Abstract:
A relatively low pressure inert gas hazard suppression system ( 20 ) is provided which is designed to protect a room ( 22 ) or the like from the effects of fire or other hazard. The system ( 20 ) includes a plurality of pressurized inert gas cylinders ( 24 ) each equipped with a valve unit ( 26 ); each valve unit ( 26 ) is in turn coupled via a conduit ( 28 ) to a delivery manifold ( 30 ). The respective valve units ( 26 ) are operable to deliver gas from the cylinders ( 24 ) at a generally constant pressure (usually around 10-100 bar) throughout a substantial portion of the time of gas delivery, to thereby provide effective hazard suppression without the need for expensive high-pressure gas handling and distribution hardware and a reduction in room venting area due to lower room over-pressurization. Each valve unit ( 26 ) has a valve body ( 48 ) and a shiftable piston-type sealing member ( 56 ). Gas pressure from the cylinder ( 24 ) and a spring assembly ( 184 ) biases the member  56  to the valve open position, this being counterbalanced by gas pressure within equalization and modulation chambers ( 180, 182 ) provided in the valve unit ( 26 ). When a hazard is detected, the valve units ( 26 ) are actuated by draining of gas from the modulation chambers ( 182 ), allowing gas flow from the cylinders ( 24 ). As gas discharge proceeds, gas flows into and out of the modulation chambers ( 182 ) so as to achieve the desired generally constant pressure gas output. Near the end of gas discharge, the spring assembly ( 184 ) becomes predominant and holds the valve unit ( 26 ) open until all gas is discharged.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to inert gas hazard suppression assemblies used to protect areas or rooms such as computer equipment rooms from hazards, and especially fire. More particularly, the invention relates to such systems, as well as pressure modulating inert gas valves forming a part thereof, where multiple high-pressure inert gas cylinders are used, with each cylinder having a valve unit operable to deliver relatively low pressure inert gas at a generally constant pressure throughout a significant period of time during which gas is delivered, thereby providing effective hazard suppression without the need for high-pressure gas handling and distribution equipment or pressure reducing orifice plates that are typical of prior inert gas hazard suppression systems. Each valve unit includes a spring assembly biasing the unit to an open, gas-flow position as well as a gas flow modulating circuit which maintains the gas pressure around the desired output pressure over a substantial part of the gas delivery cycle.  
           [0003]    2. Description of the Prior Art  
           [0004]    Hazard suppression systems have long been employed for protecting rooms or areas containing valuable equipment or components, such as computer rooms. Traditionally, these systems have made use of one or more of the Halon suppressants. These Halon suppressants are ideal from a hazard suppression viewpoint, i.e., they are capable very quickly suppressing a hazard, can be stored at relatively low pressures, and the quantity of suppressant required is relatively small.  
           [0005]    However, in recent years the adverse environmental effects of the Halon has become evident and of considerable concern. Indeed, these issues are so significant that many governmental agencies have banned any further use of Halon. In Europe for example, even existing Halon systems are being replaced by systems using other inert gases such as nitrogen, argon, carbon dioxide and mixtures thereof.  
           [0006]    In an exemplary European fire suppression system based on the use of Halon as a suppressant agent, a vessel with a nominal capacity of 150 liters filled with liquified Halon is rated to protect a volume of approximately 17,000 cubic feet. The entire piping of a Halon system need be no more than schedule  40  pipe. Where it is desired to replace a Halon installation with an inerting gas system, or in new installations based on an inerting gas, the standards require that the sufficient inert gas be delivered to a predetermined protected area so that the inert gas occupies approximately 40% by the volume of the room. This lowers the oxygen level within the room to something on the order of 10-15%, which starves the fire of oxygen. At least 95% of the requisite amount of inert gas must be delivered to the protected room in a period of 60 seconds. At the same time, the inert gas preferably should be chosen so that people can be in the room after gas delivery for a period of as much as five minutes.  
           [0007]    A European inert gas fire suppression system when configured to replace a previous Halon system or as a new installation having a rating, which is equivalent to the exemplary 17,000 cubic foot Halon protection system referenced above, will require 10 high-pressure inert gas vessels as a replacement for the single Halon vessel. The requirement for a far larger number of inert gas storage vessels in a gas inerting fire suppression system as compared with the storage vessel requirements of a Halon system is because each inert gas vessel must be of significantly greater wall thickness and therefore as a practical matter must be significantly smaller. For example, a typical 80 liter inert gas cylinder will have a wall thickness of about 16 millimeters, be about 25 centimeters in diameter and 190 centimeters in length. The single, in this instance, 150 liter Halon vessel of the example, will be 40 centimeters in diameter and 100 centimeters in length. It is therefore obvious that on the basis that as many as 10 times as many inerting gas vessels are required as compared with a required number of Halon vessels for a particular installation that the space requirements for inerting vessels are much greater.  
