"VAPOUR EXTRACTION DEVICE FOR FUEL DISPENSERS AND OPERATING METHOD THEREOF"
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FIELD OF THE INVENTION The present invention refers to a vapour extraction device for fuel dispensers and to a method for the operation of such device. BACKGROUNG OF THE INVENTION
In accordance with common national and international regulations for limiting environmental pollution, fuel-dispensing devices such as gasoline dispensers in petrol stations must be provided with a vapour extraction device for the gasoline fuel. When the tank of a vehicle is filled with liquid fuel, for instance gasoline, the fuel vapour contained in the tank is displaced by the liquid fuel and pushed to the outside through the tank opening. It is mandatory to avoid the gasoline vapour from dispersing in the atmosphere. The displaced vapour is usually collected by the vapour extraction device and fed back to the fuel supply tank, typically an underground tank of the petrol station. The volume of liquid fuel drawn from the supply tank and the volume of fuel vapour recovered from vehicles are normally balanced, i.e. the same volume of fuel is drawn from the supply tank as the vapour is displaced from the vehicle tank. In this way the vapour from the vehicle tank can be readily accumulated in the fuel supply tank. In other words the volume of fuel inside the supply tank diminishes in time as vehicles are supplied with fuel and the volume of vapour inside the supply tank correspondingly increases in time as more vapour is recovered from vehicles. Fuel supply tanks of petrol stations are usually refilled with liquid fuel carried by tankers. When the fuel supply tank is refilled, the vapour recovered from vehicles and collected in the tank is transferred in the tank of the tanker and carried to a fuel depot, where a device is available for processing this vapour. A closed system is thus obtained wherein little or no gasoline vapour enters the atmosphere.
In the known fuel dispensers the vapour displaced from a vehicle tank is usually collected by the vapour extraction device through an extraction conduit which opens in proximity of the fuel dispensing nozzle to be inserted in the tank filling opening. A gas pump is provided for sucking the vapour in the conduit and pumping it to the fuel supply tank of the service station. Because there is an open connection between the vehicle fuel tank and the extraction conduit positioned next to or within the tank filling opening, the extraction flow rate of the fuel vapour must be precisely adjusted to equal to the flow rate of the fuel being pumped into the vehicle tank. This is achieved by providing the extraction conduit with a gas pump and a control valve. The control valve is operated by an electronic device in an open control loop, in such a way that the correct amount of vapour is extracted. The electronic device converts a flow rate input signal, relative to the amount of liquid fuel dispensed through the nozzle, in an electrical signal to adapt the opening of the control valve. In this way the flow rate of the vapour recovered by the vehicle tank can be equalized to the flow rate of the fuel dispensed to the vehicle tank.
A relevant drawback of the known vapour extraction devices is in that the valve and the electronic device, which provide the control loop, may fail in equalizing the volumes of vapour and liquid fuel due to changes occurring in the characteristics of the extraction conduit and the related circuit. Such changes occur frequently and are caused by the build up of dirt or pollutants in the extraction conduit, nozzle damages, wearing of the gas pump, etc.. This would lead to a vapour extraction which does not comply any more with the prescriptions of the metrology, i.e. a vapour extraction which does not work correctly.
Thus the need is felt for a vapour extraction device for fuel dispensers which operates always in its best way of performance irrelevant of the aforesaid changes in the characteristics of the extraction conduit and the related circuit. Some prior art fuel dispensers are provided with several dispensing units for simultaneously supplying fuel to several vehicles. Typically such dispensers allow for the choice between different types of fuel, for instance diesel, high octane unleaded fuel, low octane unleaded fuel, etc., and are usually designated as multi-product dispensers (MPDs).
