GB2567772A

GB2567772A - Butterfly valve

Info

Publication number: GB2567772A
Application number: GB1902373.8A
Authority: GB
Inventors: Hayes Richard; Wakefield Richard
Original assignee: Score Europe Ltd
Current assignee: Score Europe Ltd
Priority date: 2019-02-21
Filing date: 2019-02-21
Publication date: 2019-04-24
Anticipated expiration: 2039-02-21
Also published as: AU2020201089A1; GB201902373D0; GB2567772B

Abstract

A butterfly valve comprises a valve stem 120 mounting a valve disc 116 and has first and second coaxial portions 120a, 120b at opposite sides. A valve body 112 has first and second co-axial bores 118a, 118b to receive the first and second portions 120a, 120b and permit rotation of the valve stem 120 and the disc 116 within the valve body to control fluid flow. The first portion 120a has a formation 127 to enable connection of the valve stem to an actuation mechanism. The second bore 118b is closed by a closure 122 remote from the disc 116 such that the valve stem 120 is enclosed in a pressure retaining manner. A seal arrangement is provided between the second bore 118b and the valve stem to inhibit passage of fluid to the closure 122 and a stop comprising a thrust collar 148 interacts with an abutment (164, Fig. 6) of the valve body 112 and blocks movement of the valve stem 120 in a direction away from the closure 122. Semicircular split retainer rings 140 are secured to each other and are mounted to an annular recess in the stem 120.

Description

The present teachings relate to a butterfly valve. More particularly, but not exclusively, the present teachings relate to a butterfly valve with features to inhibit the ejection of a valve stem from the valve.

BACKGROUND

Butterfly valves are used in various applications to control the flow of a fluid through pipes and other conduits. Such valves are generally simple to manufacture and reliable in operation.

The present applicant has however recognised that a particular design of butterfly valve may have certain problems relating to fluid leakage to one end ofthe valve stem allowing the valve stem to become pressurised axially, which may then allow the stem to be ejected from the valve under certain conditions. Such an ejection event is undesirable as the valve becomes non-functional, the fluid in the conduit will leak and the ejected stem may cause damage to its surroundings and/or injury to people in its path.

The present teachings seek to overcome or at least mitigate these problems.

SUMMARY

A first aspect of the present teachings provides a butterfly valve for use in fluid processing such as oil and gas production, the butterfly valve comprising:a valve stem arranged to mount a valve disc and having a first portion at one side of the valve disc and a second portion at a second side of the disc, the first and second portions being co-axially arranged;

a valve body comprising first and second co-axial bores to receive the first and second portions respectively and permit rotation of the valve stem and the disc within the valve body to control flow of a fluid through the butterfly valve;

the first portion comprising a formation to enable connection ofthe valve stem to an actuation mechanism, the second bore being closed by a closure remote from the disc such that the valve stem is enclosed in a pressure retaining manner, wherein a seal arrangement is provided between the second bore and the valve stem to inhibit passage of a fluid to the closure and a stop is arranged to interact with an abutment of the valve body and block movement of the valve stem in a direction away from the closure.

Advantageously, a butterfly valve of this configuration has a reduced risk of the valve stem being ejected from the valve body, with a consequential improvement in safety and valve durability and reliability.

Optionally, the stop comprises thrust collar mounted onto the valve stem, e.g. removably mounted on to the valve stem.

Advantageously, a thrust collar provides a convenient way of providing a stop and enabling the valve to be easily assembled.

Optionally, the stop is dimensioned so as to fit within bore.

Advantageously, this arrangement simplifies the installation of the closure.

Optionally, the abutment comprises a step projecting into the bore and of a smaller internal diameter than an external diameter of the stop.

Optionally, the butterfly valve is configured such that when the pressure therein from the fluid is axially balanced with respect to the stem, an annular face is axially spaced with respect to an opposed annular face non-rotatably mounted with respect to the valve stem, the annular faces being brought into contact if the process fluid generates a pressure urging the valve stem away from the closure.

Advantageously, by separating the surfaces in normal operation, the friction inhibiting rotation of the valve stem to open and close the valve is minimised, so less energy is required to operate the valve]

Optionally, the opposed annular faces is formed of material that visually indicates when the annular face has been brought into contact with the other annular surface and relative movement has occurred.

