Patent Publication Number: US-6708771-B2

Title: Low pressure electro-pneumatic and gate actuator

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. application Ser. No. 09/897,167, filed Jul. 2, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/810,631, filed Mar. 16, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/535,599, filed Mar. 27, 2000, now U.S. Pat. No. 6,293,348 issued Sep. 25, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to actuators for controlling the operation of valves and especially for valves used in sprinkler systems for fire protection. 
     BACKGROUND OF THE INVENTION 
     Automatic sprinkler systems for fire protection of structures such as office buildings, warehouses, hotels, schools and the like are required when there is a significant amount of combustible matter present. The combustible matter may be found in the materials from which the building itself is constructed, as well as in the building contents, such as furnishings or stored goods. 
     Of the various types of automatic sprinkler systems available, the preaction systems find widespread use. Preaction systems use an actuator which responds to a combination of signals from different detectors to trip a valve which provides water to the sprinkler piping network. Similar to the so-called “dry-pipe” systems, the piping network in the preaction system is normally filled with air or nitrogen (and not water) prior to actuation. The preaction system can thus be used in unheated environments which are subject to below freezing temperatures without fear of pipes bursting due to water within the pipes expanding upon freezing. 
     When sufficiently pressurized, the behavior of the gas within the piping network may be used to indicate a fire condition and trigger actuation of the preaction system. Heat from the fire will cause sprinkler heads to open, allowing pressurized gas to escape from the piping network and result in a pressure drop within the system. Actuation of the system may be effectively triggered by this pressure drop. 
     Specifically, double interlock preaction systems are further advantageous because an alarm may be sounded to provide a warning before the sprinklers operate. Furthermore, failure, breakage or accidental opening of the sprinklers or a pipe in the piping network will not result in an unintentional discharge of water, since there is no water in the network until the system is actuated. Actuation for double interlock preaction systems requires that two or more separate signals be sensed by the actuator. 
     Preaction systems are not without their disadvantages however. Traditional preaction systems, described above, which are triggered by a drop in air pressure within the piping network as the result of a sprinkler head opening in response to heat (along with a confirming signal from another sensor) usually maintain the sprinkler piping network at a relatively high internal pressure, typically on the order of 20% of the maximum water pressure in the system. The air pressure in such systems is used to control the release of the water to the piping network, and the valves typically operate at a mechanical advantage of about 1 to 5 air pressure to water pressure. The use of relatively high-air pressures becomes a problem with larger systems which tend to have a relatively large volume of air within the piping network. Higher air pressures and volumes require more powerful compressors, having higher capital and operating costs. Furthermore, the higher pressures mean that more air must be forced out of the piping network upon activation. The air in the network inhibits the free flow of water and, thus, increases the reaction time of the system. More air in the piping network also means that more moisture will be present, accelerating corrosion of the pipes. 
     There is clearly a need for a preaction sprinkler system having the ability to operate at relatively low system air pressures for providing a signal which activates the sprinkler system. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The invention concerns an electro-pneumatic actuator for actuating a fire sprinkler system. The system is actuated when the actuator depressurizes a piston holding a valve controlling the flow of water to the sprinkler system closed. The actuator behaves like an AND gate in a logic circuit in that it will depressurize the piston and release the valve only when two separate signals indicating a fire condition are manifest in the actuator. The actuator is, thus, connected to two separate fire detection systems, one being the piping network of the sprinkler system charged with compressed gas, the other being an electronic fire detection system having a control system in communication with a plurality of fire detectors substantially co-located with the piping network. 
     During a fire, heat-sensitive sprinkler heads on the piping network open and release pressurized air within the network to the ambient causing a pressure drop in the piping network. Because the piping network is in fluid communication with the actuator, the pressure drop is sensed by it. The pressure drop is accompanied by a signal or signals indicating a fire condition sent from one or more of the fire detectors to the control system. In turn, the control system sends an electrical signal to the actuator. In response to the concurrent pressure drop in the piping network and the electrical signal from the control system, the actuator depressurizes the piston which allows the valve to open and supply water to the piping network for release through the open sprinkler heads onto the fire. 
