GB2292982A - Double mechanical seals - Google Patents

Abstract

A double mechanical seal is disclosed adapted to seal a rotating shaft 33 to a stationary piece of equipment so as to define a substantially annular barrier fluid cavity 100 surrounding the shaft 33. The seal includes rotary parts 32, 38, 39, 34, 36 and stationary parts 43, 35, 37 which are not concentric with one another. Thus, the cavity 100 includes a region of radial constriction e.g. produced by non concentric gland 43, stationary seal faces 35, 37 and gland insert 43' and slots 46. The cavity 100 includes inlet and outlet ports (not shown) for entry and exit of barrier fluid positioned on either side of the region of constriction. The seal 31 may be cartridge mounted, i.e. mounted on a sleeve 32, and the sleeve 32 is preferably profiled on its external surface in that region which comes into contact with barrier fluid contained in the cavity 100. This arrangement allows better flow of barrier fluid and greater heads of pressure, which drive the flow, to be developed. <IMAGE>

Description

DOUBLE MECHANICAL SEALS Field of the Invention This invention relates to double mechanical seals and to seal sleeves for use in such seals.

Background to the Invention Mechanical seals are used to seal the interface between two fluids - typically a liquid product and the environment - at moving components of mechanical equipment such as, for example, a pump. The seals are mounted on rotating components of the equipment, for example pump shafts. They typically comprise at least a "rotary" and a "stationary" seal face, in constant mating contact with one another. The rotary seal face rotates with the shaft on which the seal is mounted, whereas the stationary face remains in a fixed position whilst the shaft rotates relative to it.

A cartridge mounted seal is a particular type of mechanical seal, which is provided as a single unit with all the seal components mounted on a sleeve. The unit is mounted around a rotatable shaft by fitting the sleeve around the shaft.

This greatly facilitates assembly and fitting of the seal.

A double mechanical seal, which may also be cartridge mounted, is a type of mechanical seal used in certain situations where extra security against leakage is needed. For example, where a product to be sealed from the environment is noxious (eg an acidic or carcinogenic product), or where there is a large temperature difference between two fluids under seal, the extra security provided by a double seal is often necessary or at least desirable.

In functional terms, a double seal basically comprises two single mechanical seals mounted adjacent one another. It thus has a "inner" set of seal components (ie, rotary and stationary seal face), in contact with the product under seal, and a "outer" set of seal components, in contact with the outside environment. These two seals can be mounted in a socalled "face-to-face" configuration, in which the two stationary seal faces sit directly adjacent one another.

However, a double mechanical seal may also be configured in a "back-to-back", tandem or concentric sealing arrangement.

Often, a barrier fluid is introduced into the inner cavity of a double seal so as to provide an extra barrier between the two fluids under seal. The barrier fluid is usually chosen so as to be capable of neutralising the more noxious of the two fluids (typically a product to be separated from the environment). It is also usually maintained at a higher pressure, within the mechanical seal, than the pressure of the product under seal, so that if any leakage does occur (for instance, due to seal wear) this can only be in a direction from the barrier fluid towards the product and not vice versa in the direction of the environment.

A typical overall system, in which a double mechanical seal were to be used on an item of mechanical equipment, would incorporate a supply of barrier fluid to the inner cavity of the seal, and means for pressurising the barrier fluid inside the seal.

The barrier fluid, when it comes into contact with the seal faces inside the double seal, is subject to increases in temperature. This is generally undesirable, since as the temperature of the barrier fluid rises and its density decreases. Problems also arise if the barrier fluid becomes hot enough to vaporise, since a vapour is generally much harder to contain under seal than is a liquid.

Because of these temperature build-ups it is desirable to maintain a constant flow of the barrier fluid both into and out of the double seal. This is typically achieved using a closed circuit containing the barrier fluid, the fluid then being drawn by thermal convection effects (known in this case as "thermosyphon" effects) into the seal cavity, and out again, as its temperature rises at the seal faces to the cooling region of the circuit, thence to be cooled and recycled. However, a double seal normally has components and housings which are not designed to assist the flow of hotter barrier fluid out of the seal or of cooler fluid back into it.