Advantageously, this provides a simple way for a technician to determine if the sealing at the second portion of the valve has failed and requires an overhaul.

Optionally, one of the faces is a face of a thrust washer.

Optionally, the seal arrangement comprises an O-ring seal.

Optionally, the seal arrangement comprises a lip seal, optionally an energised lip seal.

Optionally, the butterfly valve further comprises a locknut arranged to abut and retain the thrust collar.

Advantageously, using a locknut ensures that the relative positions of the stop and the abutment can be maintained.

A second aspect of the present teachings provides a butterfly valve for use in controlling the flow of a pressurised fluid in a pipe, the butterfly valve comprising:

a valve stem arranged to mount a butterfly valve disc and having a first portion at one side of the valve disc and a second portion at a second side of the valve disc, the first and second portions being co-axially arranged;

a housing comprising first and second co-axial bores to receive the first and second portions respectively, and a retainer ring arrangement comprising first and second retainer rings secured to each other and mounted to an annular recess of the valve stem, the first and second retainer rings having an outer diameter greater than a corresponding inner diameter of an adjacent aperture of the housing.

Advantageously, securing the retainer rings together reduces the wear between the rings and the annular recess, which in turn reduces the risk of the rings escaping the recess and no longer being and effective means of preventing ejection of the stem.

Optionally, the housing comprises a bridge piece releasably mounted to a valve body of the housing that comprises the first and second co-axial bores.

Advantageously, use of a separate bridge piece enables the retainer rings to be assembled in a location where they act as an effective means of inhibiting a stem ejection.

Optionally, the first and second retainer rings are housed within a protective structure.

Advantageously, by enclosing the retainer rings they can be protected from harsh external environments that may cause corrosion, and also enable more effective lubrication to be achieved.

Optionally, the protective structure abuts the housing.

Optionally, a thrust washer is interposed between the retainer rings and the housing.

Advantageously, a thrust washer reduces the friction and therefore reduces the energy required to rotate the valve between and open and a closed condition and reduces the wear on the components.

Optionally, the first and second retainer rings have a portion contacting the valve stem that is formed from a dissimilar material to the valve stem.

Advantageously, using dissimilar material reduces wear due to galling.

Optionally, the butterfly valve further comprises a lubricant in a space defined within the protective structure.

A third aspect of the present teachings provides a butterfly valve for use in fluid processing such as oil and gas production, the butterfly valve comprising:a valve stem arranged to mount a valve disc and having a first portion at one side of the valve disc and a second portion at a second side of the disc, the first and second portions being co-axially arranged;

the first portion comprising a formation to enable connection of the valve stem to an actuation mechanism, the second bore being closed by a closure remote from the disc such that the valve stem is enclosed in a pressure retaining manner, wherein a seal arrangement is provided between the second bore and the valve stem to inhibit passage of a fluid to the closure and a stop is arranged to interact with an abutment of the valve body and block movement of the valve stem in a direction away from the closure;

and a retainer ring arrangement comprising first and second retainer rings secured to each other and mounted to an annular recess of the valve stem, the first and second retainer rings having an outer diameter greater than a corresponding inner diameter of an adjacent aperture of the housing.

A fourth aspect of the present teachings provides a method of making a butterfly valve of the first aspect comprising the steps of reducing the diameter of the second portion of an existing valve stem to define a step therein and mounting a thrust collar to the reduced diameter second portion in a non-axially moveable manner.

Advantageously, this provides a cost effective way of taking an existing valve and adapting it to have increased safety and reliability.

Optionally, the method further comprises a step of mounting a sealing ring to the reduced diameter portion prior to mounting the thrust collar thereto, the sealing ring comprising at least one seal mounted thereon.

Advantageously, by enhancing the sealing the durability and reliability of the valve may be improved.

