GB2382070A

GB2382070A - Refilling liquid storage tanks

Info

Publication number: GB2382070A
Application number: GB0127806A
Authority: GB
Inventors: Carl Denby
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-11-20
Filing date: 2001-11-20
Publication date: 2003-05-21
Also published as: GB0127806D0

Abstract

A liquid storage tank 2, apparatus for installation in a liquid storage tank 2 and a method of refilling a liquid storage tank 2, comprises a gaseous outlet, a fluid flow restrictor 20 to restrict flow through said gaseous outlet 14 in a response to a filling condition within the tank exceeding a threshold level, and pressure sensitive means 30 fluidly connected to the tank. A second gaseous outlet for venting tank 2 during an excess filling condition. A fluid flow restrictor 20 comprises a lightweight float 22 and holding means (fig 4, 38) for holding the float in a first position 24, and flow restriction means for restricting a flow of liquid when the float 22 is raised to a second position, the holding means (38) includes an element for exerting an attractive force on said float. A method of enabling fluid connectivity with the interior of a liquid storage tank 2 having an access cover 8, comprising a fixing member 40 for fixing an element to said access cover 8 with a fluid passage passing into the interior of said tank, and a fluid conduit attached to said fixing member to enable fluid flow. The pressure sensitive means 30 may have an indicator response to an increase of pressure to provide an alert signal to the operator, may be provided by a whistling device 26 connected to a vent pipe 14 and a visual indicator 32. An ullage space 6 a fill pipe 12 and a suction pipe 10 may be provided.

Description

Refilling ! Liquid Stora2e Tanks This invention relates to the refilling of liquid storage tanks, both above and below ground, and in particular, but not exclusively, to apparatus and methods for preventing overfills during delivery of liquid to the tanks.

A typical overfill situation occurs when a person ordering a delivery either over-calculates the required amount or is given the wrong information by an automatic tank gauging system. Without any overfill protection, there is a risk of significant amounts of spillage occurring during delivery. In most cases, the liquid being stored will be at least to some extent pollutive, and therefore it is imperative to somehow prevent overfills.

The majority of liquid storage tanks are now fitted with overfill prevention devices. Known storage tank overfill prevention, or protection, devices (OPDs) include automatic shutoff devices installed within the fill pipe, typically inside the internal drop tube of the tank, which operate when the liquid level within the tank reaches an overfill threshold level.

Whilst these known devices do prevent overfilling of the tank, there are drawbacks to their use. The device creates a restriction in the fill pipe even when it is not tripped, which can slow delivery speeds considerably. A delivery operator receives no direct indication that an overfill situation is imminent or indeed that the overfill device has been tripped, other than a slight"jump"of the delivery hose. Generally, the first thing that the operator is aware of is that the liquid flow from the tanker to the storage tank has stopped. However, there could be a number of reasons for this stoppage. The most typical is that an air lock is present in the delivery hose or drop tube and as this is the most likely cause the delivery operator will often loosen the delivery hose connection in order to purge air out of the system. A potentially dangerous mistake is often made when the delivery operator is either given the wrong information regarding the identity of the tank to be filled or accidentally connects to the wrong tank which may

already be close to the overfill level. Unfortunately, if the cause is that the overfill device has tripped, when the hose connection is loosened there will be considerable pressure build up within the fill system which can blow the delivery hose connection off the fill pipe and saturate the operator with the delivered product. There is also a danger of the operator being severely injured through being hit with the hose and connector.

Another drawback of installing an automatic shutoff device in the drop tube of a storage tank containing volatile, inflammable and/or explosive liquids such as petroleum or petrochemical solvents is that known overfill prevention devices often destroy a liquid seal which is otherwise created within the fill pipe and drop tube. The liquid seal arrangement reduces the risk associated with fire and explosion hazards, which are significantly greater when the volatile liquids are exposed to larger volumes of gas, in particular atmospheric oxygen.

Typically, the internal drop tube terminates approximately 50mm from the bottom of the storage tank and approximately 35mm below the internal suction pipe. In effect this means that when the internal suction pipe can pull no more product from the tank, so to all intents and purposes the tank is empty, there is still approximately 85mm of product within the tank and the termination point of the fill pipe drop tube is still immersed below 35mm the stored product. This creates an effective seal between the flammable/explosive vapours within the tank and any potential sources of ignition at the tank fill point. However, known automatic shutoff devices tend to create hydraulic shock in the system when tripped and often themselves lose fluid-tightness, or even cause the internal drop tube to fall off altogether, as a result thereby destroying the liquid seal.

