Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates to pilot valves and, more particularly, to a guard for a pilot valve. 
     Automatic safety systems employing guards for pilot valves, which are also called pilot guards, are often used to control burners within fired equipment, such as to heat crude oil that has been collected in vessels in order to facilitate the separation of water droplets from the crude, which may be deployed in remote locations and be unattended (the word “control” as used herein simply means on-off accessibility to the fuel supply, i.e., access to the fuel supply is permitted in the “on” position and is precluded in the “off” position; whether fuel is actually directed to the main burner is determined by another valve, responsive to its own thermostat, interposed between this burner and the pilot guard). Such systems for both the pilot and main burner are required to avoid accumulation within the fired equipment of raw fuel discharged by unlit burners in volumes sufficient to be an explosive hazard. Because the collection vessels may be remotely located, a source of electrical power is often unavailable, or if available, is not reliable. To avoid reliance on electrical power in a control means, prior art pilot guards have utilized materials, such as mercury, which expand greatly when heated. Such arrangements are not desirable because the materials are often toxic, are susceptible to leakage, and since they have a relatively large mass from which heat must be dissipated after the removal of heat, do not react rapidly to failure of the flame being sensed. Many of these prior art devices that did use a thermocouple provided no means for emergency shutdown or means for testing the operation of the safety system. 
     U.S. Pat. No. 6,065,484 (“484”) discloses a burner and pilot guard safety and control system that provides a pilot guard having a stop shuttle normally biased to a seated position to completely block communication with a source of natural gas under pressure and a reset shuttle movable to a reset or start up position in which it unseats the stop shuttle while simultaneously permitting communication of the pilot burner with the gas source and blocking communications with the main burner. A reset latch is arranged to hold the reset shuttle in its reset position until released. A thermocouple capable of producing a voltage output proportional to its temperature is heated by the flame of the pilot burner and is connected to an electromagnet. The electromagnet, when fully energized, holds the stop shuttle in its unseated position. When the reset latch is released, the reset shuttle is then moved by the force of the gas pressure to an operational position in which both the pilot and main burners are in communication with the gas source. A momentary contact switch is arranged, when depressed to its closed position, to short circuit the thermocouple. When the thermocouple is short circuited, the holding force of the electromagnet immediately deteriorates and the stop shuttle is instantly biased to its seated position blocking all communication with the gas source. 
     The pilot guard that is disclosed in 484 works well in many applications. However, in some applications, the heat generated by the pilot flame is not adequate to energize the electromagnet sufficiently to allow it to hold the pilot guard assembly open after the pilot flame is lit. Consequently, in these applications, both the pilot guard assembly and the main burner valve assembly within the pilot guard will never “latch in” and will shut down upon release of the reset shuttle in the event of inadequate heat generated by the pilot flame. This condition of insufficient pilot flame heat could have several causes, including low BTU gas, excessive amounts of secondary air through the fire tube, low pilot pressure, and improper thermocouple alignment. Additionally, a pilot flame that is too hot could increase the time needed to de-energize the electromagnet and consequently shut off the pilot and burner gas upon occurrence of a flame-out to dangerous levels. 
     Therefore, there is a need for a pilot guard that overcomes the deficiencies of the prior art in handling the problems posed by the variable levels of heat produced by pilot flames. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pilot guard that is safer and more adaptable than prior art guards, and that has improved gas supply shut-off times in the event of a loss of pilot flame. The present invention can be adjusted in the field to accommodate a variety of levels of heat produced by pilot flames. In the preferred embodiment of the present invention described below, the adjustment is provided by a potentiometer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The following description of the preferred embodiment may be understood better if reference is made to the appended drawing, in which: 
     FIG. 1 is an elevational cross section of a pilot guard for use in an automatic safety control system according to the present invention; 
     FIG. 2 is a top plan view of the guard shown in FIG. 1 with portions thereof broken away for clarity; 
     FIG. 3 is a bottom plan view of the pilot guard shown in FIG. 1; 
     FIG. 4 is a top plan view of the pilot guard shown in FIG. 1, with the acorn nut removed from the potentiometer; 
     FIG. 5 is an enlarged view of the electromagnet housing and part of the thermocouple of the pilot guard shown in FIG. 1; and 
