Patent Description:
The invention relates to rotating shaft seals, and more particularly, to dry running mechanical shaft seals.

When a fluid-containing housing, for example the housing of a pump or turbine, is penetrated by a rotating shaft, a mechanical seal is often used to minimize leakage of a process fluid from the interior of the housing. End face mechanical seals, also referred to as mechanical face seals, are commonly used for this purpose. Document <CIT>, for example, describes a mechanical seal assembly with coolant circulation structure. Other sealing systems are known from <CIT>, and from <CIT>.

Mechanical seals rely on maintaining a closely spaced relationship between the flat and parallel surfaces of a stationary, "stator" seal face and a rotating "rotor" seal face. This generally requires that a lubricant or other "cushioning substance" occupy the small gap between the stationary and rotating seal faces, so as to minimize frictional heating and premature wear.

In circumstances where the process fluid is non-lubricating and/or hazardous, a separate, pressurized lubrication system is sometimes provided that introduces a liquid lubricant into the seal. The lubricant is applied with sufficient pressure to cause it to slowly leak through the seal into the process, thereby preventing any escape of the process fluid through the seal into the surrounding environment. While this approach can be highly effective, it is also complex and expensive.

In situations where the process fluid is a liquid with suitable lubricating properties, the process liquid itself is often used as the seal lubricant by allowing the process liquid to enter the seal and slowly leak past the seal faces. This approach has the advantage of not requiring a separate, complex and costly lubrication system. However, this approach has the disadvantage that small amounts of process fluid is allowed to leak past the seal and into the surrounding environment, which may be unacceptable in cases where the process fluid is hazardous or otherwise undesirable.

Under such circumstances, one approach is to provide a second, "dry" mechanical seal in tandem with the primary seal that will intercept the slowly leaking process fluid and allow it to be recaptured without escaping into the environment. Such dry mechanical seals may be "lift-off' seals, whereby the rotor and/or stator seal faces include lift-off features that create a pressurized air cushion during operation that fills the small gap between the two seal faces, thereby minimizing friction, wear, and heating. An example of this arrangement is shown in <FIG>, which includes a primary mechanical seal <NUM> that is lubricated by the process liquid, arranged in tandem with a secondary, dry mechanical seal <NUM> that is lubricated by a cushion of air. This approach has the added advantage that the secondary, "dry" seal <NUM> can provide temporary backup sealing in the event that the primary seal <NUM> suddenly fails, giving the operator of the equipment enough time to invoke immediate shutdown and isolation procedures during such an event, without allowing a major release of process fluid to the atmosphere.

In some applications, such as centrifugal pumps used in some petroleum refineries, the pumps are often provided in pairs, and are operated such that one pump is in operation while an adjacent pump of the same size and type is maintained in a standby mode, typically with its shaft spinning at a very low speed. Under such circumstances, the rotation speed while in stand-by mode may not be sufficient to enable a dry mechanical seal <NUM> such as the one illustrated in <FIG> to generate a lubricating air cushion, thereby causing the seal to operate in stand-by mode with the stator and rotor faces in direct mechanical contact. This can lead to damage of the lift-off features on the rotor and stator seal faces, such that they are no longer able to generate a lift-off cushion even when they are subsequently switched to normal operating mode with high shaft operating speeds. As a result, the seal faces may remain in contact when they are subsequently operated at high rotational speeds, which will cause the faces to overheat and wear at a rapid rate.

What is needed, therefore, is a dry running mechanical seal design that is able to maintain the faces in direct contact for extended periods of time at both low and high operating speeds, without requiring a lift-off feature, and without undue heating or wear of the seal faces when the seal is operated at high shaft speeds.

A dry running mechanical seal design is disclosed that is able to maintain the faces in direct contact for extended periods of time at both low and high operating speeds without including a lift-off feature and without undue heating or wear of the seal faces when the seal is operated at high shaft speeds.

Embodiments of the disclosed seal design include at least five cooperative improvements that work together to significantly reduce frictional heating of the seal faces. In particular:.

According to the present invention, heat transfer by direct physical contact from the stator seal face to the gland in which the stator seal face is mounted is enhanced by lapping of both the rear surface of the seal face and the contacting surfaces of the gland. In embodiments, these two surfaces are lapped to a flatness of flatter than three microns, and in some embodiments flatter than <NUM> microns. In various embodiments, the roughness of the lapped surfaces is less than <NUM> RMS, and in some embodiments less than <NUM> RMS, where RMS is used herein to refer to "root-mean-square" values measured in micro-inches. This improvement allows the gland to function as a virtual "heat sink" for the direct transfer of heat from the stator seal face.

