Patent Application
20090194949
The present invention relates to seals. More particularly, the invention relates to sealing systems for rotary shafts that include viscous sealants.
Many types of heavy rotary machinery, such as pumps, compressors and turbines, produce work by means of a working fluid enclosed within a working chamber or apply work to such a fluid. Such equipment is generally characterized by a main shaft that rotates with respect to a housing. Part of the shaft is coupled to the working fluid and part of the shaft protrudes from the housing.
At the aperture where the shaft exits the housing, there is a tendency for the working fluid to leak. To at least minimize, or better, to eliminate this leakage, the clearance between shaft and aperture perimeter is kept small and a seal is applied around the shaft and aperture. The seal is required to allow the shaft to rotate with minimal inhibition thereof, whilst blocking the space between the shaft and the aperture.
Numerous seal types are known. For low speed rotary machines, spring loaded gaskets, such as O-rings may be adequate. For high speed rotary machines, one common seal type is the mechanical seal which consists of radial planar surfaces normal to the shaft axis that are machined to low surface roughness. One surface is gasketed to the housing while a second surface is driven by the shaft and sealed thereon by a secondary seal such as a bellows. Mechanical seals of this type are generally expensive. They also have a tendency to fail catastrophically without warning. Furthermore, the repair of a faulty mechanical seal is costly and time consuming, since it generally necessitates extensive rotary machine downtime.
Another type of high speed rotary machine seal is the braided packing seal. This type of seal includes a braided rope that is wrapped around the shaft. Working fluid leaking from the housing keeps the rope moist and swells the fibers thereof. The working fluid prevents the rope from overheating and catching fire. Consequently, such seals cannot pump dry and their operation requires controlled leakage. The braided packing seals have a tendency to abrade the shaft surface, particularly during tightening and adjustment procedures. As the braided packing slowly loosens, it noticeably leaks, thereby provides early indication that maintenance is required, allowing the seal to be tightened. The rope packing material erodes relatively quickly due to friction and needs frequent replacement, often by time consuming replacement procedures.
To ensure tight fitting between the shaft and the solid seals, whether ‘O’ rings, braided packing seals, or other types of gaskets, the drive shaft must be cylindrical and accurately machined to a high surface finish. Pits and other irregularities result in leaking.
One sealing technique for reducing leakages fills the stuffing box surrounding the drive shaft with a viscous, non-Newtonian fluid under pressure. Seals of this type are designed to prevent the working fluid from leaking from the working chamber through the space between the rotary shaft and the perimeter of the aperture through which the shaft exits the housing. The viscous sealant material may be bounded by one or more flexible rings or rope seals at each end of the stuffing box, which serve to keep the sealant in place, and, because of their generally springy nature, tend to work with the fluid sealant to provide a leak-free seal.
Advantageously, there may be no need to compress these bounding seals around the shaft or into the housing because the viscous sealant fills small clearances.
By maintaining the pressure of the sealant within the stuffing box above the pressure of the working fluid in the working chamber, the sealant is pushed against the shaft. Thus, the sealant, optionally together with other seals at the end of the stuffing box traps the working fluid within the working chamber.
The pressure of the sealant in the stuffing box is required to be sufficient to promote adhesion of the sealant onto the rotary shaft, and to retard leakage of working fluid from the working chamber into the stuffing box through the shaft aperture. The sealant pressure may generally be slightly less than the working fluid chamber due to the contribution of the surface tension of the sealant adhering to the shaft, which retards the infiltration of working fluid to the stuffing box. If the pressure differential between the working chamber and stuffing box is greater than a threshold level, leakage of working fluid out of the shaft aperture may be noticeable.
As the sealant pressure is increased, the work expended as a result of the frictional forces between the rotating shaft and the sealant correspondingly increases, and this ‘lost work’, is directly proportional to the product of the shaft diameter, rotational speed of the shaft, and the frictional forces between the shaft and the sealant. Since the frictional forces between the shaft and the sealant are directly proportional to the sealant pressure, it follows that the lost work is also directly proportional to the sealant pressure.
