Active surface for a packing seal intended for a shaft sealing system

Abstract

A packing seal is provided for a system for sealing the shaft of a primary motor-driven pump unit of a nuclear reactor, intended to ensure sealing between the primary circuit and the atmosphere. The packing seal includes a rotary active surface and a floating active surface, and a face of the floating active surface and/or the rotary active surface is covered by a protective layer made from a material having surface energy greater than 30 mJ/m.sup.2 and an electron donor component less than 15 mJ/m.sup.2.

Claims

1. A hydrostatic packing seal for a system for sealing the shaft of a primary motor-driven pump unit of a nuclear reactor configured to ensure sealing between the primary circuit and the atmosphere, the hydrostatic packing seal comprising: a rotary active surface attached to the shaft and a floating active surface, which is free to be displaced axially to follow axial displacements of the shaft, wherein the rotary active surface and the floating active surface face each other and are separated by a water film, said active surface having a face covered with a protective layer to prevent deposition of iron oxide fouling the active surface, said protective layer being made of a material having a surface energy greater than 30 mJ/m.sup.2 and an electron donor component less than 15 mJ/m.sup.2.

2. The hydrostatic packing seal according to the claim 1, wherein the material of the protective layer has a surface energy greater than 35 mJ/m.sup.2.

3. The hydrostatic packing seal according to the claim 1, wherein the material of the protective layer has a surface energy greater than 37 mJ/m.sup.2.

4. The hydrostatic packing seal according to the claim 1, wherein the material of the protective layer has a surface energy greater than 50 mJ/m.sup.2.

5. The hydrostatic packing seal according to claim 1, wherein the material of the protective layer has an electron donor component less than 10 mJ/m.sup.2.

6. The hydrostatic packing seal according to claim 1, wherein the material of the protective layer has an electron donor component less than 5 mJ/m.sup.2.

7. The hydrostatic packing seal according to claim 1, wherein the protective layer is made of one of the following materials: silicon carbide, titanium nitride, chromium nitride, nickel, micro- or nano-crystalline diamond.

8. The hydrostatic packing seal according to claim 1, wherein the protective layer has a thickness (e) between 100 nm and 100 m.

9. The hydrostatic packing seal according to claim 1, wherein the active surface is further covered with a tie layer disposed between the protective layer and the face of the active surface.

10. The hydrostatic packing seal according to claim 1, wherein the face of at least one of the active surfaces is further micro- or nano-structured by an array of bumps, each bump having side dimensions between 10 nm and 5 m, a height between 10 nm and 5 m, the distance between two consecutive bumps being between 10 nm and 5 m.

11. A packing seal including at least one active surface according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further characteristics and advantages of the invention will appear upon reading the detailed following description, with reference to the accompanied figures, which illustrate:

(2) FIG. 1 is a cross-section view of a shaft sealing system according to one embodiment of the invention;

(3) FIG. 2 is a schematic view of a seal n1 according to one embodiment of the invention;

(4) FIG. 3 is a schematic representation of the fouling phenomenon of a seal active surface;

(5) FIG. 4 is a cross-section schematic representation of the active surfaces of seal n1 according to one embodiment of the invention;

(6) FIG. 5 is a cross-section schematic representation of the active surfaces of seal n1 according to another embodiment of the invention.

(7) For the sake of clarity, identical or similar elements are referenced by identical reference characters throughout the figures.

DETAILED DESCRIPTION

(8) FIG. 1 represents a system of mechanical packing seals for a shaft 4 of a primary motor-driven pump unit of a nuclear reactor. This shaft sealing system includes a seal n1 referenced 1 in the figure, a seal n2 referenced 2 in the figure, a seal n3 referenced 3 in the figure. Each seal 1, 2, 3 is comprised of a rotary active surface attached to the shaft 7 and of a floating active surface that can follow the axial displacements of the shaft 7 but do not rotate.

(9) Seal n1 is more precisely represented in FIG. 2. Seal n1 ensures the greatest part of the pressure drop between the primary circuit 8 and the atmosphere 9.

(10) Seal n1 is of the hydrostatic type, with a water film of a thickness in the order of 10 m. Seal n1 includes a rotary active surface 10 attached to the shaft 7 and a floating active surface 11 that can follow the axial displacements of the shaft 7. The leakage rate of seal n1 is determined by the double gradient of the floating active surface 11 or by the gradients of the rotary 10 and floating 11 active surfaces. The active surfaces are made of silicon nitride Si.sub.3N.sub.4.