Another type of known OPDs is vent line devices, including ball float valves. They typically consist of a short length of pipe that extends down into the top of the tank from a vent opening, with a wire basket fastened to the lower end of the pipe, holding a float ball capable of blocking the vent opening. As the ball closes the vent, the ullage space is compressed by the weight of the liquid in the

delivery vehicle, until no further delivery occurs when the internal pressure balances the fill pressure. Because of the gradual compression of the ullage space, there is no hydraulic shock on overfill. The operator learns that there is something wrong because the delivery seems to take too long, giving a first indication that the tank has been overfilled. In such a situation the operator closes the delivery valve, and if the hose is disconnected, the internal pressure will force liquid back up through the fill pipe. Therefore, in order to drain a full hose, the operator must manually open an entry point to the tank to release the compressed air in the ullage space, to prevent liquid spillage from the tank after disconnection of the hose.

In another known arrangement, liquid storage tanks are equipped with a "whistle"which allows the operator to determine when the tank is nearly full. The whistle consists of a constriction in the vent pipe located near the top of the tank as air is displaced by the incoming liquid. When the whistle is flooded by oil the noise stops, to indicate that the tank is full. However, the absence of the whistling signal, otherwise provided during normal filling conditions, may be missed by the delivery operator. Furthermore, if used in a public area such a whistling noise could well provide a public nuisance.

In accordance with a first aspect of the invention, there is provided a method of refilling a liquid storage tank, said method comprising providing: a gaseous outlet for removing gaseous content from the tank during delivery of liquid to the tank; a fluid flow restrictor arranged to restrict flow through said gaseous outlet in response to a filling condition within the tank exceeding a threshold level ; and pressure sensitive means fluidly connected to the interior of the tank for sensing a build up of pressure within the tank in response to operation of said fluid flow restrictor.

By restricting fluid flow at a certain level, and providing an alert signal, for example as audible and/or visual warnings to the delivery operator, can be

provided to indicate that a tank overfill situation is being counteracted by the fluid flow restrictor. If in the eventuality that these warning signals are inadvertently missed or ignored, then the fluid flow restrictor preferably will be arranged to gradually reduce the inflow of product to the storage tank to such a point that the flow will slow to a halt, or reduced to such an extent, that liquid contents tend to be kept below a predetermined safe level, for example a given fraction of the tank capacity.

According to a second aspect of the invention there is provided a method of refilling a liquid storage tank, said method comprising: defining a first gaseous outlet for venting gaseous content from the tank during delivery of liquid to the tank during a normal filling condition, the first gaseous outlet being arranged to have at least a first gaseous flow rate at a first pressure when outletting during said normal filling condition; providing a fluid flow restrictor arranged to restrict fluid flow through said first gaseous outlet during an excess filling condition, operation of said fluid flow restrictor causing an increase of pressure within the tank during the excess filling condition; and defining a second gaseous outlet for venting gaseous content from the tank during an excess filling condition, thereby to reduce excess pressure built up during an excess filling condition, the second gaseous outlet being arranged to have at least a second gaseous flow rate at a second pressure when outletting during said excess filling condition, wherein said second gaseous outlet is arranged to provide a gaseous flow which is restricted in relation to that of said first gaseous outlet, such that said second gaseous flow rate at said second pressure from said second gaseous outlet is less than one tenth of said first gaseous flow rate at said first pressure from said first gaseous outlet.

An overfill situation will typically result in the delivery truck and hoses being hydraulically locked to the fill pipe connection point. The relatively small

flow rate of the second orifice allows a hose drain down facility to be provided by a gradual pressure bypass system. Preferably, the pressure bypass system includes a pressure sensitive means for indicating when the storage tank pressure drops to a safe level. In a preferred embodiment, the pressure sensitive means is fluidly connected to the pressure bypass system.

