ELASTOMERIC RECIPROCATING COMPRESSOR VALVE SPRING
An elastomeric storage device for use in a gas compressor valve is provided. Reciprocating gas compressor valves use these storage devices to control the closing of a valve sealing element.
The present application claims priority under Title 35, United States Code, § 119 to provisional application U.S. Pat. App. Ser. No. 60/718,994 filed Sep. 21, 2005
FIELD OF THE INVENTIONThis invention is directed to dramatically improving the reliability, durability and longevity of energy storing devices such as springs in reciprocating gas compressor valves.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTNone.
THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENTNone.
REFERENCE TO SEQUENCE LISTINGNone.
BACKGROUND OF THE INVENTIONCurrent state of the art metal spring designs can withstand a tremendous number of cycles. But even the most advanced metal spring designs with the most advanced metal treatments are not able to keep up with advanced technologies in other areas of the reciprocating gas compressor valve. The spring is a weak link in the valve system.
Reciprocating gas compressor valves are currently designed with metal springs in some form or combination of forms to control the timing of the valve element closure. Reciprocating compressor valves open and close as a result of the differential pressures that are created as the compressor piston compresses the trapped gas and raises the pressure of the gas from a lower pressure to a higher pressure as it moves through its stroke. The sealing elements in a reciprocating compressor valve open and close with each stroke of the compressor for the purpose of expelling gas from the cylinder or allowing new low pressure gas to enter the cylinder. Reciprocating gas compressors typically operate at speeds between 200 rpm and 1800 rpm, gas temperatures can range from the cryogenic region to 400 F. and pressures can vary from vacuum to 10,000 psi or higher and the advancement of material technologies is increasing the size of the operating envelope.
Operators of reciprocating gas compressors demand more operating hours between overhauls. Unscheduled compressor shutdowns to replace failed parts is expensive and lowers overall plant efficiencies. A need exists therefore to replace the current state of the metal spring materials and designs with new materials and designs capable of extending the mean time between failures. A need also exists for an improved valve spring that can be used in connection with a reciprocating gas compressor valve to provide improved reliability and durability of the valve assembly.
BRIEF SUMMARY OF THE INVENTIONThe present invention is an energy storage device such as a valve spring comprising elastomeric material. Elastomeric material is a material or substance having one or more elastomers, an elastomeric compound or compounds used together, or a combination of elastomer or elastomeric compounds with other substances. The elastomeric material used in connection with the valve spring may be a single type of elastomer or may be a compound or combination of substances as described below. Hence, the valve spring may be made entirely of elastomer or as a composite where the elastomer may be bonded to or combined with other materials for improved mechanical properties. Elastomers have the inherent ability to dissipate energy from shocks and collisions, superior fatigue resistance and the ability to recover from elastic strains.
The subject invention also provides a reciprocating gas compressor valve containing at least one elastomeric valve spring. The elastomeric valve spring of the present invention can be placed between a guard and surface of a sealing element of the compressor valve to absorb and exert energy necessary to move the sealing element toward an opposing seating surface for many compression cycles without spring failure. Furthermore, the present invention provides a method of making a reciprocating gas compressor valve by assembling at least one elastomeric spring, guard, sealing element and seating surface into a valve and using the assembled compressor valve in a reciprocating gas compressor.
BRIEF DESCRIPTION OF THE DRAWINGS
It should be understood that the term “energy storing device” is the device used in a reciprocating compressor valve that controls the motion of the valve sealing element during the opening or the closing event. A “spring” is an energy storing device.
The subject invention is an elastomeric energy storage device or spring for use in a gas compressor valve. As shown in
In the gas compressor valve, a sealing element 10 will compress the stem 54 causing the web frame 56 to be displaced. Like any energy storing device, the magnitude of the resistance placed on the spring 14 is a function of the displacement. When forces are applied to the stem 54 and removed, the energy stored in the web 56 acts to close the sealing element 10 and the stem 54 returns to a neutral or biased position.
In this embodiment, the number of support members 58 depends on the performance characteristics required. For example, if a stronger spring is required, additional members may be used, the thickness of the member may be increased or the elastomer compound may be changed or any combination thereof. Alternatively, if a weaker spring is required, fewer members may be used, the thickness of the member may be decreased or the elastomer compound may be changed or any combination thereof. Also, as shown in
During valve opening, the sealing element 10 moves toward the guard 15 and deformation of the spring 14 occurs. Forces develop in the elastomer to resist deformation and energy is stored. The energy stored in the elastomer spring 14 is proportional to the magnitude of the deflection allowed. The characteristics of the spring 14 may be manipulated by varying the elastomer material or by adjusting the shape and geometry of the spring 14. When the sealing element 10 is stopped by the guard 15, the spring 14 is in the compressed state. When the valve closes and the forces holding the sealing element 10 are released, the spring will exert a force against the sealing element 10 thereby pushing it back against the seat 13 (see
In
In
As further discussed below, the springs of the subject invention are designed to fit in the space provided in a variety of compressor valves. For example, the spring of
A vast number of compressor valves have been designed and continue to operate with the prior art spring, item 100. Therefore, there is a need for the elastomeric spring to fit within the valve in order to avoid the high cost and inconvenience of replacing the guard 15. As previously described, adjusting variables such as crosslink density and geometry permit the new spring to be designed that can directly replace the prior art spring 100 without any further modifications to the existing guard 15 geometry. When new designs are created, the space restrictions of existing structures does not exist and one skilled in the art can create an elastomeric spring optimized for the service conditions and space available.
Reciprocating gas compressor valves make use of energy storing devices to control the closing of the valve sealing elements. If the valve sealing elements are in an open position (i.e., the valve sealing elements are not in contact with the opposing sealing surface) when the compressor piston changes direction, the valve sealing elements will be sucked closed leading to high energy impacts between the valve sealing element and the opposing sealing surface. The velocity at which the valve sealing element closes is directly proportional the distance the valve sealing element must travel before is makes contact with the opposing sealing surface. High impact velocities are destructive and serve to shorten compressor valve life. This condition is sometimes referred to as “late closing”. When this condition occurs, the collisions between the sealing components can become severe enough to damage the sealing components/surfaces and result in valve leaks that require the compressor be shut down for repair or replacement of the damaged compressor valves.
