Pressure relief valve with thermal trigger and movable seal plug

A thermally activated pressure relief valve includes a valve housing with an inlet communicating with an interior of a pressure vessel, and a passage from the inlet, through the housing, to an outlet for communicating with the exterior of the housing. A seal plug is disposed within a cavity of the housing and across the passage for sealing the passage. A thermal trigger engages the seal plug within the cavity to restrict movement of the seal plug and maintain a seal of the passage when a temperature adjacent to the housing is below a predetermined temperature threshold. The thermal trigger releases the seal plug, which is movable within the cavity, allowing the seal plug to relocate within the cavity when the temperature reaches the predetermined temperature threshold, thereby exposing a flow path between the inlet and outlet through which gas can escape.

BACKGROUND OF THE INVENTION 
The present invention relates to a relief valve. More particularly, the 
present invention relates to a thermally activated relief valve for use 
with a compressed gas storage cylinder (or pressure vessel). 
In high pressure compressed natural gas systems, it is a requirement to 
provide a means by which the pressure vessel can be relieved of its gas 
charge in the event of an excessively high external temperature (e.g., a 
fire near the vessel). The standard approach has been to incorporate a 
fusible plug into the design of the system (pressure vessel, valve) that 
is continuously exposed to the direct pressure of the gas charge. A 
fusible plug is a fitting that contains a slug of eutectic material that 
blocks and seals an outlet passage while the external temperature is below 
a predetermined yield point. When the temperature of the fusible plug 
reaches the yield temperature, the fusible material melts to provide a 
pathway for the pressurized gas to escape. 
In principle, this approach to temperature relief is acceptable. A problem 
arises, however, when the fusible plug is exposed to the continuous high 
pressures of the gas charge at temperatures approaching the yield point of 
the eutectic material. In such condition, extrusion of the fusible slug 
may occur, thus producing a potential leak path. 
A number of varying strategies can be applied to correct this problem 
through modification of the fusible plug (e.g., reducing bore diameter, 
increasing the yield temperature of the eutectic material). None of the 
strategies mentioned have thus far completely eliminated the problem. 
SUMMARY OF THE INVENTION 
The present invention is a thermally activated pressure relief valve which 
includes a valve housing with an inlet for communicating with an interior 
of a pressure vessel, and a passage from the inlet, through the valve 
housing, to an outlet for communicating with the exterior of the valve 
housing. A generally cylindrical-shaped seal plug is disposed within a 
cavity of the housing and across the passage between the inlet and the 
outlet, with a flat surface at a first end of the seal plug for sealing 
the passage. A radial shoulder frames the passage to provide a sealing 
surface against which the flat surface of the seal plug contacts to create 
a seal. A beveled annular exterior surface at a second end of the seal 
plug contacts a thermal trigger. 
The thermal trigger incorporates a trigger ball into a channel at a first 
end of the thermal trigger. The trigger ball, which extends partially 
beyond the first end of the thermal trigger, is fixed by a small amount of 
soft metal eutectic within the channel. The trigger ball contacts the 
beveled annular surface of the seal plug and fixes the seal plug such that 
the flat surface rests against the radial shoulder to seal the passage 
when the temperature near the valve housing is below a predetermined 
temperature threshold. When the thermal trigger reaches a predetermined 
temperature threshold, however, the eutectic melts and the trigger ball 
migrates within the channel of the thermal trigger to allow the seal plug 
to move from a first position to a second position. When the seal plug is 
in the second position, a flow path through the passage is exposed, 
allowing fluid pressure to escape to the exterior of the valve housing. 
Because the eutectic within the thermal trigger does not directly contact 
fluid pressure, the present invention avoids eutectic creep problems which 
cause premature pressure leaks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is an exploded longitudinal sectional view of a thermally activated 
relief valve 10 of the present invention. Relief valve 10 includes valve 
housing 12, seal plug 14, torque fitting 16, and thermal trigger 18. Valve 
housing 12 includes cavity 20, first end 22 and second end 24. First end 
22 defines the outer end of valve 10, and has outlet 26 which exposes 
cavity 20 to the exterior of housing 12. Second end 24 has opening 28 
through which seal plug 14 is inserted to position seal plug 14 within 
cavity 20. 
Housing 12 also includes coarse threaded exterior surface 30, which is near 
first end 22, and fine threaded exterior surface 32, which is located near 
second end 24. Coarse threaded exterior surface 30 is provided to accept a 
threaded cap or fitting (not shown) to cover first end 22. Fine threaded 
exterior surface 32 is provided to accept torque fitting 16. Valve housing 
12 also includes threaded opening 34, which is generally perpendicular to 
and communicates with cavity 20. Threaded opening 34 is provided to accept 
thermal trigger 18. 