     In the preferred embodiment, the actuator has a first chamber with a flexible first diaphragm mounted therein. The first diaphragm sealingly divides the first chamber into first and second chamber portions, both the chamber portions being in fluid communication with the cylinder. The second chamber portion has an opening providing fluid communication with the ambient, the opening being surrounded by a seat facing the first diaphragm. The first diaphragm is deflectable into sealing engagement with the seat to seal the opening when the cylinder is pressurized with a first fluid, such as the water for the sprinkler system. 
     A second chamber having a flexible second diaphragm mounted therein which sealingly divides the second chamber into third and fourth chamber portions is preferably positioned above the first chamber. The third chamber portion is in fluid communication with a source of pressurized second fluid, for example, the compressed air within the piping network, and the fourth chamber portion is in fluid communication with the ambient. The fourth chamber portion has an aperture providing fluid communication with the first chamber portion, the aperture being surrounded by a second seat facing the second diaphragm. The second diaphragm is deflectable into sealing engagement with the second seat to seal the aperture when the third chamber portion is pressurized with the second fluid, for example, the compressed air from the piping network. 
     A third chamber having a flexible third diaphragm mounted therein which sealingly divides the third chamber into fifth and sixth chamber portions is preferably mounted atop the second chamber. The fifth chamber portion is in fluid communication with the pressurized first fluid. An elongated plunger extends between the fifth and third chamber portions. One end of the plunger is positioned within the sixth chamber portion and is engageable with the third diaphragm, the other end of the plunger being positioned within the third chamber portion and engageable with the second diaphragm. The third diaphragm is deflectable into engagement with the one end of the plunger when the fifth chamber portion is pressurized with the pressurized first fluid. The plunger is forced into engagement with the second diaphragm and thereby forces the second diaphragm into sealing engagement with the second seat. 
     A passageway is located within the third chamber providing fluid communication between the fifth chamber portion and the ambient. A valve is engaged with the passageway and has a valve member movable between an open position, allowing fluid flow through the passageway and a closed position, preventing fluid flow through the passageway. The valve also has means for biasing the valve member into the closed position and an electrically operated actuator for moving the valve member into the open position upon receipt of the electrical signal form the control system. 
     In operation, the second diaphragm is deflected out of engagement with the second seat only when fluid pressures in both the fifth and the third chamber portions are lowered to respective predetermined values, pressure in the fifth chamber portion being lowered by electrically actuating the valve member into the open position, pressure in the third chamber portion being lowered by a drop in pressure of the pressurized second fluid. Upon deflection of the second diaphragm, pressurized first fluid in the first chamber portion is permitted to enter the fourth chamber portion and exit to the ambient, thereby allowing the first diaphragm to deflect out of engagement with the first seat. This allows the pressurized first fluid to flow from the cylinder through the second chamber portion and exit to the ambient, thereby depressurizing the piston and allowing it to move within the cylinder to release the valve and actuate the sprinkler system. 
     The invention also includes a reset valve for manually resetting the sprinkler system and preventing unintentional resetting during a fire. The reset valve has a valve body and a conduit extending through the valve body. One end of the conduit is in fluid communication with the third chamber portions and the other end is vented to the ambient. A valve seat is positioned in the one end of the conduit and a valve closing member is movably mounted within the conduit adjacent to the seat. The valve closing member is movable into sealing engagement with the seat to close the reset valve. The reset valve also has means for biasing the valve closing member out of engagement with the seat when fluid pressure within the one end of the conduit falls below a predetermined value. The biasing means thereby opens the reset valve and vents the third chamber portion to the ambient. Preferably, there is an identical reset valve in fluid communication with the fifth chamber portion as well. The reset valves prevent spurious pressure surges from pressurizing either of the third or fifth chamber portions and thereby accidentally resetting the system and, thus, cutting off the water supply during a fire. 
     It is an object of the invention to provide an actuator for a double interlock fire protection sprinkler system which uses pneumatic and electrical functions to actuate the system. 
     It is another object of the invention to provide an actuator, wherein the pneumatic function operates substantially independently of the system water pressure. 