Even if the sealing components in a reciprocating compressor valve could tolerate the severe forces of late closing, a certain amount of gas leaks back into the compressor cylinder while the compressor valve sealing elements move toward their opposing sealing surfaces. In other words, gas that has just been compressed can leak back into the compressor cylinder only to be compressed again. It is inefficient to compress the same gas twice and therefore “leak back” is to be avoided. In another scenario, if during the intake stroke of the compressor the inlet valves stay open to long, some of the gas just introduced to the compressor cylinder can leak back into the supply pipe. The compressor cylinder will have less than a full charge of new gas and thus, for the exact same operating conditions, the compressor suffering from “leak back” will be required to have a greater swept volume in order to move the same amount of gas moved by a compressor without a “leak back” problem. Larger equipment is typically more expensive and “leak back” drags down the overall operating efficiency of the compressor and it wastes energy.
In a reciprocating compressor valve, the spring moves the valve sealing elements toward an opposing sealing surface at some point near the end of the compressor stroke. If the spring system of a reciprocating compressor valve is set up and designed correctly, the valve sealing elements will be closed or very near the closed position when the compressor piston changes directions at the end of the stroke. A spring system that is too stiff can cause the valve sealing elements to close too early and contribute to additional hydraulic losses as the compressed gas passes through the compressor valve. These additional restrictions to gas flow cause more power to be consumed from the driver of the compressor thereby unnecessarily raising the operating costs of the compressor.
A spring system that is designed too “soft” or weak or one that becomes soft or weak after a period of time results in late closure, high valve sealing element impact energies and the potential for “leak back” to occur. Therefore it is clear that reliable energy storing devices in a reciprocating gas compressor valve not only contribute to the longevity of other components in the valve by helping to control the closing velocities of the valve sealing elements but they influence operating efficiency and operating costs as well. Springs are and have always been critical components in the reciprocating gas compressor valve. Spring selection depends on a number of variables including, but are not limited to, the gas compression ratio, compressor speed, available valve flow area, gas molecular weight, suction and discharge pressure, available compressor cylinder flow area, physical space available to place a spring, thermomechanical limitations of metals suitable for making springs and the service environment among others.
Elastomers have typically not been considered because polymer science has only recently created polymers with sufficient chemical and temperature resistance to withstand the operating environment of reciprocating gas compressors. High temperatures tend to provide the necessary energy to break the chemical bonds that make up the structure of the polymer. In service, the breaking of bonds results in a loss of physical properties and the elastomer fails in short order from fracture, tearing or the inability to retain the shape necessary to create an effective seal.
For example, chemical resistance is a critical component to the longevity of elastomer and polymers in general. Hydrogen sulfide (H2S) gas is a compound that is frequently found in oil refining operations because it is produced in subterranean the oil reservoirs. Naturally the compound can be produced by other means. However, when H2S is the presence of water, sulfuric acid is produced. Another example of an acid that forms in the presence of water is carbon dioxide. In chemical plants, aggressive compounds such as tetraethylaluminum (TEAL), strong bases like sodium hydroxide (NaOH) and other aggressive compounds like salts and oxidizers may be present. Until recently the most advanced polymers were unable to tolerate such harsh environments for any useful amount of time.
In general polymers with single, covalent bonds between the mers on the polymer chain are more chemically resistant than mers connected via double or triple bonds which are less stable and more likely to react with foreign chemical agents. For example natural rubber reacts with and dissolves in some lube oils and gasoline. Natural rubber is not a good candidate for use in compressor valves because lube oil is injected into the compressor cylinders to prolong the life of piston rings and other sliding components of a compressor cylinder.
In addition, the advancement of desk top computing has made possible the use of sophisticated analytical software to solve the partial differential equations that describe in mathematical terms the behavior of these elastomers under loads. Non-linear finite element analysis (FEA) allows those skilled in the art to apply loads, visualize the deformation and determine the stresses in the deformed elastomers. Prior to the widespread availability of these tools, trial and error techniques were these only options and this was not a practical approach due to the time and expense involved in testing a wide variety of materials.
The present invention overcomes many of the limitations of metallic energy storing devices. Elastomers are natural or synthetic polymers, polymer blends, polymer alloys, with the ability to deform elastically in response to an applied force and then return to their original shape when applied force is removed. Elastomers offer a number of advantages over metal spring designs.
First, elastomers can be molded to any shape to fit any available space.
Second, altering the chemical composition, molecular weight, tacticity, cross link density, and crystallinity of the polymer means that spring properties can be altered and varied in the laboratory. Elastomeric springs can therefore be designed for any foreseeable gas compressor valve application. Where there used to be limits with metallic designs, now there are none.
Third, unlike metals, elastomers do not lose an appreciable amount of their stiffness over time even as the number of compression, tensile, torsional or shear cycles increases. The application of elastomers to reciprocating gas compressor valves takes advantage of these inherent and valuable characteristics of elastomers.
Fourth, elastomers can be formulated to serve almost any environment. The ability to control the polymer design variables means that springs can be custom designed and optimized for a particular service environment. Metals can be manipulated to a certain degree but specialty metals are typically prohibitively expensive unless produced in commercial quantities and it is impossible to produce the “perfect” spring wire devoid of surface defects or defects at the crystalline level that lead to stress raisers and crack nucleation sites.
Fifth, metal springs store energy by applying torsional, tensile or compressive stresses to the spring geometry. For example, metal helical springs store energy by applying torsional stresses to the spring wire as the spring is compressed or extended. If the spring is compressed, the coil to coil distance diminishes. Ultimately, all the coils will touch and further compression is no longer possible. The gases compressed in a reciprocating gas compressor are rarely clean and solid debris is often carried along in the gas stream. One failure mode of metal helical springs occurs when a solid particle(s) gets trapped between adjacent spring coils and either creates a stress raiser by damaging the surface of the spring wire or the wire breaks in a mechanical fashion as the wire gets bent around the trapped particle. The novel application of elastomers to reciprocating gas compressors completely eliminates this failure mode because the elasticity is an inherent property of the polymer at the atomic and molecular level. Interatomic or polymer interchain distances are not large enough to allow for solids in the gas stream to interact with the movement of the polymer chains.