Further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, made with reference to the accompanying drawings, wherein: Figure 1 is a cross-sectional drawing of a known underground bulk liquid storage tank, as used in an embodiment of the present invention; Figure 2 is a cross-sectional drawing illustrating the use of a known overfill prevention device in the tank of Figure 1; Figure 3 is a cross-sectional drawing showing the use of an overfill prevent device in accordance with the present invention in the tank of Figure 1; Figure 4 is a perspective view of an overfill prevention device in an embodiment of the invention; and Figure 5 is a perspective view showing the connection of a pressure sensing tube to a flange stud.

Figure 1 shows an underground storage tank 2, as used in an embodiment of the invention, for storing liquid product 4, such as petroleum or other petrochemical liquids. The tank is in this embodiment installed at a vehicle refuelling station. The tank contains, in addition to the product 4, a gas-filled ullage space 6 which varies in volume depending on the liquid level within the tank. The tank 2 includes a man chamber access point cover 8 which includes connectors for a plurality of different pipes which fluidly connect space within the tank 2 to external areas. An internal suction pipe 10 connects the inside of the tank to liquid outlet points, in the form of outlet pumps with vehicle refuelling nozzles, for removing liquid 4 from the tank 2. An internal drop tube, forming part of a fill pipe 12, is used for the delivery of liquid to the tank, for example

from a delivery vehicle. Typically, a delivery vehicle includes a transportable storage tank, a hydraulic delivery valve system and a hose with a connector which is connected to the fill tube 12 and product is transferred from the transportable storage tank to the storage tank 2 via the hose and fill tube 12. Also connected to the ullage space of the tank 2 is a vent pipe 14, whereby excess pressure within the ullage space 6 is vented from the tank. The vent pipe is generally arranged to have an above ground outlet height (4.2m) which exceeds the height of the delivery vehicle tank (3m), to prevent spillage from the vent pipe in the case of an over-delivery in which any overfill prevention device used fails to operate.

Typically, where the liquid being delivered is volatile, a vapour recovery system is connected to the vent pipe 14 during delivery. The vapour recovery system can be integrated into the delivery vehicle, such that when a delivery is made, the vapours from inside the ullage space 6 are vented back to the transportable storage tank as liquid product is delivered to the storage tank 2.

Figure 2 illustrates the use of a known overfill prevention device in the form of an automatic shutoff device 16 in the storage tank 2. The overfill prevention device 16 is in the form of a pipe insert added to the internal drop tube of the fill pipe 12, containing a float-operated shut-off valve 18. When the level of the liquid product 4 within a tank reaches a pre-set safety limit, the float of valve 18 is forced upwards by the liquid as a liquid rises, thereby causing a shutoff plate within the fill pipe 12 to close the fill pipe mechanically. As noted above, the tripping of this overfill prevent device is typically accompanied by a significant mechanical shock caused by the sudden halting of the delivery, which is generally conducted at relatively high flow rates.

Figure 3 illustrates an overfill prevention system arranged in the tank of Figure 1 in accordance with the present invention. The system includes a vent shut-off valve 20, which is used in place of the fill pipe mounted valve of the prior art.

Figure 4 shows the vent shut-off valve 20 in greater detail. The valve 20 includes a double-threaded connector 34 for connecting a circular pipe, having an internal diameter of approximately 50mm, via a threaded connection, through the access cover 8 to the vent pipe 14. The valve 20 includes a float ball 22, mounted within a float ball basket 24, which rises within the basket 24 to rest against a valve seat 36 of the valve 20, thereby shutting off fluid flow. Before operation of the valve, gaseous contents from within the ullage space 6 are able to escape through the lower orifice, having a diameter of approximately 50mm, of the shutoff valve 20. When the float ball rises to its shut-off position, the orifice is closed, thus preventing further gaseous flow via the aperture.

Also included in the shut-off valve 20 is a whistling device 26, allowing a relatively very small rate of flow, compared with that through the abovementioned aperture, of gas from the ullage space 6 into the vent pipe 14. The whistling device 26 is in this embodiment formed of two axially spaced disks having central concentric apertures, each defining an orifice having a diameter of approximately 1. 5mm, whereby the whistling is produced, fluidly connected to a one-way valve located internally of the shut-off valve 20. As gas enters the whistling device 26 from the ullage space 6, an audible alert signal is produced.

