Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
VALVE FOR A SUBSEA PRESSURE CANISTER
Document Type and Number:
WIPO Patent Application WO/2015/113789
Kind Code:
A1
Abstract:
The present invention relates to a valve (20) for a subsea pressure canister (11). The valve (20) comprises a housing (21) comprising a passage (22) extending between a first port (23) and a second port (24) of the valve (20), a piston (40) arranged within the passage (22) and movable between a closed position and an open position, and a seal (41) to provide a sealing between the piston (40) and the passage (22) in the closed position of the piston (40). The piston (40) has a first sealing surface (49), and the passage (22) has a second sealing surface (50). The first sealing surface (49) is provided with a circumferential groove (61) in which the seal (41) is arranged, and with a circumferential sealing edge (62) protruding from the first sealing surface (49). In the closed position of the piston (40), the first sealing surface (49) is arranged to the second sealing surface (50) such that the seal (41) is sealingly pressed between the first sealing surface (49) and second sealing surface (50), and the circumferential sealing edge (62) is sealingly pressed between the first sealing surface (49) and second sealing surface (50).

Inventors:
ESPEDALEN HANS (NO)
Application Number:
PCT/EP2015/050198
Publication Date:
August 06, 2015
Filing Date:
January 08, 2015
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SIEMENS AG (DE)
International Classes:
F16K17/04; F16K1/38; F16K1/44
Foreign References:
JP2012207796A2012-10-25
US2307546A1943-01-05
US20100038573A12010-02-18
US4932434A1990-06-12
US20070246099A12007-10-25
JPS488717U1973-01-31
US20060071193A12006-04-06
US20060157111A12006-07-20
US20110033313A12011-02-10
Other References:
None
Download PDF:
Claims:
Patent claims

1. A valve for a subsea pressure canister, comprising:

a housing (21) comprising a passage (22) extending be- tween a first port (23) and a second port (24) of the valve (20) ,

a piston (40) arranged within the passage (22) and movable between a closed position and an open position, and

a seal (41) to provide a sealing between the piston (40) and the passage (22) in the closed position of the piston (40) ,

wherein the piston (40) has a first sealing surface (49), and the passage (22) has a second sealing surface (50) ,

wherein at least one of the first (49) and second (50) seal- ing surfaces is provided with a circumferential groove (61) in which the seal (41) is arranged, and wherein at least one of the first (49) and second (50) sealing surfaces is provided with a circumferential sealing edge (62) protruding from the corresponding one of the at least one of the first (49) and second (50) sealing surfaces,

wherein, in the closed position of the piston (40), the first sealing surface (49) is arranged to the second sealing surface (50) such that the seal (41) is sealingly pressed between the first sealing surface (49) and second sealing sur- face (50), and the circumferential sealing edge (62) is sealingly pressed between the first sealing surface (49) and second sealing surface (50).

2. The valve according to claim 1, wherein the circumferen- tial sealing edge (62) is arranged at the first sealing surface (49) and is formed integrally with the piston (40) .

3. The valve according to claim 1 or 2, wherein the circumferential sealing edge (62) is arranged along a rim of the circumferential groove (61) .

4. The valve according to any one of the preceding claims, wherein the piston (40) has a truncated conical shape provid- ing a tapered lateral surface (49) as the first sealing surface (49), and wherein the passage (22) comprises an inner wall section having a truncated conical shape providing a tapered lateral surface (50) as the second sealing surface (50) .

5. The valve according to any one of the preceding claims, wherein the piston (40) is made of at least one material of group comprising:

metal,

titanium, and

stainless steel.

6. The valve according to any one of the preceding claims, further comprising:

a spring (44) to bias the piston (40) into the closed position .

7. The valve according to any one of the preceding claims wherein, in the open position of the piston (40), the first sealing surface (49) is arranged to the second sealing surface (50) such that the seal (41) is spaced apart from at least one of the first sealing surface (49) and the second sealing surface (50), and the circumferential sealing edge (62) is spaced apart from at least one of the first sealing surface (49) and the second sealing surface (50) .