Sixth, entrained solids in the gas stream can also abrade the physical structure of metal spring resulting in a loss of volume from the metal spring causing a reduction in the load bearing area over time and a subsequent increase in stresses in the spring material until failure occurs from mechanical overload of the material. In addition, the impact damage on the surface of the spring wire represents a stress raiser and a potential site for the nucleation of cracks. Elastomeric materials tend to absorb impacts from entrained solids without a suffering surface damage or loss of volume.
Seventh, metal springs often operate in corrosive environments thereby requiring special and costly materials to be used in most applications to prevent pitting, fracturing and subsequent failure of the metal spring. The polymers in elastomeric materials can be designed in the laboratory to combat attacks from aggressive applications.
While the valves in a reciprocating gas compressor are of various types and forms, generally each valve has a seating surface, a sealing element, a guard or stop plate and one or more valve springs or energy-storing mechanisms which move the sealing element toward the opposing seating surface to form a seal before the compressor piston reaches the top or bottom dead center position. Over time, metal valve springs become fatigued from the periodic motion of the sealing element resulting in a loss of spring force. As the metal springs fail or become weaker over time, the valve sealing elements close later in the stroke and with more impact energy. Eventually the parts will fail to the point where the compressor must be shutdown for servicing.
Currently, valve springs used in reciprocating gas compressor valves include metal and metal alloy helical springs, leaf springs, nested or variable rate springs. Helical springs are generally cylindrically-shaped but conical versions are also possible and contain a number of separated coils.
In comparison, leaf springs generally include one or more strips of metal or metal alloy bent into a predetermined shape for the purpose of creating a resisting force when pressed flat. Unlike helical springs, adjacent metal strips of the leaf spring slide against one another when pressed flat by an applied force. This sliding motion creates a mechanism for wear during normal process operation. Additionally, any dirt or debris contained in the gas stream can become lodged between adjacent strips and exacerbate the problem of wear.
The shape of a spring is often dictated by the physical space limitations of the environment and the job required of the spring or springs. Elastomeric materials have a distinct advantage in they can be molded into shapes that would be difficult or impossible to duplicate with metallic materials. Elastomers permit compressor valve designs to overcome obstacles of the past. This is particularly true in the area of high speed reciprocating compressors. As machine rotative speeds increase, the compressor valve elements have less and less time afforded to them to open, dwell at full open and then close on time. Due to space restrictions, operating pressures, and operating speeds, more powerful springs are needed to fit in ever smaller valves.
The spring rate (spring constant) of metal springs and the resistive forces created by metal springs are adjusted by changing the physical parameters of the spring. For example, wire diameter, number of coils, wire material, free length, and diameter may be changed singly or in combination in order to make the spring behave as desired. It is clear that many of the parameters involve size changes so in cases where space is at a premium, spring adjustments can be anything but trivial. With limited options, there are limited solutions.
On the other hand, in the subject invention, the chemical and thermomechanical properties of the elastomeric spring are changed by adjusting the chemistry of the polymer. The molecular formula of the repeating element can be changed by formulating new repeating elements with different atoms. Different atoms result in different bond angles, differences in the magnitudes of the interatomic attractions, and different ranges of allowed motion in the polymer backbone. In addition the length of the polymer chain (molecular weight or number average molecular weight), and the magnitude of the interchain attractive forces participate in the dictating the ease at which a polymer chain can be stretched or compressed. Thus the mechanical response of an elastomeric spring is predictable by adjusting parameters that do not change the physical dimensions of the spring.
The elastic characteristics of elastomers are generated at the molecular and atomic level as the polymer chains are straightened, pulled or compressed through the application of tensile, compressive, torsional or shear forces. By definition, elastomers can undergo large, reversible deformations (6%-700% or more) at relatively low stresses. Typically elastic behavior requires that the polymer be nearly or completely amorphous with a low glass transition temperature. The glass transition of a polymer is that temperature at which a part of a polymer chain becomes mobile. Another characteristic of an elastic polymer is that the modulus of the elastic material increases with increasing strain (elongation). Furthermore the melt temperature of the crystalline regions of the elastic material must be below or not substantially above the service/use temperature of the material. The low crystalline melt temperature is a requirement and substantially influences the ability of the elastomer to reverse the deformation brought about by the applied stress. The strength of an elastomeric material is typically controlled by the degree of crosslinking, the incorporation of reinforcing inorganic fillers such as carbon black and silica among others or polymer chain geometry. Crosslinking between polymer chains inhibits the bulk flow of the polymer chains past one another thereby making it possible for the material to reverse the deformation caused by applied forces. A considerable range of elastic properties can be designed from a single polymer by simply varying the crosslink density and the percent composition of the inorganic fillers if any polymer chain length and/or the number and size of chain sidegroups.
The intermolecular and interatomic forces acting in the polymer chains and the interchain forces are repeatable and never vary unless the atomic bonds are broken chemically or physically. As long as the chemical structure of the polymer remains intact, the elastomer will continue to respond to applied forces in a consistent and repeatable manner. Unlike metals, elastomers do not suffer a continuous degradation in performance due to fatigue.
Once the polymer chain is designed for the spring, further manipulation of the mechanical response can be achieved by controlling the crosslink density between chains, the number and size of branched structures emanating from the polymer backbone, the types of fillers or reinforcements used (if any) to occupy the free volume of the bulk compound and other ingredients/compounds (i.e., plasticizers) that might be added to elicit particular behavior. In short, operating at the atomic and molecular level results in spring solutions without the constraints brought about by the physical space limitations of the service environment. If a spring with a different mechanical response in needed simply, replace it with a spring made from and elastomer with a different chemical structure.
It is acknowledged that polymer science continues to advance and desirable and effective materials may still be designed. In spite of advancing technology, the application of these materials with regards to reciprocating compressor valves falls within the scope of this invention. Commercially available and proprietary compounds that may serve as an elastomeric spring or other energy storing device include, but are not limited to, synthetic rubbers, fluoroelastomers, thermoset elastomers and thermoplastic elastomers and blends and alloys thereof.
The elastomeric material used in the present invention may be varied as necessary to satisfy the operating conditions of a particular application. Softer or harder compounds may be required or different mechanical properties may be required to meet the various service needs experienced by the reciprocating gas compressor valve. In addition, corrosion resistance and chemical attack may mandate different material blends. One skilled in the art will rely on experience and published data to make a proper material selection.
