Method for operating a turbomachine during a loading process   

Abstract: A method for increasing the operational flexibility of a turbomachine is provided. The turbomachine may include a first section, a second section, and a rotor disposed within the first section and the second section. The method may determine an allowable range of a physical parameter associated with the first section and/or the second section. The method may modulate a first valve and/or a second valve to allow steam flow into the first section and the second section respectively, wherein the modulation is based on the allowable range of the physical parameter. In addition, the physical parameter allows the method to independently apportion steam flow between the first section and the second section of the turbomachine, during the loading process.

This application is related to commonly-assigned U.S. patent application Ser. No. ______ [GE Docket 238811], filed ______; U.S. patent application Ser. No. ______ [GE Docket 238821], filed ______; and U.S. patent application Ser. No. ______ [GE Docket 238828], filed ______.

BACKGROUND OF THE INVENTION

The present invention relates generally to turbomachines and more particularly to a method for enhancing the operational flexibility of a steam turbine during a loading phase.

Steam turbines are commonly used in power plants, heat generation systems, marine propulsion systems, and other heat and power applications. Steam turbines typically include at least one section that operates within a pre-determined pressure range. This may include: a high-pressure (HP) section; and a reheat or intermediate pressure (IP) section. The rotating elements housed within these sections are commonly mounted on an axial shaft. Generally, control valves and intercept valves control steam flow through the HP and the IP sections, respectively.

The normal operation of a steam turbine includes three distinct phases; which are startup, loading, and shutdown. The startup phase may be considered the operational phase beginning in which the rotating elements begin to roll until steam is flowing through all sections. Generally, the startup phase does not end at a specific load. The loading phase may be considered the operational phase in which the quantity of steam entering the sections is increased until the output of the steam turbine is approximately a desired load; such as, but not limiting to, the rated load. The shutdown phase may be considered the operational phase in which the steam turbine load is reduced, and steam flow into each section is gradually stopped and the rotor, upon which the rotating elements are mounted, is slowed to a turning gear speed.

There are a few issues, with known methods of operating the steam turbine during the loading operational phase. Currently known methods may be disadvantageously conservative. These methods may not effectively manage competing physical requirements. Here, a single physical requirement or parameter can limit the operation of the entire steam turbine. These methods can reduce operational flexibility, require larger mechanical components, and potentially reduce the net-output delivered by the steam turbine. Therefore, there is a desire for a method and a system for increasing the operational flexibility of the steam turbine during the loading phase.

In accordance with an embodiment of the present invention, a method of unbalancing steam flow entering a turbomachine during a loading process, the method comprising: providing a turbomachine comprising at least a first section and a second section, and a rotor partially disposed within the first section and the second section; providing a first valve configured for controlling steam flow into the first section; and a second valve configured for controlling steam flow into the second section; determining whether the turbomachine is operating in a loading phase; determining an allowable turbine operating space (ATOS), wherein ATOS incorporates data on at least one, but is not limited to, of the following, but not limited to: steam flow through each section, an axial thrust limit of each section, and an HP section exhaust windage limit to approximate operational boundaries for each section of the turbomachine; determining an allowable range within ATOS of a physical parameter associated with at least one of the first section or the second section; modulating the first valve to control steam flow into the first section, wherein the modulation the first valve to control steam flow into the first section, wherein the modulation is partially limited, by the allowable range of the physical parameter; modulating the second valve to allow steam flow into the second section, wherein the modulation is partially limited by the allowable range of the physical parameter; and wherein ATOS, in real time, expands operational boundaries of the first section and the second section, and allows unbalanced steam flow between the first section and the second section of the turbomachine during the loading phase.

