WO2007032764A1

WO2007032764A1 - Fire hazard suppression system with sequential discharging of inert gas by rupturable disc and decaying pressure

Info

Publication number: WO2007032764A1
Application number: PCT/US2005/032456
Authority: WO
Inventors: Fanping Sun; Muhidin A. Lelic; Zaffir Chaudhry
Original assignee: Chubb International Holdings Limited
Priority date: 2005-09-12
Filing date: 2005-09-12
Publication date: 2007-03-22

Abstract

A controlled pressure release system (10) prevents overpressure in a protected area (14) upon delivery of gas. The system (10) includes a first and a second gas container (12a and 12b), piping (40), a first and second gas outlet 28 and 30), a first differential pressure responsive valve (16a), and a valve (22, 30). The piping (40) has a discharge outlet (24) and connects the first and the second gas containers (12a and 12b) to each other at the first gas outlet (28) and the second gas outlet (32), respectively. The first differential pressure responsive valve (16a) is positioned between the first and second gas outlets (28 and 32). The valve (22, 30) actuates the system (10) and is switchable between a closed position and an open position. When the valve (22, 30) switches to the open position, the first gas container (12a) is in communication with the discharge outlet (24). The first differential pressure responsive valve (16a) opens as a function of a gas pressure differential caused by decreasing gas pressure in the first gas container (12a).

Description

FlRE HAZARD SUPPRESSION SYSTEM WITH SEQUENTIAL

DISCHARGING OF INERT GAS BY RUFTURABLE DISC AND

DECAYING PRESSURE

BACKGROUND OF THE INVENTION Fire hazard suppression systems have long been employed for protecting areas containing valuable equipment or components, such as art galleries, data centers, and computer rooms. Traditionally, these systems utilize Halon, which is idea! for hazard suppression because it is capable of very quickly suppressing a hazard. It can be stored at relatively low pressures, and only a relatively small quantity Is required.

However. In recent years the adverse environmental effects of Halon on the ozone have become evident, and many governmental agencies have banned further use of Halon. In some countries, existing Halon systems are being replaced by systems using more environmentally friendly inert gases such as nitrogen, argon, carbon dioxide, and mixtures thereof. Unlike the Haioπ-baseo fire hazard suppression systems, Inert gas-based fire suppression systems use natural gases and do not contribute to atmospheric ozone depletion.

Combustion occurs when fuel, oxygen, and heat are present in sufficient amounts to support the ignition of flammable material. Inert gas fire hazard suppression systems are based on reducing the level of oxygen In an enclosure to a level that will not sustain combustion. In order to extinguish a fire hazard, Inert gas stored in a large number of high-pressure gas cylinders Is released into the enclosure to reduce the concentration of oxygen by displacing oxygen with the inert gas until combustion is extinguished. Typically, ambient air comprises 21% concentration by volume of oxygen. This concentration must be reduced to below 12.54% to effectively extinguish the fire hazard. To reach this objective, a relatively large volume of gas must be released. There are health and safety Implications for facility personnel, particularly sπ relation to the reduction of oxygen in the atmosphere once the system is discharged. Careful calculation Is required to ensure that the concentration of inert gas released Is sufficient to control combustion, yet not so high as to pose a serious risk to personnel. The replacement of Haloπ with Inert gas for fire hazard protection presents two Issues with the system design- First, the delivery of a large amount of gas Into a protected room within a short period time (fire codes in some countries require that the gas be delivered In less than one minute) may generate overpressure in the room that could potentially damage equipment in the room. Current Industrial practice is to use a special, expensive vent in the room to prevent the overpressure. Second, unlike Haion, Inert gas Is stored under normal room temperature in gaseous form, rather than liquid form. To reduce the storage vessel volume, a very high pressure is preferred, typically between 100 bar and 3OQ bar. As a result, the gas distribution system must be capable of withstanding extremely high pressures. These two limitations are key factors in the cost of both new Installation and retrofit.

The overpressure In a protected room Is primarily caused by an uneven discharge of the inert gas from the pressure vessel or a high pressure peak upon discharge of the inert gas greater than a safe threshold level. The pressure In the gas vessel decays exponentially during gas release, so the overpressure typically occurs in the first few seconds of the discharge. If the gas release can be throttled to a fairly uniform pressure profile over the duration of the discharge or be maintained below the threshold level at all times during the gas release, overpressure in the protected room can be prevented while ensuring that the predetermined amount of inert gas is delivered within the required time. Current systems used for throttling the gas flow require either a valve with a controllable variable opening area or an on/off valve.

