BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a pressure relief valve and in particular to a relief valve for regulating positive end expiratory pressure (PEEP) levels within a patient breathing circuit of a mechanical breathing assist device.  
           [0003]    2. Description of the Prior Art  
           [0004]    A pressure relief valve is a valve which exhibits a threshold resistance such that no fluid can flow through the valve until the fluid pressure equals a predetermined threshold value. At this time an opening force exerted by the fluid pressure on a valve element equals a counteracting threshold closing force exerted by an actuator on the valve element. Above the threshold pressure it is important that the valve should allow high rates of fluid flow without any significant increase in pressure drop across the valve. This is because such a valve is typically employed to open under conditions critical to safety where transfer of fluid pressure other than through the valve could have serious consequences. This is particularly true when the pressure relief valve is used as a safety valve within a patient breathing circuit. A pressure relief valve may also can be employed within such a breathing circuit as a PEEP valve to control the positive end expiratory pressure and typically should be able to permit the flow through of expiration gas at flow rates of up to 200 to 300 liters per minute without a significant increase in the pressure drop.  
           [0005]    A pressure relief valve, such as a PEEP valve, typically has an aperture for the flow through of a fluid toward which a moveably mounted valve element is urged with a predetermined threshold closing force generated by a biasing means. The valve element is a arranged with a sealing surface which co-operates with the aperture to regulate the flow through of fluid and which can seal against the aperture with the predetermined closing force. The sealing surface is disposed, for pressure communication with the fluid, to be movable in the direction of the fluid flow to unseal the aperture when an opening force exerted by the communicated fluid pressure exceeds the counter-acting closing force.  
           [0006]    In such a known pressure relief valve, all of the predetermined closing force is generated by the biasing means, therefore relatively large and powerful biasing means are required. Such biasing means are often space consuming and if formed by a solenoid, often consume a significant amount of energy as well. Moreover, such biasing means are relatively expensive to produce.  
           [0007]    It is known from U.S. Pat. No. 6,082,705 to provide a flow regulation valve in which fluid, the flow of which is to be regulated, is used to seal the valve in the absence of an opening force provided by a biasing element. The valve has a valve housing with a fluid inlet and a fluid outlet between which is disposed a valve face. The valve face has a valve aperture adjacent to which is a movable sealing element which is movable by an opening force exerted by the fluid in the direction of fluid flow to unseal the aperture. A biasing element is provided to control the position of the sealing element and to regulate the flow of fluid through the valve. A duct is provided to divert a portion of the fluid to act on a side of the sealing element facing away from the aperture and thus provide a closing force on the element counter to and at least as large as the opening force exerted by the fluid. In the absence of an opening force from the biasing element this will cause the element to move to seal the aperture.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to provide a pressure relief valve in which the use of a less powerful biasing element can be employed without reducing the threshold pressure of fluid required to open the valve.  
           [0009]    The above object is achieved in accordance with the principles of the present invention in a fluid pressure relief valve having an aperture through which a fluid flows, a movably mounted valve element disposed for pressure communication with the fluid and being movable to unseal the aperture when an opening force exerted by the fluid pressure on the valve element exceeds a counteracting threshold force, a biasing arrangement which provides the aforementioned bias force, in a direction for moving the valve element toward sealing the aperture, and wherein the pressure communication between the valve element and the fluid is such that a closing force, less than and counter to the opening force, is additionally exerted on the valve element by the fluid pressure, with this additional force and the bias force generating the threshold force.  
           [0010]    Arranging for the valve element to be in pressure communication with the fluid causes both an opening force and a smaller closing force are produced on the valve element by the fluid pressure. This means that the biasing element need only be powerful enough to contribute a bias force which is the difference between the closing force generated by the fluid pressure and a predetermined threshold force which must be matched by the opening force generated by the fluid pressure as it reaches a predetermined threshold pressure before fluid pressure is relieved by the valve.  
