WO2007107690A1

WO2007107690A1 - High pressure gas seals

Info

Publication number: WO2007107690A1
Application number: PCT/GB2007/000538
Authority: WO
Inventors: Alan O'brien
Original assignee: John Crane Uk Limited
Priority date: 2006-03-18
Filing date: 2007-02-19
Publication date: 2007-09-27
Also published as: GB0605510D0

Abstract

A seal has a first and second seal assembly and a barrier chamber (124) therebetween, each seal assembly has a mating ring (64) mounted in fixed axial and rotational relationship to a shaft (14) and a primary ring (l00)mounted in fixed rotatable relationship but moveable axially of a housing (20), a thrust ring (110) acting on one side of the primary ring urging a radial sealing face (116) of the primary ring into engagement with a radial sealing face (118) of the mating ring, one of said seal assemblies being located inboard, the thrust ring of said inboard seal assembly acting between the primary ring and an annular piston (84), the annular piston being pressure balanced such that when pressure in the barrier chamber is in excess of the pressure of the process fluid (P), the piston will be urged towards the seal chamber; and when pressure in the barrier chamber is lower than the pressure of the process fluid, the piston will be urged towards the barrier chamber (P).

Description

HIGH PRESSURE GAS SEALS

The present invention relates to high pressure gas seals which are capable of operating low to high speeds and low to high pressures.

In accordance with one aspect of the present invention, a seal comprises a first and second seal assembly mounted co-axially of one another in axially spaced relationship, to define a barrier chamber therebetween, each seal assembly has a mating ring mounted in fixed axial and rotational relationship to one of a pair relatively rotatable components and a primary ring mounted in fixed rotatable relationship but moveable axially of the other of said pair of relative rotatable components, a thrust ring acts on one side of the primary ring urging a radial sealing face of the primary ring into engagement with a radial sealing face of the mating ring, one of the sealing faces of the primary ring or mating ring having angularly spaced grooves extending part way across the sealing face from the periphery thereof open to the barrier chamber to a continuous dam formation, these grooves producing hydrodynamic lift upon relative rotation of said components, an inlet being provided to the barrier fluid chamber by which a barrier fluid under pressure may be supplied to the barrier chamber; one of said seal assemblies being located inboard, between the barrier chamber and a seal chamber containing process fluid, characterised in that the thrust ring of said inboard seal assembly acts between the primary ring and an annular piston, the annular piston being slidingly mounted and sealed with respect to said other component of the pair relatively rotatable components, the annular piston being pressure balanced such that when pressure in the barrier chamber is in excess of the pressure of the process fluid, the piston will be urged towards the seal chamber and when pressure in the barrier chamber is lower than the pressure of the process fluid, the piston will be urged towards the barrier chamber. In the seal disclosed above, when pressure in the barrier chamber is in excess of process fluid pressure, the thrust rings will urge each of the primary rings towards the associated mating ring. If however the pressure in the barrier chamber falls below the process fluid pressure, the annular piston will be urged towards the mating ring of the inboard seal assembly, to hydraulically urge the primary ring into engagement with the mating ring, ensuring that the seal is maintained, upon reverse pressurisation.

The present invention allows the use of compliant, free floating primary rings, which will minimise thermal and pressure distortion of the primary rings, as a result of which leakage of barrier fluid may be minimised. The construction of the seal according to the present invention also allows the use of axially narrow primary rings which reduces thermal gradients and thus thermal distortion. Moreover the cross section of the primary rings may be designed to control any thermal distortion that does occur.

The seal assemblies of the seal in accordance with the present invention may be arranged either back to back, where the primary rings are disposed between the two mating rings, or face to face where two mating rings or a single mating ring defining two sealing faces, are disposed between the primary rings of the two seal assemblies.

The seal in accordance with the present invention may be pre-assembled as a cartridge assembly.

