Patent Description:
In nuclear power generation facilities, fluid coolant, e.g., water, is circulated using pumps for control of reactor temperature and reaction rate. Pump sealing is of significant importance for performance. Moreover, effective sealing may be important for regulatory compliance, for example, to prevent leakage of radioactive material.

Within rotating machinery such as pumps, seals may be installed between stationary and rotating components. In such situations, the pressure differential that can be maintained across a seal may be proportional to the pressure with which the sealing elements are urged together. However, increased sealing pressure may lead to increased wear of sealing elements.

The effective pressure drop across a seal may depend on fluid conditions proximate the seal, such as pressure and fluid velocity. Accordingly, balancing of seal effectiveness and longevity may likewise depend on fluid conditions. <CIT> discloses an axial hydrodynamic mechanical seal for hot water high-pressure pumps, in particular for nuclear power plants. <CIT> discloses a reactor coolant pump shaft seal utilising shape memory metal. <CIT> discloses a spiral groove seal system for sealing a high pressure gas. <CIT> discloses a mechanical seal shutdown seal suitable for sealing a rotary shaft of a nuclear reactor primary coolant pump. <CIT> discloses face seals of the rotors of pump units for separation of media or pressure drop.

A coolant pump for a nuclear power generation facility comprises: a casing assembly having a gland housing; a seal assembly having a staging flow pathway defining first and second seal chambers; a rotor assembly pumping a coolant fluid through the gland housing, the rotor assembly having an acceleration surface, wherein fluid passing through the staging flow pathway is accelerated by the acceleration surface; first and second seal stages within the first and second seal chambers, each having a static sealing element and a rotating sealing element, the sealing elements engaging one another to form a fluid-tight seal; an inlet passage for feeding the coolant fluid into the staging flow pathway and past the acceleration surface.

An example method of sealing a pump in a nuclear power generation facility comprises: directing pressurized fluid through a first section of a staging flow path towards a first seal; increasing angular velocity of the pressurized fluid as it traverses the staging flow path by motion of a rotor assembly of the pump; directing the pressurized fluid through a second section of the staging flow path towards a second seal.

A seal assembly for a pump comprises: a gland housing for mounting to the pump casing; a staging flow pathway within the gland housing defining first and second seal chambers; a rotor assembly pumping a coolant fluid through the gland housing, the rotor assembly having an acceleration surface, wherein fluid passing through the staging flow pathway is accelerated by the acceleration surface; first and second seal stages within the first and second seal chambers, each having a static sealing element and a rotating sealing element, the sealing elements engaging one another to form a fluid-tight seal; an inlet passage for feeding the coolant fluid into the staging flow pathway and past the acceleration surface.

In the figures, which depict example embodiments:.

<FIG> depicts a coolant pump <NUM> for use in a nuclear power generation facility. Coolant pump <NUM> has a casing <NUM>.

Pump <NUM> includes a pump rotating assembly including a drive shaft <NUM> coupled to a pump impeller <NUM> (e.g. an impeller) for circulation of a working fluid. Drive shaft <NUM> is mounted to casing <NUM> with a hydrostatic bearing <NUM>. The working fluid may be a reactor coolant, such as water. Pump impeller <NUM> draws the working fluid into pump <NUM> through an inlet <NUM> and forces the working fluid out of the pump under pressure through an outlet <NUM>.

A gland <NUM> is installed atop casing <NUM> around hydrostatic bearing <NUM> and receives shaft <NUM>. Gland <NUM> contains a sealing assembly <NUM> for resisting leakage of fluid from pump <NUM>. Sealing assembly <NUM> provides a seal between rotating components mounted to drive shaft <NUM>, referred to as a pump rotating assembly <NUM>, and stationary components mounted to or forming part of gland <NUM>, referred to as a stator assembly <NUM>. Gland <NUM> defines an internal fluid chamber <NUM>. Fluid chamber <NUM> holds fluid under high pressure, which holds working fluid in the main impeller casing of pump <NUM>.

Fluid circulates through fluid chamber <NUM> in a staging circuit <NUM> and in a recirculation loop <NUM>. Staging circuit <NUM> directs working fluid through a series of chambers for staged sealing. Recirculation loop <NUM> provides flow of working fluid between internal fluid chamber <NUM> and a cooling reservoir <NUM>. Circulation through cooling reservoir <NUM> provides a supply of cool working fluid for circulation through staging circuit <NUM>, so that flow through staging circuit <NUM> cools components of sealing assembly <NUM>. Flow through recirculation loop <NUM> may be driven by an auxiliary rotor <NUM>. Auxiliary rotor also creates flow of pressurized fluid within fluid chamber <NUM> toward hydrostatic bearing <NUM> to ensure that fluid passing through hydrostatic bearing <NUM> flows from chamber <NUM> into the main impeller casing of pump <NUM>.

