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Патент USA US3071394

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Jan. 1, 1 963
Filed April 4, 1961
9- Sheets-Sheet 1
I; \ A
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t/ERN Mum- 5002M‘ 5am
Jan. 1, 1963
Filed April 4, 1961
2 Sheets-Sheet 2
?r Fad/42.0
United States Patent O??ce
pressures placed at opposite ends of an annular space
de?ned between a stator structure and a rotor structure
or shaft revolving with respect thereto.
Jean Marie Eugene Friberg, Bourg la Reine, Seine,
France, assignor to Societe Alsacienne de Construc
tions Mecaniques, Paris, France, a corporation of
Filed Apr. 4, 1961, Ser. No. 100,567
Claims priority, application France Apr. 7, 1960
9 Claims. (Cl. 277-3)
This invention relates to rotary mechanism having im
proved sealing means for isolating di?erent pressure zones
of the annular space de?ned between a stationary and a
Patented Jan. 1, 1963
Other objects
will appear.
vided rotary
de?ning an
thereof said
to an aspect of this invention, there is pro
mechanism including a stator and a rotor
annular space between adjacent surfaces
space including a ?rst zone containing a
?rst gas, at a low ?rst pressure and another zone remote
10 from the ?rst zone and containing a second gas at a
higher pressure, which mechanism includes sealing struc
ture for isolating said ?rst from said second gas com
prising suction means connected with said ?rst zone for
rotary part of the mechanism.
maintaining said ?rst pressure thereat, means delivering
The invention is more especially though not exclu 15 a third gas into a second zone of the space axially dis
sively directed to improvements in a form of rotary seal
placed from the ?rst zone toward the other zone at a
disclosed in French Patent No. 1,166,292 and compris
second pressure intermediate said ?rst and said higher
ing, in a rotary mechanism including a stator and a rotor
de?ning an annular space therebetween, one end of said
space containing a ?rst gas at a lower pressure and the
opposite end of said space containing a second gas at a
pressure, and further suction means connected with a
third zone of said space axially displaced from the second
zone toward said other zone for maintaining in the third
zone a third pressure somewhat lower than said second
higher pressure, sealing structure for isolating said gases
from each other comprising a ?rst helical groove of
Embodiments of the invention will now be described
tapered depth formed in one of said surfaces from an in
for purposes of illustration but not of limitation with
termediate point of said space toward said one end, and 25 reference to the accompanying diagrammatic drawings,
a second helical groove of tapered depth and reverse
pitch formed in said surface from a point adjacent said
FIG. 1 is a schematic fragmentary sectional view illus
intermediate point and extending toward said other end.
trating one half of a rotor shaft and an adjacent portion
As explained in said patent, such a sealing structure oper
of a stator structure having an improved rotary sealing
ates on the principle of a high-vacuum molecular pump 30 structure provided in the annular space de?ned between
of the Holweck type to drive back any molecules of the
?rst gas tending to move from said one end toward said
FIG. 2 is a simpli?ed fragmentary longitudinal sec
intermediate point back to said one end, and any mole
tional view illustrating a compressor having a primary
cules of the second gas tending to move from said other
seal constructed in accordance with the earlier French
end back to said other end, thereby maintaining an ex 35 patent, and a secondary seal according to the present in~
tremely high vacuum adjacent said intermediate point of
the annular space and positively preventing intermingling
FIGURE 3 is a graph indicating the pressures occur
of the gases.
ring at various points in the ?owpath of the system.
It may here be pointed out that a principal application
The embodiment of FIGURE 2, which will be described
of the rotary seal described in the earlier patent (and of
?rst, generally illustrates a compressor of a type used in
the improvement thereto according to the present in
nuclear reactors for pumping a radioactive gas such as
vention) is the application to compressors for radioac
uranium hexa?uoride. It is important not to let any
tive gas, e.g. uranium hexa?uoride, as used in nuclear
substantial amount of this gas leak out through the clear
reactors. In such an application the other gas used as a
ance space between the stator and rotor of the compres~
sealing gas is an inert gas of a high degree of purity such
sor, and to maintain the requisite low pressure in the
as recti?ed nitrogen. It is important that the radioactive
latter, and it is with this problem that the invention is
gas from the compressor be positively prevented from
chie?y concerned.
leaking out since it would otherwise raise di?icult prob
Referring to FIG. 2, there is illustrated part of a low
lems of protection and disposal in view of its highly
pressure pump or compressor including a centrifugal com
50 pressor wheel 31 formed with blading 32 and rotatable
radioactive condition.