An increase in lost work is undesirable since the overall efficiency of the rotary machine decreases as more work is lost. Furthermore, the lost work is dissipated in the form of heat energy, which causes the temperature of the sealant in the vicinity of a shaft opening to increase, risking sealant failure, and, in extreme cases, the organic sealant may even catch fire. Furthermore, heat transfer from the sealant to the working fluid may result in local boiling and cavitation within the working fluid, particularly when the working fluid is water. As the temperature increases, where the working fluid is a gas, an excessive increase in the working fluid pressure may result. This is liable to compromise the operability of the rotary machine.
It will be appreciated that although overheating may be prevented by lowering the working fluid pressure or the shaft speed, such lowering may undesirably reduce the amount of work that the machine does.
In general, therefore, as the pressure of the sealant within the stuffing box is increased, more work is required to turn the shaft and the efficiency of the system is adversely affected. Moreover, the frictional forces between the rotating shaft and the surrounding sealant erode the adhered sealant, reducing the pressure of the sealant within the stuffing box, and if the pressure drops below the pressure of the working fluid, the working fluid may leak along the drive shaft. The optimal pressure at which the sealant is maintained is typically established empirically.
Such viscous sealants are generally introduced into the stuffing box under pressure by an appropriate injection device through an aperture that can be subsequently sealed. Periodically, additional sealant must be introduced to the stuffing box to maintain the sealant pressure at a desired level. Generally the addition of such sealant and the operating pressure thereof are not optimal in that when the seal noticeably leaks, indicating too low a pressure, additional sealant is added, resulting in over-compensating generally resulting in too high a pressure. This issue is addressed in co-pending application number WO07099535A2 titled “Apparatus for delivering sealant at a predetermined pressure to a stuffing box of a shaft”.
Typically, in addition to the desired rotation about their axes, drive shafts of real systems tend to vibrate as well. It has been found that such vibrations break the adhesion between the drive shaft and sealant, causing cavities to form along the drive shaft and resulting in leakage of the working fluid. Embodiments of the present invention address this issue.
The present invention is directed to providing an improved sealing system for sealing between an aperture of a machine housing and a rotating machine shaft, protruding through the aperture, said sealing system comprising: (i) a stuffing box for encasing a segment of said shaft and the aperture of the machine housing, (ii) a viscous fluid type sealant within said stuffing box, wherein the shaft is concentric about its axis of rotation and said segment has at least one variation in its effective external diameter thereby increasing the surface area thereof, in contact with the liquid sealant.
Preferably, said segment has at least two variations in its external diameter.
Optionally, the effective external diameter of the shaft varies stepwise along at least a section of the segment.
Optionally, the effective external diameter of the shaft varies continuously along at least a section of the segment.
Optionally, the segment of the shaft has a section having a cross section along said axis having a profile selected from the group comprising crenellated, sinusoidal, dog-tooth, and jagged profiles.
Preferably sharp edges and changes of direction are avoided by beveling or curving profile of drive shaft where profile gradient changes significantly.
In one embodiment, the profile is formed on the segment of the shaft by turning the shaft on a lathe.
Alternatively, the profile is fixed to the shaft by means such as gluing, or press-fitting.
Alternatively, the shaft is cylindrical and the segment further comprises a sheath affixed around the shaft to rotate therewith, the sheath having at least one variation in external diameter along the segment thereby increasing the surface area thereof in contact with the liquid sealant.
Preferably, the sheath is sealed to the drive shaft to prevent leakage along the sheath-shaft interface.
Optionally, the effective external diameter of the sheath varies stepwise along at least a section of the segment.
Optionally, the effective external diameter of the sheath varies continuously along at least a section of the segment.
Optionally, the segment of the sheath has a cross section along said axis having a profile selected from the group comprising crenellated, sinusoidal, dog-tooth, and jagged profiles.
In one embodiment, the sheath is made of metal and the profile is formed on the sheath by a metal forming process, such as turning the sheath on a lathe or casting the sheath from a metal or alloy.