8. The valve according to any one of the preceding claims, further comprising:

- a further piston (42) arranged within the passage (22) and movable between a closed position and an open position, and

a further seal (43) to provide a sealing between the further piston (42) and the passage (22) in the closed posi- tion of the further piston.

9. The valve according to claim 8, wherein, in the closed position of the piston (40), the seal (41) and the piston (40) are arranged to hydraulically isolate the first port (23) from an intermediate space (48) provided in the passage (22) between the piston (40) and the further piston (42), and wherein, in the closed position of the further piston (42), the further seal (43) and the further piston (42) are arranged to hydraulically isolate the second port (24) from the intermediate space (48) .

10. The valve according to claim 9, wherein the intermediate space (48) is filled with a fluid, in particular with a liquid .

11. The valve according to claim 10, wherein the piston (40) is arranged relative to the further piston (42) such that, when the piston (40) is moving from the closed position to the open position, the piston (40) displaces the fluid such that the fluid displaces the further piston (42) from the closed position to the open position. 12. The valve according to any one of claims 8-11, wherein the further piston (42) has a cylindrical outer shape and the passage (22) comprises an inner wall section (51) having a cylindrical inner shape,

wherein, in the closed position of the further piston (42), the further piston (42) is fitted into the inner wall section (51) .

13. The valve according to claim 12, wherein, in the closed position of the further piston (42), the further seal (43) is arranged between the cylindrical outer shape of the further piston (42) and the cylindrical inner shape of the inner wall section (51) of the passage (22) .

14. The valve according to claim 12 or 13, wherein the pas- sage (22) comprises furthermore an enlarged inner wall section (52) arranged between the inner wall section (51) and the second port (24), wherein an inner circumference of the enlarged inner wall section (52) is larger than an inner circumference of the inner wall section (51),

wherein, in the open position of the further piston (42), the further piston (42) is arranged at least partly within the enlarged inner wall section (52) .

15. The valve according to claim 14, wherein, in the open position of the further piston (42), the further seal is arranged within and spaced apart from the enlarged inner wall section (52) .

16. The valve according to any one of claims 8-15, wherein the further piston (42) is arranged spaced apart from the piston (40) and the piston (40) is movable independently from the further piston (42) .

17. The valve according to any one of claims 8-16, wherein the further piston (42) comprises a position indicator (53) extending from the further piston (42) in a longitudinal di- rection of the passage (22), wherein the position indicator

(53) protrudes from the housing (21), when the further piston (42) is in its open position.

Description:
Description

Valve for a subsea pressure canister

The present invention relates to a valve for a subsea pressure canister, especially to a pressure relief valve for a subsea pressure canister or a subsea vessel comprising a metal seal.

For subsea applications, for example subsea oil production, subsea energy transmission and subsea telecommunications, equipment to be operated subsea must be adapted to this harsh environment. A major obstacle in deep water is the extreme high pressure experienced by the equipment at these depths. Accordingly, pressure vessels, so-called pressure canisters, may be used to protect equipment at these depths from the high pressure by providing an inside pressure of approximately one atmosphere corresponding approximately 100,000 Pascal or 1 bar.

Pressure canisters used subsea to maintain and simulate an ambient pressure similar to what is experienced at the surface are also called one atmosphere chambers, vessels or canisters. A common use for example in the oil industry for such canisters is to protect pressure sensitive components subsea. In the event that a pressure canister leaks slowly while subsea and is then withdrawn back to the surface relatively rapidly, it may contain a pressure higher than the normal atmospheric pressure. When the pressure canister has been withdrawn back from a depths of approximately 3000 metres to the surface, the pressure inside the canister may become extremely large, for example as high as 300 bar (30,000,000 Pa) . When such a pressurized canister has been retrieved from subsea and is opened at the surface, accidents may happen. To achieve a more safe operation, a hydraulic check valve acting as a pressure relief valve may be installed. However, a check valve may introduce a potential leak point for water to enter the interior of the canister. Therefore, it is an object of the present invention to enhance a safe operation of subsea equipment, especially of a one atmosphere canister.

