WO2013108006A1

WO2013108006A1 - Gas supply systems and methods

Info

Publication number: WO2013108006A1
Application number: PCT/GB2013/000021
Authority: WO
Inventors: Benjamin Johnson
Original assignee: Atlas Copco Ltd
Current assignee: Atlas Copco Ltd
Priority date: 2012-01-18
Filing date: 2013-01-18
Publication date: 2013-07-25
Anticipated expiration: 2014-07-18
Also published as: GB201200825D0; GB2500862A

Description

GAS SUPPLY SYSTEMS AND METHODS

FIELD OF THE INVENTION

The present invention relates to a gas supply system for and a method of providing a continuous gas supply to a gas dispensing system from a source of gas comprising multiple finite sources of gas, for example, in a medical gas supply within a hospital where the source of gas comprises multiple gas cylinders.

BACKGROUND ART

In medical establishments, gas supply systems are commonly used to control the supply of medical gases from at least two banks of high pressure cylinders to a medical gas dispensing system, for example, for dispensing oxygen, air, nitrous oxide, 0₂/N₂0 50%/50% v/v, He/0₂ 79%/21% v/v, carbon dioxide or nitrogen to patients in wards or in operating theatres.

Typically, such systems employ a pneumatic control assembly that has at least two independent and finite supplies of gas in the form of high pressure gas cylinders, typically with nominal fill pressures of 137 or 200 bar gauge pressure for gases and 40 to 55 bar gauge pressure (temperature dependent) for vapours stored as liquid under pressure, for example, nitrous oxide and carbon dioxide. Using two independent supplies enables gas sources of finite volume to deliver a continuous flow of gas as required in, for example, medical gas delivery applications.

It is common for the pneumatic control assembly to deliver gas to the dispensing system at a pressure level appropriate for input to medical devices connected to the dispensing system. The nominal pressure chosen is normally in the range 3.5 bar to 8 bar gauge pressure (depending on the gas type, application or system design), and is commonly required to be maintained within +/- 10% or +/- 15% of the nominal pressure throughout the dispensing system. In order to enable sufficiently accurate control of the delivered gas pressure, it is common for two separate stages of pressure regulation to be employed between each supply source and the dispensing system . Typ/cally, the first stage of pressure regulation is effected by a manifold pressure regulator and the second stage of pressure regulation is effected by a line pressure regulator.

Existing gas supply systems normally have a first gas supply designated as a 'duty bank' (e.g. a first set of gas cylinders) and a second gas supply designated as the 'standby' or 'reserve bank' (e.g. a second set of gas cylinders). The pressure in each bank is continuously monitored, typically by a pressure switch or a pressure transducer. As gas is consumed, the pressure in the duty bank tends towards zero. At a predetermined pressure level, the duty bank is considered to be empty, that is, it can no longer support the maximum volumetric flow rate that the system is designed to provide (also known as the standard discharge). When this happens, the duty bank is isolated by a control system, typically by actuating a solenoid-operated valve to a closed position. At the same time, the standby bank is selected by the control system to deliver gas, again typically by actuation of a solenoid operated valve. An indication is then sent, usually via a hardwired alarm system, to a porter, or other designated individual, informing them that the duty bank (which is now considered empty) requires the empty gas cylinders to be replaced with full cylinders.

Medical gases are supplied in this way in hospitals and other healthcare establishments, with the gas being delivered through a pipeline dispensing system to points of use throughout the facility.

Traditionally, pressure switches or transducers are used to determine when to changeover from the duty cylinder to the standby cylinder bank, with a pressure switch or transducer being used to sense the pressure within the cylinders of each bank. In some cases, the pressure downstream of a first pressure regulating valve is monitored (instead of directly monitoring cylinder pressure) as this is also related to the amount of gas remaining in the bank of cylinders (or the sufficiency of residual cylinder pressure to maintain a particular worst case (highest) flow rate (standard discharge)). Whichever method is chosen, this normally requires a relatively high threshold or changeover pressure set point, for example, typically 10-20 bar gauge pressure depending on the manufacturer, because the supply assembly is designed to provide a certain maximum design flow rate in worst case operating conditions. The changeover pressure setting is normally selected by the manufacturer (and is generally not adjustable) to enable this worst case (highest) flow rate to be provided without the system delivery pressure (to the pipeline) falling below a given low pressure level.