The hardness of elastomeric material is typically measured using the “Shore” scale. The Shore scale was developed for comparing the relative hardness of flexible elastomeric materials. The unit of measure is the “durometer”. An analogous scale would be the “Rockwell” or “Brinell” scales used in measuring the hardness of metals.
The elastomeric materials useful in connection with the invention include, but are not limited to, natural rubber, styrene butadiene, synthetic rubber, and polymers such as thermoplastic elastomers (TPE), thermoset elastomers, and fluoro-elastomers, elastomeric copolymers, elastomeric terpolymers, elastomeric polymer blends and a variety of elastomeric alloys. The particular type of elastomeric material utilized depends in part on the physical space available (geometry) and the chemical nature of the compressor valve service conditions (chemical or thermal attack).
A variety of commercially available elastomeric materials are useful with the subject invention. For example, butyl elastomer sold under the trade names of EXXON Butyl (Exxon Chemicals) or POLYSAR (Bayer Corp) performs well for MEK, silicone fluids and greases, hydraulic fluids, strong acids, salt, alkali and chlorine solutions. Ethylene and propylene are often substituted for butyl. Chloroprene sold under the trade names of BAYPREN (Bayer Corp) and NEOPRENE (DuPont Dow) performs well in petroleum oils with a high aniline point, mild acids, refrigeration seals (having resistance to ammonia and Freon), silicate ester lubricants and water. Chloroprene is also known as polychloroprene having a molecular structure similar to natural rubber. Similarly, chlorosulfonated polyethylene sold as HYPALON (DuPont Dow) performs well with acids, alkalis, refrigeration seals (resistant to Freon), diesel and kerosene. Chlorosulfonated polyethylene has good resilience and is resistant to heat, oil, oxygen and ozone. Epichlorihydrin sold under the trade name of HYDRIN (Zeon Chemicals) performs well in air conditioners and fuel systems. Epichlorihydrin is oil resistant and often used in place of chloroprene where low temperatures are a factor, having better low temperature stiffness. Ethylene Acrylic sold under the trade name of VAMAC (DuPont Dow) performs well in alkalis, dilute acids, glycols and water. This rubber is a copolymer of ethylene and methyl acrylate and has a low gas permeability and moderate oil swell resistance. Also, ethylene acrylic has good tear, abrasion and compression set properties. Ethylene propylene sold under the trade names of BUNA EP (Bayer Corp), KELTAN (DSM Copolymer), NORDEL (DuPont Dow), ROYALENE (Uniroyal) and VISTALON (Exxon Chemical) resists phosphate ester oils (Pydraul and Fyrquel), alcohols, automotive brake fluids, strong acids, strong alkalis, ketones (MEK, acetone), silicone oils and greases, steam, water and chlorine solutions. EPDM is, for example, a terpolymer made with ethylene, propylene, and diene monomer. Fluoro-elastomers sold under the names of DAIEL (Daiken Ind.), Dyneon (Dyneon), Tecnoflon (Ausimont) and VITON (DuPont Dow) perform well in acids, gasoline, hard vacuum service, petroleum products, silicone fluids, greases and solvents. Fluoro-elastomers have a good compression set, low gas permeability, excellent resistant to chemical and oils. Having high fluorine to hydrogen ratio, these types of compounds have extreme stability and are less likely to be broken down by chemical attack. Fluorosilicone sold under the trade names of FE (Shinco Silicones), FSE (General Electric) and Silastic LS (Dow Corning) performs well as static seals due to high friction, limited strength and poor abrasion resistance and particularly with brake fluids, hydrazine and ketones. Hydrogenated Nitrile sold under the trade names of THERBAN (Bayer Corp.) and ZETPOL (Zeon Chemicals) performs well in hydrogen sulfide, amines (ammonia derivatives), and alkalis, and under high pressure. Hydrogenated Nitrile is often used as a substitute for FKM materials and has high tensile properties, low compression set, good low temperature properties and is heat resistant. Natural rubber performs well in alcohols and organic acids and has high tensile strength, resilience, abrasion resistance and low temperature flexibility in addition to having a low compression set. Nitrile sold under the trade names of KRYNAC (Polysar Intl), NIPOLE (Zeon Chemicals), NYSYN (Copolymer Rubber and Chemicals) and PARACRIL (Uniroyal) performs well in dilute acids, ethylene glycol, amines petroleum oils and fuels, silicone oils, greases and water below 212° F. Also known as Buna-N, nitrile is a copolymer of butadiene and acrylonitrile. Perfluoroelastomer sold under the trade name AEGIS (International Seal Co.), CHEMRAZ (Greene Tweed), KALREZ (DuPont Dow) has low gas permeability and is resistant to a large number of chemicals including fuels, ketones, esters, alkalines, alcohols, aldehydes and organic and inorganic acids and exhibits outstanding steam resistance. Polyurethane sold under the trade names of ADIPRENE (Uniroyal), ESTAE (B.F. Goodrich), MILLITHANE (TSE Ind.), MORTHANE (Morton International), PELLETHANE (Dow Chemical), TEXIN (Bayer Corp.) and VIBRATHANE (Uniroyal) performs well under pressure, is very tough and has excellent extrusion and abrasion resistance. Silicone sold under the trade names of BAYSILONE (Bayer Corp.), KE (Shinco Silicones), SILASTIC (Dow Corning), SILPLUS (General Electric) and TUFEL (General Electric) performs well in oxygen, ozone, chlorinated biphenyls and under UV light. Silicones have great flexibility and low compression set. Tetrafluoroethylene (“TFE”) sold as ALGOFLON (Ausimont) and TEFLON (DuPont Dow) performs well in ozone and solvents including MEK, acetone and xylene. Tetrafluroethylene/propylene is a copolymer of TFE and propylene sold under the trade names of AFLAS (Asahi Glass), and DYNEON BRF (Dyneon). Tetrafluroethylene/propylene performs well in most acids and alkalis, amines, brake fluids, petroleum fluids, phosphate esters and steam.
Generally, thermoplastic elastomers (TPE) as defined in the Modern Plastics Encyclopedia (1997, 1998) are “soft flexible materials that provide the performance characteristics of thermoset rubber, while offering the processing benefits of traditional thermoplastic materials”. Hence, the thermoplastic material, a typically rigid material, is modified at the molecular level to become flexible after molding. TPE materials are popular because they are easy to make and mold.