In addition, an annular groove is formed in the gland plate that functions as a cooling annulus through which air can flow while the seal is in operation. In embodiments, slots provided in the gland plate connect the cooling annulus with the inner diameter of the gland plate, thereby allowing for entry of air into the annulus from a region surrounding the spinning shaft. In various embodiments, axial holes through which air can escape are provided that penetrate from the rear of the gland plate to the cooling annulus, thereby allowing circulation of air through the cooling annulus and in contact with the rear surface of the stator seal face.

In various embodiments, the front surface of the rotor seal face includes features such as cooling scallops on the largest OD surface that are enlarged and/or milled more deeply so as to operate as a fan-like feature that increases wind turbulence in the proximity of the seal faces and thereby enhances cooling.

In embodiments, the geometry of the rotor seal face is optimized, for example using finite element analysis, to minimize pressure distortions of the seal face at high pressure, and to hydraulically balance the seal face so as to maintain positive sealing of light hydrocarbons and other process fluids at high pressures.

As is typical for mechanical seals, a spring is provided that maintains a positive pressure between the seal faces as they are slowly worn down. This requires that the rotor be sealed to the shaft by a dynamic O-ring or other dynamic gasket that is able to slide or roll across a dynamic sealing surface so as to allow the rotor to move toward the stator. In embodiments of the present invention, this dynamic sealing surface is highly polished, so as to allow the dynamic gasket to move more freely across it with minimal drag, thereby reducing the required force of the springs and the pressure and friction between the seal faces. In embodiments, the dynamic sealing surface is polished to less than <NUM> RMS, and in some embodiments less than <NUM> RMS.

According to a first general embodiment, the present invention is a dry-running, end-face mechanical seal, comprising a gland plate in surrounding, non-contacting relationship with a rotatable shaft, a stator seal face fixed to said gland plate in surrounding, non-contacting relationship with the rotatable shaft, a rear surface of the stator seal face being in direct, parallel contact with a seal-contacting surface of the gland plate, said rear surface of the stator seal face and said seal-contacting surface of the gland plate both being flat to within a tolerance of three microns and smooth to within <NUM> RMS, a rotor mounting structure arranged in a rotationally fixed, axially movable relationship with said rotatable shaft, a rotor seal face supported by said rotor mounting structure and positioned such that a front surface of the rotor seal face is in direct parallel contact with said front surface of said stator seal face, thereby forming a dry contacting seal therebetween, a cooling channel formed in said gland plate and bounded on at least one side by the rear surface of the stator seal face, at least one inlet port configured to allow air to flow from a region proximal to said rotatable shaft into said cooling channel, and at least one outlet port configured to allow air to escape from said cooling channel.

Other embodiments included within the scope of the invention include:.

Embodiment <NUM>: The seal of embodiment <NUM>, wherein the cooling channel is formed in the seal-contacting surface of the gland plate.

Embodiment <NUM>: The seal of embodiment <NUM> or <NUM>, wherein the at least one outlet port includes at least one hole penetrating said gland plate from a rear surface thereof into said cooling channel.

Embodiment <NUM>. The seal of any preceding embodiment, wherein the seal-contacting surface of the gland plate and the rear surface of the stator seal face are both flat to within a tolerance of <NUM> microns and smooth to within a tolerance of <NUM> RMS.

Embodiment <NUM>. The seal of any preceding embodiment, wherein at least one of the front surface of the stator seal face and the front surface of the rotor seal face includes a turbulence-enhancing feature that increases air turbulence surrounding the seal faces when the rotatable shaft is rotating.

Embodiment <NUM>: The seal of embodiment <NUM>, wherein the turbulence-enhancing feature is at least one scallop.

Embodiment <NUM>: The seal of embodiment <NUM>, wherein the at least one scallop is formed in the front surface of the rotor seal face.

Embodiment <NUM>: The seal of any preceding embodiment, wherein the rotor seal face mounting structure is rotationally fixed to said rotatable shaft by an O-ring or gasket that is compressed between said rotor seal face mounting structure and a dynamic surface that is fixed to and concentric with said rotatable shaft, said dynamic surface being smooth to within <NUM> RMS.

Embodiment <NUM>: The seal of any preceding embodiment, wherein the seal is configured as a secondary seal that prevents a process fluid leaking past a primary seal from escaping into an ambient environment.