The rotary seal of the earlier French patent operates
in a casing generally designated 33‘. The compressor de
extremely well, but only provided that the sealing gas
livers at its outlet 61 a gas at a subatmospheric outlet
such as nitrogen at the outer end of the rotary seal is at
pressure, e.g. 200 mm. Hg which may be radioactive
a relatively low pressure, of the same order of magnitude
uranium hexa?uoride as used in a nuclear reactor.
as the pressure of the gas, such as uranium hexafluoride,
Mounted on the rear side of the compressor Iwheel 31 are
to be sealed off. This pressure may be on the order of
auxiliary blades 32a providing an auxiliary centrifugal
20 mm. Hg. The problem remains, therefore, of sealing
compressor device. The compressor wheel 31 is secured
off the low-pressure nitrogen (at a pressure not substan
on a drive shaft 34 and further has a sleeve 35 projecting
tially higher and preferably lower, than this pressure of
from it so as coaxially to surround shaft 34 and serving
20 mm. Hg) from the surrounding-atmosphere. This 60 the purposes of a rotor both for the primary and the sec
problem might of course be solved by pumping suf?ciently
ondary seals to be described. Sleeve 35 is journalled on
large amounts of nitrogen to maintain this low nitrogen
spaced bearings 36 around a stationary sleeve»like exten
pressure at the outer end of the rotary seal, but such a
sion 37 forming part of the casing 33 projecting into the
solution is very uneconomical especially in view of the
space between shaft 34 and rotary sleeve 35, and the
high cost of producing the high-purity nitrogen required
bearings 36 may serve as all or part of the supporting
and the large pumping rates necessary to maintain the
means for the rotational support of the compressor wheel.
low pressure.
It will be understood that to the right (in the drawing)
It is a speci?c object of this invention to solve this
problem in an economical and effective manner. Broader
of the casing 33 and immediately around the shaft 34
normal atmospheric pressure may be present, and the
objects are the provision of improved low-pressure seal 70 purpose of the structure to be described is to seal off this
pressure from the low pressure uranium ?uoride (at about
ing means, and means for isolating two gases at different
the casing sleeve extension 31. While such a lay-out is ad
vantageous in that it reduces the over-all axial dimensions
root of the compressor blading. For this purpose there
of the primary and secondary seal assembly, it should be
is provided a ?rst or primary seal arrangement which is
understood that the secondary seal may, if preferred, be
similar to that disclosed in the aforementioned French
patent, and positioned generally between the outer sur $1 disposed generally on the same cylindrical surface as the
primary seal above described, beyond the chamber 42.
face of rotary sleeve 35 and the surrounding inner stator
20 mm. Hg) present in the region designated 62 at the
surface 40 of casing 33. This primary seal comprises a
pair of helical grooves of reverse pitch 38 and 38a formed
on the outer cylindrical surface of sleeve 35 and both
As shown, to provide the secondary seal of the inven
tion the inner cylindrical surface of rotatable sleeve 35
grooves. The shorter groove 38a opens at its outer end
into an annular chamber 42 formed in the stator or
Formed in the casing portion 37 are a number of ducts,
including the three ducts 50, 51 and 52 which open at
annular seal member 41 reciprocable through rods 44 in
directions parallel to the axis of the system, and in opera
tion the ring member 41 is arranged to be automatically
displaced leftward so as to bring ?exible annular seal
strips 43 provided on its left side into sealing engagement
with the outlet of the primary seal, on the compressor de
celerating to below ,a prescribed value prior to stopping. 30
vacuum pump 56 providing a moderate vacuum of say
about 100 mm. Hg, and duct 52 connects at its outer end
with a source of high-purity nitrogen at a pressure some‘
is formed with four axially spaced serrated sections 43,
leading away from an intermediate cylindrical portion or 10 44, 45 and 46 separated by smooth annular grooves 47,
43 and 49. The serrations formed in sections 43, 44, 45
land 39 of said sleeve, in opposite axial directions there~
and 46 may be circumferential grooves or may be helical
from. The groove 38 leading toward the compressor
grooves or screw threads. While the serrations may be
wheel 31 is the longer of the two being shown as includ
omitted and the corresponding sections 43 through 46
ing 8 turns, whereas the other helical groove 38a is shown
formed as smooth lands, it is preferred according to the
with only two turns. Both helical grooves are formed
invention to provide the serrations as shown since this is
with a depth that is gradually decreasing from the inter
found to improve the effectiveness of the seal.
mediate land 39 toward the outer ends of the respective
casing 33 and connecting with a conduit 55' leading to a 20 the outer surface of casing portion 37 respectively adja
cent the grooves 47, 48 and 49'. Duct 50‘ has its free
vacuum pump 58 through a valve 59 and an uranium
end connected with a source of compressed air at a slightly
hexa?uoride trap or separator 60 as later described. Fur
superatmospheric pressure; duct 51 connects with a
ther, there is mounted within the annular chamber 42 an
This seal member 41-43 serves to provide an ordinary
static seal during idle periods of the compressor.