In accordance with an alternate embodiment of the present invention, a method of independently apportioning steam flow between sections of a steam turbine during a loading process, the method comprising: providing a power plant comprising a steam turbine, wherein the steam turbine comprises a HP section, an IP section, and a rotor partially disposed within the HP and IP sections; providing a first valve configured for controlling steam flow entering the HP section; and a second valve configured for controlling steam flow entering the IP section; determining whether the steam turbine is operating in a loading phase; determining an allowable turbine operating space (ATOS), wherein ATOS incorporates data on a least one of the following: steam flow through each section, an axial thrust limit of each section, and an HP section exhaust windage limit to approximate operational boundaries for each section of the turbomachine; determining an allowable range within ATOS of a physical parameter associated with at least one of the first section or the second section; generating a range of valve strokes for the first and second valves based on the allowable range of the physical parameter; modulating the first valve to allow steam flow into the HP section, wherein the modulation limits the range of valve strokes for the first valve; and modulating the second valve to allow steam flow into the IP section, wherein the modulation limits the range of valve strokes for the second valve; and wherein the physical parameter allows apportioning steam flow into the HP and the IP sections, independent of a received speed/load command, during the loading phase of the steam turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a powerplant site, of which an embodiment of the present invention may operate.

FIG. 2 is a chart illustrating IP section flow versus HP section flow for the steam turbine, in accordance with a known steam flow strategy.

FIG. 3 is a chart of IP section flow versus HP section flow and RH pressure versus HP section flow illustrating ATOS of the steam turbine, in accordance with an embodiment of the present invention.

FIG. 4 is a flowchart illustrating an example of a method for controlling steam flow within ATOS, in accordance with an embodiment of the present invention.

FIG. 5 is a chart of IP section flow versus HP section flow and RH pressure versus HP section flow illustrating a methodology for increasing the operability of a steam turbine within ATOS, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention has the technical effect of expanding the operational boundaries of each section of a steam turbine. As the steam turbine operates, the present invention determines the Allowable Turbine Operating Space (ATOS) of each turbine section. Next, the present invention may adjust the steam entering each turbine section during the loading phase based on ATOS. Here, the quantity of steam flow entering each turbine section is not dependent on the quantity of steam flow entering another turbine section.

The following detailed description of preferred embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention.

Certain terminology may be used herein for the convenience of the reader only and is not to be taken as a limitation on the scope of the invention. For example, words such as “upper”, “lower”, “left”, “right”, “front”, “rear”, “top”, “bottom”, “horizontal”, “vertical”, “upstream”, “downstream”, “fore”, “aft”, and the like; merely describe the configuration shown in the Figures. Indeed, the element or elements of an embodiment of the present invention may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise.

Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms, and should not be construed as limited to only the embodiments set forth herein.

Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are illustrated by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any, and all, combinations of one or more of the associated listed items.

The terminology used herein is for describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises”, “comprising”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The present invention may be applied to a variety of steam turbines, or the like. An embodiment of the present invention may be applied to either a single steam turbine or a plurality of steam turbines. Although the following discussion relates to a steam turbine having an opposed flow configuration and a cascade steam bypass system, embodiments of the present invention are not limited to that configuration. Embodiments of the present invention may other configurations that are not opposed flow.

Referring now to FIGURES, where the various numbers represent like elements through the several views, FIG. 1 is a schematic illustrating a steam turbine 102 deployed in a site 100, such as, but not limiting of: a power plant site 100. FIG. 1 illustrates the site 100 having the steam turbine 102, a reheater unit 104, a control system 106, and an electric generator 108.

As illustrated in FIG. 1, the steam turbine 102 may include a first section 110 and a second section 112. In various embodiments of the present invention, the first section 110, and the second section 112 of the steam turbine 102 may be a HP section 110, an IP section 112. In various other embodiments of the present invention, the HP section 110 may also be referred to as a housing 110 and the IP section 112 may also be referred to as an additional housing 112. Further, the steam turbine 102 may also include a third section 114. In an embodiment of the present invention, the third section 114 may be a low pressure (LP) section 114. The steam turbine 102 may also include a rotor 115, which may be disposed within the sections 110, 112 and 114 of the steam turbine 102. In an embodiment of the present invention, a flow path around the rotor 115 may allow the steam to fluidly communicate between sections 110, 112 and 114.