           [0008]    Because inerting gas is stored as a gas rather than a liquid at very high pressures, e.g., 300 bar, compared with the much lower 25 bar pressure in a typical Halon storage vessel, a manifold pipe must be provided that is connected to all of the inerting gas cylinders, which is capable of withstanding simultaneous release of the high-pressure gas from the storage cylinders for direction of the gas to the piping distribution system of the fire suppression system. The manifold pipe must be at least schedule  160  piping to accommodate the high pressure. A pressure letdown orifice plate is provided at the end of the manifold, which also must be capable of withstanding the 300 bar inerting gas pressure.  
           [0009]    Thus, in an instance where an existing Halon system is to be retrofitted using high-pressure inerting gas, not only are a significantly greater number of suppressant agent storage vessels required as explained, but in addition, there is the need for a schedule  160  manifold connected to all of the storage cylinders, and in conjunction with a high-pressure orifice plate to reduce the gas pressure to a level that can be handled by the existing schedule  40  pipe. The schedule  160  pipe needed is manifestly more expensive than schedule  40  pipe and there will be a requirement for approximately 0.3 meters of schedule  160  pipe for each inert gas vessel. Similarly, the same requirement obtained in connection with a new installation.  
           [0010]    Accordingly, there is a real and unsatisfied need in the art for improved hazard suppression systems which can make use of relatively low pressure non-Halon inert suppression gas with existing Halon system piping (or low cost, overall low pressure piping in the case of new systems) while at the same time exhibiting the performance characteristics required for rapid hazard suppression.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention overcomes the problems outlined above and provides an improved hazard suppression system capable of effectively suppressing hazards such as fire through use of relatively low pressure inert gas cylinders together with specially designed cylinder-mounted discharge valves capable of delivering the gas at generally constant pressure levels throughout a majority of the time of gas delivery. In this way, use can be made of existing piping systems designed for Halon suppressants, or in the case of new systems less expensive piping and distribution hardware may be employed.  
           [0012]    In prior high-pressure inert gas systems employing a high-pressure letdown orifice plate, release of gas from the storage cylinders for discharge from the manifold pipe through the orifice plate resulted in very high initial gas flow rates, which declined rapidly to a very low gas flow rate. As an adjunct to the initial high discharge rate of the inerting gas into the protected area, a room vent had to be provided of sufficient area to accommodate the initial gas flow. In the present instance, moderation of the gas discharge flow rate permits provision of a vent area approaching a 30% smaller flow area.  
           [0013]    Generally speaking, a hazard suppression system in accordance with the invention for use in suppressing a hazard (e.g., typically fire) within a room or the like, comprises a plurality of pressurized gas cylinders each holding a supply of hazard-suppressing gas, with a valve unit operably coupled with each of said cylinders. A distribution assembly is connected with each of the cylinder-mounted valve units for delivery of gas to the protected room or the like. Each of the valve units has a valve body presenting an inlet adapted for coupling with a source of pressurized gas (namely a cylinder in the case of the overall suppression system) and an outlet adapted for coupling with a restricted gas receiver (the distribution assembly in the complete system). Further, a shiftable valve member having a passageway therein is located between said inlet and outlet of the valve body and is shiftable between a closed, gas flow-blocking position and an open position permitting flow of gas from said source to said receiver.  
           [0014]    Each of the valve units has a spring operably coupled with the shiftable valve member for biasing the member toward the open position of the valve unit. Additionally, separate first and second operating surface areas form a part of the valve member; the first area is exposed to the pressurized gas whereas the second area is exposed to the pressurized gas through the member passageway. These first and second surface areas are oriented and correlated relative to the valve body to normally maintain the member in the closed position thereof against the bias of the spring. The valve unit is designed to present a modulating gas chamber formed between at least a part of the second surface area and adjacent portions of the valve body. Moreover, a modulating gas passage is formed in the valve body and communicates the valve unit outlet and the modulating gas chamber. An actuator is operably coupled with the modulating gas passage to normally block communication between the valve unit outlet and the modulating gas chamber. ,said actuator operable upon actuation thereof to open said passage and thereby drain gas from said modulating chamber through said passage to reduce the gas pressure within the modulating gas chamber and permit movement of said member to the open position thereof under the influence of gas pressure exerted against the first surface area. A gas flow restriction is located in the passageway and is operable to substantially limit the flow rate of gas between the modulating gas chamber and the passageway. The first and second surface areas of the shiftable valve member, the modulation chamber, the modulating gas flow passage, and the spring are correlated so that gas from the source is delivered to the receiver at a generally constant pressure over a substantial part of the time gas flows from the source to the receiver. This is accomplished by recurring flow of the gas into and out of the modulation chamber through the modulating gas flow passage.  