In order to obtain an effective extraction of gasoline fuel vapour , also when gasoline fuel is being delivered simultaneously with for instance two nozzles (of the same unit or different ones), each unit is provided with its vapour extraction device, i.e. each unit comprises an extraction conduit, a gas pump and a control valve and the respective electronic device for controlling the valve. Installing several vapour extraction devices in a fuel dispenser leads to a complicate layout and to an extremely high increase in costs (for designing, assembling, operating and maintaining the dispenser).
A multi-product dispenser provided with a single vapour extraction device is disclosed in the international patent application n. WO 02/42200. Each dispensing unit of the MPD is provided with two nozzles, a left side nozzle and a right side nozzle. The extraction device comprises an extraction conduit for each nozzle, valves, a pump and control means. The conduits of the same side of the dispenser are joined to a collector line. In each line is positioned a control valve and a pump is connected at its suction side to the collector line. The extraction device is controlled by way of a dedicated apparatus. The control apparatus comprises a first measuring unit for measuring the fuel flow rates dispensed with each fuel-dispensing unit, a first memory containing the ideal valve characteristic, i.e. the relationship between the valve position and the vapour flow rate allowed through the valve at a specific pressure drop, and a first control unit, which on the basis of the valve characteristic adjusts the valve to a position that, at the pressure drop provided by the pump, equalizes the vapour flow rate through the connector line to the fuel flow rate through the nozzles. The extraction device according to WO 02/42200 allows for extracting the fuel vapour displaced during the simultaneous operation of maximum two fuel units using one pump device, being at the same time simple to manufacture at low cost. A drawback of this embodiment is that the control unit operates on the basis of the ideal characteristic of the valve. But, as stated above, the characteristic of the extraction circuit, i.e. the characteristic of the extraction conduit, the collector line, the valve and the pump, varies in time mostly due to the build up of dirt or pollutants. Thus when the fuel dispenser operates a long time the ideal valve characteristic cannot be relied upon (its relationship with the characteristic of the circuit is lost), the vapour flow rate which effectively passes through the valve may not be the desired one and the matching between the flow rates of the fuel and the vapour may not be achieved. SUMMARY OF THE INVENTION It is an object of the present invention to provide a vapour extraction device for fuel dispensers which overcomes the drawbacks of the prior art devices and is at the same time simple to assemble and operate. It is another object of the present invention to provide a vapour extraction device for fuel dispensers which operates always in its best way of performance irrelevant of the changes in the characteristics of the extraction conduit and the related circuit.
It is still an object of the present invention to provide a vapour extraction device for fuel dispensers which provides a simple and cost saving maintenance. These and other objects are achieved by the present invention which relates to a vapour extraction device for fuel dispenser comprising at least one extraction conduit for collecting the fuel vapour displaced from a vehicle fuel tank, a pump for sucking said vapour through said extraction conduit, a valve for adjusting the flow rate of said vapour through said extraction conduit, the valve being positioned in said extraction conduit upstream of said pump, and a control unit for operating said valve, characterized in that it further comprises a vapour flow meter which senses the actual vapour flow rate through said extraction conduit, said control unit adjusting said valve on the basis of the comparison between said actual vapour flow rate and a reference value of said vapour flow rate.
Advantageously, the vapour extraction device provides a feedback loop with respect to the vapour volumetric flow rate, which loop avoids the problems of the prior art devices. The valve is always adjusted in relationship to the real vapour volumetric flow rate flowing through the extraction conduit, independently from eventual build up of dirt or pollutants in the related circuit. In other words the vapour extraction device according to the present invention allows for an effective and precise vapour recovery at fuel dispensers over time.