In other embodiments, the profiled sheath is injection molded from a plastic, a rubber, a resin, clay, or a composite, or formed from wood.
In yet other embodiments, the profile is fixed to the sleeve, by means such as glue, or press-fitting.
In preferred embodiments, the improved sealing system further comprises a conduit for a cooling fluid, for cooling the sealant.
Optionally, the conduit passes through the sealant within the stuffing box.
In another embodiment, the conduit passes around the inside of the stuffing box.
Optionally, the system comprises an inner sleeve for lining at least part of the stuffing box, wherein said conduit is defined by an inner surface of the stuffing box and an outer surface of the sleeve.
Optionally, the conduit is further defined by spacer ribs between the inner surface of said stuffing box and the outer surface of the sleeve.
Preferably the conduit is retrofittable into existing stuffing boxes.
In a preferred embodiment, the sealing system further comprises a sealant injector.
Preferably the sealant injector is configured to be loaded by replacement sealant cartridges.
Optionally and preferably, the sealant injector is coupled to the stuffing box by a sealant injection conduit whose outlet is asymmetrical to the stuffing box.
Typically the sealing system further comprises at least one sealing ring at each end of the stuffing box.
In one embodiment, a lip is provided, extending radially from proximal end of the segment of the shaft towards the stuffing box, to serve as a support for a proximal sealing ring.
Typically said lip extends radially outwards from a proximal end of the sheath around the shaft.
Optionally a lip extends radially inwards from the stuffing box towards the shaft, the lip having a rim around an inner circumference thereof, for surrounding the shaft towards a proximal end of the sheath therearound.
Preferably a lid having an inner diameter that clears the effective diameter of the shaft is boltable to a distal end of the stuffing box to cover distal sealing ring.
In one embodiment, a flange extends radially outwards from the proximal end of the stuffing box for attaching to the machine housing.
Typically, the flange is attachable to the machine housing by bolts.
A preferred embodiment of the improved sealing system for sealing between an aperture of a machine housing and a rotating shaft of the machine protruding through the aperture, comprises: (i) a sheath for fixing to the shaft for increasing the diameter of a section thereof, (ii) a stuffing box for encasing the segment of the shaft and the aperture of the machine housing through which the shaft projects (iii) at least one sealing ring at each end of the stuffing box (iv) a sealant injector for injecting a viscous fluid type sealant into the stuffing box to fill space defined by outer surface of the sheath, inner surface of the stuffing box and inner surfaces of the sealing rings, wherein diameter of sheath varies along its length.
Preferably the sealant injector is connected to the stuffing box asymmetrically to inject sealant at a position not opposite the shaft.
Preferably the improved sealant system is a self-contained unit for retrofitting onto an aperture to a machine housing about a rotating shaft protruding therefrom.
As referred to herein, the term “shaft” relates to a rotary shaft.
As referred to herein, the term “stuffing box” refers to a cavity surrounding a segment of a shaft.
Unless otherwise indicated by context, the term sealant as used herein, refers to sealing materials that are high-viscosity non-Newtonian liquids, whose viscosity varies as a function of the shear stress applied thereto. Such sealants are generally fabricated from a blend of synthetic fibers, lubricants, and binding agents. U-PAK® injectable sealant manufactured by UTEX Industries, Inc. is an example of such a sealant.
For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings.
With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:
The sealing system 10 consists of a static stuffing box 12 around the shaft 14 of a machine (not shown). The stuffing box 12 is coupled to the housing 18 of a machine about the aperture 20 thereof, and prevents working liquid 22 escaping through the clearance gap between the shaft 14 and the perimeter 24 of the aperture 20. Shaft 14 has a constant diameter D and a smooth surface finish. By maintaining a viscous non-Newtonian fluid sealant 26 within the stuffing box 12 at a sufficient pressure PS, the sealant 26 adheres to the shaft 14 with a surface tension T.
In theory, providing the sum of the surface tension T and the sealant pressure PS exceeds the working fluid pressure Pwf, i.e. T+PS>Pwf, the seal prevents leakage of the working fluid.