           [0015]    The complete hazard suppression system also normally includes a sensor assembly operable to sense a hazard within the protected room or the like and, in response thereto, to actuate each of the valve unit actuators. In the case of a fire suppression system, the sensor would normally be in the form of a smoke detector. This would be electrically coupled with a solenoid valve controlling a pilot gas source. When a fire is sensed, the solenoid valve is opened allowing flow of the pilot gas to the valve units in order to actuate the latter.  
           [0016]    The gas pressure within the cylinders, which is stored nominally at 300 bar, is released through a respective modulating valve at a constant pressure of about 20 to 50 bar at a relatively constant flow rate. Notwithstanding this relatively low controlled release pressure and flow rate, the systems of the invention are capable of supplying adequate suppression gas to the protected area within established time constraints. This represents a significant economic advantage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic representation of a hazzard suppression system in accordance with the invention, shown in a configuration for protecting a computer room or the like;  
         [0018]    [0018]FIG. 2 is a fragmentary isometric view of an inert gas cylinder equipped with a valve unit in accordance with the invention;  
         [0019]    [0019]FIG. 3 is a top elevational view of the preferred valve unit;  
         [0020]    [0020]FIG. 4 is a side elevation view of the preferred valve unit;  
         [0021]    [0021]FIG. 5 is a vertical sectional view taken along line  5 - 5  of FIG. 3 and illustrating the details of construction of the preferred valve unit;  
         [0022]    [0022]FIG. 6 is a sectional view taken along line  6 - 6  of FIG. 5;  
         [0023]    [0023]FIG. 7 is a sectional view taken along line  7 - 7  of FIG. 5;  
         [0024]    [0024]FIG. 8 is a sectional view taken along line  8 - 8  of FIG. 5;  
         [0025]    [0025]FIG. 9 is a vertical sectional view similar to that of FIG. 5, but depicting the valve unit in its open, discharge position;  
         [0026]    [0026]FIG. 10 is a sectional view taken along line  10 - 10  of FIG. 9;  
         [0027]    [0027]FIG. 11 is a fragmentary sectional view of a portion of the valve body forming a part of the preferred valve unit;  
         [0028]    [0028]FIG. 12 is a pressure versus time graph illustrating the decaying pressure characteristics of a conventional, non-modulated valve unit during discharge of very high-pressure inert gas;  
         [0029]    [0029]FIG. 13 is a pressure versus time graph illustrating a typical pressure waveform obtained using a valve unit in accordance with the invention during discharge of relatively low pressure inert gas; and  
         [0030]    [0030]FIG. 14 is a flow diagram illustrating the operation of the preferred valve unit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Turning now the drawings, a hazard suppression system  20  is schematically illustrated in FIG. 1. The system  20  is designed to protect an enclosed room  22  which may house computer equipment or other valuable components. Broadly speaking, the system  20  includes a plurality of high-pressure inert gas cylinders  24  each equipped with a valve unit  26 . Each valve unit  26  is connected via a conduit  28  to a manifold assembly  30 . As illustrated, the assembly  30  extends into room  22  and is equipped with a plurality of nozzles  32  for delivery of inert gas into the room  22  for hazard suppression purposes. The piping making up the system  30  may be conventional schedule  40  pipe as opposed to the heavy-duty schedule  160  manifold piping and pressure letdown orifice plate required of prior systems of this character. The overall system  20  further includes a hazard detector  34  which is coupled by means of an electrical cable  36  to a solenoid valve  38 . The latter is operatively connected to a small cylinder  40  normally containing pressured nitrogen or some other appropriate pilot gas. The outlet of valve  38  is in the form of a pilot line  42  which is serially connected to each of the valve units  26 . As depicted in FIG. 1, the plural cylinders  24  may be located within an adjacent room or storage area  44  in proximity to the room  22 .  