Cavity 20 includes first cavity region 36, second cavity region 38 and 
third cavity region 40. First cavity region 36 has cylindrical inner 
surface 42, which has a diameter greater than cylindrical inner surface 44 
of third cavity region 40. Second cavity region 38 has a radially tapered 
surface 46 which tapers from inner surface 42 to inner surface 44. 
Seal plug 14 is an elongated, cylindrical-shaped plug which has first end 
48 and second end 50. First end 48 has opening 52, which exposes 
uniform-diameter tubular passage 54. Outer surface 56 of seal plug 14 
varies in diameter along the length of seal plug 14 to form first section 
58, second section 60, third section 62, and fourth section 64. Section 58 
is located at second end 50 and has a diameter smaller that the diameter 
of section 60. Section 58 includes seal plug surface 66 and exhaust port 
68, adjacent to seal plug surface 66 and communicating with passage 54. In 
one embodiment, seal plug 14 includes two or more exhaust ports 68 equally 
spaced around the circumference of section 58. 
Section 60 is adjacent to section 58 and has a diameter which is slightly 
larger than section 58. Section 62 is adjacent to section 60 and has a 
diameter which diminishes from section 60 to section 64, thereby forming 
tapered shoulder 70. Section 64 is adjacent to section 62 and has a 
diameter approximately equal to the smallest diameter of section 62. 
Generally, seal plug 14 is contoured so as to closely fit within cavity 20 
of housing 12. 
Torque fitting 16 is configured to enclose seal plug 14 within housing 12 
and to attach valve 10 to a pressure cylinder. Torque fitting 16 includes 
first end 72 and second end 74. First end 72 is provided with threaded 
opening 76, which communicates with vent passage 78 and inlet 80 at second 
end 74. Second end 74 defines the inner (inlet) end of valve 10. Torque 
fitting 16 also has threaded exterior surface 82 for threading valve 10 
into a threaded opening in a pressure vessel (shown in FIG. 2). 
Threaded opening 76 has a diameter greater than vent passage 78, which 
creates shoulder 84 at the junction of vent passage 78 and threaded 
opening 76. Annular notch 86 is concentric with shoulder 84 and has an 
outer diameter less than the diameter of threaded opening 76 to provide a 
position for O-ring 88. O-ring 88 and shoulder 84 abut seal plug surface 
66 of seal plug 14 so as to block vent passage 78 and provide a 
fluid-tight seal. O-ring 90 is positioned adjacent to mounting surface 92 
to provide a fluid-tight seal when valve 10 is mounted to a pressure 
vessel. 
Thermal trigger 18 has first end 94 and second end 96. First end 94 is 
provided with threaded exterior 98 which is sized to permit threading of 
thermal trigger 18 into threaded opening 34 of valve housing 12. Thermal 
trigger 18 has channel 100 extending generally along its longitudinal axis 
from first end 94 to second end 96. Shoulder 102 divides the channel 
generally into first channel 104 and second channel 106. 
Thermal trigger 18 is also provided with trigger ball 108 which is sized to 
permit insertion into first channel 104. Trigger ball 108 is large enough, 
however, so that it abuts shoulder 102 and cannot pass into second channel 
106. Channel 104 is swaged at first end 94 to engage a diameter of trigger 
ball 108 and retain a greater portion of trigger ball 108 within channel 
104 while allowing another lesser portion to extend beyond first end 94. 
In the assembled state of thermal trigger 18, eutectic substance 110 fills 
channels 104 and 106 and holds trigger ball 108 in a fixed position within 
channel 104 and partially extending beyond first end 94 of thermal trigger 
18. 
Thermal trigger 18 includes hexagonal exterior surface 112 to accommodate 
an appropriate size wrench during installation of thermal trigger 18 into 
housing 12. Thermal trigger 18 also includes concentric ribs 114 for rapid 
heat absorption and communication to eutectic substance 110 in channels 
104 and 106. 
FIG. 2 is a longitudinal sectional view of the pressure relief valve of 
FIG. 1 shown assembled and mounted to a pressure vessel. Thermal trigger 
18 is shown mounted to valve housing 12 with threaded exterior 98 engaging 
threaded opening 34 of valve housing 12. First end 94 of thermal trigger 
18 is generally aligned with cylindrical inner surface 42 of first cavity 
region 36. Trigger ball 108, which is partially extending beyond first end 
94 of thermal trigger 18, lies within first cavity region 36 of housing 
12. 