BRIEF SUMMARY OF THE INVENTION

A controlled pressure release system prevents overpressure In a protected area upon delivery of gas. The system Includes a first gas container having a first gas outlet, a second gas container having a second gas outlet, piping having a discharge outlet, a first pressure responsive valve (such as a rupturabie disc), and a valve. The piping connects the first and the second gas containers to each other. The first pressure responsive valve ss positioned between the first and second qas outlets. The valve actuates the system and is

between a dosed position and an open position. When the valve switches to the open position, the first gas container is in communication with the discharge outlet. The first pressure responsive valve subsequently opens as a function of a gas pressure differential caused by decreasing gas pressure in the first gas container

BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a schematic representation of a fire hazard suppression system in accordance with the present invention.

FIG 2 is a front view of a first embodiment of the fire hazard suppression system In accordance with the present Invention.

FlG 3 is a front view of a second embodiment of the fire hazard suppression system in accordance with the present invention. FIG. 4 Is a graph of pressure in an enclosed room to be protected by the fire hazard suppression system In accordance with the present Invention as a function of time.

DETAILED DESCRIPTION

FIG 1 Is a schematic representation of a sequential discharging firs hazard suppression system 10. A plurality of high pressure Inert gas storage cylinders I2a-I2c are located in a storage area or room proximate an enclosed room 14 to be protected. Inert gas storage cylinders I2a-I2c contain inert gas to be released into protected room 14

In case of a fire Located between gas cylinders 12a and 12b Is a differential pressure responsive valve (rupturable disc 16a). A similar valve (rupturable disc 16b) is located between gas cylinders 12b and I2c.

When a fire is detected in protected room 14 by a detector 18 located in protected room 14, a control signal from control panel 20 opens main valve 22 Gas is then discharged into protected room 14 through delivery line 24 and discharge nozzles 26 to deplete the concentration of oxygen

In protected room 14 and extinguish the fire The discharge of gas from cylinders I2a-I 2c is sequential as a result of staggered opening times of main valve 22, rupturable disc 16a, and rupturable disc 16b. FIG. 2 is a front view of a first embodiment of sequential discharging suppression system 10. Suppression system 10 generally

Includes first gas cylinder 12a, second gas cylinder 12b, third gas cylinder

12c, first rupturable disc 16a, second rupturable disc 16b, main valve 22, delivery line 24, first gas outlet 28, first valve 30, second gas outlet 32, second valve 34, third gas outlet 36, third valve 38, and piping 40.

Suppression system 10 throttles the release of inert gas from gas cylinders 12a-12c by sequential discharging of first, second, and third gas cylinders 12a, 12b, and 12c, respectively. In order to control the pressure discharge Into protected room 14 (shown In FIG. 1).

First valve 30 is located at first gas outlet 28 and controls gas flow from first gas cylinder 12a to piping 40. First valve 30 is switchable between a closed position and an open position. When first valve 30 is in the closed position, gas from first gas cylinder 12a cannot flow through first gas outlet 28 to piping 40 When first valve 30 is in the open position, gas from first gas cylinder 12a is allowed to flow from first gas cylinder 12a through first gas outlet 28 to piping 40. First valve 30 can be activated electrically, pneumatically, or manually to open upon command from control panel 20 (shown In FlG. 1). First valve 30 is opened manually If control panel 20 Is not functioning properly. In one embodiment, first valve 30 is a solenoid valve

Second gas outlet 32, second valve 34, second gas cylinder 12b, and piping 40 Interact and function In the same manner as first valve 30, first gas outlet 28, first gas cylinder 12a, and piping 40. Also, third gas outlet 36, third valve 38, third gas cylinder 12c, and piping 40 Interact and function In the same manner as first valve 30, first gas outlet 28, first gas cylinder 12a, and piping 40.

