Abstract:
The invention is a film-riding seal for a pump shaft that is resistant to the deposition of iron oxides such as hematite and goethite on the confronting, spaced apart, seal surfaces. The seal generally comprises a ceramic runner mounted on the pump shaft and an adjacent ceramic stationary sealing ring. The runner has a sealing surface and the stationary sealing ring has sealing surface confronting the runner sealing surface. The ceramic sealing surfaces include a catalyst selected from the group consisting of rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold and mixtures thereof.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention This invention generally relates to shaft seals, and is specifically concerned with a hydrostatic, film-riding seal that is useful for sealing the shaft of a pump used for pumping liquids at high pressures and temperatures such as a coolant water pump in nuclear power plant.  
         [0002]     2. Description of the Prior Art Coolant water pumps are used in commercial pressurized water nuclear reactors to continuously recirculate 100,000 gpm or more of coolant water through reactor coolant systems at temperatures of up to about 570° F. or higher and pressures of about 2250 psi or higher. A coolant water pump maintains the reactor coolant pressure and restricts leakage of coolant water along a rotating pump shaft with a series of shaft seals, including a primary seal assembly which is a hydrostatic, “film-riding” seal and secondary and tertiary friction (or “contacting” or “rubbing”) type seal assemblies. Most of the 2250 psi pressure differential between the reactor coolant system pressure and the surrounding atmosphere is dropped across the primary seal assembly. The actual seal is formed by two confronting faceplates, one of which is stationary (known as the “ring”) and the other of which turns with the pump shaft (known as the “runner”). Water is forced between the faceplates, which causes the faceplates to ride on a thin film of water that may be on the order of a half mil thick.  
         [0003]     The correct flow of water between the faceplates of the primary seal (known as the “leak-off rate”) must be maintained within specifications for proper functioning of the seal. Investigations have shown that out-of-specification leak-off rates are often caused by iron oxide deposits on one or both faceplates. The predominant iron oxide phase deposited on the faceplates was found to be α-Fe 2 O 3 ,i.e., the mineral hematite. In addition, lesser quantities of α-FeOOH, the mineral goethite, have also been detected. The iron in both hematite and goethite is in the +3 valence state.  
       SUMMARY OF THE INVENTION  
       [0004]     In its broadest sense, the invention is an improved film-riding seal for a pump shaft that is resistant to the deposition of iron oxides such as hematite and goethite on its confronting seal surfaces. The invention generally comprises a ceramic runner mounted on the pump shaft and a ceramic stationary sealing ring. The runner and the stationary sealing ring have confronting, spaced apart, sealing surfaces. At least one, but preferably both, of the sealing surfaces include a catalyst selected from the group consisting of rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold and mixtures thereof. In preferred embodiments of the invention, the runner and the stationary sealing ring are formed of a nitride, an alumina or a zirconia In one preferred embodiment, the runner and the stationary sealing ring are formed of a nitride such as silicon nitride and the catalyst is platinum. In another preferred embodiment, the runner and the sealing ring are formed of a nitride and the catalyst is iridium. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING FIGS  
       [0005]      FIG. 1  is a cutaway perspective view of a coolant pump, illustrating a primary sealing assembly which surrounds the pump shaft; and  
         [0006]      FIG. 2  is a Pourbaix diagram indicating the stability of hematite deposits at 65° C. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]     With reference now to  FIG. 1 , the invention finds particular utility within the centrifugal water coolant pumps, represented by pump  1 , used to circulate water through the radioactive core of a nuclear power station. Such a water coolant pump  1  generally has a pump casing  3  and a No.  1  seal housing  5 . A pump shaft  7  is sealingly and rotatably mounted within the No.  1  seal housing  5 . Although not specifically shown in  FIG. 1 , the bottom portion of the pump shaft  7  is connected to an impeller, while the top portion is connected to a high-horsepower, induction-type electrical motor. When the motor rotates the shaft  7 , the impeller within the housing  9  pressurizes the water flowing through the pump housing from ambient pressure to approximately 2,250 psi. This pressurized water applies an upwardly directed, hydrostatic load upon the shaft  7  since the upper portion of the No.  1  seal housing  5  is surrounded by the ambient atmosphere.  
         [0008]     In order that the pump shaft  7  might rotate freely within the No.  1  seal housing  5  while maintaining the 2,250 psi pressure boundary between the housing interior  9  and the outside of the No.  1  seal housing  5 , the primary, secondary and tertiary sealing assemblies  11 ,  13  and  15 , respectively, are provided in the positions illustrated. Most of the necessary pressure sealing is performed by the primary sealing assembly  11 .  
