US3619724A

US3619724A - Fuel burner safety control circuit

Info

Publication number: US3619724A
Application number: US45369A
Authority: US
Inventors: Thomas W Brown; Michael A Delin; Robert D Scoville
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1970-06-11
Filing date: 1970-06-11
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09
Also published as: ES197257Y; GB1345021A; ZA713755B; ES197257U; JPS5323540B1; CA934466A

Abstract

A fuel burner safety control circuit for burner safety shutoff valves functioning in different operational modes depending upon the occurrence of an interruption in power to the circuit or an unsafe operating condition in the overall system so as to be capable of distinguishing between a momentary power interruption where the burner should not be shut down and an emergency condition where the burner should be shut down substantially immediately. The safety control circuit has a deenergize-to-trip burner safety shutoff valve with an integral time delay to maintain burner safety shutoff valve energization during momentary power interruption, and a bypass arrangement permitting the time delay to be bypassed during emergency trip conditions to provide substantially immediate burner safety shutoff valve deenergization for closing down the burner system. This capability of distinguishing between momentary power interruptions and emergency trip conditions is accomplished by using a circuit having parallel time delay relays to activate switches set in series in a time delay circuit. The time delay relays function in opposite energization senses (i.e., energize to trip and deenergize to trip) in activating the switch with which it is associated, one switch being normally closed and the other closed on application of power to the safety control circuit. On a power interruption, both relays would assume a deenergized state maintaining the switches closed for a set period of time to continue the integrity of the time delay circuit to prevent trip of the deenergize-to-trip burner safety shutoff valve. This is accomplished by the deenergize-to-trip relay holding in the powered closed switch for a discrete period of time and the energize-to-trip relay (being deenergized) having no effect on the normally closed switch. In an emergency unsafe operating condition the deenergize-to-trip relay would again be deenergized to maintain the powered closed switch closed, but the energize-to-trip relay would assume an energized state substantially immediately opening the normally closed switch to trip the deenergize-to-trip burner safety shut off valve for closing down the burner system.

Description

United States Patent [72] Inventors Thomas W. Brown Vernon; Michael A. Delin, West Simsbury; Robert D. Scoville, Torrington, all of Conn. [21] Appl. No. 45,369 [22] Filed June 11, 1970 [45] Patented Nov. 9, 1971 [73] Assignee Combustion Engineering, Inc.

Windsor, Conn.

[54] FUEL BURNER SAFETY CONTROL CIRCUIT 7 Claims, 4 Drawing Figs.

[52] U.S.Cl 317/36 TI), 251/129, 307/116,307/117 [51] Int. Cl. 1102b 3/24 [50] Field of Search 317/36,40; 307/1 16, 117, 118,236/1 A, I H; 431/66, 69, 18; 137/94; 251/129 [56] References Cited UNITED STATES PATENTS 3,254,871 6/1966 Limon 431/18 X 3,275,058 9/1966 Gage 431/69 X Primary ExaminerJ. D. Miller Assistant ExaminerHarvey Fendelman Attorneys-Carlton F. Bryant, Eldon H. Luther, Robert L. Olson, John F. Carney, Richard H. Bemeike, Edward L. Kochey, Jr. and Lawrence P. Kessler ABSTRACT: A fuel burner safety control circuit for burner safety shutoff valves functioning in different operational modes depending upon the occurrence of an interruption in power to the circuit or an unsafe operating condition in the overall system so as to be capable of distinguishing between a momentary power interruption where the burner should not be shut down and an emergency condition where the burner should be shut down substantially immediately. The safety control circuit has a deenergize-to-trip burner safety shutoff valve with an integral time delay to maintain burner safety shutoff valve energization during momentary power interruption, and a bypaw arrangement permitting the time delay to be bypassed during emergency trip conditions to provide substantially immediate burner safety shutoff valve deenergization for closing down the burner system. This capability of distinguishing between momentary power interruptions and emergency trip conditions is accomplished by using a circuit having parallel time delay relays to activate switches set in series in a time delay circuit. The time delay relays function in opposite energization senses (i.e., energize to trip and deener' gize to trip) in activating the switch with which it is associated, one switch being normally closed and the other closed on application of power to the safety control circuit. On a power interruption, both relays would assume a deenergized state maintaining the switches closed for a set period of time to continue the integrity of the time delay circuit to prevent trip of the deenergize-to-trip burner safety shutoff valve. This is accomplished by the deenergize-to-trip relay holding in the powered closed switch for a discrete period of time and the energize-to-trip relay (being deenergized) having no efiect on the normally closed switch. In an emergency unsafe operating condition the deenergize-to-trip relay would again be deenergized to maintain the powered closed switch closed, but the energize-to-trip relay would assume an energized state substantially immediately opening the normally closed switch to trip the deenergize-to-trip burner safety shut ofi valve for closing down the burner system.

