Dual diaphragm tank with telltale drain

A fluid storage and expulsion system comprising a tank 12 with an internal flexible diaphragm assembly of dual diaphragms 17,18 in back-to-back relationship, at least one of which is provided with a patterned surface having fine edges such that the diaphragms are in contact along said edges without mating contact of surface areas to thereby form fluid channels 27 which extend outwardly to the peripheral edges of the diaphragms. The interior wall of the tank at the juncture of tank sections 14,15 is formed with a circumferential annular recess comprising an outer annular recess portion which forms a fluid collection chamber 40 and an inner annular recess portion which accommodates the peripheral edge portions of the diaphragms and a sealing ring 38 in clamped sealing relation therebetween. The sealing ring is perforated with radially extending passages 39 which allow any fluid leaking or diffusing past a diaphragm to flow through the fluid channels 27 between the diaphragms to the fluid collection chamber. Ports 21,23, connectable to pressure fittings are provided in the tank sections for admission of fluids to opposite sides of the diaphragm assembly. A drain passage 42 through the tank wall to the fluid collection chamber permits detection, analysis and removal of fluids in the collection chamber.

FIELD OF THE INVENTION 
This invention relates to fluid storage and expulsion systems and more 
particularly to a fluid storage system wherein fluids stored in a tank are 
separated therein by dual diaphragms arranged in back-to-back relation and 
having patterned surfaces defining fluid channels for allowing fluid 
leaking or diffusing through said diaphragms to migrate to a collection 
chamber provided with a telltale drain. 
BACKGROUND OF THE INVENTION 
Fluid storage tanks which are provided with an elastic diaphragm therein 
for the isolated storage of two fluids are well known and have particular 
application in the aerospace industry where they have been used for the 
storage and expulsion of a fluid as necessary. Most typically, the tanks 
are of spherical shape or cylindrical with hemispheric ends and are 
provided with a single elastic diaphragm fixed within the tank at or near 
a diametric plane of the tank. The diaphragm may serve to separate a 
liquid stored in the tank on one side of the diaphragm from a pressurant 
gas contained on the other side of the tank, although it can be used to 
separate liquids or gases. When additional pressurant gas is supplied to 
the tank through an appropriate inlet port, the diaphragm is moved to 
force fluid on the other side of the diaphragm out of the tank through an 
appropriate port provided therefor. Other types of fluid storage and 
expulsion systems include a storage tank with a dual seal piston and 
cylinder arrangement with a seal-leakage drainage port located between the 
independent seals. 
The prior art systems have many disadvantages. Among the disadvantages of 
the single-diaphragm tank is the diffusion of one fluid into the other 
through the diaphragm material. Where gases are used to pressurize liquids 
in the single-diaphragm tanks, gas diffuses through the diaphragm material 
and saturates the liquid. Also, the liquid can gradually diffuse through 
the diaphragm to displace the volume reserved for pressurant gas. 
Another disadvantage of a single diaphragm tank is that one failure of the 
diaphragm allows the direct mixing of the fluids which were to be 
maintained separate. A further disadvantage associated with spacecraft 
tanks is the considerable time and effort which must be spent during 
certification of the tanks for reuse while verifying diaphragm integrity 
with pressure drop tests. With regard to the piston type accumulators, 
seal leakage frequently results from piston seal erosion. The gas seals 
ride on dry cylinder walls and fail relatively quickly. 
U.S. Pat. No. 4,784,181 discloses an expansion tank with a two part 
bladder-type diaphragm wherein liquid is stored in the bladder and 
pressurized gas is contained in the space between the diaphragm and the 
housing. 
U.S. Pat. No. 4,826,045 discloses a fluid storage and expulsion system 
designed for aerospace applications which utilizes a single diaphragm with 
reinforcement rings and guide members for controlled collapsing movement 
of the diaphragm. 
U.S. Pat. No. 4,796,676 discloses a storage tank with a secondary 
containment bladder. When a vacuum pressure is applied to the tank to mate 
the tank and bladder, an embossed surface of the bladder provides 
communication channels for air evacuation during the vacuum phase to 
reduce buckling and improve the seal between tank and bladder. 
It is to be noted that all of the above are basically single-diaphragm 
tanks and have their attendant disadvantages. 
U.S. Pat. No. 4,777,982 discloses a containment vessel with a single 
diaphragm separating wall composed of two outer layers and a porous middle 
layer. A leak or diffusion through the outer layers to the inner porous 
layer is communicated through a porous ring to an outlet. The three layers 
of the diaphragm limits its flexibility and the use of a clamping ring for 
connecting the separating wall to the container restricts its usage to low 
pressure applications. 
