This invention relates to flow control in a gas supply system. In particular it relates to a system to satisfy a variable demand for a gas mixture produced by mixing gases from different sources. In a specific embodiment it relates to the intermittent supply of a mixed gas flow of consistent proportions obtained from two or more gas sources.

Because gas demand patterns are usually variable, the supply of a consistent quality of mixed gas often creates difficulties, especially if one or more of the supply sources is a generator such as an air separation system. For example in providing a constituent gas from a generator the installation of a generating capacity equal to the peak demand flow-rate for the constituent not only tends to be uneconomic in terms of capital cost but may also involve highly intermittent operation, with frequent start-up and shut-down of the generator and a consequent increase in the rate of wear of its component parts. Instead it is common to install a generator with a capacity less than would be required to satisfy peak demand levels at all times and to include a product gas receiver with sufficient capacity to meet the peaks. Such a system can satisfy fluctuating demand, even with high peak flow-rates, and has the further advantage that the generator runs for longer periods at a steady rate.

For most duties the gas delivered from the receiver must not fall below a defined delivery pressure. This requires that refilling of the storage receiver must begin before the receiver pressure falls below the defined pressure, rather than when it becomes empty, and imposes further requirements

on the gas flow control system.

The need for intermittent supply of a mixture of selected gases arises in several instances of the manufacture and dispensing of food products, beverages and medical products. Two much-utilised examples of this are the use of mixed gases in beer dispensing and in the provision of storage gases for packaged foodstuffs. The mixing is typically effected by supplying two separate constituent gases to a mixing valve. If the mixing valve operates directly to supply the mixed gas on demand, it must include an extremely precise and stable control mechanism in order to achieve an adequately consistent composition of mixed gas over a wide range of flowrates. For example in beer dispensing the pattern of demand may involve variations of gas flowrate of up to two orders of magnitude. Additionally, any small (and thus potentially undetected) leaks in the mixed gas supply lines will cause the direct mixing valve to operate for significant periods at very low flowrates. Direct mixing valves are therefore both complex and expensive.

In one significant embodiment the invention provides an improved system for both the mixing and the intermittent delivery of a gas mixture of consistent composition, whereby the mixing of the constituent gases is effected at a constant flowrate or over a repeated and limited range of flowrates. The mixed gas is stored in a gas receiver, from which it is delivered on demand via an appropriate regulating valve to the point of use.

In its broadest aspect the present invention relates to a gas control system which ensures that supply of a gas mixture to a receiver is halted whenever the receiver pressure reaches a defined peak level and is restored whenever the receiver pressure falls to a defined base

level. This system is typically part of an installation to supply on demand a selected blend of different gases.

Thus according to the invention there is provided a control system for the supply of a gas mixture having a selected ratio of constituent gases which system comprises sources of at least two pressurised gases, a mixing zone for the gases, a control device and a gas receiver, characterised in that the control device includes an adjustable flow regulator upstream of the mixing zone for each of the gas sources and a controller which does not require electrical power and which is activated by the pressure in the receiver so as to stop all the source gas supplies when the receiver pressure reaches a pre-set upper level and to start all the source gas supplies when the receiver pressure reaches a pre-set lower level.

The invention offers particular advantages in the supply of gas mixtures of consistent quality in such fields as food packaging, beverage dispensing and medical gas supplies. It is primarily described herein with reference to mixing a carbon dioxide stream with a nitrogen-rich gas stream, which provides a mixture widely employed in beer dispensing.

Depending upon the application the sources of the constituent gases may be one or more of pressurised gas storage (for example conventional gas storage cylinders), liquefied gas storage or on-site generation, for example by pressure swing or membrane separators. The individual sources can be of a single gas, e.g. carbon dioxide, or mixture of gases, e.g. air or oxygen-depleted air.

The gas flow regulator for each of the gas sources preferably includes an adjustable flow restrictor, for example an adjustable needle valve. Each source can thereby be individually controlled to provide a desired feed rate of

constituent gas both in absolute terms and relative to the flows of the other gases. Setting the respective restrictors to the desired levels thus determines the gas ratio in the gas mixture.

Each of the flow restrictors is preferably used in association with a biased-diaphragm flow control valve. As discussed in greater detail below with regard to specific embodiments of the invention, these maintain a constant gas pressure difference across the restrictor and thus ensure a consistent gas flow for a given setting of the restrictor, regardless of the pressure in the mixed gas receiver.