The mechanical and physical properties of TPE's are directly related to the bond strength between molecular chains as well as to the length of the chain itself. Plastic properties can be modified by alloying and blending in various substances and reinforcements. The ease at which TPE's can be modified is a distinct advantage of these materials. The mechanical properties of these materials can be customized to suit a particular application or service.
Thermoset elastomers are plastic substances that undergo a chemical change during manufacture to become permanently insoluble and infusible. Thermoset polymers are a subset of thermoset elastomer materials as these materials undergo vulcanization enabling them to attain their properties. The key difference between a thermoset elastomer and a thermoplastic elastomer is the cross-linking of the molecular chains of molecules that make up the material. Thermoset materials are cross-linked and TPE materials are not.
Fluoro-elastomers polymers may be subdivided into seven categories:
-
- 1) copolymers meaning combinations or blends of two polymers;
- 2) terpolymers meaning combinations or blends of three polymers. These typically have good heat resistance, excellent sealing and good chemical resistance;
- 3) low temperature polymers, which have good chemical resistance and excellent low temperature properties;
- 4) base resistant polymers, which have superior chemical resistance to bases, aggressive oils and amines;
- 5) peroxide cure polymers, which have superior chemical resistance and excellent sealing properties;
- 6) specialty polymers; and
- 7) perfluorinated polymers, which have superior chemical resistance and excellent sealing properties.
Copolymers are materials made up of two or more different kinds of molecule chains. They are basically a combination of different materials fused into one. The individual compounds that make up the molecular chain are distinct and repeating over the length of the molecular chain. A terpolymer is a copolymer with three different kinds of repeating units. A homopolymer identifies a polymer with a single type of repeating unit. Other repeating units are possible as well. Alloys are elastomers with additives that improve the properties of the material, much like metal alloys.
The utility of rubber and synthetic elastomers is increased by compounding the raw material with other ingredients in order to realize the desired properties in the finished product. For example vulcanization increases the temperature range within which elastomers are elastic. In this process, the elastomer is made to combine with sulphur, sulphur bearing organic compounds or with other chemical crosslinking agents. Any number of ingredients can be combined in any number of ways to generate any number of mechanical or chemical properties in the finished elastomeric material.
In general, the elastomeric materials useful in the subject invention operate within the following ranges:
TEMPERATURE=−170° F. to 450° F.
PRESSURE=vacuum to 12,000 psi
DIFFERENTIAL PRESSURE=0 to 10,000 psi
SERVICE TYPE=Continuous or intermittent duty in any type of compressible gas or gas mixture.
OPERATING EQUIPMENT=Reciprocating gas compressors in any industry from any manufacturer of reciprocating gas compressors.
These ranges are typical for reciprocating gas compressors. Other elastomers can operate in more extreme temperatures and pressures depending on the characteristics of the elastomeric material used.
Other important characteristics of the elastomers are:
Durometer—range on the Shore scale or analogous scale, which is a measure of the hardness of the elastic material.
Tensile strength—which is the approximate force required to make a standard material sample fail under a tensile load.
Elongation—which is the amount of deformation that a sample will exhibit before failure. An elongation of 200% indicates that the sample will stretch 2 times its original length before failure.
Compression set—which is a measure of the elastic materials ability to withstand deformation under constant compression.
Solvent resistance—which indicates a compound's resistance to solvents that normally dissolve or degrade elastomers in general.
Tear resistance—which is the ability of the elastic material to withstand tearing and shear forces.
Abrasion resistance—which is the ability of the elastic material to withstand abrasion and rubbing against another material or itself.
Rebound resilience—which is the measure of the ability of an elastic material to return to its original size and shape after compression.
Oil-resistance—which is the relative ability of an elastic material's resistance to penetration or degradation by various hydraulic or lubrication oils commonly used in industrial services. Many reciprocating gas compressors have lubricated compressor cylinders.
Aging, weather, and sunlight resistance—which is the ability of the elastic material to withstand the elements. This is not a factor in this particular use because the elastic materials will be inside of machine components.
Hence, the specific elastomeric material used will be dictated by requirements of the reciprocating gas compressor and the valve spring. In a chemically aggressive environment, an elastomer, such as a peroxide-cured polymer, having superior chemical resistance properties is required. Similarly, unusual temperature environments mandate certain appropriate properties. Engineers and individuals experienced with gas compression may analyze a particular set of operating parameters and select a material with the appropriate properties. For this reason, there will necessarily be a large number of potential elastomer compounds that may be selected or custom designed to perform in a particular set of operating conditions. The blending and the ability to modify the mechanical and chemical properties of elastomers and/or thermoplastics offer an extensive array of possible solutions to any gas compression application. This key advantage of elastomers will yield high performance solutions to common or difficult applications where none existed prior to this invention.
The present invention provides a valve spring prepared from elastomeric material for use in connection with a reciprocating gas compressor valve. The elastomeric valve spring of the present invention includes any type of elastomeric material used to store energy when compressed, stretched, torqued or placed in shear and at the same time, exerts a force, torque when the applied force is lessened or removed. Furthermore, the elastomeric valve spring of the present invention is elastic so that it returns to its original shape or position after undergoing repeated compression or stretching.
Therefore, the elastomeric valve spring of the present invention may be a spring that, when compressed, stretched, torqued or placed in shear exerts a push, pull or torque force in response. Examples include a compression spring, a leaf spring, a helical extension spring, or a spring washer.
The elastomeric valve spring of the subject invention is used to store energy and provide force or torque necessary to move or support a sealing element of a reciprocating gas compressor. Currently, reciprocating gas compressor valves utilize several types of sealing elements.
As shown in the figures, four common forms of valves used in reciprocating gas compressors are: a concentric ring valve 60, a single element non-concentric valve 62, a channel valve 64 and ported plate valve 66.
U.S. Pat. No. 3,536,094 to Manley discloses a particular concentric ring type of valve, the MANLEY® valve. In this type of valve, the thickness of the sealing element 10 may vary by design with rounded or straight vertical edges. The MANLEY® valve has a downwardly convex protruding sealing element 10 to engage a recessed seating surface 12 in the valve seat.