         [0032]    [0032]FIG. 2 illustrates a cylinder  24 , which is conventionally a heavy-walled upright metallic cylinder having an outlet neck  46 . The inert gas within the cylinder (usually nitrogen, argon, carbon dioxide and/or mixtures thereof) is at relatively high-pressure on the order of 150-300 bar, and preferably on the order of 300 bar. The valve unit  26  is threaded into neck  46  (see FIG. 5) and includes an upright valve body  48  supporting an actuator  50 , pressure gauge  52  and rupture disc assembly  54 ; additionally, the valve unit includes an internal shiftable piston-type sealing member  56  (FIG. 5) As explained more fully hereafter, the valve unit  26  is designed so that inert gas from cylinder  24  is delivered to manifold assembly  30  at a generally constant pressure lower than the pressure within the associated cylinder over a substantial part of the time that gas flows from the cylinder.  
         [0033]    In more detail, the valve body  48  is of tubular design and has an externally threaded tubular inlet port  58  which is threadably received by neck  46 , a discharge port  60  adapted for coupling to a conduit  26 , a vent port  61  adjacent port  60 , and a stepped through bore  62  communicating with the ports  58 ,  60  and  61  and an uppermost spring chamber  64 . The bore  62  is configured to present (see FIG. 5), from bottom to top, an annular sealing ridge  66 , radially enlarged region  68 , annular shoulder  70 , annular relieved zone, shoulder  74 , and threading  76  leading to chamber  64 .  
         [0034]    The body  48  also has an extension  78  presenting a bore  80  designed to receive the inner end of actuator  50 . For this purpose, an O-ring  82  is provided within bore  80  as well as bolt connectors  84  for retaining the actuator  50  therein. A pair of passageways  86  and  88  communicate with bore  80  as best seen in FIG. 6. The passageway  86  extends from bore  80  into communication with discharge port  60  (FIG. 11). Bore  88  is dead-end bore but communicates with a passage  90  extending to threaded opening  92  which receives a plug  93 . A conventional Shrader valve  94  forming a part of the overall actuator  50  is seated within passageway and is normal blocking relation to the passage  90 . The valve  94  includes an uppermost actuator pin  96 . Another passage  95  is provided to extend from opening  92  to relieved zone  72 .  
         [0035]    Valve body  48  also includes a threaded bore  98  adapted to receive the connection end of gauge  52 . The bore  98  houses a Shrader valve  99  which is in an always-open condition when gauge  52  is installed. The bore  98  also communicates with another threaded bore  100  which receives rupture disc assembly  54 . A sensing bore  102  is provided within the body  48  and extends from bore  98  to inlet port  58 , thereby causing pressure within cylinder  24  to communicate with gauge  52  and also bore  100 .  
         [0036]    The assembly  54  comprises a threaded, somewhat T-shaped member  104  with a central relief passage  105  positioned within bore  100 . The inboard end of member  100  includes s conventional dome-shaped rupture disc  106  in normal blocking relationship to relief passage  105 . It will be appreciated, however, that if the cylinder  24  experiences an overpressure condition, such is communicated through sensing bore  102  and serves to rupture disk  106 ; this immediately vents the cylinder through the passage  105 .  
         [0037]    The actuator  50  includes a main actuator body  108 , an actuator cap  110 , and an internal shiftable piston  112 . The body  108  has a lowermost necked-down portion  114  seated within bore  80 , and a central opening  116  with an inboard, radially expanded region  117 . A vent passage  118  communicates with the opening  116  as shown. The upper end of the body  108  is internally threaded as at  120 . The cap  110  is threaded into the upper end of body  108  and has a piston chamber  122  as well as a cross passage  124 ; the latter receives the pilot line  42  as seen in FIG. 6. Piston  112  is generally T-shaped in cross-section with a latterly extending shank  126  and outer piston head  128 . Shank  126  carries a sealing O-ring  130  and a position retainer  132 , the latter extending into region  117  so as to limit the range of motion of the piston  112 . The head  128  also carries a sealing O-ring  134 . The inboard end of shank  126  is configured to engage the upper end of Shrader valve actuating pin  96  as will be explained.  
         [0038]    The sealing member  56  is positioned within valve body  48  and is selectively shiftable therein during operation of valve unit  26 . Referring to FIG. 5, the sealing member  56  includes four primary components extending from bottom to top, namely a piston seal holder  136 , bottom insert  138 , inner body section  140  and upper, outer body section  142 .  
