Abstract:
A pressure control system for controlling the pressure of a process fluid stream at a certain location, comprising: a pressure regulator disposed in the process fluid stream, through which the process fluid stream flows; a first pilot controller adapted to sense the pressure of the process fluid at the said location and receive a control pressure from a control source; a second pilot controller adapted to receive same control pressure from the control source and provide added pressure; an inspirator adapted to receive the added pressure from the second pilot controller and generate a differential pressure; wherein the differential pressure is used to control the pressure of the process fluid stream within the pressure regulator. The pressure regulator comprises a first chamber and a second chamber therein separated by a flexible element and a divider is disposed in the first chamber and operative to abut against the flexible element to divide the first chamber into separate spaces. Preferably, the pressure regulator is disposed such that the flexible element extends substantially in vertical direction and the divider extends substantially in horizontal direction.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a pressure control system. More particularly, the present invention relates to a pressure control system for low pressure operation.  
         BACKGROUND OF THE INVENTION  
         [0002]    In various industrial applications, it is often desirable to accurately control the pressure of a process fluid stream at a certain point in an industrial system. The process fluid can be gas, liquid, or mixture thereof. For example, in a fuel cell system or a fuel cell testing system, it is necessary to operate the fuel cell under a controlled pressure condition. A common technique is to control the pressure of a process fluid stream adjacent the inlet of the fuel cell for that process fluid stream. To this end, a pressure regulator is often utilized to control the pressure at this point.  
           [0003]    A known pressure control system comprises an unloading type flexible element pressure regulator and a piloting system. The pressure regulator is disposed in the process fluid stream line and in operation, allows the process fluid to flow through a chamber thereof disposed on one side of the flexible element, e.g. a diaphragm. A pilot controller senses the pressure at the point where the pressure of the process fluid stream is to be accurately controlled, and correspondingly controls the pressure of another chamber on the other side of the flexible element of the pressure regulator, thereby eventually balancing the pressure on both sides of the flexible element. By manually adjusting the preset pressure of the pilot controller, the pressure of the process fluid stream at the desired point can be controlled. Such pressure regulator and the piloting system and the way they are operated are commercially available from, for example, Mooney Controls.  
           [0004]    However, it has been found that this type of pressure control system cannot meet the increasingly strict requirement in terms of accuracy and stability, when a system operates at low pressure, e.g. less than 7 psig, partly because that the pressure regulator needs a certain pressure drop across it to activate the pressure balancing mechanism and such pressure drop is often not available when pressure of the process fluid stream is extremely low.  
           [0005]    Therefore, there remains a need for a pressure control system that accurately controls the pressure of a process fluid stream at a certain point, under a range of pressure conditions including very low pressure of the process fluid stream.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, there is provided a pressure control system for controlling the pressure of a process fluid stream at a certain location, comprising: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0007]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show a preferred embodiment of the present invention and in which:  
         [0008]    [0008]FIG. 1 is a schematic view of a known pressure control system;  
         [0009]    [0009]FIG. 2 is a schematic view of an embodiment of a pressure control system according to the present invention;  
         [0010]    [0010]FIG. 3 is an enlarged portion of the schematic view of FIG. 2 showing a regulator in greater detail;  
         [0011]    [0011]FIG. 4 is an enlarged portion of the schematic view of FIG. 2 showing a inspirator in greater detail;  
         [0012]    [0012]FIG. 5 is an enlarged portion of the schematic view of FIG. 2 showing a primary pilot in greater detail;  
         [0013]    [0013]FIG. 6 is an enlarged portion of the schematic view of FIG.  2  showing a secondary pilot in greater detail;  
         [0014]    [0014]FIG. 7 shows an end view of a portion of the regulator of FIG. 2; and  
         [0015]    [0015]FIG. 8 shows test results of pressure at a fuel cell stack inlet with various process fluid flows using the pressure control system of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    A known pressure control system is shown generally at  10  in FIG. 1. The pressure control system  10  comprises an unloading type flexible element pressure regulator  20  and a piloting system. The pressure regulator  20  is disposed in the process fluid stream line  40  and in operation, allows the process fluid to flow through a chamber  24  thereof disposed on one side of the flexible element, e.g. a diaphragm  28 . A divider  22  is disposed in the chamber  24  to adjust the pressure therein. The divider  22  initially abuts against the flexible element  28  and hence prevents the process fluid from flowing through the chamber  24 . As the process fluid is continuously fed into the chamber  24 , pressure in the chamber  24  increases and the diaphragm  28  is lifted. This permits the flow of the process fluid through the chamber  24 . A pilot controller  30  senses, via a sense line  60 , the pressure at the point  50  where the pressure of the process fluid stream is to be accurately controlled, and correspondingly controls, via a control line  80 , the pressure of another chamber  26  on the other side of the flexible element  28  of the pressure regulator  20 , thereby eventually balancing the pressure on both sides of the flexible element  28 , at a desired pressure. A further line  32 , including an orifice or throttle  34  is connected to the pilot controller  30 , and an exhaust line  36  is also provided. By manually adjusting the preset pressure of the pilot controller  30 , the pressure of the process fluid stream at the desired point  50  can be controlled. As mentioned above, this system needs a pressure differential across the pressure regulator  20  to activate. It is inadequate to achieve accurate pressure control in low pressure conditions.  