The one-way valve is arranged to allow the transmital of gas from the ullage space 6 into the vent pipe 14 at an internal gas pressure of approximately 15 millibars (mb) above atmospheric pressure and above, but to prevent return flow from the vent pipe 14 into the ullage space.

After float ball 22 shuts off the valve 20, the pressure within ullage space 6 gradually increases due to continuing delivery of product via the fill pipe 12. On reaching 15mb internal gas pressure, the generation of an audible signal by whistling valve 26 begins, which gradually increases in amplitude and frequency as the pressure within ullage space 6 increases. Conversely, the flow rate of the delivery gradually increases, until finally the gas pressure within ullage space 6 balances the head pressure of the incoming liquid, at which point delivery of the

product is effectively halted, except for a small flow rate due to a pressure bypass provided by whistling valve 26.

Note that, due to the large difference in orifice difference in orifice area between the lower orifice in the shut-off valve 20 and the orifice provided in the whistling valve 26, the gaseous flow rate through the pressure bypass provided by the whistling valve 26, even at an elevated pressure during the excess delivery conditions during an overfill situation, is significantly less than that through shut- off valve 20 during normal delivery conditions. A pressure during normal delivery conditions is less than 15mb, typically say 8mb, or less (e. g. 2mb) if no vapour recovery system is used on the delivery vehicle. On the other hand, a pressure during an excess filling condition is greater than 15mb, ranging up to 400mb, typically say 200mb. At these stated typical pressures, the gaseous flow rate through the open vent pipe 14 is typically 700 to 1,000 litres per second and though the whistling valve 26 is typically 10 to 20 litres per second. In accordance with an aspect of the invention, at these typical respective pressures, the flow rate through the whistling valve 26 is at least ten times less than that through the open vent pipe 14, to significantly reduce liquid flow rate into the tank during an overfill situation. The flow rates are preferably at least twenty times different in magnitude. To achieve this, the orifice of the whistling device is preferably at least 100 times, more preferably at least 500 times, less in area than that of the shut-off valve 20.

Gaseous flow rate through whistling device at its typical pressure (200mb) is preferably at least 5 litres per second, in order to provide pressure release within a reasonably short waiting period, say between 2 and 15 minutes, before the tanker hose can be disconnected after an excess filling condition has occurred. The orifice is preferably at least 0. 5mm in diameter.

The depth of the valve seat of the shut-off valve 20 within the tank 2, i. e. the length of the shut-off valve 20, defines the valve shutoff liquid level at which pressure within the ullage space starts to increase and, indirectly, also the

maximum possible fill level during an overfill delivery. The depth is set such that during delivery, the maximum possible fill level after valve shut-off, not taking account of the small pressure bypass provided by whistling valve 26, does not exceed a specified maximum safe level 28. A safe working capacity for an above or below bulk storage tank might for example be assumed to be 97% of the nominal capacity. Therefore, for example, the safe working capacity of a 45,000 litre tank would be approximately 43,650 litres. The length of the vent shutoff valve is calculated to take into account the maximum head pressure that could be generated from a delivery vehicle that may be full and the tank taking the delivery that might also be at full working capacity. The maximum head pressure of liquid that could be generated in this situation, using typical delivery vehicle and tank dimensions, is in the region of 400mb. This is therefore this is the amount of back-pressure that would need to be generated within the storage tank in order to stop the delivery. The valve shut-off level used takes into account the geometry of the top of the tank in which the remaining ullage space is located when the tank is nearing a full working capacity. This provides a minimum distance below the maximum fill level that the ball float shutoff level should be arranged. When freely floating, the ball float 22 itself, which is made primarily of a lightweight material such as cork, will be immersed in the liquid by less than a half of its volume, so the actual valve shutoff liquid level will be slightly below the valve seat 36.

Thus, the position of float valve seat 36 is below (in this example approximately 3cm below) maximum fill level 28 of the storage tank.

Repositioning of the float ball 22 onto the valve seat has the effect of pressurising the ullage 6 within the tank and thereby slowing delivery to an eventual stop, more precisely a very small rate due to the bypass provided by whistling valve 26, at, or below, the maximum fill level. Because of the pressure created in the ullage 6 after float valve shut-off both the audible and visible alert signals begin when the tank has been filled near to its full capacity, some time, for example a

minute or so, before the maximum fill level 28 and total flow is stopped. The position of the whistling device 26 and that of the tank entrance point for the pressure sensing tube are above the safe maximum fill level 28 of the storage tank, so that the two carry on indicating a state of alert during all stages of an overfill prevention operation.