As earlier explained herein and in the aforementioned
French patent, the action of the primary seal including
the dual helical groove arrangement described is to act
as a high-vacuum molecular pump of the Holweck type.
Thus, with the annular chamber 42 being connected with
what higher than the vacuum source 56, say a nitrogen
pressure of about 150 mm. An additional duct 54 formed
in casing part 37 and delivering at a point adjacent hear‘
ing 36 has its outer end connected with the outlet of an
oil-and-air mixer 57 which delivers a ?ne oil spray at
substantially atmospheric pressure for lubricating and
cooling the bearings, which spray traverses both bearings
as indicated by the arrow and ?nally escapes to atmos
phere or a reservoir by way of the annular space between
inner compressor shaft 34 and the inner surface of casing
part 37. The oil-air mixer 57 is shown supplied from the
a source of inert gas such as high-purity nitrogen at a rela
same source of compressed air as the duct 50.
However, the known type of operation just described
is satisfactory only if the pressure diiterential between the
out of the system, simultaneously cooling said bearings.
Another part of said air, free of oil, is passed rightward
Thus it is seen that at the inner or left hand end of the
tively low pressure approximately equal to or somewhat
lower than the uranium ?uoride gas pressure at the left 40 secondary seal arrangement described, i.e. adjacent bear
ings 36, atmospheric pressure obtains. The function of
hand end of helical groove 38, the helical groove 38 acts
the secondary seal is to isolate this atmospheric pressure
to drive any stray uranium ?uoride gas molecules tending
from the very low nitrogen pressure that is to be main
to flow rightward along the annular space between the
tained in the chamber 42 at the outer end of the primary
rotor and stator surfaces back toward the compressor
seal. The nitrogen pressure in chamber 42 is main
wheel, and similarly the reverse-pitch helical groove 38a
tained at a low value, preferably less than 20 mm. Hg, e.g.
would drive back any nitrogen molecules tending to stray
3 mm. Hg, by the action of the high-vacuum pump 58.
into said space from the chamber 42, thus maintaining an
The air at slightly superatmospheric pressure delivered
extremely high degree of vacuum in the region of land
through duct 50 adjacent groove 47, at A1, serves to pre
39 intermediate the two helical grooves and positively
vent the oil in the spray delivered through 54 from ?ow
preventing egress of the uranium hexa?uoride from the
ing rightward into the seal, and part of said air A1 there
compressor and ingress of the nitrogen into the com
fore escapes with said oil spray through the bearings and
two gaseous atmospheres (such as uranium ?uoride and
nitrogen in the case just referred to) is not too great, i.e.
provided the gas pressure in chamber 42 is not too much
higher than the compressor inlet pressure. To achieve
such a low nitrogen pressure in chamber 42 while using
across the serrated section 44 which constitutes the ?rst
land L1 of the secondary seal, and is sucked out of the
system through duct 51 by the moderate-vacuum pump
56 (eg at 100 mm. Hg).
The nitrogen as delivered through duct 52 at the
a conventional secondary seal (e.g. of the ordinary laby 60 moderate pressure of 150 mm. Hg in this example is, in
part, also drawn oif through duct 51 across the serrated
rinth type) to isolate the low-pressure nitrogen in cham
section 45 (zone L2 of the seal), but a major part of this
ber 42 from the outer atmosphere, it would, normally be
nitrogen is drawn into annular chamber 42 in view of the
necessary to pump the nitrogen at excessively high ?ow
much higher vacuum created by the high-vacuum pump
rates through said chamber, thereby seriously complicating
58. Simultaneously the pump 58 draws off through duct
the apparatus and gravely increasing operating expenses
55 any residual traces of uranium hexa?uoride that may
in view of the high cost of the extremely high grade of
have succeeded in passing out of the compressor through
purity required of the nitrogen in such nuclear installa
the primary seal, and such traces are trapped in the sep
tions. These di?iculties are overcome, in the system de
arator 60 by crystallization at very low temperature ob
scribed, by providing a secondary seal of improved char
tained e.g. by circulating liquid nitrogen between the
acter now to be described for isolating low-pressure cham
walls of the separator.
ber 42 from the atmosphere.
