Safety release pipe cap

A safety release pipe cap for emergency venting of fluids under excessive temperatures by melting out a replaceable portion. In one embodiment the replaceable portion is arranged to yield under excessive pressure differentials on its opposed faces. The cap is employed on tanks. A threaded ferrule is provided with a flange overlying the end of a pipe with which it is threadingly engaged to clamp the periphery of a separable disk of thermally fusible material against the pipe end. When the disk is of a yieldable plastic it functions as a seal gasket on the pipe end and will yield under pressure differentials to provide a visual indication of such differentials by material and wall thickness characteristics as well as the geometry of the disk. Temperature fusing can be controlled by these characteristics and particularly the melting temperature of the plastic.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to safety closures for containers and more 
particularly to closures which release under adverse conditions to vent 
the containers. Such release can be under conditions of excessive 
temperature in which a portion of the cap is melted to open the normally 
closed aperture to the container interior. In certain constructions the 
buildup of internal pressures is effective to open the vent passage or to 
distort the cap as an indication of the buildup. 
2. Description of the Prior Art 
Traditionally, storage tanks and the like have been fitted with a pipe on 
their upper surface which was sealed with a fusible metal cap. These caps 
made of metal were almost exclusively of brass, aluminum or malleable 
stainless. These caps were constructed in such a fashion as to breach a 
seal system by melting out a fusible link which sealingly retained a metal 
closure for the aperture in a metal ferrule upon the attainment of a 
certain predetermined temperature of the vessel in which the material is 
stored, and thus vent the warm high pressure volatile internal material to 
the ambient external atmosphere. This method circumvented the possibly 
explosive buildup of internal vapor pressure of these volatile and usually 
flammable materials. 
These metal cap safety devices presented a number of major practical 
disadvantages. First, should one of these metal caps be dropped 
inadvertently, there was a high probability that the impact would so 
deform the threaded areas that the cap would be functionally useless. 
Second, the extreme mass of such caps, up to six times the mass of the 
hereinafter described device, caused great inconvenience in transport with 
a working vehicle such as a tractor trailer rig. Third, these caps are 
often used in an environment rich in the vapors of the transported 
material as well as being exposed to all-weather conditions. This 
combination of corrosive forces often caused a substantial corrosion of 
the threads of the cap and its subsequent fusion or "locking" on the 
threads of the metal fitting of the storage unit. This caused major 
expense in the forced removal of the cap and the subsequent damage to the 
threads of the storage unit emergency vent pipe, which was permanently 
anchored to the storage unit. Fourth, the metal fittings generally had to 
be lubricated to facilitate easy operation of the cap threads on the pipe 
threads. In a normal use such lubricants as greases and oils are of no 
concern but in vessels containing highly purified chemicals these oils and 
greases may infiltrate the material being transported and substantially 
damage the quality and usefulness of the material. Individually, these 
problems are of import but when combined they form a substantial detriment 
to the safe economical use of these metal caps. 
Also, if a metal cap does maintain its functional integrity there are two 
further problems present. First, petro chemical transport and storage 
units are periodically cleaned on their interior surfaces with high 
pressure steam. The fusible caps are usually removed from the tank pipe 
and placed on the adjacent tank surface. As the steam cleans the interior 
of the tank it heats the tank. This elevation in temperature is 
transmitted from the tank surface to the cap fusing mechanism by the 
metallic body of the metal cap, which has a high thermal conductivity. 
Frequently the fuses are triggered in this fashion rendering them useless. 
These fuses must be shipped back to the factory for refusing which is not 
only time consuming, but also fairly expensive, when compared to the 
purchase price of an entire new fuse cap. 
An object of the present invention is to improve sealable safety vent 
release caps. 
Another object is to enable release caps to be reconstructed simply in the 
field. 
A third object is to eliminate the need for seal gaskets for safety release 
caps. 
A fourth object is to avoid accidental fusing of the fusible elements of 
safety release caps. 
