Lightweight piston accumulator

The present invention provides a hydraulic pressure accumulator having a pressure vessel and a piston chamber that is located within the interior space of the pressure vessel. In one particular embodiment, the invention relates to filament wound composite overwrapped hydraulic pressure accumulators with serviceable pistons.

FIELD OF THE INVENTION

The present invention relates to hydraulic pressure accumulators. In one particular embodiment, the invention relates to filament-wound composite overwrapped hydraulic pressure accumulators with serviceable pistons.

BACKGROUND OF THE INVENTION

A hydraulic accumulator is essentially an energy storage device. Accumulators are widely used in mobile and industrial hydraulics to store energy, dampen pulsations, compensate for thermal expansion, and/or provide auxiliary power. It generally consists of a high pressure vessel in which a non-compressible hydraulic fluid is held under pressure by an external source. These accumulators are based on the principle that gas is compressible and fluid (e.g., oil or other similar liquid) is relatively incompressible. In operation, fluid or oil flows into the accumulator and compresses the gas by reducing its storage volume. Energy is stored in the compressed gas held under pressure. If the fluid is released, it will quickly flow out under the pressure of the expanding gas, thereby dispensing the stored energy.

A bladder accumulator consists of pressure vessel with an internal elastomeric bladder with pressurized nitrogen inside the bladder and hydraulic fluid outside the bladder but contained within the vessel. The accumulator is charged with gas, typically nitrogen, through a valve installed on the top. In a bladder accumulator, the energy is stored by compressing the gas encapsulated within an elastomeric (e.g., rubber) bladder. Energy is released when the hydraulic fluid out of the accumulator's fluid port, thereby decompressing the bladder by allowing it to expand.

The main advantages of a bladder accumulator are fast acting, no hysteresis, not susceptible to contamination, lower cost and consistent behavior under similar conditions. However, bladder accumulators have limitations in applications that require extremely high flow rates, tolerance of temperature extremes, high compression ratios, ability to withstand external forces and/or mounting restrictions. In addition, bladder accumulators typically cannot provide peak power when mounted horizontally or when they are subjected to centrifugal forces perpendicular to their longitudinal direction.

Piston accumulators alleviate many of these issues. A piston accumulator has a piston which slides against the accumulator housing on seals. On one side of the piston is a gas (again typically nitrogen) and on the other side is the hydraulic fluid and connection to the system. A fill port allows pressurization of the nitrogen. One of the advantages of piston accumulators is its ability to provide higher mass flow rate of the hydraulic fluid than bladder accumulators. This means that piston accumulators promise a higher specific power (delivered power per mass of the accumulator) that can be advantageous in mobile applications. Piston accumulators also do not have a bladder that has a finite fatigue life resulting from severe deformation in each cycle, thereby requiring replacement of bladders at regular intervals. In contrast, the seals in the reciprocating piston typically do not require maintenance as frequent as bladder accumulators.

As discussed above, bladder accumulators have limitations when operating at extremely cold or warm temperatures. In contrast, depending on the type of seal used, piston accumulators can have application in a much wider temperature range.

Failure of bladder accumulators is typically sudden and results in leaking of their stored gas into the hydraulic system. In contrast, piston accumulators, because of their small seal surface, generally tend to fail gradually. Thus, even when the piston accumulators begin to fail, the migration of gas from the gas side to the fluid side is slow, leaving a sufficient time for servicing to correct gas leaks into the hydraulic fluid system.

While bladder accumulators generally perform best when mounted vertically with the fluid port at the bottom and gravity assisting in the flow of the fluid, piston accumulators can be mounted in any position. In addition, the performance of bladder accumulators is significantly reduced when subjected to centrifugal forces or Coriolis forces. A piston accumulator is not affected by these forces.

