WO1989008798A1

WO1989008798A1 - Improved lobe pump

Info

Publication number: WO1989008798A1
Application number: PCT/GB1989/000281
Authority: WO
Inventors: John Frederick Feakins
Original assignee: Ibex Engineering Company Limited
Priority date: 1988-03-16
Filing date: 1989-03-16
Publication date: 1989-09-21
Also published as: EP0440622A1; GB8806242D0

Abstract

A lobe pump is disclosed having improved "cleaning in place" characteristics enabling high sanitary standards to be obtained. Each rotor shaft (3) is sealed to the pump housing (5) by first and second sealing rings (34, 35), the first sealing ring rotating with the shaft and the second sealing ring being non-rotatable and having an inner surface which tapers inwardly away from the rotor so as to encourage recirculation of fluid from the seal back to the pump chamber. The first sealing ring is supported axially by the shaft, these parts having confronting radial shoulders with an elastomeric seal (36) interposed between them, and the second sealing ring is pressed by a spring (42) into axial abutment with the first ring. A single O-ring (46) seals a seal housing member (40) to both the pump housing (5) and the second sealing ring (35). The rotor (16) is secured to the shaft by a bolt (21) having an integral flange (20) and a seal is positioned between the periphery of the flange and rotor to resist frictionally loosening of the bolt.

Description

Improved Lobe Pump

This invention relates to pumps, and espe¬ cially, but not necessarily exclusively to positive displacement pumps suitable for use, for example, in the food processing, pharmaceutical and chemical industries, where high standards of sanitation are demanded.

A known form of sanitary positive displacement pump comprises a pump housing enclosing a pump cham¬ ber, and two rotors each having a plurality of lobes and mounted on respective shafts to be rotated in opposite directions for pumping fluid from an inlet to an outlet of the pump housing. The known pumps, sometimes called lobe pumps, can be used for pumping liquids having a wide range of viscosities and are suitable for homogeneous liquids and liquids with particles and solids in suspension.

One of the problems of the known lobe pumps arises from the need for each rotor shaft to be sealed to the pump housing. A known seal arrangement is mounted externally of the pump housing and includes a pair of sealing rings urged into axial abutment, one ring being fast for rotation with and sealed to the shaft, and other ring being sealed to the pump housing and held against rotation. O-rings are employed to seal between the sealing rings and the shaft and housing respectively. A collar is clamped to the shaft by a grub screw and provides axial support for the rotating sealing ring which is 8798 '

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urged by a spring interposed between that ring and the collar into firm contact with the stationary sealing ring. The latter ring is capable of moving axially to increase the contact pressure between the sealing rings, the spring being caused to be compressed, under the influence of liquid being pumped and under pressure penetrating into the seal arrangement and acting between the housing and stationary sealing ring. This movement ensures that the sealing force increases as the operating pressure of the pump rises so that the sealing rings do not become pushed apart by liquid becoming forced between them.

The known seal arrangement performs satisfactorily, but it is not without its drawbacks and limitations. Because of the necessary working clearances there are gaps or voids into which the liquid being pumped penetrates and in which the liquid can stagnate or even crystallise or solidify depending on the nature of the liquid. As a result, frequent cleaning operations are necessary and since effective flushing of the spaces in question cannot be ensured by passing washing solutions through the pump it is essential for manual cleaning to be carried out, which involves the cover of the pump housing and the rotors being removed. These cleaning operations are obviously inconvenient and mean significant down time of the pump and the processing plant of which it forms part.

In accordance with one aspect of the present invention there is provided a seal arrangement for sealing a rotor shaft to a pump housing, comprising a first sealing ring carried on the shaft for rotation therewith, and a second sealing ring in axial abutment with the first sealing ring and non-rotatable relative to the pump housing, the second ring having an inner surface tapering towards the shaft in the axial direction away from the pump chamber. The seal arrangement of the invention marks a significant departure from conventional thinking. Instead of trying to keep spaces within the arrangement to a minimum, the stationary sealing ring defines an annular cavity of significant volume around the shaft, the cavity being so shaped that circulation of liquid within this spaced is produced under the effect of the rotating shaft. Due to friction, liquid in the seal cavity tends to rotate with the shaft. The resultant centrifugal forces cause the liquid to move radially outwards and impinge against the inclined surface of the sealing ring and thereby to be directed to flow back towards the pump chamber. By this circulation of fluid the liquid in the seal cavity is moving continuously and stagnation or crystallisation is averted.