This is largely due to the relatively small clearances which are allowed within the seal cavity. Moreover, the universal use of standard, all purpose designs of seals means that a given type of seal may be used in a variety of different operational modes (eg, for clockwise or for anti-clockwise rotation; in horizontal or vertical orientations; etc). This again does not always provide ideal conditions for a convection circuit to operate, allowing constant flow of the barrier fluid into and out of the seal.

For these reasons, convection alone is often insufficient to drive the flow of barrier fluid into and out of a double seal at a high enough rate. This leads to a build up of heat in the barrier fluid inside the seal, which is undesirable for the reasons outlined above.

This problem is at present typically overcome by introducing a pump into the barrier fluid circuit to assist circulation.

However, the use of an external pump brings with it the need for a separate power supply and control system for the pump.

If the equipment under seal is located in a hazardous area (as may be the case when the product to be sealed is itself noxious or, for example, explosive), associated specialist equipment has to be employed with the pump to take account of fire precautions and the like.

To prevent the build up of heat in fluids around normal, usually single, mechanical seals, it is sometimes possible to machine the internal seal cavities to a larger dimension, to prevent the build up of heat at the seal faces. However, a more common method of preventing heat build up is to use flow inducement devices (often known as "pumping rings") on the seal. These tend to make use of the rotation of the shaft on which the seal is mounted, and usually take the form of an additional component which may be attached to an existing, conventional, seal. Rotating with the equipment shaft, a pumping ring acts as an impeller and increases fluid circulation in the region of the seal faces without the need to rely on convection-driven circuits or an external pump.

In this way, the build-up of heat in the region of the seal faces may be avoided.

It should be noted here that, whilst the main purpose of a flow inducer is the removal of heat, it should not be confused with other cooling components typically used in mechanical seals. For example, many seal components have finned surfaces to allow them to cool more quickly, or specially machined surfaces (for instance, provided with knurled regions) which create turbulence in the surrounding fluid so as to reduce the likelihood of localised heat build-up within a seal cavity.

A flow inducer is a device capable of generating a head of pressure in the fluid surrounding it, which drives circulation of the fluid away from the seal faces. This is in contrast to other seal cooling devices, which simply create local turbulence and hence temperature uniformity.

A typical flow inducer takes the form of a ring which is locked onto the equipment shaft independently of the mechanical seal itself. Alternatively, the flow inducer may take the form of a modification to the external surface of the rotary seal face or its holder, whichever is in direct contact with the fluid in which flow is to be induced. Both types of flow inducer have profiled surfaces, known types including square form helix threads and regularly spaced slots or similar profiling. They act as impellers, inducing flow in the fluid surrounding them.

Such flow inducers only have an effect on the product fluid under seal, which is usually in direct contact with the rotary face of the seal or its holder; most seals are of the "internal" variety, in which it is the rotary face which is on the product side of the equipment. The flow inducers are thus of no use in assisting flow of a barrier fluid in a double mechanical seal, since they are not designed to come into contact with the barrier fluid. One possible exception to this is the "back-to-back" type of double mechanical seal, where the rotary seal faces face into the inner cavity of the seal which contains the barrier fluid.However, in the faceto-face form of double seal, there is at present no way of inducing flow of the barrier fluid into and out of the seal and thus preventing temperature build-up, except (as described above) relying on the thermosyphon effect or an external pump.

Another disadvantage inherent in the use of a threaded form of flow inducer is that due to the nature of the screw thread and the way in which it is produced, its correct operation is dependent on the direction of rotation of the shaft on which it is mounted. Most mechanical seals are designed to work independently of the direction of rotation of the equipment shaft, and it is thus impractical to use with these seals a flow inducer which is dependent on the direction of its rotation.

GB-2268235-A discloses a cartridge mounted double mechanical seal in which the external surface of the sleeve is profiled in that region which would come into contact with barrier fluid contained in the inner cavity of the seal. The profiling of the sleeve is such that, when the sleeve is rotating with the shaft, the pressure of barrier fluid inside the seal cavity is increased as the fluid comes into contact with the profiled sleeve surface, thus inducing flow of the fluid around the seal sleeve.