Also included in the overfill prevention system of the present invention is a pressure sensing tube 30, connected at one end to the interior of the tank via the access cover 8, and connected at the other end to a mechanical pressure transducer in the form of a pressure indicator 32, such as an Elevated Pressure Indicator of the type made by Norgren, attached to a mounting near to the tank fill point. The visible pressure indicator 32 has a preset threshold setting, approximately 15mb above atmospheric pressure, above which an alert signal is visibly indicated to the delivery operator. The visible alert signal is given when pressure within the ullage space 6 reaches the predetermined threshold, indicating that the overfill shut-off valve 20 has operated.

Thus, by means of the indicator 32, and the whistling valve 26, the operator is both audibly and visually alerted to an overfill situation having occurred, and is prompted to shut-off delivery using the hydraulic valve system on the delivery vehicle. The whistling continues as the excess pressure within the tank is vented via the pressure bypass provided by the whistle valve. After shutoff, the operator need take no additional action to relieve the excess pressure to allow the hose to be disconnected; the pressure bypass provided by the whistle device 26 provides a gradual release of the excess gas pressure via the vent 14, until a safe pressure is reached at which point the delivery hose may be disconnected. The reaching of the safe pressure is indicated by the ceasing of the whistling signal from the whistling valve 26, and the indication of a safe working pressure by the pressure indicator 32 reverting to its non-alert indicating state.

Each of these states occur at approximately 15mb internal gas pressure.

By fluidly connecting the whistling valve 26 to the interior of the vent pipe 14, the whistling alert is made readily distinguishable through an open vented system and even more so on a closed vented, pressure/vacuum valve controlled, vapour recovery system. On a vapour recovery system all the air/vapour from the tank vents are drawn back to the delivery vehicle. The flow of air/vapour back to the vehicle carries the sound with it, which is then amplified when it enters the vehicle tank compartments. In the event of an overfill situation the whistle signal will only occur for a few minutes and because the sound is captive within the vent or the tanker it will not be carried excessively through air, thereby preventing nuisance to the general public.

The one-way valve attached on the vent side of the whistling device 26 prevents the return of fluid from the vent pipe into the ullage space 6. Thus, after an overfill situation has occurred, as the tank begins to be drained by removal of product from the tank via suction pipe 10, a gradually increasing vacuum will be created within the ullage space 6, until float ball 22 is successfully dislodged from the valve seat 36 and returned to its ambient position until such time as an overfill situation occurs again during delivery of product to the tank.

A further feature of the shut-off valve is the use of a magnetic coupling 38, between the float ball 22 and the float ball basket 24. During normal delivery conditions, with gas being vented via the aperture in the lower end of the float valve 20, an upward force is exerted by the outflowing gas, which, since the float ball is lightweight, can occasionally cause the ball to obstruct the valve seat and close the valve. This is a particular problem during rapid refilling of the tank. In order to overcome this problem, a magnetic disk is located at the base of the basket 24, and a magnetic disk arranged with opposite polarity on its external surface is implanted into the surface of the float ball 22. The basket itself is formed from non-magnetic, or weakly magnetic, material, such as non-magnetic steel. The magnetic disks are formed of non-ferrite magnets. The magnetic disk makes up only a small fraction of the weight of the float ball, for example

between one fifth and one fiftieth, but is of significantly greater density, at least double, preferably at least ten times, that of the lightweight material of the ball.

The strength of the magnet is chosen such that the float ball will release only when the incoming liquid has risen above the base of the float ball by a selected amount, for example between 100mm and 200mm of liquid (the diameter of the float ball being less than this, in this example approximately 50mm). Once the flow has been effectively shut off (other than the pressure bypass flow) the float ball will stay in place until such a time as sufficient liquid had been dispensed from the tank to allow the float ball to gradually return to the magnetically coupled inoperative position. The weight of the magnet ensures that the magnetic insert remains at the bottom of the ball and will make contact with the magnetic disk at the base of the basket when the liquid reaches the appropriate level.