         [0039]    The piston seal holder  136  includes a lower section  144  in facing relationship to bore  62  as well as an annular rib  146 . A sealing ring  148  is disposed between section  144  and rib  146 . A series of openings  149  are provided through holder  136  and merge to form a through passage  149   a . The bottom insert  138  is in the form of annular body presenting an upper radially outwardly extending flange  150  which abuts shoulder  70  of valve body  48 . The insert carries a peripheral sealing ring  152 . The inner body section  140  is threadably coupled to the upwardly projecting section of holder  136  and supports a series of vertically spaced apart sealing rings  152 - 158 . Additionally, the section  140  has a pair of vertically spaced flanged segments  160 ,  161  and an upper end provided with an internally threaded bore  162 . The section  140  has a central passageway  164  which communicates with passage  149   a . A port  166  extends from passageway  164  to a point just above flange segment  160 , and another upper port  168  extends from passageway  164  to a point just about flange segment  161 . A grub screw  169  is positioned within port  168  and serves to permit slow passage of gas therethrough from passageway  164 , while substantially blocking reverse flow into the passageway  164 .  
         [0040]    Outer body section  142  is of tubular construction and is threaded into valve body threading  76  so as to remain stationary. The section  142  has a central through bore  165  receiving inner body section  140  and external sealing rings  170 ,  172 . It will also be observed that the section  142  presents a pair of shoulders  174 ,  176 , and has a lateral passageway  178  which communicates with relieved zone  72 .  
         [0041]    The complementary design of the inner and outer body sections  140 ,  142  defines a pair of annular chambers which are important for the operation of valve unit  24 . Thus, an equalization chamber  180  is provided between the upper face of flange segment  160  and shoulder  174 , and a modulation chamber  182  is defined between the upper face of flange segment  161  and shoulder  176 .  
         [0042]    The shiftable segments of sealing member  56  (i.e., piston seal holder  136  and interconnected inner body section  140 ) are supported by means of a spring assembly  184  located within spring chamber  64 . In particular, a wave spring  186  is seated within the chamber and has at the upper end thereof an annular retainer disk  188 , the latter carrying a peripheral sealing ring  190 . A bolt  192 , seated on washer  194 , extends downwardly through disk  188  and is threadably received within bore  162 . It will be appreciated that spring assembly  184  serves to urge or bias holder  136  and section  140  upwardly as viewed in FIG. 5, that is towards the valve open position of the unit  26 .  
         [0043]    Operation  
         [0044]    It will be understood that valve unit  26  is normally in the static standby valve closed position thereof depicted in FIGS. 5-8. In this condition, the sealing member  56  is shifted downwardly as viewed in FIG. 5 so that sealing ring  152  comes into sealing engagement with ridge  66 . This is accomplished by virtue of the correlation between the first operating surface area S 1  presented by seal holder  136 , the second operating surface area S 2  presented by the sum of the equalization chamber effective surface area S 2 E (see FIG. 8, where S 2 E is the exposed portion of the face of flange  160 ) and the modulation chamber effective surface area S 2 M (see FIG. 7, where S 2 M is the exposed face of flange  161 ), and the force exerted by spring assembly  184 . That is, in the closed, static position of the valve unit  26 , a valve opening force is exerted against sealing member  56  in the form of pressure from the cylinder  24  is exerted against operating surface area S 1  through inlet port  58 , and the effect of spring assembly  184 . However, this opening force is counterbalanced and exceeded by a valve closing force exerted against operating surface S 2  (the sum of S 2 E and S 2 M), by virtue of passage of pressurized gas through the valve member via passage  149   a , passageway  164  and ports  166 ,  168  to the equalization and modulation chambers  180 ,  182 , respectively. It will be understood in this regard the grub screw  169  within port  168  permits slow passage of gas through port  168  while substantially preventing rapid reverse flow of gas from the modulation chamber  182  back into passageway  164 .  
         [0045]    In the valve close position, the actuator  50  (FIG. 6) is in its standby condition, that is, the piston  112  is elevated and Shrader valve  94  is in a flow-blocking relation relative to passage  90 .  
         [0046]    The operation of system  22  during a hazard suppression will now be described. In this discussion, reference will be made to the specific components of the system, and also to FIG. 14, which is a flow diagram of the system operation intended to facilitate an understanding of the invention.  