However, this method of control is wasteful of gas in high pressure gas cylinders, with as much as 10-15% of the pressurised contents being left unused at changeover. The unused gas is returned to the gas supplier, who will normally vent the gas to atmosphere and refill the cylinder with fresh gas. This wastage arises as the standard discharge which the assembly is designed to provide may be higher than the maximum flow rate of the dispensing system in a particular establishment requires (as supply assemblies are available in a limited number of specifications). Gas supply assemblies may, for example, be designed to provide a maximum flow rate of 100 l/m, which might be suitable for supplying oxygen to a single department, or 1200 l/m for a complete hospital. A particular gas dispensing system may, however, require a maximum flow rate of some other value and so the next higher specification (in terms of standard discharge) system has to be selected. The pre-set changeover pressure of the selected assembly thus commonly relates to a maximum flow rate which is higher than that required by a particular application.

In addition to this, the maximum flow rate is only required in worst case scenarios, for example, when an unusually high number of gas outlets in an establishment are in use or when a number clinical interventions requiring high flow rates are being practised in an establishment. Such high demands very rarely arise yet, as it is critical that such demands can be met if required (as in a medical application this can be a matter of life or death), known systems use a changeover pressure based on the need to be able to provide the required gas flow in this worst case scenario. On top of this, the maximum calculated design flow rate for a given dispensing system normally accommodates a very large safety factor. It may, for example, be designed to provide for a situation which might only be expected to occur once in ten years.

The present invention seeks to provide a gas supply system with an improved method of sensing and controlling gas supply from multiple finite sources. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a gas supply system for providing a continuous supply of gas from a gas source comprising at least first and second finite sources of gas, the system comprising at least one gas inlet for receiving gas from said gas source, seiection means for selecting which of said finite sources of gas is arranged to supply gas to the system, a gas outlet for providing a continuous supply of gas to a gas dispensing system, first sensing means for sensing a first parameter indicative of or related to the quantity of gas remaining in the finite source of gas from which the system is currently arranged to receive gas, second sensing means for sensing a second parameter indicative of or related to the pressure of gas supplied by the gas outlet to the gas dispensing system and control means arranged to actuate said selection means to select a different finite source of gas to supply gas to the system dependent upon the levels and/or rate of change of the first and second parameters.

According to a second aspect of the invention there is provided method of operating a gas supply system for providing a continuous supply of gas from a gas source comprising at least first and second finite sources of gas to a gas dispensing system, the method comprising the steps of:

sensing a first parameter indicative of the quantity of gas remaining in the finite source of gas from which the supply system is currency arranged to receive gas,

sensing a second parameter indicative of the pressure of gas being supplied by the supply system to the gas dispensing system,

and selecting a different finite source of gas to supply gas to the supply system dependent upon the levels and/or rate of change of the first and second parameters.

It should be noted that the term 'gas' as used herein includes gases and vapours whether stored as compressed gas or stored in liquid form. Pressures referred to herein are in some instances given in 'bar'. One bar is equal to 100 kPa and approximately equal to atmospheric pressure at sea level. Gauge pressures referred to herein are relative to atmospheric pressure, that is, these pressures are zero-referenced against ambient air pressure.

Other preferred and optional features of the invention will be apparent from the following description and the subsidiary claims of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a graph showing the relationship between pressure delivered to a pipeline and volumetric flow rate for a selection of supply source pressures;

Figure 2 is a schematic diagram of a typical gas supply system in which the present invention can be used; and

Figure 3 is a chart illustrating various outlet pressure thresholds that are used in a preferred embodiment of a gas supply system according to the present invention showing a typical delivered pressure response of a supply system as cylinder pressure falls over time with gas consumption.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As indicated above, medical gas systems are specified with very high safety factors for flow capacity. The average flow rates delivered by medical gas pipeline systems are therefore considerably lower than a calculated maximum flow rate which the system is designed to be capable of providing. At lower flow rates, the supply assembly can thus maintain line pressure above the minimum allowable threshold with a lower inlet pressure than that required to achieve the standard discharge because pressure loss across the supply assembly upstream of the line pressure regulator(s) will be less than the worst case for the majority of the time.