Optionally, the method further comprises a step of mounting a locknut to the reduced diameter portion after mounting the thrust collar to the reduced diameter portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with reference to the accompany drawings, in which:

FIGURE 1 is a partial cross-section view of a prior art butterfly valve;

FIGURE 2 is an enlarged view of detail A in Figure 1;

FIGURE 3 is an enlarged view of detail B in Figure 1;

FIGURE 4 is an end view of a butterfly valve of the present teachings;

FIGURE 5 is a partial cross-section through the butterfly valve of Figure 4 on the plane AA;

FIGURE 6 is a partial cross-section through the butterfly valve of Figure 5 on the plane BB; and

FIGURES 7 and 8 are enlarged views of detail C and detail D of Figure 6.

DETAILED DESCRIPTION

Figures 1, 2 and 3 show a known butterfly valve indicated generally at 10. The butterfly valve 10 comprises a valve body 12 arranged to be mounted to a conduit (not shown) via a flange 13 and have a through bore 14 that is selectively opened and closed by a valve disc 16.

The valve body 12 is provided with a pair of co-axially aligned transverse bores 18a, 18b either side of the through bore 14, acting as bearings for a valve stem 20 which rotatably mounts the valve disc 16 in the through bore and enables the disc to selectively allow the passage of fluid along the through bore.

At a non-driven portion of the stem 20 a cap 22 is bolted to the valve body 12 at the end of the transverse bore 18b.

At a driven portion of the stem a bridge piece 24 is mounted to the valve body 12 such that an end of the stem 20 protrudes from the bridge piece, and in turn has an actuator 26 mounted to thereto. The actuator 26 may be a pneumatic, electrical, hydraulic actuator, handwheel or any other suitable type. The actuator is configured to rotate the stem using a suitable linkage, typically some form of reduction gear (not shown).

It will be appreciated that fluids under pressure (e.g. of the order of 150 bar) pass through the through bore 14 and that it is desirable to inhibit the passage of such fluids along the valve stem 20, whilst allowing the free rotation of the valve stem 20 relative to the valve body 12.

With particular reference to Figure 3, the bushing and sealing arrangement at the nondriven portion of the valve stem can be seen in more detail. In series from the left hand side, it can be seen that the bushing and sealing arrangement comprises a bushing ring 28, a bushing seal 30, a bearing bush 32, five spacers 34 (with a spacer gasket 36 interposed between the fourth and fifth spacers) and the cap 22. A further gasket 38 is located between the cap 22 and the valve body 12. It has been found that the aforesaid bushing and sealing components allow fluid under pressure to build up in the space between the end of the valve stem 20 and the cap 22. In turn, this generates a force that urges the valve stem 20 in a direction towards the actuator. It can be seen from Figure 3 that the valve stem and transverse bore 18b do not have any annular abutting surfaces that block movement that may arise from this force.

The drive portion of the stem where it is supported by the transverse bore 18a has an arrangement of bushes, packers and a gland to provide an axial load, which may similarly wear over time and allow fluid to pass through. The load on the packers can be adjusted over time to prevent leakage. However if there is leakage, such fluid cannot build up at driven end face of the valve stem to generate a force in the opposing direction. In addition the valve stem 20 lacks any transverse abutting surfaces that engage with the transverse bore 18a to block axial movement of the valve stem.

In order to protect against a pressure build up resulting in an ejection of the valve stem 20 from the valve body 12 the valve 10 comprises semi-circular retainer rings 40 mounted into an annular recess 42 of the valve stem 20 at the driven portion thereof. A stepped bushing 44 in contact with the retainer rings 40 acts as a bearing mounting the valve stem 20 to the bridge piece 24.

The present applicant has recognised firstly that the retainer rings 40 are made of the same material as the valve stem 20 and that the valve stem may rotate relative to the rings, resulting in wear and/or seizure due to galling. This wear can result in the annular recess 42 becoming wedge shaped, such that axial forces caused by pressure build up tend to force the retainer rings outwardly so they no longer act as an effective measure against a stem ejection. Further, since the retainer rings 40 are mounted to the bridge piece 24 and not the valve body 12, it is possible for a maintenance technician to erroneously remove the bridge piece 24 from the valve body 12 whilst the valve is pressurised and thereby remove the one feature preventing stem ejection from occurring. This may expose the technician and others to greater danger.