         [0017]    In use, fluid is supplied through the line  32  and orifice  34  to the pilot controller  30 . When too low a pressure is sensed by the controller  30 , it is maintained in a closed position, so that flow from the line  32  is directed to the line  80  to maintain the pressure regulator  20  closed, thereby to increase the pressure in the line  40 . When sufficient pressure is present in the line  40 , this pressure is applied through the sense line  60  to the pilot controller  30  to open the controller against the action of the spring indicated therein. This permits at least part of the flow from line  32  to be exhausted through the exhaust line  36  to the outlet of the pressure regulator  20 . Consequently, the pressure on the diaphragm  28  is reduced, permitting the regulator  20  to open, thereby to reduce the pressure in the line  40 .  
         [0018]    A pressure control system according to the present invention is shown generally at  110  in FIG. 2. The pressure control system  110  has a pressure regulator  112 , an inspirator  114 , a primary (low pressure) pilot  116 , and a secondary (higher pressure) pilot  118 .  
         [0019]    The pressure regulator  112  is mounted in a process fluid line  102  to adjustably control the rate of flow of a fluid through the line  102 . More particularly, the regulator  112  has an inlet port  120   a  in fluid communication with an upstream line  102   a  of the process fluid line  102 , and an outlet port  120   b  in fluid communication with a downstream line  102   b  of the process fluid line  102 . As best seen in FIG. 3, the regulator  112  is further provided with a flow channel  122  extending between the inlet and outlet ports  102   a ,  102   b  and with a closure means  124  in the flow channel  122 . The closure means  124  is variably adjustable between a fully closed position, in which the ports  102   a  and  102   b  are fluidly isolated from each other, and a fully open position in which a maximum rate of flow is permitted through the flow channel  122 .  
         [0020]    In the embodiment illustrated, the closure means  124  comprises a sealing surface  126  attached to a flexible actuating diaphragm  128 . The diaphragm  128  serves as an actuator for advancing the sealing surface  126  towards, or retracting the sealing surface  126  away from, an engagement surface  130  provided on a divider element  132 . The divider element  132  can be in the form of a fixed wall extending into the flow channel  122 . When the sealing surface  126  abuts the engagement surface  130 , the closure means  124  is in the fully closed position and flow through the channel  122  is denied.  
         [0021]    The actuating diaphragm  128  can move in response to a pressure differential across the thickness of the diaphragm  128 . Accordingly, in the embodiment illustrated, one face  129   a  of the diaphragm  128  (i.e. the face with the sealing surface  126 ) partially defines the flow channel  122 , and the opposite face  129   b  of the diaphragm  128  partially defines a control chamber  136 . By controlling pressure in the control chamber  136  relative to that of the flow channel  122 , the position of the actuating diaphragm  128 , and hence of the closure means  124 , can be controlled, and moved between a fully lowered position  128   a  and a fully raised position  128   b  (shown in phantom in FIG. 3), corresponding to fully closed and fully open positions of the valve closure means  124 . In steady state operation, the pressures in the flow channel  122  and control chamber  136  are equal, and the diaphragm  128  is substantially stationary.  
         [0022]    According to the present invention, the pressure in the control chamber  136  is adjusted by forcing actuator fluid into, or evacuating actuator fluid from, the chamber  136  via a pressure control line  138  that extends between the chamber  136  and the inspirator  114  (FIG. 2). More particularly, the inspirator  114  has, in the form of a venturi, a converging inlet  140 , a diverging outlet  142 , and a throat  144  between the inlet  140  and outlet  142 . The pressure control line  138  extends between the control chamber  136  and the throat  144  of the inspirator  114  (FIG. 4).  