Biased-diaphragm flow control valves, which are also employed in certain other preferred aspects of the invention as discussed below, typically comprise an enclosed zone and a gas inlet and gas outlet. The outlet is closed and opened by a closure plug which in the closed position fits into a seat in the outlet. The plug is attached to a flexible diaphragm which divides the enclosed zone into a main chamber and a pilot chamber. Means are provided in the pilot chamber to bias the diaphragm and plug either towards or away from closing the valve. In the bias-closing version, gas introduced into the main chamber applies a force to the diaphragm and when this force exceeds the bias it displaces the diaphragm, thereby lifting the plug from its seat and permitting gas flow through the valve. If the inlet pressure falls to a level at which the force it applies to the diaphragm is less than the bias the plug is returned to its seat and stops the gas flow. The bias-open version operates in a similar manner but so as to keep the valve open until the pressure in the pilot chamber is sufficient to close the valve. The bias means are of two main types: a spring-loaded type> having an adjustable spring acting upon the diaphragm, and a dome-loaded type, having a gas pressurised to the desired level in a "dome"

pilot chamber. Establishing and adjusting the set pressure in the dome chamber has conventionally been achieved by employing a separate gas pressure source to inject gas to the desired pressure prior to operation of the system. In some embodiments of the present invention it is preferably achieved by supplying gas to a pilot chamber from another point in the control system.

It is generally preferred to include at least one further gas regulator in each of the gas source lines upstream of the adjustable flow regulator. Non-return valves can usefully be provided at various points in the system, for example in one or more of the the lines upstream of the adjustable flow regulators so as to prevent back-flow of one source gas into the feed line of another gas, or in the final supply line to the gas receiver so as to prevent back-flow of stored gas when the supply is stopped.

The regulators in the respective source gas lines should be adjusted to ensure that any difference between the gas pressures at the point of mixing is not sufficient to permit any significant back-flow of one source gas into the feed line of another gas.

An enlarged chamber for mixing the gases can be provided if desired but in general a sufficient degree of mixing is readily achieved by simply arranging for the lines from the respective gas source regulators to join together in a common line leading to the control device, such that the common line provides the mixing zone.

The start/stop controller for the gas mixture should include two or more control units. These are activated by the pressure in the receiver and preferably are biased-diaphragm flow control valves as described above. The controller can be arranged to stop the respective flows of source gases by

closing a valve or valves in the supply lines.

In one specific embodiment of the invention there is provided a start/stop controller for the pressurised gas mixture which comprises a pilot-controlled, biased-diaphragm valve with a main chamber and a pilot chamber, and further comprises a second pilot-controlled, biased-diaphragm valve with a main chamber and a pilot chamber, the inlet to the main chamber of the first valve is connected to the gas source, the pilot chamber of the first valve is connected by a first pilot line to the main chamber of the second valve, the pilot chamber of the second valve is connected by a second pilot line to the receiver, the outlets from the main chambers of both valves are connected to the receiver, a by-pass line containing a non-return valve leads from the outlet of the second valve to the main chamber thereof and the bias loading in the second valve in the direction of closing is greater than the bias loading in the first valve in the direction of closing.

This configuration of start/stop controller provides for the following phases of operation of the system.

(1) An initial start-up phase in which the mixed gas stream enters the main chamber of the first valve, acts against the valve-closing bias of the diaphragm to open the valve and passes through the valve to the receiver, which is initially empty, so as to fill the receiver and begin to build-up pressure therein. Gas from the first valve also passes through the by-pass line to pressurise the main chamber of the second valve and the pilot chamber of the second valve. Due to its bias loading the second valve remains closed.

(2) A receiver-pressurising phase during which, because the bias loading in the second valve is greater than the bias loading in the first valve, the first valve remains open and the feed stream continues to pass to the receiver.

This phase continues until the pressure in the receiver reaches a level (the "upper level") at which the back-pressure from the receiver to the pilot chamber of the first valve (through the by-pass line and first pilot line) combined with the bias loading therein also closes the first valve. The closure of the first valve stops the feed of gas to the receiver.

(3) A gas withdrawal phase in which gas is drawn from the receiver for use in the required application. Because no feed gas reaches the receiver during this phase the receiver pressure gradually falls from the upper level to the base level but the pressure in the pilot chamber of the first valve and in the main chamber of the second valve remains at substantially the upper level due to the action of the non- return valve in the second valve by-pass line.

