GAS SUPPLY SYSTEM

Abstract: A gas supply system in which the gas source is a permeable membrane unit and which is required to satisfy a variable demand is provided with more stable control of gas output purity, with respect to fluctuations in the pressure of feed gas applied to the membrane, by a valve configuration which comprises, in sequence from the membrane to a gas storage receiver, a flow-adjusting valve, a pressure differential maintaining device and a non-return valve. The pressure differential maintaining device comprises a main gas chamber, a pilot gas chamber, and a diaphragm between the chambers which operates a closure plug for the main chamber gas outlet. The plug is activated by the respective pressures in the main gas chamber and pilot gas chamber and has an associated means to bias it away from its seat. The pilot chamber has an associated pressurising line from a point in the pipework upstream of the pressure differential maintaining device.

Description: GAS SUPPLY SYSTEM This invention relates to gas flow control in a permeable membrane gas supply system. In particular it relates to such a gas supply system which is required to satisfy a variable demand from a steady rate of gas production. Permeable membranes for the separation of gases from gas mixtures are well known and find widespread application, especially in the generation of nitrogen-rich gas from compressed air. The advantages of membrane separation systems include operational convenience and low operating cost. Most gas demand patterns tend to be variable, often fluctuating from as low as zero to relatively high flow rates according to the time of day or stage of the process being supplied. When the supply is from a generator the provision of a generating capacity equal to the peak demand flowrate tends to be uneconomic in terms of capital cost and also may also involve unduly frequent start-up and shut-down of the generator, with resultant increase in the rate of wear, especially of the compressor components. Instead it is usual to install a generator with a capacity sufficient to meet the mean demand and to include a product gas receiver downstream of the generator. By sizing the receiver to hold sufficient stored gas to meet the peak demand the generator can satisfy the fluctuating demand, even with high peak flowrates, and runs for longer periods at a steady rate. In many applications it is also necessary for the gas to be generated at a constant purity, e.g. the concentration of residual oxygen in a nitrogen-rich product stream separated from air is usually required to be at a defined low level. The purity of the product stream from a membrane unit can be held constant by maintaining the air supply pressure, the membrane temperature and the non-permeate flowrate all at constant levels. In a typical arrangement an air compressor delivers air at constant pressure to a membrane unit to produce a nitrogen-rich gas which is stored in a receiver vessel. Downstream of the membrane unit there are usually provided, in sequence, a flow-control needle valve, a pressure-maintaining device, a non-return valve and a pressure switch. The flow-control needle valve determines the rate of gas delivery from the membrane unit; the pressure-maintaining device ensures that the membrane unit operates at a constant outlet pressure, and thus delivers gas at a constant purity; the non-return valve prevents backflow of stored gas from the receiver to the membrane unit and the pressure switch is linked to the compressor control so as to switch off the compressor when the pressure in the receiver approaches the air compression pressure. Conventionally, the pressure-maintaining device is a pressure-maintaining valve (or back-pressure valve) which is set to allow gas flow when the supply pressure to its inlet attains a preset gauge pressure. There are a number of available pressure-maintaining valves which employ a flexible diaphragm linked to a closure plug which engages a plug seat in the gas outlet. When the inlet pressure reaches the set level it acts upon the diaphragm so as to move the plug from its seat and thus permit gas flow. If the inlet pressure falls below the set level the diaphragm returns the plug to its seat and stops the gas flow. There are two main versions of such diaphragm-activated valves: the spring-loaded type, in which the level is set by an adjustable spring acting upon the diaphragm, ' and the dome-loaded type, in which the level is set by gas pressurised to the desired level in a closed "dome" chamber on the side of the diaphragm opposite to the feed gas. Establishing and adjusting the set pressure in the dome chamber is achieved by employing a separate gas pressure source to inject gas to the desired pressure prior to operation of the system. Such dome valves have low differential pressure across the diaphragm during system operation, but during shut-off periods the diaphragm has to withstand the full gauge pressure in the dome chamber. It is critical that no leakage occurs from the chamber otherwise the valve will fail. In order to make the most economical use of the system it is desirable to fill the receiver to a pressure as close as possible to the pressure of the compressed gas feed and thus to utilise the receiver's storage capacity as fully as the feed pressure permits. This is particularly important for applications requiring the product gas to be at high pressure. The level of the pressure setting of the pressure-maintaining device must therefore reflect the available pressure from the gas compressor and the optimum storage pressure in the receiver. This level should be as high as possible and just above the cut-off setting of the pressure switch. In practice the achievement of these settings for maximising the utilisation of generated gas creates a risk of failure to deliver gas: if the supply pressure to the inlet of the