         [0023]    The inspirator  114  receives a flow of actuator fluid at its inlet  140  via an inspirator supply line  146 , and discharges a flow of actuator fluid from its outlet  142  via an inspirator vent line  148 . According to one embodiment of the present invention, the fluid in the supply line  146  is supplied at a relatively constant, higher than target, pressure from a fluid source independent of the process fluid in line  102 . The flow of fluid through the inspirator  114  is accelerated in the inspirator, so that a pressure drop is generated at the throat  144  of the inspirator. At a high rate of fluid flow across the inspirator  114  (e.g. unrestricted venting), the pressure drop can generate suction in the control chamber  136  to lift the diaphragm  128  and open the closure member  124 . On the other hand, if the venting is restricted, fluid entering the inlet  140  can be directed into the chamber  136 , thereby urging the diaphragm  128  downwards and closing the closure member  124 .  
         [0024]    With respect to the outlet  142  of the inspirator  114 , the outlet  142  is connected to the inspirator vent line  148  to provide fluid communication with the primary (low pressure) pilot  116  for controlled venting of the outlet  142 . More particularly, and as best seen in FIG. 5, the primary pilot  116  has a control portion  150  and a valve portion  152 , which are in fluid isolation from each other. The valve portion  152  has an inlet port  154 , an outlet port  156 , and a valve member  158  between the inlet port  154  and outlet port  156 . The inspirator vent line  148  connects the outlet  142  of the inspirator  114  to the inlet port  154  of the primary pilot  116 . The outlet port  156  is open to atmosphere.  
         [0025]    The valve member  158  can move among a fully closed position, a fully open position, and various degrees of partially open positions. In this way, the valve member  158  controls the flow of fluid from the inlet port  154  to the outlet port  156 , and hence, the valve portion  152  of the primary pilot  116  controls the flow of fluid from the outlet  142  of the inspirator  114  to atmosphere.  
         [0026]    The control portion  150  of the primary pilot  116  adjusts the position of the valve member  158  of the primary pilot  116 . The control portion  150  has a target pressure chamber  160  and a sensed pressure chamber  162  that are separated from each other by a primary pilot diaphragm  164 . The primary pilot diaphragm  164  has a target pressure face  165   a  exposed to the target pressure chamber  160 , and a sensed pressure face  165   b  exposed to the sensed pressure chamber  162 . A pressure adjustment spring  163  is housed within the target pressure chamber and exerts a force against the target pressure face  165   a  of the diaphragm  164 , as does any fluid pressure in the target pressure chamber  160 . By changing the relative pressures in the target and sensed pressure chambers  160 ,  162 , the diaphragm  164  is moved towards one of the chamber  160 ,  162  dependent on the pressure differential and the load set on the spring  163 , which can be adjustable. The valve member  158  is connected to the diaphragm  164  by, for example, a shaft  166  and a lever  168  pivotally mounted as shown, so that movement of the diaphragm  164  adjusts the position of the valve member  158 . More particularly, a downward movement of the diaphragm  164  (movement towards the sensed pressure chamber  162 ) closes the valve member  158 , and upward movement of the diaphragm  164  (towards the target pressure chamber  160 ) opens the valve member  158 . In the embodiment illustrated, the spring  163  is relatively soft and the surface area of the faces  165   a  and  165   b  of the diaphragm  164  are relatively large so that the diaphragm  164  has greater sensitivity to small pressure differentials across the diaphragm  164 .  
         [0027]    The target pressure chamber  160  of the primary pilot valve  116  is in fluid communication with a target (or control) pressure input  170 . More particularly, the control input  170  (FIG. 2) can be a pressurized supply of fluid regulated by, for example, but not limited to, an electronic pressure regulator, and connected to a target port  172  of the target pressure chamber  160  via a control line  174 .  
         [0028]    The sensed pressure chamber  162  of the primary pilot valve  116  is in fluid communication with a point  178  in the process fluid line  102  where the process pressure is to be controlled. In the embodiment illustrated, the point  178  where the system pressure is to be controlled is located in the upstream line  102   a  of the process fluid line  102 . In other embodiments, the point  178  can be located in other positions of the system, such as, for example, but not limited to, the downstream line  102   b  of the process fluid line  102 . A sense line  180  extends from a sense port  182  of the sensed pressure chamber  162  to an orifice in the line  102  at the point  178 . In this way, the pressure of the fluid stream in the line  102  at the point  178  is reflected by the pressure in the sensed pressure chamber  162 .  