(4) A feed restoring step in which the receiver pressure initially falls to just below the base level. At this point the combined bias loading of the second valve and the back pressure to its pilot chamber from the receiver fall below the pressure in its main chamber and the second valve reopens. The gas pressure in the main chamber and in the first valve pilot chamber is thus released to the receiver. The system thus reverts to phase 1, the feed gas supply stream is restarted and, since the feed gas pressure is greater than the combined bias loading of the first valve and the reduced pressure in its pilot chamber, the first valve reopens, restoring gas supply to the receiver.

The controller thus cycles from phases 1 to 4 and back to 1. The receiver pressure thereby cycles between the base and upper levels. The range between these pressure levels is equivalent to the difference in bias loading between the first and second valves. The bias loading of the respective valves can accordingly be selected to provide a desired frequency of starting and stopping the gas supply for a given duty.

If required, gas can also be withdrawn from the receiver during phase 2, i.e. while supply from the source is continuing. A rate of withdrawal which reduces the reservoir pressure to below the base level is however generally to be avoided and the system will normally be sized relative to the required duty so as to avoid this possibility.

In one convenient configuration of this first version of start/stop controller the by-pass line and non-return valve are constructed within the casing of the second valve.

In a further specific embodiment of the invention there is provided a version of start/stop controller which comprises two pilot-controlled, biased-diaphragm valves each having a main chamber and a pilot chamber, the first valve being biased to the open position and the second valve being biased to the closed position, the inlet to the main chamber of the first valve is connected to the gas source, the pilot chamber of the first valve is connected by a first pilot line to the main chamber of the second valve, the pilot chamber of the second valve is connected by a second pilot line to the receiver, the outlet from the main chamber of the first valve is connected to the receiver and the inlet to the main chamber of the second valve is connected to a gas source having a pressure above the receiver pressure.

This alternative configuration of start/stop controller provides for the following phases of operation.

(1) An initial start-up phase in which the mixed gas stream passes through the main chamber of the first valve through the open valve to the receiver, which is initially empty, so as to fill the receiver and build-up pressure therein. This phase continues until the receiver pressure reaches a level (the "upper level") at which the

back-pressure from the receiver to the pilot chamber of the second valve combined with the bias loading in the second valve opens the second valve and connects the higher-pressure gas source to the pilot chamber of the first valve, thereby closing the first valve and stopping the flow of mixed gas to the receiver.

(2) A gas withdrawal phase in which gas is drawn from the receiver for use in the required application. Because no feed gas reaches the receiver during this phase the receiver pressure gradually falls from the upper level to a base level at which the second valve recloses and the first valve re-opens. Supply of gas to the receiver is thereby restored.

The controller thus cycles between phases 1 and 2 and the receiver pressure thereby cycles between the base and upper levels. The range between these pressure levels is again equivalent to the difference in bias loading between the first and second valves which again determines the frequency of starting and stopping the gas supply for a given duty.

The gas pressures in the respective source and supply lines are largely dictated by the specific sources and the required duty. For most applications in the food, beverage and medical markets the pressures can be relatively low, typically 550 to 900 kPa (80 to 130 psig) for the source gases and 70 to 550 kPa (10 to 80 psig) for the withdrawn gas mixture.

The invention provides an improved control system for gas mixture supply which offers the advantages of consistent product quality, ease of adjustment to give ^' a desired product mix and infrequent switching of component parts with a resultant saving in wear and corresponding increase in reliability. It is especially well suited to application in the on-site mixing of gases such as carbon dioxide and

nitrogen in beverage and food packaging duties. Such applications may call for the supply of more than one gas mixture and these can readily be supplied according to the invention. In certain duties more than one of the gas mixtures can be supplied from a single membrane separator sys em.

In principle, the gases are to be mixed at a controlled ratio and the mixture supplied to charge one or more receivers up to a pre-determined pressure, at which point the system will automatically, and without external electrical power, interrupt the flow of both feed gases. When the usage of the mixed gas from the receiver(s) reduces the pressure therein to a lower, predetermined, level then the system resumes supply of the mixed gas and the working cycle is repeated.

The invention is illustrated below with reference to the accompanying Figures 1 and 2, which are flow diagrams of two different systems according to the invention. The figures are presented so as to show clearly the flow control features of the invention but are not to scale.

The system illustrated in Figure 1 comprises two gas supply lines 110 a/b and 120 a/b. Each line has a pressure regulator (112, 122 respectively) and a flow controller 114, 124 with a flow adjustment restrictor valve 115, 125. The pressure regulator valve 122 is of a conventional, screw-adjustable type but the regulator 112 is of the pilot-operated, diaphragm type (with the diaphragm biased towards the closed position) .