With reference now to Figures 4 to 8, a butterfly valve 110 according to an embodiment of the present teachings is shown.

A butterfly valve 110 may be manufactured by adaptation of a butterfly valve 10 of the prior art by carrying out machining and/or repair operations and fitting certain new components. Alternatively the butterfly valve 110 may be assembled from entirely new components as described below.

The butterfly valve 110 comprises a valve body 112 arranged to be mounted to a fluid conduit (not shown) via flanges 113. The valve body 112 has a through bore 114 that is selectively opened and closed by a valve disc 116.

The valve body 112 is provided with a pair of co-axially aligned transverse bores 118a, 118b either side of the through bore 114. The transverse bores act as bearings for a valve stem 120 which rotatably mounts the valve disc in the through bore 114 and enables the disc to selectively allow the passage of fluid along the through bore. The valve stem 120 is provided with two opposed axially arranged keys 115 to mount the valve disc thereto. In other embodiments, splines or other drive formations may be used.

At a non-driven portion of the valve stem 120 a closure in the form of a cap 122 is bolted to the valve body 112 at the free end of the transverse bore 118b.

A first portion of the valve stem on a first side of the valve disc can be defined as a driven portion 120a of the valve stem 120. A bridge piece 124 is mounted to the valve body 112 by a plurality of threaded connecters 125 at the driven portion 120a. The driven portion 120a of the valve stem 120 protrudes from the bridge piece 124. The protruding portion is provided with a pair of opposed actuator keys 127 to permit drive to be transmitted from an actuator (not shown, but similar to the actuator 26 of the prior art) to rotate the valve stem 120 through 90° between a closed position of the valve disc where it is seated against the periphery of the through bore 114 and fluid flow in the through bore is substantially prevented, and an open position in which fluid can flow relatively unhindered.

The butterfly valve 110 is designed to control the flow of fluids under pressure (e.g. of the order of 150 bar) that pass through the through bore 114. The valve stem 120 and associated sealing and bushing at the non-driven portion is configured so as to enhance sealing and axial stem retention as described below, whilst allowing the rotation of the valve stem 120 relative to the valve body 112.

Referring to Figures 6 and 7, the bushing and sealing arrangement at a second portion of the valve stem 120 on the other side of the valve disc can be defined as a non-driven portion 120b of the valve stem, can be seen in more detail. In contrast with the butterfly valve 10 of the prior art, in this embodiment the bushing and sealing arrangement is configured with an abutment arrangement with respect to the valve body 112 that acts to inhibit stem ejections, as described in more detail below.

Firstly, it should be noted that the non-driven portion 120b of the valve stem 120 has a smaller (reduced) diameter than the main body of the valve stem. In some embodiments this may be achieved by machining a stem 20 of the prior art to have this reduced diameter. This allows a sealing ring 146 and a thrust collar 148 to be located radially between bearing bush 132 and the valve stem 120.

The thrust collar 148 may be manufactured from nitride treated stainless steel. The bearing bush 132 may be manufactured from aluminium bronze and be shrink fitted into position in the bore 118b. In this embodiment, the sealing ring 146 is formed from nitride treated stainless steel and is formed with an annular recess 150 approximately mid-way along its length which receives an O-ring seal 152 (e.g. made from a suitable material such as nitrile rubber, PTFE, elastomer or polymer). This O-ring seal 152 is dynamic, maintaining seal integrity between the moving sealing ring 146 and static bearing bush 132. The sealing ring 146 abuts a shoulder 154 of the valve stem 120 where it increases in diameter. A further O-ring seal 156 is located in an annular recess 158 of the valve stem 120 and seals against an inner face of the sealing ring 146. This O-ring seal 156 is static.

A backup static O-ring seal 157 may optionally be provided in a further annular recess 159 of the valve stem 120.

