NO344443B1

NO344443B1 - A metal-to-metal stem seal system

Info

Publication number: NO344443B1
Application number: NO20181342A
Authority: NO
Inventors: Ole Jørgen Rongved
Original assignee: Kvenna Emt As
Priority date: 2018-10-19
Filing date: 2018-10-19
Publication date: 2019-12-09

Description

A metal-to-metal stem seal system

Field of the invention

The invention concerns a stem seal system, having a stem and a stem seal with metal-tometal sealing faces. The invented stem seal system is particularly useful in dynamic applications, such as in actuators and valves. The invention is particularly useful in subsea applications, in association with the transportation of hydrocarbons.

Background of the invention

A typical valve has a body with a flow passage bore extending through it. The flow passage, which may be connected to pipes or other conduits, intersects a central cavity in the valve body, and a movable valve closure member (obturator) is arranged in this central cavity. A stem bore is arranged in a part of the valve body which is commonly referred to as a bonnet or a gland, and is in communication with the central cavity. A stem extends through the stem bore and into the central passage where the stem is connected to the obturator. Selective movement of the stem causes the obturator to move between closed and open positions that obstruct flow or allow flow, respectively, through the flow passage.

In one valve type – a gate valve – the obturator is a gate that is arranged perpendicular to the flow passage. In another valve type – a ball valve – the obturator is a ball that is arranged for rotation in the central cavity.

In a typical gate valve, the stem travels linearly (reciprocates) in the stem bore to move the gate between closed and open positions. In other types of gate valves, the stem may also rotate in the stem bore to rotate the gate in the central passage.

In the case of a ball valve, the ball comprises a through-conduit bore, and a stem extends through the stem bore and into the central passage where the stem is connected to the ball. Rotation of the stem causes the ball to move between closed and open positions that obstruct flow or allow flow, respectively, through the flow passage.

Since the central cavity of the valve and the stem bore are in communication, the space between the stem and the stem bore must be sealed to avoid leakage. This stem seal must effectively cope with pressures inside the valve, but also with pressures exerted on the valve from the environment (seawater) outside the valve. The ability to withstand such external pressures are of particular importance for valves that are installed at large water depths.

The prior art includes US 5293943 A, which describes a circular metal seal having a coat of hard metal in the engaging region, and the metal seal has an external seal. The stem also has a coat of hard metal.

Polymeric and metal-to-metal seals are commonly used to seal between the stem and the stem bore. However, as valves in the oil and gas industry are now being used in new environments in which the temperatures and pressures are very high, demands for more robust and durable seals arise. It is therefore a need for a seal system that has an improved service life and sealing capacity to prevent any leakage of hydrocarbons to the environment.

Summary of the invention

The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a metal seal, comprising a circular seal ring forming a through-going bore which is configured for receiving at least a portion of a stem, and a circular seal lip arranged externally of said seal ring, a stem-engaging region which is arranged circumferentially on the seal ring, said stem-engaging region comprising a portion having a seal coating of a hard material, such as a metal or a ceramic having a high hardness, characterized in that the seal ring and seal lip extend in the generally same direction along a seal axis, and the seal lip comprises a coating of a lubricating and sealing material, whereby the assembly procedure and sealing properties are improved.

In one embodiment, the seal coating comprises tungsten carbide. The coating on the seal lip may comprise a layer of silver plating.

In one embodiment, the seal ring and seal lip extend in the generally same direction along a seal-axis, forming a U-shaped profile on the seal.

In one embodiment, the metal seal further comprises a second circular seal lip seal extending in a generally opposite axial direction to that of the seal ring and seal lip.

It is also provided a metal-to-metal valve seal system, characterized by the metal seal according to the invention and a stem configured for operating an obturator in a valve flow passage; said stem further comprising a stem-circumferential region having a stem coating of a material similar to that of the seal coating.