     FIG. 6 is a schematic diagram of the electrical components of the pilot guard shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 through 5, there is shown a pilot guard indicated generally at  10 , having a body  12 , which for ease of manufacture and assembly is composed of an electromagnet housing  14 , an inlet disc  16 , an output disc  18 , and a bottom cover disc  20  all of which are joined together as a unitary structure by screws  22 , shown in FIG. 2, that extend through aligned holes in the electromagnet housing  14  and the inlet disc  16  to engage tapped holes in the output disc  18 , and by screws  24 , shown in FIG. 3, that extend through holes in the bottom cover disc  20  to engage the same tapped holes. An O-ring seal  26  positioned in a peripheral groove in the inlet disc  16  contacts the inner diameter of the electromagnet housing  14  to prevent the escape of gas between the housing  14  and the inlet disc  16 . Another O-ring seal  28  is positioned between the output disc  18  and the inlet disc  16  to prevent the escape of gas between the adjacent surfaces of the inlet and output discs  16  and  18  respectively. The inlet disc  16  is provided with an inlet port  30  which is arranged in a conventional manner to connect with a natural gas supply line  32  through an in-line filter  33 , which may be any of the commercially available types, such as a sintered bronze filter, for example, for removal of water and solid contaminates that could otherwise interfere with the proper operation of the pilot guard  10 . The inlet port  30  communicates with a central cavity  34  formed in the inlet disc  16 . A central longitudinal bore  36  in the outlet disc  18  communicates with the cavity  34  and with a pilot port  38  and a main burner port  40 . A stop shuttle  42  extends through and reciprocates in a central bore  44  in the inlet disc  16 . The diameter of central bore  44  is slightly larger than the diameter of shuttle  42  to ensure that pressure in central bore  44  is always equal to that in bore  45  defined by housing  14 . The lower end  46  of the shuttle  42  is frusto-conically shaped for engagement with a counterbore  48  to assure alignment of the longitudinal axis of the shuttle  42  with the bore  36 . An O-ring  49  carried in a groove in the shuttle  42  is engageable with the intersection of the counterbore  48  with the upper surface of the output disc  18  to block communication between the cavity  34  and the bore  36 . A compression spring  50  trapped between the inlet disc  16  and a collar  52  on the shuttle  42  urges the O-ring  49  into sealing engagement with the outlet disc  18 . As the O-ring  49  is deformed by the force of the spring, i.e., takes a permanent set, the lower end  46  will simply travel downward further, so the sealing capability of the O-ring  49  is retained. 
     A reset shuttle  54  is reciprocal in the bore  36  but with sufficient clearance to permit an adequate flow of gas therebetween to provide the fuel requirement of both the pilot and main burners. An O-ring seal  56  is carried between lands  58  and  60  formed on the reset shuttle  54 , which lands  58  and  60  engage and reciprocate in a counterbore  62 . The engagement of the lower land  60  with the upper surface of the bottom cover disc  20  limits the downward travel of the reset shuttle  54 , in which position the seal  56  is below the main burner port  40  permitting communication of the bore  36  with the port  40 . An extension  64  is formed on the reset shuttle  54  and extends through a bore in the bottom cover disc  20 , the lower end of which protrudes to function as a reset button  66 . Pushing upward on the reset button  66  first causes the O-ring  56  to isolate the burner port  40  and then the upper face of the shuttle  54  to engage the end  46  to push the stop shuttle  42  upward, against the bias of the spring  50 , disengaging the O-ring  49  from its seat. Communication between the inlet port  30  and the pilot port  38  is thereby established. 
     A groove  68  is formed in the extension  64  and is engageable by the inner end  70  of a latch pin  72  which is reciprocably retained in a radial bore  79  in the bottom cover disc  20  by a bushing  74  that is screwed into a threaded counterbore  78  in the disc  20 , and that bears against a collar  80  formed on latch pin  72 . A compression spring  76  is trapped between the bottom of the bore  79  and collar  80  and urges the pin  72  toward the right, as viewed in FIG. 1, so that the inner end of the latch pin  72  clears the extension  64  and the opposite end thereof protrudes beyond the bushing  74  to function as a latch button  82 . The inner end  70  of the latch pin  72  has a frustroconical shape with the largest diameter at the extreme end thereof A complementary shape is provided to the upper surface of the groove  68  so that the force of the compression spring  50  will retain the inner end  70  of the latch pin  72  within the groove  68 , when upward manual force on the reset button  66  is released before the release of inward manual force on the latch button  82 , to hold the reset shuttle  54  in the raised position previously described, i.e., with the inlet port  30  in communication with the pilot port  38  but with the main burner port  40  isolated from the inlet port  30 . Gas is thereby permitted to flow to the pilot but not to the main burner. Subsequently manually pushing the reset button  66  upward, without any force being applied to latch button  82 , will permit compression spring  76  to release the end  70  from the groove  68 . Upon release of such upward manual force on the reset button  66 , the downward force of the gas pressure acting on the reset shuttle  54  will cause shuttle  54  to move downward until the land  60  engages the upper surface of bottom disc cover  20 . In this position of the reset shuttle  54 , the bore  36  is in communication with both ports  38  and  40 . Gas would thereafter be supplied to both ports  38  and  40  if, and only if, the stop shuttle  42  did not move downward under the force of the compression spring  50  so that the O-ring seal  49  precludes communication between the inlet port  30  and the bore  36 . 
