Receptacles – Closures – Floating closure
Reexamination Certificate
1998-11-23
2001-04-24
Cronin, Stephen K. (Department: 3727)
Receptacles
Closures
Floating closure
Reexamination Certificate
active
06220469
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the manufacture of an antistatic ball and its use as flotation in a cover of a tank holding flammable liquid.
Balls, particularly light weight balls of plastic, are employed in numerous situations ranging from children's toys to flotation in industrial equipment. Hollow balls, be they of thin walled metallic or plastic material, have excellent flotation characteristics. Plastic balls are preferred for flotation due to ease of manufacture and resistance to corrosion.
A situation of considerable interest in the use of flotation in the construction of tanks containing flammable or explosive liquid, such as an oil tank in a tank farm as found in the petrochemical industry. One form of construction of such a tank is the provision of a circular cylindrical sidewall open at the top, and being closed off by a cover which floats on liquid contained within the tank. The cover is constructed with pontoons which enable the cover to float. It is well known that such tanks may require maintenance after an extended period of use, particularly with respect to insuring the integrity of the flotation employed in the cover. Access ports, large enough to admit entry of a person, are provided at the top of each of the pontoons to an able the person to enter the pontoon to inspect the pontoon.
Generally, the cover is fabricated of metal and has a hollow chamber divided by walls into an array of pontoons to provide sufficient flotation to carry the weight of the cover plus additional weight, such as the weight of snow which might be on the cover. In older oil tank equipment, the cover was constructed of a metal plate with the pontoons mounted beneath the cover plate, while modem tanks have the pontoons located above the metal cover plate. Repairs to the cover may require welding equipment which can be used only after the tank has been taken out of service so as to insure that the cover is clean and that there are no flammable vapors present. If any flammable vapors be present during repair work on the cover, such as the repair of a pontoon of the cover, a spark from the welding may ignite an explosive burning of the vapor.
Repairs may be made also without taking the tank out of service. For example, one of the pontoons, may have sustained a relatively small opening through which liquid can seep resulting in a loss of buoyancy. By means of the access port a person may enter the pontoon and apply foamed urethane plastic as a liquid which later hardens to maintain buoyancy. Use of the plastic is not intended as a permanent repair because the plastic may become impregnated with the flammable liquid. Also, the plastic is disadvantageous because, at the conclusion of the service interval when reconditioning is mandatory, it is very difficult to remove the plastic so as to be able to clean the cover and make any permanent repairs. Clearly, welding cannot be employed for repair until all liquid and liquid soaked flotation, such as the foamed plastic, has been removed.
As an alternative procedure of repair, one might consider insertion in the pontoons of hollow non-foamed plastic bodies to provide sufficient buoyancy so that it is not necessary to repair the leak in the pontoon. However, the use of a plastic hollow body, such as a hollow ball, has been avoided in the petrochemical industry because such a plastic body is electrically insulating and susceptible to developing a static electric charge. There is a danger that the flotation body may suddenly discharge via a spark which can ignite an explosion.
SUMMARY OF THE INVENTION
The foregoing danger is overcome and other advantages are provided by the construction of a hollow plastic body, particularly a plastic ball, which has antistatic properties and, therefore, can be used in flotation of oil tank covers without danger of developing a static charge followed by subsequent hazardous discharge. The balls are constructed, in accordance with the invention, of a non-foamed high-density plastic resin, HDPE (high density polyethylene), mixed with an antistatic additive or agent. The antistatic agent is effective to convert the electrically insulating plastic to an electrically conductive material which does not develop a static electrical charge. In this sense, the antistatic plastic balls may be likened to metallic balls, but without danger of corrosion and with greater facility of manufacture. In a preferred embodiment of the invention, the balls are hollow and are formed by a process of blow molding. It is noted also that, if desired, the balls can be used for purposes other than flotation, including use by children as a toy.
In accordance with a feature of the invention, it is noted that the antistatic agent has a melting point, and the HDPE plastic has a melting point higher than the melting point of the antistatic agent. Mixing of the plastic resin with the antistatic agent produces a resultant plastic material wherein the melting point has been reduced from the melting point of the plastic. In the use of the invention in the construction of a tank cover of an oil-storage tank on a tank farm, elevated temperatures are experienced due to a heating of the tank by rays of the sun. The melting point of the HDPE plastic is well above environmental temperatures experienced on the tank farm so as to insure integrity of the flotation in the tank cover. However, use of an excessive amount of the antistatic agent in the mixture can produce a resultant plastic material having too low a melting temperature such that the integrity of the flotation can no longer be assured in the presence of intense heating of the tank by the sun's rays. It is observed in the practice of the invention that only a relatively small amount of the antistatic agent, less than 10 percent or even less than one percent by weight, is sufficient to provide the desired antistatic property to the flotation body or ball. Such a small fraction of the antistatic agent in the final plastic material has no more than a minimal effect on the melting temperature, thereby to retain the integrity of the flotation in the presence of the elevated temperature.
REFERENCES:
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patent: 3346138 (1967-10-01), Tubbs
patent: 3366266 (1968-01-01), Heartstedt
patent: 3401818 (1968-09-01), Hagen
patent: 3687329 (1972-08-01), Baum
patent: 3998204 (1976-12-01), Fuchs et al.
patent: 4329918 (1982-05-01), Kuhtreiber
patent: 5320887 (1994-06-01), Moss et al.
Cronin Stephen K.
Warren David M
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