EP0065922A1 - Primary pump for a pressurized water reactor, having a seal arrangement for its shaft - Google Patents

Abstract

The invention relates to a sealing device for the drive shaft (27) of a pump for high pressure fluid. It comprises a set of seals (33, 34, 35) of which at least one (33) is a hydrostatic seal. Between the seal (33) and the interior space of the volute (21) is disposed an auxiliary seal (45) of the mechanical type. The joint (45) separates two chambers and. The two chambers and can be placed in communication or isolated by a pipe on which a valve (48) is arranged. The invention applies, in particular, to the primary pumps of a pressurized water nuclear reactor.

Description

The invention relates to a sealing device for the drive shaft of a pump for high pressure fluid.

In pressurized water nuclear reactors, the cooling circuit of the reactor core, or primary circuit, comprises at least two cooling loops each comprising a steam generator and a primary pump.

The primary pumps consist of a volute inside which rotates an impeller rigidly attached to the lower end of a drive shaft connected to a motor.

Sealing along the drive shaft is ensured by a set of seals arranged in an annular space between the shaft and a housing which surrounds this shaft from its exit from the volute to the drive motor.

The sealing device of the drive shaft of the primary pumps generally consists of three seals comprising a fixed part secured to the housing and a mobile part secured to the shaft.

The facing surfaces of these sealing elements are either in rubbing contact, the seal is then of the mechanical type, or separated by a layer of fluid in circulation between the surfaces of the seal, the seal is then of the hydrostatic type.

Mechanical seals are generally used to seal between two zones whose pressures are not too different, while hydrostatic seals can be used in the case of a very large pressure difference on either side of the joint.

In the case of primary pumps, the water circulated by the pump is at a very high pressure, of the order of 150 bars. The seal placed most upstream on the drive shaft, that is to say closest to the internal part of the pump, is therefore a hydrostatic seal which allows a significant pressure drop between its upstream part and its downstream part, while the seals arranged downstream are generally mechanical type seals.

A circuit for supplying cold water under high pressure makes it possible to bring in the annular space limited by the housing, upstream of the hydrostatic seal, water, part of which is discharged towards the volute of the pump and of which another part provides the leakage current of the seal hydrostatic. After passing through the hydrostatic seal, this water is also used to cool the mechanical seals.

A hydrostatic seal as used on primary pumps as an upstream seal has for example been described in French patents 1,435,568 and 2,049,690.

In order for such a hydrostatic seal to function properly, that is to say without the elements arranged facing each other limiting the leak coming into contact, it is necessary that the pressure drop across this hydrostatic seal, called Δp , or above a certain limit.

In the case of the primary pumps currently used, this pressure limit is of the order of 14 bars.

In the normal operation of the nuclear reactor, the cooling water of the reactor 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 and generally around 150 bars. The proper functioning of the hydrostatic seal is then ensured.

It is no longer the same when the pressure of the primary circuit decreases for example in the case of a reactor shutdown, since it is necessary to decrease the injection pressure of cold water when the pressure of the primary circuit decreases. It is indeed necessary to balance the flows injected into the volute and into the joint 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 telΔP greater than 14 bars and the hydrostatic seal can no longer function correctly.

In the case of primary pumps used in pressurized water nuclear reactors currently in service, it is considered that the minimum pressure of the primary fluid below which the hydrostatic seal can no longer be operated is of the order of 26 bars.

When shutting down a pressurized water nuclear reactor, it is necessary to leave at least one primary pump in operation to allow the circulation of the primary fluid and ensure good cooling.

At the end of cooling, we are in the presence of water at 26 bars and at a temperature of 70 ° C. At this temperature, the pressure of 26 bar can no longer be maintained by using the liquid / vapor balance in the reactor pressurizer and we are forced to use charge pumps from an auxiliary circuit to maintain the pressure.