A further object is to combine thermally fusible venting with pressure 
sensitive venting in a replaceable cap. 
SUMMARY OF THE INVENTION 
The above objectives of the invention may typically be achieved by a 
fusible pipe cap system formed of polymers to be used in the control of 
pressure in a volatile material storage unit and in association with the 
emergency vent system of said storage unit comprising a cap device having 
a threaded ferrule and a separable fuse insert to be fitted into the 
ferrule which can be replaced on site without the return to the factory of 
the entire unit. The fuse insert may be in the form of a disk of various 
polymeric materials, geometries and wall thicknesses to melt out of the 
ferrule at a desired venting temperature and/or distort at a desired 
venting pressure differential.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Referring to the drawings wherein like reference numerals designate similar 
parts throughout, there is illustrated a two part fusible cap device to 
regulate the maximum pressure level in a storage unit for volatile 
materials. As seen in FIGS. 1 and 2 the invention consists of a ferrule 1 
with an individually separable insert 2 which is composed of a mixed 
polymeric material typically consisting of a 80/20 percent by weight 
mixture of low density polyethylene and ethylvinylacetate, respectively. 
However, any suitable polymer mix or native copolymer may be used if such 
composition provides the necessary thermal and materials strength 
properties. The fuse insert or fusing disk composed of the 80/20 mixture 
above will have an average melting out temperature of 220.degree. F., but 
in no case will the melting out temperature exceed 250.degree. F. The 
ferrule 1 is composed of the material polypropylene and is individually 
separable from the fuse insert 2, and maintained therein by frictional 
engagement of its outer edge with the minor diameter of internal threads 
4. The polypropylene ferrule is mechanically and thermally stable to 
substantially higher temperatures than disk 2. 
Typically ferrule 1 has cylindrical sidewalls 3 internally threaded to 
accommodate a standard pipe thread as at 4. An internal flange 5 provides 
on its inner face 6 one seat for the peripherial face 7 of a fusible disk 
2 so that the periphery 7 is clamped between the face 6 and the end 8 of a 
vent pipe 9 upstanding from a closed container 11 as at its top. 
Application and removal of the ferrule 1 and thus the cap assembly is 
facilitated by radially extending lugs 12 which provide a suitable 
purchase to turn the threaded ferrule on and off the threaded vent pipe. 
In one embodiment a three inch fusible vent cap has a polypropylene 
ferrule having an outside diameter of three and seven eights inches, a 
internal depth of about one inch, a flange thickness of about one quarter 
inch and a flange face width of about three eights of an inch. 
Fusible disk 2 is arranged with an upstanding bead 15 which fits within the 
inner diameter limits of flange 5. The periphery 7 of the disk outside 
bead 15 is the portion which tends to yield either by melting in response 
to an elevated temperature or a combination of melting and distorting in 
response to a combination of temperature and pressure within the closed 
vessel 11. In the exemplary three inch fusible vent cap intended to 
rupture at 225.degree. F., the peripheral wall of the disk of 80/20 low 
density polyethylene and ethylvinylacetate is about one tenth of an inch 
thick and has an outer diameter which frictionally fits within the ferrule 
1 and is retained therein by friction. Such disks are sufficiently 
flexible to be introduced and removed from within the ferrule with finger 
pressure. Their wall thickness of the central portion 16, that inside the 
stiffening bead 15, is about one eighth inch. 
In practice a ferrule 1 and disk 2 require no gasketing to the vent pipe 
since the resilience and flexibility of the disk material enables it to be 
compressed as a gasket between face 6 of the ferrule internal flange 5 and 
the end 8 of the vent pipe 9. This enables the metal of the vent pipe to 
conduct heat in the container structure or its contents directly to the 
thinner walled portion of the fusible element, disk 2, in the region 
designed to melt out as the releasing mechanism. The melting of the 
periphery 7 can be augmented where the temperature is increased slowly in 
the vicinity of the melting temperature of the material of disk 2 by the 
softening of that material to enable the buildup of pressure within the 
vessel to cause the periphery to flow as the center portion baloons. (See 
FIG. 5) Thus, the periphery, even though clamped between its seats, 
innerface 6 and pipe end 8, will be withdrawn from between those seats 
when softened by elevated temperatures and subjected to the forces of the 
internal pressure. When the periphery is withdrawn the seal is ruptured 
and the container is vented. 