Figure 5 illustrates an arrangement used to provide fluid connectivity between the pressure sensing tube 30 and the tank ullage 6, when retro-fitting. In order to maintain the integrity of the tank, the connectivity is provided via a threaded flange stud fixing 40, which is used to fix a flange holding any of the pipes 10,12, 14 to the access cover. The flange fixing already includes apertures which extend through the cover. In order to make use of these apertures, the fixing 40 includes an axial bore 42, which is internally threaded at the head of the stud, allowing a matching connector 44 to be used to attach the tube 30 to the stud 40. When the stud in fixed in place on access cover, the pressure sensing tube is fluidly connected to the interior of the tank via the bore 42.

It should be noted that at the commencement of a delivery to a storage tank fitted with an internal drop tube pressure peaks are often seen. Although these peaks in pressure last only a few seconds it is sufficient enough time to destabilise the visual indicator 32 and to confuse the delivery operator. Therefore in order to stabilise the pressure, a pressure delay switch is installed in the pressure tube 30. An additional pressure stabiliser may also be fitted into the internal drop tube, such as that described in International Patent Publication WO

96/38375, the contents of which are incorporated herein by reference, to purge trapped air from the fill system on commencement of a delivery.

Note that the overfill prevention system of the present invention can be readily retro-fitted into existing below and above ground storage tank systems and by calculation it can be set to slow and eventually stop the fill at any given level. It is particularly suitable for cylindrical storage tanks. Most cylindrical storage tanks regardless of the nominal capacity are manufactured to a standard outside diameter (typically 3m). In order to achieve the required nominal storagecapacity it is only the length of the tank that is adjusted. A vent shut-off valve of a standard length can be provided for use in all cylindrical storage tanks of the same outside diameter, regardless of capacity.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, the whistle valve 26, instead of being fluidly connected to the vent pipe 14, may instead be situated adjacent the fill point to increase audibility of the signal to the delivery operator. A pressure line, similar to pressure line 30, may be used to convey the gas under pressure from the ullage space 6 to the whistle valve.

Furthermore, a pressure sensitive valve, fluidly connected to the ullage space 6 via a similar pressure line, may be placed in the delivery system to automatically shut off liquid flow through the delivery system in response to detection of excess pressure within the ullage space due to operation of the float valve 20. The pressure sensitive valve may take the form of a pressure transducer switch, located on the delivery vehicle, capable of operating the hydraulic delivery valve system on the vehicle to stop liquid delivery in response to detection of excess pressure within the tank 2.

It is to be understood that any feature described in relation to one embodiment may also be used in other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without

departing from the scope of the invention, which is defined in the accompanying claims.

Claims

Claims:

1. A method of refilling a liquid storage tank, said method comprising providing: a gaseous outlet for removing gaseous content from the tank during delivery of liquid to the tank; a fluid flow restrictor arranged to restrict flow through said gaseous outlet in response to a filling condition within the tank exceeding a threshold level ; and pressure sensitive means fluidly connected to the interior of the tank for sensing a build up of pressure within the tank in response to operation of said fluid flow restrictor.

2. A method according to claim 1, wherein said pressure sensitive means comprises an indicator responsive to an increase of pressure within the tank to provide an alert signal to a delivery operator.

3. A method according to claim 2, wherein said pressure sensitive means comprises a whistling device.

4. A method according to claim 3, wherein said tank includes a vent pipe and said whistling device is fluidly connected to said vent pipe.

5. A method according to claim 2, wherein said pressure sensitive means comprises a visual indicator.

6. A method of any preceding claim, wherein said fluid flow restrictor comprises a float valve.

7. A method of any preceding claim, wherein said pressure sensitive means comprises a pressure switch arranged to operate a delivery valve in response to sensing a build up of pressure within the tank.

8. A method of refilling a liquid storage tank, said method comprising : defining a first gaseous outlet for venting gaseous content from the tank during delivery of liquid to the tank during a normal filling condition, the first gaseous outlet being arranged to have at least a first gaseous flow rate at a first pressure when outletting during said normal filling condition; providing a fluid flow restrictor arranged to restrict fluid flow through said first gaseous outlet during an excess filling condition, operation of said fluid flow restrictor causing an increase of pressure within the tank during the excess filling condition; and defining a second gaseous outlet for venting gaseous content from the tank during an excess filling condition, thereby to reduce excess pressure built up during an excess filling condition, the second gaseous outlet being arranged to have at least a second gaseous flow rate at a second pressure when outletting during said excess filling condition, wherein said second gaseous outlet is arranged to have a gaseous flow which is restricted in relation to that of said first gaseous outlet, such that said second gaseous flow rate at said second pressure from said second gaseous outlet is less than one tenth of said first gaseous flow rate at said first pressure from said first gaseous outlet.