The object of the invention is therefore to propose a device for sealing the drive shaft of a pump for high pressure fluid comprising along the length of this shaft a set of seals, at least one of which is hydrostatic type with liquid leakage between two elements limiting this leakage, one connected to the shaft and the other to a box surrounding the shaft and providing an annular space around it, this seal disposed most upstream, that is to say towards the inside of the pump, requiring, for its operation, a sufficient pressure difference between the part of the annular space which is upstream of the seal, forming a chamber in communication with the internal part of the pump and the part of this annular space which is downstream of the seal, this chamber being supplied with fluid under high pressure, by a supply circuit, the sealing device having to allow maintenance in operation of the pump, even if the pressure of the fl uide which is pumped reaches low values, for example less than 26 bars.

• - un joint auxiliaire du type mécanique dont les pièces frottantes en regard sont liées, l'une à l'arbre et l'autre au boitier, disposé dans la chambre, à l'amont de l'arrivée de fluide sous pression, séparant cette chambre en une partie amont et une partie aval,
• - et une canalisation diposée entre la partie amont et la partie aval de la chambre, sur laquelle est placée une vanne permettant d'isoler ou de mettre en communication les deux parties de la chambre.

To this end, the sealing device according to the invention further comprises:

• an auxiliary seal of the mechanical type, the rubbing facing parts of which are linked, one to the shaft and the other to the housing, arranged in the chamber, upstream of the arrival of pressurized fluid, separating this chamber in an upstream part and a downstream part,
• - And a pipe available between the upstream part and the downstream part of the chamber, on which is placed a valve making it possible to isolate or to put the two parts of the chamber in communication.

• La figure 1 représente dans une vue en perspective éclatée, une pompe primaire suivant l'art antérieur.
• La figure 2 représente de façon schématique le dispositif d'étanchéité suivant l'invention.
• La figure 3 représente dans une demi-vue en coupe par un plan vertical de symétrie, la partie supérieure d'une pompe primaire pour réacteur nucléaire comportant un dispositif d'étanchéité suivant l'invention.

In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the attached figures, of a primary pump of a pressurized water nuclear reactor equipped with a device for sealing according to the invention.

• Figure 1 shows in an exploded perspective view, a primary pump according to the prior art.
• Figure 2 shows schematically the sealing device according to the invention.
• FIG. 3 shows in a half-view in section through a vertical plane of symmetry, the upper part of a primary pump for a nuclear reactor comprising a sealing device according to the invention.

In FIG. 1 we see a pump comprising a pump body or volute 1 pierced with a suction opening 2 and a discharge opening 3.

Inside the volute is dipose a diffuser 4 inside which rotates the impeller 5 integral with the shaft of ^t Drives for 7 connected to the drive motor of the non représehté pump.

The upper part of the pump body has a coupling flange 8 making it possible to connect the pump to its motor unit.

The shaft 7 is surrounded by a box 9 which spares around it an annular space 10.

At its upper part, the housing 9 has a flange 12 for connection to the pump drive motor.

In the annular space 10 are arranged seals 14 making it possible to seal along the shaft 7.

The shaft 7 carries at its end penetrating the volute the impeller 5 and leaves this volute at a labyrinth seal 16 at which is also placed a thermal barrier .17 traversed by a cooling coil. The shaft 7 then passes through a bearing 15 ensuring its maintenance and its guidance.

The seal 14 disposed most upstream, that is to say towards the inside of the pump, and therefore placed 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 by injection pipes 19, into the annular space 10 in which the seals are placed.

In FIG. 2, we see a schematic representation of the sealing device associated with a primary pump of the same type as the pump shown in FIG. 1.

Inside the volute 21 of this pump comprising a suction opening 22 and a discharge opening 23, the impeller 25 integral with the end of the shaft 27 is rotated by means of this tree. On leaving the volute 21, the shaft passes through a set of two labyrinth seals 26 themselves surrounded by the thermal barrier 28 traversed by a coil 29 supplied with cooling water.

The shaft 27 then passes through the annular space 30 delimited by a box disposed around the shaft along its entire length until it is connected to the drive motor 31.

Inside this annular space are arranged the bearing 32 allowing the shaft to be guided and the seals 33, 34 and 35.

The first seal 33 disposed upstream is of the hydrostatic type with liquid leakage between its rotating part linked to the shaft 27 and its fixed part linked to the housing.