In one embodiment, the stem coating extends between axially spaced apart end points on the stem, and a first coated portion of the region has a first stem diameter which is greater than a stem diameter outside of a second coated portion. The second coated portion is outwardly inclined at an inclination angle to form an entering portion for the stem seal when the seal is energized. The seal coating and the stem coating are configured to abut and form a sealing interface when the stem and seal are installed and energized. On stems designed for linear movement, the stem coating may cover the entire stroke length, or the stem coating is applied at both ends of the stem stroke, to provide a back-seat metal sealing function.

The invention thus provides a self-energizing, fully elastic double metal stem seal having a U-profile for High Temperature and/or High Pressure (HT/HP) applications in subsea valves and actuators in deep water subsea production systems (SPS) and deep water subsea transport systems (STS).

The self-energizing function is achieved during installation by the stiffness of the seal ring, to provide initial seal contact between the inner seal face and stem and between the outer seal lip and the bonnet or gland seal face. After installation, the seal is capable of sealing both from internal and external pressure.

The stem seal face and the seal face of the stem seal both engage the stem, and are in one embodiment hard-coated by a High Velocity Oxygen Fuel (HVOF) process, using tungsten carbide. Surface roughness, tungsten carbide thickness and geometrical tolerances are carefully selected to achieve a gas tight seal. While the outer seal lip is of a static nature, the installation onto the cylindrical seal face of bonnet or gland is achieved by carefully selected entering zone geometry. The inner seal against the stem is of dynamic nature. Assembly of the stem seal onto the stem is achieved by carefully selected entering zone on the stem to locate the seal onto the sea face diameter.

During pressurization, the stem seal is further pressure energized. Contact pressures are selected to achieve a gas tight seal at both low and high pressure and temperature, without compromising the strain and surface pressure limits defined by the tungsten carbide seal coating. The geometry of the inner seal profile includes a stabilizing section below the tungsten carbide coated seal coating to enhance optimal seal contact pressure at high pressure. For valve application with a rotating stem (e.g. a ball valve), the top flange of the stem seal, which supports the seal, includes two anti-rotation tabs to prevent rotation of the stem seal during operation of the valve. The outer seal lip contact band will under internal pressure be progressively extended, and the lip angle will ensure that the seal is in the elastic range at both high and low pressures. The seal lip preferably comprises a silver-plated coating to ease assembly and enhance gas sealing properties. With the addition of a second seal lip, the metal seal may also be capable of sealing against high external pressure in ultra-deep waters in the range between 2000 m and 4000 m.

Brief description of the drawings

These and other characteristics of the invention will become clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein:

Figure 1 is a sectional drawing of a ball valve in which an embodiment of the stem seal system according to the invention is installed, the section taken in the plane defined by the valve flow axis X-X and the stem longitudinal axis Y-Y (i.e. the valve vertical axis);

Figure 2 is an enlarged view of the area marked "A" in figure 1, and shows i.a. an embodiment of the stem seal system according to the invention;

Figure 3 is a perspective view of an embodiment of the stem seal system according to the invention, illustrating an embodiment of the stem seal according to the invention installed on a portion of a valve stem having a seal-engaging region according to the invention (a section of the stem seal having been removed to illustrate seal faces);

Figure 4 is a sectional drawing of an embodiment of the stem seal system according to the invention, illustrating the stem seal system in an energized state in which the stem and stem seal are installed in a gland, the section taken along the stem longitudinal axis Y-Y;

Figure 5 is a part-sectional elevation view of the stem seal and a portion of the stem; illustrating the stem seal in an installed position on the stem;

Figure 6 corresponds to figure 4, but shows a state in which the stem seal is not engagement (i.e. the seal system is not energized);

Figure 7 is an exploded view of the parts illustrated in figures 4 and 6;

Figure 8 is a perspective view of an embodiment of the stem seal according to the invention;

Figure 9 is a perspective sectional drawing of the stem seal illustrated in figure 8, the section taken along the stem longitudinal axis Y-Y (cf. figure 6);

Figure 10 is an enlarged plan view of the area marked "B" in figure 9;

Figure 11 is a side view of an embodiment of the stem according to the invention, intended for rotational motion;

Figure 12 is a sectional drawing of the section F-F in figure 11;

