GB2329432A - Seal for pump shaft with adjustable barrier fluid pressure - Google Patents

Abstract

A seal assembly for a pump includes stator and rotor components 28, 20 annularly and coaxially disposed about the pump shaft 23 axially adjacent to the impeller with complementary axially tapered surfaces 27, 25. One of the complementary surfaces (25) is formed with a continuous groove 26 which extends in a spiral configuration having open entry and exit mouths at opposite axial ends. The complementary surfaces positively pump a barrier fluid toward the impeller fully across the surface upon rotation of the rotor. A device in the housing acts on one of the rotor and stator components to adjust the clearance between the complementary surfaces for regulating the pressure of the barrier fluid, the device consisting of a nut 38 for adjustment of the bias of a spring 32 in the embodiment of Fig. 1.

Description

Title; SEAL FOR PUMP, WITH PRESSURE RELIEF ADJUSTMENT This invention concerns fl development of the technology as described in patent publication W0-95/35457.

In WO-95/35457,there is shown a shaft seal for a rotary pump, in which tapered sleeves are arranged in rotor/stator association. The rotor sleeve is provided with a spiral groove, and the groove serves to pump a barrier liquid from a reservoir at an entry-end of the groove, along the groove, to an exit chamber at the other end.

In WO-95/35457, it is arranged that the exit chamber is adjacent to the process chamber of the pump. The designer might arrange that the exit chamber is open to the process chamber, but it is usually preferred to arrange that the exit chamber be sealed from the process chamber. For this purpose, the designer may favour a seal of the mechanical seal type.

In WO-05/35457,tapered sleeve are shown in various configurations of rotor and stator, including, in Fig 1 thereof, a configuration that may be described as a pressure-relief configuration. In that configuration, the axially-slidable one of the tapered sleeves is spnng-biassed in the direction towards the higher-pressure exit chamber. As the pressure in the exit chamber rises, the slidable sleeve is increasingly pushed back against the spring, until, at a certain pressure in the exit chamber, the force on the slid able sleeve is sufficient to push the slidable sleeve herd enough that the two sleeves break apart. When that happens, the pressure in the exit chamber drops. Thus, during operation, an equilibrium develops at a certain pressure in the exit chamber, the pressure being determined by the force of the spring.

When this happens, the friction torque drops to virtually nothing. The tiny amount of heat generated in the seal is easily dissipated. The barrier liquid can even be allowed to be static, i.e not to circulate. This can simplify piping etc, although ports usually need to be provided to ensure that air bubbles can be bled out of the various chambers.

The invention will now be further described with reference to the accompanying drawings, in which: Fig 1 is a cross-section of an impeller shaft for a rotary pump, which embodies the invention; Fig 2 is a corresponding cross-section of another pump that embodies the invention; Fig 3 is a corresponding cross-section of another pump that embodies the invention; Fig 3a is a corresponding cross-section of another pump that embodies the invention; Fig 4 is a corresponding cross-section of another pump that embodies the invention; Fig 4a is a corresponding cross-section of another pump that embodies the invention; Fig 5 is a corresponding cross-section of another pump that embodies the invention; Fig 6 is a corresponding cross-section of another pump that embodies the invention; Fig 7 is a corresponding cross-section of another pump that embodies the invention In Fig 1, the spring force on the axially-slidable sleeve, which in this case is the stator sleeve, can be adjusted from outside the pump. In Fig 1, the adjustment is effected manually. In fact, the spring force on the sleeve can be adjusted manually even while the pump is running.

Fig 1 shows a shaft-seal arrangement for a rotary pump. In Fig 1, a rotor sleeve component 20 is clamped to a shaft 23 by means of grub screws 24. A tapered surface 25 of the rotor 20 is provided with a spiral groove 26, as explained in W0-95/35457. The surface 25 is in hyd,odyna.mic bearing relationship with plane tapered surface 27 of a female stator sleeve 28, the surfaces being separated by a bearing film. The film is of a barrier liquid, which is supplied from a reservoir to an entry chamber 29 via port 30.

The stator sleeve 28 is mounted for axial sliding. A spring 32 pushes the stator sleeve 28 to the right. and therefore pushes the plain tapered surface 27 of the stator towards contact with the grooved tapered surface 25 of the rotor. The hydrodynamic film that develops between the surfaces prevents actual metal-to- metal contact. The barrier liquid is selected on the basis that the film should have enough viscosity to prevent contact, although in many cases the requirement for viscosity is so easily met that the barrier liquid can be water, or a mixture of water and a thin lubricant.