Furthermore, the pump can be cleaned well enough to meet sanitation regulations by washing through with cleaning solutions and without need for the pump to be disassembled. Another disadvantage of the known seal arrangement described above is that the grub screw locking the collar to the shaft can loosen and/or slip with the result that the seal leaks. In addition, accurate setting up is necessary to ensure correct location of the collar along the shaft when the seal arrangement is installed, the preload of the spring being determined by the collar position.

According to a second aspect of the invention there is provided a seal arrangement for sealing a rotor shaft to a pump housing, comprising a first sealing ring carried on the shaft for rotation therewith, and a second sealing ring in axial abutment with the first sealing ring and non-rotatable relative to the housing, the first ring having a shoulder axially supported by a confronting shoulder formed on the shaft facing the second sealing ring. rreferably the first sealing ring is sealed to the shaft by a cylindrical seal element having an external flange portion engaged between the shoulders to be compressed therebetween. According to a third novel aspect of the invention there is provided a seal arrangement for sealing a rotor shaft to a pump housing, comprising a first sealing ring carried on the shaft for rotation therewith, and a second sealing ring in axial abutment with the first sealing ring and non-rotatable relative to the housing, the second sealing ring being received in a seal housing member mounted to the pump housing, and a single sealing element being arranged to seal the housing member to the pump housing and to the second sealing ring. The sealing element can conveniently take the form of an O-ring seal or other similar elastomeric sealing member. In the preferred construction the seal housing member has a rebate in the inner surface thereof and arranged to define an inwardly opening groove with the outer surface of the pump housing, the O-ring being received in the groove and projecting therefrom into sealing contact with an outer surface of the second sealing ring. With this construction the O-ring is over- compressed in as much that it seals against four sides rather than three which is usual, and as a result a highly effective seal is obtained and liquid is prevented from penetrating between the seal and the surfaces with which it is in contact.

To prevent rotation of the second sealing ring it can be keyed to the seal housing member. Preferably the sealing ring has at least one notch in its outer periphery and the housing has an integral dog which engages in each notch. Conveniently, the dogs can serve to retain a spring which urges the second sealing ring axially into abutment with the first sealing ring, which facilitates assembly of the seal arrangement. For some applications it is necessary to maintain a fluid barrier between ambient atmosphere and the sealing faces of the sealing rings in order to to prevent any possibility of the fluid being pumped coming into contact with the atmosphere. In fehe prior art there is known a construction in which an enclosure defines a chamber surrounding the seal arrangement and into which a barrier fluid can be introduced through a port in the enclosure. At one end the enclosure is sealed to the pump housing and at the other end it is sealed to the shaft by a lip seal which contacts the rotating shaft. This arrangement is inconvenient because the lip seal can wear the shaft over the long term use, and disassembly of the seal arrangement is complicated. In accordance with a f rther aspect of the present invention there is provided a seal arrangement for sealing between a rotor shaft and a pump housing, comprising a first sealing ring carried on the shaft for rotation therewith, a second sealing ring in axial abutment with the first sealing ring and non-rotatable relative to the housing, an enclosure defining a chamber surrounding the sealing rings and including a port for supplying a barrier fluid to the chamber, the enclosure being sealed to the pump housing at one end and being sealed to the outer surface of the first sealing ring at the other end.

A lip seal can be used to seal between the enclosure and the first sealing ring. Unlike the shaft, the sealing ring is relatively simple and cheap to replace should it become worn by the seal. In addition disassembly is facilitated by this sealing arrangement. The enclosure can be constituted by the aforementioned seal housing member whereby the manufacture and assembly may be facilitated. In the known lobe pumps as described hereinabove, the rotors are fastened to their shafts so as to enable easy removal for cleaning purposes. In particular, the rotors are splined on the shafts and are fixed axially by nuts which screw onto threaded end portions of the shafts. Problems can arise due to the nuts working loose, for example as a result of being insufficiently tightened after a cleaning operation, and ^"any penetration of the liquid into dead spaces between the rotors and shafts is a disadvantage since these areas cannot be cleaned effectively without removing the rotors.