Unfortunately, traditional piston accumulators are made of steel with elaborate machining operation and are generally very heavy. Typically, thick steel cylindrical chambers are used to support the structural load as well as to house the reciprocating piston. Some accumulator manufacturers have attempted to reduce the overall weight of these piston accumulators by substituting steel with structural composite overwrapped over legacy steel chamber designs.

Despite many different hydraulic pressure accumulators that are available, there is a continuing need for a lightweight serviceable hydraulic pressure accumulator.

SUMMARY OF THE INVENTION

Some aspects of the invention provide a hydraulic pressure accumulator that is lightweight, adaptable for servicing, while providing various advantages of piston accumulators.

In one particular embodiment, the invention utilizes a hydraulic pressure accumulator that comprises a vessel body and a piston chamber that is disposed within the interior space of the vessel body. The piston chamber acts as a piston accumulator. However, unlike conventional piston accumulators, the piston chamber does not require a thick steel cylindrical member to support the structural load. The majority of pressure exerted by the fluids in hydraulic pressure accumulator of the invention is carried by the vessel body and not the piston chamber itself.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with regard to the accompanying drawings which assist in illustrating various features of the invention. In this regard, the present invention generally relates to hydraulic pressure accumulators. That is, the invention relates to hydraulic pressure accumulators that comprise a piston accumulator within the interior of a composite pressure vessel. In this manner, the pressure carrying capability of piston accumulators is separated from the piston accumulator's functionality. The majority, if not all, of stored energy of compressed gas and the pressurized fluid is endured by the composite pressure vessel itself and not by the piston accumulator chamber.

In some embodiments, the composite pressure vessel comprises a metal lined composite pressure vessel with a large, metallic port opening that allows facile assembly, servicing and maintenance of a lightweight and detachable cylindrical piston (i.e., piston accumulator) housed inside of the composite pressure vessel.

Exemplary embodiments of hydraulic pressure accumulators are generally illustrated in the accompanyingFIGS. 1 to 4, which are provided solely for the purpose of illustrating the practice of the present invention and which do not constitute limitations on the scope thereof.

It has been found that the performance of bladder accumulators does not meet its full potential when these accumulators are mounted horizontally. The present inventors have discovered that piston accumulators would provide better performances over bladder accumulators in certain applications, such as those discussed above. Some of these applications experience variable mounting positions (e.g., mobile applications of earth movers and excavators), centrifugal forces (e.g., pitch control of wind turbine blades), Coriolis forces (e.g., aeronautical applications such as airplanes and helicopters) and operation in extreme temperatures (e.g., hydraulic hybrid) that can benefit from lightweight piston accumulators.

FIGS. 1 to 4depict some of the embodiments of the invention for hydraulic pressure accumulator100. As can be seen, hydraulic pressure accumulator100comprises a vessel body200and a piston chamber300. Suitable materials for vessel body200comprise carbon fiber composite, glass fiber composite, or one of many other strong and lightweight composite materials, such as may be found for high pressure composite pressure vessels. As can be seen inFIG. 3, in one embodiment, vessel body200comprises a composite overwrap250(e.g., carbon fiber, glass fiber, or other strong and lightweight materials) and a liner254. Suitable materials for liner254include, but are not limited to, metals, alloys, ceramics, plastics, or any other strong and non-permeable materials. Typically, the liner254comprises a ductile and fatigue resistant material. As used herein, the term “non-permeable” refers to a material that does not allow gas to leak under the normal operating pressure conditions. Typically, the non-permeable material shows no significant (e.g., <0.1% over a period of one week) gas leakage at the vessel's operating pressure.

Vessel body200can also include polar bosses400A and400B that reside at the ends of vessel body200to provide access to the interior of vessel body200. Typically, polar bosses400A and400B are embedded or conjoined within liner254, if liner254is provided. One particular embodiment of polar bosses include those disclosed in the commonly assigned U.S. patent application Ser. No. 14/282,160, which is incorporated herein by reference in its entirety.

