GB2217877A - Gas pressure control apparatus - Google Patents

Abstract

Gas pressure control apparatus comprises two streams 12A and 12B connected in parallel between a high pressure inlet 10 and a low pressure outlet 11. Stream 12A includes a slam-shut valve 15A followed by pressure regulating means 16A; stream 120 is similar. Control means sense the failure of the regulating means in a stream and close the slam-shut valve of that stream. If the regulating means of a channel fails, the control means closes both slam-shut valves on a rise of outlet pressure (by path 25A to control valve means 26A, 27A, 21A, and 20A and likewise for stream 120), determines the resulting pressure difference between the two streams following the slam-shut valves (by valve 30, controlled via 34A and 340), and re-opens one of the slam-shut valves (via path 31 to the appropriate control valve means). An electronic control system can alternatively be used. <IMAGE>

Description

Gas Pressure Control Apparatus The present invention relates to gas pressure control apparatus.

In gas supply systems, there is often a requirement to supply a low pressure (typically 30 psi) distribution network from a high pressure (typically 300 psi) gas supply. (1 psi is roughly 7 kPa; the unit psi is used here as it is the accepted unit in this field.) In principle, this can be achieved by using a pressure regulator, which is a device which provides gas at its outlet at a substantially constant pressure regardless of the pressure at its inlet.

Such pressure regulators may however malfunction. There is therefore a need for a pressure regulating system in which such malfunction is compensated for. It is of course possible to connect two pressure regulators in series.

However, with such an arrangement it is difficult to determine whether both pressure regulators are sound. If one pressure regulator fails, the arrangement will continue to work satisfactorily, but its reliability will then be no more than that of a single pressure regulator.

A system has therefore been developed which provides two parallel gas paths or streams, each including regulating means consisting of one or more pressure regulators. In this system, both streams are normally functional (though in practice imbalance between the two streams will result in only one passing gas).

Each stream includes a valve of the type commonly referred to as a slam-shut valve. Means are provided for detecting the failure of a stream and shutting down that stream, by means of its slam-shut valve, as a result. The fact that a stream has been closed down is an indication that a pressure regulator failure has occurred, and that servicing is therefore required.

In the known double stream system, a one-way flap valve is provided at the downstream end of each stream, and for each stream, a control line is connected from between the pressure regulating means and the one-way flap valve to the slam-shut valve in that stream, the control line operating if the pressure rises above a preset limit. Normally, the pressures in both streams are below the limit, and both slam-shut 'valves remain open. If the regulating means in one stream fails, then the pressure in that stream rises above the limit, and the slam-shut valve of that stream is therefore closed. The pressure at the outlet of the system also rises, tending to produce a reverse gas flow into the second stream. The one-way flap valve in the second stream therefore closes, preventing the pressure in that stream from rising, so the slam-shut valve of that stream remains open.Gas flow therefore continues normally through the second stream.

This arrangement suffers froni various drawbacks. Economically, a flap type valve suitable for high pressure systems and having acceptable pressure drop characteristics, compatible with safe slam-shut operating pressure needs, is costly. Technically, if the gas flow rate is abnormally high, a back pressure build up against the of the one-way flap valve can cause the slam-shut valve to close. Also, with such flap valves the flap has been known to become detached, which can lead to loss of gas supply security.

There is therefore a need for a more effective system for controlling double stream high pressure systems, maintaining security of safe gas pressure to consumers.

According to the present invention there is provided gas pressure control apparatus comprising two streams connected in parallel between a high pressure inlet and a low pressure outlet, each stream including a slam-shut valve followed by pressure regulating means, and control means for closing the slam-shut valve of a stream on failure of its regulating means, characterized in that the control means closes both slam-shut valves on a rise of outlet pressure, and re-opens one of the slam-shut valves in dependence on the resulting pressure difference between the two streams following the slam-shut valves.

The control means may be pneumatic or electronic. Means may be provided for preventing repeated cycling if the pressure regulating means fail in both streams.