According to a further aspect of the present invention there is provided a pump having a rotor removably mounted on a shaft and secured thereto by a threaded connector engaged with an end of the shaft, said connector having an integral external flange received in a counterbore of the rotor, and a sealing means being received between the peripheral surface of the flange and the counterbore.

The sealing means may be an O-ring and apart from providing an effective seal between the connector, and hence the shaft, and the rotor it also acts as a friction device to resist unintentional loosening of the connector due to vibrations and the like occurring during operation of the pump. Another disadvantage associated with the known lobe pumps and connected with the problems of cleaning stems from the fact that a seal located between the end face of the pump housing and the detachable cover is at least at some locations spaced at an appreciable distance from the inner edge of the housing end face. As a consequence small gaps can occur between the cover and housing which are open to the pump chamber and into which liquid being pumped can enter and remain. Once again these dead spaces cannot be effectively cleaned by so-called cleaning in place, i.e. passing washing solutions through the pump, which is another reason for the cover having to be removed for cleaning. One of the reasons for the seal being displaced from the edge of the housing is that the pump chamber has two portions of part circular configuration and each extending through more than 180° about the axis of a respective rotor. At each side these portions are connected by wall portions made slightly concave.

In accordance with yet another aspect the present invention provides a pump comprising a pair of rotors with lobes and arranged to cooperate with each other and with a pump housing to pump fluid from an inlet to an outlet upon rotation of the rotors in opposite directions, the pump housing defining a chamber including a pair of opposed straight parallel wall portions interconnected at opposite ends by substantially semi-cylindrical wall portions generally centred on the axes of the respective rotors, the housing having a cover attached thereto and closing the chamber, and an annular seal interposed between the housing and the cover and following in close proximity the edge of the pump chamber around the entire periphery thereof.

By the shaping of the chamber the seal can follow very closely the inner edge of the end face of the housing and as a result dead spaces between the housing and the end cover are precluded.

By combining the different aspects of the invention as explained above a pump having several benefits over the prior art can be obtained especially regarding the ability to be cleaned efficiently enough to meet sanitary regulations without need to disassemble any parts of the pump. In a preferred form of pump embodying the invention cleaning operations are further facilitated by the pump being self draining, which is achieved, firstly by the inlet and outlet ports being located at the top and the bottom (rather than at the sides which is conventional) so that liquid can drain under gravity through the lower port when the pump is not operating, and secondly by liquid being free to flow under gravity out of the annular cavity of each shaft seal arrangement.

The various aspects and features of the invention will become clearer from the following detailed description of some embodiments, given with reference to the accompanying drawings in which:-

Figure 1 is a side view of a positive displacement pump in accordance with the invention;

Figure 2 is a top plan view of the pump shown partly in cross-section taken along the line x-x of Figure 1 ;

Figure 3 is a sectional view showing one seal arrangement of the pump on an enlarged scale;

Figure 4 is a cross-section taken along the line A-A of Figure 3; Figure 5 is an enlarged schematic sectional view illustrating the liquid movement within the seal arrangement;

Figure 6 is a view corresponding to Fig. 3 but showing a modified construction; and Figure 7 is a front view of the pump with the cover of the pump housing detached.

Illustrated in Figure 1 is a positive displacement pump of the so-called lobe pump type. The pump has a timing gearbox 1 with a single input shaft 2, and twin, parallel output shafts 3 which are driven at equal speeds, but in opposite directions, when the input shaft is rotated. Secured to the forward end of the gearbox housing by bolts 4 is a pump housing 5 and a front end cover 6 which closes the pump chamber 7 defined within the pump housing.The pump chamber has integral connections 8, 9 for pipes to conduct liquid being pumped to and from inlet and discharge ports of the pump. The two output shafts 3 are journalled in the gearbox housing by bearings 10 (Fig. 2) and extend through seals 11 fitted to an end plate 12 which is bolted to the gearbox housing and closes its front end. The rear_wail of the pump housing 5 includes a flange 13 which locates within a rim 14 of the gearbox housing to ensure correct alignment between the pump chamber 7 and the axes of the shafts 3, which is important -te ensure that the rotors 16 attached to the shafts (as described in more detail below) do not come into contact with the wall of the pump chamber during operation of the pump. In this connection it should be mentioned that it is a novel feature of the illustrated pump that the rim 14 of the gearbox housing is machined, at its opposite sides, about respective centres coincident with the axes of the shafts 3. Hitherto, it has been normal to machine the rim about one centre, which is the centre line of the pump, i.e. midway between the axis of shafts 3. By using two centres, which machining can be conveniently performed when drilling the bores in the gearbox housing to receive the shafts 3, a better or more positive location of the pump housing is obtained, whereby further means previously necessary to ensure correct alignment between the housing is obviated.

