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EP 0 033 726 B1 |
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EUROPEAN PATENT SPECIFICATION |
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Mention of the grant of the patent: |
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25.07.1984 Bulletin 1984/30 |
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Date of filing: 09.08.1979 |
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International Patent Classification (IPC)3: F04C 19/00 |
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International application number: |
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PCT/US7900/586 |
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International publication number: |
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WO 8100/438 (19.02.1981 Gazette 1981/05) |
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TWO STAGE LIQUID RING PUMP
FLÜSSIGKEITS-RINGPUMPE MIT ZWEI STUFEN
POMPE A ANNEAU LIQUIDE A DEUX ETAGES
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Designated Contracting States: |
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DE GB SE |
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Date of publication of application: |
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19.08.1981 Bulletin 1981/33 |
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Applicant: THE NASH ENGINEERING COMPANY |
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Norwalk
Connecticut 06856 (US) |
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Inventor: |
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- HAAVIK, Harold K.
South Norwalk, CT 06854 (US)
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Representative: Baillie, Iain Cameron et al |
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Ladas & Parry,
Altheimer Eck 2 80331 München 80331 München (DE) |
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[0001] This invention is concerned with a liquid ring pump. Such a pump essentially comprises
a bladed rotor mounted within an eccentric casing into which ring liquid or seal liquid
is introduced and, under the centrifugal force produced by rotation of the rotor,
is caused to form a ring following the interior contour of the casing. The blades
of the rotor and the inner surface of the ring define working chambers or buckets
which are alternately brought into communication with inlet and outlet ports and into
which a gas is admitted during a suction stroke and from which the gas is expelled
as the bucket volume contracts.
[0002] Different porting techniques are adopted for admitting gas to the buckets and for
allowing the exit of gas from the buckets. For example, a so-called center port pump
is known in which the gas enters and leaves the buckets radially. A side port pump
is known in which the gas enters and leaves the pump axially. Combinations of those
two types of pump are also known in which one of the inlet and outlet ports communicates
radially with the buckets while the other communicates axially with the buckets.
[0003] Additionally, the pumps may have casings which define a single lobe such that there
be one operational cycle per revolution of the rotor or the casing may define multiple
lobes, there being as many cycles per revolution as there are lobes. It will be recognized
from the following description that the subject matter herein is applicable to the
various kinds of pumps.
[0004] The invention is specifically concerned with pumps of the two stage kind that is
to say is concerned with pumps comprising a first pumping stage, the outlet of which
is connected to the inlet of a second pumping stage, the second pumping stage being
of lesser capacity than the first pumping stage. One pump of this general kind is
described in British Patent 691,425.
[0005] A two stage liquid ring pump comprises first and second stages, each including a
rotor within a pump casing in which a quantity of seal liquid forms an annular ring
around the inner periphery of the casing and having intake and discharge stroke areas
within the casing, and interstage conduit means connecting the first stage discharge
stroke area to the second stage intake stroke area.
[0006] It is well known that in two stage pumps the first stage is constructed with several
times the volumetric displacement capacity of the second stage. At low vacuum levels
the first stage pump discharges a gas volume rate greater than that which can be handled
by the second stage, i.e. a gas volume rate in excess of the capacity of the second
stage. If the second stage has a full liquid ring and the interstage, i.e. the connection
between the first and second stages, is not otherwise vented, the excess volume of
gas passed from the first stage over that which can be accommodated by the second
stage, is trapped which results in high pressures between the first and second stages.
This situation results in such performance problems as a high power requirement, reduced
first stage capacity and surging, i.e. unstable pumping action. Attempts to solve
this problem have included the provision of a bypass check valve to vent the excess
of the pump's capacity, the check valve opening whenever the volume of gas moved by
the first stage exceeds that which can be handled by the second stage. This solution
is generally adequate but, of course, relies upon a mechanical device for its effectiveness
and the value is subject to breakdown or malfunction.
[0007] An alternative technique is described in the aforementioned British patent specification.
In that patent there is provided an unloader hole in the second stage which is effective
to bleed seal liquid from that stage to decrease the thickness of the liquid ring
and necessarily, therefore, to increase the capability of the second stage to permit
the gas to pass directly from its inlet to its discharge. This method also relies
preferably on a valve to regulate the amount of liquid discharge from the second stage
and, of course, one configuration or valve setting providing for a specific discharge
at a nominal operating condition does not give good performance characteristics at
different speeds and seal flow rates. Further, this latter method is one which does
not provide tolerance to seal flow variations and to rotor speed variations.