The sealant 26 may be provided by a sealant injector 28 configured to maintain the pressure of the sealant 26 at a suitable level. A sealant injector 28 is described in WO07099535A2, which is incorporated herein by reference.
Now in practical rotary machine systems it has been found that the shaft 14 not only rotates, but also vibrates. Such vibrations tend to cause the shaft 14 to separate from the sealant 26 therearound, resulting in a path for working fluid leakage along the shaft 14.
To overcome adhesive failure between sealant 26 and shaft 14, the sealant pressure PS is generally more than otherwise desirable, resulting in increased work to overcome the pressure, particularly when such systems are started up or the rotation speed of the shaft 14 varies.
With reference to
In the embodiment shown, sheath 216 varies in diameter in a sinusoidal manner along a section of its length. This provides a greater surface area for sheath 216—sealant 226 adhesion therealong. Additionally, a labyrinth type sealing effect results since even if vibration of the shaft 214 and sheath 216 results in local failure of the interface between sheath 216 and sealant 226, the section of the shaft 214 with varying profile helps ensure that there is no direct pathway for leakage therealong. Therefore, the varying profile of the shaft may create a sealing surface that extends radially, and may reduce extensive run-outs if the sealant detaches from the cylindrical part of the shaft, in consequence of vibrations, for example.
It will be appreciated that the varying profile of the sheath 216 need not vary in a sinusoidal manner. Indeed, as shown in
The variation need not be regular, and, as shown in
At least theoretically, instead of the sealing system comprising a sheath around the shaft, the diameter of the shaft itself may be varied by having a change in diameter tooled thereunto, which may be one or more sloping sections, circumferential notches, lips, or one or more sinusoidal waveforms, crenellations, dog-teeth, steps or jagged sections added thereto. Practically, however, it is generally much easier to add a sheath, and this enables a sheath 216 with at least one effective change in diameter therealong, to be retrofitted to the shaft 214 in the segment 215 thereof, where the stuffing box 212 is fitted.
Thus in contradistinction to prior art shafts—viscous sealant—stuffing box systems, in the present invention, the effective diameter of the drive shaft 214 is not kept constant but is, instead, characterized by having at least one variation in its effective external diameter along the segment 215 and preferably the effective external diameter of the shaft 214 varies continuously along at least a section of the segment 215.
Where the varying diameter is formed on the shaft 214 itself, it may be machined thereunto by turning on a lathe, for example. A retrofittable sheath 216 may be turned on a lathe, but may, instead, be fabricated by other means, such as casting where the sheath is fabricated from a metal or alloy or injection molding where the sheath is fabricated from a material such as a plastic, a rubber, a resin, a clay, or a composite, or carved from wood It will be appreciated that the feature of a non-constant diameter drive shaft may be coupled with additional features to improve the performance of the sealing system and to increase the operating range thereof. For example, the sealant may be cooled by a cooling fluid, to allow the seal system to operate at faster rotation speeds.
As shown in
With reference to
To increase the effectiveness of the conduits, the effective length thereof should be maximized. The space between insert 854 and stuffing box 812 may be configured into a long pathway therearound, by addition of ribs spanning there between.
In a preferred embodiment of the sealant system 1010, shown in
In this manner, the sealing system can be refilled without switching off the machinery to which it is attached.
As explained hereinabove, and referring back to
Referring to
It will be appreciated that typically sealant rings for maintaining the viscous sealant within the stuffing box, contact lips extending from the static elements towards the shaft. Referring back to
Referring back to
The present invention is capable of much variation and features shown with some specific embodiments may be incorporated with other embodiments. Thus the scope of the present invention is defined by the appended claims and includes both combinations and sub combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.
In the claims, the word “comprise”, and variations thereof such as “comprises”, “comprising” and the like indicate that the components listed are included, but not generally to the exclusion of other components.
The present invention claims priority to Provisional Application No. 61/026,019 filed on Feb. 4, 2008.
| Number | Date | Country | |
|---|---|---|---|
| 61026019 | Feb 2008 | US |