Embodiment <NUM>: The seal of embodiment <NUM>, wherein the seal is configured to prevent leakage of said process fluid into said ambient environment even in case of failure of said primary seal.

Embodiment <NUM>: The seal of any preceding embodiment, wherein the seal is able to maintain vapor emissions to less than <NUM> ppm and to prevent substantially all liquid leakage of the process fluid for at least <NUM> minutes when a pressure of the process fluid is about <NUM> kPa (<NUM> psi) and the rotatable shaft is rotating at a speed of <NUM> RPM.

Embodiment <NUM>. The seal of any preceding embodiment, wherein the seal is able to maintain an average temperature of the seal faces of less than <NUM> degrees higher than ambient when the rotatable shaft is rotating at <NUM> RPM and the rotor seal face is exposed to propane vapor at about <NUM> kPa (<NUM> psi) pressure.

Embodiment <NUM>. The seal of any preceding embodiment, wherein the rotor seal face is made of carbon graphite.

Embodiment <NUM>. The seal of any preceding embodiment, wherein the stator seal face is made of silicon carbide.

The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.

A dry running mechanical seal design is disclosed that is able to maintain a direct contact dry mechanical seal for extended periods of time at both low and high operating speeds without undue heating or wear of the seal faces.

With reference to <FIG>, the disclosed seal <NUM> is suitable for operation in tandem with a primary seal <NUM>, so that under normal operating conditions it is subject to only very small amounts of process fluid that leak past the primary seal <NUM>.

<FIG> is a simplified cross sectional diagram that illustrates the primary features of the embodiment <NUM> of <FIG>. The seal <NUM> includes a rotor seal face <NUM> having a front surface <NUM> that is pressed in direct contact against the front surface <NUM> of a stator seal face <NUM> mounted in a gland plate <NUM>. In embodiments, the rotor seal face <NUM> is made of carbon graphite, and/or the stator seal face is made of silicon carbide. In some embodiments, the seal <NUM> is a secondary seal that prevents process fluid leaking past a primary seal <NUM> from reaching the ambient environment.

In some of these embodiments, the seal <NUM> is configured to prevent leakage of the process fluid into the ambient environment even in case of failure of the primary seal. For example, in embodiments the seal <NUM> is able to maintain vapor emissions to less than <NUM> ppm and to prevent substantially all liquid leakage of the process fluid for at least <NUM> minutes when the pressure of the process fluid is about <NUM> kPa (<NUM> psi) and the shaft <NUM> is rotating at a speed of <NUM> RPM. In various embodiments, the seal <NUM> is able to maintain an average temperature of the seal faces <NUM>, <NUM> of less than <NUM> degrees higher than ambient when the shaft <NUM> is rotating at <NUM> RPM and the rotor seal face <NUM> is exposed to propane vapor at about <NUM> kPa (<NUM> psi) pressure.

Both the front surface <NUM> and the rear surface <NUM> of the stator face <NUM> are lapped, and the rear surface <NUM> of the stator face <NUM> is pressed against a lapped surface <NUM> of the gland plate <NUM>. The lapping of both of these surfaces <NUM>, <NUM> allows heat to be conducted very efficiently between them, and thereby allows the gland plate <NUM> to serve as a heat sink that removes heat by direct conduction from the seal faces <NUM>, <NUM> and helps to prevent them from overheating. In embodiments, the rear surface <NUM> of the stator seal face and the corresponding lapped surface <NUM> of the gland plate <NUM> are lapped to a flatness of flatter than three microns, and in some embodiments flatter than <NUM> microns. In various embodiments, the roughness of the lapped surfaces <NUM>, <NUM> is less than <NUM> RMS, and in some embodiments less than <NUM> RMS.

In addition, a "cooling annulus" <NUM> is provided behind the stator face <NUM>. This cooling annulus <NUM> is formed in embodiments by milling an annular groove in the gland plate <NUM> directly behind the rear surface <NUM> of the stator face <NUM>. In embodiments, slots <NUM> are milled or otherwise provided at periodic, spaced-apart locations about the cooling annulus <NUM> which extend from the cooling annulus <NUM> to the inner diameter of the gland plate <NUM> and allow air <NUM> to flow from a region near the inner diameter into the cooling annulus <NUM>. This flow is enhanced due to the proximity of these slots <NUM> to the rotating sleeve <NUM> that surrounds the shaft <NUM> of the pump or other apparatus that is being sealed.