Seal plug 14 lies within cavity 20 of valve housing 12. Outer surface 56 of 
sections 60 and 64 of seal plug 14 are configured to fit closely within, 
and permit movement within, cylindrical inner surfaces 42 and 44, 
respectively, of cavity 20. Seal plug 14 is positioned within cavity 20 
such that tapered shoulder 70 rests against trigger ball 108. In this 
"loaded" position, tapered shoulder 70 and first end 48 of seal plug 14 
are spaced apart from tapered inner surface 46 and first end 22 of housing 
12, respectively, so as to permit seal plug 14 to move outward should 
trigger ball 108 relocate within channel 104 due to temperatures exceeding 
a predetermined temperature threshold. 
Torque fitting 16 is connected to valve housing 12 by engaging threaded 
opening 76 over fine threaded exterior surface 32 of valve housing 12. A 
rotational force is applied in a first direction to torque fitting 16 to 
fully seat seal plug surface 66 against shoulder 84 and O-ring 88, thereby 
forming seal 116 and creating vent space 118 between cylindrical inner 
surface 42 and outer surface 56 of section 58 of seal plug 14. Seal 116 is 
a fluid-tight seal which separates vent passage 78 from vent space 118. 
Vent space 118 communicates with exhaust port 68. 
Relief valve 10 is connected to pressure vessel 120 by engaging threaded 
exterior surface 82 of torque fitting 16 within threaded opening 122 of 
pressure vessel 120. A rotational force is applied to torque fitting 16 in 
a second direction to seat mounting surface 92 and O-ring 90 against outer 
surface 124 of pressure vessel 120. In this assembled state, inlet 80 of 
torque fitting 16 is exposed to interior 126 of pressure vessel 120. 
FIG. 3 is a longitudinal sectional view of the relief valve of FIG. 2 shown 
in the thermally triggered position. When thermal trigger 16 is exposed to 
temperatures that exceed a predetermined temperature threshold, eutectic 
substance 110 of channel 100 melts, thereby liberating trigger ball 108 
from its fixed, semi-extended position within cavity 20 of housing 12. As 
eutectic substance 110 melts and trigger ball 108 becomes liberated within 
first chamber 104 of thermal trigger 18, gas pressure from interior 126 of 
pressure vessel 120 exerts an outward force on seal plug surface 66 of 
seal plug 14; this outward force causes seal plug surface 66 to move 
outward, away from shoulder 84, which coincidentally causes an outward 
force to be exerted against trigger ball 108 by tapered shoulder 70. 
Accordingly, trigger ball 108 relocates to a position primarily within 
chamber 104, against shoulder 102. In this position, it should be noted is 
that trigger ball 108 acts as a check valve. In other words, since trigger 
ball 108 is too large to pass into second channel 106, it rests against 
shoulder 102 to prevent any gas communication between the passage through 
valve 10 and the second end 96 of thermal trigger 18. 
Seal plug 14 continues to move outward until tapered shoulder 70 contacts 
tapered inner surface 46. In this position, with seal plug surface 66 
disengaged from shoulder 84 and O-ring 88, vent passage 78 is exposed to 
first cavity region 36, thereby allowing gas to escape through vent space 
118, exhaust port 68, passage 54 and outlet 26 of valve housing 12. The 
flow of gas out of relief valve 10 can be halted by engaging a threaded 
seal cap (not shown) over coarse threaded exterior surface 30 to seal 
outlet 26 of valve housing 12. Alternatively, the escaping gas can be 
recaptured by engaging a threaded fitting, which is connected to a second 
vessel, over coarse threaded exterior surface 30. 
FIG. 4 is an exploded longitudinal sectional view of a second embodiment of 
the thermally activated relief valve of the present invention. As shown in 
FIG. 4, relief valve 150 includes valve housing 152, blocking poppet 
assembly 154, torque plug 156 and thermal trigger 158. 
Valve housing 152, which is fashioned from a solid block of stainless 
steel, includes first end 160, second end 162 and cavity 164. Cavity 164 
is generally cylindrical and is created by boring into the block from 
second end 162 to create first cavity region 166, second cavity region 168 
and third cavity region 170. First cavity region 166 is near second end 
162 and includes threaded opening 172. First cavity region 166 has inner 
surface 174, which has a diameter greater than the diameter of inner 
surface 176 of third cavity region 170. Second cavity region 168 has a 
radially tapered surface 178 which tapers from inner surface 174 to inner 
surface 176. Cavity 164 terminates at end surface 180 of third cavity 
region 170. 
