when the valve stem 30 is fully depressed by the main pressure relief rod, the tip of the stem will project below the bottom of the body 58 of the non-return valve for reasons discussed below.

A bottom end face of the body 58 has a seal 68 provided therein so as to encircle the through-bore 60.

A lower portion of the outer face of the auxiliary non-return valve is threaded in this embodiment in order to receive a corresponding collet or cowl 70, which in conjunction with the end face of the body 58, defines a Tee-shaped slot 72 by virtue of inward facing Bps 74 on the cowl. As the cowl is progressively tightened on to the body 58 the axial spacing between the lips 74 and the end face of the body 58 reduces.

Approximately mid-way along the body 58, in a location not obscured either by the body 12 of the main stinger 11 or the cowl 70 at least a pair of flats 75 are provided on the outer face for the releasable attachment of a suitable wrench or spanner so that the auxiliary non-return valve can be turned relative to the cowl. As can be seen in Figures 2 and 3, the cowl also has a pair of flats 76 so as to hold the cowl fast with a second wrench during the tightening operation.

The upper end of the auxiliary non-return valve is also provided with a buttonhead flange of suitable standard dimensions as well as an upper end face arranged to contact and seal against the seal 36.

In operation, the stinger assembly 10 may be used as follows: The auxiliary non-return valve 16 is first connected to a non-return valve 8 of a main valve such as ball valve 2 as illustrated in Figure 1. In the described embodiment by loosening the cowl 70 to enable the lips 74 to be slid under the buttonhead of the non-return valve 8. The auxiliary non-return valve 16 is then tightened down onto the seal 68 in order to provide a substantially pressure tight connection between the non-return valve 8 and auxiliary non-return valve 16. This step may be undertaken either with the main stinger 11 not attached, or attached thereto with the main pressure relief rod retracted. If not attached, at this point the main stinger 11 may be tightened down onto the auxiliary non-return valve such that the seal 36 is seated on the upper end face of the body of the non-return valve 58 and this interface is also sealed.

At this stage, suitable instrumentation (not shown) is connected to the monitoring port, optionally together with a collecting vessel (not shown) for pressurised fluid to be bled off, and if present, an injector device is connected to the injector port 50. The integrity of the stinger assembly may then be checked by pressurising it using the connected instrumentation, prior to its use.

The handle 22 is then turned so as to lower the tip 28 of the stem 26 and depress the valve stem 30. In turn, this lowers the tip of rod 30b which contacts the valve member (not shown) of non-return valve 8 so as to open the non-return valve 8 and allow the chamber 15 to be in fluid flow communication with the body cavity of the main valve 2.

The integrity of the ball 4 to seal assembly 6 connection in the main ball valve 2 may be checked by measuring the pressure of fluid in the body cavity 7 via the instrumentation attached to the monitoring port 38. Excess pressure, indicating a leaking seal may then be released to a suitable container attached to the monitoring port 38.

If it is determined that injection of lubricant or sealant is required, the needle valve 40 is then closed to isolate the instrumentation etc. connected to the monitoring port 38. With the main pressure relief rod still in its lowered state, the lubricant or sealant is injected through the injector fitting 52 and injector port 50.

Once injection is complete, further testing may be carried out via the monitoring port 38 to determine if firstly the injection operation has been successful in restoring the operation of the main valve 2, and secondly to confirm if the non-return valve fitted to the housing 3 of the main valve 2 is itself functioning to prevent the release of fluid passing through the valve 2 in to the atmosphere. If the testing determines that the non-return valve 8 is itself defective, then the auxiliary non-return valve 16 may be left in place connected to the non-return valve 8 such that the integrity of the seal from the valve body to the atmosphere is maintained by the auxiliary non-return valve 16 and not the non-return valve 8. The main stinger 11 is however removed for re-use with a new auxiliary non-return valve on other main valves.

Thus, the auxiliary non-return valve and stinger assembly incorporated in such a non-return valve as described above may both enhance the safety of valve maintenance This is achieved by ensuring that testing and in situ maintenance via injection of sealant or lubricant is undertaken using equipment that retains the potential high pressures that may be found within the body of a valve, and to which a stinger may be subjected if sealant or lubricant is injected through it under pressure.