pressure-maintaining device inlet does not reach the set pressure of the device, no gas will flow through the device to the receiver. The present invention relates to an improved type and disposition of pressure differential maintaining device to overcome this risk and to improve the control of purity of gas output from a permeable membrane gas generator. Thus according to the invention there is provided a gas supply system which comprises a permeable membrane gas generator, a storage receiver for generated gas, pipework from the said generator to the said receiver having in sequence a flow-adjusting valve, a pressure differential maintaining device and a non-return valve, wherein the pressure differential maintaining device comprises a main gas inlet, a main gas chamber, a main gas outlet, a pilot gas chamber, a closure plug with an associated seat in the main gas outlet, a flexible diaphragm between the main gas chamber and pilot gas chamber, the said diaphragm carrying the closure plug and being activated by the respective pressures in the main gas chamber and pilot gas chamber, characterised in that the closure plug has an associated means to bias the plug away from the seat and in that the pilot chamber has an associated pressurising line from a point in the pipework upstream of the pressure differential maintaining device. The invention further provides a gas generation process in which compressed gas from a permeable membrane generator is passed through a flow-adjusting valve, a pressure differential maintaining device and a non-return valve to a storage receiver wherein the pressure differential maintaining device comprises a main gas inlet, a main gas chamber, a main gas outlet, a pilot gas chamber, a closure plug with an associated seat in the main gas outlet, a flexible diaphragm between the main gas chamber and pilot gas chamber, the said diaphragm earring the closure plug and being activated by the respective pressures in the main gas chamber and pilot gas chamber, characterised in that the closure plug has an associated means to bias the plug away from the seat and in that the pilot chamber has an associated pressurising line from a point upstream of the pressure differential maintaining device to maintain the pressure in the pilot chamber. The flow-adjusting valve, which can for example be a needle valve or other adjustable orifice valve, complements the flow control imposed by the pressure differential maintaining device and permits fine tuning thereof. The bias means is preferably a spring, most preferably a coil spring, located between the diaphragm and the valve seat. Conveniently the spring is shaped and located so as to bear upon the closure plug rather than the diaphragm as such, so that the force of the spring is carried directly by the plug. In the system and process according to the invention the pressure setting of the pressure differential maintaining device is determined by the difference between the pressure in the upstream pipework, which acts through the pressurising line to move the diaphragm and the closure plug towards the valve seat, and the force applied to the diaphragm by the bias means, which acts to move the diaphragm and the closure plug away from the valve seat. The pressure setting is therefore automatically adjusted by the upstream pressure. The level of force applied by the bias means is small relative to the upstream'pressure, being required in the shut-off mode simply to hold the diaphragm and plug away from the associated seat, and is selected according to the pressure difference in the system between the main gas chamber and the point at which the pressurising line meets the upstream pipework so that at the desired operating pressure of the separator the combined forces applied to the diaphragm by the gas pressure in the main chamber and the bias means exceed the force applied to the diaphragm by the gas pressure in the pilot chamber. The pressure differential maintaining device is closed if the pilot chamber pressure exceeds the combination of gas pressure in the main chamber and the biasing force, i.e. if the product gas pressure from the generator has not reached the required level for the receiver. The invention offers the important functional advantage over a conventional pressure-maintaining valve in that it provides for operation of a membrane gas generator at a more stable gas output purity with respect to fluctuations in the pressure of feed gas applied to the membrane. The reason for this can better be understood by considering the basic physical relationship between pressures and flowrate for flow restrictors such as needle valves or other adjustable orifice valves which are commonly used in association with pressure maintaining valves to set the output gas purity and gas flowrate from membrane gas generators. F = K . [(P P 2> P 2 ] where F = gas flowrate, expressed at standard atmospheric pressure and temperature, P, = absolute pressure upstream of the adjustable orifice valve, P_ = absolute pressure downstream of the adjustable orifice valve, K a constant which includes geometrical factors of the valve, as well as the temperature and relevant physical properties of the gas flowing through the adjustable orifice valve. For the conventional configuration of an adjustable orifice valve and pressure maintaining valve, P_ is held constant. For the configuration of the present invention where an adjustable orifice valve is associated with the pressure differential maintaining valve, P_ is not held constant, but instead the differential, (P, - P_), is held constant. As the above expression shows, these two alternatives are mathematically identical only when the pressure P_ in the case of the pressure maintaining valve is numerically equal to (P. - P_) in the case of the pressure differential maintaining valve. In practice, however, it is highly desirable that maximum effective use is made of the receiver (or optionally more than one receiver) installed downstream of the membrane gas generator, and therefore in the case of a pressure maintaining valve its setting must be kept as high as possible, typically not more than 1 or 2 bar below the supply pressure applied to the membrane's inlet port. In the case of a pressure differential maintaining valve, similar considerations apply in respect of the downstream receiver(s), but here it is instead the pressure differential (P.