[0008] According to a preferred embodiment of this invention, it is proposed that both stages
of a two stage liquid ring vacuum pump be provided with unloader orifices. The angular
and radial location of the unloader orifices has been determined to be of crucial
importance and can be selected to render the pump substantially immune to the effects
of variations in rotor speed or variations in the delivery rate of fresh seal liquid.
The maintenance of a seal water reservoir for recirculation of the unloader liquids
at high vacuum further renders the pump less sensitive to fresh seal liquid rate changes.
These advantages and others are discussed in greater detail infra.
[0009] Embodiments of the present invention are illustrated in the accompanying drawings
in which:
Fig. 1 shows, schematically and in eccentric longitudinal cross section, a pump according
to this invention.
Fig. 2 is a diagram illustrating the disposition of the unloader orifices according
to the present invention; and
Figs. 3 and 4 are plots showing the results of tests conducted with equipment according
to the present invention.
[0010] The pump in Fig. 1 is a two-stage liquid ring pump of the center head kind. It comprises
a shaft 10 having a drive end 12 by which drive is imparted to it from a motor through,
if necessary, an appropriate transmission. The shaft is mounted in bearing 14 within
bearing bracket 22 at its drive end and in bearing 16 within bearing bracket 19 at
its idle end.
[0011] The first stage of the pump comprises a casing 20 which is mounted between the drive
end head structure 25 and a center head structure 24 described in more detail hereinafter.
In the particular embodiment of the invention illustrated the pump is of the circular
lobe type and the interior surface 26 of the casing is cylindrical and is eccentric
to the shaft 10. The casing is closed by port plates 28, 28' provided with inlet openings
30 and discharge ports 32, 32' shown schematically in Fig. 1 and of known configuration.
Keyed to the shaft 10 within casing 20 is a rotor 33 comprising a hub portion 34,
and a plurality of radially disposed and axially extending blades 38 the free radial
edges of which have a close clearance with the port plates 28, 28'.
[0012] Gas is admitted to the buckets through the first stage inlet passage 40 to the center
head 24 and through the end head 25 and then on through inlet ports 30, 30'. The gas
is then discharged through the first stage discharge ports 32, 32' on either side
of the first stage and then into interstage passages 43, 43' which are interconnected
by an external connection 43" shown schematically in Fig. 1 but which, of course,
may be integrally cast within the casing 20. Connection 43" leads to an inlet port
44 of port plate 46 of the second stage pump. The second stage pump is constituted
by a casing 48 and is closed by port plate 46 which includes a discharge port 50.
[0013] Within the casing 48 there is a rotor 52 comprising a hub 54 and a plurality of radially
disposed and longitudinally extending blades 56. The casing is eccentric to the shaft
and to the rotor on that shaft. Gas entering the second stage pump from the first
stage pump through inlet port 44 is transported to discharge port 50 and thence to
a discharge passage 60 in the center head.
[0014] The discharge passage 60 connects with a seal liquid reservoir 60' via channel 60"
within the center head so that the reservoir constantly receives ring liquid discharged
through the port 50 and is maintained at the discharge pressure of the second stage
pump.
[0015] Both the first stage and the second stage pumps of the embodiment of Fig. 1 are provided
with relatively small unloading orifices, the locations of which are discussed in
greater detail hereinafter with particular regard to Fig. 2 and those orifices depicted
diagrammatically in Fig. 1 as the drive-end first stage unloader orifice 70, idle-end
first stage orifice 71 and second stage unloader orifice 73, communicate with the
seal liquid reservoir 60' either directly, through external tubing 75 or by means
of channels 60' formed in the center head.
[0016] Passage 60" and reservoir 60' are so positioned that they are continually flooded
and when either or both the first or second stage casing pressures in the unloader
orifice locations go to vacuum, the reservoir liquid will recirculate from the second
stage discharge to the first and second stage casings. At high vacuum this recirculation
renders the pumps less sensitive to fresh seal liquid rate changes.
[0017] Fig. 2 illustrates, diagrammatically, the disposition of the unloader orifices. Specifically,
Fig. 2 is in effect a diagrammatic end view of the pump with the shaft axis at 200
and the lobe axis at 202 so that the eccentricity Y is the spacing between the two
axis. The land (i.e., the point at which the outer periphery of the rotor is closest
to the inner periphery of the casing), of course, is at the zero degrees position.