At very low cylinder (supply) pressures, a first stage pressure regulating valve (also commonly known as a manifold regulator) is fully open and line (output) pressure is controlled by a line pressure regulator only. At volumetric flow rates lower than the system design flow, the loss of pressure between the supply and the output will also be lower. The pressure loss between the cylinders and the line pressure regulator inlet will also be lower at volumetric flow rates lower than the design flow, as, within the range of volumetric flow rates of interest, pressure loss across this portion of the pneumatic control assembly is roughly proportional to the square root of the volumetric flow rate.

Figure 1 is a graph showing a typical example of a delivered outlet outlet pressure P3 against flow at various supply source pressures PI (1400, 1200, 1000, 800 and 600 kPa). It will be seen that, at higher flow rates, the delivered pressure P3 falls off (due to the pressure drop across the assembly) and that the fall off or 'droop' is smaller at higher supply pressures (PI). Thus, if the system is designed to be capable of providing a high flow rate (such as 500 l/min), the changeover threshold (Lower Limit) has to be set at a relatively high level (1400 kPa in the example shown in Fig 1) to avoid the effect of these pressure fall-offs at high flow rates. It will be appreciated that a single low pressure threshold of this nature for initiating bank changeover based on inlet supply pressure PI alone is therefore wasteful when the system is supplying less than the maximum rated design flow as the bank will be automatically isolated and the cylinders replaced whilst they still contain potentially useful gas.

Figure 2 illustrates a typical gas supply system in which the present invention can be used. Only the features relevant to explaining the present invention will be described. Figure 2 shows a first bank 1 of gas cylinders and a second bank 2 of gas cylinders. Each bank typically comprises a plurality of cylinders (two in the example shown). These are connected to a supply system 3 (indicated by dashed lines) which is used to supply a continuous supply of gas to an outlet pipeline 4, e.g. a gas distribution network in a hospital. The supply system 3 typically comprises at least one inlet 7 for receiving gas from the finite gas sources 1, 2, first stage regulators 5 and second stage regulators 6 and at least one outlet 8 for supplying gas to a pipeline 4 of gas dispensing system. First sensing means in the form of pressure sensors SI are provided for directly sensing the supply pressure (one for each cylinder bank in the arrangement shown). Alternatively (or additionally), pressure sensors S2 may be provided for sensing an intermediate pressure P2 (between the first and second stages of pressure regulation). Whilst not a direct measurement of cylinder pressure PI, this intermediate pressure P2 is related to the cylinder pressure PI (or the sufficiency of the cylinder pressure to maintain a particular maximum volumetric flow rate). Second sensing means in the form of a further pressure sensor S3 is provided for sensing the outlet pressure P3, that is, the pressure of gas supplied to the outlet 8 and hence to the pipeline 4.

It should be noted that gas supply systems having the pressure sensors described above are known. They are not, however, used to provide the changeover control system described below. In prior art systems, the pressure sensor S3 may, for example, simply be provided for initiating an alarm indication, for example, a low pressure alarm corresponding to that described below in relation to Figure 3 (when the pressure falls below threshold T6).

The present invention takes account of these factors by monitoring both supply source pressure PI (or P2) and line pressure P3 in order to maximise the use of gas from finite sources such as cylinders by effecting changeover from the duty to the standby source only when both of the following conditions are satisfied:

1. The duty supply source pressure PI (or P2) is below a pre-determined low pressure level, and

2. The line pressure P3 falls below a predetermined level and/or a predetermined trend is detected that indicates a condition in which it may be necessary to changeover from the duty to the standby gas source. The above applies to cases in which the line pressure P3 tends to fall as the supply pressure PI falls. However, in cases in which a significant 'supply pressure effect' occurs (for example, with unbalanced pressure regulating valves) whereby the line pressure P3 tends to rise as the supply pressure PI falls, the second condition is .instead:

2. The line pressure P3 rises above a predetermined level and/or a predetermined trend is detected that indicates a condition in which it may be necessary to changeover from the duty to the standby gas source.