-P_) which should not exceed 1 or 2 bar. The relative flow control responses with respect to changes in supply pressure both for the conventional (pressure maintaining valve) and for the valve of the present invention are shown in the diagram of Figure 4. Here, the two flowrates have been equalised at P,= 14 bar with the respective conditions P„ = 12 bar (for the pressure maintaining valve) and (P, - P 2 ) « 1 bar (for the valve of the present invention) . It can be seen that the control achieved with the pressure differential maintaining valve is superior for a wide range of fluctuation of P, , and thus for similar changes in the gas pressure supplied to the membrane separator inlet. A further advantage of the flowrate control achieved with the pressure differential maintaining valve arises from the fact that, for limited fluctuations of the compressor pressure experienced in practice (i.e. P-.), the flowrate varies approximately linearly with P,. This characteristic matches the control condition which is required to maintain a constant purity of gas produced from a membrane gas generator if its operating pressure varies. The slope of the flowrate vs. P. characteristic is increased by increasing the bias force in the pressure differential maintaining valve, and it is thus possible to select springs to match particular membranes in order to minimise changes in gas purity over a wide range of operating pressures. During shut-off periods, due to the action of the bias means in holding the plug away from the plug seat, the diaphragm experiences only light pressure loading. During operation of the generator the diaphragm experiences only the difference in pressure between the pilot chamber and the main chamber. Thus during operation there is little or no net differential pressure across the diaphragm such that inexpensive lightweight material can be used for its manufacture. The lightweight material has the advantage of a low hysteresis in the small physical movements involved in functioning of the valve. Moreover because the plug is mostly out of contact with its seat a simple rubber or plastic material can be used for the seat without danger of developing deformation and consequent variations in sealing performance. The system optionally further includes automatic shut-off means comprising a pressure switch upstream of the receiver which is activated by a set pressure approaching the maximum desired receiver pressure and which is linked to the compressor control so as to switch off the compressor when this set pressure is reached. In one convenient construction the pressure differential maintaining device additionally includes the non-return valve as an integral part thereof. This configuration reduces the space required for the device and valve and may also reduce their combined cost. The pressure differential maintaining device requires no adjustment or maintenance for different compressor pressure, and can be manufactured with spring settings to give gas flow with reliable small pressure differentials between the gas chambers on different sides of the diaphragm. This feature maximises the pressure to which the receiver may be charged with gas at constant flowrate without risk of failure of gas generation, and also enables coarser and less expensive needle control valves to be utilised without sacrificing the accuracy of control over gas purity. The invention is further described below with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a membrane gas separation system incorporating a pilot-controlled pressure differential maintaining device according to the invention. Figure 2 is a sectional view of the pilot-controlled pressure differential maintaining device of Figure 1 shown on a larger scale. Figure 3 is a sectional view of an alternative type of pilot-controlled pressure differential maintaining device according to the invention. The system illustrated in Figures 1 and 2 includes a compressor 11 to deliver air, at constant pressure, through a pipe 12 to a membrane separation unit 14. Nitrogen-rich gas product gas from the membrane unit 14 passes via a pipe 15 through a needle control valve 16 to the inlet 18 of a pressure differential maintaining device, indicated generally by the numeral 20. Gas leaving the pressure differential maintaining device 20 passes through a pipe 39 to a storage receiver 40 which has a controlled outlet 42. The pressure differential maintaining device 20 comprises a diaphragm 22 and associated valve stem 24 located between an upper body member 21a and a lower body member 21b so as to separate chambers 25 and 26 enclosed by the body members 21a and 21b respectively. The stem 24 has a corresponding seat 28 with a resilient sealing ring 29. A fixed spring 34 imparts to the stem 24 a small upward force corresponding to a low equivalent differential pressure across the diaphragm 22. When the pressures above and below the diaphragm 22 are substantially the same the spring 34 holds the stem 24 off its seat 28 and thus allows gas flow through the valve. The lower chamber 26 is connected at inlet 18 to the outlet of the needle valve 16. The upper chamber 25 is connected via a port 35 and pipe 33 to the pipe 12 upstream of the membrane unit 14. In an alternative configuration the upper chamber 25 can be connected to pipe 15 at a point just downsteam of the membrane unit 14. This alternative configuration