         [0029]    In operation, a pressure drop in the sensed system pressure (i.e. in pressure chamber  162 ) may be caused by an increase in fluid flow through the regulator  112  (i.e. an increase in fluid consumption by the system). The higher pressure in the target pressure chamber  160  of the pilot  116  presses the diaphragm  164  towards the sensed pressure chamber  162 , which therefore moves the valve member  158  to a more closed position. This restricts the flow of fluid from the outlet  142  of the inspirator  114  to the atmosphere, which in turn increases the pressure in the pressure control line  138  and in the pressure control chamber  136 . As a result, the actuator diaphragm  128  advances towards the engagement surface  130  of the divider  132 , thereby restricting flow through the flow channel  122  of the regulator  112  and increasing the pressure at the sensing point  178 .  
         [0030]    An increase in the sensed system pressure (at point  178 , and in the sensed pressure chamber  162 ) may be caused by a decrease in fluid flow through the regulator  112  (i.e. a decrease in fluid consumption by the system). The higher pressure in the sensed pressure chamber  162  pushes the pilot diaphragm  164  towards the target pressure chamber  160 , so that the valve member  158  is moved to a more open position. This increases the flow of fluid from the outlet  142  of the inspirator  114 , which in turn reduces the pressure in the pressure control line  138  and the pressure control chamber  136  of the regulator  112 . As a result, the diaphragm  128  retracts away from the engagement surface  130  of the divider  132 , which increases flow through the channel  122  and decreases the pressure at the sensing point  178 .  
         [0031]    Accordingly, in response to a sensed pressure that is either higher or lower than the target pressure, the pressure control system  110  reacts to counteract the pressure deviation.  
         [0032]    Further details of the fluid supply to the inlet  140  of the inspirator  114  will now be provided. A supply of actuator fluid through the inspirator supply line  146  can be provided by an independent (or auxiliary) fluid supply  188  that is passed through the secondary (higher pressure) pilot  118 . The actuator fluid supply  188  can be a supply of any fluid that is isolated from the process fluid line  102 , and is preferably supplied at a pressure greater than the target pressure input  170 .  
         [0033]    As best seen in FIG. 6, the secondary pilot  118  has a control portion  190  and a valve portion  192 , which are fluidly isolated from each other. The valve portion  192  has an inlet port  194  and an outlet port  196 , and a valve member  198  between the inlet port  194  and outlet port  196 . An auxiliary supply line  199  connects the auxiliary fluid supply  188  to the inlet port  194  of the auxiliary pilot  118 . The outlet port  196  is connected to the inspirator supply line  146 .  
         [0034]    The valve member  198  can move among a fully closed position, a fully open position, and various degrees of partially open positions, so that the valve portion  192  of the secondary pilot  118  controls the flow of fluid from the auxiliary fluid supply  188  to the inlet  140  of the inspirator  114 .  
         [0035]    The control portion  190  of the secondary pilot  118  adjusts the position of the valve member  198  of the secondary pilot  118 . The control portion  190  has a target pressure chamber  200  and a sensed pressure chamber  202  that are separated from each other by a secondary pilot diaphragm  204 . The secondary pilot diaphragm  204  has a target pressure face  205   a  exposed to the target pressure chamber  200 , and a sensed pressure face  205   b  exposed to the sensed pressure chamber  202 . A pressure adjustment spring  203  is housed within the target pressure chamber  200  and exerts a force against the target pressure face  205   a  of the diaphragm  204 , as does any fluid pressure in the target pressure chamber  200 . The force of the spring  203  can be adjusted by turning an adjustment screw  207  to change the preload on the spring  203 . By changing the relative pressures in the target and sensed pressure chambers  200 ,  202 , the diaphragm  204  is moved towards the chamber  200 ,  202  with the reduced pressure. The valve member  198  can be connected to the diaphragm  204  by, for example, a shaft  206 , so that movement of the diaphragm  204  adjusts the portion of the valve member  198 . In the embodiment illustrated, the spring  203  is relatively stiff and the surface area of the faces  205   a  and  205   b  of the diaphragm  204  are relatively small (compared to those of the primary pilot  116 ).  