[0018] The outer peripheral edge of the rotor 33-52 is indicated at 204 and the rotor radius
is indicated at R
R. The casing or lobe radius is indicated at R
L and the inner surface of the lobe is indicated at 206.
[0019] The rotor turns in the direction indicated by the arrow w.
[0020] The intake or inlet port is indicated at I and the discharge port at D. It is to
be appreciated that the diagram in Fig. 2 is common to the pump of Fig. 1 and as such
inlet port I corresponds to ports 30, 30' and 44 of the embodiment of Fig. 1 and to
the discharge ports 32, 32' and 50.
[0021] It is to be noted that in the particular embodiment illustrated, the intake stroke
essentially from 0 to 180° with the intake port normally contained within these angles.
The discharge stroke is from 180 to 360° with the discharge port normally defined
within these angles.
[0022] In the particular embodiment of the invention illustrated it has been determined
that for the considerations herebelow, the optimum disposition of the first stage
unloader orifice 1 UO is approximately 230° and slightly beyond the periphery of the
rotor.
[0023] For the second stage unloader orifice 2UO, it has been determined that the optimum
position between 20 and 180° from land and at the outermost portion of the lobe defining
casing.
[0024] The effects of the unloader orifices and the positions of those orifices can be determined
from the consideration of Figs. 3 and 4.
[0025] From a consideration of the graphs now to be discussed, it is concluded that the
particular disposition of the unloader orifices in the first stage gave good tolerance
to a variation of seal flow from a nominal flow rate or over a range of approximately
plus and minus 25%. Similarly, the configuration worked well for the rotor tip speed
ranging from 50 to 65 FPS (15 to 20 MPS) over the whole operating range of the pump
without poor performance characteristics such as excessive power, surging, and/or
excessive capacity loss.
[0026] Fig. 3 is a graph in which the capacity of the pump for different locations of the
unloader orifice in the first stage are shown and upon which also the horsepower per
capacity measure is plotted against angular location of the first stage unloader orifice.
Referring to the curves at the upper part of Fig. 3 they show the operation of the
pump for a nominal seal delivery rate plus or minus 25%. Nominal seal delivery rate
means the seal delivery rate which gives maximum or near maximum pump efficiency in
cubic meters per hour per kilowatt at the (vacuum) pressure at which the pump is expected
to operate most of the time. The full line plots show the capacity of 27.5" HgA (93.1
KP) while the dash line curves show the capacity of 2" HgA (6.8 KPa).
[0027] The three lower plots show the performance expressed in horsepower per actual cubic
feet per minute (kilowatt per actual cubic meter hour) at 27.5" HgA (93.1 KPa). Again,
the curves are of the conditions prevailing for the nominal seal rate plus or minus
25% delivered to the pump.
[0028] The minimum value for horsepower per cubic feet per minute (kilowatts per cubic meter
per hour) for the nominal seal rate is used as a base to compare the HP/CFM (Kw/m
3/h) at different unloader orifice locations. In this respect the value of 1.00 is
determined for the pump having the unloader orifice at 250° with the nominal seal
rate and at the rotor tip speed of 61 FPS (18.6 MPS). As can be seen approximately
40% more power is required with the orifice at 180° than at 250° with the nominal
seal liquid rate.
[0029] The parts on the graph in Fig. 4 are substantially similar to those in Fig. 3 but
are the results obtained by testing a pump rotated at a tip speed of 52 feet per second
(15.8 meters per second).
[0030] The graphs demonstrate the effect of the angular location of the unloader orifice
of the first stage on the overall pump performance, i.e., the low and high vacuum
displacement figures and the low vacuum power requirement per unit of volumetric displacement.
[0031] The primary consideration is the hp/cfm (Kw/m
3/h) figures which for both speeds is at or close to a minimum for angular locations
greater than 215° and up through 250°. The second most significant effect is seen
to be on low vacuum capacity which, especially for the lower seal rate drops rapidly
as the angular location increases from 180 to 250°. The optimum location of around
230° balances a combination of low hp/cfm (Kw/m
3/h) and higher low vacuum capacity. It is to be noted that the high vacuum cfm (m
3/h) is not greatly affected by the angular location of the unloading orifice.
[0032] From a consideration of these curves, a pump designer would almost always want the
first stage unloader orifice to be located at 215° or further from land. According
to present considerations an optimum location is 230° from land. However, if one ignores
the low vacuum capacity criteria, the optimum location would be closer to 250° from
land which gives the lowest power requirement on the hp/cfm (Kw/m
3/h) curve and gives slightly better overally high vacuum capacity. Also, a pump operated
at low speed only will operate well with the hole location at 250° from land.