As can be seen, piston chamber300resides within vessel body200. Piston chamber300can be welded to vessel body200(e.g., at polar boss400A by means of a weld joint) or can be threaded into polar boss400A. When piston chamber300is attached to vessel body200via a thread, a seal (not shown) can be used to prevent any leakage of gas and/or hydraulic fluid. Other joining means, e.g., a male-female joint connection with an appropriate sealing means) may alternatively be employed.

In one embodiment, end plugs210A and210B are used to seal the ends of vessel body200. In particular, end plugs210A and210B are placed within polar bosses400A and400B to place piston chamber300in place within vessel body200. End plugs210A and210B can also include gas port orifice214and fluid port orifice224, respectively.

Fluid port orifice224communicates with fluid sources (not shown) external to hydraulic pressure accumulator100. Similarly, gas port orifice214communicates with gas sources (not shown) external to hydraulic pressure accumulator100. Typically, gas port orifice214and fluid port orifice224are closeable or resealable such that the orifices can be opened or closed to allow pressure variation.

Piston chamber300also includes a cylindrical non-permeable body. The inner diameter of vessel body200is greater than the outer diameter of cylindrical body of piston chamber300and forms annular volume500between vessel body200and piston chamber300. Generally, the ratio of the inner diameter of vessel body200and the outer diameter of piston chamber300is at least about 2:1, typically at least about 1.5:1, and often at least about 1.25:1.

As can be seen, piston chamber300is disposed within the interior space of vessel body200and is substantially concentric with the cylindrical vessel wall of vessel body200. The piston chamber300also includes a piston304within the interior space of piston chamber300. Piston304is slidably disposed within the interior space of piston chamber300thereby separating piston chamber300into a first chamber310and a second chamber320. First chamber310contains a gas adapted to be compressed under pressure, and second chamber320contains pressurized fluid in fluid communication with an external fluid source through fluid port orifice224.

Within first chamber310, typically near polar boss area400A, piston chamber300also includes an orifice314configured to allow communication between first chamber310and annular volume500. In some instances, a plurality of orifices314can be present in first chamber310. This configuration allows the majority, if not substantially all, of the pressure exerted by the gas to be supported by vessel body200rather than by piston chamber300. Orifice314is typically located within first chamber310such that even when piston304is at the extreme end of first chamber310, no hydraulic fluid can leak through orifice314due to the thickness and/or the design of the piston304.

In some embodiments, first chamber310can also include a foam, an elastomeric material, a traditional coiled metallic spring, a set of composite bellow springs, a bellow or other compressible device or material in addition or alternative to gas.

While not shown, it should be appreciated that because hydraulic pressure accumulator of the invention is designed to allow communication of gas between first chamber310and annular volume500, one can design piston chamber300such that the length of piston chamber300is shorter than the length of vessel body200from one polar boss end to the other (e.g., from400A to400B). Such a configuration would result in the end of first chamber310“hanging” or dangling within annular volume500. In such embodiments, orifice314is not required.

Typically, however, the length of piston chamber300runs at least from gas port orifice214to fluid port orifice224.

As hydraulic fluid enters and exits via port224, piston304moves longitudinally within piston chamber300in reaction to forces resulting from the balancing of pressure between the gas in first chamber310and the fluid in second chamber320. Charge gas can be prevented from contacting the fluid by means of piston seal (not shown). In some instances, dynamic radial seals (not shown) are present encircling piston304. Such dynamic radial seals are well known in the art and act to facilitate its longitudinal movement within piston chamber300.