Further, the stinger assembly reduces the need for fittings to be interchanged in order to undertaken such maintenance of operations and the use of an auxiliary non-return valve that is adapted to be connected to the usual non-return valves fitted to a main valve enables defective non-return valves to be left in place and seal integrity to be maintained without subjecting the main valve to expensive, and potentially off-site repair with the attendant potential downtime of the pipeline or system to which the main valve is fitted.

Terms such as inner and outer, upper and lower, top and bottom as used herein are used for ease of understanding and should not be regarded as limiting. It will be further understood that where the terms "pressure retaining" or "sealed" are used, this is at the pressure for which the relevant device is designed to be used -for example 690 Bar (10,000 PSI) It will be understood that numerous changes may be made within the scope of the present invention. For example, the interfaces between the auxiliary non-return valve and a main valve to which it is to be fitted may be altered as required, as may the interface between Me main stinger and the auxiliary non-return valve. In order to guard against accidental subsequent removal of the auxiliary non-return valve from a non-return valve, a grub screw or other security arrangement may be provided to lock the cowl into place on the body of the auxiliary non-return valve.

Claims

Claims An auxiliary non-return valve for mounting to a further non-return valve of a main valve; the auxiliary non-return valve comprising; a body having a through-bore defining a longitudinal axis, a valve seat and a fitting for mounting to the further non-return valve; a valve stem mounted in the bore and having a valve member arranged to seat on the valve seat and a tip and moveable axially to project into the further non-return valve and actuate the further non-return valve; a biasing member arranged to urge the valve member against the valve scat; and a seal arranged to seal the auxiliary non-return valve against the further non-return valve and retain pressure within the bore.

2. An auxiliary non-return valve according to claim I wherein the fitting comprises a Tee slot coupling.

An auxiliary non-return valve according to claim 2 wherein the Tee slot coupling comprises a cowl threadably connected to the body.

An auxiliary non-return valve according to claim 2 or claim 3 wherein the body comprises an arrangement for applying torque thereto such as first and second opposing flats on an external face thereof for the releasable attachment of a spanner, wrench or the like.

5. An auxiliary non-return valve according to any preceding claim wherein the body further comprises a buttonhead flange at an end thereof opposing the fitting.

6. An auxiliary non-return valve according to any preceding claim wherein the body further comprises an externally threaded portion.

7. A stinger assembly comprising an auxiliary non-return valve according to any preceding claim and a main stinger mounted thereto.

8. A stinger assembly according to claim 7 wherein the main stinger is relcasably mounted thereto.

9. A stinger assembly according to claim 8 wherein the stinger is threadably mounted thereto.

10. A stinger assembly according to any one of claims 7 to 9 wherein the main stinger comprises a stinger body having a chamber therein, a stinger stern arranged within the chamber such that a tip thereof is ranged to selectively depress the valve stem.

11. A stinger assembly according to claim 10 wherein the chamber is a through-bore and the stinger stem is threadably mounted within the through-bore.

12. A stinger assembly according to claim 11 wherein the stinger stem further comprises an input, such as a handle, arranged such that rotation of the input results in axial movement of the stinger stem.

13. A stinger assembly according to any one of claims 10 to 12 further comprising a seal arrangement sealing between the stinger stem and stinger body.

14. A stinger assembly according to any one of claims 10 to 13 wherein a seal arrangement is configured to seal between the stinger and the auxiliary non-return valve so as to substantially isolate the chamber and the through-bore.

15. A stinger assembly according to claims 7 to 14 wherein the stinger further comprises a monitoring port in fluid flow communication with the chamber via a valve.

16. A stinger assembly according to claim 15 wherein the valve is a needle valve.

17. A stinger assembly according to any one of claims 7 to 16 further comprising an injector port for the fitment of an injector.

18. A stinger assembly comprising: a stinger body having a through-bore defining a longitudinal axis and a chamber therein; a fitting for mounting to a non-return valve of a main valve; a stinger stem arranged within the through-bore and having a tip moveable axially to project into the non-return valve and actuate the non-return valve to selectively depress a valve stem of the non-return valve; a seal arrangement sealing between the stinger stem and stinger body to substantially prevent the passage of fluid from the chamber therebetween; a seal arranged to seal the stinger assembly against the non-return valve and retain pressure within the chamber; and a monitoring port in fluid flow communication with the chamber via a valve.

19 A stinger assembly according to claim 18 further comprising an auxiliary non-return valve according to any one of claim 1 to 6.