           [0011]    The biasing element can generate a variable bias force. The valve element can have a first surface and a smaller second surface over which surfaces the fluid pressure acts and which are mechanically connected and disposed to define opposing end wall portions of a fluid receiving chamber. This chamber contains the aperture for the flow through of the fluid which is sealable by the first surface when moved against the direction of flow through of the fluid. This achieves a relatively simple valve construction. Moreover, by using the larger surface to seal the aperture then the aperture can be dimensioned to provide sufficient flow through of fluid without a significant increase in the size of the valve.  
           [0012]    Preferably, the valve is adapted to receive expiration gas from within an expiration line of a patient breathing circuit and to provide a PEEP level within the breathing circuit which may be made adjustable by providing a bias element capable of providing a variable bias force. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic illustration of an embodiment of the pressure relief valve according to the present invention.  
         [0014]    [0014]FIG. 2 is a schematic illustration of a further embodiment of the pressure relief valve according to the present invention.  
         [0015]    [0015]FIG. 3 is a schematic illustration of a patient breathing circuit according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    As shown in FIG. 1, an embodiment of a pressure relief valve  2  according to the present invention is illustrated partly in section. The valve  2  has a valve housing  4  having integrated with a solenoid housing  6  in which is housed a solenoid  8 . The valve housing  4  encloses a fluid receiving chamber  10  which communicates with the exterior of the valve housing  4  via an inlet  12  through which fluid, such as an inspiration or an expiration gas, can be received into the chamber  10 . The chamber  10  has opposing wall sections defined by a movable first plate  14  and a movable second plate  16  which are mechanically connected by a rigid member  18  so that they are movable in concert. The first plate  14  has a first surface  14 ′ exposed to fluid pressure within the chamber  10  and the second plate  16  has a second surface  16 ′ which is also exposed to the same fluid pressure. The area A 1  of the first surface  14 ′ is greater than the area A 2  of the second surface  16 ′. The first plate  14  is movable to seal (solid lines in FIG. 1) and unseal (broken lines in FIG. 1) against a sealing ring  20  located around a periphery of an aperture  22  in the chamber  10 . The aperture  22  is connected to an outlet  24  through which fluid can exit the valve  2 . The second plate  16  forms a rigid end wall of a collapsible wall section  26 , the opposite end of which is sealed to a rigid wall section  28  of the chamber  10 . The collapsible wall section  26  is arranged such that it is in an expanded state (as shown in FIG. 1) when the first plate  14  seals against the sealing ring  20  and is in a collapsed state when the plate  14  moves to unseal the aperture  22 . A magnetically susceptible shaft  30  has an end  32  connected to the plate  14  and acts as a movable core of the solenoid  8  such that by passing a current through the solenoid  8  via leads  34  the shaft  30  can urge the plate  14  toward the aperture  20  with a bias force Fb, determined by the magnitude of the current flowing through the leads  34 . Thus by varying the current through the leads  34  the bias force Fb, can be varied. A sealing ring  36  is provided to prevent any leakage of fluid from the outlet  24  to the solenoid housing  6 .  
         [0017]    In use fluid at a pressure P enters the chamber  10  and produces a force, F 1  on the first surface  14 ′ of the first plate  14  having an area A 1  over which the pressure P acts. The force, F 1 , is calculated as:  
           F 1 =P·A 1  1)  
         [0018]    This force F 1  acts on the first plate  14  to provide an opening force tending to unseal the aperture  22 .  
         [0019]    Simultaneously, the fluid pressure P acts on the smaller surface  16 ′ of the second plate  16  which has an area A 2 , less than A 1 , to produce a force F 2  on the plate  16 . The force F 2  is calculated as:  
           F 2 =P·A 2  (2)  
         [0020]    The force F 2 , because of the rigid connecting member  18 , acts on the second plate  16  to provide a closing force on the first plate  14 .  