offers the advantage that the gas on both sides of the diaphragm 22 has the same composition. The body member 21b includes a further chamber 27, downstream of the valve seat 28, with a port 37 connected to the pipe 39 leading to the pressure switch 38 and receiver 40. Within the chamber 37 a perforated carrier 50 is sealed to the body 21b by an O-ring 51 and fitted with a flexible moulding 52 to form an integral non-return valve, thus preventing back-flow of gas from the receiver 40 through the pressure differential maintaining device 20 when the receiver 40 has been filled and the membrane generator has been switched off. The diaphragm 22 allows gas to be conducted from port 36 past the valve seat 38 when the pressure in chamber 26 plus the force from the spring 34 equals the pressure in the chamber 25. The non-return valve moulding 52 prevents generated gas being released through port 37 to the receiver 40 until the pressure at port 36 exceeds the pressure in the receiver 40. A pressure switch, indicated generally by the numeral 38, is connected to the pipe 39 to respond to the pressure therein. The setting of the pressure switch 38 is such that when the pressure in the pipe 39 (and thus in the receiver 40) reaches the maximum intended pressure in the receiver 40 the switch activates a control to shut down the compressor 11. The constant pressure at the pressure differential maintaining device 20 ensures that the flowrate of nitrogen-rich gas from the membrane unit 14 is held constant until the pressure in the receiver 40 builds up to the said constant pressure, at which level the compressor 11 is automatically shut off by the pressure switch 38. The non-return valve 50/52 prevents backflow of stored gas from the receiver 40 to the membrane unit 14. Figure 3 shows a pressure differential maintaining valve 20' which combines the functions of both the needle valve 16 and pressure valve 20 into a single component. The numbering of Figure 3 is generally the same as in Figure 2, but the valve 20* differs in additionally having aligned ducts 25b, 26b drilled in the body parts 21a and 21b respectively. The integral needle valve 16 comprises a carrier 60 for an adjustable concentric needle 61 located upstream of an annular seat 62 incorporated in the entry port 18. The carrier 60 has a removable closure cap 63 and a threaded internal bore 64 to engage with threads on the shaft of the needle 61 and thus permit linear adjustment thereof. Adjustment of the flowrate is achieved by temporarily removing the cap 63 and inserting a rotary-sealed key tool (not shown) . Pressure seals 65 and 66 are formed by O-rings between the carrier 60 and the body part la and between the carrier 60 and the cap 63. Another feature of the valve 20a is that the valve stem 24 is provided with a self-aligning mechanism having a stem-retaining cap arrangement 70, which offers the advantage of allowing a more durable hard valve seat 28' to be used in place of the resilient seat 28/29 of Figure 2. A further advantage of the Figure 3 valve is that it has only 2 ports, thus providing a system with simplified pneumatic connections.

Claims: CLAIMS 1. A gas supply system which comprises a permeable membrane gas generator, a storage receiver for generated gas, pipework from the said generator to the said receiver having in sequence a flow-adjusting valve, a- pressure differential maintaining device and a non-return valve, wherein the pressure differential maintaining device comprises a main gas inlet, a main gas chamber, a main gas outlet, a pilot gas chamber, a closure plug with an associated seat in the main gas outlet, a flexible diaphragm between the main gas chamber and pilot gas chamber, the said diaphragm earring the closure plug and being activated by the respective pressures in the main gas chamber and pilot gas chamber, characterised in that the closure plug has an associated means to bias the plug away from the seat and in that the pilot chamber has an associated pressurising line from a point in the pipework upstream of the pressure differential maintaining device. 2. A gas supply system as claimed in claim 1, wherein the flow-adjusting valve is a needle valve. 3. A gas supply system as claimed in claim 1 or claim 2, wherein the bias means is a spring located between the diaphragm and the valve seat. 4. A gas supply system as claimed in claim 3, wherein the spring is shaped and located so as to bear upon the closure plug rather than the diaphragm as such. 5. A gas supply system as claimed in claim 3 or claim 4, wherein the spring is selected to match the membrane. 6. A gas supply system as claimed in any preceding claim, wherein the pressure differential maintaining device additionally includes the non-return valve as an integral part thereof. 7. A gas generation process in which.-compressed gas from a permeable membrane generator is passed through a flow-adjusting valve, a pressure differential maintaining device and a non-return valve to a storage receiver wherein the pressure differential maintaining device comprises a main gas inlet, a main gas chamber, a main gas outlet, a pilot gas chamber, a closure plug with an associated seat in the main gas outlet, a flexible diaphragm between the main gas chamber and pilot gas chamber, the said diaphragm earring the closure plug and being activated by the respective pressures in the main gas chamber and pilot gas chamber, characterised in that the closure plug has an associated means to bias the plug away from the seat and in that the pilot chamber has an associated pressurising line from a point upstream of the pressure differential maintaining device to maintain the pressure in the pilot chamber.

Inventors: PAGE JOHN KENNETH RURIK, PAGE, JOHN, KENNETH, RURIK
Applicants: CALOR AIR SEPARATION LTD, CALOR AIR SEPARATION LIMITED
Application year: 1992
Main classification: F17C13
IPC codes: B01D53, F17C13, G05D16