The shafts 3 project through respective openings in rear wall of the pump housing 5, to which wall the shafts are sealed by sealing arrangements 18. The ends of the shafts 3 received in the pump chamber are splined and receive the respective rotors 16, each of which has a plurality of lobes 19, which are four in number in the illustrated embodiment as best seen in Fig. 7. The rotors 16 cooperate, without coming into direct contact with each other or with the pump housing, to cause liquid to be transferred from the inlet port to the discharge port, when the rotors are contra-rotated by the shafts 3. To prevent any of the liquid entering the interstices of the rotors each is sealed to its shaft. As may be seen from Figure 3, one O-ring seal 22 is positioned between the rear face of the rotor 16 and a radial shoulder on the shaft 3, and a second O-ring 23 is located between the periphery of a_flange 20 and a surrounding surface of the rotor adjacent its front face. The flange 20 is integral with a front bolt 21 which screws into a tapped blind hole -provided in the end of the shaft 3 to secure the rotor 16 on the shaft. The rotor has a shallow counterbore for accommodating the flange and in addition to sealing between the flange and counterbore the O-ring 23 serves as a friction device to resist rotation of the bolt 21 to help prevent it undoing because of vibrations occurring within the pump. It will be appreciated that by virtue of the seals 22, 23 liquid is prevented from penetrating into and occupying any dead spaces within the rotors 16.

The seal arrangements 18 are the same for each shaft 3 and one will now be described in detail making particular reference to Figs 3 and 4. The shaft is stepped with two cylindrical portions 30,31 separated by a radial shoulder 32, and the opening in the rear wall of the pump housing 5 is of sufficient diameter to define a substantial clearance with the portion 31 of smaller diameter. As shown, the opening is slightly smaller in diameter than the hub of the rotor 16.

A pair of sealing rings 34, 35 are provided and have opposed plain faces adapted to abut in sealing cooperation with each other. One sealing ring 34 is carried on the shaft 3 and is fast for rotation with the shaft. The sealing ring 34 _has stepped internal surface with smaller and larger diameter surface portions separated by a radial shoulder which is axially.supported by the shoulder 32 of the shaft 3. A sealing element 36 is interposed between the ring 34 and the shaft, the element having the form of a cylindrical sleeve with an external flange which is held between the confronting shoulders. A short axial slot 38 is provided in the ring at the end opposite the sealing face, and a pin 39 fixed into a hole in the shaft engages in this slot so that the ring 34 rotates with the shaft 3 b t-is not held against axial displacement.

The second sealing ring 35 is mounted in a seal housing 40 which has a central boss received in a recess of the pump housing wall, and a peripheral flange which is fastened to the pump housing wall by the bolts 41. The boss has a stepped bore defining a radial shoulder facing away from the pump housing and a wave spring 42 is inserted between this shoulder and an external flange formed on the sealing ring 35 for urging the rinq 35 towards the first sealing ring 34, and hence biasing the sealing faces of these rings into sealing abutment. To prevent the ring 35 rotating.the housing 40 is formed with a plurality (three as seen in Fig. 5) of dogs 44 which engage in complementary notches provided in the flange of ring 35. To accommodate the spring 42, slots are provided behind the dogs 44 and as a consequence the dogs serve to retain the spring in the housing, which can facilitate assembly and disassembly of the seal arrangement. The seal housing 40 is sealed to both the wall of the pump housing and the sealing ring 35 by a single O-ring seal 46. The central boss of the seal housing has a rebate which defines with the outer surface of the pump housing wall a groove of rectangular cross- section in which the 0-ring 46 is received. As well as being compressed between the opposite side walls of the groove, the O-ring is compressed between the bottom wall of the groove and the outer surface of the sealing ring 35 adjacent the end of the sealing ring nearest the pump chamber. By being "over-compressed" in this way, the 0- ring provides an effective seal between the components concerned. Furthermore, the seal 46 can not be deformed under the effect of pressurised liquid so that the liquid will enter the spaces left between the O-ring and the surfaces with which it cooperates.