As shown in
In
The MOPPET® valve 69 shown in
The sealing element 10 in the MOPPET® valve 69 (
Because the poppet valve 62 does not have a central hole in the sealing element 10, the gas can only flow over the outer circumferential surface of the sealing element 10. During the opening event, the valve sealing element 10 presses directly on the spring 100 compressing it to create a resisting force to the motion of the valve sealing element 10. The resisting force in the helical spring occurs as a result of torsional forces in the spring wire that grow as the spring 100 is compressed. As the differential pressure in the compressor decreases, there is a point in the compressor stroke where the spring forces can influence the motion of the sealing element 10 and begin to move it toward the sealing element 13. This opening and closing action occurs with each stroke of the compressor and in each valve suction 28 or discharge 30.
As shown in
Some of the advantages of the ported plate valve 66 include the valve spring 14 that supports the sealing element 10 and acts on the entire sealing element 10 rather than just the ring under which they are placed. Since the rings are all connected, the design permits the use of larger and possibly fewer valve springs than a valve with concentric rings that are not all connected. In non-connected concentric ring valves, the individual rings are supported by their own valve springs and generally the diameter of the valve spring 14 is limited to the width of the particular sealing element 10 or ring.
Ported plate valves 66 operate in a slightly different manner than non-connected types. While the basic function is the same (to alternately open and close), the gas dynamics in the reciprocating gas compressor cylinder are such that flow through a compressor valve is rarely perfect. In other words, because of the various geometries of the gas compressor cylinders themselves, the gas forces acting on the ported plate may not be equally distributed across the entire plate and one side of the plate may open ahead of the other side.
Ported plate valves 66 and concentric ring valves 60 are generally known to have rather large flow areas and lower pressure drops, representing efficiency advantages. However ported plate valves, by their nature, are difficult to form into aerodynamic shapes. What cannot be achieved with improved aerodynamics is achieved with more generous flow areas. Concentric rings as used in the MANLEY® valve can be made into aerodynamic shapes and the minor loss in flow area can be restored with better aerodynamics. The function is the same, but the path to achieve it is slightly different.
In the valves of
Leaf springs are undesirable because they require a clean environment. The action of pressing the spring 100 flat induces a sliding motion between the top of the spring 100 and the under side of the channel/element 10. Abrasion always occurs between the moving parts and the introduction of solids entrained in the gas stream exacerbates the problem. Common to all metal spring is the problem of fatigue and a gradual loss of spring forces as the number of opening cycles accumulates.
The sealing element 10 is further directed by a guide 40. The guides serve to keep the channels from moving laterally in the space afforded between the seat 13 and the guard 15. Motion in one direction is assured while the valve is in operation. Guides also exist for the springs 100 so as to keep them in position under the channels 10 during operation.
During operation of the compressor, there is a point in the compressor stroke where each spring 100 moves the sealing element toward the seat 13. It is important to have contact with the seat 13 at the time of or before the compressor piston changes direction so as to avoid the high velocities and high impact energies associated with late closure as previously described.
Poppet valves do not have a hole in the central part of the element 10 and thus gas flow can only occur over the outer circumference of the element 10. Hence, the spring 100 is shielded to some extent from solids entrained in the gas, the curved top surface of the element 10 affords some aerodynamic advantages but the element 10 must move much farther from the seat 13 in order to maintain acceptable pressure drops as the gas moves through the valve. The lack of a central flow hole reduces the area through which the gas can flow and to compensate the element 10 must be allowed to move farther from the seat 13.
However, in this valve like others, the metal helical spring 100 is subject to fatigue and a reduction in forces as the number of opening and closing events increases. In addition, the spring 100 has the possibility of rubbing along the inside of the hollow stem of the element 10 thereby reducing the wire diameter of the spring 100 over time. A reduction in wire diameter increases local stress in the spring 100 leading to a failure mode. When the spring 100 fails in any compressor valve, there is no mechanism to move the element 10 toward the seat 13 before the compressor piston changes direction. Spring failures result in the elements 10 striking the seat 13 with increased and destructive velocities.
As shown in Figures, the reciprocating gas compressor valve includes a sealing element 10 and a seating surface 12 having an opening 20 for intake and exhaust of gas. The seating surface 12 surrounds the periphery of the opening 20. The sealing element 10 is sized and shaped to correspond with, and fully close the opening 20 when engaged against the seating surface 12.
The intake or exhaust gas flows into or out of the reciprocating gas compressor through the opening 20. Operation of the reciprocating gas compressor requires that the opening 20 of the reciprocating gas compressor valve be alternately opened and closed. The opening 20 is closed when the sealing element 10 is moved into contact with the seating surface 12 and closes the opening 20. When the sealing element 10 is moved out of contact with the seating surface 12, the opening 20 is opened and gas is permitted to flow into or out of the reciprocating gas compressor cylinder depending on whether the valve is located in the suction or discharge position of the reciprocating gas compressor cylinder.
The opening 20 and sealing element 10 are often cylindrical or spherical; however, the opening 20 and sealing element 10 of reciprocating gas compressor valve may be of any geometric configuration. The only requirement is that the size and shape of the sealing element 10 must correspond to the opening 20 in order to effectuate a seal.
The movement of a sealing element 10 is often limited by a guard 15 (also referred to as a “stop plate”). Typically, the reciprocating gas compressor geometry is such that when the seat plate 10 and the guard 15 are joined together, there is space available between the two for the sealing element 10 to move away from the seating surface 12 and against the guard. In modern reciprocating gas compressor designs it is possible to control the total travel of the sealing element 10 by adjusting the geometry of the guard and/or varying the thickness of the sealing element 10. The distance traveled by the sealing element is generally decided by the manufacturer of the reciprocating gas compressor valve after analysis of the operating conditions.
As noted above, the spring 14 is placed adjacent the guard 15 for the purpose of pushing the sealing element 10 toward the seating surface 12. Thus, the spring is interposed between the guard 15 and sealing element 10. Although not required, the compressor valve may also include a spring button 34 interposed between the spring 14 and sealing element 10. The spring 14 stores energy and provides force or torque that will push the sealing element 10 against the seating surface 12, resulting in a gas tight seal when the compressor valve is in a static, non-pressurized condition. During operation the purpose of the elastomeric valve spring 14 is to push the sealing element 10 toward the seating surface 12 at some point in time before the compressor piston reaches top or bottom dead center. Top or bottom dead center refers to the position of the compressor piston within the compressor cylinder.