Piping 40 of suppression system 10 includes first intermediate line 40a, second intermediate line 40b, and third intermediate line 40c. First intermediate line 40a Is located between main valve 22 and first rupturable disc 16a. First gas outlet 28 connects first gas cylinder 12a to piping 40 at first Intermediate line 40a. Second intermediate line 40b Is located between first rupturabie disc 16a and second rupturable disc 16b. Second gas outlet 32 connects second gas cylinder 12b to piping 40 at second intermediate line 40b. Third intermediate line 40c is located between second rupturable disc 16b and third gas cylinder 12c. Third gas outlet 36 connects third gas cylinder 12c to piping 40 at third intermediate line 40c. Main valve 22 Is located between piping 40 and delivery line 24 and controls the release of gas from gas cylinders 12a-12c through first intermediate line 40a of piping 40. When main valve 22 is powered off or deactivated (closed), gas cannot leave piping 40. When main valve 22 is activated (open), gas Is allowed to leave through piping 40 to protected room 14. Main valve 22 may be activated electrically or manually When main valve 22 Is activated electrically, main valve 22 opens upon command from control panel 20 (shown in FIG. 1). Main valve 22 may also act as a manual override If control panel 20 is not functioning properly. In one embodiment, main valve 22 Is a solenoid valve First rupturable disc 16a is located along piping 40 between first intermediate line 40a and second intermediate line 40b and controls the release of gas from second gas cylinder 12a through first intermediate line 40a to protected room 14. First rupturabie disc 16a controls the release of gas from second gas cylinder 12b by acting as a barrier between second gas outlet 32 and delivery line 24 When first rupturable disc 16a Is Intact, gas cannot pass from second gas cylinder 12b to delivery line 24. Upon rupture of first rupturable disc 16a, gas is free to pass from second gas outlet 32 through first rupturabie disc 16a and first intermediate line 40a to delivery line 24 First rupturable disc 16a ruptures as a function of a pressure differential between first intermediate line 40a and second Intermediate line 40b, which are based on the pressures in first gas cylinder 12a and second gas cylinder 12b, respectively. As gas leaves first gas cylinder 12a and the pressure in first gas cylinder 12a decreases, the pressure differential on first rupturable disc 16a increases First rupturabie disc 16a ruptures when the pressure differential between first Intermediate line 40a and second intermediate line 40b reaches a predetermined value

Second rupturable disc 16b is located along piping 40 between second intermediate line 40b and third intermediate line 40c and controls the release of gas from third gas cylinder 12c through first and second intermediate line 40a and 40b to protected room 14. Second rupturable disc 16b functions in the same manner as first rupturable disc 16a and acts as a barrier between third gas outlet 38 and delivery line 24, When second rupturable disc 16b Is intact, gas cannot pass from third gas cylinder 12c to main valve 22. Upon rupture of second rupturable disc 16b, gas is free to pass from third gas outlet 36 through second rupturabie disc 16b, second Intermediate line 40b, first rupturabie disc 16a, and first intermediate line 40a to delivery line 24. Second rupturable disc 16a ruptures as a function of a pressure differential between third intermediate line 40c and first and second Intermediate lines 40a and 40b. which are based on the pressures In third gas cylinder 12c and first and second gas cylinders 12a and 12b respectively. Rupturable discs 16 can be manufactured to nave varying rupture limits and rupture at any pressure differential depending on the delay desired between firing of gas cylinders 12a-12c. In one embodiment, rupturable discs 16 are burst discs. Rupturabie discs 16 can be any type of barrier that breaks or opens as a function of a differentia! pressure without departing from the intended scope of the present invention. To install gas cylinders 12a-12c to piping 40, gas cylinders 12a-12c are transported with first, second, and third valves 30, 34, and 38 In the dosed position. First gas cylinder 12a is connected to piping at first gas outlet 28, second gas cylinder 12b Is connected to piping 40 to second gas outlet 32, and third gas cylinder 12c is connected to piping 40 at third gas outlet 36. Once gas cylinders 12a-12c are connected to piping 40, first, second, and third valves 30, 34, and 38 are opened to pressurize first and second rupturable discs 16a and 16b. First, second, and third valves 30. 34. and 38 must be opened slowly and simultaneously while closely monitoring the pressure rise at first, second, and third gas outlets 28, 32, and 36 to ensure that the pressure differential across rupturable discs 16a and 16b do not approach the rupture limit. Because the rupture limit is based on the pressure differential on either side of rupturable discs 16, the pressures at first, second, and third gas outlets 28, 32, and 36 should be substantially equal. Thus, there will be a minimal pressure differentia! across rυpturabie discs 16. keeping them intact. In one embodiment, rupturable discs 16 are one-way burst discs.