     It is yet another object of the invention to provide an actuator which will not trigger the system in the event of an electrical power failure. 
     It is again another object of the invention to provide an actuator which will not reset itself during a fire due to an air pressure surge. 
     These and other objects of the invention will be apparent upon consideration of the following drawings and detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of a preaction double interlock fire protection sprinkler system using a low pressure electro-pneumatic AND gate actuator according to the invention; 
     FIG. 2 is a longitudinal sectional view of a valve and control piston used in the preaction fire protection system shown in FIG. 1; 
     FIG. 3 is a longitudinal sectional view of a low pressure electro-pneumatic AND gate actuator according to the invention; and 
     FIG. 4 is a longitudinal sectional view of an alternate embodiment of a low pressure electro-pneumatic AND gate actuator according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a double interlock preaction fire protection sprinkler system  10  having a low pressure AND gate electro-pneumatic actuator  12  according to the invention. System  10  comprises a piping network  14  having a plurality of automatic sprinkler heads  16  which open when the air surrounding the head reaches a predetermined temperature due to a fire. Network  14  is normally dry and is connected to a valve  18  which controls the flow of water from a pressurized water supply source  20  to the network  14 . 
     As shown in FIG. 2, the valve closing member of valve  18  is preferably a pivoting clapper  22  held in the closed position against the pressure of the water supply  20  by a latch  24  controlled by a piston  26  reciprocable within a cylinder  28 . Cylinder  28  is in fluid communication with water supply  20  via a conduit  30 , the water supply  20  pressurizing piston  26  against compression spring  27  to hold the latch in position keeping the clapper  22  closed. As shown in both FIGS. 1 and 2, cylinder  28  is also in fluid communication with the electro-pneumatic actuator  12  via a conduit  32 , the actuator hydraulically controlling the action of the piston  26  to actuate valve  18  as described below. The piping network  14  is in fluid communication with a compressed air supply  38  at a point  40  downstream of the clapper  22 . The piping network  14  is also connected to the electro-pneumatic actuator  12  via a conduit  42  as described in detail below. 
     As further shown in FIG. 1, sprinkler system  10  also includes a plurality of fire detectors  36 . Detectors  36  are substantially co-located with sprinkler heads  16  and may be virtually any type of transducer which detects a fire condition and generates an electrical signal in response thereto. For example, detectors  36  may detect smoke, heat, rate of temperature rise, visible, infra-red or ultra-violet light and generate an electrical signal in response. The signal is transmitted via a communication link  34  to a control system  44 . Preferably, control system  44  is a microprocessor which receives electrical signals from one or more of the detectors  36  indicating a fire condition and in response sends an electrical signal to the electro-pneumatic actuator  12  via another communication link  35 . 
     In operation, both the piping network  14  and the detectors  36  act as sensors to trigger the sprinkler system  10  in the event of a fire. The piping network  14  is charged with compressed air from the compressed air supply  38 . Heat from the fire causes the automatic sprinkler heads  16  nearest the fire to open. Substantially concurrently, detectors  36  nearest the fire sense its presence and generate a signal which is sent to the control system  44  via communication link  34 . Opening of the heads  16  of the piping network  14  permits a drop in the pressure of the compressed air in the network which is communicated to the electro-pneumatic actuator  12  through conduit  42 . In response to the signal from the detectors  36 , control system  44  sends an electrical signal to the electro-pneumatic actuator through communication link  35 . Upon receiving the combination of the pressure drop within the piping network  14  and the electrical signal from the control system  44 , the electro-pneumatic actuator  12  depressurizes piston  26  which, under the biasing force of spring  27  releases latch  24  permitting clapper  22  to open and supply water to the piping network  14 . Upon reaching the open sprinkler head or heads  16 , the water is discharged onto the fire. The operation of the electro-pneumatic actuator  12  is described in detail below. 
     As shown in cross-section in FIG. 3, the low pressure electro-pneumatic actuator  12  has a housing  46  preferably comprised of brass. Housing  46  has three chambers, a top chamber  48 , a middle chamber  50  and a bottom chamber  52 . Although the chambers are shown positioned one above another and are named top, middle and bottom, it is understood that the orientation of the actuator is irrelevant to its operation and the naming of its parts is for convenience and by way of example only and places no limitations on the structure or configuration of the actuator. 