The pressure conditions in the system can be more
The secondary seal which forms'the main feature of
readily grasped from the chart of'FlG. 3 where pressures
this invention is generally positioned, in the embodiment
are plotted in ordinates on a logarithmic scale, while vari
shown, within the annular space de?ned between the inner
ous important points of the system considered along a
surface of revolving sleeve 35 and the outer surface 53 of
?ow-path extending from the outlet point 61 of the com
pressor, across the primary seal and then the secondary
across a sealing section such as L1, L2 or L3‘ is approxi
mately proportional to the difference between the squares
of the pressures present at the opposite ends of such sec
tion. Hence the leakage ?ow across sealing zones L2 and
seal and to atmosphere, are indicated on the abscissa axis
on an arbitrary scale. The full line graph indicates the
conditions during rotation of the compressor, while the
dotted-line graph relates to the idle condition of the sys
tem. Points 63 and 64 respectively refer to the starting
points of the two opposite helical grooves 38 and 38a of the
primary seal, i.e. to points on either side of the smooth
land 39 intermediate said grooves. It is noted that at 10
both these points, and particularly at point 63 which is
the origin of helical groove 38, an extremely high vacu
L3 is much lower than would occur across a single seal
ing zone, such as L1, across which a large pressure differ
ence would have to be maintained. Thus the seal of the
invention saves a considerable amount of the valuable
gas that would otherwise be lost through leakage across
the zone L2.
What I claim is:
1. In rotary mechanism including a stator and rotor
de?ning an axially elongated annular space between
um is present, indicating a substantially complete isola
tion between the uranium hexa?uoride atmosphere in the
adjacent surfaces thereof, the combination comprising:
compressor and the nitrogen atmosphere pumped into the 15 suction means connected with a ?rst zone of said space
outer end of the primary seal. The chart simultaneously
for maintaining a low ?rst pressure in said ?rst zone;
shows, in the portion of the graph relating to the second
means delivering a relatively valuable gas into a second
ary seal i.e. from point 42 to point 50 (or 54) of the
zone of said space axially displaced from the ?rst zone
graph, that the relatively low nitrogen pressure which is
at a second pressure substantially higher than said ?rst
required to be maintained at point 42 to permit the afore 20 pressure; further suction means connected with a third
said isolation to be effectively accomplished by the op
zone of said space axially displaced from the second zone
eration of the primary seal, is obtained as the result of
away from the ?rst zone for maintaining in said third
only a moderate drop in nitrogen pressure from 150 to
zone a third pressure somewhat lower than said second
3 mm. Hg while the seal between the higher-pressure ni
pressure, and means delivering a low-valued gas into said
trogen and the atmospheric or higher-pressure air is pro 25 space in a fourth zone thereof axially displaced from the
vided principally by way of the low-vacuum suction
third zone away from the ?rst and second zones at a
through duct 51, which provides an effective separation
fourth pressure substantially higher than said second
between the atmospheric or superatmospheric pressure
lubricating oil spray, and the high-purity nitrogen serving
2. The combination claimed in claim 1 wherein said
as the sealing meduim.
30 low-valued gas comprises air and said fourth pressure is
A schematic presentation of a secondary seal according
substantially atmospheric pressure.
to this invention is given in FIG. 1 apart from the pri
3. The combination claimed in claim 1 wherein said
mary seal, for providing a better understanding of the
duct means are formed in said stator.
fundamental structure and operation of the invention. In
4. The combination claimed in claim 1, including
this ?gure, there is shown in axial section the lower half 35 generally circumferential serrations formed in one of said
of a shaft or other rotor structure 21 and a portion of a
adjacent surfaces over at least part of the axial extent
casing or other stator structure 22, and a seal arrange
ment according to the invention is provided in the adja
thereof from said ?rst to said fourth zone.
5. The combination claimed in claim 1, including
cent surfaces of the rotor and stator to isolate the left side
generally circumferential serrations formed in the surface
of the assembly, which may be at atmospheric pressure, 40 of said rotor between adjacent ones of said zones.
from the right side which is at a lower gaseous pressure.