FUEL BURNER SAFETY CONTROL CIRCUIT BACKGROUND OF THE INVENTION The complexity of the modern power generating facility requires that automatic systems be employed for control and safety purposes. Such automatic systems are necessary to assure that combustion rate is accurately matched to power demand for maximum plant efiiciency and that certain operating conditions are not exceeded for overall system protection. Automatic limit controls for safety purposes can, in and of themselves, however, create safety problems.

This is particularly true with regard to burner arrangements wherein shutdown and reignition presents a potentially explosive condition due to unburned fuel accumulation in the fur nace. It is, therefore, necessary to provide an automatic burner control arrangement which is capable of distinguishing between an emergency condition wherein substantially immediate system shutdown is necessary and a transient condition which appears to exceed safe operating limits but wherein delayed action for a specific period may be tolerated to permit normal conditions to be reestablished. One such transient condition which gives a false signal of shutdown necessity may occur when electrical power input to the burner control system is switched from one power bus to another. Presently used methods to prevent shutdown of burner control systems due to transients include using continuous DC, using an inverter with zero transfer time, using a capacitor bank, or employing energize-to-trip systems (no trip on power loss possi ble).

SUMMARY OF THE INVENTION It is the purpose of this invention to accomplish control of a fuel burner system which is capable of distinguishing between certain transient conditions, such as the above-noted momentary power interruptions, which give a false indication of the necessity for system shutdown and true emergency conditions, and operate in a more efficient and economical manner than that of the above-noted methods. There is herein provided a safety control circuit which relies upon a deenergize-to-trip burner safety shut off valve with an integral time delay to maintain valve energization during a momentary power interruption to prevent burner shutdown. The safety control circuit has a bypass feature which permits the time delay to be bypassed during emergency trip conditions to provide substantially immediate burner safety shut off valve deenergization and resulting burner system shutdown. The capability of distinguishing between momentary power interruptions and emergency trip conditions is accomplished by using a circuit having parallel time delay relays to activate switches set in series in a time delay circuit. The time delay relays function in opposite energiration senses (i.e., energize to trip and deenergize to trip) in activating the switch with which it is associated, one switch being normally closed and the other closed on application of power to the safety control circuit. On a power interruption both relays would assume a deenergized state maintaining the switches closed for a set period of time to continue the integrity of the time delay circuit to prevent trip of the deenergize-to-trip burner safety shut off valve. This is accomplished by the deenergize-to-trip relay holding in the powered closed switch for a discrete period of time and the energize-totrip relay (being deenergized) having no effect on the normally closed switch. Thus, if power loss is momentary, circuit integrity is maintained and burner shutdown is avoided; however, if power loss exceeds the set period of time, the powered closed switch will open deenergizing the deenergize-to-trip burner safety shut off valve closing down the burner system. In an emergency unsafe operating condition, the deenergize-totrip relay would again be deenergized to maintain the powered closed switch closed, but the energize-to-trip relay would assume an energized state substantially immediately opening the normally closed switch to trip'the deenergize-to-trip burner safety shut off valve for closing down the burner system. The energize-to-trip relay does have a slight time delay action in order to prevent accidental tripping of the normally closed valve on resumption of power after a momentary power interruption.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the fuel burner safety control circuit according to this invention.