Summary of the Invention

According to the present invention, this object is achieved by a valve for a subsea pressure canister as defined in claim 1. The dependent claims define preferred and advantageous embodiments of the present invention.

According to an embodiment, a valve for a subsea pressure canister is provided. The valve comprises a housing with a passage extending between a first port and a second port of the valve. The housing may be attached to the subsea pressure canister such that the first port is in a fluid communication with an interior of the subsea pressure canister and the second port is in a fluid communication with an environment outside the subsea pressure canister. Thus, the passage is configured to provide a fluid communication between an interior and an exterior of the subsea pressure canister. The valve comprises furthermore a piston which is arranged within the passage and which is moveable between a closed position and an open position. The valve comprises furthermore a seal configured to provide a sealing between the piston and the passage in the closed position of the piston. The piston has a first sealing surface and the passage has a complementary second sealing surface. At least one of the first and second sealing surfaces is provided with a circumferential groove in which the seal is arranged. Furthermore, at least one of the first and second sealing surfaces is provided with a circumferential sealing edge protruding from the corresponding one of the at least one of the first and second sealing surfaces. For example, the circumferential groove may be provided at the first sealing surface, i.e. at the piston, and the circumferential sealing edge may be provided also at the first sealing surface. Furthermore, in the closed position of the piston, the first sealing surface is arranged to the second sealing surface such that the seal is sealingly pressed between the first sealing surface and the second sealing surface, and additionally, the circumferential sealing edge is sealingly pressed between the first sealing surface and the second sealing surface. For example, in the closed position of the piston, the first sealing surface may be arranged in parallel to the second sealing surface. In the example described above, in which the circumferential groove as well as the circumferential sealing edge are arranged at the piston, in the closed position of the piston, the seal is pressed against the second sealing surface of the passage and the circumferential edge is also pressed against the second sealing surface of the passage. However, in the open position of the piston, the seal as well as the circumferential sealing edge may be arranged spaced apart from the second sealing surface of the passage. The circumferential groove may comprise an endless and continuous groove arranged in the piston or the passage. Furthermore, the circumferential sealing edge may comprise an endless continuous sealing edge protruding from the passage or the piston. For example, the piston may be moveable in a longitudinal direction between the open and the closed position. In this case, the groove as well as the sealing edge may be provided in a plane perpendicular to the longitudinal direction. For example, the piston may comprise a poppet-like piston with the circumferential groove as well as the circumferential sealing edge being arranged at a sealing surface of the poppet. By providing two sealing mechanisms, i.e., the seal and the sealing edge, a fluid communication between the first port and the second port may be inhibited, thus providing a reliable sealing in the closed position of the valve. Therefore, an interior of the subsea pressure canister may be protected from pressurized fluid at an outside of the pressure canister in a subsea environment. In other words, the two sealing mechanisms provide a double barrier protecting the interior of the subsea pressure canister from high subsea pressure such that the subsea pressure canister may be arranged in a depth of approximately 3000 me- ters subsea with an interior pressure of approximately one atmosphere. By providing two sealing mechanisms in a serial arrangement, a reliable sealing over a long lifetime of the valve, for example up to thirty years, may be provided.

According to an embodiment, the circumferential sealing edge is arranged at the first sealing surface of the piston and is formed integrally with the piston. Furthermore, the circumferential sealing edge may be arranged along a rim of the circumferential groove. By integrally forming the circumferential sealing edge with the piston, the number of parts of the valve may be reduced and assembly of the valve may be simplified resulting in lower cost of the valve. Furthermore, the piston may be made of, for example, metal, titanium, or stainless steel. Hence, the circumferential sealing edge may also be made of a metal material, thus providing a metal seal when being pressed between the first sealing surface and the second sealing surface. The metal seal may provide a long lifetime in harsh subsea environments. By providing the circumferential sealing edge along the rim of the circumferential groove in which the seal is arranged, a sealing effect of the seal may be enhanced, especially when the seal is arranged as a first sealing barrier and the circumferential sealing edge is arranged as a second sealing barrier with respect to the pressure acting in the direction from the second port to the first port in subsea environments.

