Sealing device for the drive shaft of a high pressure fluid pump

Sealing device for the drive shaft (27) of a high pressure fluid pump comprising a system of seals (33, 34, 35) at least one of which (33) is a hydrostatic seal. Between this hydrostatic seal (33) and the interior of the volute (21) is disposed an auxiliary seal (45) of the mechanical type. The auxiliary seal (45) separates two chambers (46a) and (46b) which can be brought into communication or isolated by a system of pipes (47) on which a valve (48) is disposed. The invention is particularly applicable to the primary pumps of a pressurized water nuclear reactor.

FIELD OF THE INVENTION 
The invention relates to a sealing device for the drive shaft of a high 
pressure fluid pump. 
BACKGROUND OF THE INVENTION 
In pressurized water nuclear reactors, the reactor core cooling circuit, or 
primary circuit, comprises at least two cooling loops, each containing a 
steam generator and a primary pump. 
The primary pumps are composed of a volute, inside which a bladed wheel 
turns which is rigidly fixed to the bottom end of a drive shaft connected 
to a motor. 
Leaktightness along the drive shaft is achieved by a system of seals 
disposed in an annular space between the shaft and a casing surrounding 
this shaft from the point where it passes out of the volute, as far as the 
drive motor. 
The sealing device for the primary pump drive shafts is generally composed 
of three seals comprising a fixed portion fastened to the casing and a 
movable portion fastened to the shaft. 
The facing surfaces of these sealing elements are either in rubbing 
contact, in which case the seal is of the mechanical type, or separated by 
a layer of fluid circulating between the surfaces of the seal, in which 
case the seal is of the hydrostatic type. 
Seals of the mechanical type are generally used for ensuring leaktightness 
between two zones in which the pressures are not two different from one 
another, while hydrostatic seals can be used when there is a very great 
difference in pressure between the two sides of the seal. 
In the case of the primary pumps, the water circulated by the pump is at a 
very high pressure, of the order of 150 bars. The seal disposed in the 
most upstream position on the drive shaft, i.e., the nearest to the 
internal part of the pump, is therefore a hydrostatic seal which permits a 
substantial pressure drop between its upstream side and its downstream 
side, whereas the seals disposed downstream are generally seals of the 
mechanical type. 
A circuit supplying cold water under high pressure makes it possible to 
introduce into the annular space delimited by the casing, upstream of the 
hydrostatic seal, water one part of which is delivered towards the pump 
volute and another part of which supplies the leakage current of the 
hydrostatic seal. After passing through the hydrostatic seal, this water 
is also used for cooling the mechanical seals. 
A hydrostatic seal of the kind used as the upstream seal in primary pumps 
has, for example been described in French Patents Nos. 1,435,568 and 
2,049,690. 
If a hydrostatic seal of this kind is to function correctly, i.e., without 
bringing into contact the elements disposed opposite one another and 
limiting leakage, it is necessary that the pressure drop across this 
hydrostatic seal, called .DELTA.p, should exceed a certain minimum. 
In the case of the primary pumps used at the present time, this pressure 
limit is of the order of 14 bars. 
When the nuclear reactor is operating normally, the reactor cooling water 
is at a pressure of the order of 150 bars and the cold water injected 
upstream of the hydrostatic seal is at a slightly higher pressure, so that 
the pressure drop across the hydrostatic seal is very high, usually close 
to 150 bars. Satisfactory functioning of the hydrostatic seal is then 
ensured. 
This is no longer the case when the pressure of the primary circuit falls, 
e.g., when the reactor is stopped, because the cold water injection 
pressure must be reduced when the primary circuit pressure falls. The 
flows injected into the volute and into the seal must in fact be balanced, 
and these flows depend on the pressure of the primary circuit. 
Below a certain value of the pressure in the primary circuit, the injection 
pressure upstream of the hydrostatic seal is no longer sufficient to 
ensure a .DELTA.p higher than 14 bars, and the hydrostatic seal can no 
longer function correctly. 