The internal surface of the--sealing ring 35 is made frusto-conical so that it tapers inwardly in the axial direction towards the first sealing ring. Thus, the ring 35 defines with^" the shaft 3 an annular cavity 48 of generally triangular cross-section. In operation of the pump, this cavity fills with liquid and, due to friction, is caused to be rotated by the shaft. Under the effect of the resultant centrifugal forces the liquid moves radially outwards and impinges against the inclined surface of the sealing ring 35 which directs the liquid back towards the pump chamber. The direction of liquid flow is illustrated by the arrows in Figure 5. By virtue of this circulation effect, the liquid is moving continuously within the seal arrangement and there are no dead spaces in which the liquid can collect and stagnate or crystallise. The axial force exerted on the sealing ring 35 by the liquid presses it into stronger abutment with the ring 34 so that the sealing force between the two rings increases as the liquid pressure rises. It may be noted .that because there is a pressure drop generally diametrically across the rotor 16, from the inlet side to the discharge side, there is a tendency for liquid also to flow into and out of the cavity 48 so that the liquid within the cavity is being constantly renewed.

In the pump of Figs 1-4 the space 50 surrounding the sealing arrangements 18, between the gearbox and the pump head, is connected to atmosphere via openings defined between the housings at the top and bottom of the pump. For some applications it is necessary to maintain a fluid barrier between the sealing rings 34, 35 and atmosphere to preclude any chance of the liquid being The modified seal arrangement depicted in Figure 6 is suitable for such applications. Most of the compo¬ nents are identical to those of the previous embodi¬ ment and are designated with the -same reference numer- als in the drawing. The seal housing 40 is in this case arranged to enclose a fluid compartment 55 around the sealing rings 34, 35, and has a port 56 for sup¬ plying barrier fluid to this compartment. As before, the forward end of the housing 40 is sealed to the pump housing 5 and the ring 35 by the O-ring 46. At its_. rear end the housing 40 carries a lip seal assem¬ bly comprising an annular sealing lip 52 which is in contact with the outer surface of ring 34. The ring 34 is of wear resistant material and is relatively cheap to replace. Thus, the cooperation of the lip seal with the ring 34, instead of with the shaft 3 as in prior art constructions, has a significant advan¬ tage. In addition, assembly and disassembly of the seal arrangement are much more convenient. The described pump is entirely self-draining.

With lobe pumps it is usual for the inlet and outlet ports to be positioned horizontally, i.e. at opposite sides of the pump housing. The embodiment of the present invention has ports 8, 9 located at the top and bottom of the pump housing, as shown in Figure 1 , so that liquid can drain from the pump chamber and out through the port 8 when the pump is not operating. Also, the cavity 48 of each seal arrangement (see Figs 3 and 6) is open to the pump chamber so that liquid drains from this cavity under the influence of gravity and will pass out through the port 8 with the other liquid when the pump operation is interrupted. In other words, the inner edge of the rear wall of the pump housing 5 does not extend inwardly beyond the seal 46 and as a result it is without any lip directed inwardly towards the shaft (a commonfeature of prior art seal arrangements) and forming a weir preventing liquid within the seal arrangement from draining back into the pump chamber. It will be understood that the self- draining characteristics contribute significantly to the ease of cleaning the pump.

In Figure 7, there is shown on the left-hand side the front view of the pump of the invention with the cover 6 removed, whereas on the right-hand side there is shown a typical prior art pump. In each case the other half of the pump chamber would be of symmetrical configuration. In the prior art pump, the chamber has two portions 60 centred on the rotor axes and of greater than 180° extent, these portions being connected by opposed concave portions 61. The seal which seals the

• cover to the pump housing follows the line 63 and, as can be seen, is spaced at a significant distance from the edge of the pump chamber, especially at the interconnections between the part cylindrical portions 60 and the concave portions 61. The pump of the invention has a chamber with semi-cylindrical portions 65 connected together by straight parallel portions 66. As made possible by this shape of chamber, the O-ring seal 67 which seals the cover 6 to the pump housing follows a line 68 which is closely adjacent the edge of the chamber around its entire periphery. Small dead spaces between the cover and housing in which liquid may collect are thereby obviated.