Exhaust passage 182 exposes first cavity region 166 to first side 184. 
Exhaust passage 182 is perpendicular to cavity 164 and includes outlet 186 
through which gas pressure can escape. 
Valve housing 152 also includes threaded opening 188, which is generally 
perpendicular to and communicates with cavity 164. Threaded opening 188 is 
exposed at second side 190, which is opposite first side 184, and is 
provided to accept thermal trigger 158. 
Blocking poppet assembly 154 includes blocking poppet 192 and restraining 
member 194. Blocking poppet 192 is formed from a solid cylindrical piece 
of (metal) having first end 194, second end 196 and cylindrical surface 
198. Second end 196 of blocking poppet 192 includes blocking surface 200. 
First end 194 is bored to provide first bore region 202 and second bore 
region 204. First bore region ends at first base 206. Second bore region 
204, which is concentric with first bore region 202, ends at second base 
208. 
A multiplicity of holes 210 are bored into cylindrical surface 198, with 
sides 212 of holes 210 generally aligned with second base 208. Holes 210 
have four locations, radially spaced 90.degree. apart, around the 
circumference of cylindrical surface 198. Holes 210 are perpendicular to 
and communicate with first bore region 202. Each hole 210 is provided with 
ball 214, which is moveable within hole 210. 
Restraining member 194 is configured to fit within first and second cavity 
regions 202 and 204. Restraining member 194 includes bevelled edge 216, 
main body region 218 and pin 220. Main body region 218 is provided with 
rounded edge 222. In an alternative embodiment, rounded edge 222 is 
replaced by a radially tapered surface. When retaining member is inserted 
within first and second boring regions 202 and 204, main body 218 lies 
within first bore region 202 and pin 220 lies within second bore region 
204. When pin 220 is fully seated against second base 208 of second bore 
region 204, rounded edge 222 contacts balls 214 and exerts an outward 
force on balls 214, thereby causing balls 214 to extend beyond cylindrical 
surface 198. 
Torque plug 156 is configured to enclose blocking poppet assembly 154 
within valve housing 152 and to attach relief valve 150 to a pressure 
vessel. Torque plug 156 includes first end 224 and second end 226. First 
end 224 is provided with threaded exterior surface 228 for threading 
torque plug 156 within threaded opening 172 of valve housing 152. Second 
end 226 is provided with threaded exterior surface 230 for threading 
relief valve 150 into a threaded opening in a pressure vessel (shown in 
FIG. 5). Second end 224 has inlet 231, which exposes cylindrical passage 
232 to the interior of a pressure vessel (shown in FIG. 5). Passage 232 is 
exposed at first end 224 and is framed by annular notch 234 and annular 
shoulder 236. Annular notch 234 is concentric with passage 232 and has a 
diameter slightly larger than passage 232, which provides surface 238 
against which O-ring 240 rests. Shoulder 236 has a diameter greater than 
annular notch 234 and is concentric with annular notch 234. Annular 
shoulder 236 provides a flat metal surface against which blocking surface 
200 of blocking poppet 192 is sealed when the temperature near housing 152 
is below the predetermined temperature threshold. O-ring 240 also contacts 
blocking surface 200 to provide an impermeable seal across passage 232. 
O-ring 237 is positioned adjacent to mounting surface 239 to provide a 
fluid-tight seal when valve 150 is mounted to a pressure vessel. 
Thermal trigger 158 is similar to thermal trigger 18 as shown in FIGS. 1-3. 
Thermal trigger 158 includes first end 242 and second end 244. First end 
242 is provided with threaded exterior surface 246, which is sized to 
permit threading of thermal trigger 158 into threaded opening 188 of valve 
housing 152. Thermal trigger 158 has channel 248 extending generally along 
its longitudinal axis from first end 242 to second end 244. Shoulder 250 
divides channel 248 generally into first channel 252 and second channel 
254. 
Thermal trigger 158 is also provided with trigger ball 256 which is sized 
to permit insertion into first channel 252. Trigger ball 256 is large 
enough, however, so that it abuts shoulder 250 and cannot pass into second 
channel 254. First channel 252 is swaged at first end 242 to engage a 
diameter of trigger ball 256 and retain a greater portion of trigger ball 
256 within channel 252, while allowing another lesser portion to extend 
beyond first end 242. In the assembled state of thermal trigger 158, 
eutectic substance 258 fills channels 252 and 254, and holds trigger ball 
256 in a fixed position within channel 252 and partially extending beyond 
first end 242 of thermal trigger 158. 