[0033] It will be quite apparent to one skilled in the art by appropriately selecting the
position of the unloader orifice in the first stage one can derive a combination of
characteristics as desired.
[0034] As noted hereabove, by the appropriate selection of the position of the unloader
orifice, certain advantages accrue, those advantages including, the tolerance of a
two-stage vacuum pump to speed change; the tolerance of such a pump to variations
in seal liquid flow; good performance of a two-stage vacuum pump with a fixed seal
flow rate over its entire operating range and good performance with no interstage
bypass check valve which, of course, is subject to mechanical failure or misfunction.
[0035] At low vacuums, the first stage discharges a gas volume rate in excess of the capacity
of the second stage. If the second stage has a full liquid ring and the interstage
is not otherwise vented, the excess capacity is trapped in the interstage resulting
in high interstage pressures. This situation creates the problems discussed supra.
As noted hereabove, the techniques adopted to correct this situation have been the
inclusion of an interstage bypass check valve and the unloading of the second stage
as described in the aforementioned British patent 691,425. According to the teaching
of that British patent, proper sizing of the unloading orifice relieves sufficient
water from the second stage lobe at low vacuum to decrease the liquid ring thickness
and increase the capacity of the second stage to bypass gas.
[0036] The seal unloading system of the present invention works well because, among other
things, it takes advantage of naturally occurring pressure distributions within the
pump. In the first stage, the placing of the unloader orifice in the compression zone
approximately 230° from the land is optimized because in this position it senses the
highest internal air pressure occurring in the first stage. Over compression in the
bucket occurs in this region of the compression stroke at the low vacuum high mass
flow rate condition. According to test measurements the unloader pressure is high
in the range of 15 to 30" HgA (50.8 to 101.6 Kpa) first stage inlet pressure and then
tends to fall off quickly at inlet pressures less than 15" HgA (50.8 Kpa). Since the
rate of seal discharge through the unloader is proportional to the square root of
this pressure, once one has resolved to unload the first stage, it can be seen that
the seal unloading rate from the first stage will be high at high absolute pressures
and will decrease rapidly at pressures less than 15" (50.8 Kpa).
[0037] It is believed that this flow characteristic accounts in part for the good power
characteristic of the pump at low vacuum since the second stage receives a small supply
of seal liquid which is easily discharged through its unloader orifice. Thus, the
second stage liquid ring replenishment is minimized at low vacuum.
[0038] In comparison to the method described in the British patent 691,425 this new method
allows a much smaller orifice diameter for the second stage unloader since at low
vacuum it must unload only a small percentage of the seal flow to the pump (because
of the diverted first stage flow), whereas the British proposal must be capable of
discharging nearly 100% of the flow. This is important when the second stage is operating
at high compression ratio where it is desirable to minimize the unloading rate from
the second stage to obtain peak performance of this unit. The larger orifice of the
British patent design (especially since it is located on the discharge side) is sensitive
to pump speed and delivered seal flow to the pump and has to be throttled or otherwise
controlled by some external means for operation under conditions other than those
for which the pump is specifically designed. This requirement for a throttling valve
in line is, of course, a disadvantage since it is a part subject to mechanical failure
and also requires adjustment for varying conditions.
[0039] Another characteristic of the present invention is the fact that the second stage
unloading orifice can be located anywhere within the intake stroke because the pressure
distribution is essentially constant with respect to circumferential location in this
region. Location on the inlet side also tends to minimize the discharge pressure and
flow of this unloader as the second stage goes to high compression ratio since the
lobe pressure tracks the second stage suction pressure.
1. A two-stage liquid ring pump comprising first and second stages, each including
a rotor (33, 52) within a pump casing (20, 48) in which a quantity of seal liquid
forms an annular ring around the inner periphery of the casing and having intake (30,
44) and discharge (32, 32', 50) stroke areas within the casing, and interstage conduit
means (43, 43") connecting the first stage discharge stroke area (32, 32') to the
second stage intake stroke area (44), characterized by a seal liquid unloading orifice
(70, 71) in the first stage casing disposed so that the unloading orifice is normally
covered by the first stage liquid ring and connected to seal liquid conduit means
(60', 60", 75) outside the first stage casing which communicates with the discharge
of the second stage and which is maintained at the discharge pressure of the second
stage.
