SPECIFICATIONA storage system for a volatile liquidThis invention relates to a system for storing a volatile liquid, e.g. petrol or gasoline. In particular, but not exclusively, the invention relates to a system for storing petrol in an underground storage tank at a petrol service station.
At present it is normal practice at a petrol service station to store petrol in an underground storage tank provided with a vent pipe terminating approximately 5 m above ground level. Vapour from the petrol in the storage tank mixes with the air in the storage tank and the air/vapour mixture vents directly into the atmosphere through the vent pipe. This loss of petrol vapour via the vent pipe is undesirable both in terms of cost and in terms of pollution of the atmosphere.
The present invention seeks to provide a storage system for a volatile liquid in which evaporation losses are minimised.
According to the present invention there is provided a system for storing a volatile liquid comprising a storage tank for a volatile liquid provided with volatile liquid supply and withdrawal means, a reservoir for storing under pressure vapour derived from the volatile liquid in the tank, first valved conduit means connected between the storage tank and the reservoir for withdrawing said vapour from the storage tank and supplying it under pressure to said reservoir during supply of volatile liquid to the storage tank, second valved conduit means connected between the reservoir and the storage tank for returning said vapour from the reservoir, via expansion means, to the storage tank during withdrawal of volatile liquid from the storage tank, and control means for controlling the flow of said vapour in the system during supply of volatile liquid to, and withdrawal of volatile liquid from, the storage tank.
Suitably the vapour is conveyed round the system with the assistance of a suitable carrier gas (e.g. air, nitrogen or carbon dioxide). Fire risk is reduced to a minimum of the carrier gas comprises nitrogen or carbon dioxide.
Conveniently the first valved conduit means includes cooling means and a transfer pump for pumping vapour through the cooling means and into the reservoir for storage under pressure.
Typically the system includes a cooling circuit for cooling the vapour contained in the storage tank. Suitably the cooling circuit comprises third valved conduit means, including a circulating pump (preferably thermostaticallycontrolled), connected between the storage tank and the said expansion means, and further cooling means forming part of the second valved conduit means and situated between the expansion means and the storage tank.
The invention will now be described by way of example with reference to the accompanying drawing the sole Figure of which is a schematic representation of a system for storing petrol at a petrol service station.
The drawing shows an underground storage tank 1 for petrol having connected thereto a supply pipe (not shown) for supplying petrol, e.g. from a petrol tanker, to the tank and a withdrawal pipe (not shown) for supplying one or more petrol pumps (not shown). The supply and withdrawal pipes are of a type which are conventional for known underground storage tanks.
The storage tank 1 is not fitted with a conventional vent pipe but instead is provided with a petrol vapour circulating system consisting of a closed main circuit and a closed auxiliary circuit. In the drawing the arrows indicate the direction of circulation of petrol vapour in the system.
The main circuit of the circulating system comprises a transfer pump 2, a cooler 3, a reservoir 4 for storing vapour under pressure, an expansion tank 5 and a further cooler 6.
The storage tank 1 is connected to the reservoir 4 by a conduit 7 connecting the tank 1 to the pump 2, a conduit 8 connecting the pump 2 to the cooler 3 and a conduit 9 connecting the cooler 3 to the reservoir 4.
The return circuit back from the reservoir 4 to the storage tank 1 is provided by a conduit 10 connecting the reservoir 4 to the expansion tank 5, a conduit 11 connecting the expansion tank 5 to the cooler 6 and a conduit 12 connecting the cooler 6 to the storage tank 1. A pressure regulating valve 1 3 is provided in the conduit 10 adjacent the inlet of the expansion tank 5 and a pressure reducing valve 14 is provided in the conduit 11 adjacent the outlet of the expansion tank 5. Stop valves 15, 16, 17 and 18 are provided in the conduits 7, 9, 10 and 12, respectively, the stop valve 1 7 being located between the reservoir 4 and the pressure regulating valve 1 3.
