Embodiments of the invention relate to sealing systems for pressure vessels and more particularly, to sealing systems for high pressure fuel cylinders and additionally, incorporating means for maintaining and monitoring seal integrity.
High pressure cylinders are known typically for carrying and supplying alternative fuels to vehicles such as compressed natural gas (CNG) vehicles and for hydrogen fuel cells. Due to the rigid safety requirements for such vessels, it is desirable to provide systems for sealing the boss or neck of the vessel that are robust and capable of maintaining the high pressure in the cylinders without fear of leaking.
Typically, conventional cylinders are sealed at an outer aspect of the boss using a face seal to provide sealing between an insert, such as a plug or a valve assembly, and the cylinder. The seal is rated to be able to contain at least the maximum pressure in the cylinder. Should the seal fail however, the contents of the vessel would be vented to atmosphere causing a loss of fuel from the cylinder and creating a combustion hazard, particularly if the vehicle is in an enclosed structure such as a garage. Failure of the seal is typically without warning and may result in a situation where a vehicle is stranded without sufficient fuel. Most often the operator is unaware of such a leak or failure of the seal until after a significant portion of the fuel within the cylinder has already escaped.
In an incident reported in the media, Toyota recalled a number of fuel cell vehicles as a result of leaking. Applicant believes that it was determined to be a leak at the valve-cylinder interface and when examined resulted due to failure of an O-ring.
Conventional seals used in high pressure fuel cylinders are subjected to large pressure differentials across the seal between the high pressure interior of the cylinder, as high as 700 bar (10,000 psi) for hydrogen storage cylinders, and atmosphere. The high pressure differential acts to reduce the life of the seal, despite using highly effective sealing materials.
The automotive industry has attempted to provide more reliable sealing by utilizing different seal materials, seal types and seal gland configurations. Further, tapered threads combined with sealing paste or tape has been used to prevent leaking however, Applicant believes this induces additional circumferential stress in the cylinder neck.
Ideally, a sealing system for high pressure fuel cylinders is capable of withstanding large pressure differentials for extended lifetimes without need for frequent replacement and more preferably is equipped with a means for detecting seal failure before the contents of the cylinder are vented.
In one embodiment a unique apparatus, method and system for sealing a high pressure from a lower pressure, such as a high pressure fuel cylinder from atmospheric pressure, utilizes an inner seal spaced from an outer seal by an intermediate space. An intermediate pressure is maintained in the intermediate space to reduce the pressure differential across both the inner seal and the outer seal and thus extend seal life.
Typically, the intermediate space is formed about an insert fit within a boss of the high pressure fuel cylinders and the inner and outer seals seal between the insert and the cylinder boss.
In another embodiment a means for monitoring the intermediate pressure is fluidly connected to the intermediate space to permit detection of changes in the intermediate pressure, which are indicative of a leak in either or both the inner and outer seal.
Thus, in one broad aspect of embodiments of the invention a method for sealing a fluid at a first high pressure from a second lower pressure comprises: providing an inner seal capable of sealing the fluid at the first high pressure; providing an outer seal capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; and providing means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal.
In another broad aspect of embodiments of the invention, apparatus for sealing a fluid at a first high pressure from a second lower pressure comprises: an inner seal capable of sealing the fluid at the first high pressure; an outer seal capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; and means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal.
Further, in another broad aspect of embodiments of the invention a system adapted for sealing a boss in a high pressure cylinder and for indicating the integrity of said sealing comprises: an insert adapted to fit within the boss; an inner seal, adapted to be positioned between the insert and the boss, and capable of sealing the fluid at the first high pressure; an outer seal, adapted to be positioned between the insert and the boss, and capable of sealing the fluid at the first high pressure, the outer seal being spaced from the inner seal for forming an intermediate space therebetween, the intermediate space having an intermediate pressure being lower than the first high pressure and higher than the second lower pressure; means for maintaining the intermediate pressure in the intermediate space for reducing a pressure differential at the inner seal; and means for monitoring the intermediate pressure fluidly connected to the intermediate space for detecting a change in the intermediate pressure being indicative of a lack of integrity of the inner seal, the outer seal or both.
Preferably the intermediate pressure is maintained by balancing an inflow and outflow from the intermediate space, typically as a result of permeation across and around the inner and outer seals. Permeation is balanced by selecting a seal material for the inner and outer seals and the differential pressure, by selecting a seal geometry for the inner and outer seals or by providing a pressure relief device fluidly connected to the intermediate space for releasing pressure therefrom.
The means for monitoring the pressure in the intermediate space may be any suitable pressure monitoring means such as a pressure switch or a mechanical pressure gauge fluidly connected to the intermediate space.
a-2c are dimensional sectional views according to
a is a section view of the cylinder boss;
b is a detailed section view of an outer valve seat; and
c is a detailed view of a taper or chamfer adjacent an inner sealing surface to permit insertion of the insert into sealing engagement with the cylinder boss; and
a-3c illustrate a plug insert adapted for insertion into the cylinder boss according to
a is a side view of the plug insert;
b is a detailed sectional view of an annular groove adjacent a bottom end of the plug insert for accommodating the inner seal and a backup-ring;
c is a plan view of a top of the plug insert;
d is a partial sectional view of an insert illustrating parameters for calculation of a barrier thickness;
Embodiments of the invention are described herein in the context of high pressure cylinders used to fuel vehicles. One of skill in the art would understand that the sealing arrangement described herein is applicable to any situation wherein gases are stored in vessels at high pressure.