Since reciprocating gas compressor cylinders may be double acting, the reference to top or bottom dead center is relevant only after it is determined which end of the compressor cylinder is being analyzed. When the piston reaches top or bottom dead center at the conclusion of the discharge or suction stroke, the piston changes direction, and pressures inside the compressor cylinder reverse. Pressure that was increasing starts to decrease (and vice versa) as soon as the piston reverses direction. If this occurs and the valve sealing element(s) 10 is some distance away from the seating surface 12 the valve sealing element(s) 10 can be forced against the seat plate in a violent manner by the changing gas pressure. Differential pressure forces can be substantial. The spring 14 is therefore installed behind the sealing element 10 to push the sealing element 10 toward the seating surface 12 well before top or bottom dead center occurs so that the pressure changes resulting from the change in direction of the compressor piston do not accelerate the valve sealing elements to excessive or destructive speeds. By varying the spring forces, the valve designer can influence the velocity of valve sealing elements and thereby control (to some extent) the impact forces between the seating surface 12 and sealing element 10.
As referred to herein, the elastomeric energy storage device or spring 14 may be useful in reciprocating gas compressors that are driven by electric motors, gas or liquid fuel engines, steam turbines or any other energy conversion device that provides power to a shaft for the purposes of imparting a rotating motion to a crankshaft. The reciprocating gas compressor may be directly coupled or indirectly coupled to the driver through the use of gears, belts, etc.
Reciprocating gas compressors include one or more compressor cylinders attached to a common crankshaft for the purpose of raising the gas from one pressure to another higher pressure. The reciprocating gas compressors may operate as a single stage unit or they can be designed for multistage operation. The gas cylinders can be oriented in any direction in relation to the crankshaft or to each other. Reciprocating gas compressors may be designed to operate in series or parallel with other compressors.
There are many manufacturers of reciprocating gas compressors. Each reciprocating gas compressor, however, performs the same task but varies in form and size. Currently known manufacturers of reciprocating gas compressors include: ABC Compressor; Ajax (Cooper); Aldrich Pump; Alley; Ariel; Atelier Francois; Atlas Copco; Bellis & Morcam; Blackmer Pump; Borsig; Broomwade; Bryan Donkin; Burckhardt; Burton Corbin; C.P.T.; Chicago Pneumatic; Clark; Consolidated Pneumatic; Corken; Crepelle; Creusot Loire; Delaval; Demag; Du Jardin; Ehrardt & Schmer; Einhetsverdichter; Energy Industries; Essington; Framatome; Frick Bardieri; Gardner Denver; Halberg; Halberstadt; Hitachi; Hofer; IMW; Ingersoll Rand; Ishikawajima-Harima Heavy Industries (IHI); Iwata Tosohki; Japan Steel Works; Joy; Kaji Iron Works; Khogla; Knight; Knox Western; Kobe Steel; Kohler & Horter; Mannesmann Meer; Mehrer; Mikuni Heavy Industries; Mitsubishi Dresser; Mitsui; Neuman & Esser; Norwalk; Nuovo Pignone; Pennsylvania Process Compressor (Cooper); Pentru; Penza; Peter Brotherhood (FAUR); Quincy; Reavell; Sepco; Siad; Suction Gas Engine Company; Sulzer; Superior (Cooper); Tanabe; Tanaise; Thomassen; Thompson; Undzawa Gumi Iron Works; Vilter; Weatherford Enterra (Gemini); Whitteman; and Worthington.
In the case of metal springs, the spring responds to forces and deflections in a mathematically linear fashion and the spring performance can be readily predicted for any amount of deflection by the formula: Force=Kx where “F” is the force applied to the spring, “K” is the spring constant or spring “rate” and “x” is the distance the spring deflects when the force “F” is applied. With some algebraic manipulation, one can measure the deflection of a spring and determine the magnitude of the force applied. In order to determine the spring constant “K”, the force and deflection of an existing spring must be measured at two points and graphed on a an x-y axis. If a line is drawn to connect the two points, the line will have a slope and the slope of the line is the spring constant “K”.
When designing a spring for the first time, of course, the aforementioned procedure cannot be used. Formulas, procedures and rules of thumb for designing new springs of almost any geometry is presented with great clarity in “Mechanical Springs” by A. M Wall (published by the Spring Manufacturer's Institute). Mechanical spring design is an iterative process and through experience and good judgment, the physical parameters of the material and geometry can be manipulated to produce the desired mechanical spring.
Elastomeric springs present a much more complex set of variables and require substantially more sophisticated mathematics and computational power to accurately predict and behavior. As forces are applied to an elastomeric material, the materials will deflect as previously noted but as the force is increased the modulus of the material will increase with increasing strain. In other words, the “spring constant” for elastomeric springs or elastomeric materials is not constant; it is variable and it increases with increasing strain. It is this non-linear behavior that increases the complexity of selecting an effective design. Every elastomer has a different modulus response to applied loads and therefore each elastomer will respond to applied loads in a unique way. Accommodating the variable modulus is but one significant complication associated with elastomeric materials. Elastomers, therefore provide the potential for an infinite number of solutions to a particular application but each possible solution must be evaluated individually which can make material selection a time consuming task.
The advent of finite element analysis (FEA), mathematical numerical techniques and high power computers makes solving the controlling partial differential equations relatively easy. Even though the software and computers handle most of the complex computations, properly modeling the material characteristics, applied loads and geometry in the software takes skill and requires considerable time. Accuracy in all phases of design results in actual springs being produced that match the performance predictions of the FEA.
FEA is computer modeling of product designs in order that a design can be analyzed to specific specifications and results. Over the years and at least as early as 1970, FEA has become a solution to the task of predicting failure due to unknown stresses by showing problem areas in a material and allowing designers to see all of the theoretical stresses within. This method of product design and testing is far superior to the manufacturing costs which would accrue if each sample was actually built and tested The proposed design performance characteristics can be verified prior to manufacturing or construction. FEA may be used for structural, vibrational, heat transfer and fatigue analysis. For example, the spring 14 of the subject invention can be modified and analyzed using FEA to qualify it for use in any type of valve. In case of structural failure, FEA may then used to help determine the design modifications to meet the new condition.