2. A pump according to claim 1, characterized in that the portion (60") of the seal
liquid conduit means adjacent the unloading orifice (70, 71) is constantly flooded
with seal liquid during operation of the pump.
3. A pump according to claim 1, characterized by a seal liquid unloading orifice (73)
in the second stage casing disposed so that the second stage unloading orifice is
normally covered by the second stage liquid ring and connected to the seal liquid
conduit means (60').
4. A pump according to claim 3, characterized in that the first stage unloading orifice
(70, 71) is disposed at an angular location included within the angular location of
the first stage compression stroke area.
5. A pump according to claim 4, characterized in that the first stage unloading orifice
(70, 71) is disposed at an angular location of between 215° and 250° from the land
in the direction of rotation.
6. A pump according to claim 4, characterized in that the first stage unloading orifice
(70, 71) is disposed adjacent the outer periphery of the first stage rotor (33).
7. A pump according to claim 5, characterized in that the first stage unloading orifice
(70, 71) is disposed adjacent the outer periphery of the first stage rotor (33).
8. A pump according to claim 5, characterized in that the first stage unloading orifice
(70, 71) is disposed at an angular location of about 230° from the land in the direction
of rotation and adjacent the outer periphery of the first stage rotor (33).
9. A pump according to claim 3 characterized in that the second stage unloading orifice
(73) is disposed at an angular location included within the angular location of the
second stage intake stroke area and adjacent the outer periphery of the second stage
liquid ring.
10. A pump according to claim 3, characterized in that the first (70, 71) and second
(73) stage unloading orifices are normally covered by seal liquid in the seal liquid
conduit means (60', 60", 75).
1. Zweistufige Flüssigkeitsringpumpe mit einer ersten und einer zweiten Stufe, die
je einen Rotor (33, 52) in einem Pumpgehäuse (20, 48) besitzen, an dessen Innenumfang
eine Menge einer Sperrflüssigkeit einen Ring bildet, in dem ein Einlaßhubbereich (30,
44) und ein Druckhubbereich (32, 32', 50) angeordnet sind, ferner mit einer die Stufen
verbindenden Leitungsanordnung (43, 43") zwischen dem Druckhubbereich (32, 32') der
ersten Stufe und dem Einlaßhubbereich (44) der zweiten Stufe, dadurch gekennzeichnet,
daß in der ersten Stufe eine zur Abgabe von Sperrflüssigkeit dienende Drosselöffnung
(70, 71) so angeordnet ist, daß die Drosselöffnung normalerweise von dem Flüssigkeitsring
der ersten Stufe bedeckt ist, wobei die Drosselöffnung mit einer außerhalb des Gehäuse
der ersten Stufe vorgesehenen Sperrflüssigkeits-Leitungsanordnung (60', 60" 75) verbunden
ist, die mit der Druckseite der zweiten Stufe in Verbindung steht und unter dem Austrittsdruck
der zweiten Stufe gehalten wird.
2. Pumpe nach Anspruch 1, dadurch gekennzeichnet, daß der der Drosselöffnung (70,
71) benachbarte Teil (60") der Sperrflüssigkeits-Leitungsanordnung im Betrieb der
Pumpe ständig mit Sperrflüssigkeit überflutet wird.
3. Pumpe nach Anspruch 1, dadurch gekennzeichnet, daß in dem Gehäuse der zweiten Stufe
eine Drosselöffnung (73) zur Abgabe von Sperrflüssigkeit derart angeordnet ist, daß
die Drosselöffnung der zweiten Stufe normalerweise von dem Flüssigkeitsring der zweiten
Stufe bedeckt ist, und daß die Drosselöffnung der zweiten Stufe mit der Leitungsanordnung
(60') für die Sperrflüssigkeit verbunden ist.
4. Pumpe nach Anspruch 3, dadurch gekennzeichnet, daß die Drosselöffnung (70, 71)
der ersten Stufe im Winkelbereich des Druckhubes der ersten Stufe angeordnet ist.
5. Pumpe nach Anspruch 4, dadurch gekennzeichnet, daß die Drosselöffnung (70, 71)
der ersten Stufe in einem Winkelbereich von 215 bis 250 Grad von der dem Steg, in
der Drehrichtung gemessen, angeordnet ist.
6. Pumpe nach Anspruch 4, dadurch gekennzeichnet, daß die Drosselöffnung (70, 71)
der ersten Stufe im Bereich des Außenumfanges des Rotors (33) der ersten Stufe angeordnet
ist.