According to an embodiment, the piston has a truncated conical shape which provides a tapered lateral surface as the first sealing surface and the passage comprises an inner wall section having a truncated conical shape providing a tapered lateral surface as the second sealing surface. In the closed position of the piston, the first sealing surface is arranged essential in parallel to the second sealing surface. Furthermore, the seal and the sealing edge are arranged between the first sealing surface and the second sealing surface such that the seal is pressed between the first and second surfaces and the sealing edge is also pressed between the first and second surfaces, thus providing a double barrier fluid tight sealing between the first and second sealing surfaces. The seal may comprise an O-ring arranged in the groove at the tapered lateral surface of either the piston or the passage. In other words, the piston provides in combination with the inner wall section of the passage a poppet valve. The poppet valve may provide a robust sealing inhibiting a fluid flow in the direction from the second port to the first port. On the other hand, when the pressure inside the subsea pressure canister increases and is larger than a pressure outside the canister, the poppet valve may be easily opened by the pressure difference.

According to another embodiment, the valve comprises a spring to bias the piston into the closed position. The spring may be configured to keep the piston in the closed position until a pressure difference between a pressure at the first port and the second port crosses a predetermined pressure threshold. The pressure threshold may comprise a value in a range from one to ten bar (10,000 to 1,000,000 Pa). By biasing the piston with the spring, the valve will open only in case of a significant overpressure within the subsea pressure canister, thus avoiding an unnecessary opening of the valve.

According to a further embodiment, the valve comprises a further piston arranged within the passage and moveable between a closed position and an open position. Furthermore, the valve comprises a further seal to provide a sealing between the further piston and the passage in the closed position of the further piston. In other words, the valve comprises two pistons arranged in the passage, with each piston having a seal for sealing the corresponding piston in its closed position. The two pistons may be arranged in a serial arrangement such that the piston and the further piston must be positioned both in their open positions to enable the fluid communication between the first port and the second port, whereas the fluid communication between the first port and the second port is inhibited when at least one of the pistons is arranged in its closed position. By providing two pistons which are each configured to inhibit the fluid communication between the first port and the second port, a reliable sealing in the closed position of the valve can be provided to protect an interior of the subsea pressure canister from the pressurized fluid at an outside of the canister in a subsea environment. The two seals of the piston and the seal of the further piston provide a triple barrier protecting the interior of the subsea pressure canister from high subsea pressure. By providing two independent pistons in a serial arrangement, the pressure in the passage between the piston and the further piston may be balanced by an independent movement of the further piston. This enables each piston to provide an optimized sealing.

According to an embodiment, in the closed position of the piston, the seal and the piston are arranged to hydraulically isolate the first port from an intermediate space provided in the passage between the piston and the further piston. Furthermore, in the closed position of the further piston, the further seal and the further piston are arranged such that the second port is hydraulically isolated from the intermediate space. Therefore, the seal, the sealing edge and the further seal provide a triple barrier and the pressure difference between a pressure at the first port and a pressure at the second port is balanced via the intermediate space between the piston and the further piston. The intermediate space may be filled with a fluid, in particular a liquid, for example with oil, for example silicon oil. By using a fluid like silicon oil which is hardly compressible, a pressure balance between the double sealing at the piston and the further seal at the further piston may be accomplished with an approximately constant volume of the intermediate space in pressurized and non-pressurized conditions. Due to the fluid in the intermediate space, the further seal may be pressure- balanced. This compensation may eliminate pressure differentials on the further seal to keep the further seal fit and operational during lifetime of the valve. According to another embodiment, the piston is arranged relative to the further piston such that, when the piston is moving from the closed position to the open position, the piston displaces the fluid and the fluid displaces the further piston from the closed position to the open position. For example, when the valve is arranged at a subsea pressure canister which has a leak and is retrieved from subsea, there may be a high pressure inside the canister compared to atmospheric pressure outside. Due to the pressure difference between the interior of the pressure canister and the one atmospheric pressure outside the canister, the piston may be moved from the closed position to the open position. Due to the enclosed intermediate space provided between the piston and the further piston, the silicon oil in the intermediate space is urged by the piston, and the silicon oil urges the further piston from the closed position to the open position. Finally, when both pistons are in their open positions, the high pressure inside the subsea pressure canister may be released via the valve.