(11) The fouling process of the active surfaces 10, 11 in the absence of the characteristics of the invention is explained in FIG. 3. Water circulates between the active surfaces 10, 11. This water brings Fe.sup.2+ ions which preferably adsorb at the face of the active surfaces into Si.sub.3N.sub.4. Fe.sup.2+ is a Lewis acid, it reacts with oxygen groups present on the face of the active surfaces and can in turn react with colloidal or particulate Fe.sub.2O.sub.3 which has a strong electron donor component. Fe.sup.2+ ions can then adsorb at the face of the hematite particles and the reaction continues as a chain reaction, which causes the active surfaces to be fouled.

(12) In order to avoid this fouling process, with reference to FIGS. 4 and 5, the face 12 of at least one of the active surfaces 10, 11 is covered with a protective layer 13. This protective layer 13 is made of a material on which Fe.sup.2+ ions seldom adsorb, if at all. To this end, the protective layer 13 is made of a material having a surface energy greater than 30 mJ/m.sup.2 and an electron donor component less than 15 mJ/m.sup.2.

(13) The protective layer can thus be made of nano- or micro-crystalline diamond, which has a surface energy of 50 mJ/m.sup.2 and a low electron donor component of 3 mJ/m.sup.2.

(14) A set of active surfaces covered with a carbon layer in the form of microcrystalline diamond of a 2 m thickness has been tested on a model simulating the deposition in real conditions. For prior art raw active surfaces of silicon nitride, the deposition of iron oxide occurs after 250 h. When the active surfaces are covered with the carbon layer in the form of microcrystalline diamond of a 2 m thickness, the deposition only occurs after 750 h. In this case, the deposition is strongly visually reduced with respect to an active surface of silicon nitride tested during 250 h.

(15) The protective layer can also be made of titanium nitride which has a total surface energy of 44 mJ/m.sup.2 and an electron donor component of 0.3 mJ/cm.sup.2.

(16) The protective layer can also be made of chromium nitride which has a total surface energy of 41 mJ/m.sup.2 and an electron donor component of 0.4 mJ/cm.sup.2.

(17) The protective layer can also be made of chemical nickel, which has a surface energy of 33 mJ/m.sup.2 and an electron donor component of 9 mJ/m.sup.2. When the protective layer is made of nickel, a tie layer is preferably disposed between the face of the active surfaces and the protective layer so as to improve the strength of the protective layer and to initiate the autocatalytic reaction of nickel deposition. This tie layer is preferably made of platinum or of palladium.

(18) The protective layer can also be made of silicon carbide (SiC). Silicon carbide can have a variable surface energy as a function of its composition, so that the silicon carbide composition is selected in order to have a surface energy greater than 30 mJ/m.sup.2 and a sufficiently low electron donor component.

(19) The material is not limited to the previously mentioned materials.

(20) The protective layer preferably has a thickness greater than 100 nm so as to be continuous, and less than 100 m so as to reduce the risk of crack and limit the disturbances in case of disconnection. Deposition preferably has a thickness between 1 and 5 m.

(21) Besides, the protective layer can be micro- or nano-structured by an array of holes or pillars.

(22) Each hole has side dimensions between 10 nm and 5 m, and a depth of 10 nm to 5 m. The distance between two consecutive holes is between 10 nm and 5 m.

(23) Each pillar has side dimensions between 10 nm and 5 m. The aspect ratio (height/side dimension ratio) must preferably be less than 2 and more preferably be less than 1 in order to avoid erosion phenomena. The distance between two consecutive pillars is between 10 nm and 5 m. This micro- or nano-structuration makes it possible to prevent Fe.sub.2O.sub.3 particles from tying to Fe.sup.2+ ions by limiting the tie points in the case where Fe.sup.2+ ions would nevertheless be tied to the face of the protective layer.

(24) The micro- or nano-structuration of the protective layer can be performed by lithography by using a mask of micro- or nano-particles or even by block copolymers or by any other known micro- or nano-structuration method.

(25) Structuration can be hierarchical by combining microstructures and nanostructures.

(26) In the case of nano- or micro-structuration, the protective layer is preferably put in place and then structured.

(27) Of course, the invention is not limited to the embodiments described with reference to the figures and alternatives could be contemplated without departing from the scope of the invention. Especially, other materials than those mentioned in the detailed description could be used.