Working fluid may be admitted to staging circuit <NUM> under pressure from chamber <NUM> and routed past components of sealing assembly <NUM> to step down the pressure in one or more sealing stages. Flow through staging circuit <NUM> may also cool and lubricate the sealing interfaces and other components. In particular, each seal stage may include an interface between a rotating sealing element and a stationary sealing element, which may be urged together for a tight seal. Relative motion of such sealing components while being urged together may lead to wear on the sealing components and buildup of heat due to friction.

The forces acting on a seal may be represented as the balance ratio, namely, a ratio of forces tending to close the seal and forces tending to open the seal. The balance ratio of a seal may correlate to the seal's propensity to leak or fail - higher balance ratios correspond to tighter (more leak-resistant) seals, and lower balance ratios correspond to looser (less leak-resistant) seals.

Urging sealing elements together tends to increase the resistance of the seal to leakage. However, it also tends to increase the wear rate of the seal and the heat produced as sealing elements move relative to one another. Typically, seals are designed to target a particular balance ratio in order to achieve a compromise between leak resistance and longevity. The balance ratio selected for a particular seal application may depend on the pressure differential which will be created across the seal and the required service life of the seal.

The balance ratio of a seal may be influenced, among other factors, by the seal geometry, such as the inner and outer diameters of the stator ring, fluid pressures on the high-pressure and low-pressure sides of the seal, as well as any mechanical force applied to urge the seal elements together.

<FIG>, <FIG> and <FIG> depict sealing assembly <NUM> in greater detail. <FIG> is a cross-sectional view of the complete sealing assembly <NUM>. <FIG> is an enlarged view of region II of <FIG>, showing components of pump rotating assembly <NUM> that are mounted for rotation with drive shaft <NUM>. <FIG> is an enlarged view of region II of <FIG>, showing components of stator assembly <NUM> that are stationary and mounted to gland <NUM>.

In the depicted embodiment, sealing assembly <NUM> has two sealing stages <NUM>, referred to individually as sealing stages <NUM>-<NUM> and <NUM>-<NUM>. Sealing stages <NUM>-<NUM>, <NUM>-<NUM> form first and second sealing stages. Sealing stages <NUM>-<NUM>, <NUM>-<NUM> have respective rotor seal members <NUM>-<NUM>, <NUM>-<NUM> (collectively, rotor seal members <NUM>). Sealing stages <NUM>-<NUM>, <NUM>-<NUM> have respective stator seal members <NUM>-<NUM>, <NUM>-<NUM> (collectively, seal members <NUM>).

As best shown in <FIG> and <FIG>, rotating assemblies <NUM> are received within primary staging chamber <NUM> and secondary staging chamber <NUM>, which form part of staging flow circuit <NUM>.

Stator seal members <NUM> are mounted within block structures <NUM>, which are mounted within chamber <NUM> and hold stator seal members <NUM> securely in a fixed position. Block structures <NUM> define a plurality of inner staging chambers. As depicted, block structures <NUM> define two staging chambers, namely primary staging chamber <NUM> and secondary staging chamber <NUM>. Each staging chamber corresponds to a seal stage of sealing assembly <NUM>. In other embodiments, more than two staging chambers may be defined, e.g. one chamber to each seal stage <NUM>.

In the depicted embodiment, the rotating assemblies <NUM> of seal stages <NUM>-<NUM>, <NUM>-<NUM> are identical. Such a design may allow rotating assemblies <NUM> to be interchangeable, limiting the number of unique parts and assemblies in pump <NUM>.

<FIG> is a simplified schematic diagram of seal stages <NUM>-<NUM>, <NUM>-<NUM> of seal assembly <NUM>, showing staging flow of cooling fluid.

Primary staging chamber <NUM> and secondary staging chamber <NUM> are in communication by way of staging flow passage <NUM>. Staging flow passage <NUM> receives flow from primary staging chamber <NUM> and includes a convoluted flow conduit, e.g. a coil. The convoluted flow path cooperate to restrict flow through passage <NUM>, such that cooling fluid traverses the passage at a defined rate and with a specific pressure drop. In some embodiments, the convoluted flow path may have an orifice sized to impose a pressure drop.