The thrust collar 148 abuts the end face of the sealing ring 146 at the opposite end to the shoulder 154. A portion of this abutting surface at the radially outer corner is removed to form an annulus 160 defined by the sealing ring, thrust collar 148 and bearing bush 132. A lip seal 162 is positioned in the annulus 160 to act as a secondary seal. The lip seal 162 may be an energised lip seal, for example graphite filled PTFE and an Elgiloy^R™ spring lip seal. The lip seal is arranged to act as a back-up in case of failure of the O-ring seal 152 and as such only becomes energised by pressurised fluid if is escapes past the O-ring seal 152. In this embodiment the wear collar is manufactured from nitride treated stainless steel. It will be appreciated that by more effectively sealing the non-driven portion 120b the valve stem 120 remains pressure balanced in normal operation, and so is not subjected to an axial load that may lead to stem ejection. Further the fact the stem is pressure balanced may lead to a reduced torque being required to actuate the valve 110.

In other embodiments the sealing ring 146 may be omitted and the wear collar may extend into abutting contact with the shoulder 154 of the valve stem 120. In this case, the lip seal 162 may also be omitted. The annular recess 150 and associated O-ring seal 152 may be provided in the thrust collar 148 rather than the sealing ring 146.

With reference to Figures 5 and 6 it can be seen that an abutment 164 is provided on the transverse bore 118b adjacent to where the transverse bore meets the through bore 114. The abutment 164 acts as a stop that blocks movement of the bearing bush 132 towards the through-bore 114.

The thrust collar 148 has a female thread along a portion of its length and is screwed onto a corresponding male thread cut into the non-driven portion 120b of the valve stem 120. The thrust collar 148 is therefore mounted to the reduced diameter second portion in a non-axially moveable manner. Two or more axially extending blind bores 166 are provided in the end face of the thrust collar 148, to be engaged by a suitable tool (not shown) to apply torque thereto.

With particular reference to Figure 7 the thrust collar 148 is provided with a wear collar 168 that extends to substantially the same outer diameter as the bearing bush 132. The axial lengths and positions of the bearing bush 132 when abutting the abutment 164, and the thrust collar 148 when seated against the sealing ring 146, which in turn is seated against the shoulder 154 of the valve stem 120 is such that an axial spacing exists between the opposing annular faces of the bearing bush and the thrust collar. A thrust washer 170 is located in this space and has a thickness less than the spacing such that a clearance still remains between the opposing annular faces of the thrust washer and the bearing bush or wear collar. As a result, in normal operation no contact occurs between these opposing annular faces and no wear occurs on the thrust washer 170.

The thrust washer 170 is made from a low friction material in this embodiment, advantageously, a material that enables wear thereof to be seen from a visual inspection. A suitable material for the thrust washer 170 is graphite, for example.

However, in a situation where the sealing fails and fluid in the through-bore 114 is able to pass to the end of the valve stem 120 under pressure, the pressure will urge the stem and the thrust collar mounted on the stem towards the actuator. In turn this removes the clearance and causes the thrust washer to be clamped between the end of the bearing bush 132 and the wear collar 168. When rotation of the valve stem 120 occurs, the washer acts to reduce friction between the parts, but also will become marked, and therefore act as a visual indication of the failure of the seal, and the pressure acting on the stem axially. Such a seal failure may otherwise not be apparent if the butterfly valve 110 is stripped down for inspection or maintenance.

With further reference to Figures 5 and 6, a thrust collar locknut 172 is also screwed on to the threaded portion of the valve stem 120 behind the thrust collar 148. The locknut helps to ensure that the thrust collar 148 is prevented from rotation in operation, which would otherwise result in its axial misalignment to the other components at the non-driven portion 120b of the valve stem 120. The locknut 172 is provided with at least two blind bores 174 to be engaged by a suitable tool (not shown) to apply torque thereto to tighten the locknut.

A cap 122 closes the end of the transverse bore 118b to close and seal the bore from the external environment.

The arrangement described above at the non-driven portion 120b of the valve stem 120 is arranged to act as the primary structure for inhibiting the ejection of the valve stem in the event of the failure of sealing at that end of the of the valve stem and the consequent pressure building up between the cap 122 and the end of the valve stem 120.

The drive portion 120a of the valve stem 120 where it is supported by the transverse bore 118a has a similar arrangement of bushes, packing rings and gland as in the prior art, and will therefore not be discussed in more detail.