STATEMENT OF THE INVENTION 
The invention is a fluid storage and expulsion system comprising a storage 
tank formed of two tank portions which are joined along mating edge 
surfaces in fluid tight relation. The system includes an internal flexible 
diaphragm assembly which is joined in sealing relation to the tank along 
the mating edge surfaces of the two tank portions and serves as a 
separating wall to two fluids introduced to the tanks through ports in 
said tank portions. The diaphragm assembly comprises dual diaphragms in 
back-to-back relationship, at least one of which is provided with a 
patterned surface having fine edges such that the diaphragms are in 
contact at points along said edges without contact of surface areas 
therebetween and form fluid channels which extend in an outward direction 
to the peripheral edges of the diaphragms. Each tank portion is provided 
with a port for accommodating a fitting whereby a first fluid may be 
introduced to the tank on one side of the diaphragm assembly and a second 
fluid introduced to the tank on the other side of the diaphragm assembly. 
Each tank portion is also provided with an annular groove formed in the 
mating edge surface thereof to open inwardly of the tank and form with the 
annular groove of the other tank portion an annular recess comprising an 
outer annular portion which forms a fluid collection chamber and an inner 
annular portion which accommodates the peripheral edge portions of said 
diaphragms and a sealing ring in clamped sealing relation therebetween. 
The sealing ring is provided with a plurality of radially extending 
passages whereby when the tank portions are joined together, the radial 
passages allow any fluid which may have diffused or leaked past a 
diaphragm to flow outwardly through the fluid channels between the 
diaphragms to the fluid collection chamber. The collection chamber is 
provided with a drain passage and a fitting at the outer end of the drain 
passage for accommodating a removable cap whereby the presence in the 
collection cavity of the fluids stored in the tank can easily be 
determined by inspection and would indicate a diaphragm failure. A mixture 
of fluids would indicate failures of both diaphragms. Any fluids which 
have leaked or diffused through the diaphragm material can be easily 
drained away from a tank in service thereby preventing saturation of 
liquids with the pressurant gas and the displacement of gas volume by 
liquid diffusion.

DETAILED DESCRIPTION OF THE INVENTION 
Referring more particularly to the drawings, there is shown in FIG. 1 a 
fluid storage and expulsion system 10 which comprises a spherical tank 12 
formed of two hemispherical sections 14,15 joined together by an annular 
weld 13 at their junction in a diametral plane. The system includes a 
flexible diaphragm assembly 16 of dual flexible diaphragms 17,18 placed in 
back-to-back relation and joined about their peripheral edge portions to 
the inner wall of the tank by means of a sealing connection to be 
hereinafter described. The upper hemispherical tank section 14 is provided 
with a pressure port 21, which is adapted to be provided with a fitting 
whereby a fluid, such as a pressurant gas, may be delivered to the tank 
and contained therein on one side of the flexible diaphragm assembly 16. 
The lower hemispherical tank section 15 is similarly provided with a port 
23 which is adapted to be provided with a fitting whereby a working fluid, 
such as a liquid fuel, may be delivered to the tank and contained therein 
on the other side of the flexible diaphragm assembly 16. Both ports have 
perforated baffles 22 made to prevent extrusion of diaphragms through the 
port holes during high differential pressures. 
The tank 12 may be formed of any suitable material in accordance with 
intended use. For many aerospace applications, a high strength steel would 
be appropriate. The diaphragms may be constructed from a wide variety of 
flexible materials, such as a synthetic rubber or other elastomer, so long 
as it is compatible with the fluids with which it will be used. The areal 
extent of the diaphragms should be such as would conform to the interior 
surface of a hemispherical tank section, as when the tank is filled with a 
pressurant fluid and the working fluid has been completely eliminated. As 
shown in FIG. 1, the tank is approximately half-filled with a pressurant 
gas and the remainder of the tank is filled with a working fluid such as a 
liquid fuel. Because the areal extent of the diaphragms assembly exceeds 
the diametral cross section of the tank, folds are present in the 
diaphragm assembly for any position thereof which is intermediate the 
positions wherein the diaphragm assembly is in conforming engagement with 
either of the walls of the hemispherical tank sections. 
The diaphragm 17, shown in FIGS. 2 and 5, is provided with a patterned 
surface comprising a plurality of ridges 25, each in spaced collinear 
segments with fine edges. The ridges 25 are shown in parallel array, but 
it is to be understood they could be random. The diaphragm 18, shown in 
FIG. 5 is placed in facing contact with the diaphragm 17 and contacts the 
diaphragm 17 along the edges of ridges 25 without mating contact of 
surface areas such that the diaphragms and ridges 25 define a plurality of 
fluid channels 27 which extend outwardly to the peripheral edges of the 
diaphragms. The diaphragm 18 may also be provided with a patterned surface 
for contact with ridges 25 of diaphragm 17 or it may be a planar surface. 