Figure 13 is an enlarged view of the area marked "G" in figure 12;

Figure 14 is a sectional drawing of a gate valve in which an embodiment of the stem seal system according to the invention is installed, the section taken in the plane defined by the valve flow axis X-X and the stem longitudinal axis Y-Y;

Figure 15 is a perspective sectional drawing of another embodiment of the stem seal according to the invention, the section taken along the stem longitudinal axis Y-Y (cf. figure 17);

Figure 16 is a perspective sectional drawing of yet another embodiment of the stem seal according to the invention, the section taken along a stem longitudinal axis;

Figure 17 is a side view of another embodiment of the stem according to the invention, intended for linear (and reciprocal) motion; and

Figure 18 is a side view of yet another embodiment of the stem according to the invention, intended for linear (and reciprocal) motion.

Detailed description of a preferential embodiment

The following description will use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, ”upper”, “lower”, “inner”, “outer”, “forward”, “rear”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader’s convenience only and shall not be limiting.

Figure 1 and figure 2 (the latter being an enlargement of the area "A" in figure 1) show an embodiment of the invented stem seal system installed in a ball valve 100. The valve may for example be a valve designed for subsea use (the illustrated valve corresponds to figure B5 of the standard API (American Petroleum Institute) 6D SS), but the invention shall not be limited to such applications. The ball valve 100 comprises a valve body 101 having a through-conduit flow passage 101a. A ball (obturator) 106 comprises a through-conduit bore 106a and is rotatable in the flow passage 101a by rotation of a stem 200. Such operation is well known and needs therefore not be described in further detail. The valve also comprises a closure 104 and a bonnet 102; components that a well-known in the art. A gland 103 is connected to the valve body and comprises a stem bore 107, in which a secondary stem seal 105 is arranged. The stem 200 extends through the stem bore 107 and is connected to the ball 106 via a ball interface portion 202. The stem free end, extending outside the stem bore 107, comprises tool interface portion 201 by means of which the stem may rotated to open or close the flow passage 101a. Also arranged in the stem bore is a metal stem seal 300, which is described in further detail below.

In a typical seal arrangement in a subsea valve, the metal stem seal 300 is defined as a primary barrier against internal pressure. The secondary stem seal 105 may typically comprise spring-energized polymeric lip seals of PTFE (polytetrafluoroethylene)-based material, with support rings in PEEK (polyether ether ketone). External seal against sea water will also typically include two barriers.

Figure 3 shows how the stem seal 300 is installed on a seal-engaging region 206 on the valve stem 200. The seal-engaging region 206 on the stem is defined by first and second (e.g. upper and lower) limits 203 and 204, respectively, forms a raised portion extending around the stem and being coated by a hard material, such as a metal or a ceramic having a high hardness suitable for the application at hand. One example of such material, particularly suited for subsea valves, is tungsten carbide. It should be understood that the mentioned "raised portion" is a stem-circumferential portion having a diameter greater than the diameter of the stem outside of the first and second limits. The seal-engaging region 206 is described in further detail below.

Figure 5 shows a corresponding state to that of figure 3, and reference number 305 designates the stem-engaging region on the seal 300. This stem-engaging region 305 and the seal-engaging region 206 on the stem, together define the sealing region S. Although the sealing region S and the seal-engaging region 206 (figures 3 and 5) thus far have been described with reference to a rotating stem (i.e. for use in a ball valve), it should be understood that the same principles and features apply to reciprocating stems (i.e. for use in e.g. a gate valve). Such stems are described below, with reference to figures 17 and 18.

Figure 4 shows the stem seal system in an energized state. The stem (here: a rotatable stem) and stem seal are installed in a gland 103. Figure 6 corresponds to figure 4, but shows a state in which the stem seal 300 is not fully engaged with the gland 103, and the stem-engaging region 305 and the seal-engaging region 206 are not fully engaged. In this position of the stem seal and the stem, the seal system is not energized. In the case of a gate valve or an actuator, the stem seal may be either fully engaged with the stem during the complete stroke, or it may be energized in end positions, fully open and fully closed valve, to provide metal back-seat sealing in these positions.