Briefly, the operation of this apparatus is that when a pressure regulating means fails, the outlet pressure rises. Both the slam-shut valves are closed, isolating the outlet completely from the inlet. The outlet pressure falls in 'due course. The closing of the slam-shut valves traps high pressure gas in each stream between the slam-shut valve and the pressure regulating means of that stream. As the outlet pressure falls, so this trapped high pressure gas leaks away through the failed regulating means, but is retained by the sound regulating means in the other stream. The resulting pressure difference is used to select which stream is re-opened.

A double stream gas pressure reducing system embodying the invention will now be described, by way of example, with reference to the drawing, which is a schematic diagram of the system.

The main gas flow is along the channels shown by heavy lines, from a high pressure inlet 10 to a low pressure outlet 11 via two parallel streams 12A and 12B. In stream 12A, the gas passes in succession through a first manual valve 13A, a filter 14A, a slam-shut valve 15A, first and second pressure regulators 16A-1 and 16A-2, and a second manual valve 17A. A pressure relief valve 18A of limited capacity is connected between the pressure regulator 16A-2 and the manual valve 17A. Stream 12B contains a similar sequence of components. In normal operation, both slam-shut valves 15A and 15B are open, and gas is able to flow through both streams. In practice, however, the pressure regulating means in the two streams may have slightly different characteristics, in which case gas will only flow through one stream.

The slam-shut valve 15A is operated by a double-sided piston actuator 20A, the two sides of the piston being fed from a spool valve 21A which passes an operating pressure from the high pressure inlet 10 via a line 22A and a valve 23A which controls at a preset pressure. One end of the spool valve 21A is fed with the pressure at the outlet side of the regulating means in the stream 12A via a line 25A, a valve 26A which opens at a preset pressure, and a bleed restriction 27A. Valve 15B has similar control arrangements.

In normal operating conditions, the pressure at the outlet 11, and hence at the Junction between the regulating means 16A in stream 12A and the second manual valve 17A in that stream 12A, is low - typically not much above 25 psi, the nominal outlet pressure. This pressure is below the preset operating pressure of the valve 26A, and the pressure at the right-hand end of the spool valve 21A is therefore low, because the bleed restriction 27A allows any high pressure to bleed away. In this condition, the spool valve 21A allows the operating pressure from line 22A to pass to the right-hand side of the piston 20A, and slam-shut valve 15A remains open. The slam-shut valve 15B in stream 12B similarly remains open.

In the event of the failure of the regulating means in either of the streams, the operation is controlled by a stream select spool valve 30, which is a high pressure differential piston valve. This is fed with the outlet pressure at outlet 11 via 'a line 31 through a manual valve 32 and a valve 33 which controls at a preset pressure. The valve 30 is controlled by two lines 34A and 34B connected to the streams 12t and 12B between the slam-shut valves 15A and 15B and the regulating means as shown. Its position is therefore determined by the pressure difference between the two streams at those points.If the pressure in stream 12A is higher than that in stream 12B, line 31 is connected to the upper outlet of valve 30, and the lower outlet is connected to a vent; conversely, if the pressure is stream 12B is higher than that in stream 12A, line 31 is connected to the lower outlet of valve 30 and its upper outlet is vented. The two outlets of valve 30 are connected to the left-hand ends of the control valves 21A and 21B as shown.

The operation of the system is as follows. If the regulating means in either stream fails, the pressure at the outlet 11 rises from the normal value of around 25 psi towards the high pressure of say 300 psi at the inlet 12. This rise in pressure is fed through the lines 25A and 25B and overcomes the preset pressure limits of the valves 26A and 26B in those lines to cause both the slamshut valves 15A and 15B to close, at say 30 psi. The pressure at the outlet 11 will fall in due course, as a result of consumption by the consumers fed from it.

As it falls, so the pressure in lines 25A and 25B will fall, and when it falls below the preset pressure limits of the valves 26A and 26B, these valves will close. The high pressure on the right-hand side of the spool valves 21A and 21B will therefore fall as it bleeds away through the bleed restrictions 27A and 27B.

The closing of the slam-shut valves 15A and 15B traps a quantity of high pressure gas on the downstream side of each of them. The pressure at the outlet 1 1 will normally not have risen as high as the pressure of this trapped gas, and is falling as the result of consumption by the consumers, so there will be a pressure difference across the regulating means in both streams 12A and 12B. One of these regulating means has failed, and will therefore be unable to maintain a significant pressure difference across itself, while the other will still be sound and able to hold the pressure on its high pressure side. As the pressure at the outlet 11 falls, therefore, the pressure immediately upstream of the failed reguating means will fall with it, while the pressure immediately upstream of the sound regulating means will remain high.