In the case of the primary pumps used in the pressurized water nuclear 
reactors in service at the present time, it is considered that the minimum 
primary fluid pressure below which the hydrostatic seal can no longer be 
operated is of the order of 26 bars. 
On the stoppage of a pressurized water nuclear reactor, it is necessary to 
leave at least one primary pump in operation in order to permit the 
circulation of the primary fluid and to ensure good cooling. 
At the end of the cooling, the water is at a pressure of 26 bars and a 
temperature of 70.degree. C. At this temperature it is no longer possible 
to maintain the pressure of 26 bars by utilizing the liquid/vapor 
equilibrium in the pressurizer of the reactor, and it becomes necessary to 
use loading pumps of an auxiliary circuit in order to maintain the 
pressure. 
OBJECT OF THE INVENTION 
The object of the invention is therefore to propose a sealing device for 
the drive shaft of a high pressure fluid pump, which sealing device 
comprises, along the length of this shaft, a system of seals of which at 
least one is of the hydrostatic type with a leakage of liquid between two 
elements which limit this leakage, one of which is connected to the shaft 
while the other is connected to a casing surrounding the shaft and forming 
an annular space around the latter, this seal, which is disposed in the 
highest upstream position, i.e., towards the interior of the pump, 
requiring for its operation a sufficient pressure difference between the 
portion of the annular space which is upstream of the seal, forming a 
chamber in communication with the interior of the pump, and the portion of 
that annular space which is downstream of the seal, this chamber being fed 
with fluid under a high pressure by a supply circuit, while the sealing 
device must enable the pump to be kept in operation even if the pressure 
of the fluid being pumped reaches low values, for example lower than 26 
bars. 
To this end, the sealing device according to the invention comprises, in 
addition: 
(a) an auxiliary seal of the mechanical type, in which one of the mutually 
facing parts in rubbing contact is joined to the shaft and the other to 
the casing, and which is disposed in the chamber, upstream of the 
pressurized fluid inlet, thus dividing this chamber into an upstream 
portion and a downstream portion, and 
(b) a pipe disposed between the upstream and downstream portions of the 
chamber, a valve enabling the two portions of the chamber to be isolated 
or brought into communication being provided on this pipe.

DESCRIPTION OF PREFERRED EMBODIMENT 
In FIG. 1 can be seen a pump comprising a pump body or volute 1 having a 
suction opening 2 and a delivery opening 3. 
Inside this volute is disposed a diffuser 4, inside which rotates the 
bladed wheel 5 fixed to the drive shaft 7 connected to the pump drive 
motor (not shown). 
The top part of the pump body is provided with a coupling flange 8 enabling 
the pump to be joined to its motor unit. 
The shaft 7 is surrounded by a casing 9 which forms an annular space 10 
around it. 
The top the casing 9 is provided with a flange 12 for connection to the 
pump drive motor. 
Seals 14 enabling leaktightness to be ensured along the shaft 7 are 
disposed in the annular space 10. 
The shaft 7 carries the bladed wheel 5 at its end which penetrates into the 
volute, and it passes out of the volute through a labyrinth seal 16 at the 
level of which is also disposed a heat barrier 17 through which a cooling 
coil passes. The shaft 7 then passes into a bearing 15 which supports and 
guides it. 
The seal 14 disposed in the most upstream position, i.e., towards the 
interior of the pump and therefore closest to the bearing 15, is of the 
hydrostatic type, cold water under a pressure slightly higher than the 
water pressure in the pump being injected through injection pipes 19 into 
the annular space 10 in which the seals are disposed. 
FIG. 2 shows a diagrammatic representation of the sealing device associated 
with a primary pump of the same type as the pump shown in FIG. 1. 