         [0009]     The primary sealing assembly  11  generally includes a sealing ring  17  which is stationarily mounted within the No.  1  seal housing  5  by a clamping ring  19 , and a runner  23  which is mounted onto a flange  25  of shaft  7  by means of another clamping ring  21 . The bottom surface of the sealing ring  17  and the top surface of the runner  21  form sealing surfaces  18  and  22 , which are biased toward one another as a result of the fluid pressure load on the pump shaft  7 . However, the sealing surfaces  18  and  22  normally do not frictionally engage on another, since the sealing surface  18  of the sealing ring  17  is tapered at a shallow angle with respect to the substantially flat and horizontal sealing surface  22  of the runner  23 . Such tapering provides a flowing film of water between the sealing surfaces  18  and  22  of the sealing ring and the runner  21 , which in turn allows the sealing ring  17  and runner  23  to rotate relative to one another in a “film-riding” mode. Because the primary sealing assembly  11  normally operates in a film-riding mode, some provision must be made for handling the water which “leaks off ” in the annular space between the shaft  7 , or within the No.  1  seal housing  5  which rotatably mounts the shaft  7 . Accordingly, No.  1  seal housing  5  includes a primary leak-off port  27 . Leak-off ports  29  and  31  accommodate water leakage from secondary and tertiary sealing assemblies  13  and  15 , respectively. Coolant water pumps designed for use in plants designed by the assignee of the present invention may have leak-off rates of from about 1 to about 6 gallons/minute. See, in this regard, U.S. Pat. No. 4,693,481 which is incorporated by this reference for a more detailed description of the structure of reactor coolant pumps  1  and shaft seals.  
         [0010]     The sealing ring  17  and the runner  23  may be formed of a ceramic material such as a nitride, an alumina or a zirconia. Preferably, the sealing ring  17  and the runner  23  are formed of a nitride. In a preferred embodiment both the sealing ring  17  and the runner  23  are formed from 98% pure, hot-pressed silicon nitride. Advantageously, such a material permits that the primary sealing assembly  11  to operate for a period of time in a non-film-riding mode should the coolant pump  1  fail to provide water pressurized enough to create the aforementioned film of flowing water between the sealing surfaces  18  and  22 . The use of hot-pressed silicon nitride, in combination with the taper of the sealing surfaces  18  and  22 , provides a shaft seal which is not only capable of operating in a non-film-riding mode (in the event of a malfunction or other emergency within the coolant pump  1 ), but also provides a shaft seal which is extremely durable and resistant to a variety of deleterious mechanical effects (e.g., as a result of “thermal shock”). See, in this regard, U.S. Pat. No. 5,057,340, which generally discloses a method for forming a ceramic coating on a sealing surface for a coolant water pump  1 .  
         [0011]     At least one, and preferably both, of the sealing surfaces  18  and  22  comprise one or a mixture of the following catalysts: rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold. Preferably the top 20 angstroms (20 Å) of the sealing surface  18  and/or  22  include at least 1%, and more preferably at least 15%, by weight of catalyst. Desirably, there is substantial particle-to-particle contact between catalyst particles for increasing the electrical conductivity of the sealing surfaces. In some embodiments, the catalyst particles form a coating (i.e., a film or thin layer) on the sealing surfaces  18  and  22 .The catalyst may be added to the sealing surface  18  and/or the sealing surface  22  by any suitable technique, including precipitation from a molten salt or an aqueous solution, and plasma assisted vapor deposition.  
         [0012]     A catalyst is added to the sealing surface  18  of the sealing ring  17  and/or the sealing surface  22  of the runner  23  for the following reasons. It is known that the iron oxide deposits tend to form on the sealing surfaces  18  and  22  of coolant water pumps  1  used in nuclear power stations at the end of on-line operations notwithstanding the use of very pure coolant water. Thus, it is theorized that the deposition of iron oxide is caused by the difference in conditions at the sealing surfaces  18  and  22  at the end of on-line cycles. Alloys in the system piping (including the chemical and volume control system and its supporting subsystems in light water nuclear reactors) corrode and release iron corrosion products into the coolant water. The released iron is a mixture of dissolved iron in the +2 valence state and colloidal FeOOH hydrates that are smaller than the pore size of the injection filters.  
         [0013]     The colloidal FeOOH particles or FeOOH hydrates deposit on the sealing surfaces  18  and  22  through an electrophoretic mechanism. Thus, the small charged particles are attracted to the charged surfaces  18  and  22  of the stationary sealing ring  17  and the runner  23  in the region of highest flow. Also, the electrophoretic deposition is facilitated by low ionic strength and alkaline pH, which conditions occur at the end of on-line operations. The totally dissolved iron in the +2 valence state react with dissolved oxygen in the water at the sealing surfaces  18  and  22  to form insoluble iron in the +3 valence state. The iron in the +3 valence state precipitates on the sealing surfaces  18  and  22  to hold the particles in place. The iron oxide hydroxides then convert to a more thermodynamically stable hematite.  
