Shutoff valve for cryogenic liquid storage tank

There is disclosed an improved internal pressure relief valve for filling a cryogenic liquid storage tank. The internal pressure relief valve consists of a housing with a sealing seat at one end, an inlet hole at the other end, and a ball enclosed therein. The internal pressure relief valve is constructed of materials which can withstand the heat encountered during fabrication without melting. Consequently, the material for the ball is more dense than the cryogenic liquid in the tank so that the ball will not float in the cryogenic liquid. Consequently, the ball and the housing are dimensioned so that the momentum of the cryogenic liquid as it flows into the housing toward a vent port during the filling operation is sufficient to drive the ball into engagement with the sealing seat, closing the vent port, and assuring the termination of the filling process when the pressure in the cryogenic tank builds up to that of the delivery pressure.

BACKGROUND OF THE INVENTION 
This invention relates generally to a cryogenic liquid storage tank, and 
more particularly concerns a cryogenic liquid storage tank having means 
for relieving the internal head pressure during filling in order to fill 
completely the cryogenic tank with cryogenic liquid. 
Gases, having low boiling points at atmospheric pressures, such as carbon 
dioxide (CO.sub.2) and oxygen (O.sub.2), for example, present many 
difficulties and problems not encountered handling ordinary gases. In 
order to provide CO.sub.2 gas for use in fast food restaurants for 
carbonating soft drinks, for example, it has been necessary in the past to 
provide the compressed CO.sub.2 in single or clustered high pressure 
containers which are really best suited for customers with low consumption 
or sporadic use. Such service is expensive because of the high cost of 
handling the necessary heavy containers the weight of which is very high 
in comparison to the weight of the compressed gas contained therein. 
Customers having high or moderately high demands for O.sub.2 or CO.sub.2 
gas have also been serviced by means of high pressure tubular receivers 
installed on their premises. Such receivers are periodically serviced by 
means of a pump equipped liquid tank truck which transports the material 
to the customer's premises in cryogenic liquid form and charges such 
receivers with high pressure gas drawn from the vaporized cryogenic liquid 
in the tank truck. These tank trucks must be specifically equipped for 
this service and represent a large capital expense. Furthermore, the 
delivery of gas is time consuming because of the limiting capacity of the 
portable high pressure pumps. 
Another way of storing such low boiling point gases, such as O.sub.2 and 
CO.sub.2, is to store them in a cryogenic storage tank in liquid form on 
the user's premises. Such a cryogenic liquid storage tank includes an 
inner vessel which holds the cryogenic liquid and an outer vessel within 
which the inner vessel is supported. There is an insulating space between 
the inner and outer vessels in which a vacuum is drawn and insulating 
material is positioned. Because of the low heat transfer from the ambient 
atmosphere outside of the outer vessel to the contents of the inner 
vessel, the liquid O.sub.2 or CO.sub.2 can remain in liquid form for some 
period of time before heat vaporization causes the vapor pressure of the 
O.sub.2 or CO.sub.2 to exceed a maximum pressure and to activate a 
regulator system for maintaining the vapor pressure within a safe range. 
When such a cryogenic tank is installed on a customer's premises, such as a 
CO.sub.2 tank at a fast food restaurant, it is necessary periodically to 
refill the cryogenic tank with liquid CO.sub.2. The cryogenic CO.sub.2 
tank is filled by means of a delivery truck carrying CO.sub.2 cryogenic 
liquid which makes its rounds from one customer to the next. In order to 
achieve the greatest efficiency, it is important to be able to fill the 
customer's tank as nearly full as possible without resorting to 
sophisticated high pressure pumps and/or regulator systems. 
One way of filling a cryogenic tank on the customer's premises is to attach 
a single hose from a cryogenic tank on a transport truck to the inlet of 
the customer's cryogenic liquid storage tank. The vapor pressure in the 
transport truck's tank forces the liquid from the transport tank into the 
cryogenic tank on the customer's premises. As the liquid flows into the 
customer's cryogenic tank, the increase volume of liquid in the customer's 
cryogenic tank compresses the vapor above the liquid into a smaller and 
smaller space until the vapor pressure in the customer's tank exactly 
equals the vapor pressure in the transport tank. At that point, transfer 
from the transport tank to the customer's tank ceases even though the 
customer's tank may only be partially full. 