There are generally two types of finite element analysis that are used: 2-D modeling, and 3-D modeling. While 2-D modeling conserves simplicity and allows the analysis to be run on a relatively normal computer, it tends to yield less accurate results. 3-D modeling, however, produces more accurate results while sacrificing the ability to run on all but the fastest computers effectively. Within each of these modeling schemes, the programmer can insert numerous algorithms (functions) which may make the system behave linearly or non-linearly. Linear systems are less complex and generally do not take into account plastic deformation. Non-linear systems will account for plastic deformation, and are capable of testing a material all the way to fracture.
FEA uses a complex system of nodes (points) which make up a mesh (or grid). Through computer programming, the mesh identifies the material and structural properties which in turn define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. For example, areas or regions which will receive large amounts of stress usually have a higher node density than those which experience little or no stress. Points of interest may consist of: fracture point of previously tested material, fillets, comers, complex detail, and high stress areas. From each node, there extends a mesh element to each of the adjacent nodes. The vector carries the material properties to the object, creating many elements.
Mass, volume, temperature, strain, stress, force, displacement, velocity, acceleration can each be varied within the FEA system and minimized or maximized. Furthermore, multiple loading conditions may be applied including point, pressure, thermal gravity and centrifugal static loads, thermal loads from solution of heat transfer, enforced displacements, heat flux and convection, point, pressure and gravity dynamic loads. Each FEA program may come with an element library, or one is constructed over time including rod, beam, plate/shell/composite, shear panel, solid, spring, mass, rigid, and viscous damping elements. Many FEA programs also are equipped with the capability to use multiple materials within the structure such as: isotropic, orthotropic, and general anisotropic.
If the material properties are known, FEA can be used to determine, in an iterative manner, what the geometry of the spring needs to be in order to perform the desired task. Conversely if the geometry of the spring and the loads are known, the material properties required of the elastomer can be predicted and polymers with matching characteristics and be used or new polymers can be designed having the desired properties. The key is being able to accurately model the materials, geometries and loads in order to make accurate predictions about the final product.
To design the spring of
To select a material having a potential efficacy, a stress-strain curve is developed for the material. To prepare such curves, certain standard mechanical tests are performed including uniaxial tension, biaxial tension, shear stress and volume change. These are standardized tests that use well defined geometries of the elastomer compound being studied.
As shown in
Therefore, once a potential elastomer has been selected, tensile and biaxial tests are performed. These are standardized tests usually performed by a laboratory or compounder of elastomeric materials for the purpose of creating stress-strain plots of the material as it is pulled in tension along its axis or biaxially (two directions). The X-Y plot of the measured stresses and strains cannot usually be characterized by a straight line connecting the points. Each elastomer has its own characteristic curve. Therefore, in order to model the mechanical response of an elastomeric material (via finite element analysis), stress-strain curves are needed for each and every compound proposed.
As mentioned above, once a suitable material is selected, the next task is to prepare a finite element analysis to determine how the material will respond mechanically to loads and deflections that are anticipated to occur in service. Preferably, the finite element analysis is prepared via computer modeling. Examples of useful software currently available are: ANSYS, Abaqus, COSMOS, ALGOR, NASTRAN, FEMsys and NISA among others.
As noted above, springs are energy storing devices and the forces generated as a result of being subjected to pushes, pulls or torques must be designed to meet the needs of the service or duty. For compressor valves, the proper spring forces result in the valve sealing elements closing at the proper time thereby reducing the harmful effects of high velocity impacts with the sealing seat at valve closure. When replacing a currently existing spring, the forces and deflections of the new spring must match in order to duplicate the performance of the spring being replaced.
The critical material in an elastomeric spring is the elastomer; it provides the energy storage capability and the elastic response. Stretching an elastomer will cause stresses internal to the material and if these stresses exceed the limits of the material, a rupture/failure of the material will occur. Therefore prior to actually producing a spring, it is convenient to model the geometry, apply the anticipated loads and service conditions and evaluate the performance of the spring to determine if the design is suitable for the operating environment. The non-linear behavior of elastomers significantly increases the complexity of the mathematics often requiring partial differential equations and numerical methods to converge to a solution.
The solutions from the software can show strains, strains and deformations at any point in the geometry being modeled. The analyst can compare the predicted results to the published mechanical properties of the elastomer to determine if the mechanical limits of the material have been exceeded. If the computer model shows that the stresses are beyond what the elastomer material can tolerate, the geometry, the material or both can be changed and the FEA analysis can be run again.
Claims
1. A valve for a reciprocating gas compressor, the valve comprising:
- a sealing element repetitively moveable between an open and a closed position in response to the differential pressure across the sealing element; and
- an elastomeric energy storage device in operational connection with the sealing element for controlling the timing of the opening and the closing of the sealing element.
2. In a reciprocating gas compressor valve, an elastomeric valve spring in operational connection with a sealing element repetitively moveable between a seated and unseated position in response to differential pressure across the sealing element.
3. An energy storage device for controlling the timing of the repetitively opening and closing of a valve in a reciprocating gas compressor, the energy storage device comprising:
- an elastomeric spring.
4. A valve spring assembly for a reciprocating gas compressor valve, the assembly comprising:
- a stem having a shaft and a first end;
- a spring housing; and
- an elastomer web connected between at least a portion of the shaft and the housing;
- wherein the first end of the stem is adapted for operational connection with a sealing element of the reciprocating gas compressor valve for controlling the timing of the repetitive seating and unseating of the sealing element in response to a differential pressure across the sealing element.
5. In a reciprocating gas compressor valve, a spring comprising elastomeric material wherein said spring operably engages a sealing element to open and close the valve.
6. The spring of claim 5 wherein the elastomeric material is selected from the group consisting of natural rubber, styrene butadiene, synthetic rubber, thermoplastic elastomers, thermoset elastomers, fluoro-elastomers, elastomeric copolymers, elastomeric terpolymers, elastomeric polymer blends and elastomeric alloys.
7. A spring comprising a stem having a first end and a second end, a web frame comprising at least a first support member and a second support member extending radially from the stem and longitudinally along a substantial portion of the length of the stem, and a wall structure wherein said web frame is contained within said wall structure so as to enclose the support members and the stem.
Type: Application
Filed: Sep 20, 2006
Publication Date: Mar 22, 2007
Inventor: Kevin Durham (Stafford, TX)
Application Number: 11/533,668
International Classification: F04B 53/10 (20060101);