         [0036]    The target pressure chamber  200  of the secondary pilot valve  118  can be in fluid communication with the control pressure input  170 . More specifically, the control (or target) pressure input  170  can be in communication with the chamber  200  via a secondary pilot control line  210  connecting the control line  174  of the primary pilot  116  to a target port  212  of the chamber  200 . A stabilizing needle valve  213  can be provided in the secondary pilot control line.  
         [0037]    The outlet port  196  of the secondary pilot valve  118  is in fluid communication with the inspirator supply line  146 . A sense line  214  extends from a sense port  216  of the sensed pressure chamber  202  to an orifice in the inspirator supply line  146  between the outlet port  196  of the valve portion  192  of the secondary pilot  118  and the inlet  140  of the inspirator  114 .  
         [0038]    In operation, the auxiliary fluid supply  188  can be supplied to the pressure control system  110 , via the line  199 , at a pressure that is greater than the target pressure (which is set as desired by the target pressure input  170 ). Without any additional force exerted on the diaphragm  204  by the spring  203 , the fluid pressure to the outlet  196  of the secondary pilot  118  (and hence, the fluid pressure to the inlet  140  of the inspirator  114 ) is generally equal to the system target pressure. By adjusting the screw  207  to increase the force on the target face  205   a  of the diaphragm  204 , fluid at a higher pressure can be passed to the inspirator. Accordingly, a greater pressure difference can be generated across the diaphragm  128  of the regulator  112 .  
         [0039]    Referring again to FIG. 1, in known pressure control systems using regulators  20 , the regulator  20  is usually positioned so that the diaphragm  28  is facing vertically upwards, or in other words, the diaphragm  28  is generally horizontal having the chamber  26  above and the chamber  24  below. The divider  22  is oriented generally vertically, and a partial gap or flow channel is provided between the top of divider  22  and the bottom of diaphragm  28 . The inventors have observed that when a process fluid stream comprises a saturated gas stream, a problem can occur if liquid is condensed in the chamber  24  of the pressure regulator  20 . As the gas stream flows through the chamber  24 , liquid can condense and accumulate in the upstream side (left side in FIG. 1) of the divider  22  causing partial blockage of the flow passage which can in turn create a significant pressure change in the gas stream. This can result in instability of pressure control and is therefore undesirable.  
         [0040]    To overcome this problem, the pressure regulator  112  of the control system  110  can be rotated approximately 90 degrees around its axial direction, i.e. the direction of the process fluid stream therethrough (FIG. 7). As a result, during operation, the divider  132  is disposed generally in horizontal position, and the diaphragm  128  extends in a generally vertical direction, with the flow channel  122  on one side and the chamber  136  on the other side. In this configuration, condensed liquid can more easily flow through the flow channel  122  between the surfaces  126  and  130 . This can eliminate any significant pressure change due to condensation, so that the pressure of the process gas stream can be controlled constantly at a steady level.  
         [0041]    In one application of the pressure control system  110 , a fuel cell stack was placed between the pressure control point  178  in the line  102   a  and the inlet  120   a  of the pressure regulator. Test results of this example are provided hereinafter. It is to be appreciated, however, that the present system  110  can be used to control pressure of the process fluid stream at any arbitrary location along the process fluid stream line. It is also to be appreciated that the present invention can be applied in many industrial applications, where accurate control of the pressure of a process fluid stream is desirable, especially under low pressure conditions, including but not limited to a fuel cell system.  
       EXAMPLE  
       [0042]    In a test run, the control pressure at inlet  170  was set at about 20 psi. The auxiliary supply stream was provided at about 100 psi. The adjustment screw  207  was adjusted so that the total force exerted on the target pressure face  205   a  of the diaphragm  204  was equivalent to about 25 psi. A fluid consuming device in the form of a load cell stack was disposed in the upstream fluid line  102   a , between the control point and the regulator  112 .  
         [0043]    [0043]FIG. 4 shows test results when the present pressure control system in employed in a fuel cell system. As can be seen, although the process gas flow changes dramatically from near zero slpm to 200 slpm, the pressure of the process fluid at a certain point in the system is maintained at reasonably constant level.  
         [0044]    While the above description constitutes the preferred embodiments, it will be appreciated that the present invention is susceptible to modification and change without departing from the fair meaning of the proper scope of the accompanying claims.