7. Pumpe nach Anspruch 5, dadurch gekennzeichnet, daß die Drosselöffnung (70, 71)
der ersten Stufe im Bereich des Außenumfanges des Rotors (33) der ersten Stufe angeordnet
ist.
8. Pumpe nach Anspruch 5, dadurch gekennzeichnet, daß die Drosselöffnung (70, 71)
der ersten Stufe im Bereich des Außenumfanges des Rotors (33) der ersten Stufe unter
einem Winkel von etwa 230°, von dem Steg in der Drehrichtung gemessen, angeordnet
ist.
9. Pumpe nach Anspruch 3, dadurch gekennzeichnet, daß die Drosselöffnung (72) der
zweiten Stufe im Bereich des Außenumfanges des Flüssigkeitsringes der zweiten Stufe
im Winkelbereich des Einlaßhubes des Flüssigkeitsringes der zweiten Stufe angeordnet
ist.
10. Pumpe nach Anspruch 3, dadurch gekennzeichnet, daß die Drosselöffnungen der ersten
(70, 71) und der zweiten (73) Stufe normalerweise von Sperrflüssigkeit in der Leitungsanordnung
(60', 60", 75) bedeckt sind.
1. Pompe annulaire à liquide à deux étages comprenant des premier et second étages,
comportant chacun un rotor (33, 52) disposé dans un boîtier de pompe (20, 48), dans
lequel un volume de liquide formant joint constitue une boucle annulaire autour de
la périphérie interne du boîtier, et possédant des zones de course d'admission (30,
44) et d'évacuation (32, 32', 50), à l'intérieur du boîtier, des moyens formant conduit
inter-étages (43, 43") reliant la zone (32, 32') de course d'évacuation du premier
étage à la zone (44) de course d'admission du second étage, caractérisée par un orifice
(70, 71) de décharge d'un liquide formant joint, dans le boîtier du premier étage,
disposé de telle sorte que l'orifice de décharge soit normalement recouvert par la
boucle de liquide du premier étage, et relié à des moyens (60', 60", 75) qui constituent
un conduit pour le liquide formant joint, disposés à l'extérieur du boîtier du premier
étage, qui communiquent avec l'évacuation du second étage et qui sont maintenus à
la pression d'évacuation du second étage.
2. Pompe selon la revendication 1, caractérisée en ce que la portion (60") des moyens
constituant un conduit pour le liquide formant joint, adjacents à l'orifice de décharge
(70, 71), est submergée en permanence avec du liquide formant joint pendant le fonctionnement
de la pompe.
3. Pompe selon la revendication 1, caractérisée par un orifice de décharge (73) pour
le liquide formant joint dans le boîtier du second étage, disposé de telle sorte que
l'orifice de décharge du second étage soit normalement recouvert par la boucle de
liquide du second étage et relié aux moyens (60') constituant un conduit pour le liquide
formant joint.
4. Pompe selon la revendication 3, caractérisée en ce que l'orifice (70, 71) de décharge
du premier étage est disposé dans une position angulaire comprise dans le secteur
angulaire de la zone de course de compression du premier étage.
5. Pompe selon la revendication 4, caractérisée en ce que l'orifice de décharge (70,
71) du premier étage est disposé à une position angulaire comprise entre 215° et 250°
à partir du point mort dans la direction de rotation.
6. Pompe selon la revendication 4, caractérisée en ce que l'orifice de décharge du
premier étage (70, 71) est disposé adjacent à la périphérie extérieure du rotor du
premier étage (33).
7. Pompe selon la revendication 5, caractérisée en ce que l'orifice de décharge du
premier étage (70, 71) est disposé adjacent à la périphérie extérieure du rotor du
premier étage (33).
8. Pompe selon la revendication 5, caractérisée en ce que l'orifice de décharge du
premier étage (70, 71) est disposé à une position angulaire d'environ 230° à partir
du point mort dans la direction de rotation et adjacent à la périphérie extérieure
du rotor du premier étage (33).
9. Pompe selon la revendication 3, caractérisée en ce que l'orifice de décharge du
second étage (73) est disposé à une position angulaire comprise dans le secteur angulaire
de la zone de course d'admission du second étage et adjacent à la périphérie extérieure
de la boucle de liquide du second étage.
10. Pompe selon la revendication 3, caractérisée en ce que les orifices de décharge
du premier (70, 71) et second (73) étages sont normalement recouverts par du liquide
formant joint dans les moyens constituant le conduit (60', 60", 75) pour le liquide
formant joint.