Preparing hydraulic pressure accumulator100for operation generally involves pre-charging the gas side. During pre-charging a gas (e.g., nitrogen or any other suitable gas known to one skilled in the art) is introduced into first chamber310and annular volume500(via orifice314) through gas port orifice214at a designated pre-charge pressure, e.g., 1000 psi, 2000 psi, 5000 psi or even up to 10000 psi. The pressure of the initial gas charge causes piston304to move longitudinally toward the opposite end of piston chamber300, expelling fluid from second chamber320if any present as the piston sweeps through it. To retain the charge gas, gas port orifice214is sealed by conventional gas valve means as is known in the art. In this manner, hydraulic pressure accumulator100is brought to its proper pre-charge pressure. At the beginning of the operation of the hydraulic system, hydraulic fluid is introduced into the second chamber320through fluid port orifice224so as to cause second chamber320to be filled with fluid and piston304slides towards the first chamber310. As one skilled in the are can readily recognize, the piston304slides towards first chamber310until the pressure equilibrium is reached between first chamber310(which also includes annular volume500) and second chamber320. To store energy in hydraulic pressure accumulator100, fluid is pumped into second chamber320through fluid port orifice224by a hydraulic pump/motor or other means as is known in the art. Also as known in the art, this causes charged gas in first chamber310(and annular volume500) to become compressed as fluid causes piston304to move towards first chamber310.

Change in pressure in some instances causes temperature change within the charge gas. In order to avoid or reduce temperature increase during energy storage process (i.e., hydraulic fluid charging process), in some embodiments, interior of vessel body200comprises or is coated with a phase changing material (“PCM”). In another embodiment, first chamber310can be filled with PCM elastomer or foam that gets compressed during the energy storage process as piston304moves towards first chamber310. Suitable PCMs are well known in the art. See commonly assigned U.S. Pat. No. 8,662,343, issued Mar. 4, 2014, which is incorporated herein by reference in its entirety. Briefly, typical PCM comprises a material that melts (i.e., changes phase from solid to liquid) at a certain temperature. The useful PCMs of the invention have a melting point in the range of from about 0° C. to about 80° C. typically from about 20° C. to about 50° C. In some embodiments, the piston chamber300can also include (or coated with) a PCM. Exemplary PCMs that are suitable for the invention include, but not limited to, organic materials such as paraffin and fatty acids, salt hydrates, water, eutectics, naturally occurring hygroscopic materials, metals and metallic particles, nano-materials. Some of the particular PCMs suitable for the invention include, but are not limited to, heptanone-4®, n-Unedane®, TEA_16®, ethylene glycol, n-dodecane, Thermasorb 43®, Thermasorb 65®, Thermasorb 175+®, Thermasorb 215+®, sodium hydrogen phosphate, Micronal®, and an assortment of other polymeric PCMs.

By utilizing composite overwrap pressure vessel, the present invention avoids gas loss problems while providing a relatively light weight piston accumulator system.

As discussed herein, in general piston chamber300that houses piston304is enclosed within a pressure vessel, i.e., vessel body200. In one particular embodiment, pressure vessel (i.e., vessel body)200is a composite overwrapped pressure vessel and piston304comprises a material selected from the group consisting of a metal, a composite material, a ceramic, a reinforced polymer, and a combination thereof.

In another embodiment, vessel body200has port openings that can be at least partially closed using appropriate plugs. In one particular instance where vessel body200is of composite overwrapped pressure vessel, the port opening is facilitated by polar boss integrated with the liner and composite structure.

Assembly of hydraulic pressure accumulator100typically involves inserting piston chamber300into the pressure vessel200through the port opening. First chamber310is then charged with a compressible gas and second chamber320is filled with hydraulic fluid. Piston304, optionally with radial seal(s), separates the gas and the fluid in the two compartments. One of the key elements of the invention is that annular area between piston chamber300and vessel body200(i.e., annular volume500) is fully or partially filled with compressible gas. Having a communication pathway between first chamber310containing compressible gas and annular volume500allows the pressure load to be supported by vessel body100.

The pressure in the compressed gas is structurally supported by vessel body200. In one embodiment, vessel body200is a composite pressure vessel. In another embodiment, vessel body200is a composite shell overwrapped over impermeable liner such as metal or polymer.