It has been found possible to clean the illustrated pump well enough by normal "cleaning in place" techniques in which washing solutions are passed through the pump without any dismantling of the pump components, for sanitary requirements to be satisfied. Thus, the need to disassemble the pump regularly, e.g. after each few hours of use, can be eliminated with great commercial advantages being obtained. Modifications of the described pump are of course possible without departing from the scope of the invention. For example, O-ring seals are not essential and where employed other suitable elastomeric seals could be used instead.

Claims

CLAIMS:-

1. A seal arrangement for sealing a rotor shaft (3) to a pump housing (5), comprising a first sealing ring (34) carried on the shaft for rotation therewith, and a second sealing ring (35) in axial abutment with the first sealing ring and non-rotatable relative to the pump housing, characterised in that the second sealing ring (35) has an inner surface tapering to¬ wards the shaft (3) in the axial direction away from the pump chamber (7) and towards the first sealing ring, said taper terminating at the inner diameter of the second sealing ring.

2. A seal arrangement according to claim 1 , where¬ in the second sealing ring (35) is movable axially relative to the pump housing (5), and spring means

(42) acts between the housing and second- sealing ring (35) to urge the second sealing ring (35) into abut¬ ment with the first sealing ring.

3. A seal arrangement according to claim 2, where¬ in the spring-means comprises a spring element (42) extending about the second sealing ring (35) and acting against an external radial shoulder on the second sealing ring.

4. A seal arrangement according to claim 3, where¬ in the second sealing ring (35) and the spring element (42) are mounted in a seal housing member (40), the member (40) being arranged to retain the spring ele- ment upon removal of the second sealing ring (35) from the housing member.

5. A seal arrangement according to claim 4, where¬ in the housing member has inwardl projecting keying means (44) engaging in complementary recess means in the second sealing ring (35) to prevent rotation thereof, said keying means (44) also serving as retention means for holding the spring element (42) in the housing member.

6. A seal arrangement according to claim 4 or 5 wherein the seal housing member (40) is sealed to the pump housing (5) and to the second sealing ring (35) by a single common sealing element (46).

7. A seal arrangement for sealing a rotor shaft (3) to a pump housing (5), comprising a first sealing ring (34) carried on the shaft for rotation therewith, and a second sealing ring (35) in axial abutment with the first sealing ring and non-rotatable relative to the pump housing, characterised in that the second sealing ring (35) is received in a seal housing member (40) mounted to the pump housing, and a single sealing element (46) seals the housing member (40) to the pump housing (5) and to the second sealing ring.

8. A seal arrangement according to claim 6 or 7, wherein the sealing element is an elastomeric ring seal (46).

9. A seal arrangement according to claim 6, 7 or 8, wherein the sealing element is located in a radial¬ ly inwardly facing annular groove having opposite side walls defined respectively by the pump housing (5) and the seal housing member (40), the sealing element (46) being compressed between said side walls and between the bottom wall of the groove and the outer surface of the second sealing ring (35).

10. A seal arrangement according to claim 9, where¬ in the sealing element engages the second sealing ring (35) adjacent the end thereof nearest the pump chamber (7).

11. A seal arrangement according to any one of the preceding claims, wherein the first sealing ring (34) has an internal shoulder firmly supported axially by a confronting shoulder (32) formed on the shaft (3) and facing towards the second sealing ring (35).

12. A seal arrangement for sealing a rotor shaft (3) to a pump housing (5) comprising a first sealing ring (34) carried on the shaft for rotation therewith, and a second sealing ring (35) in axial abutment with the first sealing ring and non-rotatable relative to the pump housing (5), characterised in that the first: sealing ring (34) has an internal shoulder firmly supported axially by a confronting shoulder (32) formed on the shaft (3) and facing towards the second sealing ring (35) and the pump chamber (7).