Secondary staging chamber <NUM> receives flow from staging flow passage <NUM> and discharges staging flow through a discharge passage <NUM>. As depicted, discharge passage <NUM> extends through stator block <NUM>. Flow exiting through discharge passage <NUM> is subjected to a further restriction and pressure drop to approximately atmospheric pressure. Discharge passage <NUM> may be configured to impose flow restrictions similar to those of passage <NUM>. For example, discharge passage <NUM> may include a conduit with convolutions to impose flow restrictions or may have an orifice with small cross-sectional area restricting flow at the inlet or at another location. Alternatively, such flow restriction may be imposed externally to discharge passage <NUM>. Additional staging and cooling flow may pass through a collection duct <NUM>, which may communicate with discharge passage <NUM> by way of a plenum <NUM>.

Hydraulic chambers <NUM>, <NUM> are defined behind seal stages <NUM>-<NUM>, <NUM>-<NUM>, respectively. Each seal stage <NUM> is designed to permit passage of a small quantity of fluid into the respective hydraulic chamber. That is, seal stage <NUM>-<NUM> permits slight leakage of fluid into hydraulic chamber <NUM> and seal stage <NUM>-<NUM> permits slight leakage of fluid into hydraulic chamber <NUM>. Hydraulic chamber <NUM> drains into secondary staging chamber <NUM>. Hydraulic chamber <NUM> drains to outlet <NUM>.

Hydraulic chambers <NUM>, <NUM> are the low-pressure sides of seal stages <NUM>-<NUM>, <NUM>-<NUM>. The pressure differential across seal stage <NUM>-<NUM> is approximately equal to the pressure drop that occurs through staging flow passage 134a, 134b. The pressure differential across seal stage <NUM>-<NUM> is approximately equal to the pressure drop between the entry to discharge passage 138a, 138b and the pressure at outlet <NUM> (e.g. atmospheric pressure).

Staging flow circuit <NUM> defines a flow path from chamber <NUM> into primary staging chamber <NUM>, then through staging flow passage <NUM>, into secondary staging chamber <NUM> and to through outlet passage <NUM> and ultimately through discharge outlet <NUM>. The flow path of staging flow circuit <NUM> extends generally axially, that is, in a direction parallel to the axis of the drive shaft of pump <NUM>.

Fluid within chamber <NUM> is pressurized at a high pressure, typically in excess of the discharge pressure of pump <NUM>. Fluid discharged to outlet <NUM> is at a lower pressure, which may be approximately atmospheric pressure. In an example, the pressure drop through staging flow path <NUM> is approximately <NUM> bar (<NUM> psi). In other embodiments, the pressure drop may be larger, e.g. <NUM> bar (<NUM> psi), or smaller, e.g. <NUM> bar (<NUM> psi).

Seal stages <NUM>-<NUM>, <NUM>-<NUM> and staging flow circuit <NUM> are configured to divide the pressure drop. In other words, multiple stages <NUM> may be configured such that each stage bears only a portion of the total pressure drop across assembly <NUM>. A first pressure drop occurs across seal stage <NUM>-<NUM> and through staging flow passage <NUM>, and a second pressure drop occurs across seal stage <NUM>-<NUM> and through discharge passage 138a, 138b. Total stagnation pressure of fluid at the inlet to primary staging chamber <NUM> is approximately the same as the total stagnation pressure at the sealing interface of seal stage <NUM>-<NUM> (point B in <FIG>). The total stagnation pressure at the inlet to primary staging chamber <NUM> is higher than that at the inlet of secondary staging chamber <NUM> by approximately the pressure drop through staging flow passage <NUM>. Atmospheric pressure, for example, at point D in <FIG> is lower than that at the sealing interface of seal stage <NUM>-<NUM> (point C in <FIG>) by approximately the pressure drop through discharge passage 138a, 138b. As depicted, staging flow passage <NUM> and outlet passage <NUM> impose about the same pressure drop, i.e. about <NUM> bar (<NUM> psi). As used herein, the term "stagnation pressure" refers to the sum of the static pressure and the dynamic pressure of a fluid flow, namely, the pressure that would result from decelerating the fluid flow to stagnation without losses.