The control unit is supplied by the related fuel dispenser with a signal indicative of the volumetric flow rate of the fuel supplied to the vehicles at each instant of time. The control unit has memorized the vapour flow rate characteristic of said flow meter, for instance the characteristic of the flow rate versus the pressure drop through the same flow meter. In order for the vapour extraction device to balance the volumetric flow rate of the dispensed fuel and the volumetric flow rate of the recovered vapour, the reference value of said vapour flow rate is chosen by the control unit which value equals the value of the flow rate of fuel being distributed to vehicles. The value of the vapour flow rate sensed by the flow meter depends on the temperature of the vapour. The characteristic of the flow meter is set at a reference temperature, for instance 20 centigrades. Thus the flow rate signal provided by the flow meter is processed to compensate the difference between the reference temperature and the vapour actual temperature. This is done by sensing the vapour temperature and processing the flow meter signal. The flow meter comprises a temperature detector for sensing the actual temperature of the vapour. The extraction device comprises a temperature compensation unit for processing the vapour flow rate signal from the flow meter on the basis of the difference between the actual temperature sensed by the temperature detector and said reference temperature. Preferably the vapour flow meter is a Venturi provided with a pressure transducer. The characteristic of this flow meter is vapour flow rate versus pressure drop. The Venturi is preferably positioned in the extraction conduit upstream of the valve.
The vapour extraction device according to the present invention is provided with a second control loop for checking the valve operation. The control unit has memorized the vapour flow rate versus the aperture (or resistance) characteristic of the valve. For each value of the fuel flow rate provided by the fuel dispenser, the control unit sets the correspondent value of the valve characteristic as the reference value. This reference value, i.e. the opening of the valve which corresponds to the vapour flow rate equal to the flow rate of the fuel dispensed to the vehicle, is compared with the value of the valve opening previously set by the control unit to adjust the valve. If the difference between the reference valve opening and the valve opening set by the control unit exceeds a preset value an error signal is generated which indicates malfunctioning of the vapour extracting device. Preferably the vapour extraction device of the present invention is associated to a multi-product dispenser MPD, for instance a traditional MPD having several units, each of which is provided with left side nozzles and right side nozzles dispensing different fuels such as diesel, unleaded gasoline, etc.. The extraction conduit of the device of the invention collects at least the gasoline vapour extraction conduits of the same side nozzles of the MPD, i.e. the gasoline conduits which are associated to the left or right nozzles of the MPD join the extraction conduit of the device according to the present invention. Preferably the extraction device of the invention is duplicated for serving both sides of an MPD, i.e. a first vapour extraction device is associated to the left side of the MPD and a second vapour extraction device is associated to the right side of the same MPD.
The device of the present invention provides several advantages over prior art devices.
The vapour recovery is continuously corrected for wear and extraction conduits pollution by an automatic feedback loop. The efficiency of the extraction device is kept at its maximum in time.
The vapour extraction device of the present invention is also simple to manufacture and to install in common fuel dispensers. In fact the device components such as the Venturi, the valve, the temperature sensor, the control unit, etc., are easy to find on the market at low cost and also provide a simple maintenance. Moreover, thanks to its "auto-adapting" feature, i.e. to the possibility of adapting the operation to the wearing conditions of the extraction circuit, the vapour extraction device requires minimum technical assistance in time in comparison with prior art devices. This permits to extremely reduce overall maintenance costs of the fuel dispenser or MPD. The vapour extraction device of the present invention, installed in a MPD, allows for the simultaneously vapour recovery from several extraction conduits associated to the nozzles of the same MPD side. The present invention has also the aim of providing a method for recovering fuel vapour displaced from vehicle tanks, wherein the flow rate of the extracted vapour is continuously maintained equal to the flow rate of the fuel delivered to the vehicles regardless of the operating conditions of the fuel dispenser.
This objective is achieved by a method for recovering fuel vapour from vehicle tanks by way of the afore mentioned extraction device, the method comprising the steps of:
- comparing said actual vapour flow rate provided by said flow meter with a reference value of said vapour flow rate;
- adjusting the aperture of said valve in dependence of said comparison. The method according to the present invention is for the feedback loop on the flow rates of the fuel and the recovered vapour. This feature permits the device to automatically adapt itself to changes occurring in the extraction circuit in time, due to crusting or build up of dirt or waste within the extraction conduits, the valve, etc.. Moreover the present invention provides a vapour extraction device for multi- product dispensers (MPD's) which allows for an independent open control loop comparing in a continuous way an ideal vapour extraction parameters with the really measured ones and generating an error signal for remote diagnostics, if need be.