The auxiliary circuit of the circulating system comprises the branch of the main circuit provided by the conduit 11, the cooler 6 and the conduit 12 and a further branch consisting of a circulating pump 19, a conduit connecting the storage tank 1 to the pump 1 9 and a conduit 21 connecting the pump 19 to the expansion tank 5 and provided with a stop valve 22. The motor of the pump 1 9 is controlled via a thermostat 23 mounted in the storage tank 1 for sensing the temperature of vapours contained therein.
In operation the empty storage tank 1 is first filled with petrol. As petrol enters the tank 1 through the supply pipe (not shown), the normally closed valves 1 5 and 1 6 are opened and the transfer pump 2 is activated causing the air and petrol vapour derived from the petrol entering the tank 1 to be withdrawn from the tank 1 via the conduit 7 and passed via the conduit 8 through the cooler 3 and into the reservoir 4 via the conduit 9. When the storage tank 1 is filled with petrol and the cooled air/petrol vapour mixture has been completely transferred under pressure to the reservoir 4, the pump 2 is de-energised and the stop valves 1 5 and 16 are closed.
When it is desired to pump petrol from the storage tank 1 via the withdrawal pipe (not shown), the normally closed stop valves 1 7 and 18 are opened and the air/petrol vapour mixture flows from the reservoir 4 via the conduit 10, the expansion tank 5, the conduit11, the cooler 6 and the conduit 12 into the space at the top of the storage tank 1 which is vacated as the petrol level in the tank 1 drops as petrol is withdrawn therefrom. The inside of the storage tank 1 is maintained substantially at atmospheric pressure and the flow of air/petrol vapour mixture from the reservoir 4 to the tank 1 is made possible by the mixture being stored under pressure in the reservoir 4. The expansion tank 5 reduces the pressure of the air/petrol vapour mixture flowing from the reservoir 4 to the required pressure for enabling admission of the mixture into the tank 1.
The temperature of the air/petrol vapour mixture in the storage tank 1 is kept low to reduce evaporation losses during supply of petrol to, and withdrawal of petrol from, the tank 1 by passing the mixture through the coolers 3 and 6 of the aforementioneå main circuit. When the tank 1 is not being supplied with petrol not having petrol withdrawn therefrom, the temperature of the air/petrol vapour mixture is kept low to reduce evaporation losses by circulating the mixture through the previously described auxiliary circuit.The auxiliary circuit is brought into operation when the thermostat 23 senses that the temperature inside the tank 1 has risen above a set value causing the pump 19 to be energised. When the pump is energised, the normally closed valves 18 and 22 are opened and the air/petrol vapour mixture is pumped around the auxiliary circuit and is cooled as it passes through the cooler 6. When the thermostat 23 senses that the temperature inside the tank 1 has dropped below the set value, the pump 19 is de-energised and the valves 18 and 22 are closed.
The thermostat 23 is part of an automatic control system which, in addition to controlling the operation of the pump 19 and the valve 22 also controls the operation of the pump 2 and the stop valves 15 to 18.
Circulation of the air/control vapour mixture through the main circuit is controlled in dependence on supply and withdrawal of petrol to and from the storage tank 1. Although the control system has not been described in detail, an automatic control system for the petrol storge system could easily be designed by a person skilled in the art.
For safety purposes the tanks 1 and 5 and the reservoir 4 are provided with safety valves 24, 25 and 26, respectively. The expansion tank is provided with a pressure gauge 27 and the reservoir is provided with a pressure gauge 27 and the reservoir is provided with a pressure gauge 28 and a drain valve 29.
The system described above is designed to reduce evaporation losses from the storage tank 1 with petrol vapour, admixed with air as a carrier gas, being circulated in a closed system. In order to prevent or reduce to a minimum fire or explosion risks, carbon dioxide or nitrogen may be used as a carrier gas instead of air, carbon dioxide or nitrogen when admixed with petrol vapour being relatively non-inflammable.