While the example embodiments described herein illustrate two stages <NUM>-<NUM>, <NUM>-<NUM>, in other embodiments, three or more stages are included. The third and/or additional stages are positioned in series with the first two stages in the flow pathway. In some embodiments, the components in and interfacing with the third section or stage of the flow path can be configured such that each stage bears a desired portion of the total pressure drop across the assembly. These components and interfaces can include chamber geometry, staging flow values, sealing element configurations, surface features, flow path lengths, etc. and/or any other feature described herein with respect to the two stage design.

References to an assembly with first and second stages can, in some embodiments, refer to assemblies with more than two stages.

<FIG> shows an enlarged schematic view of sealing elements of a sealing stage <NUM>. As noted, seal stages <NUM> are designed to leak slightly. Accordingly, a thin leaking layer <NUM> of fluid is typically present between the sealing elements. The sealing elements are urged apart by fluid pressure of the leaking layer and are urged together by fluid pressure on the high-pressure sides of the seal stages and by urging of the rotor assemblies <NUM> toward block structures <NUM>. Flow of fluid around the sealing elements <NUM>, <NUM> and between the sealing elements <NUM>, <NUM> through leaking layer <NUM>, provides cooling of the sealing elements, which tends to promote consistent and/or predictable temperatures of the sealing elements <NUM>, <NUM>.

Rotor assemblies <NUM> may be free to move axially, such that the position of rotor assemblies <NUM> and pressure with which they urge the sealing elements together depends on fluid pressure acting on the rotor assemblies <NUM>.

As shown in <FIG>, seal members <NUM>, <NUM> have opposing faces 126a, 128a, high pressure faces 126b, 128b, and low pressure faces 126a, 128a. High pressure faces taper away from one another in a direction extending into staging chamber <NUM>/<NUM> (radially outward in the depicted embodiment).

Generally, pressure acting on opposing faces 126a, 128a and high pressure faces 126b, 128b tends to urge the seal members <NUM>, <NUM> apart. Loading conditions may cause deformation of seal members <NUM>, <NUM>, which may alter the exterior profile of the seal members and thus, the forces acting on their faces. The balance ratios of seal stages <NUM> is therefore affected by fluid pressure in the vicinity of the sealing elements, as well as fluid pressure acting on rotor assemblies <NUM> and the pressures on the stationary components (e.g. stator <NUM>).

During operation, the propensity of seal stages <NUM> to leak may be related to the balance ratio of the seal stages <NUM>. Seals may be more likely to leak at lower balance ratios and less likely to leak at higher balance ratios.

On the other hand, the rate at which the seal members wear may also be related to balance ratio. Specifically, seals may wear faster at higher balance ratios. Rotor seal member <NUM> and stator seal member <NUM> of each seal stage <NUM> move relative to one another and cause wear in proportion to the balance ratio of the seal stage.

Thus, effective seal design may rely on accurate determination of balance ratio. Under-estimating a seal's balance ratio may lead to a design that is prone to leaking. Overestimating a seal's balance ratio may lead to a design that wears prematurely. In multi-stage configurations, the sealing performance and longevity of seals at each stage may depend on the pressure drop at that stage. In embodiments, it may be desired to balance the pressure drop and balance ratio at each stage, such that sealing elements wear evenly and provide equivalent sealing performance. For example, for seals with two stages, the stages tend to wear evenly when each stage bears approximately one half of the pressure drop, and the stages have approximately equal balance ratios. Conversely, if one stage bears a greater pressure drop and has a greater balance ratio, that stage may tend to wear and fail faster than other stages, while the other stage may be more prone to leaking.

Typically, seal parameters are selected based on static analysis. That is, pressures and balance ratios are typically calculated based on an assumption of negligible dynamic pressure.

However, the inventors observed that seal stages of previous designs tended to wear unevenly and leak more than expected. That is, seal stages <NUM> with a single seal that were predicted to have equal balance ratios were observed to wear unevenly relative to one another. In addition, some seal stages <NUM> were observed to perform as if they had balance ratios lower than expected.