The steam turbine 102 may include a first valve 116 and a second valve 118 for controlling the steam flow entering the first section 110 and the second section 112, respectively. In various embodiments of the present invention, the first valve 116 and the second valve 118 may be a control valve 116 and an intercept valve 118 for controlling the steam flow entering the HP section 110 and the IP section 112, respectively.

During the operation of the steam turbine 102, steam exiting from the HP section 110 may flow through the reheater unit 104 where the temperature of the steam is raised before flowing into the IP section 112. Subsequently, the steam may exit from the reheater unit 104, via the intercept valve 118, and flow into the IP section 112 and the LP section 114, as illustrated in FIG. 1. Then, the steam may exit the IP section 112 and the LP section 114, and flow into a condenser (not illustrated in figures).

FIG. 2 is a chart 200 illustrating IP section flow versus HP section flow for the steam turbine 102, in accordance with a known steam flow strategy. The X-axis illustrates steam flow through the HP section 112 and the Y-axis illustrates steam flow through the IP section 114. As illustrated by a line 202, the known flow strategy seeks to balance the steam flow between the sections 110, 112. This typically involves maintaining equal steam flow through the HP section 112 and the IP section 114 during the loading process of the steam turbine 102. The line 202 connecting the points A, B, and C represent the variation of the steam flow through the HP section 112 with the steam flow through the IP section 114 during the loading process of the steam turbine 102. Line 202 may be considered the natural pressure line; which indicates equal or balanced flow through the HP and IP sections 110, 112.

A speed/load governor may generate a speed/load command, which may represent the desired steam flow through the sections of the steam turbine 102. The speed/load command may be subsequently provided to the control valve 116 and the intercept valve 118. This known balanced flow strategy may be maintained during the loading process of the steam turbine 102, as illustrated by the line 202. Here, the speed/load command may be provided to the control valve 116 and the intercept valve 118 during the entire loading process.

FIGS. 3 through 5 are schematics illustrating a method of using ATOS to expand the operability space of each section 110, 112, in accordance with an embodiment of the present invention. As discussed, balance flow may be considered a methodology and/or control philosophy that seeks to provide the same quantity of steam flow to each section 110, 112. Embodiments of the present invention seek to replace the balanced flow approach and expand the operating boundaries of the steam turbine 102. As the steam turbine 102 operates, the control system may determine ATOS. ATOS may be considered the current operational boundaries of the steam turbine 102. As ATOS changes, embodiments of the present invention may adjust the positions of valves 116, 118 to change the amount steam flow into the sections 110, 112.

The following should be considered when reviewing the FIGS and corresponding discussion on ATOS. All figures should be considered non-limiting examples that may be associated with certain steam turbine 102 configurations. Furthermore, the numerical ranges on each figure are for illustrative purposes only. The FIGS may not reflect the length of time the steam turbine 102 may operate or traverse each limiting boundary. ATOS should be considered a region within which a steam turbine 102 may operate. Each ATOS boundary, discussed and illustrated below, should not be considered a fixed or limiting boundary. ATOS, and its associated boundaries should be considered a changing and dynamic operating environment. This environment is determined, in part, by the configuration, operational phase, boundary conditions and mechanical components and design of the steam turbine 102. Other directions, shapes, sizes, magnitudes, and sizes of ATOS and its boundaries, not illustrated in the figures, do not fall outside of the nature and scope of embodiments of the present invention. Therefore, the direction, magnitude, shape, and size of ATOS and its boundaries, as illustrated in the figures, are merely illustrations of non-limiting examples.