<FIG> is a cross sectional illustration of the embodiment of <FIG>, where the cross section is taken at a different angular orientation about the central shaft <NUM>. It can be seen that the slot <NUM> is not intersected by this cross section, but instead the cross section is taken through a hole <NUM> that penetrates through the gland plate <NUM> and to the cooling annulus <NUM>. In embodiments, a plurality of these holes <NUM> are provided, and are offset angularly from the plurality of slots <NUM>. Since the rotation of the shaft <NUM> and sleeve <NUM> causes the air pressure to be higher than ambient, air naturally flows during operation through the slots <NUM> into the cooling annulus <NUM>, through the cooling annulus <NUM> to the holes <NUM>, and out through the holes <NUM>.

The relationship between the cooling annulus <NUM>, the slots <NUM>, and the holes <NUM>, and the path of the air <NUM> flowing through them, can be more easily seen in <FIG>, which is an end view of the gland plate <NUM> of <FIG> and <FIG>.

Referring again to <FIG>, the seal faces <NUM>, <NUM> are pressed against each other by springs <NUM> that extends between a dynamic rotor seal face housing <NUM> and a spring housing <NUM>. In particular, as the seal faces wear, the dynamic rotor seal face housing <NUM> and the rotor seal face <NUM> are able to move toward the stator seal face <NUM> because they are sealed to the shaft sleeve <NUM> by a dynamic gasket <NUM> that is able to roll or slide over a corresponding dynamic surface <NUM> of the spring housing <NUM>. In embodiments, this dynamic surface <NUM> is highly polished, thereby reducing the amount of spring force that is required to maintain firm contact between the seal faces <NUM>, <NUM>. In embodiments, the dynamic sealing surface <NUM> is polished to less than <NUM> RMS, and in some embodiments less than <NUM> RMS. Due to this reduction in spring force, the friction between the seal faces <NUM>, <NUM> is reduced, and the consequent heating of the seal faces <NUM>, <NUM> is further reduced.

In embodiments, the shape of the rotor seal face <NUM> is optimized to adjust its center of mass and minimize pressure distortions of the seal face <NUM> at high pressure. In the embodiment of <FIG>, <FIG>, and <FIG>, this is evident in the shaping of the rear portion <NUM> of the rotor seal face <NUM>.

With reference to <FIG>, in embodiments the front surface <NUM>, <NUM> of the rotor face <NUM> includes enlarged scallops <NUM> and/or other features that increase the turbulence of the air surrounding the seal faces <NUM>, <NUM> when the shaft <NUM> is rotating. This enhanced air circulation improves direct cooling of the exposed surfaces of the rotor face <NUM> and stator face <NUM>, and the increased pressure and turbulence of the air near the spinning shaft sleeve <NUM> also increases the flow of air into and through the cooling annulus <NUM>.

In summary, at least five cooperative improvements work together in embodiments of the present invention to significantly reduce frictional heating of the seal faces at high rotational speeds. In particular:.

The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application.

Claim 1:
A dry-running, end-face mechanical seal, comprising:
a gland plate (<NUM>) in surrounding, non-contacting relationship with a rotatable shaft (<NUM>);
a stator seal face (<NUM>) fixed to said gland plate (<NUM>) in surrounding, non-contacting relationship with the rotatable shaft (<NUM>), a rear surface (<NUM>) of the stator seal face (<NUM>) being in direct, parallel contact with a seal-contacting surface (<NUM>) of the gland plate (<NUM>), said rear surface (<NUM>) of the stator seal face (<NUM>) and said seal-contacting surface (<NUM>) of the gland plate (<NUM>) both being flat to within a tolerance of three microns and smooth to within <NUM> RMS;
a rotor mounting structure arranged in a rotationally fixed, axially movable relationship with said rotatable shaft (<NUM>);
a rotor seal face (<NUM>) supported by said rotor mounting structure and positioned such that a front surface (<NUM>) of the rotor seal face (<NUM>) is in direct parallel contact with a front surface (<NUM>) of said stator seal face (<NUM>), thereby forming a dry contacting seal therebetween;
a cooling annulus (<NUM>) formed in said gland plate (<NUM>) and bounded on at least one side by the rear surface (<NUM>) of the stator seal face (<NUM>);
at least one inlet port configured to allow air to flow from a region proximal to said rotatable shaft (<NUM>) into said cooling annulus (<NUM>); and
at least one outlet port configured to allow air to escape from said cooling annulus (<NUM>).