The valve stem 120 is assembled by first shrink fitting bearing bushes 132 into the transverse bores 118a and 118b. The sealing ring 146 is then fitted to the valve stem together with its associated O-ring seals 152 and 158. The valve stem is then inserted through the transverse bores 118a and 118b. The thrust collar 148 and thrust washer 170 are then mounted on to the valve stem 120 and screwed into contact with the sealing ring 146. The locknut 172 is then screwed home behind the thrust collar 148. The cap 122 is then bolted on to the valve body 112 to close the end of the transverse bore 118b.

In this embodiment, a further structure to protect against an ejection of the valve stem 120 from the valve housing, is additionally provided at the driven portion 120a of the valve stem.

Referring to Figures 5, 6 and 8, in this embodiment this structure comprises semi-circular retainer rings (split collar) 140 mounted into an annular recess 142 of the valve stem 120 at the drive portion 120a thereof. Specifically, the retainer rings 140 are mounted proximate to a wall 145 of the bridge piece 124.

The semi-circular retainer rings 140 have a T-shaped cross-section and are provided with chordally arranged bores that are aligned across the circumferential end faces of the of each ring. One of each of the aligned pairs of bores has an internal female thread such that a fastener, e.g. a bolt 175 may be inserted into each pair and tightened to clamp the retainer rings together and hold the rings within the annular recess 142 to prevent relative axial movement between the rings and the valve stem 120. The non-threaded bores may be countersunk so the heads of the bolts 175 are recessed when fitted.

Additionally, the clamping may frictionally inhibit relative rotational movement between the valve stem 120 and the retainer rings 140. Further, the retainer rings 140 are advantageously manufactured from a different material to the valve stem 120 and/or coated with a suitable material (e.g. Nitride) to inhibit galling. The valve stem 120 may for example be manufactured from stainless steel and the retainer rings from a nickel alloy.

In one embodiment the retainer rings 140 may be manufactured as two pieces that are clamped together and machined to ensure an optimum fit.

In some embodiments, to further inhibit relative rotation of the retainer rings 140 and the valve stem 120 one or more grub screws (not shown) may be inserted radially inwardly in suitable threaded bores (not shown) through the retainer ring(s) and into a complementary depression(s) (not shown) in the valve stem 120.

To minimise friction at the end faces of the retainer rings 140, these may be manufactured with grooves therein to minimise the contact surface and allow the distribution of lubricant.

To protect the retainer rings 140 from the surrounding environment, which may include corrosive chemicals, they are enclosed by a protective structure, which in this embodiment includes a cover plate 176 and transition piece 178 which are bolted together. O-ring seals 180 and 182 are located at the mating faces between the cover plate 176 and transition plate 178 and cover plate and valve stem 120 respectively.

To minimise friction between the retainer rings 140 and the protective structure 176, 178, thrust washers 184 are located between the axial faces of the retainer rings and the opposing faces of the structure. These washers 184 may, for example be bronze or aluminium bronze material for low friction and galling resistance.

The space around the retainer rings 140 may also be packed with lubricant such as grease and a grease nipple 186 is in this embodiment provided on the cover plate 176 to enable the introduction of grease into the space.

In this embodiment a location ring 188 is mounted to the valve stem 120 where it extends through the wall 145 of bridge piece 124. Both the wall 145 and the transition plate 178 have a corner recess 190, 192, and the location ring 188 is seated on each of these corner sections to ensure the concentricity of the valve stem 120 and retainer rings 140 with respect to these components.

A dynamic O-ring seal 194 is located between the valve stem 120 and the location ring 188 and a static O-ring seal 196 between the location ring and the transition piece 178.

It will be appreciated that this structure provides a (further) mechanism to prevent an ejection of the valve stem 120 in the event of a seal failure, since any such axial load is reacted through the retainer rings 140, transition piece 178, bridge piece 124 and into the valve body 112.

The further structure is assembled on to the valve stem 120 by first mounting the location ring 188 and transition plate 178 on the bridge piece 124. The bridge piece 124 is then slid over the valve stem 120 together with the cover plate 176 and the thrust washers 184, but without the cover plate secured to the transition plate 178. The bridge piece 124 is secured to the valve body 112. The retainer rings 140 are then mounted to the annular recess 142 in the valve stem 120 using the bolts 174. The cover plate 176 is then secured to the transition plate. Alternatively, the structure including the retainer rings 140, transition plate 178 and cover plate 176 may be pre-assembled on to the valve stem 120 prior to the fitting of the bridge piece 124.