FIG. 2 is a schematic representation of the novel safety control circuit of FIG. 1 showing the circuit in its armed, poweron mode.

FIG. 3 is a schematic representation of the novel safety control circuit of FIG. 1 in its power-interruption mode.

FIG. 4 is a schematic representation of the novel safety control circuit of FIG. 1 in its emergency trip mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 is a schematic representation of the fuel burner safety control circuit 10 according to this invention. The circuit 10 comprises lead lines 12, 12' across which an AC power supply (not shown) is located. Between the lead lines 12, 12' there is a number of parallel paths for the circuit elements. It is noted that the legend identifies the symbolsused throughout the drawings representing various elements within the circuit 10.

The first parallel path comprises a relay actuated contact switch 14 which is a control switch dependent upon satisfactory accomplishment of necessary boiler operating conditions, such as feedwater flow and proper fan operation, for actuation thereof. In series therewith is a boiler purge control relay actuated contact switch 16 and a master fuel trip relay l8. Across the switch 16, so as to be in parallel relationship with it, is a master fuel trip locking switch 20. A first time delay relay 22 is located in parallel with the master fuel trip relay l8 and in series with the

contact switches

16 and 20. Additionally, a time delay switch 24, in series with a proof of trip safety shut off valve open contact switch 25, is in parallel with the master fuel trip locking switch 20 for the purposes to be explained hereinbelow.

The second parallel path includes a first master fuel trip relay actuated contact switch 26 which is maintained in a normally closed position. A second time delay relay 28 is in series with switch 26, which, of course, controls the energization of the relay 28.

The third parallel path of the circuit 10 includes a relay ac tuated contact switch 30 which is a control switch dependent upon burner operating conditions, such as fuel availability and power for ignition, for the actuation thereof. In series therewith is a second master fuel trip relay actuated contact switch 32 and a deenergize-to-trip burner safety shut off valve 34 for shutting off fuel to the burner system. A burner safety shut off valve time delay loop 36 is in parallel with the burner safety shut off valve 34. The loop 36 consists of a first time delay switch 40 actuated by relay 22 (upon deenergization thereof) and a second time delay switch 38 actuated by relay 28 (upon energization thereof). In series with the two

time delay switches

38 and 40 is a trip delay unit 42, for the purpose of maintaining power supply to the burner safety shut off valve 34 for a discrete period of time, the unit 42 being of any well-known type such as the Maxon "I'D trip delay unit made by the Mason Premix Burner Company Inc.

The above-described elements of the safety control circuit 10 are interrelated to operate in the following manner. An AC power source is connected across the leads l2, l2 and when energized the circuit is armed leading to the mode as represented in FIG. 2 in the following sequence. Upon satisfactory accomplishment of all necessary boiler operating conditions, the switch 14 will be actuated to its closed position. With all boiler operating conditions satisfied, the boiler is purged of latent gases by forcing of air therethrough for a period of time. The purge control relay actuated switch 16 is closed at completion of satisfactory boiler purge (switch 16 remains closed only for a short period of time) to energize the master fuel trip relay l8 and the first time delay relay 22. The master fuel trip relay 18 in turn actuates the master fuel trip locking switch 20 to a closed position and at the same time serves to open the first master fuel trip relay actuated switch 26 while closing the second master fuel trip relay activated switch 32. With the switch 26 in its open position, power is prevented from flowing through the second parallel path. The second time delay relay 28 is thus maintained in a deenergized state which in turn maintains the switch 38 which it controls in its normally closed position.

At the same instant at which power is supplied to the first time delay relay 22, the relay 22 actuates the first time delay relay locking switch 24 and first time delay switch 40 to closed positions. The first time delay relay switch 24, in series with the proof of trip burner safety shut off valve open contact switch 25, serves to maintain circuit integrity and a power path for relays l8 and 22 upon momentary power interruptions. The switch 25 permits this circuit integrity to exist only when the burner safety shut off valve is open (and it is correspondingly closed).