It has now been discovered that a relatively simple design change in the seal as a whole allows a greater and predictable flow of barrier fluid within the seal cavity than is possible with the slotted seal sleeve previously described.

Statements of the Invention According to the present invention there is provided a double mechanical seal adapted to seal a rotating shaft to a stationary piece of equipment so as to define a substantially annular barrier fluid cavity surrounding the shaft, of which the outer surface is stationary with respect to the piece of equipment and the inner surface is stationary with respect to the shaft, and in which the cavity includes a region of radial constriction.

It has been found that the region of radial constriction in the substantially annular cavity enables appreciable flow to be induced in barrier fluid present within the cavity, even if both the inner and outer surfaces of the cavity are smooth or devoid of flow-inducing profiles.

The inner and outer surfaces of the cavity may be substantially circular, in which case the region of radial constriction may be provided by offsetting the centre of the inner surface from that of the outer surface. Initial investigations have shown that the amount of flow developed is greater, the greater the degree of eccentricity. The seal may be adapted to seal the shaft to the equipment such that the rotary parts of the seal are not concentric with the stationary parts of the seal. Alternatively, a shaped insert may be introduced into a barrier fluid cavity between concentric stationary and rotating parts to provide the required overall shape.

The cavity will typically include inlet and outlet ports for entry and exit of barrier fluid and these ports are preferably positioned on either side of the region of constriction. This has been found to promote circulation of the barrier fluid into and out of the cavity.

The seal may be a cartridge mounted seal including a seal sleeve. The external surface of the sleeve may be profiled in at least part of that region which would come into contact with barrier fluid contained in the cavity.

The profiled seal sleeve acts as an impeller and induces flow of the barrier fluid around the outside of the sleeve. This pumping action allows the pressure of the barrier fluid to be maintained within the seal cavity, without the build-up of undue heat in the regions of the seal faces. Continued circulation of the barrier fluid is thus maintained, into and out of the seal cavity, without the need to rely on convection alone, without the need for an external pumping means and whatever the orientation of the seal in use.

The cartridge mounted double mechanical seal may be derived from any conventional cartridge mounted double seal, having either a back-to-back or a face-to-face configuration of seal components. If, for example, the seal gland is modified so as to attach to the piece of equipment at a position slightly offset from being concentric with the shaft, a cavity of the requisite shape will be formed.

If a profiled sleeve is used, the profiling preferably comprises a series of recesses circumferentially spaced about the sleeve. The rotating sleeve then acts in a similar fashion to a water wheel, each recess driving a volume of fluid from one position to another around the sleeve as it rotates. In this way, the sleeve acts as an impeller when it rotates in contact with a fluid, inducing a flow of the fluid around its circumference. The impelling action will be particularly pronounced in the region of radial constriction of the substantially annular cavity.

The profiling on the sleeve surface may thus take the form of one or more recesses in the external surface of the sleeve; one or more raised portions protruding from the surface, effectively defining recesses therebetween; one or more slots cut out of the sleeve surface; etc. These profiled portions are preferably elongate in shape, running axially along at least part of the length of the sleeve. They then serve to induce flow in a surrounding fluid regardless of the direction in which they rotate, and will function correctly as part of a seal, no matter what the direction of rotation of a shaft on which the seal is mounted. The sleeve can thus be used in most different types of seal.

A preferred design of sleeve in accordance with the invention comprises a number of elongate slots, running axially along at least part of the length of the sleeve and circumferentially spaced from one another. A preferred number of such slots is four. However, the number of profiled portions provided on a sleeve in accordance with the invention, and their dimensions, will be chosen in any particular case so as to maximise performance and also to take into account the conditions of use of the seal sleeve and production constraints. In the case of profiled portions comprising slots cut into the sleeve, the greater the number of slots, the greater the flow that the sleeve will be able to induce in a surrounding fluid. However, also to be borne in mind is that the greater the number of slots, the weaker the sleeve overall and the harder it is to produce.