The invention is now described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows a sectional side elevation of a seal in accordance with the present invention; Figure 2 shows a partial sectional side elevation of a modified version of the seal shown in figure 1

Figure 1 shows a seal cartridge 10 which may be secured to a vessel 12 which defines a sealed chamber containing a process fluid "P", to provide a seal with a rotating shaft 14 which passes through a bore 16 in the wall of the vessel 1 2.

The seal cartridge 10 comprises a housing 20 formed from two parts, an inboard housing portion 20' and an outboard housing portion 20". The inboard and outboard housing portions 20', 20" are bolted together by a plurality of angularly spaced bolts 22 and define a stepped bore 24, which reduces in diameter adjacent each end. The housing 20 is secured to the vessel 12 by means of bolts which engage through angularly spaced holes 26 which extend axially through the housing portions 20', 20". The inboard end 28 of inboard housing portion 20' has a reduced external diameter and engages in an increased diameter outer end portion of the bore 16 in vessel 1 2. An elastomeric O-ring 30 provides a seal between the vessel 12 and housing 20.

A sleeve 40 is mounted coaxially of the housing 20, the sleeve 40 extending beyond the outboard end 42 of housing 20 and has a radial clearance with the reduced diameter of bore 24 at the outboard end. Before installation of the seal cartridge 10 on the vessel 12 and shaft 14, the sleeve 40 is located coaxially of the housing 20 by means of a plurality of angularly spaced brackets 44 which are bolted to the housing 20 by bolts 46 and to a drive collar 48 secured to the outboard end of the sleeve 40 by means of bolts 50 which engage threaded apertures 52 which extend through the drive collar 48 and sleeve 40. When installed, the brackets 44 are removed and set screws 54 engage the threaded apertures 50, to clamp the sleeve 40 to the shaft 14. An elastomeric O- ring 56 engages in a groove 58 in the internal diameter of the sleeve 40, to seal the sleeve 40 with respect to the shaft 14.

A flange formation 60 extends radially from the external diameter of sleeve 40, the flange formation 60 being located centrally of the larger diameter central portion of bore 24. The flange formation 60 has annular recesses 62 one in each of its radial end faces. Annular mating rings 64 are located in the recesses 62 and sealed with respect thereto by elastomeric O-rings 66. A pin 68 engages through an axially extending bore 70 in the flange formation 60 and engages slots in the mating rings 64, to prevent rotation of mating rings 64 relative to the flange formation 60 and sleeve 40. The mating rings 64 are thereby mounted with respect to the shaft 14, in fixed axial and rotational relationship.

Adjacent the inboard end, the bore 24 reduces in diameter in two steps to produce inner and outer radial faces 80 and 82, which are separated axially from one another. A correspondingly stepped annular piston 84, slidingly engages this portion of bore 24 and is sealed with respect thereto by elastomeric O-ring 86. The inboard end 88 of piston 84 abuts the inner radial face 80, to limit movement of the piston 84 towards the seal chamber. A pin 90 is located in a bore 92 in the portion of housing 20' defining the inner radial face 80 and engages a slot 93 in the inboard end 88 of piston 84, to locate the piston 84 rotationally with respect to the housing 20, while permitting axial movement of the piston 84 in the bore 24.

At the outboard end of bore 24, the reduced diameter portion forms a single step defining a single radial face 94. Annular recesses 96 are provided in the inboard end of piston 84 and in the radial face 94. Primary rings 100 are slidingly located one in each of the recesses 96. An axially extending rib 102 is provided in the outer circumferential wall of each recess 96 for engagement of a corresponding axial recess 104 in the outer circumferential surface of primary ring 100, in order to prevent rotation of the primary ring 100 relative to the piston 84 or housing 20. A snap ring 106 is located in a groove 108 in the outer circumferential wall of each recess 96 to retain the primary rings 100 in the recesses 96.