     Each chamber is divided into upper and lower chamber portions by respective top, middle and bottom diaphragms  54 ,  56  and  58 . Preferably, diaphragms  56  and  58  comprise a metal ring  60  surrounding a metal plate  62 . Both the plate  62  and ring  60  are encapsulated in a flexible sheath  64  and are attached to one another by a membrane portion  66  of the sheath  64  which extends between the plate and the ring. Ring  60  stiffens the perimeter of the diaphragm and provides a means for attaching it to the housing, the ring being sandwiched between various segments  70 ,  72  and  74  forming the housing. The sheath is preferably EPDM or a similar flexible polymer and provides for a fluid tight seal between the segments. Plate  62  stiffens the diaphragm and the sheath surrounding it ensures a fluid tight seal between the diaphragm and various seats as described below. The membrane portion  66  provides flexibility allowing the diaphragm to deflect in response to fluid pressure on one side or another. 
     Top diaphragm  54  is preferably a simple membrane which performs a sealing function between the upper and lower chamber portions of the top chamber  48 . 
     While the diaphragms as described above are preferred, it is understood by those of skill in the art that other types of diaphragms may also be used without adversely affecting the operation of the actuator. 
     Bottom chamber  52  is divided by bottom diaphragm  58  into an upper chamber portion  76  and a lower chamber portion  78 . Both chamber portions  76  and  78  are in fluid communication with cylinder  28  through conduit  32 . Conduit  32  has a large diameter duct  80  which connects with the lower chamber portion  78  and a smaller diameter duct  82  which connects to the upper chamber portion  76 . Lower chamber portion  78  has a hole  86  surrounded by a seat  88 , the hole  86  allowing the lower chamber portion to vent to the ambient through a port  89 , the seat  88  being engageable by the bottom diaphragm  58  to seal the hole  86  when the force exerted by the pressure in the upper chamber portion  76  is greater than the force exerted by the pressure in the lower chamber portion  78 . Preferably, a biasing means in the form of a spring  90  is positioned within upper chamber portion  76  to bias bottom diaphragm  58  into sealing engagement with seat  88 . 
     Middle chamber  50  is divided into upper and lower chamber portions  92  and  94  respectively by middle diaphragm  56 . Upper chamber portion  92  is in fluid communication with piping network  14  through conduit  42  (see also FIG.  1 ), and lower chamber portion  94  is in fluid communication with the ambient through a duct  98  connecting to port  89 . Lower chamber portion  94  is further in fluid communication with upper chamber portion  76  through an aperture  100 . A seat  102  surrounds aperture  100 , the seat being engageable by middle diaphragm  56  to seal the aperture  100 . A biasing means in the form of a spring  104  is positioned within the lower chamber portion  94  to normally bias the diaphragm out of engagement with seat  102 . 
     Top chamber  48  is divided into upper and lower chamber portions  106  and  108  by top diaphragm  54 . Upper chamber portion  106  is in fluid communication with pressurized water source  20  through a conduit  68  which branches from conduit  32 . Preferably, conduit  68  has a restrictor element  69  which restricts fluid flow to the upper chamber portion  106  but allows the full fluid pressure of pressurized water source  20  to be developed within the upper chamber portion  106 . 
     The upper chamber portion is also in fluid communication with a passageway  110  in fluid communication with the ambient. A valve  111  is engaged with the passageway  110  and has a valve member  113  movable between an open position allowing fluid flow from the upper chamber portion  106  through passageway  110  and to the ambient and a closed position preventing such flow. The valve  111  has a means for normally biasing the valve member into the closed position and an electrically operated actuator for moving the valve member into the open position in response to the electrical signal from the control system  44  carried over communication link  35 , which is connected to the valve  111  as shown in FIG.  3 . Preferably, valve  111  comprises an electrically actuated solenoid valve and valve member  113  is an armature of the solenoid which is moved into the open position when the solenoid is energized by the electrical signal from the control system  44 . 