BEST WAY TO CARRY OUT THE INVENTION Further advantages and features of the present invention will become apparent from the following detailed description with reference to the drawing enclosed as a non-restrictive example, where figure 1 shows a schematic representation of a fuel dispenser according to the present invention. In particular Figure 1 shows two vapour extraction devices according to the present invention installed in a multi-product dispenser MPD provided with three fuel dispensing pump units, unit 1 , unit 2 and unit 3. Each unit has two nozzles for delivering different types of fuel to vehicles positioned on the left side or the right side of the unit. A first extraction device is for recovering gasoline fuel vapour from the vehicles located on the right side of the units and a second extraction device is for recovering gasoline fuel vapour from the vehicles located on the left side of the units.
For clarity's sake only the first extraction device will be described, being understood that the second extraction device is a duplication of the first one and operates in the same manner.
The skilled man will appreciate that the vapour extraction device according to the present invention may be associated not only to MDPs but also to single pump unit dispensers. The vapour extraction device comprises at least one extraction conduit 5 which is in fluid communication with an aperture 4A provided next to a fuel nozzle 6 for the suction of the vapour displaced from vehicle tanks. As shown in figure 1 , in a MPD the extraction conduit 5 communicates with several openings 4A through a corresponding number of extension conduits 4. For instance the openings 4A may be coaxial openings around the throat of each nozzle 6 to be presented close to the filling opening of the gasoline tank of a vehicle when filling with gasoline. The extension conduits 4 may be coaxial to the related fuel hose supplying the nozzle 6. In figure 1 fuel hoses are not shown for clarity reasons since they are coaxial to conduits 4. The extraction device comprises a vacuum pump 1 to suck fuel vapours from the apertures 4A and collect such vapours to a storage tank T through the extension conduits 4 and the extraction conduit 5. The pump 1 may be a traditional vacuum pump of the type usually operating in fuel dispensers. As shown in figure 1 , the extraction conduits 5 of the first and second extraction device join together upstream of the suction side of the pump 1. This means that when multiple extraction devices are installed, a single vacuum pump may be used.
In common multi-product fuel dispensers only one nozzle of a given side can supply fuel to a vehicle per time, i.e. only one vehicle can be served at each side of the MPD. Thus, when a fuel hose is supplying fuel to a vehicle on the right side of the MPD the remaining hoses are blocked. Normally this is achieved by providing each gas conduit part of the extension conduit 4 with an on/off electrovalve 3 which is separately operated from other valves 3. In other words the on/of electrovalves 3 are normally closed, only the one connected to the nozzle 6 which is filling fuel and sucking gas is opened. The valves 3 shown in figure 1 intercept the gas in the hoses positioned inside the conduits 4 and not the fuel.
A valve 2 is provided in the extraction conduit 5 upstream of the pump 1 for adjusting the flow rate of the vapour sucked by the same pump 1. Preferably the valve 2 is a proportional electrovalve. The extraction circuit (conduits 4 and 5, valve 2 and pump 1 ) is similar to the one disclosed in WO 02/42200 but for the elements shown in figure 1 upstream of the valve 2 which will be explained later on. The basic idea in patent WO/42200 is a constant pressure difference over the proportional valves. This is obtained by the combination of a pressure transducer, the proportional valve and a vacuum pump. When the pressure difference over a proportional valve is kept constant, the flow rate through the valve only depends on its opening, thus depends on the amplitude of the control signal set as input to the same valve. In this system the flow rates of both extraction conduits 5 are interdependent and necessitates a constant pressure difference over the proportional valves to provide for their mutual independence. It also does not take into account flow resistance changes in the conduits and the nozzle or changes in the valve characteristic due to build up of wastes. Thus, as such changes occur in time, it will happen that the effective flow rate of vapour flowing within the extraction conduit is different from the desired flow rate (the input signal to the valve is wrong). The extraction device according to the present invention obviates this problem. The device provides a feedback loop on the volumetric flow rate of vapour flowing through the extraction conduit 5. This is achieved by way of a flow meter 7 provided within the conduit 5 to sense the vapour volumetric flow rate. The actual vapour flow rate provided by the flow meter 7 is compared with a reference value and in dependence of said comparison the valve 2 is adjusted. This comparison is operated by a control unit CTR of the extraction device.