13. A seal arrangement according to claim 11 or claim 12, wherein the shoulders of the first sealing ring (34) and the shaft (3) are separated by a seal element (36) sealing the first sealing ring to the shaft.

14. A seal arrangement according to claim 13, wherein the seal element (36) is cylindrical with an external flange positioned between the said confront¬ ing shoulders.

15. A seal arrangement according to claim 16, wherein the seal element (36) extends axially from the shoulders substantially to the abutting faces of the first and second sealing rings (34, 35).

16. A seal arrangement according to any one of claims 11 to 15, wherein the first sealing ring (34) is keyed to the shaft (3) for rotation therewith by means (38, 39) which do not impede axial displacement of the first sealing ring (34) on the shaft (3).

17. A seal arrangement according to claim 16, wherein the key means comprise an axial slot (38) in the first sealing ring (34) and a pin (39)- fixed to the shaft and engaged in the slot (38).

18. A seal arrangement according to claim 17, wherein the slot (38) is open at the end of the first sealing ring (34) remote from the second sealing ring.

19. A seal arrangement according to any one of the preceding claims, wherein an enclosure defines a chamber (55) surrounding the sealing rings (34, 35) . and has a port (56) for supplying barrier fluid to the chamber, the enclosure being sealed to the pump hous- • ing at one end and being sealed at the other end thereof to the outer peripheral surface of the first sealing ring (34).

20. A seal arrangement according to claim 19, and as defined in any of claims 3 to 10, wherein the enclosure is constituted by the seal housing member (40).

21. A seal arrangement for sealing between a rotor shaft (3) and a pump housing (5) comprising a first sealing ring (34) carried on the shaft for rotation therewith, a second sealing ring (35) in axial abut- ment with the first sealing ring and non-rotatable relative to the pump housing, and an enclosure defin¬ ing a chamber (55) surrounding the sealing rings and including a port (56) for supplying barrier fluid to the chamber, the enclosure being sealed to the pump housing (5) at one end and being sealed relative to the shaft (3) at the other end, characterised in that the said other end of the enclosure is sealed to the outer peripheral surface of the first sealing ring (34).

22. A seal arrangement according to claim 19, 20 or 21 , wherein the enclosure is sealed to the first sealing ring by a lip seal (52) non-rotatably sup¬ ported by the enclosure.

23. A pump having a rotor shaft (3) sealed to a pump housing (5) by a seal arrangement as defined in any one of the preceding claims, and comprising a rotor (16) detachably mounted on the shaft (3) in the pump chamber (7), and a threaded connector (20) en¬ gaged with the end of the shaft to secure ^'the rotor thereon, the connector having an integral flange (20) received in a counterbore of the rotor, and a sealing means (23) being positioned between the peripheral surface of the flange and the counterbore.

24. A pump having a rotor (16) removably mounted on a shaft (3) and secured thereto by a threaded connec- tor (21) engaged with an end of the shaft, characterised in that said connector has an integral flange (20) received in a counterbore of the rotor, and a sealing means (23) is interposed between the peripheral surface of the flange and the counterbore.

25. A pump as claimed in claim 23 or 24, wherein the sealing means is an elastomeric ring.

26. A pump comprising a housing (5), a pair of rotors (16) carried on respective shafts (3), the rotors having lobes (19) and being arranged to cooper¬ ate with each other and the housing (5) to pump liquid from an inlet to an outlet upon rotation of the rotors in opposite directions, the pump housing (5) defining a chamber (7) including arcuate end wall portions (65) generally centered on the axes of the respective rotors, and the housing having a cover (6) attached thereto and closing the chamber, and an annular seal (67) interposed between the housing and cover, characterised in that the end wall portion (65) of the chamber are substantially semi-cylindrical and are interconnected by opposed straight parallel wall portions (66), and the seal (67) follows the peripher- al edge of the pump chamber in close proximity thereto and at substantially constant spacing therefrom around the entire circumference of the chamber.

27. A pump according to claim 26, wherein each rotor shaft (3) is sealed to the pump housing (5) by a seal arrangement as defined in any one of claims 1 to 22.

28. A pump according to claim 26, wherein the pump is also as defined in claim 23, 24 or 25.