After the purge cycle is completed and when all necessary burner operating conditions have been met, the relay actuated contact switch 30 will be closed to power the deenergize-totrip burner safety shut off valve 34 to its active (open) position, the proof of trip safety shut off valve open contact switch 25 simultaneously being closed. At the same time, the safety shut off valve time delay loop 36 will be brought to an energized state due to the fact that the first and second time delay switches (40, 38) are in their closed positions.

Upon an electrical power interruption to the burner control system, the novel safety control circuit is activated to the mode shown in FIG. 3. Power across leads 12, 12' ceases caus ing the boiler operating condition switch 14 and the burner operating condition switch 30 to return to their open positions which would ordinarily deenergize the master fuel trip relay 18, first time delay relay 22 and the deenergize-to-trip burner safety shut off valve 34. However, the deenergize-to-trip burner safety shut off valve is maintained energized by the trip delay unit 42 in the time delay loop 36 for a period of time in the following manner. The first time delay relay 22, upon deenergization thereof, will maintain the time delay switch 24 and first time delay switch 40 of the time delay loop 36 closed for a discrete period of time (e.g., one second). This will retain the integrity of the electrical path of the burner safety shut off valve time delay loop 36 so that energy stored in the time delay unit 42 may maintain the burner safety shut off valve 34 in an energized state so as to prevent the tripping thereof and resultant burner shutdown.

The purpose of maintaining switch 24 in its closed position for the discrete period of time is to provide a path for power through the master fuel trip relay l8 and time delay relay 22 upon the resumption thereof. The

switches

24 and 40 as noted remain closed only for a period of time so as to compensate for transient power interruptions. That is to say, if the power interruption is only momentary, the deenergize-to-trip burner safety shut off valve 34 will be maintained in its energized state (by trip delay unit 42) and the circuit will remain armed so that upon the return of power, the system will return to its mode shown in FIG. 2. if, however, the power interruption is not transient but is of a significant time duration, the

switches

24 and 40 will open thus deactivating the deenergize-to-trip burner safety shut off valve 34 closing down the burner operations and dearrning the safety control circuit 10.

Under certain circumstances, an emergency condition may arise at which time the deenergize-to-trip burner safety shut off valve 34 must be substantially immediately deenergized to shut down burner operation rather than permitting an extended delay as above. The emergency condition may either occur in the burner operating conditions such as flame failure or the boiler operating conditions such as feedwater interruption. The burner shutdown is accomplished by activating circuit 10 to the mode represented in FIG. 4 in the following manner. Upon the occurrence of an emergency condition, the

switches 14 (or switch 14 in cooperation with switch 30 if the emergency is in the burner system) will be actuated to its open position, thus cutting off power to the master fuel trip relay l8 and the first time delay relay coil 22. As a result thereof, second master fuel trip relay actuated switch 32 will open interrupting power to the deenergize-to-trip burner safety shut off valve 34. As in the power interruption mode represented in FIG. 3, the deenergization of the first time delay relay 22 will cause the

switches

24 and 40 to remain in their closed positions to preserve circuit integrity. This, of course, will maintain the energization of the burner safety shut off valve time delay loop 36 by the trip delay unit 42.

in order to promptly interrupt this energization and cause the valve 34 to trip to shut down the burner, the switch 38 is substantially immediately opened in the following manner. Upon the opening of switch 14 and the deenergization of master fuel trip relay 18, the first master fuel trip relay actuated switch 26 is returned to its normally closed position. Since the power supply between the leads l2 and 12' is still being maintained, the path through the first master fuel trip relay actuated switch 26 will be powered to energize the second time delay relay 28. Upon energization thereof, the second time delay switch 38 in the time delay loop 36 will be maintained closed for a discrete period of time (e.g., onetenth of a second) and then will open to deenergize the deenergize-to-trip burner safety shut off valve 34 so as to substantially immediately shut down burner operation. The small time delay in the opening of switch 38 by the time delay relay 28 (as opposed to an instantaneous response) is necessary for proper rearming of the burner safety control circuit after a momentary power interruption. The time delay permits the circuit to return to the mode represented in FIG. 2 after the momentary power interruption by keeping switch 38 closed until the second time delay relay 28 is deenergized. if the time delay were not present, upon resumption of power, the second time delay relay 28 would be momentarily energized (thus opening switch 38 disrupting the integrity of time delay loop 36) before it could be deenergized by opening of switch 26 by master fuel trip relay 18.