The seals 34 and 35 are of the mechanical type comprising two friction parts, one of which is integral with the shaft and the other of the housing.

A pressurized cold water supply circuit 36 makes it possible to bring this cold water to a pressure slightly higher than the pressure of the water circulated by the pump, in the annular space 30, upstream of the seal 33.

The pressure of this cold water is adjusted by means of a bypass valve 38 and a charge pump 40 placed in bypass on the main pipe of the circuit 26. A pressure gauge 39 makes it possible to check the pressure in the circuit 36. Pipes 41, 42 and 43 make it possible to recycle the water recovered downstream of the seal 33 in the supply circuit 36.

The sealing device according to the invention further comprises an auxiliary seal 45 disposed upstream of the hydrostatic seal 33 in the part of the annular space 30 constituting a chamber 46 in communication with the internal part of the volute 41, by the intermediate labyrinth seals 26.

The auxiliary seal 45 is a mechanical seal with rubbing surfaces which separates the chamber 46 into two parts, 46a situated upstream of the seal 45 and 46b situated downstream of the seal 45 and upstream of the seal 33.

A pipe 47 makes it possible to join the two parts 46a and 46b of the chamber 46. A valve 48 is arranged on the pipe 47 to isolate or put the two parts of the chamber 46 into communication.

A valve 49 is placed in bypass with respect to the valve 48.

Referring to FIG. 3, we find the same elements as those represented in FIG. 2 and with the same references, the primary pump being of the same type as the pump represented in FIG. 1.

In FIG. 3, the circuit 36 has not, however, been shown in order to avoid complicating the representation.

The auxiliary seal 45 consists of a fixed part 50 secured to the housing 29 and a movable part 51 secured to the shaft 27, the surfaces of which face to face are in rubbing contact.

The hydrostatic seal 33 consists of a floating packing and a rotating packing separated by a film of water with controlled leakage. The thickness of the water film (filtered and injected water upstream of the seal 33 in the chamber 46b by the circuit 36) is regulated by the geometric profile of the active parts as a function of the pressure in the chamber 46b. The water leak from this seal 33 is partly evacuated through the seal 34 and the remainder towards the circuit 36 via the pipe 41 (FIG. 2).

We will now describe, with reference to Figures 2 and 3, the operation of the sealing device according to the invention.

During normal operation of the pump, the nuclear reactor being in service, it causes the circulation of the water in the primary circuit which is at a pressure of the order of 150 bars and at a temperature above 300 ° C. The valve 48 disposed on the pipe 47 connecting the two parts of the chamber 46 is open. Cold water is brought by the circuit 36 in the part 46b of the chamber, at a pressure slightly higher than the pressure of the primary circuit. The placing in communication of the two parts 46a and 46b of the chamber 46 by the pipe 47 produces a pressure balancing between these two parts of the chamber, so that the pressure difference across the auxiliary seal 45 is negligible. It is therefore possible to provide a rubbing contact between the surfaces facing the seal 45, with a low application pressure so that the wear of this seal is very limited. On the other hand, the seal 45 is cooled by the water of the circuit 36 and works at moderate temperature.

The sealing device then functions like the devices of the prior art, the pressure difference across the hydrostatic seal 33 being practically equal to the overpressure of the primary circuit.

When the reactor is shut down, the pressure in the primary circuit can drop to a low value, for example less than 26 bars. To maintain the stern in working order, it is then sufficient to close the valve 48 and to regulate the flow rate and the pressure of the cold water injected by the circuit 36, by acting on the valve 38 and the pump 39, so as to maintain sufficient pressure in the chamber 46b delimited by the hydrostatic seal 33 and by the auxiliary seal 45. Preferably, this pressure will be chosen equal to 26 bars so as to be just above the minimum threshold of Δ p allowing the operation of the attached 33.

Under these conditions, the pressure difference 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 pressure drop of on either side of the joint 45 is at most equal to this value.

This is compatible with operation of the seal 45 under good conditions.