Where the sleeve comprises slots rather than recesses, it will usually need to be sealed, against a rotatable shaft upon which it is mounted, at both ends of the profiled region of its surface. This is because barrier fluid inside a seal cavity needs to be prevented from escaping from the seal, via the sleeve, into either of the two fluids which the seal is intended to separate from one another. The sleeve will preferably be provided with at least one annular seal groove running around its inner surface, the groove being adapted to hold a sealing element such as an O-ring to provide a seal between the inner surface of the sleeve and a rotatable shaft on which it is mounted.

The seal may in all other respects be of conventional design.

It will comprise an inner and an outer set of seal components each set itself comprising a rotary and a stationary seal face, all of which are mounted, in use, around the seal sleeve, which in turn is capable of being mounted around a rotatable shaft in equipment to be sealed.

A double seal in accordance with the invention is a selfcontained unit, capable of operating as an efficient seal without the usual problems associated with the use of double seals and barrier fluids. There is no need to add on extra components to the seal, for use as flow inducers to prevent temperature build-up in a barrier fluid contained in the seal.

The present invention will now be described by way of example only, with reference to the accompanying illustrative drawings, of which: Figure 1 is a schematic cross-sectional view of a conventional cartridge mounted double seal; Figure 2 is a schematic cross-sectional view of a cartridge mounted double seal including a conventional flow inducer; Figure 2A is a end view on that part of the seal of Figure 2 indicated by the arrow in Figure 2; Figure 3 is a schematic cross-sectional view of a cartridge mounted double seal in accordance with the present invention; Figures 4-6 show three alternative types of seal sleeve; and Figure 7 is a schematic lateral cross-section showing an eccentric gland insert in accordance with the present invention.

Detailed Description of the Drawings Referring firstly to Figure 1, there is shown a conventional cartridge mounted double mechanical seal 1. This comprises a seal sleeve 2, on which the main seal components are mounted, and which is itself adapted for mounting about a rotatable shaft 3 in equipment to be sealed. The seal components comprise an inner rotary seal face 4, which rotates with the shaft 3 when the seal is in use, and an inner stationary seal face 5, which remains fixed with regard to the rotating shaft 3. These inner seal faces are, in use, in contact with a product liquid under seal. They are in constant mating contact with one another. The seal also comprises an outer rotary seal face 6 and an outer stationary seal face 7, again in mating contact with one another, these being on the atmosphere side of the seal.

Component 8 is a holder for inner rotary seal face 4, and component 9 a holder for outer rotary seal face 6. Component 10 is a grub screw, by means of which the seal is fixed to rotating shaft 3. The seal also comprises O-ring seals, as shown in Figure 1, in particular O-ring 11, which seals the interface between seal sleeve 2 and rotating shaft 3.

The above-mentioned stationary seal faces 5 and 7 are located within gland plate 13 and gland plate insert 13A, the former being bolted to the body 12 of equipment, for instance a pump, thereby attaching the stationary seal faces to the stationary equipment body.

Cavity 14, within component 13, usually contains a barrier fluid, which is free to enter all regions of the seal labelled 15 in Figure 1. This barrier fluid thus comes into contact, in use of the seal, both with the external surface of seal sleeve 2 and also with the seal faces 4, 5, 6 and 7, the temperature of which can cause undesirable increases in the temperature of the barrier fluid.

Figure 2 shows the double seal of Figure 1, with an additional pumping ring 20 fitted to it. An end view of the pumping ring 20 is shown in Figure 2A at the right hand side of Figure 2.

It is designed to be fitted to or machined around the outside of the rotary seal face holder 8, and thus rotates with the shaft 3, sleeve 2 and rotary seal face 4. As the pumping ring rotates, its profiled surface (having raised portions 21 and recessed portions 22 spaced circumferentially about the external surface of the ring) drive the product fluid in contact with the outside of the ring, thus inducing a flow of the fluid around the outside of the seal faces 4 and 5. In this way, the build-up of excessive heat in the product fluid surrounding the inner seal faces 4 and 5 can be prevented.

However, pumping ring 20 is only effective to induce flow in the product fluid on the inside of the double seal, and does not affect in any way a barrier fluid contained in the seal cavity 14.