Having reference to
Having reference to
Seal permeability is a factor to be considered when attempting to maintain the pressure Pm between the inner and outer seals 10,11. Seal permeability is dependant primarily on two factors, a material from which the seal 10,11 is made and a geometry of the seal 10,11 itself. As shown in Tables A and B, reproduced in part, respectively, from Peacock, R. N. “Practical selection of elastomer materials for vacuum seals.” Journal of Vacuum Science Technology Vol. 17 No. 1 (January/February 1980):330-336 and Parker Seals, Parker O-Ring Handbook, Table 3-19, Gas Permeability Rates, Pages 3-27-3-35, Parker Hannifin Corporation, 2360 Palumbo Drive, Lexington Ky. 40509 USA, the entirety of which are incorporated herein by reference, different elastomers have different gas permeability rates for different fuel types.
K=permeation constant in sccms−1cm−2 cmatm−1
Std cc cm/cm2sec.bar
Further, as demonstrated in the following formulas, permeation can be calculated dependant upon the material selected or upon the geometry or size of the barrier presented by the seal 10, 11, for a particular material. Permeation is defined as the passage of a gas under pressure into, through and out a solid material by diffusion and solution to the low pressure side. Assuming an equilibrium state, the rate of gas permeation can be calculated using the following formula:
where,
For example, if the gas to be contained is helium, the permeation rate from a 2.000″ port, on a 350 bar cylinder using a nitrile (Buna-N) seal, can be estimated using the following values in the above equation:
The average permeation constant for helium gas through nitrile (Buna-N):
The area of the barrier:
A=π·D·t=π·5.44·0.27=4.6 cm2
The high and low side pressures:
The barrier thickness, as shown in
Substituting the above values into the permeation rate equation yields:
In a preferred embodiment of the invention, for use in a 700 bar (10,000 psi) cylinder 2, the inner and outer seals 10,11 are selected or configured to maintain a maximum intermediate pressure Pm of 350 bar (5000 psi) in the intermediate space 12 therebetween, thus significantly reducing the pressure differential ΔP at the inner seal 10. Each of the seals 10,11 is selected to be capable of containing the full operating pressure or first high pressure Pop of 700 bar (10,000 psi) so that in the event of a failure of the inner seal 10, the contents of the cylinder 2 are not vented to atmosphere. The inner and outer seals 10,11 however, are selected so that the permeation across the inner seal 10 is compensated for or balanced by the permeation at the outer seal 11, effectively maintaining the lower intermediate pressure Pm therebetween.
Optionally, as shown in
As shown in
Preferably, a backup ring 106 is positioned adjacent the inner seal 10 and in the annular groove 102. The backup ring 106 is typically manufactured from a material, such as nitrile, having a greater durometer rating than the inner seal 10 so as to provide a surface against which the seal 10 may be compressed and to prevent extrusion of the seal 10 from the annular groove 102. The backup ring 106 may be a split ring or a deformable ring.
In the preferred embodiment, as shown in
Further, if for some reason the outer seal 11 should fail, rather than the inner seal 10, the change or drop in the intermediate pressure Pm would be observed and signal a need for service. In both cases, the redundancy in the sealing arrangement would allow the cylinder 2, and ultimately, a vehicle, (not shown) to which the cylinder 2 is supplying fuel, to remain in use until it could be removed from service and the inner and outer seals 10,11 replaced.
Optionally, additional apparatus (not shown) such as a burst disc, an on-off valve, gas sensors, flow restrictors or regulators, pressure regulators, check valves or gas filters, may be fit within the monitoring port 110.
In one embodiment of the invention, wherein the insert 100 and the cylinder boss 101 are manufactured using aluminum, threads 120 used for threading the insert 100 into the boss 101 are not self-centering to avoid sharp edges which may result in the insert 100 galling to the boss 101 during installation or removal. Further, the sealing surface 103 is polished to remove any spiral or radial tool marks, scratches or gouges which would impair sealing thereto.
An additional advantage of positioning the inner seal 10 into the boss 101 of the cylinder 2 is realized during manufacturing high pressure cylinders 2 which undergo autofrettage as part of the manufacturing process. Autofrettage pressures have the potential to cause deformation of the boss 101 of the cylinder 2, thus, positioning a seal 10 at an inner surface adjacent the containment portion of the cylinder 2 acts to protect the boss 101 from the high pressure Pop, preventing costly rework of the boss 101 or rendering the cylinder 2 defective.
This application is a regular application claiming priority of U.S. Provisional Patent application Ser. No. 60/564,605 filed on Apr. 23, 2004, the entirety of which is incorporated herein by reference.
Number | Date | Country | |
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60564605 | Apr 2004 | US |