In all cases, the seals with rubbing surfaces of the sealing device shown in FIGS. 2 and 3 operate in good conditions because the pressure drops on either side of these seals are weak and the circulation of water coming from in contact with these seals allows lubrication of the friction 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 a part of the water passing through the seal 33. This water allowing the cooling and the lubrication is finally recycled by lines 41, 42 and 43.

The valve 49 placed in bypass with respect 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 valve opens only in the case of an overpressure in the chamber 46a relative to the chamber 46b. During normal operation of the reactor, this valve therefore remains closed since the circuit 36 introduces water slightly suppressed with respect to the water in the primary circuit.

During a shutdown of the reactor, the pressure being brought to a low value in the primary circuit and the valve 48 being closed, if damage occurs on the supply circuit 36, the pressure in the chamber 46a becomes higher at the pressure in the chamber 46b which is no longer supplied and the valve 49 opens, so that the water of the primary circuit cooled by the thermal barrier 28 can penetrate into the chamber 46b and perform the cooling and lubrication of seals with rubbing surfaces replacing the cold water injected by the circuit -36.

It is therefore seen that an advantage of the device according to the invention is to allow the operation of the primary pumps of a nuclear reactor at low pressure. During the reactor shutdown phases, it becomes possible to continue circulating the water in the primary circuit without using auxiliary means to maintain its pressure above 26 bars.

On the other hand, the reactor cooling circuit which makes it possible to lower the temperature and the pressure of the primary circuit during the cold shutdown phases of the reactor can be used at pressures below 26 bars, which was not the case. not possible previously since circulation of primary water must be maintained during the shutdown phases.

As a result, this cooling circuit can be used until the final phases of the cold shutdown of the reactor.

However, the invention is not limited to the embodiment which has just been described; on the contrary, it includes all the variants thereof.

Thus, the hydrostatic seal disposed upstream of the sealing device can be associated with seals of any type and that the number of these seals is not limited.

One can also imagine the use of a sealing device according to the invention in the case of a pump for high pressure fluid different from a water circulation pump of the primary circuit of a nuclear reactor with pressurized water.

Claims (3)

1.- Sealing device of the drive shaft (27) of a pump for high pressure fluid comprising, along the length of the shaft (27) a set of seals (33, 34, 35) of which l '' at least one (33) is of the hydrostatic type with liquid leakage between two elements limiting this nite connected one to the shaft (27) and the other to a housing surrounding the shaft (27) and providing a space annular (30) around the latter, this seal (33) disposed most upstream, that is to say towards the interior of the pump, requiring for its operation, a sufficient pressure difference between the part of the annular space (30) which is located upstream of the seal (33), forming a chamber (46) in communication with the internal part of the pump and the part of this annular space which is located downstream of the seal ( 33), this chamber (46) being supplied with fluid under high pressure, by a supply circuit (36) characterized in that it further comprises: - An auxiliary seal (45) of the mechanical type, the facing parts of which are linked one to the shaft (27) and the other to the housing, arranged in the chamber (46), upstream of the fluid inlet under pressure, separating this chamber into an upstream part (46a) and a downstream part (46b), - And a pipe (47) disposed between the upstream part and the downstream part of the chamber, on which is placed a minne (48) making it possible to isolate or to put the two parts (46a and 46b) of the chamber into communication. 2.- sealing device according to claim 1, characterized in that a valve (49) is placed in bypass with respect to the valve (48) allowing the isolation or the communication of the two parts (46a and 46b) of the chamber (46), this valve (49) being designed to open when the pressure in the part (46a) of the chamber situated upstream is greater than the pressure in the part (46b) of the chamber situated downstream of the auxiliary seal (45). 3.- sealing device according to any one of claims 1 and 2, in the case of a primary water circulation pump of a pressurized water nuclear reactor, comprising from the pump to the engine for driving the latter a thermal barrier (28), a bearing (32), a hydrostatic seal (33), a first mechanical seal with friction surfaces (34) and a second mechanical seal with friction surfaces (35) characterized by the fact that it further comprises a mechanical seal with rubbing surfaces (45) interposed between the thermal barrier (28) and the bearing (32) which constitutes the auxiliary seal.

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