It will be appreciated that numerous changes may be made within the scope of the present teachings. For example a pressure gauge may be provided in fluid flow communication with the space between the cap and the valve stem to monitor for the leakage of fluid into the space. A butterfly valve of the present teachings may be fitted with either or both of the stem ejection structures described above.

Claims

1. A butterfly valve for use in fluid processing such as oil and gas production, the butterfly valve comprising:- a valve stem arranged to mount a valve disc and having a first portion at one side of the valve disc and a second portion at a second side of the disc, the first and second portions being co-axially arranged;

the first portion comprising a formation to enable connection of the valve stem to an actuation mechanism, the second bore being closed by a closure remote from the disc such that the valve stem is enclosed in a pressure retaining manner, wherein a seal arrangement is provided between the second bore and the valve stem to inhibit passage of a fluid to the closure and a stop is arranged to interact with an abutment of the valve body and block movement of the valve stem in a direction away from the closure.

2. A butterfly valve according to claim 1 wherein the stop comprises thrust collar mounted onto the valve stem, e.g. removably mounted on to the valve stem.

3. A butterfly valve according to claims 1 or 2 wherein the stop is dimensioned so as to fit within bore.

4. A butterfly valve according to claim 3 wherein the abutment comprises a step projecting into the bore and of a smaller internal diameter than an external diameter of the stop.

5. A butterfly valve according to claims 1 to 4 wherein the butterfly valve is configured such that when the pressure therein from the fluid is axially balanced with respect to the stem, an annular face is axially spaced with respect to an opposed annular face nonrotatably mounted with respect to the valve stem, the annular faces being brought into contact if the process fluid generates a pressure urging the valve stem away from the closure.

6. A butterfly valve according to claim 5 wherein at least one of the opposed annular faces is formed of material that visually indicates when the annular face has been brought into contact with the other annular surface and relative movement has occurred.

7. A butterfly valve according to claims 5 or 6 wherein one of the faces is a face of a thrust washer.

8. A butterfly valve according to any preceding claim wherein the seal arrangement comprises an O-ring seal.

9. A butterfly valve according to any preceding claim wherein the seal arrangement comprises a lip seal, optionally an energised lip seal.

10. A butterfly valve according to any one of claims 2 to 9, further comprising a locknut arranged to abut and retain the thrust collar.

11. A butterfly valve for use in controlling the flow of a pressurised fluid in a pipe, the butterfly valve comprising:

12. A butterfly valve according to claim 11 wherein the housing comprises a bridge piece releasably mounted to a valve body of the housing that comprises the first and second co-axial bores.

13. A butterfly valve according to claim 11 or claim 12 wherein the first and second retainer rings are housed within a protective structure.

14. A butterfly valve according to claim 13 wherein the protective structure abuts the housing.

15. A butterfly valve according to any one of claims 11 to 14 wherein a thrust washer is interposed between the retainer rings and the housing.

16. A butterfly valve according to any preceding claim wherein the first and second retainer rings have a portion contacting the valve stem that is formed from a dissimilar material to the valve stem.

17. A butterfly valve according to any of claims 11 to 16 further comprising a lubricant in a space defined within the protective structure.

18. A butterfly valve comprising a seal arrangement and stop according to any of claims 1 to 10 and a retainer ring arrangement according to any of claims 11 to 17.

19. A method of making a butterfly valve of any of claims 2 to 10 comprising the steps of reducing the diameter of the second portion of an existing valve stem to define a step therein and mounting a thrust collar to the reduced diameter second portion in a nonaxially moveable manner.

20. A method according to claim 19 further comprising a step of mounting a sealing ring to the reduced diameter portion prior to mounting the thrust collar thereto, the sealing ring comprising at least one seal mounted thereon.

21. A method according to claim 19 or claim 20 further comprising a step of mounting a locknut to the reduced diameter portion after mounting the thrust collar to the reduced diameter portion.