     Preferably, the water pressure within upper chamber portion  106  comprises the means for biasing the valve member  113  into the closed position. Solenoid valve  111  comprises a fluid tight valve chamber  115  which is in fluid communication with upper chamber portion  106 . Valve member  113  is positioned within the valve chamber  115  and is biased into the closed position, closing off passageway  110 , when the upper chamber portion and the valve chamber are pressurized by the pressurized water source  20  communicated through conduits  32  and  68 . When the solenoid valve  111  is electrically actuated by the control system  44 , the valve member  113  is moved against the pressure within valve chamber  115  away from the passageway  110  allowing the fluid within the valve chamber  115  and the upper chamber portion  106  to flow through the passageway  110  to the ambient. 
     An elongated plunger  112  extends between lower chamber portion  108  and upper chamber portion  92  of middle chamber  50 . One end  114  of the plunger is engageable with top diaphragm  54 . The other end  116  of the plunger is engageable with middle diaphragm  56 . The plunger is slidably movable within the housing  46 , and the lower chamber portion  108  of the top chamber  48  is isolated from the upper chamber portion  92  by a seal  118  surrounding the plunger  112 . 
     Preferably, the upper chamber portion  92  of the middle chamber  50  vents to the ambient through a reset valve  120  positioned in fluid communication with conduit  42 , which has a flow restrictor  43  positioned between the reset value and the piping network  14 . Flow restrictor  43  helps isolate the actuator  12  from major pressure fluctuations in the piping network and ensures that upper chamber portion  92  vents rapidly through the reset valve  120  when this valve triggers. Reset valve  120  has a valve body  122  through which a conduit  124  extends providing fluid communication between the upper chamber portion  92  and the ambient. A valve seat  126  is positioned at the end of the conduit  124  which is in fluid communication with the conduit  42 , and a valve closing member  128  is movably mounted within the conduit and is movable into sealing engagement with the valve seat  126 . In the example shown in FIG. 3, valve closing member  128  is mounted on the end of a shaft  130  which is slidably movable within the valve body  122 , although other configurations are also feasible. 
     Shaft  130  extends outwardly from the valve body  122  and has a knob  132  which may be manually grasped to pull valve closing member  128  into engagement with valve seat  126 . A biasing means in the form of spring  134  is positioned around shaft  130  to bias the closing member  128  out of engagement with seat  126 . Preferably, conduit  124  is sized larger than the valve closing member over a region  136  between seat  126  and the conduit  42  for reasons explained below. 
     Low Pressure Electro-Pneumatic AND Gate Actuator Operation 
     The low pressure electro-pneumatic AND gate actuator  12  according to the invention is used in the preaction fire protection system  10  to reset the system (make it ready for actuation) and to actuate the system upon receipt of the appropriate signals. The appropriate signals preferably comprise a pressure drop in the sprinkler piping network  14  caused by one or more sprinkler heads  16  opening in response to the heat of a fire and an electrical signal from the control system  44  in response to signals from one or more fire detectors  36 . 
     System Reset Function 
     The sprinkler system  10  is made ready for action by resetting both the electrical and the pneumatic functions of the electro-pneumatic actuator  12 . 
     Water from the pressurized water supply  20  acting through conduits  32  and  68  flows to the upper chamber portion  106  of top chamber  48  and into the valve chamber  115  of solenoid valve  111 . Assuming the solenoid valve  111  is energized by a signal from the control system  44 , valve member  113  is held in the open position and water flows from the upper chamber portion  106  through passageway  110  to the ambient. The electrical function of the sprinkler system  10  is then reset by removing the signal from the control system  44  to the solenoid valve  111 . This releases valve member  113  which moves in response to the water flow through the valve chamber  115  into the closed position preventing further flow of water through passageway  110  to the ambient. Water pressure increases within the valve chamber  115  as well as within upper chamber  106 , the pressure securely seating the valve member  113  closed and deflecting the top diaphragm  54  toward the middle chamber  50 . The top diaphragm  54  engages end  114  of plunger  112 , forcing the opposite plunger end  116  into engagement with the middle diaphragm  56  and causing it to deflect into lower chamber portion  94  against biasing spring  104 . Middle diaphragm  56  sealingly engages seat  102  to close the aperture  100  between the lower chamber portion  94  and the adjacent upper chamber portion  76 . Air in lower chamber portion  94  is vented to ambient through duct  98  and port  89 . 