In the interior of the volute 21 of this pump, which has a suction opening 
22 and a delivery opening 23, the bladed wheel 25 fixed to the end of the 
shaft 27 is rotated by means of this shaft. On leaving the volute 21 the 
shaft passes into an arrangement comprising two labyrinth seals 26, which 
are in turn surrounded by the heat barrier 28, through which passes a coil 
29 fed with cooling water. 
The shaft 27 then passes through the annular space 30 delimited by a casing 
disposed around the shaft over its entire length, as far as its connection 
to the drive motor 31. 
The bearing 32 permitting the guiding of the shaft, and the seals 33, 34 
and 35, are disposed inside this annular space. 
The first seal 33 disposed upstream is of the hydrostatic type with leakage 
of liquid between its rotating part joined to the shaft 27 and its fixed 
part joined to the casing. 
The seals 34 and 35 are of the mechanical type, comprising two parts in 
rubbing contact, one of which is fixed to the shaft and the other to the 
casing. 
A pressurized cold water supply circuit 36 enables cold water, at a 
pressure slightly higher than the pressure of the water circulated by the 
pump, to be introduced into the annular space 30, upstream of the seal 33. 
The pressure of this cold water is regulated with the aid of a bypass valve 
38 and a loading pump 40 connected in a bypass on the main line of the 
circuit 36. A pressure gauge 39 enables the pressure in the circuit 36 to 
be checked. Pipes 41, 42 and 43 enable the water recovered downstream of 
the seal 33 to be recycled to the supply circuit 36. 
The sealing device according to the invention contains in addition an 
auxiliary seal 45 disposed upstream of the hydrostatic seal 33 in that 
portion of the annular space 30 which constitutes a chamber 46 in 
communication with the interior of the volute 21 by way of the labyrinth 
seals 26. 
The auxiliary seal 45 is a mechanical seal with surfaces in rubbing 
contact, which divides the chamber 46 into two parts, namely a part 46a 
situated upstream of the seal 45 and a part 46b situated downstream of the 
seal 45 and upstream of the seal 33. 
A pipe 47 enables the two parts 46a and 46b of the chamber 46 to be 
connected. A valve 48 is disposed on the pipe 47 in order to enable the 
two parts of the chamber 46 to be isolated or brought into communication. 
A non-return valve 49 is connected in a bypass relative to the valve 48. 
Referring to FIG. 3, the same elements are found as those shown in FIG. 2 
and are given the same reference numerals, the primary pump being of the 
same type as the pump shown in FIG. 1. 
In FIG. 3, however, the circuit 36 has been omitted in order to avoid 
complicating the drawing. 
The auxiliary seal 45 is composed of a fixed part fastened to the casing 9' 
and a movable part 51 fastened to the shaft 27, the facing surfaces of 
these parts being in rubbing contact. 
The hydrostatic seal 33 consists of a floating packing and a rotating 
packing, these packings being separated by a controlled leakage water 
film. The thickness of the film of water (filtered water injected upstream 
of the seal 33 into the chamber 46b by the circuit 36) is regulated by the 
geometric profile of the operative parts in dependence on the pressure in 
the chamber 46b. Water leaking from this seal 33 is partly discharged 
through the seal 34, the remainder passing towards the circuit 36 by way 
of the pipe 41 (FIG. 2). 
A description will now be give, with reference to FIGS. 2 and 3, of the 
operation of the sealing device according to the invention. 
During the normal operation of the pump, with the nuclear reactor in 
service, the pump circulates the water of the primary circuit, which is at 
a pressure of the order of 150 bars and at a temperature higher than 
300.degree. C. The valve 48 disposed on the pipe 47 and bringing the two 
parts of the chamber 46 into communication is open. Cold water is supplied 
by the circuit 36 into the part 46b of the chamber, at a pressure slightly 
higher than the pressure of the primary circuit. Establishing 
communication between the two parts 46a and 46b of the chamber 46 by way 
of the pipe 47 brings about pressure equilibrium between these two parts 
of the chamber, so that the difference in pressure across the auxiliary 
seal 45 is negligible. Rubbing contact can thus be provided between the 
mutually facing surfaces of the seal 45, with a low application pressure, 
so that wear on this seal is very limited. Moreover, the seal 45 is cooled 
by the water from the circuit 36 and works at a moderate temperature. 