Preferably, as shown in figure 1 , the flow meter 7 comprises a Venturi and a pressure transducer PT. The flow meter 7 provides the control unit CTR with signal indicative of the flow rate of the vapour through the Venturi, corrected by the temperature sensor 8 and the correction unit 9 for temperature.
The device control unit CTR is also supplied with a signal from the MPD indicative of the actual fuel flow rate, i.e. the volumetric flow rate of the fuel instantly delivered to vehicle tanks. The control unit CTR of the vapour extraction device according to the present invention processes the various signals. Figure 1 shows several modules 10- 13 implemented within the control unit CTR. The skilled man will appreciate that subdivision of the control unit is made for clarity's sake and that, indeed, the control unit CTR may be a single electronic device. The module 10 has memorized the characteristic of the flow meter 7. In the case shown in figure 1 the module 10 has memorized the volumetric flow rate versus the pressure drop characteristic of the flow meter 7. In other words the relationship between the flow rate through the Venturi and the pressure difference measured by the pressure transducer PT is stored in the module 10. The fuel flow rate signal provided by the MPD is input to the module 10 which in turn, on the basis of the aforesaid flow meter 7 characteristic, provides the value of the pressure drop which is to be set in the extraction conduit 5 in order to equalize the volumetric flow rates of the delivered fuel and the recovered vapour.
The pressure transducer PT provides a module 11 of the control unit CTR with a signal indicative of the effective pressure drop occurring through the Venturi when the fuel flow rate signal is supplied by the MPD. The module 11 is a comparator which amplifies and compares the signals of the effective pressure drop sensed by the transducer PT and the pressure drop to be set in the extraction conduit 5 (signal provided by module 10). If the values corresponding to such signals are different, the comparator 11 operates by adjusting the aperture of the valve 2, otherwise the aperture of the valve remains unchanged.
The characteristic of the flow meter 7 stored in the module 10 is the characteristic at a reference temperature, for instance 20 centigrades. A temperature detector 8 senses the temperature of the vapour flowing through the flow meter 7 and provides a correspondent signal to a temperature compensation module 9 which processes the signal provided by the pressure transducer PT on the basis of the difference between the reference temperature and the sensed or actual temperature. In this way a compensation of temperature differences in taken into account. Processing of the signals for temperature differences may be implemented in a known manner. The temperature detector 8 may be, for example, a thermocouple. Tests on the vapour extraction device of the present invention proved that the device provides an accurate vapour flow rate measurement independently from the above mentioned changes in the extraction circuits. Advantageously, the present invention does not require a constant pressure drop at the outlet of the proportional valves 2 (or at the inlet of the vacuum pump 1 ). Moreover, the pressure drop through the Venturi, measured by the pressure transducer PT, provides the exact vapour flow rate. The comparator/amplifier 11 controls the operation of the valve 2 to match the flow rate stored in the module 10 which is itself set equal to the fuel flow rate. If, for instance, the flow resistance in the extraction conduit 5 is increasing due to a partial obstruction, the proportional valve 2 is opened more than in case of the obstruction being not present, so to match the ideal real vapour flow rate with the ideal (requested) one. In other words, the device according to the invention provides an automatic self correction capability, this feature permitting the device to maintain in time the metrology prescriptions provided for the related fuel dispenser. In fact the comparator 11 continuously compares the signals from the flow meter 7 and the module 10 thus instantly controlling the valve 2 to equalize the vapour and fuel flow rates. The module 12 of the control unit CTR provides an output signal which serves as a reference for the integrated open loop control requested by the metrology. Preferably the characteristic flow rate versus aperture of the proportional valve 2 is stored in the module 12 at the moment of the installation of the MPD, thus when the whole circuit is new and conform with the metrology prescriptions.