         [0021]    The closing force F 2 , and the bias force Fb, combine to generate a threshold force Ft which the opening force F 1  must reach before the plate  14  can move under the influence of a threshold fluid pressure to unseal the aperture  22  and relieve the fluid pressure. The threshold force Ft is then:  
           Ft=F 2+ Fb=F 1  (3)  
         [0022]    Thus, if the valve of FIG. 1 is used as a PEEP valve so that pressures above a selected PEEP pressure P(PEEP) cause the plate  14  to unseal the aperture  22  then the valve  2  must be devised such that from equations (1) to (3):  
           Fb= ( A 1− A 2)· P ( PEEP )  (4)  
         [0023]    Since it is reasonable to expect the areas A 1 ,A 2  of the plates  14 , 16  are known or readily obtainable and fixed, then equation (4) can be used to calculate the necessary bias force Fb which the solenoid  8  must generate. This is less than the force which the solenoid  8  of known valves must generate since a part of the threshold force Ft in the valve  2  of the present invention is generated by the fluid pressure itself, acting on the smaller plate  16 .  
         [0024]    Considering now FIG. 2, a further embodiment of a pressure relief valve  38  according to the present invention is illustrated partly in section. The valve  38  has a valve housing  40  integrated with a spring housing  42  in which is housed a bias spring means  44 . The valve housing  40  encloses a fluid duct which is connected an externally accessible inlet  48  to an externally accessible outlet  50 . An aperture  52  for the flow through of fluid is disposed within the duct  46  between the inlet  48  and the outlet  50  and is sealable by a first movable plate  54  which is located directly in front of the outlet aperture  52  in the fluid flow direction through the valve  38 , from inlet  48  to outlet  50  and is movable to seal and unseal the aperture  54 . The first plate  54  is mechanically connected to a second movable plate  56  via a rigid rod  58  so that the two plates  54 , 56  can only move in concert. As in FIG. 1 the first plate  54  has a first surface  54 ′ and the second plate  56  has a smaller second surface  56 ′ over which the fluid pressure acts. The second plate  56  is located within the spring housing  42  and is operably connected to the spring  44  such that a bias force Fb′, which is generated by the compression of the spring  44 , is transmitted via the second plate  56  and the rod  58  to the first plate  54  to urge the plate  54  toward sealing the aperture  52 . Also located within the spring housing  42  is a variable volume container  60  which is sealed at one end to the second plate  56  so as to vary its volume as the second plate  56  moves and encloses the spring  44 . The interior of the container  60  is in fluid communication with the fluid duct  46  by a bypass duct  62 . In this manner the second, smaller plate  56  is placed in pressure communication with fluid in the fluid duct  46 , the pressure of which is to be regulated by the valve  38 , only on one side of the plate  56 .  
         [0025]    The second surface  56 ′ of the plate  56  has an area A 2 ′ and is urged upwardly when exposed to a fluid at a pressure P′ with a closing force F 2 ′, as given by substitution into equation (2). The same fluid pressure within the fluid duct  46  generates an opening force F 1 ′ on the first plate  54  whose first surface  54 ′, has an area A 1 ′ at a level given by substitution into equation (1). If the valve  38  is to function as a PEEP valve operating at a pressure P′(PEEP) then the bias force Fb′ necessarily provided by the spring  44  is, from equation (4):  
           Fb′= ( A 1′− A 2′)· P′ ( PEEP )  (5)  
         [0026]    The required bias force Fb′ can be obtained by varying the compression of the spring  44  by rotating a threaded knob  64  attached to an end of the spring  44  either manually or automatically.  
         [0027]    It will be appreciated by those skilled in the art that other known biasing arrangements can be substituted for those  8 , 44  described in FIG. 1 and FIG. 2 and that the described biasing arrangements  8 , 44  can be substituted for one another without departing from the invention. Indeed, it may be preferable for safety reasons to substitute the solenoid  8  of the valve  2  of FIG. 1 for a spring bias if the valve is used as a safety pressure release valve as a break in supply to the solenoid  8  may cause the valve to malfunction under safety critical conditions. Moreover, it will be further appreciated that the valve according to the present invention can be simply modified without departing from the invention so that as an alternative the second, smaller, movable plate  16 ,  56  is movable to seal and unseal a through flow aperture.  