When the rotor turns, the barrier liquid is driven by the groove from the entry chamber 29 towards exit chamber 34. The exit chamber 34 is sealed from an inner process chamber 35 of the pump by means of a mechanical seal 36.

When the pressure in the exit chamber rises, the pressure acts on the slidable stator sleeve 28, and urges the stator sleeve to the left. The stator sleeve is biassed to the right by means of a biassing spring 32. The pressure in the exit chamber rises until an equilibrium condition of the stator sleeve is reached, in which the force due to the exit chamber pressure pushing the stator sleeve to the left is just balanced by the force due to the spring 32 pushing the stator sleeve to the right. At that condition, the sleeves are in a pressure-relief condition, and the pressure in the exit chamber cannot rise higher.

It is recognised that the pressure-relief condition, as described, is desirable as a running condition. In the pressure relief condition, the tapered surfaces of the sleeves are just on the point of breaking contact. The hydrodynamic film holds the sleeves well apart in this condition, in that the film maintains itself to a thickness that is just enough to support the pressure in the exit chamber.

The pressure at which the pressure-relief condition is reached is determined by the force of the spring 32. (Actually, spring 32 is made up of a number (e.g ten) springs disposed around the circumference of the stator sleeve 28. The springs reside in holes 37 in the sleeve 28.) The left end of the spring 32 engages a stator adjustment ring 38. It is arranged that the adjustment ring 38 can be adjusted axially. The ring 38 is in screw-thread engagement with the housing 39. The housing 39 bolts to the fixed frame 40 of the pump. When the ring 38 is turned, the ring moves left or right relative to the housing. Thus, the force developed by the spring 32 acting on the stator sleeve 28 is adjusted by turning the adjustment ring 38.

Pin 42 serves to turn the stator sleeve 28 In unison with the adjustment ring 38.

Thus, the sleeve 28 is a stator only in contrast to the rotor component; the stator sleeve 28 can turn when the adjustment ring 38 turns. After adjustment has beon effected, the adjustment ring 38 is locked to the housing 39 by means of a grub screw 43.

The pressure in the exit chamber 34 at which the stator sleeve 28 backs off from the rotor sleeve 20 can therefore be adjusted by screwing the ring 38 in and out.

Turning the screw is done manually. Adjustment can even be effected while the pump is running (though of course making adjustments in the vicinity of a rotating shaft would require that the usual safety precautions be taken).

Port 45 provides access to the exit chamber 34, whereby the pressure in the exit chamber during running can be measured on a pressure gauge, and adjustments made accordingly. Generally, the pressure in the exit chamber 34 should be higher than the pressure in the inner process chamber 35, so that, if the mechanical seal 36 should leak, the leakage is or barrier liquid into the process, rather than the other way round.

For the best service life of the mechanical seal 36, the pressure differential across the seal 36 should be a minimum. A gauge may be provided for indicating the pressure in the inner process chamber 35, and the operator adjusts the ring 38, which raises the pressure developed in the exit chamber 34, until the pressure in the exit chamber is just a little higher. Calibrations may be marked in relation to the movement of the ring 38, to serve as an indication of the pressure developed in the exit chamber. Such calibrations would not be accurate, but might provide a useful initial setting which can be made before the pump is running; the setting would be refined during running, as the pressure actually being developed in the exit chamber was measured and indicated.

If running conditions changed, e.g if the operational process pressure requirements were to change, the operator can simply re-adjust the ring 38 to suit the new pressure, and the mechanical seal 36 again sees only a small pressure differential.

The designer should see to it that the range of movement of the adjustment ring 38 relative to the housing 39 is fairly long. The ionger the range, the finer the degree of control over the force on the spring 32, and therefore of the pressure in the exit chamber. The designer should ensure that even when the adjustment ring 38 is at the extreme left of its travel, that still some force remains on the spring 32, loading the rotor and stator sleeves together, as otherwise the hydrodynamic film might not be initiated. The iower half of Fig 1 shows the adjustment ring 38 adjusted leftward to its maximum extent.