         [0047]    In the event of a hazard condition such as a fire in room  22 , the sensor  34  (e.g., a smoke detector) operates (Step  196 ) and sends an opening signal to solenoid valve  38  (Step  198 ). Compressed gas (usually nitrogen) then passes through pilot line  42  (Step  200 ) so as to actuate each of the valve units  26  respectively coupled to the corresponding cylinders  24  (Step  202 ). Turning to FIG. 10, upon introduction of pilot gas through line  32 , the piston  112  is shifted downwardly so that the inboard butt end thereof engages and shifts actuating pin  96  of Shrader valve  94 . As a consequence, the passage  90  is opened. When this occurs, gas flows from modulating chamber  182  into and through a modulating passage made up of annular relieved zone  72 , passage  95 , opening  92 , and passage  90  to discharge port  60  (Step  204 ). At this point, the valve opening force exerted by gas pressure against surface area S 1  and the spring assembly  184  is sufficient to move the sealing member  56  to the valve open position depicted in FIGS. 9-10. Therefore, gas from the cylinder  24  passes from inlet port  58  through discharge port  60 , conduit  28 , manifold  30  and nozzles  32  (Step  206 ).  
         [0048]    As indicated previously, a problem with prior discharge valves in the context of high-pressure hazard suppression systems is the tendency of such valves to exhibit a pronounced pressure decay pattern as illustrated in FIG. 12. This characteristic decay pattern results in an initial “burst” of inert gas delivery owing to the high pressure of the gas (on the order of 200 bar or around 3000 psi) with exponential decline in pressure during the course of remaining gas discharge. While these prior systems are capable of delivering adequate volumes of inert gas within the hazard suppression time frame, use of the high-pressure gas cylinders entails considerable expense in terms of piping and related gas handling and distribution hardware.  
         [0049]    This problem is overcome by the present invention which exhibits the general pressure wave form of FIG. 13, i.e., gas is delivered at a generally constant pressure lower than the pressure of gas within the cylinder  24 , but over a substantial period (at least about 50%, more preferably at least about 75%) of the time during which gas is discharged by the valve unit  26 . This type of pressure waveform enables release of gas at a much lower inert gas pressure, on the order of from about 10 to about 100 bar, or from around 150 to 1500 psi, and as a consequence use can be made of low-cost gas handling and distribution equipment, often the existing equipment in systems heretofore employing Halon as suppressants. In a preferred system, the release pressure is about 50 bar.  
         [0050]    Specifically, as gas from the cylinders  24  is initially delivered to the discharge port  60 , a back pressure is generated within the valve unit which causes gas from the cylinder to travel back through the above-described modulating passage comprising passage  90 , opening  92 , passage  95 , relieved zone  72  and into modulating chamber  182 . This serves to move the sealing member  56  back toward the closed position of the valve unit. This in turn creates a restriction to gas flow from the cylinder  24 , which continues until the pressure within discharge port  60  is reduced. Thereupon, gas from the modulation chamber  182  flows along the described modulating passage to the discharge port. This back and forth gas flow pattern along the modulating passage recurs throughout a majority of the time gas flows from the cylinders  24 . The result is a pressure modulation of gas flow from the cylinder  24  to create the generally horizontal portion of the FIG. 13 wave form. Towards the end of discharge of gas from the cylinder  24 , the spring force exerted from assembly  184  becomes greater than the sum of the forces exerted in the equalization and modulation chambers, so that the spring becomes the sole operating element in the valve unit and the latter remains full open until gas discharges completely. It will be understood in this respect that while FIG. 13 depicts an essentially straight line, constant pressure condition with a rapid tail-off at the end of gas discharge, in practice the wave form would exhibit fluctuations generally around the straight line portion of the straight line.  
         [0051]    The modulation operation of unit  26  is illustrated in FIG. 14 within the dotted line box  208 , in the form of a logic diagram. Thus, in Step  210 , if the cylinder force (i.e., the force exerted by the cylinder gas against surface area S 1 ) plus the spring force (i.e., the force exerted by spring assembly  184 ) equals the counterforce exerted against second surface area S 2  (the sum of the S 2 E and S 2 M surface areas) in the equalization and modulation chambers  180 ,  182 , the system is balanced, Step  212 . If the cylinder force plus the spring assembly force is less than the counterforce (Step  214 ), the sealing member is moved toward the valve closed position thereof (Step  216 ), to restrict the flow of gas from the cylinder. If the cylinder force plus the spring force is greater than the counterforce (Step  218 ), then the sealing member is moved toward the valve open position (Step  220 ). This modulation continues by the effective determination of the cylinder force, spring force and counterforce (Step  222 ) until, in Step  218 , the spring force is greater than the counterforce exerted through the equalization and modulation chambers (Step  224 ). At this point, the spring assembly fully extends (Step  226 ), which is generally corresponds to the downwardly directed “knee” portion of the FIG. 13 wave form. This completes the system operation Step  228 .