In order to control the rate of gas eπtenπg protected room 14 from suppression system 10, main vaive 22 remains closed until hazard suppression is needed in protected room 14 When main valve 22 is closed, gas cannot pass from first gas cylinder 12a through main valve 22 to delivery line 24. First rupturable disc 16a thus remains Intact because the pressure in first intermediate line 40a and the pressure in second intermediate line 40b are substantially equal. Due to the minimal or no pressure differential across first rupturable disc 16a, first rupturable disc 16b does not rupture and acts as a barrier between first and second intermediate hnes 40a and 40b.

When first rupturable disc 16a is intact, gas Is not allowed to pass from second or third intermediate lines 40b and 40c to first intermediate line 4Qa. The pressure in suppression system 10 therefore remains equalized and second rupturable disc 16b is also left intact. When second rupturable disc 16b is intact, the pressure in second Intermediate line 40b and the pressure in third Intermediate line 40c are substantially equal. Because there Is minimal or no pressure differential across second rupturabie disc 16b, second rupturable disc 16b does not rupture and prevents the gas in second and third gas cylinders 12b and 12c from leaving suppression system 10. when main valve 22 Is activated (opened), gas Is allowed to flow from first gas cylinder 12a through first intermediate line 40a, main valve 22, and delivery line 24 Into protected room 14 The pressure in first gas cylinder 12a decreases as gas Is being expelled from first gas cylinder 12a into protected room 14 and the pressure differential across first rupturable disc 16a begins to Increase as the pressure in first intermediate line 40a decays and the pressure In second Intermediate line 40b remains constant. As the pressure in first Intermediate line 40a continues to decay, the pressure differential at first rupturable disc 16a eventually reaches a predetermined level and first rupturable disc 16a reaches Its rupture limit. When first rupturable disc 16a reaches Its rupture limit, first rupturabie disc 16a ruptures, allowing gas from second gas cylinder 12b to flow through second intermediate line 40b and first rupturable disc 16a to join the gas from first gas cylinder 12a.

After first rupturable disc 16a ruptures, gas from second gas cylinder 12b joins the gas from first gas cylinder 12a and leaves suppression system 10. The pressures sn first and second gas cylinders 12a and 12b combine and quickly reach a new pressure equilibrium due to the gas cross-charging. As gas is discharged into protected room 14, the combined pressure in first and second gas cylinders 12a and 12b continually decreases Thus, the pressure differential across second rupturable disc 16b begins to Increase as the pressure in first and second intermediate lines 40a and 40b decay and the pressure rπ third intermediate line 40c remains constant. When the pressure in first and second gas cylinders 12a and 12b has decayed below a second predetermined level, the pressure differential across second rupturable disc 16b reaches a maximum limit and second rupturable disc 16b reaches its rupture limit Second rupturable disc 16b then ruptures and gas from third gas cylinder 12c is allowed to flow to delivery line 24, along with the gas from first and second gas cylinders 12a and 12b.

FIG. 3 snows a second embodiment of suppression system 100 which Is identical to the first embodiment of suppression system 10 (discussed in FIG 2} in design and functionality, with the exceptions that main valve 22 is not necessary and that when gas cylinders I2a-12c are installed, first, second, and third valves 30, 34, and 38 remain closed. Because second embodiment of suppression system 100 may not Include a main valve 22, first, second, and third valves 30, 34, and 38 remain closed so that no gas is allowed to pass into piping 40. When a fire Is sensed in protected room 14, control panel 20 successively sends signals VI , V2, and V3 to open first, second, and third valves 30, 34, and 38, respectively When inert gas is needed in protected room 14 to suppress a fire, control panel 20 first sends a signal VI to open first valve 30. When first valve 30 opens, gas is allowed to pass from first gas cylinder 12a through discharge outlet 24 into protected room 14. Shortly after first valve 30 opens, control panel 20 sends a signal V2 to second valve 34 and a signs! V3 to third valve 38, which are opened sequentially. Although FIG. 3 does not depict main valve 22 in the second embodiment of suppression system 100. a main valve 22 may be added to act as a safety to ensure that qas does leak from gas eyhnders 12a-12c pπor to activation of suppression system 100. if main vaive 22 is installed in second embodiment of suppression system 10, coπiroi paπei 20 sends a signal to main valve 22 and first valve 30 either simultaneously, or in dose sequence to each other. Rupturabie discs 16a and 16b function in the same manner In the second embodiment of suppression system IQQ as in the first embodiment of suppression system IQ.