In Figure 3, there is shown a cartridge mounted double mechanical seal, generally labelled 31, in accordance with the present invention. This is in many respects identical to the seal 1 shown in Figure 1. It comprises an inner rotary seal face 34, an inner stationary seal face 35, an outer rotary seal face 36, an outer stationary seal face 37, inner and outer rotary holders 38 and 39 respectively, gland components 42 and 43 and an inner cavity 44 within component 43. Again, this cavity 44 would in use contain a barrier fluid, as extra protection between the product fluid to be sealed - on the left hand side of the seal - and the atmosphere - on the right hand side.

Seal 31 differs from seal 1 in that it is designed to be mounted on the equipment with the stationary parts of the seal - i.e. the gland 43, gland insert 43' and stationary seal faces 35, 37 - non-concentric with the rotary parts - i.e. the sleeve 32, rotary holders 38, 39, rotary seal faces 34, 36, etc. - and the equipment shaft 33. A relatively simple modification of the gland insert 43', such as moving any mounting spigots (not shown) off centre will achieve this object. As only one half of the seal is shown in fig.3, the shape of the substantially annular barrier fluid cavity 100 defined between the sleeve 32 and the stationary seal faces 35, 37 cannot immediately be appreciated.The other half of the seal will look similar, but with a greater distance between the sleeve 32 and the stationary seal faces 35, 37 and a corresponding adjustment of the relative position of the rotary and stationary seal faces.

The arrangement is shown in lateral cross-section in fig. 7.

The eccentric gland insert 43' is shown schematically in broken lines and the equipment shaft 33 is illustrated in solid lines. The substantially annular cavity 100 having a constriction at point 101 can be seen clearly from fig. 7.

Inlet and outlet ports A, B for the barrier fluid are let into the cavity 100 via the gland 43 and for optimum pumping of the fluid, the ports are positioned at either side of the narrow part of the cavity, this being the part through which the section of fig. 3 is taken. Thus, one port will, in three dimensions, be somewhat above the plane of the paper; the other will be somewhat below. If the shaft 33 rotates in the direction of the arrow, the former will be the outlet; the latter the inlet.

Seal 31 also differs from seal 1 in the design of its sleeve 32, which comprises a number of elongate slots such as 46, cut into its external surface. Sleeve 32 actually has four such slots, regularly spaced about its circumference, each extending axially along part of the length of the sleeve.

When this shaft is rotating, sleeve 32 also rotates with it, and slots 46 thus induce a flow in barrier fluid contained within cavity 44 which comes into contact with the external surface of sleeve 32. This causes an increase in pressure of the barrier fluid as it comes into contact with the rotating seal sleeve, generating a "head" of pressure in the fluid which assists its circulation around a closed cooling circuit in association with which the seal is used.

Thus, in use of the seal, relatively cool barrier fluid is supplied to seal cavity 44. When this fluid comes into contact with the seal faces 34, 35, 36 and 37 it will increase in temperature. However, it will also be induced to flow, by the rotation of slots 46 in seal sleeve 32 and by the exaggeration of this effect in the constricted region of the cavity 100, around the outside of the seal sleeve. Thus, as the barrier fluid increases in temperature at the seal faces, it will be driven away from the seal and into the cooling part of its circuit, to be cooled and ultimately recycled back into cavity 44.

The barrier fluid is never allowed to become hot enough to vaporise within the seal, or to reduce its pressure, which would render it less effective as security against leakage of product fluid. A constant flow of the barrier fluid around its own circuit is maintained because of the head of pressure generated.

Because the slots 46 are cut out of the sleeve 32, the interface between the sleeve and the rotating shaft 33 needs to be sealed at both ends of the seal, by means of O-ring seals 47 and 48.

Figure 4 shows in more detail a seal sleeve similar to sleeve 32 shown in Figure 3. Figure 4A is an end view of the sleeve, Figure 4B a longitudinal cross-section and Figure 4C a side view.