Piston304slides towards first chamber310and compresses the gas when fluid enters second chamber320to bring equilibrium in pressure between the gas and fluid. Energy is stored in the compressed gas. When the pressure in second chamber320drops or when fluid leaves second chamber320, piston304slides towards second chamber320thereby decompressing the gas and recovering the stored energy and allowing equilibrium in pressure between first chamber310and second chamber320.

When first chamber310is partially or fully filled with elastomeric material, foam or other compressible material, such a material can also include a phase change material. When the gas is compressed quickly, it results in temperature rise. When the temperature settles, the pressure in the gas compartment drops. This results in less-than-desirable fluid volume that is expelled when the stored energy is recovered. Use of PCM in the gas compartment (i.e., first chamber310) provides improved thermal management of the compressed gas during each energy storage and recovery cycle, and therefore allow hydraulic pressure accumulator of the invention to deliver peak power and operate more efficiently during each cycle.

In some embodiments, first chamber310comprises a spring like device that stores energy by compression. The spring can be made of metal, polymer, elastomer, PCM or composite. The spring can also be a metal, composite or elastomeric bellow.

One of the advantages of the invention is that piston chamber300is in neutral equilibrium, i.e., there is no pressure differential between the interior of piston chamber300and the exterior, i.e., annular volume500). This net pressure differential allows, a wide variety of materials to be used as piston chamber300. Suitable materials for piston chamber300include, but are not limited to, metal, metal alloy, ceramic, polymer, composite, etc. It can be machined or net formed to allow for circularity demanded for piston operation. In one particular embodiment, piston chamber300is made from metal that has been machined, honed and lapped to produce smooth interior surface. In another embodiment piston chamber300is made from thin metal, polymer or ceramic shell overwrapped with composite.

In some embodiments, after insertion inside pressure vessel200, piston chamber300is sealed against the polar boss using radial seals. This prevents leakage of gas or fluid past the port opening of pressure vessel200. The installation of piston chamber300inside of pressure vessel200can be achieved by using threads, special mechanical locks or attachments operated from outside of pressure vessel200. End caps can be threaded or locked on to the port openings to allow for one gas filling port and a fluid port on each end of the accumulator.

Once the threads, special mechanical locks or attachments are removed from the port opening, piston chamber300can be retracted and removed from pressure vessel200for servicing of its interior surface, piston304or radial seals on piston304.

The compression ratio and the energy and power storage capacity of hydraulic pressure accumulator of the invention is generally determined by the relative ratio between the diameter of piston chamber300and the diameter of pressure vessel200. In one embodiment, the outer diameter of piston chamber300is very large (70-85%) compared to the inner diameter of pressure vessel200to keep the compression ratio greater than or equal to 2. This requires the polar opening of pressure vessel200to be a significant fraction of the inner diameter of pressure vessel200.

Unlike monolithic and isotropic material like steel, a composite overwrapped pressure vessel with a large port opening can be designed to withstand very high internal pressure. This is enabled by an optimized design of the structural shape and composite layup such that the composite material is adequately and optimally placed to support the internal pressure. In one particular embodiment, the dome shape (e.g., non-cylindrical portion) of the composite pressure vessel can be selected that allows for geodesic filament winding of unidirectional composites with helical wind angle that optimizes the pressure carrying capability of the dome. In such case, the winding pattern allows for complete coverage of the pressure vessel with a polar opening diameter that is a significant fraction (between 50 and 90%) of the diameter of pressure vessel200. In another embodiment as schematically shown inFIG. 4, a polar opening diameter that is 80% of the diameter of pressure vessel200can be achieved by utilizing a helical wind angle of ±54.5° resulting in a composite structure without the need for hoop or circumferential plies.

Piston chamber300can be designed to be integral to the structure of pressure vessel200. The polar blowout load imposed on the end caps can be fully or partially supported by the wall of piston chamber300in its axial direction. The wall of piston chamber300can be designed to fully or partially support this axial load by optimizing the thickness of a metallic shell or a combination of metallic, polymeric, ceramic and composite shell.