     Compressed air is supplied to the actuator  12  from the system air supply  38  through conduit  42 . Assuming reset valve  120  is open, the air flows through it to the ambient. To reset the pneumatic function of the electro-pneumatic actuator  12 , an operator pulls upwardly on the reset knob  132  on the reset valve  120 , moving the valve closing member  128  against biasing spring  134  and seating the valve closing members against valve seat  126 . When the valve closing member  128  is in the unseated (open) position as shown in FIG. 3, compressed air normally flows around it due to the enlarged regions  136  of conduit  124 . Enlarged conduit region  136  prevents an air pressure surge in the system from unintentionally resetting the system during a fire (and thereby cutting off the water to the sprinkler heads) by inadvertently seating the valve closing member  128 . Because of the enlarged conduit region  136 , the valve closing member in valve  120  must be held in the seated position until sufficient pressure is achieved within upper chamber  92  and conduit  42  to exert a force on the valve closing member  128  which exceeds the biasing force of spring  134 . The spring  134  and valve closing member  128  are designed such that a pressure above about 6.5 psi in upper chamber  92  and conduit  42  is sufficient to keep the valve closing member seated. The reset valve is, thus, used to establish a relatively low pressure trip point for the system as described in more detail below. 
     With the reset valve  120  closed, air pressure increases in the upper chamber portion  92 . This pressure will cause middle diaphragm  56  to deflect into the lower chamber portion  108  forcing it to engage seat  102  and close off aperture  100  independently of the action of the top diaphragm  54  acting through plunger  112  described above. Together the top and middle diaphragms  54  and  56  provide the AND gate logic function of the actuator  12  in that both diaphragms must be allowed to independently deflect to allow the bottom diaphragm  58  to unseat and open aperture  100  to actuate the main valve  18  supplying water to the sprinkler heads as described further below. Either diaphragm alone, however, can exert sufficient force to keep the bottom diaphragm  58  seated and prevent actuation of the system. 
     Bottom diaphragm  58  is normally biased into engagement with seat  88  by spring  90 , thus, sealing hole  86  which would otherwise vent the lower chamber portion  78  to the ambient through port  89 . As shown in FIGS. 1 and 2, water pressure taken from upstream of valve  18  through conduit  30  pressurizes the piston  26  within cylinder  28  against spring  27  into engagement with latch  24 , keeping clapper  22  closed and cutting water off from the sprinkler piping network  14 . The cylinder  28  is in fluid communication with lower chamber portion  78  of actuator  12  through conduit  32  and with upper chamber portion  76  through the small diameter duct  82  fed from conduit  32 . Water pressure within the cylinder  28  which keeps clapper  22  closed also forces bottom diaphragm  58  against seat  88  to close hole  86 . The water pressure in upper chamber portion  76  exerts greater force on the bottom diaphragm  58  than the same pressure in lower chamber portion  78  since the water pressure in the lower chamber portion  78  does not act over the entire area of the diaphragm as it does in the upper chamber portion  76 . This is because the central portion of diaphragm  58  is exposed to atmospheric pressure through hole  86  when the diaphragm  58  is seated against seat  88 , and the water pressure within chamber  78  cannot act against this central portion isolated by seat  88 . 
     The system is now set and ready to supply water to sprinkler heads  16  as called for to suppress a fire. 
     System Actuation 
     Heat from a fire will cause sprinkler heads  16  on the piping network  14  in the immediate vicinity of the fire to open. This allows compressed air within the piping network to vent to the ambient, causing a pressure drop in the piping network. As shown in FIG. 3, the upper chamber portion  92  of the middle chamber  50  is in fluid communication with the piping network  14  through conduit  42 . A pressure drop in the piping network  14  will, thus, be communicated to the chamber portion  92  within the actuator  12 . 