The seal sleeve 50 shown in Figure 4 is for use as part of a cartridge mounted double mechanical seal such as that shown in Figure 3. It is in many respects identical to conventional seal sleeve inserts, such as that labelled 2 in Figure 1.

However, it is provided with four longitudinally extending slots 51, regularly spaced about its circumference. These are cut out from the sleeve walls. As the sleeve rotates with an equipment shaft on which it is mounted in use, the slots 51 serve to drive fluid in contact with the external surface of the sleeve around its circumference. In this way, the sleeve as a whole acts as an impeller, inducing flow within the cavity of a seal in which the sleeve 50 is incorporated. The dimensions of slots 51 are carefully chosen so as to generate the desired flow rate in barrier fluid contacting the sleeve in use.

The sleeve 50 is also provided with O-ring grooves 52 and 53, on its inside surface. These are adapted to receive O-ring seals, which seal the interface between the inner surface of the sleeve and a rotatable equipment shaft on which it is mounted.

Figure 5 shows an alternative seal sleeve. Again, Figure 5A is an end view of the sleeve, Figure 5B a longitudinal crosssection and Figure 5C a side view.

Sleeve 60 is provided with four longitudinally extending recesses 61, cut into its outer surface at regular intervals about its circumference. In a similar manner to slots 51 in the sleeve shown in Figure 4, recesses 61 serve to drive fluid around the circumference of sleeve 60 as it rotates. Each slot 61 picks up a volume of fluid as it rotates, and transfers this volume to another point further round the circumference of the sleeve.

Sleeve 60 is provided with a single O-ring groove 62. Because recesses 61 do not pass through the entire thickness of the walls of the sleeve 60, there is no need for an additional 0ring groove at the opposite end of the sleeve, in order to maintain an effective seal when the sleeve is used as part of a cartridge mounted double seal. 63 is a grub screw hole, for receiving a grub screw used to secure the sleeve to other seal components mounted upon it in use, and to an equipment shaft around which it is fitted.

Figure 6 shows, in views corresponding to those of Figures 4 and 5, a third seal sleeve 70. This is provided with four recesses 71 in its outer surface, each recess being rectangular in plan view. Again, because the recesses do not cut through the entire thickness of the sleeve wall, only a single O-ring groove 72 is needed on the sleeve. This receives an O-ring which, when the sleeve is in use as part of a cartridge mounted double seal, seals the interface between the sleeve and a rotating equipment shaft on which it is mounted. 73 is a grub screw hole.

Claims (9)

Claims

1. A double mechanical seal adapted to seal a rotating shaft to a stationary piece of equipment so as to define a substantially annular barrier fluid cavity surrounding the shaft, of which the outer surface is stationary with respect to the piece of equipment and the inner surface is stationary with respect to the shaft, and in which the cavity includes a region of radial constriction.

2. A seal according to claim 1 in which the inner and outer surfaces of the cavity are substantially circular, the centre of the inner surface being offset from that of the outer surface.

3. A seal according to claim 1 or claim 2 in which the seal includes rotary parts adapted to be affixed to and rotate with the shaft and stationary parts adapted to be affixed to the piece of equipment and is adapted to seal the shaft to the equipment such that the rotary parts of the seal are not concentric with the stationary parts of the seal.

4. A seal according to claim 1 or claim 2 in which the seal includes rotary parts adapted to be affixed to and rotate with the shaft, stationary parts adapted to be affixed to the piece of equipment and a shaped insert adapted to be positioned between the stationary and rotating parts.

5. A seal according to any preceding claim in which the cavity includes inlet and outlet ports for entry and exit of barrier fluid positioned on either side of the region of constriction.

6. A cartridge mounted seal according to any preceding claim including a sleeve to be mounted on the shaft, in which the external surface of the sleeve is profiled in at least part of that region which comes into contact with barrier fluid contained in the cavity.

7. A seal claim 6, in which the profiling on the external surface of the sleeve comprises a series of recesses circumferentially spaced about the sleeve.

8. A seal according to any claim 6, in which the profiling on the external sleeve surface comprises one or more slots cut out of the surface.

9. A double mechanical seal substantially as described herein with reference to the accompanying drawings.