Changeover is effected by selection means which disconnect a first finite source of gas from the system and connect a -second finite source thereto. The selection means may, for example, comprise one or more solenoid operated shut off valves 9.

The supply system has control means that is arranged to control the changeover from duty to standby as supply source in accordance with the above criteria. The control means may, for example, comprise pneumatic and/or electronic components to actuate the selection means as required.

In a preferred embodiment, dome loaded 1st stage pressure regulators are employed (rather than directly controlling gas flow by solenoid operated shut-off valves). In this way, pressure loss across the supply assembly in minimised so a greater amount of the gas in a finite source can be used.

The control means preferably comprises at least one processor (not shown) arranged to continuously monitor the rate of change of line pressure P3 over time, so enabling a prediction of future line pressure to be made (and hence when a changeover may be required). The same processor may also be used to monitor pressure PI (or P2) and is preferably to carry out the logic functions described herein to determine if a changeover should be initiated.

As indicated above, the control system monitors the levels of both the inlet and outlet pressures PI and P3. Alternatively, or additionally, it may also monitor the rate of change of the pressures PI and/or P3. In a preferred arrangement, the rate of change of the outlet pressure P3 is monitored so a prediction can be made of future line pressures whereby appropriate action can be taken based on the prediction to maintain continuity of the gas supply.

The estimated future line pressure P3 may also be used by the processor to effect switching to at least one new supply source (in addition to the duty bank), to overcome the risk of a transitory high or low line pressure condition occurring before or during supply source changeover. Where there is a finite time interval between the changeover to a new supply source being initiated and the new supply source providing gas at sufficient pressure to the dispensing system, a risk of an unacceptable, transitory high or low pressure condition may exist. By monitoring the rate of change of pressure P3 with time, the future pressure can be predicted by evaluating the direction (increasing or decreasing) and rate of change of pressure over at least one, but preferably multiple consecutive and relatively short time intervals, and so the risk of such transitory conditions arising can be reduced. -

In addition to monitoring PI and P3, the system maybe arranged to monitor the pressure of one or more standby or reserve supply sources to check that the pressure of the reserve supply source does not fad below a predetermined level - and if it does, to provide an indication that a leak exists, thereby prompting early corrective action (and so reduce further the risk of loss of gas supply).

The system may also be provided with a temperature sensor (not shown) to monitor ambient air temperature, so that a high or low temperature warning can be initiated in the event of extreme conditions that might lead to the risk of hazardous situations or damage to the equipment. The temperature thresholds set will depend on the circumstances and the gases used. For example, below -6 °C, there is a risk of separation of a normally homogenous 50%/50% v/v 0₂/N₂0 mixture into two distinct volumes of the constituent gases within a cylinder.

One way of effecting changeover is by the use of a dome (bonnet) loaded pressure regulating valve as the first stage of pressure regulation. In a preferred embodiment, the pressure regulator has a bias spring that is compressed to a predetermined length to provide a known force acting to displace a diaphragm or piston in communication with the regulating valve pin. Opposing displacement of the valve pin, regulated pressure downstream of the valve applies a force due to pressure acting over the area on the opposite side of the piston or diaphragm, such that, at some known regulated pressure equal to or lower than T5, the forces are in equilibrium and the regulated pressure is constant when no gas is being consumed by the dispensing system. The pressure at which the pressure regulating valve opens can be increased by introducing a bias pressure to a sealed dome (bonnet), resulting in additional force acting in the same direction as the bias spring to oppose the force applied over the diaphragm or piston due to regulated pressure. Accordingly, regulated pressure is then a function of the sum of the forces due to compression of the bias spring and the bias pressure within the dome. By this means, selection of a supply source can be effected by applying a dome pressure to one first stage regulator, which then provides gas to the dispensing system, whilst reducing the dome (bonnet) pressure within another (standby) first stage pressure regulator to atmosphere. In a preferred embodiment, the delivered pressure from two or more first stage regulators are delivered to a common conduit such, that in normal operation, following one finite gas supply being emptied and a new gas source being connected, as the regulated pressure from the duty supply source is opposing only the force of the bias spring of the standby pressure regulator, the diaphragm or piston is held in a position clear from contact with the valve pin. The valve pin is therefore protected from the damage that could arise when a high pressure gas cylinder valve is opened rapidly by causing the pin to rapidly strike the piston or diaphragm.