6. In rotary mechanism including a stator and a rotor
Said right side may be (but not necessarily is) connected
with the outer end of a primary seal of the type earlier
speci?ed. Thus the annular chamber 24 here shown as
formed in the rotor 21 corresponds in function to the
chamber 42 in FIGURE 2 in which a low pressure of pure
de?ning an axially elongated annular space between adja
cent surfaces thereof, and wherein one end of said space
is at a substantially lower gaseous pressure than the
45 surrounding atmosphere, means for isolating said one end
of said space from the atmosphere comprising: a ?rst
helical groove formed in one of said surfaces and extend
ing from a ?rst point spaced from said one end of said
tions 25 and 26 of the rotor in which serrations, e.g. screw
space toward said one end with the depth of the groove
threads, are provided, separated by the smooth portion 50 decreasing along said ?rst groove; a second helical groove
27. Formed through the stator are the ducts 28, 29 and
formed in said one surface and extending from a point
30. Duct 27 delivers into the annular space adjacent the
axially spaced from said ?rst point in reverse sense from
smooth section 27, and is connected at its outer end with
said ?rst groove with the depth of the groove decreasing
a source of moderate vacuum E1, corresponding to the
along said second groove; suction means connected with
moderate vacuum pump 56 of FIG. 2. Duct 29 delivers 55 a first zone of said space adjacent the end of the second
at a point intermediate the length of the serrated sec
groove remote from the ?rst groove for maintaining a
tion 26 and is connected at its outer end with a source
low pressure in said ?rst zone of similar order of mag
A2 of the valuable sealing gas (e.g. nitrogen) at a pres
nitude to the low pressure at said one end of said space;
nitrogen, or other valuable sealing gas, is to be main
tained. The secondary seal comprises two spaced sec
sure only slightly higher than E1, this duct therefore cor
means delivering a ?rst gas into a second zone of the
responding to duct 52 of FIG. 2. Duct 30 delivers into 60 space axially displaced from the ?rst zone away from
the low-pressure chamber 24 and is connected at its outer
end with a source E2 of high-vacuum such as the pump
said one end thereof at a second pressure intermediate
said ?rst zone pressure and the pressure of the surround
58 of FIG. 2 for maintaining the requisite low pressure in
atmosphere; further suction means connected with a
chamber 24. Thus it is seen that the rotor 21 is sub
divided into three seal sections L1, L2 and L3. Atmos 65 third zone of said space axially displaced from the second
zone away from the ?rst zone for maintaining in said
pheric air is admitted at A1 into zone L1. This air to
zone a third pressure somewhat lower than said
gether with a minor proportion of the valuable gas is
second pressure; and means applying the pressure of said
withdrawn at moderately low pressure at E1. Most of
surrounding atmosphere to a fourth zone of said annular
the sealing gas discharged into the system from A2 is
space axially displaced from the third zone and adjacent
passed into the chamber 24 and serves to maintain there
the opposite end of said space.
a low pressure of the sealing gas, while requiring a con 70
7. In rotary mechanism including a stator and a
siderably lower amount of this valuable gas to be de
rotor de?ning an annular space between adjacent surfaces
livered per unit time than would be required in the ab
thereof and wherein one end of said space contains a ?rst
sence of the sealing structure just described.
gas at a lower pressure and the opposite end of said space
In this connection it is recalled that the leakage flow 75 contains a second gas at a higher pressure, means for
isolating said gases comprising: a ?rst helical groove of
tapered depth formed in said rotor surface from an inter
mediate point of said space towards said one end; a
second helical groove of tapered depth formed in said
rotor surface from a point adjacent said intermediate
point towards said other end; suction means connected
8. The combination claimed in claim 6 wherein said
rotor comprises a tubular extension projecting between
coaxial surfaces of said stator, said extension having an
inner and an outer cylindrical surface, and wherein
said helical grooves are formed on one of the cylindrical
surfaces of the rotor extension and said zones are gen
erally provided adjacent the other of said cylindrical
with said space at a zone adjacent the shallow end of
rotor surfaces.
the second groove for maintaining a low pressure in said
9. The combination claimed in claim 8 wherein said
zone generally similar to said lower gas pressure; means
grooves are formed on the outer cylindrical sur
delivering a third gas into said space at another zone
face of the rotor extension.
displaced from said zone away from said one end at a
pressure intermediate said lower and higher gas pres
References Cited in the ?le of this patent
sures; further suction means connected with said space
at a point displaced from said other zone for applying
at said last point a further pressure somewhat lower than l5 2,721,747
Whit?eld ___________ __ Oct. 25, 1955
said intermediate pressure; whereby said second gas
applied at said opposite end will be drawn out by said
further suction means and said third gas will be drawn
out by said ?rst suction means.
Cuny _______________ __ Sept. 8, 1959
Patent N00 3,,O71v384
January 1' 1963
Jean Marie Eugene Friberg
It is hereby certified that error appears in the above numbered pat
ent requiring correction and that the said Letters Patent should read as
corrected below.
Column 41,
line 59, for "as" read —— gas ——.
Signed and sealed this 5th day of November 1963.
Attesting Officer
AC ting Commissioner of Patents
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