FIG. 3 is a chart 300 of IP section flow versus HP section flow illustrating ATOS of the steam turbine 102, in accordance with an embodiment of the present invention. FIG. 3 illustrates a non-limiting example of ATOS 302 of the steam turbine 102, in accordance with an embodiment of the present invention. Here, the ATOS boundaries are lines 2-6 (which is a combination of the intersection of lines 1-2 and 5-6) and line 3-4. Line 1-2 may be considered an IP/LP Thrust Line and indicates the maximum allowable IP section flow as a function of the HP section flow to maintain axial thrust within limits. Line 3-4 may be considered an HP Thrust Line; and indicates the maximum allowable HP section flow as a function of the IP section flow to maintain axial thrust within limits. Line 5-6 may be considered an HP section Exhaust Windage Line and indicates the maximum allowable RH pressure to prevent undesirably high temperatures at the exhaust of the HP section.

The X-axis illustrates steam flow through the HP section 110. The left Y-axis illustrates steam flow through the IP section 112 and the right Y-axis illustrates a RH pressure. The natural pressure line 202, passing through the points A, B, and C illustrates the balanced flow strategy, as previously discussed.

The thrust lines 1-2 and 3-4 are a function of steam flow through the opposing HP and IP sections 110, 112. Lines 1-2 and 3-4 may represent the allowable flow imbalance that a specific steam turbine 102 may tolerate before experiencing an undesirably high axial thrust load. The actual shape and associated values of these lines depend, inter alia, on the thermodynamic design of each section 110, 112 and the size of the associated thrust bearing. Advanced steam turbine designs may increase the axial thrust force and limit the allowable flow imbalance, reducing ATOS 302. Similarly, increasing the thrust bearing size may allow greater flow imbalance and increase ATOS 302.

The HP section Exhaust Windage Line, line 5-6, may be a function of the minimum HP flow required to prevent undesirably high temperatures at the latter stages of the HP section 110; as a function of the RH pressure and HP inlet steam temperature. Higher RH pressure may drive higher pressure at the HP section exhaust. This may decrease the pressure ratio through the HP section 110, for a given flow and a given HP inlet steam temperature. This may also increase the HP section exhaust temperature. Similarly, higher HP inlet steam temperature may also increase the HP section exhaust steam temperature, for a given steam flow at a given RH pressure.

During the operation of some steam turbines 102, the HP section exhaust temperature may approach material-specific limiting values when the RH pressure reaches a higher than desired condition with high HP inlet steam temperature. However, as the steam turbine 102 operates at reduced inlet steam temperatures, the likelihood of high HP section exhaust temperature is lessened even with high RH pressure. Here, the enthalpy of HP inlet steam reduces significantly with reduced temperature. Therefore, the HP section windage considerations may be limiting in certain conditions, such as, but not limiting of, when the steam temperature is high.

As discussed, lines 1-2, 3-4, and 5-6 are boundaries that may define ATOS 302 at a given operational condition. These lines are dynamic in nature. Embodiments of the present invention may determine, in real time, ATOS 302; and allow greater operational flexibility. In practical terms, each ATOS boundary may be considered a physical parameter that defines ATOS 302 of a specific steam turbine 102. The physical parameter may include, but is not limiting to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, or the like.

As illustrated in FIG. 3, areas 304, 306, and 308 denote the regions where the operation of the steam turbine 102 may exceed the preferred limits of the HP section exhaust temperature and/or the axial thrust. In an embodiment of the present invention, the allowable steam flow through the IP section 112 may be determined as the minimum of the steam flow indicated by the line 1-2 or the line 5-6. Here, a range of valve strokes may be generated for the first valve 116 and the second valve 118 based on ATOS 302. In comparison with FIG. 2, embodiments of the present invention allow a greater utilization of ATOS 302 versus the balanced flow approach.

FIG. 4 is a flowchart illustrating an example of a method 400 for controlling steam flow within ATOS, in accordance with an embodiment of the present invention. As discussed, embodiments of the present invention incorporate an unbalanced flow method to increase the utilization of ATOS 302. Here, the steam flow entering each section 110, 112 is intentionally unbalanced to expand the operational boundaries and flexibility of the steam turbine 102. This may be accomplished by independently controlling the amount of steam entering each section 110, 112, in real-time. The method 400 may be integrated with a control system that operates the steam turbine 102.