Turning now to figures 8, 9 and 10, an embodiment of the stem seal 300 will be described in further detail. The stem seal 300 comprises a generally circular body with a through-going bore 310, and is formed of a metal or metal alloy, such as an agehardenable, high-strength, nickel alloy. For typical subsea applications, the seal material may be chosen according to the commonly known standard UNS (Unified Numbering System) N7725 (nickel-chromium-based alloy for high-temperature application in sour service.

The stem seal 300 comprises an outer seal lip 303, configured for sealing against the stem bore (formed in e.g. a bonnet or gland), and an inner seal ring 304, configured for sealing against a stem. The seal lip 303 and the seal ring 304 are interconnected via a shoulder 302. Holes 308 in the upper portion of the seal are threaded (not shown), and are designed for interface with tools for disassembly of the seal. Reference number 308 denotes a stabilizing section, and a stem-engaging section 305 forms a raised ring on the inner wall of the seal ring 304, i.e. inside the through-going bore 310. It should be understood that the mentioned "raised ring" is a circular structure forming a portion on the seal inside that has a diameter smaller than the diameter of the seal ring inner wall. As the illustrated embodiment is configured for installation in a valve in which the stem may be rotated (e.g. in a ball valve), the shoulder is furnished with tabs 301 that will abut with a complementary structure (not shown) within the stem bore, whereby rotation of the stem seal is prevented when the stem is operated (rotated). Similar tabs may be advantageous for a seal installed in gate valve in which the stem also may be rotated (e.g. for a rotating-stem type gate valve).

Referring in particular to figure 10, the seal lip 303 and an outward region of the seal ring 304 comprises a coating 309 of a lubricating and sealing material. In the illustrated embodiment, in a valve for subsea hydrocarbon applications, the coating 309 comprises a layer of silver plating. This material is particularly advantageous in hydrocarbon applications, as the silver plating provides an enhanced metal-to-metal sealing function, even for gases. A silver plating coating is furthermore advantageous during valve assembly, as is has a lubricating effect which enhances the assembly process. It also allows repeated disassembly and assembly. The coating 309 covers the seal lip 303 extends between boundaries SP1and SP2, i.e. covering virtually the entire seal lip.

Figure 10 also shows how a layer 307 is arranged on the stem-engaging region 305. The layer 307, hereinafter referred to as the "seal coating", comprises a coating of a hard material, such as a metal or a ceramic having a high hardness suitable for the application at hand. One example of such material, particularly suited for subsea valves, is tungsten carbide. It should be understood that hardmetals with hardness properties similar to those of tungsten carbide, may also be used.

The seal-engaging region 206 on the stem 200 will now be described in further detail, with reference to figures 11 to 13. Figure 11 shows a rotating stem (i.e. for use in a ball valve), having a seal-engaging region 206 defined by first and second limits 203 and 204. Referring to figure 13, the seal-engaging region 206 is defined as a region C which is comprises a layer 207 of a hard metal coating, hereinafter referred to as the “stem coating”. The stem coating 207 comprises a material similar to that of the seal coating 307, as described above (i.e. for example tungsten carbide or an equivalent material). The stem coating 207 extends between end points 203 and 205. A first portion Caof the coated region C has a first stem diameter d1which is greater than the stem diameter d2outside of the second coated portion Cb. The second coated portion Cbis outwardly inclined (inclination angle α) and thus forms an entering portion for the stem seal when the seal is energized. The transitions T1, T2between the first and second diameters d1, d2, are preferably polished and rounded. Reference number 205 denotes the sealentering region (conical). The thicknesses of the stem coating 207 and the seal coating 307 are determined based on applicable design parameters and standards, defined by the coating process.