     Concurrently with the opening of sprinkler heads  16 , the detectors  36  in the immediate vicinity of the fire will sense the fire and signal the control system  44  through communications link  34 . In response, control system  44  sends a signal via communications link  35  to the solenoid valve  111 , energizing the solenoid and moving the valve member  113  against the biasing pressure within valve chamber  115  to open the passageway  110  and allow the water within upper chamber portion  106  to flow through the passageway to the ambient, thus, relieving the pressure deflecting the top diaphragm  54  toward the middle chamber  50 . This also relieves the force on plunger  112  and allows the middle diaphragm to deflect away from seat  102 , thus, opening aperture  100 , provided that the air pressure within upper chamber portion  92  is also reduced. 
     The reduction in air pressure within upper chamber  92  occurs due to the opening of sprinkler heads  16  in response to the fire as described above. When the air pressure in upper chamber portion  92  drops to a predetermined value (preferably about 6.5 psi), the reset valve  120  opens (valve closing element  128  unseats from seat  126  and is biased into enlarged conduit region  136 ) venting the upper chamber portion  92  to the ambient and causing a rapid pressure drop in the upper chamber portion. As the pressure in upper chamber portion  92  drops, it falls below a second predetermined value which allows biasing spring  104  to deflect both the top and middle diaphragms  54  and  56  upwardly, unseating middle diaphragm  56  from seat  102  and opening aperture  100 . This allows water under pressure in upper chamber portion  76  to flow through aperture  100 , into lower chamber portion  94  and out to the ambient through duct  98  and port  89 . With the water pressure in upper chamber portion  76  thus relieved, the bottom diaphragm  58  is deflected by water pressure within lower chamber portion  78 , the bottom diaphragm is unseated from seat  88 , allowing water from conduit  32  to vent to the ambient. Deflection of the bottom diaphragm  58  away from seat  88  is ensured by making the diameter  80  of conduit  32  feeding lower chamber portion  78  relatively large as compared with the diameter of duct  82  which feeds the upper chamber portion  76 . Despite being at the same pressure, water from conduit  32  cannot flow fast enough through small diameter duct  82  to pressurize upper chamber portion  76  and deflect the bottom diaphragm  58  into engagement with seat  88 . 
     Conduit  32  is in fluid communication with cylinder  28  (see also FIG.  2 ). Thus, when the conduit  32  is vented to ambient by the action of bottom diaphragm  58 , piston  26  is depressurized. This allows spring  27  to move the piston  26  and release latch  24 , allowing clapper  22  to open under the pressure of water source  20  and supply water to the piping network  14  where the water is released from the open sprinkler heads  16  onto the fire. 
     Based upon the foregoing description of the actuator and its operation, it is possible to view the actuator as comprised of a plurality of pressure actuated valves. Bottom chamber  52  and its associated bottom diaphragm  58  comprise an example of a first pressure actuated valve controlling the flow of the pressurized fluid through the actuator. This first valve has a first valve closing member (diaphragm  58 ) with opposite sides both in fluid communication with the pressurized fluid. The first valve is normally closed and prevents the fluid flow which depressurizes the piston  26 . The first valve closing member opens to permit the depressurizing flow when the fluid pressure on one side of the first valve closing member exceeds the fluid pressure on the opposite side of the first valve closing member. 
     The middle chamber  50  and its middle diaphragm  56  comprise an example of a second pressure actuated valve controlling the fluid pressure on the opposite side of the first valve closing member. The second valve has a second valve closing member (diaphragm  56 ) which is movable from a closed position, which maintains fluid pressure on the opposite side of the first valve closing member, to an open position, which releases fluid pressure from the opposite side of the first valve closing member. The second valve closing member has a side in fluid communication with a first source of compressed fluid and is movable from the closed to the open position in response to a decrease in pressure of the first source of compressed fluid. 