Both flow controllers 114, 124 are of the pilot-operated, spring-loaded diaphragm type. Each of them has a non-return valve (116, 126) in its outlet line. The pilot chambers of the flow controllers 114, 124 and of the pressure regulator

112 are all pressurised, via pilot lines 111, Ilia, 111b and 121, by the gas pressure in the feed line 120b downstream of the regulator valve 122.

Downstream of the flow controllers 114, 124 the lines 110 and 120 join to form a common line 117 leading to a relay 132 which leads in turn to a second relay 142. The relays 132 and 142 are also of the pilot-operated, spring-loaded type and together with their associated pipework, form the start/stop controller for the system. Each of them has a main chamber, 136, 146 respectively and a pilot chamber 137, 147 separated by a flexible diaphragm 130, 140 which contacts a closure plug 139, 149 and is biased towards closure of the main chamber 136, 146 by a spring 131, 141. Each further includes a gas inlet 133, 143 and a gas outlet 134, 144 leading respectively to and from the main chamber 136, 146. The outlets 134, 144 are both directly connected via an outlet line 118 to a receiver 151.

Each of the relays 132, 142 additionally has a port 138, 148 into its pilot chamber 137, 147. The pilot chamber 137 of relay 132 is connected via ports 138 and 143 and line 119 to the main chamber of relay 142. The pilot chamber 147 of relay 142 is connected via port 148 and a line 152 to the line 118 leading to the receiver 151. Finally the relay 142 has incorporated in its structure a line 153, with a non-return valve 154, to by-pass the outlet port 144.

The operation of the unit shown in Figure 1 is described below with reference to the use of nitrogen as the gas supplied from a membrane source through the feed line 110a and carbon dioxide as the gas supplied from compressed storage through the feed line 120a.

In the following description the quoted pressures are included by way of example; operation is not restricted to

such values.

With their respective pressure regulators (112, 122) both set to 550 kPa [80 psig] , uniform flows of nitrogen and of carbon dioxide both at this pressure are supplied to the flow controllers (114, 124) for all common downstream pressures up to approximately 35 kPa [5 psig] below the regulator (112, 122) pressure, i.e. up to 515 kPa [75 psig] . The two flows mix in line 117 then open the relay 132 to commence charging of the receiver 151 (which is empty prior to the initial start-up) .

The regulator 112 acts as a pressure equalising valve which controls the nitrogen pressure supplied to the restrictor 115 associated with flow controller 114 and keeps it equal to the regulated carbon dioxide pressure supplied to the pilot chambers of both flow controllers 114 and 124. The inlet nitrogen pressure in line 110a is preferably arranged to be always greater than the regulated carbon dioxide pressure in line 111.

The gas mixture accordingly passes through relay 132 to the receiver 151 at substantially constant flow and gradually builds up the pressure in the receiver 151.

The pressure in the receiver 151 acts on both sides of the diaphragm 140 in the relay 142, via the pilot line to the pilot chamber 148 and via the by-pass line 153 and check valve 154, and thus the spring 141 (which typically represents a force of about 100 kPa [15 psig]) keeps the relay 142 closed. The pressure in the pilot line 119 continues to build up with the increasing receiver pressure due to continuing gas flow back through the by-pass line 153. The relay 132 continues to allow passage of gas to line 118, as the receiver pressure at its pilot inlet 138 continues to build up.

When the receiver pressure reaches 480 kPa [70psig] , the combination of pilot pressure and spring loading in relay 132 shuts it off and thus stops the mixed gas feed to the receiver The relay 142 remains shut under the action of its spring force (equivalent to 100 kPa) . As gas is drawn out of the receiver 151, the pressure at ports 134 and 144 falls, and the pressure in the pilot chamber 147 of relay 142 begins to fall. When the receiver pressure falls to 380 kPa [55psig] , the relay 142 suddenly opens, connecting this 380 kPa receiver pressure to port 138 of relay 132, causing it to open and thus reopening the feed path through relay 132 for gas from the supply line 117 to the receiver 151.

The receiver 151 accordingly recharges from 380 to 480 kPa and the cycle is repeated.