In other embodiments, the control system may be arranged to connect an additional finite gas source to an inlet should it be predicted by risk analysis that a demand for gas supply could occur that exceeds that which can be provided with a single finite gas source. Two or more sources of supply, e.g. cylinder banks, can be temporarily brought online if there is a peak demand greater than the system can supply. An alarm indication is also preferably given where a low line pressure P3 is detected when the primary supply source PI is above the pressure necessary to provide its rated design flow rate (as this indicates the flow rate demanded by the dispensing system is higher than that which can be provided within defined pressure limits, thereby indicating that the supply system was, if only temporarily, undersized for the application).

In order to safeguard against gas flow being initiated from a standby bank before changeover has occurred, and therefore without pressure being applied to a dome valve of the first stage regulator designated as standby at that time (as could otherwise happen when flow demand is very low and the primary supply source is tending towards empty but the line pressure P3 has never tended towards its high or low pressure set point), the primary supply pressure PI is preferably monitored so that when the pressure of the duty bank has fallen to just above the value of first stage pressure regulator outlet pressure P2 when no pressure is applied to the dome changeover is initiated after a predetermined (and relatively short) time interval, for example, after 2 s. This corresponds to the situation described below for threshold T5.

A further safeguard is preferably provided for gaseous sources of supply whereby standby or reserve supply source pressures are continuously monitored so that if the primary supply source pressure PI remains below a predetermined low pressure and is substantially constant but there is a reduction in pressure of the standby or reserve sources of supply, changeover should be initiated (as this indicates that the standby source is supplying gas in lieu of it being selected to do so). This safeguard is desirable so that an alarm can be given to indicate that the operation of one or more pressure regulators may have 'drifted', e.g. due to spring relaxation, thereby indicating that the system requires recommissioning or that maintenance may be required.

The control system described is also preferably provided with a user interface, for example, a display screen, providing both qualitative and quantitative data of operating parameters and alarm conditions. Preferably, this data can also be transmitted to a remote location via a network or by other means.

The gas supply system described above thus provides a control system which enables more efficient use of gas within finite sources such as gas cylinders by enabling more gas to be suppied therefrom before they are replaced by full cylinders and thus reduces the amount of gas that is otherwise wasted. At the same time, this is achieved without compromising the reliability or safety of the gas supply.

For a typical system having a nominal 4 bar pipeline pressure, such a control system can enable cylinder pressures to be depleted to as little as 5 bar when the flow demanded by the pipeline system is very low. This can significantly reduce the number of gas cylinders used by a hospital, e.g. by as much as 8% or, in other terms, reduce the amount of waste gas returned to the supplier by as much as 75%. This also provides a substantial cost saving.

Figure 3 shows a diagram that illustrates the thresholds used in the control system of a typical gas supply system of the type described above. The outlet pressure P3 is monitored in relation to a variety of pressure thresholds as will be described further below. The diagram shows the outlet, or delivered, pressure P3 on the vertical axis and the horizontal axis represents the inlet pressure PI as this gradually falls over time (as the gas is used).

The following thresholds are shown:

Tl: A high pressure alarm limit. This is a predetermined gauge pressure at which an alarm condition is initiated if the delivered pressure P3 of the pipeline rises above it. Figure 3 shows a high pressure alarm limit of 504 kPa (which is given as 120% of the nominal delivery pressure of 420 kPa as required by the relevant industry standard)

T2: A static set pressure. This is the pressure at which the line pressure regulator valve(s) will close completely. The system is said to be static when there is no gas being taken from the pipeline system. Assuming sufficient gas supply is available, the delivered pressure will reach the static set pressure when the system is static. In figure 3, the static set pressure is 467 kPa.

T3: Delivered pressure at rated flow. This is the gauge pressure that the supply system will deliver at a stated maximum volumetric flow rate. In figure 3, the pressure at maximum rated flow is given at 90% of the static set pressure, which is 420 kPa. The pressure at maximum rated flow is also the nominal pipeline pressure (as described in the industry standard referred to above).