Althouqh FiGS. 1 and 2 represent suppression system IQ having three gas cylinders and two rupturabie discs, more gas cyiiπders and rupturabie discs may be used as needed to adequately protect the enclosed room without departing from the intended scope of the present invention. For example, more than one gas cylinder may be attached between rupturabie discs if necessary to adequately suppress the fire in the enclosed room. In addition, if more gas cylinders are needed, more rupturabie discs may also be needed to ensure that overpressure does not occur In the protected room More gas cylinders and rupturabie discs may be needed depending on the size of the protected room, the volume of the gas cylinders, and other factors

FIG. 4 Is a qranh of the pressure in an enclosed room dunπg sequential discharging from gas cylinders 12a-12c as a function of time. As can be seen in FIG 4, suppression system IQ releases gas into the enclosed room at a controlled rate so that there is no overpressure in the enclosed room. While there is a relatively high initial peak

pressure PI In the enclosed room y»hen main valve 22 Is activated and gas from first qas cylinder 12a discharges into the enclosed room, the initial high pressure Increase is still below a pre-speclfied threshold limit. As the gas In first cylinder 12a is released and the level of gas remaining in first gas cvlinder 12a decays, the pressure differential at first rupturabie disc 16a increases until first rupturabie disc 16a ruptures and there ss a second peak In pressure P2 in the enclosed room as gas is also being emitted from second gas cylinder 12b. The pressure in the enclosed room then continues to decrease as the level of gas remaining in first and second gas cylinders 12a and 12b decays. As gas from first and second gas cylinders 12a and 12b is discharged Into the enclosed room, the pressure differential across second rupturable disc 16b Increases until second rupturabie disc 16b ruptures and gas Is also emitted from third gas cylinder 12c, causing a third pressure peak P3 in the enclosed room. Gas continues to discharge from gas cylinders 12a-12c until there Is a nominal amount of gas left in suppression system 10 and no more gas is emitted.

The number and values of pressure peaks PI . P2, and P3 depend on the number of gas cylinders and their volumes, respectively. Various fire codes mandate that the requisite amount of gas for suppressing a fire or other hazard in an enclosed room must be discharged Into the enclosed room within sixty seconds. As can be seen in FIG. 4, even with the time delay between pressure peaks PI . P2. and P3, all of the gas Is emitted from suppression system 10 within 60 seconds. The time delays between pressure peaks Pl P2, and P3 are controlled based on the pressure differential required to rupture rupturable discs 16 and can be manufactured to meet any specification. The optimal pressure differentials will vary depending on the size of the room, the number of gas cylinders, the volumes of the gas cylinders, and the time delay desired between sequential firing of the gas cylinders In one embodiment, the pressure differential required to rupture rupturable discs 16 Is between approximately 15 bar and 50 bar.

The sequential discharging fire hazard suppression system of the present invention efficiently prevents overpressure In a protected room upon discharge while reducing equipment. Installation costs, and maintenance costs. The fire hazard suppression system automatically discharges inert gas and controls the flow rate of the released inert gas by disposable rupturabie discs as a function of the pressure differential across the rupturable discs without using an expensive throttling valve. When a main valve Is activated, gas from a first gas cylinder discharges into the protected room. As the pressure from the first gas cylinder decays, the pressure differential at a first rupturable disc acting as a barrier between the first gas cylinder and a second gas cylinder ruptures and allows gas from Vn.e second gas cylinder to also discharge Into the protected room. The pressures in the first and second gas cylinders quickly reach a combined pressure equilibrium due to the gas cross- charqlng, and form a new pressure. As the new pressure in the first and second gas cylinders decays, the pressure differential at a second rupturable disc located between the second gas cylinder and a third gas cylinder ruptures so that qas from the third gas cylinder is allowed to pass Into the protected room. The qas from all three gas cylinders continue to discharge into the protected room until there is a nominal amount of gas left In the suppression system.

Although the present Invention has been described with reference to preferred embodiments, workers skilled In the art will recognize that changes may be made In form and detail without departing from the spiπt and scope of the invention.