These two pressures are those fed to the stream select spool valve 30, which will therefore move up or down according as the regulating means in the lower stream 12B or the upper stream 12A has failed. This directs the outlet pressure through line 31 to the control means for the control valve 21A or 21B for the slam-shut valve of the upper stream 12A or the lower stream 12B respectively. This pressure, as soon as the counteracting pressure through line 25A or 25B has decayed as described above, causes the associated slam-shut valve 15A or 15B to open.

In summary, the sequence of events is as follows. When a pressure regulating means fails, the outlet pressure rises, and this causes both the slam-shut valves to close, isolating the outlet completely from the inlet. The closing of the slam-shut valves traps high pressure gas in each stream between the slamshut valve and the pressure regulating means of that stream. The outlet pressure falls in due course, and as it falls, so this trapped high pressure gas leaks away through the failed regulating means, but is retained by the sound regulating means in the other stream. The resulting pressure difference is used to select which stream is re-opened.

The other stream remains closed, and the position of its slam-shut valve is an indication that the associated pressure regulating means has failed and requires servicing. When the regulating means has been serviced, the slam-shut valve of that stream is re-opened by manual operation of its associated control valve 21A or 21B.

With the system as described.so far, if both streams fail in succession, the system will oscillate. As the pressure at the outlet rises, both slam-shut valves will be closed, and one or other will then be opened as the outlet pressure falls; if both pressure regulating means have failed, whichever stream is opened will cause the outlet pressure to rise, and the sequence of events will repeat. However, means can readily be added to prevent this; for example, counter means can be added to the valves 21A and 21B to prevent them being moved to the right for a second time, or these two valves can be cross coupled to prevent either being moved to the right if the other is already at the right.

This will of course cut off the consumers, whereas allowing the system to cycle will result in heavy wear of the system components but will maintain a supply of sorts to the consumers.

The system described operates purely pneumatically, but it will be realized that an electronically operated system could easily be designed. For this, the two slam-shut valves 15A and 15B would be electrically controlled. A pressure transducer would be required for measuring the pressure at the outlet 11, and a pressure difference transducer would be required for measuring the pressure difference between the outlet sides of the two slam-shut valves. (Two simple pressure transducers with their outputs electrically differenced could of course be used instead of a pressure difference transducer.) Control circuitry would cause both slam-shut valves to close on the outlet pressure rising above a preset limit, and then select one or other for re-opening in dependence on the output of the pressure difference transducer.

Claims (5)

Cla im s

1 Gas pressure control apparatus comprising two streams connected in parallel between a high pressure inlet and a low pressure outlet, each stream including a slam-shut valve followed by pressure regulating means, and control means for closing the slam-shut valve of a stream on failure of its regulating means, characterized in that the control means closes both slam-shut valves on a rise of outlet pressure, and re-opens one of the slam-shut valves in dependence on the resulting pressure difference between the two streams following the slamshut valves.

2 Gas pressure control apparatus according to claim 1, wherein each pressure relating means comprises a plurality of pressure regulating devices connected in series.

3 Gas pressure control apparatus according to either previous claim, wherein the control means comprises: for each stream, valve control means for the slamshut valve fed with the high pressure at the inlet and the pressure from downstream of the regulating means; and a valve which passes the pressure at the outlet to one or other of the two valve control means in dependence on the pressure difference between the two streams following the slam-shut valves.

4 Gas pressure control apparatus according to either .of claims 1 and 2 wherein the slam-shut valves are electrically controlled and the control means comprise a pressure transducer for measuring the pressure at the outlet, pressure difference transducer means for measuring the pressure difference between the outlet sides of the two slam-shut valves, and circuitry for causing both slam-shut valves to close on the outlet pressure rising above a preset limit and then selecting one or other for re-opening in dependence on the output of the pressure difference transducer means.

5 Gas pressure control apparatus substantially as herein described and illus treated.