The operation of the vapour extraction device according to the invention will now be described with reference to a typical situation of a car refilling the tank with gasoline at a MPD. As an example, the nozzle 6 of pump unit 1 is taken out from its nozzle booth (not shown in figure 1 ) on the right side of the MPD. The microswitch in the nozzle booth is activated which starts the motor of the pumping units and the vacuum pump 1. The appropriate on/off valve 3 opens. As long as the nozzle is closed there is no pressure difference detected in the Venturi of the flow meter 7 and no control voltage is applied to the proportional valve 2 which remains closed.
When the nozzle 6 is opened by the user and delivers gasoline in the vehicle tank, the MPD computer generates a signal indicative of the fuel flow rate and supplies such signal to the modules 10 and 12. The module 10 stores the Venturi characteristic (flow rate versus pressure drop). According to the Venturi characteristic, the module 10 generates an output signal which is needed to be matched by the pressure transducer PT, corrected for temperature differences, in order to equalize the vapour flow rate through the Venturi and the flow rate of gasoline flowing through the nozzle 6. The proportional valve 2 is consequently opened by the comparator/amplifier 11 to allow the desired vapour flow rate. The comparator/amplifier 11 amplifies the difference of the signals from the module 10 and module 9 in such way that the control signal to the proportional valve 2 adjusts the opening of the same valve 2 until the signal provided by the module 9 is equal to the signal provided by the module 10. If the difference between both signals is zero, the comparator/amplifier 11 keeps its output control signal constant, i.e. there is no variation in the valve operation.
When the user partially chokes the nozzle, the delivered fuel flow rate changes, i.e. the working point on the Venturi characteristic stored in the module 10 changes correspondingly. The module 10 sets the new value of the pressure signal to input to the comparator/amplifier 11 on the basis of the flow rate signal coming from the MPD computer. This creates again a difference at the two inputs of comparator/amplifier 11 resulting in a new control signal provided to the proportional valve 2. The valve 2 is consequently operated until the output signal from the compensation module 9 equals the output signal of module 10.
Not only a change in the opening position of the nozzle produces a signal difference at the input of the comparator/amplifier 11. In the MPD shown in figure 1 also the operation of a nozzle 6 on the left side of the MPD (opposite to the side of the example above) causes a change in the fuel flow rate through the nozzle already operative on the right side. Thus again a difference between the two input signals of the comparator/amplifier 11 arises. Again the comparator/amplifier 11 immediately send the proper control signal to the proportional valve 2 for the vapour flow rate to match the new fuel flow rate.
The vapour extraction device according to the present invention provides another inventive feature over prior art systems. A supplementary control loop, independent from the above mentioned feedback loop, is provided relative to the operation of the valve 2.
The module 12 generates a signal related to the valve characteristic only when the other side (in this example is the left side) of the MPD does not deliver any gasoline, so that its proportional valve 2 is closed. The module 13 compares this output signal from module 12 with the signal supplied by the comparator/amplifier 11 to the proportional valve 2 which, in the above mentioned situation, is the only valve controlling the gas flow from the vacuum pump 1. If the difference between the values corresponding to those signals (from the module 12 and the comparator 11) becomes bigger than a preset value an error signal is generated.
The error signal may be for instance sent to the kiosk of the petrol filling station for indicating that technical assistance is needed to correct for the malfunctioning of the system.