If a patterned surface is provided, it is to be such that the two 
diaphragms contact only along edges whereby there is no surface areal 
contact which could establish seals therebetween. Other patterned surfaces 
which could be provided the diaphragms 17, 18 are shown in cross section 
in FIGS. 6 and 7 wherein uniformly spaced ridges 24 and grooves 26 may be 
provided as shown in FIG. 6 or a plurality of adjacent ridges forming a 
saw tooth configuration 29 of ridges and grooves as shown in FIG. 7. It is 
important, however, that whatever design is chosen, the fluid channels 27 
formed between the diaphragms extend outwardly to the edges of the 
diaphragm whether in radial or non-radial fashion and that there is no 
mating contact between the diaphragms as would establish sealed areas. 
As shown in FIG. 3, the two tank sections 14, 15 are joined by welding 
along their diametral edge surfaces 31, 32, as indicated by the annular 
weld 13. The tank sections could, however, be clamped together with 
appropriate bolts and clamp flanges, as necessary. Each of the tank 
sections' edge surfaces is formed with an annular groove 34 which opens 
toward the interior of the tank and when the tank sections are joined 
together, forms an annular recess which extends circumferentially around 
the tank interior at the junction of the tank sections. The annular recess 
is comprised of an inner annular portion which is designed to accommodate 
the peripheral edge margins of the diaphragms in a fluid-tight sealing 
relation therewith when a sealing ring 38 of circular radial cross section 
is disposed therebetween. For this purpose, the walls of the tank edges 
which define the inner portion of the annular recess are arcuately grooved 
in radial cross section so as to grip and retain the diaphragm edge 
portions and sealing ring 38. The outer portion of the annular recess 
forms an annular fluid collection chamber 40. 
The sealing ring 38 should be formed of the same structural steel as the 
tank wall to ensure uniform thermal expansion characteristics. During 
assembly of the system 10 and joining of the tank sections 14, 15 
sufficient clamping pressure is applied between the tank sections so that 
a tight seal is established between the tank sections and the diaphragms. 
Although the sealing ring 38 establishes seals with the diaphragms 17, 18 
it is perforated by a plurality of radially extending passages 39 which 
establish fluid communication between the fluid channels 27 and the fluid 
collection chamber 40. 
Referring to FIG. 3, it is to be seen that a drain passage 42 extends from 
the exterior of the tank to the fluid collection chamber 40 and is 
provided with a removable threaded cap 44. It is also to be seen that the 
patterned surfaces of the diaphragms, whether on only one or both 
diaphragms, allows leakage or diffusion of fluids through either diaphragm 
to migrate and flow outwardly through the channels 27 to the low pressure 
collection chamber 40 which extends circumferentially about the sealing 
element 38. The drain 42 allows an inspection of the chamber 40 to detect 
the presence of the tank fluids which could indicate a failure, such as a 
tear in the material, of one or both of the diaphragms 17, 18. A fluid 
mixing would indicate at least two failures, and at least one in each 
diaphragm. The diffusion of fluids through the diaphragm material can 
therefore be easily drained away from a tank when in service, thereby 
preventing saturation of a liquid tank fluid with a pressurant gas or the 
displacement of gas volume with liquid diffusion. 
A modified form of sealing ring, which may be more suitable for 
particularly high pressure applications, is shown in cross section in FIG. 
4. The sealing ring 46, shown therein, is provided with angular surfaces 
on either side which clamp the diaphragms 17, 18 to similarly configured 
surfaces provided on the edge surfaces 31, 32 of the tank sections. 
It is therefore to be appreciated that a new and improved fluid storage 
tank and expulsion system is disclosed herein which uses a dual diaphragm 
arrangement of flexible back-to-back diaphragms for separating one fluid 
in the tank from another fluid in the tank. The two diaphragms have 
nonmating surfaces, at least one of which is provided with a pattern of 
ridges and/or grooves so as to define fluid channels therebetween without 
any sealed surfaces as would restrict flow therebetween. The pattern may 
be random or non-random so long as the channels extend to the edges of the 
diaphragms whereby any leakage fluid will flow to the collection chamber, 
which may then be inspected or drained as appropriate. 
It is also to be understood that the foregoing description of a preferred 
embodiment of the invention has been presented for purposes of 
illustration and explanation and is not intended to limit the invention to 
the precise form disclosed. For example, the tank may have a different 
shape than described and modified forms of sealing elements may be 
appropriate for different applications. It is to be appreciated therefore, 
that various materials and structural changes may be made by those skilled 
in the art without departing from the spirit of the invention.