According to an embodiment, the further piston has a cylindrical outer shape and the passage comprises an inner wall section having a cylindrical inner shape. In the closed position of the further piston, the further piston is fitted into the inner wall section. Furthermore, in the closed position of the further piston, the further seal may be arranged between the cylindrical outer surface of the further piston and the cylindrical inner surface of the inner wall section of the passage. More precisely, the cylindrical further piston may be arranged within a tubular section of the passage with the further seal providing a fluid-tight sealing between the further piston and the passage.

The passage may comprise furthermore an enlarged inner wall section arranged between the tubular inner wall section and the second port. An inner circumference of the enlarged inner wall section is larger than an inner circumference of the tu- bular inner wall section. In the open position of the furthe piston, the further piston is arranged at least partially within the enlarged inner wall section, especially, the further piston may be arranged such that the further seal is ar ranged between the cylindrical outer surface of the further piston and the enlarged inner wall section. Due to the large inner circumference of the enlarged inner wall section, a fluid-tight sealing is not provided anymore by the further seal and thus a fluid may pass along the passage in the open position of the further piston.

Furthermore, the further piston may comprise a position indicator extending from the further piston in a longitudinal direction of the passage. The piston indicator protrudes from the housing when the further piston is in its open position. As described above, when the further piston is moved from the closed position to the open position, the position indicator is moved together with the further piston in the longitudinal direction of the passage and protrudes from the housing such that the position of the further piston can be visually perceived from outside of the valve and therefore also from outside of the subsea pressure canister. Furthermore, as in the open position of the further piston the further seal is arranged in the enlarged inner wall section, even when the pressure difference between the first port and the second port is balanced, the further piston may remain in the open position and thus the position indicator indicates that the valve has been in the open position before. Thus, after retrieving the subsea pressure canister from subsea, a technician can easily recognise that a pressure relief has occurred .

According to another embodiment, the further piston is arranged spaced apart from the piston and the piston is movable independently from the further piston. By providing a space between the piston and the further piston, which may be filled with a fluid, in particular a liquid, like silicon oil, a pressure balance between the pistons and the sealings may be achieved. Furthermore, as the piston and the further piston are movable independently from each other, the piston may be moved back via the spring after a pressure relief, whereas the further piston may remain in the open position tc indicate via the position indicator that a pressure relief has occurred.

Finally, according to the present invention, a subsea pressure container comprising the valve described above is pro- vided.

Although specific features described m the above summary and the following detailed description are described in connection with specific embodiments and aspects of the present invention, it should be understood that the features of the exemplary embodiments and aspects can be combined with each other unless specifically noted otherwise.

Brief Description of the Drawings

The present invention will now be described in more detail with reference to the accompanying drawings.

Fig. 1 shows schematically a subsea pressure canister according to an embodiment of the present invention.

Fig. 2 shows schematically a perspective view of a valve according to an embodiment of the present invention.

Fig. 3 shows schematically a top view of the valve of Fig. 2.

Fig. 4 shows schematically a sectional view of the valve of Fig. 2 in a closed condition.

Fig. 5 shows schematically an exploded view of the valve of Fig. 2. Fig. 6 shows schematically an enlarged sectional view of a piston of the valve of Fig. 5.

Fig. 7 shows schematically an enlarged top view of the piston of Fig. 6.

Fig. 8 shows schematically an even more enlarged sectional view of a detail of the piston of Fig. 6.