The inventors discovered that dynamic effects may be significant in pump <NUM>. Specifically, static pressures may vary due to dynamic effects within the staging flow circuit. For example, the pressure exerted on steal stage <NUM>-<NUM> depends on the stagnation pressure of fluid at point B of <FIG>, as well as the velocity of the fluid at point B, such as the velocity of the average fluid streamline. Likewise, the pressure exerted on seal stage <NUM>-<NUM> depends on the stagnation pressure at point C of <FIG> and the fluid velocity at point C, e.g. the velocity of the average fluid streamline. Thus, the balance ratio of each seal stage <NUM> may depend on the pressure drop across the seal stage, pressure losses through flow passage <NUM> and the static pressure acting on the sealing elements , (which in turn depends on fluid velocity). Generally, high fluid velocities on the high-pressure side of a seal stage <NUM> reduce the force acting on high-pressure faces 126b, 128b. In addition, velocity (and thus, pressure) at the bottom of staging chamber <NUM> (point A in <FIG>) relative to velocity and pressure near seal stage <NUM>-<NUM> (point B in <FIG>) may impact the balance ratio and sealing effectiveness of seal stage <NUM>-<NUM>. Specifically, lower velocity and higher pressure between the rotor assembly and stator assembly (point B in <FIG>) may tend to urge the rotor assembly away from the stator assembly, while lower velocity and higher pressure at the entry to staging chamber <NUM> (point A in <FIG>) may tend to urge the rotor assembly towards the stator assembly.

Each of rotor seal elements <NUM> forms part of a rotating assembly <NUM>. Fluid flow in chambers <NUM>, <NUM> passes over rotating assemblies <NUM> and is accelerated. Therefore flow entering a chamber <NUM>, <NUM> has lower velocity and greater static pressure than flow exiting a chamber <NUM>, <NUM>.

While a static analysis would balance seal stages <NUM>-<NUM>, <NUM>-<NUM> based on expected losses through passages <NUM>, <NUM>, the depicted design compensates for both losses through passages <NUM>, <NUM> and dynamic effects. For example, based on simulation of the flow regime, chambers <NUM>, <NUM> and flow passages <NUM>, <NUM> are designed to balance the effects of fluid acceleration at stages <NUM>-<NUM>, <NUM>-<NUM>. Specifically, chambers <NUM>, <NUM> are shaped and the flow resistance in passages <NUM>, <NUM> is such that fluid flows in a consistent direction - on average, in the direction indicated by arrow F in <FIG>.

As fluid flows through primary staging chamber <NUM>, it traverses a path adjacent to rotating assembly <NUM> and thus is accelerated by the motion of rotating assembly <NUM> such that average velocity increases. As the fluid accelerates, its dynamic pressure increases and its static pressure correspondingly decreases. Accordingly, static pressure is reduced between the inlet of staging chamber <NUM> (point A in <FIG>) and the region proximate sealing stage <NUM>-<NUM> (point B in <FIG>). Likewise, as fluid flows through secondary staging chamber <NUM>, it is accelerated by motion of rotating assembly <NUM> such that velocity increases and static pressure is reduced between the outlet of staging flow passage <NUM> and the region proximate sealing stage <NUM>-<NUM> (point C in <FIG>).

As will be apparent, the tangential velocity at any point on the surface of rotating assembly <NUM> depends on rotational speed, and on radius from the axis of rotation. In the depicted embodiment, rotating assemblies <NUM> have an outer cylindrical surface <NUM> at a relatively large radius. During operation, the tangential velocity of the rotating assembly at points on cylindrical surface <NUM> is relatively high and cylindrical surface <NUM> therefore causes relatively significant acceleration of the surrounding fluid. Cylindrical surface <NUM> may therefore be referred to as an accelerating surface.

In some embodiments, chambers <NUM>, <NUM> and rotating assemblies <NUM> may be configured to control the velocity (and thus, the static pressure) of fluid proximate seal stages <NUM>. In particular, fluid may be accelerated as it traverses a path adjacent to one or more acceleration surfaces <NUM>. Accordingly, chambers <NUM>, <NUM> may be configured to control the cumulative acceleration due to acceleration surfaces <NUM>, <NUM>. In some embodiments, fluid proximate seal stages <NUM>-<NUM>, <NUM>-<NUM> may be accelerated to approximately equal velocity. In other words, fluid velocity and dynamic pressure proximate seal stages <NUM>-<NUM>, <NUM>-<NUM> may be approximately equal. For example, chambers <NUM>, <NUM> may have equal cross sectional area proximate seal stages <NUM>-<NUM>, <NUM>-<NUM>, respectively.

In some embodiments, one or more acceleration surfaces <NUM>, <NUM> include surface features configured to enhance the fluid velocity. In some embodiments, the features maintain the cooling flows in and around the rotating assembly. In some embodiments, the surface features can include wedges, troughs, and/or any other suitable protrusions, indentations or other surface features. In some embodiments, the surface features can include machined slots and/or windows. In some embodiments, the acceleration surfaces <NUM> include windows, and the acceleration surfaces <NUM> include machine slots.