In some applications it is advantageous to avoid the cavity 17 within the 
center of the ferrule above the disk since that cavity tends to accumulate 
dirt and other debris and, in winter, ice and snow. Such accumulations can 
obscure the disk, can impede the pressure distortion of the disk center 16 
and, if material works into the space between the disk and face 6, may 
impair the seal of the cap. The construction shown in FIG. 3 mitigates 
against this accumulation of foreign matter by an extension of the center 
portion 21 of the fusible disk 22 through the open center 23 of the 
ferrule 24 to a location at least level with the outer face 25 of the 
ferrule flange 26. In this embodiment no chamfer as at 18 of FIG. 2 is 
provided on the ferrule flange inner diameter so that space between the 
ferrule and the disk center portion is minimized to avoid accumulations 
between those elements. 
The embodiment of FIGS. 4 and 5 illustrate another aspect of the fusible 
pipe cap assembly wherein the disk is also arranged as a pressure 
responsive fuse. A disk 31 having a center wall portion 32 of a thickness 
of the same general magnitude as its peripherial portion 33 is shown with 
a weakening cross score 34 in FIG. 4. That disk may be of either the 
generally planar form of FIGS. 1 and 2 or of the top hat form of FIG. 3 
where the center portion is maintained with a thin wall by having its 
internal contours follow the external contours (not shown). In the 
exemplary disk size, material, and thickness described with respect to 
FIGS. 1 and 2, the crossed score lines 34 on the outer face of disk 31 
extending about two inches across the diameter of the central wall portion 
16 of about two and one quarter inch diameter inside bead 15, when one 
sixteenth inch deep in a one eighth inch thick wall, will blow out by 
tearing along the scores at a pressure differential of from ten to fifteen 
pounds per square inch. It is to be appreciated that the venting pressure 
can be adjusted from the seventy pounds per square inch of an unscored 
disk of FIGS. 1 and 2 downward to less than that illustrated by choice of 
the length and form of the score line or lines, the disk thickness, and 
the score line depth. 
The effect of a pressure buildup within the container 11 is illustrated in 
FIG. 5. The center portion 32 of the disk 31 baloons outward prior to 
either pulling the peripheried flange 33 free of the clamping flange 5 and 
vent pipe end 8 or to rupturing the center, thereby providing a 
preliminary visual indication of the buildup prior to pressure relief. As 
can be seen at the score line 34, the disk 31 is weakened and tends to 
rupture along the line 34 when under pressure. 
The above described fusible cap system is particularly effective for fusing 
over-the-road tankers in accordance with ICC regulations. It does not 
become bonded in place by corrosion since the polypropylene of the ferrule 
is relatively inert. The high temperatures of steam cleaning do not 
adversely affect the thermally fusible disk when the cap assembly is 
placed on the tank since the polypropylene of the ferrule does not conduct 
sufficient heat to the disk to raise it to its melting temperature. The 
assembly is resistant to damage by impact. Since the disks are 
inexpensive, thin and lightweight, spare disks are carried conveniently 
and can be replaced manually. Therefore, there is less tendency to 
substitute non fusible caps on the vent pipes of truck tanks. 
Fusible cap assemblies according to this invention have been used on 
storage tanks and even drums. In the case of storage vessels subjected to 
high temperatures such as imposed by tropical sun the vent caps have been 
employed to avoid imposing destructive pressures on the vessels. 
It is to be appreciated that fusible cap assemblies of different materials, 
dimensions and geometries can be employed without departing from the 
spirit and scope of this invention. While threaded couplings have been 
shown, alternative forms of securing means for the cap assembly are 
contemplated.