9. A method according to claim 8, wherein said second gaseous flow rate is less than one twentieth of said first gaseous flow rate.

10. A method according to claim 8 or 9, wherein said second orifice has an area at least 100 times less than that of the first orifice.

11. A method according to claim 10, wherein said second orifice has an area at least 500 times less than that of the first orifice.

12. A fluid flow restrictor comprising a lightweight float and holding means for holding the float in a first position in which fluid flow is relatively unrestricted, and a flow restriction means for restricting a flow of fluid when the float is raised to a second position in response to liquid immersion, wherein said holding means includes an element for exerting an attractive force on said float, thereby to inhibit movement of said float from said first position to said second position.

13. A method of enabling fluid connectivity with the interior of a liquid storage tank having an access cover, comprising providing a fixing member fixing an element to said access cover with a fluid flow passage passing into the interior of said tank, and attaching a fluid conduit to said fixing member to enable fluid flow between the interior of the tank and the fluid conduit via said fluid flow passage.

14. A liquid storage tank comprising: a gaseous outlet for removing gaseous content from the tank during delivery of liquid to the tank; a fluid flow restrictor arranged to restrict flow through said gaseous outlet in response to a filling condition within the tank exceeding a threshold level; and pressure sensitive means fluidly connected to the interior of the tank for sensing a build up of pressure within the tank in response to operation of said fluid flow restrictor.

15. Apparatus for installation in a liquid storage tank, said apparatus, when installed in said tank, comprising: a gaseous outlet for removing gaseous content from the tank during delivery of liquid to the tank; a fluid flow restrictor arranged to restrict flow through said gaseous outlet in response to a filling condition within the tank exceeding a threshold level; and pressure sensitive means fluidly connected to the interior of the tank for sensing a build up of pressure within the tank in response to operation of said fluid flow restrictor.

16. A liquid storage tank comprising: a first gaseous outlet for venting gaseous content from the tank during delivery of liquid to the tank during a normal filling condition, the first gaseous outlet being arranged to have at least a first gaseous flow rate at a first pressure when outletting during said normal filling condition; a fluid flow restrictor arranged to restrict fluid flow through said first gaseous outlet during an excess filling condition, operation of said fluid flow restrictor causing an increase of pressure within the tank during the excess filling condition; and a second gaseous outlet for venting gaseous content from the tank during an excess filling condition, thereby to reduce excess pressure built up during an excess filling condition, the second gaseous outlet being arranged to have at least a second gaseous flow rate at a second pressure when outletting during said excess filling condition, wherein said second gaseous outlet is arranged to have a gaseous flow which is restricted in relation to that of said first gaseous outlet, such that said second gaseous flow rate at said second pressure from said second gaseous outlet

is less than one tenth of said first gaseous flow rate at said first pressure from said first gaseous outlet.

17. Apparatus for installation in a liquid storage tank, said apparatus, when installed in said tank, comprising: a first gaseous outlet for venting gaseous content from the tank during delivery of liquid to the tank during a normal filling condition, the first gaseous outlet being arranged to have at least a first gaseous flow rate at a first pressure when outletting during said normal filling condition; a fluid flow restrictor arranged to restrict fluid flow through said first gaseous outlet during an excess filling condition, operation of said fluid flow restrictor causing an increase of pressure within the tank during the excess filling condition; and a second gaseous outlet for venting gaseous content from the tank during an excess filling condition, thereby to reduce excess pressure built up during an excess filling condition, the second gaseous outlet being arranged to have at least a second gaseous flow rate at a second pressure when outletting during said excess filling condition, wherein said second gaseous outlet is arranged to have a gaseous flow which is restricted in relation to that of said first gaseous outlet, such that said second gaseous flow rate at said second pressure from said second gaseous outlet is less than one tenth of said first gaseous flow rate at said first pressure from said first gaseous outlet.