         [0014]     This process would be necessary for the deposition of layers more than one particle deep because iron oxide hydroxide deposits and iron oxide hydroxide colloidal particles will have the same electrostatic charge and will repel each other.  
         [0015]     The chemistry conditions where hematite deposits are stable can be illustrated by means of a Pourbaix diagram (sometimes known as a Eh-pH plot). In such a diagram, the electrochemical potential (Eh or ECP) is plotted against pH, the negative log of the hydrogen concentration. The diagram is divided into regions of stability for different chemical species.  FIG. 2  is a Pourbaix diagram that was generated for a temperature of 65° C., which is representative of typical coolant water pump seal temperatures. The iron concentration of  FIG. 2  is 1.0 × 10 −7  mol/kg or 0.56 ppb.  FIG. 2  shows that, even at this low iron concentration, hematite has a wide range of stability. At the end of a nuclear power station&#39;s on-line cycle, the pH of 65° C. seal injection may be in the 6.5 to 7 range. Because hematite commonly forms at the end of a cycle, the electrochemical potential must be positive.  
         [0016]     The circle  40  imposed on  FIG. 2  shows a likely Eh and pH for the seal injection water at the end of a nuclear power station&#39;s on-line cycle. The arrows  42  and  44  show the chemical changes that could most easily destabilize hematite and prevent deposition. As is shown by arrow  42 , the injection fluid at the sealing surfaces  18  and  22  could be made more acid. As is shown by arrow  44  the electrochemical potential could be reduced to a negative value. The electrochemical potential can be lowered on the coolant pump seal  11  by changing the relative rates of the different surface chemical reactions on the sealing surfaces  18  and  22 . The electrochemical potential at the sealing surfaces  18  and  22  is a mixed potential that is set primarily by the oxidation of hydrogen and the reduction of oxygen. Hydrogen is frequently controlled in the injection water during on-line operations at a concentration of from 25 to 50 cc/kg H 2 O and oxygen concentrations in the ppb to ppm range can be introduced through the station&#39;s make-up system. 
 
H 2 +2OH-         2H 2 0 +2 e   (Equation 1) 
 
½O 2  +2H++2 e-H   2 O  (Equation 2) 
 
 These reactions do not occur appreciably in solution at 65° C. Rather, they take place at surfaces. The oxidation of hydrogen (Equation 1) tends to push surfaces to negative potentials, while the reduction of oxygen (Equation 2) tends to push surfaces to positive electrochemical potentials. 
 
         [0017]     The oxygen reduction reaction (Equation 2) is kinetically favored on most oxide surfaces such as on thin SiO 2  layers that may form on ceramic sealing rings  17  and runners  23  in water or hematite that may deposit on the sealing surfaces  18  and  22  of sealing rings  17  and runners  23 . Thus, even thought the amount of oxygen in the water is typically low compared with the hydrogen, the oxygen reduction reaction (Equation 2) still pushes the electrochemical potential at the sealing surfaces  18  and  22  in the positive direction. As a result, the hematite remains stable and any Fe +2  in solution may be oxidized to form additional Fe+ 3 .  
         [0018]     The hydrogen oxidation reaction can be accelerated by an oxidation catalyst on the surfaces  18  and  22 . Thus, a catalyst such as platinum or iridium may be added to change the chemistry at the sealing surfaces  18  and  22 . The catalyst catalyzes the hydrogen oxidation reaction (Equation 1) by promoting dissociation of adsorbed hydrogen gas to produce hydrogen adatoms. The adatoms can easily give up electrons or can react directly with oxygen adatoms on the surface of the platinum. The catalytic element does not need to be chemically combined with the ceramic material in the sealing ring  17  or the runner  23 . In preferred embodiments, the catalytic element may fill the pores of the ceramic material. For example, the outer 20 microns adjacent silicon nitride surfaces  18  and  22  may be more than 40% porous. Preferably, in applications where the injection water contains substantial amounts of dissolved hydrogen, silicon nitride sealing rings  17  and runners  23  contain platinum. In applications where the injection water contains hydrazine, silicon carbide sealing rings  17  and runners  23  preferably contain iridium.  
         [0019]     The catalyst results in the injection of more electrons into the sealing surfaces  18  and  22 . Thus, the electrochemical potential becomes more negative. At the same time, the surfaces  18  and  22  become more acidic because OH ions are consumed in the hydrogen oxidation reaction (Equation 1). In addition, Fe+2 is no longer oxidized to form Fe +3  deposits. Instead, the process is reversed and the Fe +3  deposits on the sealing surfaces  18  and  22  are converted to dissolved Fe +2  species in solution. Also, the lower surface pH also helps to keep iron from precipitating and will help dissolve any iron deposits that may form by some other mechanism. Accordingly, the catalytic element is added in an amount that is effective to increase the electrical conductivity of the sealing surfaces  18  and  22 .  
         [0020]     While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.