The sub-assembly, comprising the rotor component 20, the stator sleeve 28, the adjustment ring, the mechanical seal, and the housing 39, make up a self-contained cartridge unit. This cartridge unit is assembled and set up in the factory. To assemble tho cartridge into the pump, the housing 39 is bolted to the frame 40, and the grub-screws 43 are tightened to the shaft. It is simple for the designer to ensure that the rotor is locked at the correct position relative to the stator. An installation-spacer 46 is provided, which fits between grub-screw-ring 47, and the entry-chamber-seal 48. The installation instructions call for the adjustment-ring 38 to be screwed to the rightwards extremity (as shown in the upper half of Fig 1) during installation. After installation, i.e after the grub screws 24,43 have been locked, the spacer 46 is discarded, and the adjustment setting of the ring 38 can be carried out as required. The internal components of the cartridge are not handled at all during installation; installation requires only a minimum of skill and attention, which is useful when the pump is at a remote location.

Fig 2 shows another arrangement, in which the screw-threaded adjustment ring 49 is separate from an intenmedja.e ring 50. The intermediate ring has a key, which engages a slot 52 in the housing 53, whereby the intermediate ring 50 and the stator sleeve 54 are not required to rotate with the adjustment riny 49 during adjustment.

The designer can provide for the automation of the movement of the adjustment ring, if desired, by providing a suitable means for motorising the rotation of the adjustment sleeve.

Alternatively, the adjustment of the force on the stator sleeve (and therefore the adjustment of the pressure in the exit chamber at which the stator sleeve backs off, and relieves that pressure) can be automated in the manner as will now be described.

Fig 3 is a section of another pump having a shaft seal of the type as described in W0-95/35457. Fig 3 is similar to Fig 1 except that in Fig 3 the stator adjustment ring 56 is moved, not by being screwed manually, but under the action of a hydraulic piston.

Hydraulic pressure in the line 57, and in the cylinder 58, turns the adjustment ring 56 into a piston. The piston force moves the ring 56 to the right, the piston force adding to the force of the spring 59 which is tending to push the stator sleeve 60 to the right.

The hydraulic pressure in the line 57 can be derived from the pressure in the exit chamber 62, or from the pressure in the inner process chamber 63, or from the pressure in the process chamber outlet 64 (the pressure in the inner process chamber sometimes can be no more than a quarter of the pressure in the process chamber outlet), or from an outside source.

Using hydraulic pressure to set the biassing load on the stator sleeve 60 can be useful during start-up. It may be noted that the hydrodynamic film is non existent at slow speeds, whereby, undssirably, the yrooved tapered surface on the rotor might make direct metal-to-metal contact against the plain tapered surface on the stator, at slow speeds. Therefore, it can be desirable to keep the contact force low, until running speed has been attained Thus, the load on the spring should be kept low until the shaft is up to running speed. The designer can arrange for the pressure in the cylinder 58 to be derived from the pressure in the exit chamber 62.

As the rotor speed builds up, pressure builds up in the exit chamber 62, and the cylinder 58 moves to the right against the force of the spring 59, compressing the spring 59, and thereby increasing its force on the stator sleeve 60, as the rotor speed builds up.

The force acting on the stator sleeve 60 should not be allowed to go on increasing in proportion to the pressure in the exit chamber, without limit. The designer can arrange for the ring/piston 56 to strike a stop 65, i.e for the piston to reach the end of its travel, at a point where the spring S9 can still act with elastic resilience on the stator sleeve 60. Once the piston 56 has reached the stop 65, the force on the spring 59 remains constant, and the pressure corresponding to that spring force remains as the relief pressure for the exit chamber 62.

Fig 3a shows the provision of a setting-up plate xx, by which the travel stops of the cylinder 58 can be adjusted and set.

In some cases, it may be arranged that the pressure at which the exit chamber relieves should be dictated by the process pressure, at 64, or (preferably) by the pressure in the inner process chamber 63. In that case, the cylinder 58 can be fed with hydraulic pressure frorn the inner process chamber 63. In that case, also, the spring 59 might be dispensed with. in that the force on the stator sleeve at which the stator sleeve moves to the left, end relieves the pressure in the exit chamber, is the force arising directly from the inner process pressure acting on the piston 56.

In that case, even though there is no spring, the stator sleeve is nevertheless in pressure-relief configuration, in that the stator sleeve can move (i.e back off) to the left, to relieve the pressure in the exit chamber, if the pressure in the exit chamber should give rise to a leftwards force on the sleeve greater than the rightwards force on the sleeve arising from the pressure in the cylinder 58. However, the no-spring option is not preferred; the tapered sleeves should be loaded together, if only very lightly, at zero and low speeds, otherwise the hydrodynamic film might not initiate itself at all.