The sealing device then functions like the devices of the prior art, the 
difference in pressure across the hydrostatic seal 33 being practically 
equal to the overpressure of the primary circuit. 
During a stoppage of the reactor the primary circuit pressure may decrease 
to a low value, for example lower than 26 bars. In order to keep the pump 
in working order, it is then sufficient to close the valve 48 and regulate 
the flow and pressure of the cold water injected through the circuit 36, 
by acting on the valve 38 and the pump 40, in such a manner as to maintain 
an adequate pressure in the chamber 46b delimited by the hydrostatic seal 
33 and the auxiliary seal 45. This pressure will preferably be selected to 
be equal to 26 bars, so as to be just above the minimum threshold of the 
.DELTA.p permitting the operation of the seal 33. 
Under these conditions the difference in pressure between the chamber 46b, 
subjected to a pressure close to 26 bars, and the chamber 46a, subjected 
to a pressure close to that of the primary circuit, is at most equal to 26 
bars, so that the drop in pressure from one side of the seal 45 to the 
other is at most equal to that value. 
This is compatible with satisfactory operation of the seal 45. 
In all cases the rubbing surface seals of the sealing device shown in FIGS. 
2 and 3 operate under good conditions, because the pressure drops from one 
side of these seals to the other are low and the circulation of the water 
coming into contact with these seals permits the lubrication of the 
rubbing surfaces. The seal 45 is cooled and lubricated by the water 
injected by the circuit 36, while the seals 34 and 35 are cooled and 
lubricated by part of the water passing through the seal 33. This water, 
permitting cooling and lubrication, is finally recycled through the pipes 
41, 42 and 43. 
The non-return valve 49 connected in a bypass relative to the valve 48 is 
designed to remain closed as long as the pressure in the chamber 46a is 
lower than the pressure in the chamber 46b. This non-return valve opens 
only if the pressure in the chamber 46a is higher than that in the chamber 
46b. During the normal operation of the reactor this non-return valve 
therefore remains closed because the circuit 36 introduces water at a 
slightly higher pressure than the pressure of the primary circuit water. 
During a stoppage of the reactor, the pressure being brought to a low value 
in the primary circuit and the valve 48 being closed, if damage occurs in 
the supply circuit 36 the pressure in the chamber 46a becomes higher than 
the pressure in the chamber 46b, which is no longer being fed, and the 
clack valve 49 opens, so that the primary circuit water cooled by the heat 
barrier 28 can penetrate into the chamber 46b and take over the function 
of cooling and lubricating the rubbing-surface seals in place of the cold 
water injected by the circuit 36. 
One advantage of the device according to the invention is that it permits 
the operation of the primary pumps of a nuclear reactor at low pressure. 
During the stoppage phases of the reactor it becomes possible to continue 
to circulate the primary circuit water without needing auxiliary means to 
keep its pressure above 26 bars. 
Moreover, the reactor cooling circuit which makes it possible to lower the 
temperature and pressure of the primary circuit in the course of the 
phases of the cold stoppage of the reactor can be used at pressures lower 
than 26 bars, which previously was not possible because a circulation of 
the primary water must be maintained during the stoppage phases. 
This has the consequence that this cooling circuit can be used until the 
last phases of the cold stoppage of the reactor. 
However, the invention is not limited to the embodiment just described, but 
on the contrary includes all variants thereof. 
Thus, seals of any type may be associated with the hydrostatic seal 
disposed upstream of the sealing device, and the number of these seals is 
not limited. 
It is also possible to conceive the use of a sealing device according to 
the invention in the case of a pump for highly pressurized fluid of a type 
different from a pump for circulating the primary circuit water in a 
pressurized water nuclear reactor.