From the foregoing it is apparent that there is herein described a novel fuel burner safety control circuit operating so as to be capable of discriminating between transient power interruptions where burner shutdown is not desirable and actual emergency conditions where substantially immediate burner shutdown is necessary. This is accomplished by constructing a control circuit with parallel time delay relays functioning in opposite energization senses to activate series switches in a time delay loop in parallel with a deenergize-to trip burner safety shut off valve. Upon a power interruption, both relays are deenergized maintaining the switches closed for a set period of time permitting a time delay device in the loop to maintain the safety shut off valve energized; under emergency conditions, one relay will be deenergized while the other will be energized opening one of the switches in the time delay loop, thus deenergizing the loop in order to provide substantially immediate tripping of the burner safety shut off valve and corresponding burner shutdown.

While this preferred embodiment of the invention has been shown and described, it will be understood that it is merely illustrative and that changes may be made without departing from the scope of the invention as claimed.

We claim:

1. A fuel burner safety control circuit operable so as to be capable of distinguishing between power interruptions and emergency operation conditions necessitating closing a burner safety shut off valve, said safety control circuit including: a deenergize-to-trip burner safety shut off valve; a power source across said burner safety shut off valve to energize said valve; a time delay loop in parallel relationship to said burner safety shut off valve, said time delay loop having a time delay unit and a first and second switch in series relationship with one another; a first time delay relay operatively associated with said first switch to maintain said switch closed for a discrete period of time upon deenergization of said relay; a second time delay relay operatively associated with said second switch to maintain said switch closed for a discrete period of time upon energization of said relay, said discrete period of said second switch being substantially less than said discrete period of said first switch; and means connected with said power source for controlling the energization state of said first and second time delay relays so that both time delay relays are deenergized on power interruption and during an emergency condition said first time delay relay is deenergized and said second time delay relay is energized.

2. The safety control circuit of claim 1 wherein said means for controlling the energization state of said first and second time delay relays includes a first control switch in series relationship with said first time delay relay, a master fuel trip relay in series relationship with said first control switch and in parallel relationship with said first time delay relay, and a second control switch in series relationship with said second time delay relay, said second control switch operatively associated with said master fuel trip relay so as to be activated to an open position from a normally closed position upon activation of said master fuel trip relay through said first control switch.

3. The safety control circuit of claim 2 wherein said first control switch is dependent upon satisfaction of all necessary boiler operating conditions before actuation thereof to a closed position is initiated.

4. The safety control circuit of claim 2 wherein there is further included a third control switch to selectively permit initial energization of said deenergize-to-trip burner safety shut off valve, activation of said third control switch to its closed position being dependent upon satisfaction of all necessary burner operating conditions.

5. The safety control circuit 'of claim 2 wherein there is provided a locking switch in series between said first control switch and said master fuel trip relay, said locking switch operatively associated with said first time delay relay to remain closed for a discrete period of time to maintain integrity of the power path to said master fuel trip relay.

6. The safety control circuit of claim 1 wherein said discrete period of time that said first switch remains closed is on the order of 10 times the discrete period of time that said second switch remains closed.

7. The safety control circuit of claim 5 wherein said discrete period of time that said first switch remains closed is one second and said discrete period of time that said second switch remains closed is 0.10 second.

Claims

5. The safety control circuit of claim 2 wherein there is provided a locking switch in series between said first control switch and said master fuel trip relay, said locking switch operatively associated with said first time delay relay to remain closed for a discrete period of time to maintain integrity of the power path to said master fuel trip relay.