Detailed Description of Preferred Embodiments

In the following, exemplary embodiments of the invention will be described in more detail. It is to be understood that the features of the various exemplary embodiments described herein may combined with each other unless specifically noted otherwise. Same reference signs in the various drawings refer to similar or identical components.

Fig. 1 shows schematically a subsea pressure canister 11 in a typical subsea environment 10. The subsea canister or subsea vessel 11 may be used for housing components 14 in an interior 13 for protecting the components 14 from high pressure present in deep sea water 12. For coupling the components 14 with other equipment, the subsea canister 11 may comprise a port 16, for example an oil-filled pressure compensated chamber 16, and an electrical penetrator 15 providing electrical lines arranged in an oil-filled conduit 17. The other end of the conduit 17 may comprise a connector 18 for coupling the conduit 17 to another subsea or land based equipment.

The canister 11 may be arranged in deep water, for example in a depth of 3000 metres. Therefore, a pressure exerted on the canister 11 by the seawater 12 may become as large as for example 300 bar. For enabling operation of standard components 14 in such a harsh environment, the subsea pressure canister 11 may provide a pressure in the interior 13 of the canister 11 of for example approximately one bar. In the event that the subsea pressure canister 11 leaks while being arranged subsea and is then retrieved from subsea to an environment of normal atmospheric pressure relatively rapidly, it may still contain a very high pressure resulting from the leakage subsea. Such a pressurised canister may explode when being exposed or opened. Therefore, a pressure relief valve 20 is arranged at the canister 11 comprising a first port 23 in a fluid communication with the interior 13 of the canister 11 and a second port 24 in a fluid communication with the environment of the canister 11. A passage 22 is extending between the first port 23 and the second port 24. When the pressure difference between a pressure at the first port 23 and a pressure at the second port 24 crosses a predefined threshold, the valve 20 opens and a high pressure from the interior 13 of the canister 11 is released via the passage 22 into the environment. However, the valve 20 is closed when the pressure outside the canister 11 is higher than the pressure inside the canister 11 to avoid introduction of for example seawater 12 into the canister 11 when the canister 11 is arranged and operated in the subsea environment 10.

Fig. 2 shows the valve 20 in more detail. The valve 20 comprises a housing or body 21 in which the above described check valve mechanism is arranged to control the fluid flow from the first port 23 to the second port 24.

Fig. 3 shows a top view of the valve 20. A diameter of the valve may be in the range of 10 to 30 millimetres, preferably 18 millimetres. The length of the valve may be in the range from 20 to 40 millimetres, preferably 28 millimetres.

Fig. 4 shows a sectional view of the valve 20 along the line A-A of Fig. 3. The valve 20 comprises at an outside of the housing 21 a seal 45, for example an O-ring, for providing a fluid-tight sealing between the housing 21 and the canister 11 when the valve 20 is mounted at the canister 11 as shown in Fig. 1. Furthermore, at the housing 21, a threaded section 25 is formed and a nut 26 as well as a collar 27 may be provided to enable mounting the valve 20 at the canister 11. As described above, the housing 21 provides the passage 22 which extends from the first port 23 to the second port 24. Inside of the passage 22 a first piston 40 and a second pis- ton 42 are arranged. The first piston 40 is movable between a closed position as shown in Fig. 4 and an open position in which the first piston is moved in an upward direction in the Fig. 4. The second piston 42 is movable between a closed position shown in Fig. 4 and an open position in which the sec- ond piston is also moved in an upward direction in the Fig. 4. The pistons 40 and 42 are not coupled directly with each other such that they may be moved independently from each other. Therefore, there is a space in the passage 22 between the first piston 40 and the second piston 42 which will be called in the following intermediate space 48. Within the passage 22 near the first port 23, a spring 44 is arranged which biases the first piston 40 such that the first piston 40 is urged by the spring 44 into the closed position, i.e. downwards in Fig. 4. For assembling the valve 20, the housing 21 comprises a main body and a cap 47 arranged at an upper end of the main body. The second port 24 is formed as a hole or a plurality of holes in the cap 47. Furthermore, a spring retainer 46 is arranged at the first port 23 retaining the spring 44 in position. The second piston 42 comprises an ex- tension 53 which protrudes from the cap 47 in the open position of the second piston 42, and which does not protrude from the cap 47 in the closed position of the second piston 42 as shown in Fig. 4. Thus, the extension 53 acts as a position indicator 53 indicating the position of the second pis- ton 42. Furthermore, a first seal 41 is arranged in a continuous circumferential groove 61 at the first piston 40 and a second seal 43 is arranged in a continuous circumferential groove at the second piston 42. Finally, a sealing edge 62, which will be described in more detail in the following in connection with Figs. 6 and 8, is provided at the first piston 40. Fig. 5 shows the components of the valve 20 described above in connection with Fig. 4 in an exploded view.