Fig 4 shows a version of the adjustable pressure-relief configuration of the tapered sleeves, in which the exit-chamber pressure at which the sleeves relieve is set, not be a spring, but by an imposed hydraulic pressure. In Fig 4, a pressure source is derived from the line 68, which contains the pressure in the exit chamber, and from the lines 69 and 70, which contain pressure in different parts of the process chamber. The pressure source is measured and regulated, in a pre-determined manner, end applied to the cylinder 72. It should be noted that the spring 73 in Fig 4 is very light, end is present only to ensure an inItial bias of the sliding stator sleeve to the right, to ensure the sleeves are in contact even when no pressure is applied. That is to say: the force urging the stator sleeve to the right, i.e the pressure relief force, in Fig 4, during normal running, at speed, is derived almost wholly from the pressure applied to the cylinder 72, and not from the spring 73.

Fig 4a shows a bracket 74, which is used to ensure that the rotor and stator lie in their correct relative positions prior to start-up. The bracket must be removed before the motor is switched on to rotate the shaft.

It should be pointed out that the rotor and stator should be set up so that the sliding stator sleeve is free to slide far enough to perform its pressure relief function. The rotor should be secured to the shaft at such a location that the axial force loading the stator sleeve and the rotor sleeve together is the force derived from the pressure in the cylinder; the cylinder should not, for instance, be allowed to move so far to the right that the cylinder bottoms against the housing. The bracket 74 serves to ensure the components are in their correct axial relationships prior to start-up, whereby when the components have been clamped into position, the bracket removed, and the shaft up to running speed, the cylinder is free to travel to perform its pressure-relief function.

In an alternative version, a system can be provided having a combined manualadjustment/ automatic-adjustment of the force acting on the stator sleeve.

Fig 5 shows a version of the adjustable pressure-relief system, in which the sleeve that moves to relieve the excess pressure in the exit chamber 75, is the rotor sleeve 76. The stator sleeve 78 remains fixed in the housing 79. The rotor sleeve 76 is biassed to the right by means of a spring 80. The force on the spring is adjusted by adjusting the position of an adjustment ring 82. The adjustment ring, once set in position on the shaft 83, can be secured to the shaft by means of a grub screw.

Alternatively, the adjustment ring may be secured by means of a spring 80, which is tightly coiled around the shaft. The other end of the spring 80 may be rotated on the shaft, to open the coils and permit the spring to move on the shaft, by means of a turn ring 84. Of course, in this case, the position of the rotor sleeve 76 cannot be adjusted while the shaft is rotating.

Fig 6 shows a double-tapered-sloeve or back-to-back-tapered-sleeve configuration.

Here, the tapered sleeves serve as the sole bearings for the shaft; i.e there are no other bearings on the shaft. Here, the pressure applied to the cylinder 85, and to the piston Isleeve component 86, is derived from the exit chamber 87.

Alternatively, or additionally, the pressure can be derived from the intermediate chamber 89. It will be understood that the other ways of providing for adjustment of the biassing force on the sleeve, as previously described, may be applied in respect of the double-tapered-sleeve configuration also.

Fig 7 shows a magnetic-drive pump. A feature of such pumps is that there are no rotary seats in respect of which, if the seal were to faii, process fluid could leak out to the environment. In conventional mag-drive pumps, one major problem has been in how to provide bearings for the impeller shaft. The double-tapered-sleeve assembly, in pressure-relief configuration, is especially suitable for installation in a mag-drive pump.

Claims (2)

1. Seal assembly apparatus for a rotating shaft having an impeller and mounted for rotation within a housing comprising a stator component and a rotor component adapted for rotation about the axis of rotation of the shaft, characterized in that the apparatus includes the following features, in combination: the stator and rotor components are annularly and coaxially disposed about the shaft axially adjacent to the impeller and have complementn axially tapered surfaces for fitting together in a male-female configuration, the rotor being secured for rotation with the shaft and the stator being secured to the housing; one of the complementary surfaces is formed with a continuous groove which extends in a spiral configuration around the surface, the spiral groove having open entry and exit mouths at opposite axial ends, and the complementary surfaces are configured for positively pumping a barrier fluid for sealing toward the impeller fully across the surface upon rotation of the rotor; a means for receiving a barrier fluid from a source of barrier fluid, and for reliably conveying the barrier fluid to the open entry mouth of the spiral groove; and a device in the housing acting on one of the stator and rotor components to adjust a clearance between the cornplementary surfaces for reguiatzg the pressure of the barrier fluid between the complementary surfaces.

2. A seal assembly substantially as hereinDefore described with reference to and as illustrated in any one of the accompanying drawings.