     The solenoid valve  111  comprises an example of a third pressure actuated valve. The third pressure actuated valve has a third valve closing member  113  with a mechanical link to the second valve closing member through top diaphragm  54  and plunger  112 . The third valve closing member has a side in fluid communication with a source of compressed fluid and is movable from a first position which maintains a force through the mechanical link onto the second valve closing member (thereby maintaining the second valve closing member in the closed position) to a second position removing the force from the second valve closing member. The third valve closing member is electrically actuated and moves to the second position in response to an electrical signal from the control system  44 . However, both the third and second valve closing members move into their respective open positions only upon a concurrent pressure decrease in the piping network and an electrical signal to the electro-pneumatic actuator, as occurs when the piping network  14  is vented when one or more sprinkler heads open and one or more of the detectors  36  send a signal to the control system  44  in the event of a fire. Motion of both the second and third valve closing members allows the first valve closing member to move into its open position and permit flow of the pressurized fluid through the actuator, thereby depressurizing piston  26  and triggering the sprinkler system. 
     Use of the actuator according to the invention provides the following advantages. The actuator will not trigger the system as a result of an electrical power failure. Second, the sprinkler system may operate at a relatively low air pressure, the air pressure design parameters being chosen independently of the source water pressure needed. This is made possible by controlling the ratio of the area of the middle diaphragm  56  to the cross-sectional area of the aperture  100 . By keeping this ratio relatively large, for example, substantially greater than 8/1, a modest air pressure may be used to control a much larger water pressure. Preferably, the ratio is on the order of 20/1 or greater and may range between 20/1 and 700/1 in practical applications. Other ranges of this area ratio, for example, from about 20/1 to about 100/1 or 20/1 to about 600/1 are also useful in practical sprinkler system designs. A preferred embodiment of the actuator uses a ratio of about 528/1. For the various ranges of ratios described above, the system air pressure is effectively independent of the system water pressure. Thus, regardless of the system water pressure (typically 100-120 psi) the system air pressure may be kept relatively low (preferably about 10 psig maximum), and the volume of air in the piping network  14  may be kept to a minimum. This results in less corrosion due to the presence of water vapor in the piping system. Furthermore, water traveling from the source to the sprinkler heads also will arrive sooner because there will be less air under lower pressure to displace out of the system. Third, the actuator acts as an AND gate in a logic circuit in that both the top diaphragm  54  and middle diaphragm  56  must deflect for actuation to occur. Inadvertent depressurization of the piping network, such as may occur if a sprinkler head is damaged, will not trip the system in error. Fourth, unintended resetting of the system, for example, during a fire, is prevented by the use of the reset valve  120 , which must be manually held in place until sufficient pressure is achieved to hold the valve closing member  128  seated. This is accomplished by the enlarged conduit region  136  which permits relatively large surges of compressed air to flow without closing the reset valve and shutting down the system. Fifth, the reset valve also eliminates the need for auxiliary means to accelerate system activation since it rapidly depressurizes the chamber portion with which it is associated upon opening. 
     In an alternate embodiment of the electro-pneumatic actuator  12  shown in FIG. 4, the actuator is modified to operate in a purely pneumatic mode in the event of a power failure. Solenoid actuator  111  is modified so that power supplied to the solenoid holds the valve closing member  113  in a closed position, sealing passageway  110  against the force of a biasing spring  140 . Under normal operation, if a true fire condition is present, the proper electric and pneumatic signals will be generated, the electric signal from the control system  44  causing the solenoid  111  to release the valve closing member  113 , the valve closing member moving under biasing spring  140  and opening passageway  110  (as shown in FIG. 4) and thereby relieving the pressure in the upper chamber portion  106 . The pneumatic signal from the piping network concurrently relieves the pressure in upper chamber portion  92 , thereby actuating the sprinkler system  10  as described above. 
     In the event of a power failure, however, the solenoid  111  releases the valve closing member  113  and spring  140  moves it to open passageway  110 , allowing fluid from upper chamber portion  106  to vent to ambient, relieving the pressure in the upper chamber portion  106 . Thus, no force is placed on the middle diaphragm  56  by the top diaphragm  54  during a power failure. This condition allows the actuator  12  to respond to a purely pneumatic signal through a pressure drop in piping system  14  transmitted to upper chamber portion  92  through conduit  42  and activate the fire protection sprinkler system. While it is true that the AND gate function of the actuator is lost during a power failure, the system fails safely in that it will still respond to suppress an actual fire condition.