     The stop shuttle  42  will move downward only if a horseshoe electromagnet  90  is not energized. A disc  92 , which is made of a magnetic material, is attached to the top of the stop shuttle  42  by a screw  94  extending through washer  96 . When electromagnet  90  is energized, the disc  92  will be held by magnetic attraction thereagainst, holding the stop shuttle  42  in its upward, open position against the bias of the spring  50 . The electromagnet  90  is energized by a thermocouple  100 , which is held by a suitable bracket  102  in a position to be heated by the flame of the pilot burner (not shown). Referring to FIGS. 2 and 5, lead wires  110  and  112  from the thermocouple  100  extend through a flexible sleeve  104  and terminate in a connector  106  which mates with a complementary socket  108  secured to the top of the electromagnet housing  14 . Wire  110  from thermocouple  100  connects with wire  116  leading from one terminal of socket  108 , and is connected to one terminal of the windings  119  of electromagnet  90 . Wire  112  is connected to wire  120  leading from the remaining terminal of socket  108 , and is connected to one terminal of a potentiometer  130 , which is mounted in any suitable fashion to the top of housing  14 . Potentiometer  130  can be a 100 ohm,  20  turn potentiometer manufactured by Spectrol, 4501 Greystone Drive, Ontario, Calif. 91761 as part no. 043P101 (and available from Mouser Electronics, Inc., 1000 North Main Street, Mansfield, Tex. 75063, 800-346-6873, as part no. 594-43P101). The resistance of potentiometer is decreased by turning it in the counterclockwise direction and increased by turning it in the clockwise direction. Wire  118  from the remaining terminal of windings  119  of electromagnet  90  is connected to the second terminal (or wiper) of potentiometer  130 , thus completing the series connection of the electromagnet  90 , potentiometer  130  and thermocouple  100 . FIG. 6 shows this arrangement schematically. Electromagnet  90 , potentiometer  130 , and connectors  116 ,  118  and  120  are potted within housing  14  as shown in FIG.  5 . 
     When thermocouple  100  is heated by the flame of the pilot burner, it will generate a voltage across wires  110  and  112 . Potentiometer  130  is adjusted to allow enough current to pass through windings  119  to cause electromagnet  90  to hold stop shuttle  42  in its upper position, in which natural gas is provided to both main burner port  40  and pilot burner port  38 . Referring to FIG. 4, potentiometer  130  is adjusted in well-known fashion by unthreading acorn nut  137  from post  133  of potentiometer  130  and inserting the blade of a screwdriver into slot  131  defined by post  133  of potentiometer  130 , and rotating the screwdriver to rotate post  133  about its longitudinal axis. If the resistance of potentiometer  130  is set too high, the current through windings  119  will be reduced to a level that is insufficient to allow electromagnet  90  to produce sufficient magnetic force to hold mating disk  92  against the opposing force of compression spring  50  in its upper position. Thus, O-ring  49  will prevent the flow of gas to both ports  38  and  40 , and neither the pilot burner nor the main burner will receive gas. Also, main burner port  40  also will not receive gas if the pilot flame is extinguished, the thermocouple  100  is not positioned properly in the pilot flame, or thermocouple  100  is defective. In all these cases, thermocouple  100  will not produce any significant voltage to the circuit shown in FIG. 6, and gas can never be supplied to main burner port  40 . 
     It has been discovered that using sintered metal oxide ferrite for the core of the horseshoe electromagnet  90  and sintered phosphorous iron (available, for example, as product number PSP-45 from Sintered Parts, LLC, of Tulsa Oklahoma) for the disc  92  will produce a magnetic force sufficient to hold the stop shuttle  42  against the bias of the spring  50  even at the low voltage and current output of a conventional thermocouple. A thermocouple producing an output voltage of 600 to 750 millivolts and a current of 100 milliamps has been found to reliably hold the stop shuttle  42  against a spring force of two pounds. 