Fig. 6 shows the first piston 40 in a sectional view in more detail. The dimensions shown in Fig. 6 are only exemplary and the first piston 40 may have other dimensions than shown in Fig. 6. The first piston 40 has a first sealing surface 49 which may be a tapered lateral surface of a truncated conical shape. As shown in Fig. 4, the passage 22 comprises a second sealing surface 50 having a complementary tapered lateral surface of a complementary truncated conical shape. In the closed position of the first piston, as shown in Fig. 4, the first sealing surface 49 and the second sealing surface 50 may be arranged in parallel and in closed vicinity. The first sealing surface 49 is provided with a continuous endless circumferential groove 61 in which the first seal 41 is provided. At a rim of the groove 61, a circumferential sealing edge 62 is protruding from the first sealing surface 49. As shown in Fig. 8 in more detail, the sealing edge 62 may pro- trude, for example, 0.1 mm and may be formed integrally with the first piston 40. The first piston 40 as well as the second sealing surface 50 of the passage 22 may be made of a metal material, for example titanium or stainless steel.

Therefore, in the closed position of the first piston 40, the first seal 41 as well as the sealing edge 62 are pressed against the second sealing surface 50 of the passage 22 providing a double barrier sealing to prevent a fluid communication between the intermediate space 48 and the first port 23. The metal-to-metal seal provided by the sealing edge 62 and the second sealing surface 50 offers a significantly improved reliability and safety enabling a lifetime of the valve 20 of approximately thirty years .

Fig. 7 shows a top view of the first piston 40. As can be seen from Fig. 7, the shape of the first piston 40 may be a circular shape. In this case, the groove 61 as well as the sealing edge 62 may extend in a circular direction along the first sealing surface 50 in a plane perpendicular to a longitudinal axis of the first piston 40.

Operation of the valve 20 will be described in connection with Fig. 4 in the following in more detail.

Fig. 4 shows the valve 20 in the closed position. The conical tapered first sealing surface 49 of the first piston 40 is arranged in parallel to the complimentary tapered second sealing surface 50 of the housing 21. In the groove 61 arranged along the conical tapered first sealing surface 49 in a circumferential direction the first seal 41 is arranged. Due to a pressure exerted by the spring 44 on the first piston 40 in a downward direction in Fig. 4, the first seal 41 is pressed against the tapered second sealing surface 50. Therefore, a fluid communication between the first port 23 and the intermediate space 48 is interrupted. The second piston 42 has a cylindrical shape and the second seal 43 is arranged in a groove in a circumferential direction of the cy- lindrical surface such that the second seal 43 is in contact with a cylindrical inner wall 51 of the housing 21. Due to an elastic pressure of the second seal 43, a sealing between the second piston 42 and the cylindrical section 51 of the housing 21 is achieved, and a fluid communication between the second port 24 and the intermediate space 48 is interrupted. The intermediate space 48 is filled with a fluid, in particular a liquid, for example a silicon oil.