Like thermal trigger 18 shown in FIGS. 1-3, thermal trigger 158 includes 
hexagonal exterior surface 260 to accommodate an appropriate size wrench 
during installation of thermal trigger 158 into valve housing 152. Thermal 
trigger 158 also includes concentric ribs 262 for rapid heat absorption 
and communication to eutectic substance 258 in channels 252 and 254. 
FIG. 5 is a longitudinal sectional view of the relief valve of FIG. 4 shown 
assembled and mounted to pressure vessel 270. Thermal trigger 158 is shown 
mounted to valve housing 152, with threaded exterior 246 engaging threaded 
opening 188 of valve housing 152. First end 242 of thermal trigger 158 is 
generally aligned with inner surface 176 of third cavity region 170. 
Trigger ball 256, which is partially extended beyond first end 242 of 
thermal trigger 158, lies within third cavity region 170 of valve housing 
152. 
Blocking poppet assembly 154 lies within cavity 164 of valve housing 152. 
Restraining member 194 is positioned within cavity 164 such that beveled 
edge 216 rests against trigger ball 256 when pin 220 and main body region 
218 are seated within first bore region 202 and second bore region 204, 
respectfully, with pin 220 contacting second base 208 of second bore 
region 204. In this "loaded" position, rounded edge 222 contacts balls 214 
and exerts an outward force on balls 214 which causes balls 214 to extend 
beyond cylindrical surface 198 and engage radially tapered surface 178 of 
second cavity region 168. With balls 214 engaging tapered surface 178, 
blocking poppet 192 is restricted from moving within cavity 164 toward end 
surface 180. Ample space exists between beveled edge 216 of restraining 
member 194 and end surface 180 of third cavity region 170 so as to permit 
blocking poppet assembly 154 to move toward end surface 180 should trigger 
ball 256 relocate within channel 252 due to temperatures exceeding a 
predetermined temperature threshold. 
Torque plug 156 is connected to valve housing 152 by engaging threaded 
exterior surface 228 within threaded opening 172 of valve housing 152. A 
rotational force is applied in a first direction to torque plug 156 to 
full seat blocking surface 200 against annular shoulder 236 and O-ring 
240, thereby forming seal 272. Seal 272 is a fluid tight seal which 
separates passage 232 from cavity 164 of valve housing 152. 
Relief valve 150 is connected to pressure vessel 270 by engaging threaded 
exterior surface 230 of torque plug 156 within threaded opening 274 of 
pressure vessel 270. A rotational force is applied to torque plug 156 in a 
second direction to seat mounting surface 239 and O-ring 237 against outer 
surface 276 of pressure vessel 270. In this assembled state, inlet 231 of 
torque plug 156 is exposed to interior 278 of pressure vessel 270. 
FIG. 6 is a longitudinal sectional view of the relief valve of FIG. 5 shown 
in the thermally triggered position. When thermal trigger 158 is exposed 
to temperatures that exceed a predetermined temperature threshold, 
eutectic substance 258 of channel 248 melts, thereby liberating trigger 
ball 256 from its fixed, semi-extended position within cavity 164 of valve 
housing 152. As eutectic substance 258 melts and trigger ball 256 becomes 
liberated within first chamber 252 of thermal trigger 158, restraining 
member 194 moves toward end surface 180 of third cavity region 170. As 
restraining member 194 moves towards end surface 180, pin 220 withdraws 
from second bore region 204, and rounded edge 222 moves away from balls 
214, thereby allowing balls 214 to disengage from tapered surface 178 of 
second cavity region 168 and retreat within holes 210 of blocking poppet 
192. With balls 214 disengaged from tapered surface 178, gas pressure from 
interior 278 of pressure vessel 270 exerts an outward force on blocking 
surface 200 of blocking poppet 192, which moves blocking poppet 192 
towards third cavity region 170. As blocking poppet 192 moves towards 
third cavity region 170, seal 272 is broken, thereby exposing passage 232 
to first cavity region 166. With first cavity region 166 exposed to 
passage 232, a right angle flow path is exposed from inlet 231, through 
passage 232 to first cavity region 166, and through exhaust passage 182 to 
outlet 186. 
It should be noted that like trigger ball 108 shown in FIGS. 1-3, trigger 
ball 256 acts as a check valve. In other words, since trigger ball 256 is 
too large to pass into second channel 254, it rests against shoulder 250 
to prevent any gas communication between the passage through valve 150 and 
the second end 244 of thermal trigger 158. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.