     While the pull-in force, i.e., the magnetic force attracting the disc  92  toward the magnet  90  when an air gap exists between them, with the described arrangement is small, the holding force, i.e., the magnetic force generated when these two elements are in contact with each other, has been found to be quite large. The reason is that, when in contact, the disc  92  completes a magnetic circuit between the ends of the horseshoe magnet  90 , efficiently transferring the magnetic flux therebetween. Holding force, rather than pull-in force, is important since the disc  92  will be moved into contact with the ends of the electromagnet  90  by the manual upward movement of the reset button  66  to permit release of the latch pin  72 . The groove  68  is positioned so that when the end  70  of the latch pin  72  is in engagement therewith a small air gap exists between the disc  92  and the ends of the electromagnet  90 . Subsequent manual upward movement of the reset button  66 , which is necessary to release the latch pin  72 , will close this gap. 
     The pilot guard  10  with the thermocouple  100  properly positioned by attachment of the bracket  102  to be heated by the flame of a pilot burner, is placed in operation by initially introducing an ignition source adjacent the pilot burner. The reset button  66  is then depressed, i.e., manually moved upwardly, while simultaneously depressing the latch button  82 , i.e., manually urging the latch button  82  inward. When the operator feels the inner end  70  move into the groove  68 , the reset button  66  is released, while pressure on the latch pin is, at least momentarily, maintained. The engagement of the tapered end  70  with the upper surface of the groove  68  will retain the latch pin  72  in the groove  68  holding the reset shuttle  54  in an elevated position in which the stop shuttle  42  is unseated and only the pilot port  38  is provided with gas. The force of the gas pressure acting on the reset shuttle  54  and the force of the spring  50  will retain the end  70  within the groove. Once the thermocouple  100  is heated, the potentiometer  130  is adjusted such that the current supplied to electromagnet  90  is just above the latch-in current. Upon setting the potentiometer  130 , the reset button  66  is depressed again, with no force being applied to latch button  82 . The spring  76  will cause the latch pin to move outward extracting the end  70  from the groove  68  and allowing gas pressure to move the reset shuttle  54  downwardly connecting both the pilot port  38  and the main burner port  40  to be supplied with gas. Of course, this assumes the thermocouple  100  has produced sufficient voltage to energize the electromagnet  90  in order for magnetic force to hold the stop shuttle  42  in its unseated position. If the pilot flame has failed, or if the thermocouple is defective or improperly positioned, the spring  50  will immediately return the stop shuttle  42  to its seated position blocking all communication with the gas source. While the pull in, or latch in, and drop out currents for the guard will depend on the configuration of the guard and are readily ascertainable for those of ordinary skill in the art, latch in currents of 65 to 75 milliamps, and drop out currents of 40 to 45 milliamps are typical for common configurations. 
     A typical method of operating follows: 
     Clear the area of combustibles; 
     Close shut-off valves in the main burner line and pilot line, and wait for gas to vent from the system; 
     Decrease the effective resistance of potentiometer  130  by turning potentiometer  130  20 turns counterclockwise; 
     Stand to the side of the burner and light a torch; insert the torch into the fire tube next to the pilot burner; 
     Open the pilot shut off valve; 
     Depress reset button  66  until it and reset shuttle  54  latches to allow gas to flow to the pilot burner and ignite the pilot; 
     When thermocouple  100  comes up to temperature (usually 60 to 90 seconds after ignition), fully depress and then slowly release reset button  66 , at which point pilot guard  10  should latch in the open position, allowing gas to flow to both the pilot port  38  and main burner port  40 ; 
     Increase the resistance of potentiometer  130  by (a) slowly turning potentiometer  130  clockwise until pilot guard  10  drops out (disc  92  becomes disengaged from magnet  90 ), (b) turning potentiometer  130  counterclockwise 4 turns, (c) relight the pilot by following the preceding steps, except that the potentiometer is not turned 20 turns in the counterclockwise direction; 
     If unable to latch pilot guard  10  in the open position, turn potentiometer  130  1 more turn in the counterclockwise direction, and repeat this procedure until pilot guard  10  remains latched open, and then turn potentiometer  130  2 turns in the clockwise direction; 
     This procedure should result in a 12 to 20 second shut down time after loss of pilot flame. To obtain a shorter drop out time, slowly turn potentiometer  130  clockwise to find the maximum number of turns that can be made before pilot guard  10  drops out; 
     Slowly open the manual shut-off valve to the main burner line to light the main burner; 
     Test for proper operation by extinguishing the pilot flame and, with the manual shut-off valve to the pilot open, observing that gas pressure to the pilot and main burner control is shut off within 45 seconds; to shorten the drop out time, slowly turn potentiometer  130  further in the counterclockwise direction; use the foregoing procedure to relight the burner. 
     While a preferred embodiment of the present invention has been illustrated and described herein, it is to be understood that various changes may be made therein without departing from the spirit of the invention, as defined by the scope of the appended claims.

Technology Category: f