In some embodiments, one or more acceleration surfaces <NUM>, <NUM> provide for the recirculation of fluid to control the temperature of surfaces. In some embodiments, the one or more acceleration surfaces <NUM>, <NUM> can additionally or alternatively serve to generate desired velocities for the force balance.

In some embodiments, the length of flow paths through chambers <NUM>, <NUM> and adjacent to rotating assemblies <NUM> may be the same, such that fluid resides in chambers <NUM>, <NUM> for a similar length of time and is therefore accelerated to a similar velocity by rotating assemblies <NUM>. The flow path length may depend on the shapes of chambers <NUM>, <NUM> and rotating assemblies <NUM>, and the location at which fluid flow enters each respective chamber <NUM>, <NUM>.

In some embodiments, the velocities to be provided by one or more of the components described herein are based on absolute velocities and/or relative velocities. Absolute velocities can be targeted to adjust pressure, and relative velocities can be targeted to adjust heat transfer.

In some scenarios, the staging flow is a parameter which can be adjusted in new designs or installations. In other scenarios, such as retrofits, the staging flow parameter can be predefined by the system in place.

Referring again to <FIG>, primary staging chamber <NUM> has an inlet passage <NUM> through which fluid is drawn from chamber <NUM> into chamber <NUM>. Inlet passage <NUM> extends generally radially through block structure <NUM>. Some of fluid drawn through passage <NUM> flows generally in the direction indicated by arrow F toward seal stage <NUM>-<NUM>. Some of fluid drawn through passage <NUM> may be drained generally in the reverse flow direction indicated by arrow R through a staging chamber throat <NUM>, which lies in an axial plane. In some embodiments, this can be driven by an auxiliary impeller.

As shown, inlet passage <NUM> is located at an axial position between staging chamber throat <NUM> and acceleration surface <NUM>. The acceleration experienced by fluid flowing to seal stage <NUM>-<NUM>, may depend on the position of passage <NUM>. For example, if passage <NUM> were moved in direction F, relative to the depicted position, fluid acceleration due to rotating assembly <NUM> may be decreased if fluid traversed a shorter path adjacent to acceleration surface <NUM>. Conversely, if passage <NUM> were moved in direction R, more fluid may be drained generally in direction R, and acceleration of fluid that flows to seal stage <NUM>-<NUM> may be increased.

Staging flow circuit <NUM> is configured so that seal stages <NUM>-<NUM>, <NUM>-<NUM> are approximately at their desired balance ratio during operation. That is, the flow and pressure regime within staging chambers <NUM>, <NUM> causes mechanical loading of seal stages <NUM>-<NUM>, <NUM>-<NUM> to their desired balance ratio. The flow and pressure regimes are in turn affected by the location of inlet passage <NUM>. For example, the location of inlet passage <NUM> influences the static pressure profile within the primary staging chamber <NUM>. High pressure proximate throat <NUM> urges rotating assembly <NUM> against block assembly <NUM>, tending to increase balance ratio of the seal stage. Conversely, lower pressure proximate throat <NUM> exerts less force on rotating assembly <NUM>. In some embodiments, by balancing dynamic pressure, along with the losses through passages <NUM>, <NUM>, loading of the seal elements is likewise balanced. In other words, designing for both losses and dynamic effects provides for a consistent balance ratio of seal stages <NUM>-<NUM>, <NUM>-<NUM>. Performance and longevity of the seal stages is likewise balanced. Accordingly, seal stages <NUM>-<NUM>, <NUM>-<NUM> wear at similar rates and may be serviced or replaced at the same service interval. Conversely, uneven wear between stages <NUM>-<NUM>, <NUM>-<NUM> may result in premature failure or may require replacement of a seal stage <NUM> at a shorter interval. In some scenarios, when one stage is replaced, both stages are replaced so uneven wear can result in a less worn seal being replaced before it has been utilized to its full potential.

<FIG>, <FIG> and <FIG> are cross-sectional views of seal assembly <NUM>. For simplicity, some components of seal assembly <NUM> are depicted schematically in planar orientation. However, as will be apparent, such structures may extend radially, e.g. in a direction normal or skewed relative to the plane of the depicted cross section. Likewise, flow paths depicted as planar in <FIG>, <FIG> and <FIG> may extend around the radius of seal assembly <NUM>. For example, chambers <NUM>, <NUM> and fluid passages <NUM>, <NUM> and <NUM> may be generally annular. Additionally or alternatively, structures shown as planar and described as singular may, for example, be series of radially-extending passages spaced at different angular orientations. For example, passage <NUM> may extend radially. A single passage <NUM> may be present, or a plurality of passages <NUM> may be present, spaced at even intervals around seal assembly <NUM>.