Thrust rings 1 10 are located in the recesses 96, between the bases of the recesses 96 and the primary rings 100. The thrust rings 1 10 are resiliently biased by means of a plurality of angularly spaces compression springs 1 12 which are located in closed bores 1 14 in the bases of the recesses 96 to urge radial sealing faces 1 16 the primary rings 100 into sealing engagement with radial seal faces 1 18 of the mating rings 64. The thrust rings 1 10 are sealed with respect to the primary rings 100 and the inner circumferential wall of the recesses 96, by means of elastomeric O-rings 120, which locate in recesses 1 21 on the inner periphery of the thrust ring 1 10.

A series of angularly spaced grooves 150 are provided in the sealing faces 1 18 of the mating rings 64. These grooves 150 open to the external diameter of each of the mating rings 64, but extend only part way across the sealing face 1 18, to provide a continuous dam formation 1 52 adjacent the internal diameter of the mating ring 64. The grooves 1 50 are configured to provide hydrodynamic separation of the sealing faces 1 16, 1 18, when the shaft 14 rotates and may be configured for unidirectional operation, for example as disclosed in EP 578577, the disclosure of which is incorporated herein by reference thereto; or bidirectional operation as disclosed in EP499370, the disclosure of which is incorporated herein by reference thereto.

An inlet 1 22 is provided through the housing 20, into the barrier chamber 1 24 defined between the seal assemblies 64,100, by which a barrier fluid, for example nitrogen, steam or water, may be introduced under pressure, to the barrier chamber 124. An outlet 126 is also provided from chamber 1 24 through which barrier fluid may be drained from the barrier chamber 124, for maintenance purposes, the outlet 1 26 being plugged during normal operation of the seal.

In operation, barrier fluid at a pressure of at least 1 .δbarg above the process fluid pressure, is supplied to the barrier chamber 1 24, from a suitable source. While the shaft 14 is stationary, the springs 1 12 urge the primary rings 100 into engagement with the mating rings 64, seal face 1 16 sealingly engaging the continuous dam formation 1 52 radially inwardly of the grooves 1 50 in sealing face 1 1 8 of the mating ring 64. The balance diameter of the primary rings 100 is also such that the pressure differential between the process fluid and the barrier fluid, acting on the primay ring 1 10, will apply a load thereto, which will reinforce the load applied by the springs 1 12. Under these conditions, any leakage across the sealing faces 1 16,1 18 will be of barrier fluid from the barrier chamber 124 to the process fluid chamber.

When the shaft 14 rotates, barrier fluid is entrained into the grooves 1 50 in the mating rings 64, creating an increase in pressure at the inner ends of the grooves 150, causing the sealing faces to move apart, so that wear of the sealing faces 1 16, 1 18 is minimised. A film of barrier fluid between the sealing faces 1 16, 1 18 prevents the escape of process fluid, although there is some leakage of barrier fluid into the process fluid chamber. In accordance with the seal design, the primary rings 100 are free floating and may be made compliant, so that there is minimal pressure and thermal distortion, thereby minimising leakage of barrier fluid and dilution of the process fluid. If the pressure of the barrier fluid should fall, so that the process fluid pressure exceeded the barrier fluid pressure, then the balance diameter of the piston 84 is such that the excess process fluid pressure acting on the piston 84, will apply a load to the piston 84 towards the barrier chamber 124. This load will compress springs 1 12 and hydraulically force the primary rings 100 into engagement with the mating rings 64, and sealing faces 1 16 into sealing engagement with sealing faces 1 18, to maintain a seal.

In the embodiment illustrated in figure 2 the elastomeric O-ring 86 is replaced by a polymer seal 1 30. The polymer seal 130 is formed from a ring 132 of low friction polymer material of channel section, the walls 134 of the channel section being forced apart and into sealing engagement with opposed cylindrical surfaces of the piston 84 and housing 20, by means of a garter spring 136. In order to ensure pressure equalisation across the polymer seal 130, radially extending slots are provided in the closed radial face 138 of the ring 132.