Figure 14 shows an embodiment of the invented stem seal system installed in a gate valve 400. The valve may for example be a valve designed for subsea use (the illustrated valve corresponds to figure B2 of API 6D SS), but the invention shall not be limited to such applications. The gate valve 400 comprises a valve body 401 having a through-conduit flow passage 401a. A gate (obturator) 406 is movable into and out of the flow passage 401a by movement of a stem 200, as described above (e.g. reciprocal movement). Such operation is well known and needs therefore not be described in further detail here. The gate valve also comprises a spool 407, a bonnet 402 and a gland 403; components that a well-known in the art. A stem bore 407, in which a secondary stem seal 405 is arranged. The stem 200 extends through the stem bore 407 and is connected to the gate 406 via an interface portion (not shown in figure 14). The stem free end, extending outside the stem bore 407, comprises tool interface portion 201 by means of which the stem may be moved reciprocally and/or rotated to open or close the flow passage 401a. Also arranged in the stem bore is a metal stem seal 300’, an example of which is illustrated in figure 15. It will be seen that the stem seal 300’ does not have anti-rotation tabs (e.g. as reference number 301 in figure 9), but it should be understood that the stem seal 300’ is similar to the stem seal 300 in all other aspects. The description above with reference to figures 3 to 13 shall therefore also apply to the embodiment of the seal 300’ illustrated in figure 15.

Figure 16 illustrates yet another embodiment of the stem seal 300” according to the invention. This embodiment of the stem seal 300” differs from the stem seal embodiments 300 and 300’ in that the anti-rotation tab is absent and in that it additionally comprises a second seal lip 306, extending in the opposite direction from that of the seal lip 301. Although not illustrated, it should be understood that the second seal lip 306 may comprise a coating similar to the coating 309 on the seal lip 303. Apart from these differences, it should be understood that the stem seal 300” is similar to the stem seals 300 and 300’ in all other aspects. The description above with reference to figures 3 to 13 shall therefore also apply to the embodiment of the stem seal 300” illustrated in figure 16. This stem seal 300” is particularly favorable for use in water depths of between 2000 m and 4000 m. In this "double lip seal" configuration, the seal lip 303 (when bearing against the gland or bonnet) forms an internal pressure barrier, while the second seal lip 306 (when bearing against the gland or bonnet) forms an external pressure barrier.

Figures 17 and 18 illustrate embodiments of the stem according to the invention, intended for linear (and reciprocal) motion.

The stem 200’ shown in figure 17 comprises two seal-engaging regions, wherein a first seal-engaging region 206’a is arranged at an upper end of the stem, and second sealengaging region 206’b is arranged at a lower end of the stem. The two seal-engaging regions (both comprising respective stem coatings 207, as described above). The first (upper) seal-engaging region 206’a comprises two, oppositely arranged seal-engaging regions 205 (conical), one on each side of the first portion Caof the coated region C with the first stem diameter d1(see figure 13). The second (lower) seal-engaging region 206’b may be similar to the seal-engaging region 206 described above with reference to figure 11. The seal is thus energized twice when the stem is move axially (reciprocating)

In the stem 200” shown in figure 18, the seal-engaging region 206” (comprising the stem coating 207, as described above) covers the entire stem stroke length. The seal is thus energized once, when encountering the entering region 205.

With all of the embodiments of the stem seal described above, the invention provides a novel seal coating 307 of a material having a high hardness suitable for the designed application (for example tungsten carbide, as described above) on the stem-engaging region 305. The invented seal provides a large and robust seal ring, and the entire seal is operating the elastic range (during installation and during operation). There are no permanent deformations. The invention provides a dual seal, with a dynamic seal on inside (facing the stem) and a static sealing on the outside (facing the bonnet or gland).

Possible applications for the invented seal systems comprise:

a. Gate valves (slab gate and double expanding gate valves) and actuators for SPS (Subsea Production Systems). Design code (example): API 6A/ISO10423, typical pressure ratings API 6A: 5k, 7.5k and 10k with typical temperature rating: -46 ºC to 177 ºC .

b. Ball valves for SPS. Design code Example): API 6A/ISO10423, typical pressure ratings API 5k and 7.5 k and temperature rating range from -46 ºC to 150 ºC.

c. Ball valves for STS (Subsea Transport Systems) Design code (example): API 6DSS/ISO14723, typical pressure ratings ASME class 900, 1500 and 2500 and temperature rating range from -29 ºC to 121 ºC.