When the valve 20 is arranged at the canister 11, and the canister 11 is immersed subsea, the pressure at the second port 24 rises with respect to a pressure inside the canister at the first port 23. The higher outer pressure at the second port 24 urges the second piston 42 in Fig. 4 in a downward direction. Due to the fluid in the intermediate space 48, also the first piston 40 is pressed in a downward direction. Due to the fact that the fluid in the intermediate space 48 is essentially incompressible, the volume of the intermediate space 48 remains essentially unchanged and therefore the dis- tance between the second piston 42 and the first piston 40 remains unchanged. However, due to the high pressure urging the first piston 40 in the downward direction, a reliable sealing is achieved prohibiting intrusion of seawater from the second port 24 into the canister 11. The second seal 43 is pressure balanced due to the fluid in the intermediate space 48. This balancing compensation may eliminate essentially pressure differentials on the second seal 43 to maintain the second seal 43 operable for sealing purposes during lifetime of the valve 20. Due to the first seal 41, the second seal 43 and especially due to the metal-to-metal sealing of the sealing edge 62, a triple barrier sealing with a high reliability and long lifetime of 30 years or more may be provided .

Under normal conditions, when the canister 11 does not leak and the canister 11 is brought back to the surface such that the pressure at the port 24 corresponds essentially to the pressure at the port 23, the valve 20 does not open and even when the pressure inside the canister rises somewhat above the outside pressure at port 24, the valve 20 does not open due to the biasing force exerted on the first piston 40 by the spring 44. However, in case in subsea conditions the canister 11 leaks, the pressure inside the canister 11 may increase far above one atmosphere. When the canister 11 is then brought back to the surface rapidly, a considerably higher pressure than one atmosphere would be present inside the canister 11 without the valve 20, for example some 10 or some 100 atmosphere or bar. However, when the pressure inside the canister 11 rises with respect to the outside pressure, the valve 20 opens. For example, the spring 44 may be dimensioned such that the valve 20 opens when the pressure at the first port 23 is 1 to 10 bar higher than the pressure at the second port 24. The higher inside pressure at the first port 23 urges the first piston 40 in Fig. 4 in an upward direction resulting in a space between the tapered first and second sealing surfaces 49 and 50, and the first seal 41 as well as the sealing edge 62 lose contact to the tapered second sealing surface 50. Fluid from inside the canister 11 flows through the first port 23 along the tapered first and second sealing surfaces 49 and 50 into the intermediate space 48. This increases the pressure in the intermediate space 48 and the second piston 42 is urged in an upward direction. The cylindrical surface 51 changes over to a conical tapered surface 52 as shown in Fig. 4. When the second piston 42 is being urged in the up- ward direction, the second seal 43 is moved from the cylindrical surface 51 to the tapered surface 52 and beyond. Thus, the sealing of the second seal 43 leaks and a fluid communication between the intermediate space 48 and the second port 24 is provided. Finally, the higher pressure from inside the canister 11 can be reliably released via the passage 22 of the valve 20 to the second port 24.

The position indicator 53 is coupled to the second piston 42 and moves therefore along with the second piston 42 in an up- ward direction when the valve 20 is opened. In the open position of the second piston 42, the position indicator 53 protrudes from the upper surface of the cap 47 of the valve 20. Thus, a person handling the canister 11 can immediately determine that the valve 20 has opened. When the pressure be- tween the inside and the outside of the canister 11 is balanced, the first piston 40 may be urged back into its closed position by the spring 44 preventing seawater from intruding into the canister. However, the second piston 42 may remain in the open position still indicating that the valve 20 has opened in the meantime.

The above-described pressure relief valve 20 may be designed for a working depth of 3000 metres. The housing 21, the first piston 40, and the second piston 42 may be made of titanium or a stainless steel, for example AISI 316. The bias force of the spring 44 may be configured such that the valve 20 opens at a pressure difference, a so-called crack pressure, of 1 to 10 bar. The passage 22 may be designed such that in the open state of the valve 20 a fluid flow of at least one litre per minute of gas or water may be achieved at a pressure difference of 10 bar. It shall be noted that in the following claims, the "first piston" is called "piston", the "second piston" is called "further piston", the "first seal" is called "seal", and the "second seal" is called "further seal".