Extended range proportional valve method

A method for operating a proportional flow valve with a pilot valve member over an extended range and at a low flow rate. The pilot valve member may be operated, by varying the frequency or duty cycle or both of the pulse width modulated current applied to the solenoid, without thereby causing the main valve member to open.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention provides a method for operating a proportional flow valve at low flow rates and over a continuous range. In one embodiment, the valve is operated such the main valve member does not lift off the main valve seat. The valve is operated by providing either pulse width or frequency modulation, or both, of the current applied to the solenoid which operates the pilot valve. By providing a pulse width modulated voltage signal to the solenoid, and varying the duty cycle of the signal, the combination of duty cycle and frequency of the current causes the pilot valve member to open and close at a desired time-opened to time-closed ratio. The time-opened to time-closed ratio may be maintained below the value at which the main valve member opens, such that the fuel flow is only through the pilot valve member, and no fuel flows through the main valve member. In operation, such control of the current in the solenoid coil causes the pilot valve member to bounce up and down on the pilot valve seat, or oscillate, at a rate that allows a specific and controllable amount of fuel to flow through the pilot opening. Further, such fuel flow rate may be continuously controlled at rates between no fuel flow, when the pilot valve member does not disengage from the pilot valve seat, through the fuel flow rate which causes the main valve member to open. Such control may be effected by modifying either the pulse width or frequency modulation, or both, of the current applied to the solenoid which operates the pilot valve. In one embodiment, a proportional flow valve including a diaphragm pressure member is employed in the fuel valve subsystem of a natural gas operated microturbine system. The fuel valve subsystem includes a fuel valve manifold and four associated fuel valves, two of which are shut-off valves, and two of which are proportional flow control valves. Operated under the operating frequency conditions specified by the manufacturer, such proportional flow valves have a minimum level of controllable fuel flow of about 7 pounds per hour (pph) of natural gas. That is, on actuating the proportional flow valve, such valve would immediately open from no flow, or 0 pph, to about 7 pph. Similarly, on decreasing the fuel flow, the same valve would immediately close at a flow rate of about 7 pph. Thus fuel flow at a rate of between about 0 pph and about 7 pph could not be obtained either on initiating flow or decreasing flow. This results in undesired explosive turbine engine starts and stops, with associated undesirable noxious emissions. Such proportional flow valve including a diaphragm pressure member, operated at the manufacturer's specified frequencies and current, had a maximum flow rate, with the main valve fully open, of about 100 pph of natural gas. This thus provided a turn-down ratio of about 100 pph to about 7 pph, or about 14:1. By applying an appropriate pulse width or frequency modulation to the solenoid actuating the pilot valve member, it is possible to bounce or oscillate the pilot valve member on the pilot valve seat at a rate such that the main valve does not open. Thus the pressure above the diaphragm is such that the main valve member is held against the main valve seat, and does not open. Further, by modifying either the frequency or the pulse width, or both, the rate of opening or oscillation of the pilot valve member on the pilot valve seat is varied, thereby causing the flow rate through the pilot valve to vary. By this means the rate of opening or oscillation of pilot valve member can be controlled such that the flow rate through the pilot valve is from zero to that flow required to cause the main valve member to open. Further, this method does not affect the maximum flow rate that can be achieved with the main valve fully open. Utilizing a proportional flow valve including a diaphragm pressure member, operated using natural gas, a flow from less than 1 pph to about 7 pph was achieved, all at such flow rates as did not cause the main valve to open. Thus a turn-down ratio of about 100 pph to less than 1 pph, or more than about 100:1, can be achieved. In one embodiment, the method is used with a valve that includes a flexible diaphragm connected to the valve body and to a movable main valve unit. The diaphragm can be an inexpensive annular diaphragm mounted between the main valve member and the valve housing as a pressure member to counterbalance the force of the inlet fluid pressure on the main valve member. The valve includes a bleed passageway connecting the regions above and below the diaphragm, thereby permitting fluid entering the inlet port to occupy both regions. When the region above the diaphragm is pressurized by fluid from the inlet port of the valve it opposes the force of the inlet fluid pressure in the region below the diaphragm. The area of the main valve unit exposed to the region above the diaphragm has a larger effective area than the area of the main valve unit exposed to the region below the diaphragm, so that when the main valve unit is subjected to inlet fluid pressure, the main valve unit is urged toward the main valve seat to keep the valve closed. When the region above the main valve unit is open to the outlet port, the fluid in that region is permitted to escape faster than the bleed passageway can supply fluid, and the resulting pressure decrease in the region above the main valve unit causes a net force urging the main valve unit to move away from the main valve seat to open the valve, and keep it open. The valve includes a pilot valve with a pilot valve sealing member carried by the solenoid armature and a pilot valve seat fixed to and moveable with the main valve unit. The pilot valve controls the pressure of inlet fluid in the region above the main valve unit and the flow of inlet fluid to the outlet port. As a result, the position of the armature indirectly controls the position of the main valve unit. The bleed passageway continuously bleeds pressurized fluid from the inlet port to the reservoir above the main valve unit so as to keep the main valve unit sensitive to the control of inlet fluid pressure by the pilot valve. Utilizing such a valve with the method of this invention, a pulse width modulated voltage signal may be applied