A polymer seal 140 is also used between thrust ring 1 10 and prinary ring 100, on place of the elastomeric O-ring 120. The polymer seal 140 is of similar construction as polymer seal 130 and the same reference numerals are used for corresponding components of the seal. In order to reduce friction between the polymer seal 140 and primary ring 100, to allow the primary seal to float radially, annular grooves 142 are provided in the close radial face 144 of the polymer seal 140, to minimise contact between the polymer seal 140 and primary ring 100.

Various modifications may be made without departing from the invention. For example while in the embodiment disclosed above, each seal assembly has a separate mating ring 64, a single mating ring defining two radial sealing faces 1 18, one at each axial extremity of the mating ring, may be secured to the sleeve 40, in suitable known manner. The single mating ring will thereby be common to both seal assemblies.

Claims

1 . A seal comprising a first and second seal assembly mounted co- axially of one another in axially spaced relationship, to define a barrier chamber (124) therebetween, each seal assembly having a mating ring (64) mounted in fixed axial and rotational relationship to one of a pair relatively rotatable components (12,14) and a primary ring (100) mounted in fixed rotatable relationship but moveable axially of the other of said pair of relative rotatable components (12, 14), a thrust ring (1 10) acts on one side of the primary ring (100) urging a radial sealing face (1 16) of the primary ring (100) into engagement with a radial sealing face (1 18) of the mating ring (64), one of the sealing faces (1 16,1 18) of the primary ring (100) or mating ring (64) having angularly spaced grooves (1 50) extending part way across the sealing face (1 16,1 18) from the periphery thereof open to the barrier chamber (1 24) to a continuous dam formation(152), these grooves (150) producing hydrodynamic lift upon relative rotation of said components (12, 14), an inlet (122) being provided to the barrier fluid chamber (1 24) by which a barrier fluid under pressure may be supplied to the barrier chamber (124); one of said seal assemblies being located inboard, between the barrier chamber (1 24) and a seal chamber containing process fluid, characterised in that the thrust ring (1 10) of said inboard seal assembly acts between the primary ring (100) and an annular piston (84), the annular piston (84) being slidingly mounted and sealed with respect to said other component (12) of the pair relatively rotatable components (12,14), the annular piston (84) being pressure balanced such that when pressure in the barrier chamber (1 24) is in excess of the pressure (P) of the process fluid, the piston (84) will be urged towards the seal chamber and when pressure in the barrier chamber (124) is lower than the pressure of the process fluid (P), the piston (84) will be urged towards the barrier chamber (124).

2. A seal according to claim 1 characterised in that the primary rings (100) are free to float.

3. A seal according to claim 1 or 2 characterised in that the primary rings (100) are compliant.

4. A seal according to any one of the preceding claims characterised in that barrier fluid is supplied to the barrier chamber (124) at a pressure in excess of the pressure (P) of process fluid in the seal chamber.

5. A seal according to claim 4 characterised in that the pressure of the barrier fluid is at least 1 .5barg, above the pressure (P) of the process fluid.

6. A seal according to any one of the preceding claims characterised in that the piston (84) is slidingly sealed to its associated component (12) and/or the thrust ring (1 10) is sealed with respect to the primary ring (100) by means of an elastomeric O-ring seal (86,120).

7. A seal according to any one of claims 1 to 5 characterised in that the piston (84) is slidingly sealed to its associated component (12) and/or the thrust ring (1 10) is sealed with respect to the primary ring (100) by means of a polymer seal (130,140).

8. A seal according to claim 7 characterised in that the polymer seal (130,140) comprises a channel shaped ring (132,142), with spring means (136) located in the channel to urge walls (134) of the channel apart and into sealing engagement with opposed parallel cylindrical walls.

9. A seal according to claim 8 characterised in that the radial face (138) at the closed end of the channel shaped ring (132) has radially extending slots to permit pressure equalisation across the polymer seal.

10. A seal according to claim 7 characterised in that the radial face (144) at the closed end of the channel has annular grooves ( 142).

1 1 A seal substantially as described herein, with reference to and as shown in figures 1 and 2 of the accompanying drawings.