In the illustrated embodiments, the seal’s stem-engaging region 305 and the stem’s sealengaging region 206 comprise respective coatings 307, 207, as described above. In the case where the seal coating 307 and stem coating 207 comprises respective layers of tungsten carbide, these layers are applied by means of any known coating method, such as a High Velocity Oxygen Fuel (HVOF) method according to the NORSOK standard M-630 EDS NHF2. The coating 309 (silver plating or similar) provides a fully gas-tight metal seal. In a practical subsea applications, an external design pressure may for example be 200 bar for the outer seal lip 303. The double seal lip design (figure 16) may for example have a design pressure of 400 bar (external pressure). But the invention shall not be limited to such design pressures.

In a typical subsea application, the materials of the bonnet and/or gland may be either ASTM A694 grade F60/F65 with Alloy 625 clad on the seal face against the outer seal lip, or ASTM A182, grade F5525%Cr Duplex (ASTM = American Society for Testing and Materials). In the seal arrangement in subsea valves, the metal stem seal is normally defined as the primary barrier against internal pressure. A secondary stem seal may typically include spring energized polymeric lip seals of PTFE based material, with support rings in PEEK. External seal against sea water will also be built up of two barriers, where the metal stem seal may be defined as primary.

Although the invention has been described with reference to valves designed for handling of fluids produced from subsea subterranean hydrocarbon reservoirs, the invention shall not be limited to such applications. The invention is applicable to both dynamic and static operations.

Although the invention has been described above with reference to a stem being arranged in a stem bore in a gland, it should be understood that the stem bore may also be formed in a gland or other parts of a stem-operated valve.

Although the invention has been described with reference to a ball valve and a gate valve, it should be understood that the invented stem seal system is equally applicable for other valves as well, in which the obturator is operated by a stem.

Claims

1. A metal seal (300), comprising a circular seal ring (304) forming a throughgoing bore (310) which is configured for receiving at least a portion of a stem (200), and a circular seal lip (303) arranged externally of said seal ring; a stem-engaging region (305) which is arranged circumferentially on the seal ring, said stem-engaging region (305) comprising a portion (305) having a seal coating (307) of a hard material, such as a metal or a ceramic having a high hardness, characterized in that the seal ring (304) and seal lip (303) extend in the generally same direction along a seal axis (Y-Y), and the seal lip (303) comprises a coating (309) of a lubricating and sealing material, whereby the assembly procedure and sealing properties are improved.

2. The metal seal of claim 1, wherein the seal coating (307) comprises tungsten carbide.

3. The metal seal of any one of claims 1-2, wherein the coating (309) comprises a layer of silver plating.

4. The metal seal of any one of claims 1-3, wherein the seal ring (304) and seal lip (303) extend in the generally same direction along a seal axis (Y-Y), forming a U-shaped profile on the seal.

5. The metal seal of any one of claims 1-4, further comprising a second circular seal lip seal (306) extending in a generally opposite axial direction to that of the seal ring (304) and seal lip (303).

6. A metal-to-metal valve seal system, characterized by

- the metal seal according to any one of claims 1-5, and

- a stem (200; 200’; 200”) configured for operating an obturator (106; 406) in a valve flow passage (101a; 401a); said stem further comprising at least one stemcircumferential region (206; 206’a, 206’b; 206”) having a stem coating (207) of a material similar to that of the seal coating (307).

7. The seal system of claim 6, wherein stem coating (207) extends between axially (Y-Y) spaced apart end points (C1, C2) on the stem, and wherein a first coated portion (Ca) of the region has a first stem diameter (d1) which is greater than a stem diameter (d2) outside of a second coated portion (Cb).

8. The seal system of claim 6 or claim 7, wherein the second coated portion (Cb) is outwardly inclined at an inclination angle (α) to form an entering portion for the stem seal when the seal is energized.

9. The seal system of any one of claims 6-8, wherein the seal coating (307) and the stem coating (207) are configured to abut and form a sealing interface when the stem and seal are installed and energized.