20. A method of perfoi ling a maintenance operation on a main valve comprising the steps of: a) attaching a stinger according to claim 18 to a non-return valve of the main valve; b) actuating the stinger stem to depress the valve stem of the non-return valve; and c) performing the maintenance operation.

21. A method according to claim 19 wherein the stinger assembly further comprises an auxiliary non-return valve according to any one of claim 1 to 6 and in step b) the stinger stem depresses the valve stem of the auxiliary non-return valve and the valve stern of the auxiliary non-return valve depresses the valve stem of the non-return valve.Amendment to the claims have been made as follows: Claims An auxiliary non-return valve for mounting to a further non-return valve of a main valve; the auxiliary non-return valve comprising; a body having a through-bore defining a longitudinal axis, a valve seat and a fitting for mounting to the further non-return valve; a valve stem mounted in the bore and having a valve member arranged to seat on the valve seat and a tip and moveable axially to project into the further non-return valve and actuate the further non-return valve; a biasing member arranged to urge the valve member against the valve scat; and a seal arranged to seal the auxiliary non-return valve against the further non-return valve and retain pressure within the bore.2. An auxiliary non-return valve according to claim I wherein the fitting comprises a Tee slot coupling.An auxiliary non-return valve according to claim 2 wherein the Tee slot coupling comprises a cowl threadably connected to the body.An auxiliary non-return valve according to claim 2 or claim 3 wherein the body comprises an arrangement for applying torque thereto such as first and second opposing flats on an external face thereof for the releasable attachment of a spanner, wrench or the like.5. An auxiliary non-return valve according to any preceding claim wherein the body further comprises a buttonhead flange at an end thereof opposing the fitting.6. An auxiliary non-return valve according to any preceding claim wherein the body further comprises an externally threaded portion.7. A stinger assembly comprising an auxiliary non-return valve according to any preceding claim and a main stinger mounted thereto.8. A stinger assembly according to claim 7 wherein the main stinger is relcasably mounted thereto.9. A stinger assembly according to claim 8 wherein the stinger is threadably mounted thereto.10. A stinger assembly according to any one of claims 7 to 9 wherein the main stinger comprises a stinger body having a chamber therein, a stinger stern arranged within the chamber such that a tip thereof is ranged to selectively depress the valve stem.11. A stinger assembly according to claim 10 wherein the chamber is a through-bore and the stinger stem is threadably mounted within the through-bore.12. A stinger assembly according to claim 11 wherein the stinger stem further comprises an input, such as a handle, arranged such that rotation of the input results in axial movement of the stinger stem.13. A stinger assembly according to any one of claims 10 to 12 further comprising a seal arrangement sealing between the stinger stem and stinger body.14. A stinger assembly according to any one of claims 10 to 13 wherein a seal arrangement is configured to seal between the stinger and the auxiliary non-return valve so as to substantially isolate the chamber and the through-bore.15. A stinger assembly according to claims 7 to 14 wherein the stinger further comprises a monitoring port in fluid flow communication with the chamber via a valve.16. A stinger assembly according to claim 15 wherein the valve is a needle valve.17 A stinger assembly according to any one of claims 7 to 16 further comprising an injector port for the fitment of an injector.18. A stinger assembly comprising: a stinger body having a through-bore defining a longitudinal axis and a chamber therein; a fitting for mounting to a non-return valve of a main valve; a stinger stem arranged within the through-bore and having a tip moveable axially to project into the non-return valve and actuate the non-return valve to selectively depress a valve stem of the non-return valve; a seal arrangement sealing between the stinger stem and stinger body to substantially prevent the passage of fluid from the chamber therebetween, a seal arranged to seal the stinger assembly against the non-return valve (r) 15 and retain pressure within the chamber and a monitoring port in fluid flow communication with the chamber via a valve.19 A stinger assembly according to claim 18 further comprising an auxiliary non-return valve according to any one of claim 1 to 6.20. A method of performing a maintenance operation on a main valve comprising the steps of': a) attaching a stinger assembly according to claim 18 to a non-return valve of the main valve; b) actuating the stinger stem to depress the valve stem of the non-return valve, and c) performing the maintenance operation.21 A method according to claim 20 wherein the stinger assembly further comprises an auxiliary non-return valve according to any one of claim I to 6 and in step b) the stinger stem depresses the valve stem of the auxiliary non-return valve and the valve stern of the auxiliary non-return valve depresses the valve stem of the non-return valve.