In order to relieve the vapor pressure in the customer's cryogenic tank, 
the prior art suggests various ways of liquifying the CO.sub.2 vapor in 
the top of the customer's tank by means of eductors, J-shaped bubbler 
tubes, or J-shaped sprinkler tubes, all of which are shown in Remes et 
al., Ser. No. 448,729, filed Dec. 10, 1982 now abandoned. 
Another way of assuring complete filling of the customer's cryogenic tank 
is disclosed in the applicant's prior patent, Gustafson U.S. Pat. No. 
4,625,753, in which an automatic pressure relief means vents the CO.sub.2 
vapor while the tank is being filled so that the tank may be completely 
filled. The automatic pressure relief means includes a cylindrical housing 
which is connected to the lower end of a vent tube. The housing has 
perforations to allow entry of cryogenic liquid. A buoyant float ball is 
enclosed within the housing. As the cryogenic tank is filled, the buoyant 
float ball floats upwardly into contact with an O-ring seat at the bottom 
of the vent tube thereby causing the pressure relief means automatically 
to close the vent tube. In an alternative embodiment of that invention, 
the automatic pressure relief means is combined with an eductor attached 
to the tank's inlet. The eductor entrains and condenses vapor within the 
tank to minimize the amount of vapor vented by the automatic pressure 
relief means. 
While the automatic pressure relief means disclosed and claimed in 
Gustafson U.S. Pat. No. 4,625,753 performs the function of venting the 
tank so that the tank may be filled completely, the automatic pressure 
relief means uses a float ball which floats on the cryogenic liquid to 
close off the vent port once the cryogenic liquid has reached the vent 
tube at the top of the tank. In order to float in liquid CO.sub.2 or 
O.sub.2, the material for float ball is general confined to a plastic 
material. The use of a plastic material for the float ball limits the 
temperature in the tank the melting point of the plastic. During the 
manufacture of the vessel, the necessary welding operations cause the 
inside temperatures in the tank to exceed the melting points of available 
plastics. Consequently, the automatic pressure relief means with its 
plastic float ball can only be positioned inside the inner vessel after 
the inner vessel has cooled. Consequently, it is necessary that the 
automatic pressure relief means with its plastic float ball be attached to 
the outlet pipe by means of a threaded connector which can be installed 
after the welding operations have ended and the tank has had an 
opportunity to cool. Such an assembly constraint increases assembly time 
and results in a threaded connection that may be subject to failure. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an automatic 
pressure relief means for a cryogenic liquid storage tank which does not 
employ materials that will melt at the temperatures encountered during the 
fabrication process of the cryogenic tank. 
In order to achieve the foregoing objective, the automatic pressure relief 
means includes a cylindrical housing attached to a vent tube having a seat 
at one end. The other end of the vent tube is attached to a safety 
regulator valve to vent gas to the atmosphere. The cylindrical housing is 
closed except for a hole at its lower end. A steel ball, which has a 
diameter greater than the hole in the lower end of the housing, is 
contained within the housing below the seat. As the cryogenic tank is 
filled, the steel ball first responds to the momentum of the gas escaping 
through the vent tube and is lifted toward the seat. When the cryogenic 
liquid reaches the steel ball, the addition momentum of the liquid with 
its greater mass drives the steel ball into engagement with the seat and 
substantial halts the escape of gas through the vent tube. Once the steel 
ball has engaged the seat, the difference in pressure between the gas in 
the tank and the setting of the safety regulator valve on the vent tube 
holds the steel ball in place against the seat until the filling operation 
is complete, and the pressure in the tank has decayed to the operating 
pressure of the tank. Although the steel ball does not float in the 
cryogenic liquid, the momentum of the liquid against the steel ball in the 
enclosed housing is sufficient to drive the steel ball against the seat 
and substantially seal off the vent tube. 
Other objects and advantages of the invention will become apparent upon 
reading the following detailed description and upon reference to the 
drawings.