During operation of pump <NUM>, temperatures within seal assembly <NUM> may increase, causing expansion of components. The rates and direction of expansion may depend on factors such as part shape and material properties.

Differential expansion of components may cause changes in sealing geometry. Such changes could result in increased clearance or reduced pressure between sealing elements <NUM>, <NUM> and thereby reduce the stability or sealing effectiveness of a seal stage <NUM>.

Therefore, in order ensure sealing performance and to manage component wear, components of seal assembly <NUM> may be configured to promote or maintain dimensional stability of seal stages <NUM>-<NUM>, <NUM>-<NUM> and their respective sealing elements <NUM>, <NUM> throughout the typical range of temperatures that would be occur during normal operation of pump <NUM>. In some examples, during normal operation, components of seal assembly <NUM> may typically be subjected to temperatures between approximately 32º C and 93º C (90º F and 200º F). Seal components may be formed of a material such as silicon carbide or titanium carbide with a relatively low coefficient of thermal expansion in the anticipated operational thermal range. As will be apparent, operating temperatures may vary depending on reactor design and material choices may likewise vary. Suitable materials are those which deliver adequate sealing and wear performance, with sufficiently low thermal expansion such that sealing performance is not excessively degraded by normal operating temperatures. For example, if sealing elements <NUM>, <NUM> are designed to be self-relieving, the part geometry and materials may be designed such that the self-relieving feature is maintained throughout a normal operating temperature range.

According to the invention, the rotating sealing element <NUM> is constrained by first and second expansion-control rings <NUM>. Expansion-control rings <NUM> may be high-strength metallic components installed around seal elements <NUM> or <NUM>. Expansion-control rings <NUM> may be sized with a nominal inner diameter sized to radially constrain seal elements <NUM>, <NUM> around which they are installed. Expansion-control rings <NUM> may be formed of a material with a low coefficient of expansion at normal operating temperatures of pump <NUM>. At such temperatures, expansion-control rings <NUM> mechanically resist thermal expansion of seal elements <NUM>, <NUM> such that the shapes of seal elements <NUM>, <NUM> are maintained. For example, in the depicted embodiment, seal elements <NUM>, <NUM> taper slightly away from one another at the radially-outermost portion of their interface. Pressure acting against the tapered portions of the seal elements tends to urge the seal elements away from one another. Thus, the shape of the seal elements may serve a self-relieving function.

In some embodiments, as the temperature rises, the seal surfaces are configured to deflect such that they open more and allow more leakage. This leakage serves to cool the rubbing surfaces thereby reducing the hear going through the seal parts.

Certain operational conditions in a nuclear power generation facility may lead to a loss of coolant flow. In such events, operation of coolant pump <NUM> may cease. Fluid temperature in pump <NUM> may increase dramatically and may significantly exceed normal operational ranges. Safety and regulatory considerations may require protection against leaks in such events. In some examples, fluid temperatures up to or exceeding 260º C (<NUM>° F) may be experienced in a loss of coolant flow event such as a blackout. Expansion-control rings <NUM> may have a greater coefficient of thermal expansion than sealing elements <NUM> and rotor assemblies <NUM>. Thus, as temperature increases, clearance between sealing elements <NUM> and expansion-control rings <NUM> may increase. Alternatively, expansion control rings <NUM> may be configured to have an increased coefficient of thermal expansion above a threshold temperature, in a range expected during a loss of coolant flow event and higher than normal operating temperatures. In a loss of coolant flow event, expansion-control rings <NUM> expand, creating clearance between sealing elements <NUM> and expansion-control rings <NUM>. This in turn allows sealing elements <NUM>, <NUM> to expand into one another. Expansion of sealing elements <NUM>, <NUM> may be sufficient to eliminate leakage through seal stages <NUM>. Further temperature increases may reinforce the seal by causing sealing elements <NUM>, <NUM> to continue expanding together.

In some embodiments, materials for these components are selected based on their material properties. In some scenarios, selecting different materials can allow for selective disengagement of rings so that beyond a given threshold there is a change in the overall behaviour of the rotating assembly.