to the solenoid, which signal has a selectable maximum voltage and a selectably variable duty cycle. The signal induces a current in the solenoid, which current has an average value over time. By varying the duty cycle, and thereby varying the average value of the current, the pilot valve member is caused to open and close at a desired time-opened to time-closed ratio. The time-opened to time-closed ratio can be maintained below a value at which the main valve member opens, thereby permitting very low flow, below the minimum rate with the main valve member opened. Further, by varying the time-opened to time-closed ratio, the flow can be continuously adjusted from no flow to that flow rate which causes a pressure decrease in the region above the main valve unit, thereby causing a net force urging the main valve unit to move away from the main valve seat and to thereby open the main valve member. One pilot-operated proportional control valve with which this method may be used is disclosed in U.S. Pat. No. 5,538,026, entitle Pilot-Operated Proportional Control Valve, to S. A. Kazi (the '026 patent). In FIG. 1 , the pilot-operated proportional control valve of the '026 patent is shown at 10 , and is as described in the '026 patent. During operation, the housing inlet 42 is connected to a fluid supply and the housing outlet 48 is connected to a fluid-operated device. Fluid through the inlet 42 travels through a first flow path from radial entrance 58 , through radial inlet orifices 79 at the lower end of pilot valve member 74 , axially upward through pilot valve member 74 , and radially outward through radial outlet orifices 84 into pilot valve chamber 62 . When the solenoid is de-energized, axial opening 78 to bore 75 in pilot valve member 74 is closed because of pilot valve member 74 abutting the shank of screw 64 . Radial outlet orifices 90 along the length of pilot member 74 are out of alignment with radial bore 66 in main valve member 56 , and hence are also closed. The fluid in chamber 62 is therefore at inlet pressure and acts to bias main valve member 56 downwardly against valve seat 50 . When the solenoid is initially energized the armature 20 is caused to move upwardly against spring 94 , which pulls pilot valve member 74 upward. As pilot valve member 74 moves upwardly, it moves away from screw 64 , thus providing greater fluid flow through axial inlet opening orifice 78 into central bore 75 . At the same time, radial outlet orifices 90 along the length of pilot valve member 74 become aligned with radial bore 66 in main valve member 56 , such that a second flow path is provided through longitudinal bore 68 to housing outlet 48 . When orifices 90 are opened, the pressure within pilot valve chamber 62 and central bore 75 is reduced. As main valve member 56 moves away from valve seat 50 , fluid flows through a third flow path from housing inlet 42 directly to housing outlet 48 through the radial passages 70 in the main valve member 56 . Spring 95 provides that main valve member 56 moves smoothly and evenly in proportion to the movement of pilot valve member 74 , and hence in proportion to the input signal amplitude or current on the solenoid. As the current or voltage through the solenoid is increased, the main valve member 56 moves further upwardly and increases the flow to housing outlet 48 . This continues until full voltage or current is reached and the housing outlet pressure approaches the housing inlet pressure and full flow is seen at housing outlet 48 . Using the method of this invention, an appropriate pulse width or frequency modulation may be applied to the solenoid coils of the '026 patent and all similar valves, such that the armature 20 is moved to open the pilot valve member, but not to such extent that the pressure within the pilot valve chamber 62 is reduced such that the main valve member 56 is caused to open. By applying such appropriate pulse width or frequency modulation, the pilot valve member 74 will oscillate or bounce on the screw 64 , thereby rapidly cycling on and off, and controlling flow at a lower level, and over a range, heretofore not possible. Yet another type of pilot-operated proportional control valve with which this method may be used is disclosed in U.S. Pat. No. 5,716,038, entitle Proportional Flow Control Valve, to M. F. Scarffe (the '038 patent). In this valve a diaphragm is provided. In FIG. 2 , the valve 100 of the '038 patent is shown, and is as disclosed therein. As the armature 101 begins to lift, the valve pad 103 connected thereto is lifted from the end of the pilot tube 104 and liquid in the pilot chamber 120 flows through the tube 104 in the main valve member 102 and to the outlet port 108 . This flow reduces the pressure in the pilot chamber 120 as compared to the pressure on the other side of the diaphragm 118 , and hence the diaphragm 118 begins to lift away from the main valve seat 112 and in doing so lifts the main valve member 102 . Flow of liquid from the inlet port 106 to the outlet port 108 through the main valve seat 112 therefore commences. However, the pilot chamber 120 remains in communication with the liquid inlet 106 via the small hole in the diaphragm 118 . Using the method of this invention, an appropriate pulse width or frequency modulation may be applied to the solenoid coils of the '038 patent and all similar valves, such that the armature 101 is moved to open the valve pad member 103 , which functions as a pilot valve member, from the end of the pilot tube 104 , but not to such as extend that the pressure within the pilot chamber 120 is reduced such that the main valve member 102 is caused to open. By applying such appropriate pulse width or frequency modulation, the valve pad member 103 will oscillate or bounce on its seat 104 , thereby rapidly cycling on and off, and controlling flow at a lower level, and over a range, heretofore not possible. The pulse width or frequency modulation on the solenoid to initially open the pilot valve member, and then the further pulse width or frequency modulation range over which the pilot valve member can be operated without also opening the main valve member, can vary depending upon the spring constant on springs associated with the pilot valve member and springs associated with the main valve member, or the equivalents thereof, the size of the armature, if provided, the length and number of coils on the solenoid, and other factors as should be known to those skilled in the art. Such modifications and variations can be made to a valve, in order to optimize the application of the method of this invention to the valve. Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.