         [0028]    [0028]FIG. 3 shows an embodiment of a patient breathing circuit  66  according to the present invention. The illustrated breathing circuit  66  represents a circuit as is generally known in the art which includes a known PEEP valve. An inventive difference of the breathing circuit  66  of FIG. 3 is that the known PEEP valve is substituted for a PEEP valve according to the present invention, for example a valve  2 , 38  according to the above described exemplary embodiments. For this reason the breathing circuit  66  need not be described in great detail.  
         [0029]    The breathing circuit  66  has a ventilator unit  68  to which ends of an inspiration gas line  70  and an expiration gas line  72  are connected for the transportation of gas respectively to and from an open end  74  of a patient tube  76 , which open end  74  is intended in use to connect to the airways of a patient (not shown). One-way valves  78 ,  80  are respectively connected in-line in an inspiration line  70  and an expiration line  72  to ensure gas flows within the inspiration line  70  only in a direction toward the open end  74  and in the expiration line  72  only in a direction from the open end  74 . A PEEP valve  2  according to the present invention is arranged in-line with the expiration line  72  with its inlet  12  connected to the expiration line  72  between the one way valve  80  and the ventilator  68  and its outlet  24  is connected via the expiration line  72  to the ventilator  68 . The gas pressure within the expiration line  72  which will cause the valve  2  to open is controlled by current supplied by a control unit  84  (which may be incorporated within the ventilator  68 ) along leads  34  and is set so that the aperture  22  will remain unsealed until the pressure of the expiration gas falls to the PEEP level. A second relief valve  38  is incorporated in the inspiration line  70  and is arranged to act as a pressure release safety valve which operates to ensure that the pressure of inspiration gas supplied to the open end  74  does not exceed a predetermined safe maximum which may be set by adjusting the knob  64  (see FIG. 2) of the valve  38  to vary the extension of the spring bias  44  (see FIG. 2). To this end, and different to the PEEP valve  2  connection with the expiration line  72 , only the inlet  48  of the valve  38  is connected (such as via a T-piece  86 ) to the inspiration line  70  with the outlet  50  venting to atmosphere (or a recovery system—not shown). In this configuration the aperture  52  will remain sealed until the inspiration gas pressure within the valve  38  exceeds a threshold set at the safe maximum value. Until this threshold is exceeded the inspiration gas continues to flow from the ventilator  68 , along the inspiration line  70  and to the open end  74  of the patient tube  76 .  
         [0030]    It will be appreciated that the identical valves  2  or  38  may be used in the inspiration line  70  and the expiration line  72  of the breathing circuit  66  or that the valves  2  and  38  may be interchanged to provide the functionality described above with respect to FIG. 3 without departing from the invention. Additionally or alternatively a valve  2 ,  38  according to the present invention may be included in-line in the inspiration line  70  and arranged in a manner similar to that shown for valve  2  in the expiration line  72  of FIG. 3 to act as a lower pressure delimiter for gas supplied from the ventilator unit  68 .  
         [0031]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.
A fluid pressure relief valve has an aperture for the flow through of a fluid, the aperture being disposed between a fluid inlet and a fluid outlet. A movably mounted valve element is disposed for pressure communication with the fluid and is movable to unseal the aperture when an opening force exerted by the communicated fluid pressure on the valve element exceeds a counteracting threshold force. A biasing source provides a bias force for moving the valve element toward sealing the aperture. The valve element is disposed for pressure communication with the fluid such that a closing force less than and counter to the opening force is additionally exerted on the valve element by the communicated fluid pressure. This force in combination with the bias force generates the threshold force.