DETAILED DESCRIPTON OF THE INVENTION 
While the invention will be described in connection with the preferred 
embodiment, it will be understood that I do not intend to limit the 
invention to that embodiment. On the contrary, I intend to cover all 
alternatives, modifications, and equivalents as may included within the 
spirit and the scope of the invention as defined by the appended claims. 
Turning to FIG. 1, there is shown a cryogenic tank 10 having an outer 
vessel 12 and an inner vessel 14. The inner vessel is suspended within the 
outer vessel by means of a neck 16 and a base support 18. An insulating 
space 20 located between the inner vessel and the outer vessel is 
evacuated to create a vacuum and is insulated thereby minimizing the 
amount of heat transfer from the ambient atmosphere outside of the tank 10 
to the contents of the inner vessel 14. The inner vessel 14 contains 
liquified gas, such as CO.sub.2, in the liquid phase 22 with a vapor phase 
32 disposed above the liquid 22. 
The neck 16 provides a sealable port from outside of the tank 10 to the 
inside of inner vessel 14. An inlet/outlet pipe 24 for filling and 
emptying vessel 14 extends through the neck 16 and has an inlet/outlet 
port 26 at the top of the tank 10. The pipe 24 also extends nearly to the 
bottom of the inner vessel. A self-closing coupling 27 and an outlet 
pressure regulator 29 are connected to the inlet/outlet port 26 by means 
of pipe 31. 
A pressure relief means 25 includes a vent tube 34 which extends through 
the neck 16 and which has an exhaust port 28 at its top end and an 
internal pressure relief valve 30 at its lower end. The internal pressure 
relief valve 30 only extends a short distance into the vapor space at the 
top of the inner vessel 14. A safety pressure relief valve 33 is connected 
to exhaust port 28 and has a pressure set point above the operating 
pressure (emptying pressure) of the tank and below the higher delivery 
pressure (filling pressure) of the tank. 
Turning to FIG. 2, the internal pressure relief valve 30 includes a 
cylindrical housing 36 which is connected to the bottom end 37 of the vent 
tube 34. The housing 36 is enclosed except that its lower end 38 has a 
hole 40 therein to allow entry of gas 32 and liquid 22. Near the top of 
the housing 36 there is provided a tapered seat 44 which surrounds an 
opening 39 leading to the vent tube 34. Enclosed within the housing 36 is 
a ball 42 which is formed of steel for example. Other materials may be 
used as long as they have sufficiently high melting temperatures so that 
the ball can remain within the housing 36 while the tank is being welding 
during fabrication. Materials that can withstand the heat of fabrication 
are too dense to float in the cryogenic liquid in the inner vessel 14. 
Consequently one cannot rely on the buoyancy of the ball 42 to force the 
ball into contact with the tapered seat and close the internal pressure 
relief valve 30. I have discovered that the momentum of the cryogenic 
liquid flowing into the housing 36 can be used to propel the ball 42 into 
engagement with the tapered seat 44 and substantially close the internal 
pressure relief valve 30. 
In order to fill the tank 10 (FIG. 1), a single delivery hose 41 from a 
transport tank (not shown) is connected to the inlet/outlet port 26 by 
means of the coupling 27 and pipe 31. The vapor pressure in the transport 
tank causes the cryogenic liquid in the transport tank to flow through the 
hose 41, through coupling 27, through pipe 31, through pipe 24, and into 
the inner vessel 14. As the cryogenic fluid 22 rises in the inner vessel 
14, the vapor pressure increases until it exceeds the set point of safety 
pressure relief valve 33. Once the set point of safety pressure relief 
valve 33 is exceeded, the vapor 32 escapes (as shown by arrows 35 in FIG. 
2) through the internal pressure relief valve 30, the vent tube 34, the 
exhaust port 28, and the safety pressure relief valve 33, which has its 
set point below the pressure of the transport tank. Consequently, the 
vapor 32 is vented to the atmosphere instead of being compressed above the 
liquid 22 and creating back pressure sufficient to counteract the vapor 
pressure in the transport tank. As the vapor 32 escapes through the vent 
tube 34, it necessarily passes through the hole 40 and the housing 36. As 
the gas 32 passes through the housing 36, the momentum of the escaping gas 
causes the steel ball 42 to rise within the housing 46 toward the tapered 
seat 44. The steel ball and housing are dimensioned such that the momentum 
of the gas is not sufficient to drive the steel ball into engagement with 
the tapered seat 44. Once the liquid CO.sub.2 22 rises to the opening 40 
of the enclosed housing 36, liquid is forced through hole 40 and into the 
enclosed housing 36. Because the liquid CO.sub.2 has a greater mass and 
therefore greater momentum than the gas 32, the momentum of the liquid 
flowing into the housing 36 is sufficient to drive the steel ball 42 into 
engagement with the tapered seat 44. 