The method 400 may control the first valve 116 and the second valve 118 for controlling steam flow through the first section 110 and the second section 112 respectively. In various embodiments of the present invention, the first valve 116 and the second valve 118 may be the control valve 116 and the intercept valve 118 that control steam flow through the HP section 110 and the IP section 112 respectively, as previously discussed.

In step 410, the method 400 may determine the operating phase of the steam turbine 102. As discussed, the steam turbine 102 normally operates in the three distinct, yet overlapping, phases; startup, loading, and shutdown. Embodiments of the present invention may function during the loading phase; in which the quantity of steam entering the sections 110, 112 is increased until the output of the steam turbine 102 is approximately a desired load; such as, but not limiting of, the rated load.

In step 420, the method 400 may determine whether the steam turbine 102 is operating in the loading phase. Here, the method 400 may receive operating data or operational data from a control system 106 that operates the steam turbine 102. This data may include, but is not limited to, output of the generator 108. If the steam turbine 102 is operating in the loading phase then the method 400 may proceed to step 430; otherwise, the method 400 may revert to step 410.

In step 430, the method 400 may determine the current ATOS 302. Here, the method 400 may receive current data related to the ATOS boundaries, as described. The method 400 may receive data on the physical parameter associated with the ATOS boundaries. This data may be compared to the allowable or the preferred limits and the boundaries. For example, but not limiting of, an ATOS boundary may include an axial thrust. Here, the method 400 may determine the current axial thrust and allowable axial thrust for the current operating conditions.

In an alternate embodiment of the present invention, the method 400 may incorporate a transfer function, algorithm, or the like to calculate, or otherwise determine ATOS 302.

In step 440, the method 400 may determine an allowable range of a physical parameter associated with at least one of the first section 110 of the steam turbine 102. The physical parameter may include, but is not limiting to, an operational and/or physical constraints. These constraints may include, but are not limited to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, or the like. The method 400 may then generate a range of valve strokes for the first valve 116 based on the allowable range of the physical parameter.

In step 450, the method 400 may modulate the first valve 116 to allow steam flow into the first section 110 of the steam turbine 102. The method 400 may modulate the first valve 116 based on the allowable range of the physical parameter.

In step 460, the method 400 may determine an allowable range of a physical parameter associated with at least one of the second section 112 of the steam turbine 102. The physical parameter may include, but is not limiting to, an operational and/or physical constraints. These constraints may include, but are not limited to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, or the like. The method 400 may then generate a range of valve strokes for the second valve 118 based on the allowable range of the physical parameter.

In step 470, the method 400 may modulate the second valve 118 to allow steam flow into the second section 112 of the steam turbine 102. The method 400 may modulate the second valve 118 based on the allowable range of the physical parameter.

Embodiments of the present invention allow for real time determination of a change in the physical parameters that bound ATOS 302. Therefore, after steps 450 and 470 are completed, the method 400 may revert to step 410.

FIG. 5 is a chart 500 of IP section flow versus HP section flow and RH pressure versus HP flow illustrating a methodology for increasing the operability of a steam turbine 102, within ATOS 302, in accordance with an embodiment of the present invention. Essentially, FIG. 5 illustrates the potential results of an application of the method 400 of FIG. 4. As discussed, embodiments of the present invention provide an unbalanced flow methodology, which determines the allowable steam flow for each section 110, 112, based on the current ATOS 302.

Similar to FIG. 3, the X-axis illustrates steam flow through the HP section 112. The left Y-axis illustrates steam flow through the IP section 114 and the right Y-axis illustrates the RH pressure. The line 202 illustrates the natural pressure line, as discussed in FIG. 2. In an embodiment of the present invention, a transfer function, algorithm, or the like may determine the current operational ranges of a physical parameter associated with the HP section 112 and/or the IP section 114 based on the determined ATOS 302. As discussed, lines 1-2, 3-4, and 5-6 are boundaries that may define ATOS 302 at a given operational condition. These lines are dynamic in nature. Embodiments of the present invention may determine, in real time, ATOS 302; and allow greater operational flexibility. Practically, each ATOS boundary may be considered a physical parameter that defines ATOS 302 of a specific steam turbine 102.