In the depicted embodiment, the expansion-controls rings <NUM> installed around a particular sealing element <NUM> may be formed of different materials, with different thermal expansion rates. For example, one ring <NUM> may be formed of nitronic <NUM> and another ring of stainless steel. Differential expansion rates of the expansion-control rings <NUM> may lead to one ring <NUM> disengaging from sealing element <NUM> before the other ring <NUM>. This may, in turn cause the sealing element <NUM> to deform as it expands above the normal operating temperature. For example, sealing element <NUM> may deflect toward sealing element <NUM>, such that any self-relieving properties of the seal are removed.

In other embodiments, expansion-control rings <NUM> may be formed of the same material, but may be sized for differing nominal clearance (or interference) with sealing element <NUM>. In such designs, one expansion-control ring <NUM> may expand out of contact with sealing element <NUM> before the other. Alternatively or additionally, sealing element <NUM> may be shaped such that its diameter varies and the nominal inner diameters of expansion-control rings <NUM> differ. In such configuration, an equal percentage change of the expansion-control rings <NUM> may correspond to a greater absolute change in diameter in the larger of the two rings. Accordingly, expansion control rings <NUM> may be sized for the same amount of nominal clearance or interference with sealing element <NUM>, and may be formed of the same material, but the larger ring may release the sealing element <NUM> at a lower temperature than the smaller ring.

In some embodiments (not claimed), geometry of sealing elements <NUM>, <NUM> may be such that a single expansion-control ring may be used, or such that multiple rings may be used that are configured to release the sealing element <NUM> at the same temperature. Specifically, sealing elements <NUM>, <NUM> may be designed and expansion constrained such that, at normal operating temperatures, sealing elements <NUM>, <NUM> permit a small amount of leakage, are self-relieving under thermal expansion, or both, while at temperatures above the normal operating range, one or more expansion-control rings <NUM> releases and sealing element <NUM> is permitted to expand towards sealing element <NUM> and thereby reduce or eliminate leakage or self-relieving properties.

Sealing assembly <NUM> therefore provides a failsafe against leakage. However, because components have relatively low coefficients of expansion, the balance ratio of the seal during regular operation is not materially affected, nor is the rate of wearing or the sealing performance under normal operational conditions affected.

As described above, fluid flow enters each respective one of primary staging chamber <NUM> and secondary staging chamber <NUM> at an inlet end, while seal stages <NUM>-<NUM>, <NUM>-<NUM> are positioned proximate the outlet ends of staging chamber <NUM>', <NUM>'. Fluid flow traverses chamber <NUM> or chamber <NUM> before reaching the respective sealing stage <NUM> and is accelerated during such traversal. The effects of acceleration, i.e., decreased static pressure, are balanced between stages. Moreover, the amount of acceleration, and thus, the change in static pressure at sealing stages <NUM> may be controlled by changing the length of the fluid flow path proximate accelerating surface <NUM>, <NUM>. As described herein, in some embodiments, this is also controlled by the shape of the chambers <NUM>,<NUM> and by features of the surfaces <NUM>, <NUM>.

Although the embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that fall within the scope of the claims and that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claim 1:
A seal assembly (<NUM>) for a pump (<NUM>), comprising:
a gland housing for mounting to the pump casing;
a staging flow pathway within the gland housing defining first and second seal chambers (<NUM>, <NUM>);
a rotor assembly pumping a coolant fluid through said gland housing, the rotor assembly having at least one acceleration surface (<NUM>, <NUM>), wherein fluid passing through the staging flow pathway is accelerated by said at least one acceleration surface (<NUM>, <NUM>);
first and second seal stages (<NUM>) within said first and second seal chambers (<NUM>, <NUM>), each having a static sealing element (<NUM>) and a rotating sealing element (<NUM>), said sealing elements engaging one another to form a fluid-tight seal, wherein said sealing elements are configured to thermally expand;
an inlet passage (<NUM>) for feeding the coolant fluid into said staging flow pathway and past said at least one acceleration surface (<NUM>, <NUM>); characterised by
expansion-control rings (<NUM>) configured to restrict expansion of said rotating sealing element (<NUM>) below a threshold temperature,
wherein said expansion-control rings (<NUM>) comprise a first expansion control ring around said rotating sealing element (<NUM>) and a second expansion control ring around said rotating sealing element (<NUM>), wherein said first expansion control ring is configured to expand out of contact with said rotating sealing element (<NUM>) at a lower temperature than said second expansion control ring.