The close "tracking" imposed by the common pilot line pressure on the flow-rates from flow controllers 114 and 124 as the pressure in common line 117 approaches its limiting value (i.e. when the relay 132 is about to close) is of advantage in the operation of the relay 142 because its bias spring has the smaller equivalent pressure and thus defines the start/stop pressure range of supply to the receiver 151. Any variation between the respective flows through flow controllers 114 and 124 as the quantity of gas delivered to the receiver 151 decreases is an increasing proportion of the total quantity of gas delivered. In the decreased flow conditions minor differences between the physical characteristics of flow controllers 114 and 124, which may be undetectable at their normal constant flow condition, become important. The tracking ensures that any variations are proportionately reduced as the flow decreases,

In addition to ensuring closer matching of flow controllers 114 and 124 over their whole operating range, the common

pilot line configuration ensures that a failure of the carbon dioxide supply will prevent any gas being delivered to the downstream receiver. It also simplifies the control and setting of the gas blending system since only one regulator has to be adjusted initially.

The system illustrated in Figure 2 is similar to that of Figure 1 but has some modifications, particularly in regard to the start/stop controller. Components which are generally the same or similar to those described with reference to Figure 1 utilise the same reference numerals as in that figure, but in the 200 series.

The system again comprises two gas supply lines 210a/b and 220a/b, with pressure regulators 212, 222, flow controllers 214, 224 and associated flow adjustment restrictors 215, 225. In this Figure 2 version the non-return valves 216a, 226a, are not in a downstream location relative to the flow controllers 214, 224. The non-return valve 216a in the nitrogen supply line 210a is upstream of the regulator 212 and the non-return valve 226a in the carbon dioxide supply line 220 is located between the the regulator 222 and controller 224. The pilot chambers of the flow controllers 214, 224 and of the pressure regulator 212 are again pressurised by the gas pressure in the feed line 220b downstream of the regulator valve 222.

In this version the common line 217 leads to a relay valve 282 which with an associated relay valve 292 forms the start/stop controller for the system. The valves 282 and 292 (shown by conventional pneumatic valve symbols) each have a diaphragm dividing them into a main chamber (286, 296) and a pilot chamber. 282 is a valve which when the pressure applied via line 298 to its pilot port 287 reaches a pre-determined level will exhaust via the outlet port 284. 292 is a further valve which when the pressure applied

to its pilot port 297 reaches a pre-determined level will switch to its main chamber outlet port (connected to line 298) a pressurised gas from a source 289.

The outlet port 284 from the valve 282 leads through a non-return valve 299 and outlet line 218 to the receiver 251. The pressure source 289 applied to valve 292 is a secondary source, in this example carbon dioxide gas at 950 kPa (140 psig). A particularly convenient source of this carbon dioxide is the gas in line 220a supplying the regulator 222.

The operation of the unit is again described with reference to the use of nitrogen as the gas supplied from a membrane source through the feed line 210a and carbon dioxide as the gas supplied from compressed storage through the feed line 220a.

The gas flow ratios (and therefore the gas blend ratios) are set by adjustable restrictors 215 and 225.

The regulators 212, 222 and flow controllers 214, 224 maintain constant pressures across the restrictors 215 and 225, thereby enabling set flow rates (and therefore the gas proportions in the blend) to remain constant regardless of the pressure in receiver 251 or the nitrogen or carbon dioxide supply pressures in lines 210a and 220a respectively. Non-return valve 216a prevents escape of carbon dioxide in the event of loss of pressure in line 210a.

In valves 214 and 224 a pilot pressure is applied in the pilot chambers and a lower, predetermined pressure is maintained in the main chambers, regardless of the pressure in line 217. The regulator 212 keeps the nitrogen supply pressure down to that of the carbon dioxide supplied by the

regulator 222.

The gases delivered from the restrictors 215 and 225 and valves 214 and 224 in the required proportions are mixed in line 217. The mixed gas passes through the normally open valve 282 and via line 218 into the receiver 251.

The pressure in the receiver 251 is sensed by the pilot chamber of the normally closed valve 292. Once the pressure in the receiver 251 reaches a predetermined level the output port of valve 292 is switched to connect with the pressure source 289 and this same pressure is thus applied to the pilot port 287 of valve 282.

Pressure in its pilot chamber causes the valve 282 to switch, thus interrupting the flow of mixed gas to the receiver. The non return valve 299 prevents mixed gas from the receiver 251 discharging through valve 282.

Mixed gas is withdrawn on demand from the receiver 251. As this happens the receiver pressure falls. When the receiver pressure, sensed by the pilot 297, reaches a predetermined level the valve 292 switches, thus exhausting the established pressure in the pilot line 298 which, in turn, causes valve 282 to reset and to open the flow again via line 217 to the receiver 251.