In use, an embodiment of the present invention provides a new loading phase methodology for the steam turbine 102; which may include multiple stages. In an embodiment of the present invention, each stage may be based, at least in part, on a current ATOS boundary.

As discussed, the numerical ranges discussed and illustrated on FIG. 5 are for illustrative purposes of a non-limiting example. Each ATOS boundary should not be considered a fixed or limiting boundary. ATOS 302, and its associated boundaries should be considered a changing and dynamic operating environment; which are determined, in part, by the configuration, operational phase, boundary conditions and mechanical components and design of each steam turbine 102. Therefore, the direction, magnitude, shape, and size of ATOS 302 and its boundaries, as illustrated in FIG. 5, is merely an illustration of a non-limiting example, discussed below. Other directions, shapes, sizes, magnitudes, and sizes of ATOS 302 and its boundaries, not illustrated in the FIG. 5, do not fall outside of the nature and scope of embodiments of the present invention.

The following provides a non-limiting example of an embodiment of the present invention, in use during a loading phase. In an embodiment of the present invention, the loading process of the steam turbine 102 may include five stages as illustrated by region 502. A first part of the path, from startup to stage A′, may include the initial loading of the steam turbine 102. In an embodiment of the present invention, after initial loading, steam flow through the HP section 112 and the IP section 114 may be about 25%.

Next, from stage A to stage B, steam flow through the IP section 114 may be increased to the current operational range of the IP section 114 while the steam flow through the HP section 112 is maintained at a nearly constant rate. In an embodiment of the present invention, steam flow through the IP section 114 may be increased to approximately 37%, the boundary defined by ATOS 302. As illustrated by comparing FIGS. 3 and 5, an embodiment of the present invention, may result in increased output. The loading path of FIG. 3 incorporates equal steam flow through the HP and IP sections 112 and 114. However, an embodiment of the present invention, illustrated in FIG. 5, may result in increased output from the steam turbine 102, resulting from the additional steam flow through the IP section 114.

Next, from stage B to stage C, steam flow through the HP section 112 and the IP section 114 may be increased to respective operational ranges, based on the current ATOS 302. In an embodiment of the present invention, steam flow through the HP section 112 may be increased to approximately 52% and steam flow through the IP section 114 may be increased to approximately 68%, which may be the boundary defined by ATOS 302. As illustrated by comparing FIGS. 3 and 5, an embodiment of the present invention, may result in increased output. The loading path of FIG. 3 incorporates equal steam flow through the HP and IP sections 112 and 114. However, an embodiment of the present invention, illustrated in FIG. 5, may result in increased output from the steam turbine 102, resulting from the additional steam flow through the IP section 114.

Next, from stage C to stage D, steam flow through the HP section 112 may be increased to the current operational range of the HP section 112 and steam flow through the IP section 114 may be increased to approximately 100%, which may be the boundary defined by ATOS 302. In an embodiment of the present invention, steam flow through the HP section 112 may be increased to approximately 82%. As illustrated by comparing FIGS. 3 and 5, an embodiment of the present invention, may result in increased output. The loading path of FIG. 3 incorporates equal steam flow through the HP and IP sections 112 and 114. However, an embodiment of the present invention, illustrated in FIG. 5, may result in increased output from the steam turbine 102, resulting from the additional steam flow through the IP section 114.

Next, from stage D to baseload, steam flow through the HP section 112 may be increased to approximately 100% while maintaining steam flow through the IP section 114 nearly constant at 100%.

The IP and LP sections 112, 114 may generate the majority of the total output on some steam turbines 102. Therefore, the load path A→B→C→, taught by embodiments of the present invention, may increase the output of the steam turbine 102 by 5% to 15%. In addition, embodiments of the present invention may utilize the steam produced by the steam turbine 102, reducing waste, and improving the transient efficiency.

Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.