Claims

CLAIMS:

1. A controlled pressure release system for preventing overpressure In a protected area upon delivery of gas from a plurality of gas containers, the system comprising- a first gas container; a second gas container; piping contacting the first sπd second gas containers, the piping having a discharge outlet; a first gas outlet and a second gas outlet connecting the first gas container and the second gas container to the piping. respectively; a first differential pressure responsive valve positioned between the first and second gas outlets; and a valve for actuating the system, the valve switchable between a closed position and an open position, wherein when the valve switches to the open position, the first gas container ss

In communication with the discharge outlet, and wherein the first differential pressure responsive valve opens as a function of a decreasing gas pressure in the first gas container

2. The system of claim 1 , wherein a first gas pressure \n the first gas container is approximately equal to a second gas pressure In the second gas container when the valve is in the closed position.

3. The system of claim 1 , wheresn the first differential pressure responsive valve comprises a first rupturable disc.

4. The system of claim 3, wherein the first rupturable disc ruptures when a gas pressure in the ϋrst gas container fails below a predetermined level.

5. The system of claim 4, wherein the second gas outlet Is in communication with the discharge outlet when the first rupturable disc ruptures.

6. The system of claim 1 , wherein a first gas pressure in the first gas container decreases at a first decreasing velocity and a second gas pressure In the second gas container decreases at a second decreasing velocity.

7 The system of claim 1. and further comprising a second valve located between the second gas container and the discharge outlet, the second valve swltehable between an open position and a closed position, wherein when the second valve switches to the open position and the first differential pressure responsive valve opens, the second gas container is in communication with the discharge outlet.

8. The system of claim 1 , and further comprising: a third gas container; a third gas outlet connecting the third gas container to the piping; and a second differential pressure responsive valve positioned between the second and third gas outlets.

9. The system of claim 8, wherein the second differential pressure responsive valve comprises a rupturable disc that ruptures as a function of a decreasing gas pressure in the second gas container.

10. The system of claim 9, wherein a third gas pressure in the third gas container Is equal to a first gas pressure In the first gas container and a second gas pressure in the second gas container when the valve Is in the closed position.

11. The system of claim 9, wherein the second differential pressure responsive valve opens when a pressure In the first and second gas containers fall below a predetermined level.

12. The system of claim 9, wherein the third gas outlet Is in communication with the discharge outlet when the second differential pressure responsive valve opens.

13. A method for automatically and sequentially releasing gas from a plurality of gas containers Into a protected room, the method comprising- actuating a valve connected between a first gas container of the plurality of gas containers and a discharge outlet to release gas from the first gas container; and releasing gas from a second gas container In response to opening of a first differential pressure responsive valve positioned between the first gas container and the second gas container opening.

14. The method of claim 13, wherein the first differentia! pressure responsive valve opens based on a pressure differentia! between a first gas pressure in the first gas container and a second gas pressure In the second gas container.

15. The method of claim 13, wherein actuating the vaive opens communication between the first gas container and the discharge outlet, and wherein opening the first differentia! pressure responsive va!ve opens communication between the second gas container and the discharge outlet.

16. The method of claim 13 and further comprising: releasing gas from a third gas container In response to a second differentia! pressure responsive valve positioned between the second gas container and the third gas container.

17 The method of claim 16, wherein the second differentia! pressure responsive valve opens based on a pressure differential between a second gas pressure In the second gas container and a third gas pressure in the third gas container.

18. The method of claim 16, wherein communication opens between the third gas container and the discharge outlet when the second differentia! pressure responsive valve opens.

19. A method for automatically and sequentially delivering gas from a plurality of gas containers separated by a plurality of valves and a plurality of pressure responsive valves to a protected room, the method comprising: actuating a valve connected to a first set of gas containers to deliver gas from the first set of gas containers to the protected room; and delivering gas from a second set of gas containers after a first pressure responsive valve positioned between the first set of gas containers and the second set of gas containers opens In response to a pressure differentia! across the first pressure responsive valve, wherein the second set of gas containers Is positioned upstream from the first set of αas containers.

20. The method of ciaim 19, and further comprising: delivering gas from any remaining plurality of gas containers after any remaining plurality of valves and plurality of pressure responsive valves successively open in response to pressure differentials across the remaining plurality of pressure responsive valves, the gas from the remaining plurality of gas containers being delivered successively to the main valve.