To assure proper operation of the internal pressure relief valve 30, the 
dimensioning of the steel ball and the enclosed housing 36 is critical. 
Particularly, in a preferred embodiment of the present invention in which 
a steel ball is used, I have found that a 1/2 inch steel ball performs 
satisfactorily when the inside diameter of the housing 36 is 5/8 inch. 
Moreover, for a steel ball with a diameter of 1/2 inch, the inside 
diameter of the housing must be 5/8 inch plus or minus 1/32 inch. If the 
inside diameter of the housing 36 is too small, the momentum of the gas 
will be sufficient to drive the steel ball into engagement with the 
tapered seat 44 prematurely closing the internal pressure relief valve 30 
and preventing the tank from being completely filled. If, on the other 
hand, the inside diameter of the housing 36 is too large, the momentum of 
the liquid 22 will not be sufficient to drive the steel ball 42 into 
engagement with the tapered seat 44, and the internal pressure relief 
valve 30 will not close allowing escape of liquid as the tank continues to 
fill. 
If another heat resistant material other than steel is used for the ball 
42, such as a lighter weight heat resistant material, the tolerances on 
the inside diameter should be less critical. Likewise, using a tapered 
housing which has a greater diameter at the top and a smaller diameter at 
the bottom (much like the tapered seat 44) should likewise provide for 
less critical dimensional tolerances between the inside diameter of the 
housing and the diameter of the ball. 
Once the float ball 42 engages the tapered seat 44, further escape of 
liquid 22 or vapor 32 from within the inner vessel 14 is substantially 
inhibited although the metal to metal contact between the ball and the 
tapered seat will not provide a complete seal. In the context of the 
present invention the tapered seat 44 which substantially inhibits the 
escape of liquid and gas is considered a sealing seat. The use of a 
sealing gasket such as the O-ring employed by the prior art has been 
eliminated because such sealing gaskets are generally made of materials 
that cannot stand the heat of manufacture or if capable of withstanding 
such heat, are expensive. Moreover, the seal created by the ball and 
tapered leaks only a small amount of gas and only for the time that the 
vapor pressure exceed the set point of the safety pressure relief valve. 
Even after the valve 30 has been substantially closed by means of the steel 
ball 42, cryogenic fluid will continue to flow into the tank until the 
vapor pressure in the vapor space in the inner vessel 14 equals that of 
the vapor pressure in the transport tank. That further increase in the 
level of the liquid in the tank 10 increases the internal vapor pressure 
inside the inner vessel 14, and the resulting pressure differential 
between the inside of the inner vessel 14 and the set point of the safety 
pressure relief valve 33 insures that the steel ball is securely seated 
against the tapered seat 44 to insure against further significant venting 
of vapor or escape of liquid during the filling process. Once the 
cryogenic tank 10 has been filled, the transport tank hose 41 is uncoupled 
from self-closing coupling 27. 
As the gas 32 is subsequently withdrawn from vessel 14 through regulator 
29, the pressure of the vapor 32 will be reduced below the set point of 
safety pressure relief valve 33, and the steel ball 42 will drop from 
engagemnet with the tapered seat 44. With the steel ball 42 disengaged 
from tapered seat 44, the tank 10 is ready for the next filling operation. 
In filling tank 10 in FIG. 1, the small amount of CO.sub.2 vapor and liquid 
that escapes during the filling operation through port 28 is less 
economically important than the cost of providing a skilled transport 
operator and/or sophisticated pumping and venting apparatus. Also, the 
cost of the vented CO.sub.2 vapor and liquid is small when compared to the 
cost of additional delivery visits that would result if the tank 10 is 
only partially filled.