ΊΑ; Rate of change measurement limit. This is a gauge pressure below which the control system monitors the rate of change of the delivered pressure with time. When the rate of change of delivered pressure with time is greater than a predetermined rate, changeover is initiated (as this indicates that the cylinder pressure is quickly tending towards a level that is considered too low for the supply system to maintain the delivered line pressure for a long enough period to prevent a low a)arm pressure condition being initiated before transition to a standby supply source can be completed).

T5: Automatic changeover pressure. This is a gauge pressure at which changeover from the current supply source to the standby supply source is automatically initiated (if the cylinder pressure PI is also below a predetermined minimum level). NB At this pressure, the rate of change of delivered pressure with time has no influence in determining whether or not a changeover is required. In a simple embodiment of this invention, the automatic changeover pressure threshold T5 may be used in lieu of monitoring the rate of change of delivered pressure with time.

T6: Low pressure alarm limit. This is a gauge pressure at which both a low pressure alarm condition is initiated and the control system selects at least two supply sources as the duty supply. After a sustained interval (during which the delivered pressure has been maintained above the automatic changeover pressure), the supply system may then revert back to using only the original duty supply source.

The key feature of the gas supply system according to the present invention is that it monitors both supply and outlet pressures (or parameters indicative of these) and uses both measurements to determine whether a changeover should be initiated. In the preferred arrangement described above, the control system is arranged to initiate changeover when PI (or P2) falls below a predetermined level (TO) AND the outlet pressure P3 falls below T4 and the rate of decease is greater than a predetermined rate, or the outlet pressure falls below T5. Whilst just P3 could be monitored, e.g. in relation to thresholds T4 and T5 described above, this could result in a changeover being initiated when the gas cylinder was still full, or nearly full during a period of particularly high flow, PI (or P2) should therefore also be monitored to avoid this happening.

Figure 3 also illustrates how the present invention enables more gas to be used compared to the prior art. The prior art initiates a changeover at point X solely upon measurement of PI (or P2), e.g. when PI falls below a threshold (TO) of 1400 kPa as shown, whereas the present invention initiates the changeover at point Y when PI is about 600 kPa (the changeover in this example being triggered by the pressure P3 falling below T4 and falling at a rate which is greater than a predetermined rate).

The system preferably also has alarms which indicate if high or low pressure conditions occur (as indicated by thresholds Tl and T6). These are not however used in the control system described for determining when to effect a changeover.

In a simple embodiment of the invention, e.g. employing dome loaded first stage pressure regulators, changeover from a primary to a standby gas source is initiated when the primary supply source pressure PI falls below the 1st stage regulator set pressure AND the line pressure P2 falls below (or above) a predetermined threshold.

Various aspects of the gas supply system described above can be altered without departing from the scope of the invention. For example:

1. The system may control more than two finite sources of gas supply

2. Solenoid valves positioned within gas delivery conduits may be used to directly switch supply sources. These may be provided at any position within the supply assembly i.e. high pressure, intermediate pressure or line pressure.

3. As indicated above, the supply source may be indirectly switched, e.g. by applying a bias pressure to a dome loaded first or second stage pressure regulator.

4. Gas sources may be switched by means of single directional control valves with multiple positions and ways.

5. Pressure switches may also be used as opposed to pressure transducers. 6. The first sensing means may sense some other parameter, besides pressure, which is indicative of the quantity of gas remaining in a finite source of gas and the second sensing means may sense some other parameter, besides pressure, which is indicative of the quantity of gas being supplied to the dispensing system (or the ability to supply gas to meet the demand of the dispensing system).

A gas supply system has been described with particular reference to its application to a the supply of gases in a medical establishment but it will be appreciated that it can be used in many other applications and in other fields that use gas from multiple finite supply sources.

Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

Claims

1. A gas supply system for providing a continuous supply of gas from a gas source comprising at least first and second finite sources of gas, the system comprising:

at least one gas inlet for receiving gas from a gas source comprising at least first and second finite sources of gas;

a selector for selecting which of the finite sources of gas is to supply gas to the gas supply system;

a gas outlet for providing a continuous supply of gas to a gas dispensing system;

a first sensor for sensing a first parameter indicative of or related to a quantity of gas remaining in the finite source of gas from which the gas supply system is currently arranged to receive gas;

a second sensor for sensing a second parameter indicative of or related to a pressure of gas supplied by the gas outlet to the gas dispensing system; and a controller arranged to actuate the selector to select a different finite source of gas to supply gas to the gas supply system dependent upon the levels and/or rate of change of the first and second parameters.

2. The system as claimed in claim 1, wherein the first parameter is indicative of or related to an inlet pressure (PI) of gas at the at least one gas inlet and the second parameter is indicative of or related to an outlet pressure (P3) of gas at the gas outlet.

3. The system as claimed in claim 2, being of a type in which the outlet pressure (P3) tends to fall as the inlet pressure (PI) falls, and wherein the controller is arranged to actuate the selector when the following conditions are satisfied: the inlet pressure (PI) falls below a first threshold, and either the outlet pressure (P3) falls below a second threshold (T4) and the rate of change of the outlet pressure (P3) is greater than a predetermined rate, or the outlet pressure (P3) falls below a third threshold (T5) which is lower than the second threshold (T4).

4. The system as claimed in claim 2, being of a type in which the outlet pressure (P3) tends to rise as the inlet pressure (PI) falls, and wherein the controller is arranged to actuate the selector when the following conditions are satisfied: the inlet pressure (PI) falls below a first threshold, and either the outlet pressure (P3) rises above a second threshold and the rate of change of pressure (P2) is greater than a predetermined rate, or the outlet pressure (P3) rises above a third threshold which is higher than the second threshold.

5. The system as claimed in any of claims 1 to 4, wherein the selector comprises valves.

6. The system as claimed in claim 5, wherein the selector comprises solenoid- actuated shut-off valves.

7. The system as claimed in claim 5, wherein the selector comprises dome-loaded pressure regulators.

8. The system as claimed in any of claims 1 to 7, wherein the controller comprises a processor for determining when changeover is to be initiated and actuating the selector.

9. The system as claimed in claim 8, wherein the controller is arranged to monitor the outlet pressure (P3) relative to a plurality of thresholds to determine when changeover is to be initiated and/or an alarm signal is to be generated in respect of one or more other conditions.

10. The system as claimed in claim 8 or 9, wherein the controller is arranged to monitor the rate of change of the outlet pressure (P3) so that a prediction can be made of future outlet pressures, whereby appropriate action can be initiated, based on the prediction, to maintain continuity of the gas supply.

11. The system as claimed in claim 10, wherein the control system is also arranged to connect an additional finite gas source to an inlet where a demand for gas supply is predicted that exceeds that which can be provided with a single finite gas source.

12. A method of operating a gas supply system for providing a continuous supply of gas from a gas source comprising at least first and second finite sources of gas to a gas dispensing system, the method comprising the steps of:

sensing a first parameter indicative of or related to a quantity of gas remaining in the finite source of gas from which the gas supply system is currently arranged to receive gas;

sensing a second parameter indicative of or related to a pressure of gas being supplied by the gas supply system to the gas dispensing system; and selecting a different finite source of gas to supply gas to the gas supply system dependent upon the levels and/or rate of change of the first and second parameters.

13. The method as claimed in claim 12, wherein the first parameter is indicative of or related to the inlet pressure (PI) of gas being supplied to the gas supply system and the second parameter is indicative of or related to the outlet pressure (P3) of gas being supplied to the gas dispensing system.

14. The method as claimed in claim 14, wherein a different finite source of gas is selected when the following conditions are satisfied:

the inlet pressure (PI) falls below a first threshold, and either the outlet pressure (P3) falls below a second threshold (T4) and the rate of change of the outlet pressure (P3) is greater than a predetermined rate, or the outlet pressure (P3) falls below a third threshold (T5) which is lower than the second threshold (T4).

15. The system or method of any of claims 1 to 14, wherein the gas comprises gases and vapours whether stored in compressed gaseous form or stored in liquid form.