US3310039A - Safety control system for steam generators - Google Patents

Description

March 2l, 1967 H. H. HOLT 3,30,039

SAFETY CONTROL` SYSTEM FOR STEAM GENERATORS Filed oct. 11, 1965 i i L BY @QW ATTORNEY .5

United States Patent O 3,310,039 SAFETY CNTROL SYSTEM GENERATORS Harry H. Holt, llroaches Grocery, Rte. 4, Florence, S.C. 29501 Filed Oct. 11, 1965, Ser. No. 494,505 7 Claims. (Cl. 122-504) FOR STEAM the defective generator from creating abnormal operatingv conditions in its companion generators. Thus, the present control system, as set forth, is intended to and will respond to several conditions and/ or modes of operation, to cut-off feed water to a disabled generator. As will be seen, in some cases the end result is to prevent robbing of feed water from companion generators, as well as prevent explosion, while in other cases, particularly in the case of solid fuel or waste red generators, the primary concern is to prevent explosion.

Boiler tubes in the usual modern steam generator are usually divided into two groups, water tubes and superheater tubes. As the names imply the water tubes are so called because the initial conversion of water to steam occurs in these tubes which are directly and initially exposed to combustion heat. The steam is passed from these tubes to a feed water header or steam drum and then through superheater tubes to the point of final discharge from the generator. In the superheater tubes the steam is dried and brought to final operating characteristics.

Steam generators of the general type described above may be tired by combustible fuels derived from several sources, for example, by oil or gas burners; by solid fuels or by waste material derived from the very process which the steam generator may be supplying, i.e., for example, the residue from a pulp mill.

irrespective of the fuel used and whether a single generator or one of a family of generators supplied by common fuel and water systems, or, whether a water or superheater tube ruptures, such a reputure creates dangerous and troublesome conditions which are common to all modern automati-c or semi-automatically controlled steam generators. For example, in a solid fuel red generator, a water tube rupture can result in explosion as water is dumped into the combustion chamber and ashes into steam, or -due to the violent reaction when water hits the re bed. In another situation, bursting of one superheater tube can cause rupture of more tubes as the high pressure steam literally cuts into adjacent tubes resulting in a cascading failure as tube after tube is ruptured. Also, as has been mentioned, the defective generator can and will rob Water from companion generators in the situation where a common supply system is used to provide feed water to several generators operating in common.

The present invention is, then, designed to prevent the development of uncontrollable conditions in steam generators when and if a tube, water or superheater, ruptures during normal operation of the generator.

The present invention, then, has for an object, the

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provision of an explosion preventing safety system for steam generators.

Another object of the invention is to provide a feed water cut-off system which operates only in the case of a tube rupture in the furnace.

Still a further object of the invention is to provide a safety control system particularly useful in lsteam generating systems utilizing a common feed water system for multiple generators.

An additional object of the invention is to provide a safety shut-down system which is readily installed into and is compatible with existing control systems.

These and other objects of the invention, not specifically referred to but inherent herein, are accomplished in a control system for -a steam generator including means for monitoring and generating a pressure signal in direct relation to steam flow and pressure, means for monitoring and generating a pressure Isignal in direct relation to feed water flow; and means for monitoring and generating a pressure signal in direct response to furnace pressure by the improvement comprising a plurality of pressure signal responsive switches responsive to the drum pressure, feed water ow, steam llow and furnace pressure, said switches being connected in a series-parallel system to a control means for shutting off feed water supply to the generator and fuel or air supply to the combustion chamber whereby, upon rupture of a tube in the generator, pressure signals are generated to operate in combination and simultaneously to cut off 0r reduce the feed water supply to the generator and also air and/or fuel, to regulate the rate of combustion in the combustion chamber.

Having defined the invention in broad terms, the following descriptive matter will enable one skilled in the art to understand and uitilize same, reference being made to the drawing wherein:

FIG. l is a schematic illustration of a steam generator installation in which the invention may be utilized to advantage, and

FIGS. 2a and b is a schematic diagram illustrating the manner of installation of the invention in conventional steam generator or boiler control system.

As shown in FIG. l, two generators 1 and 3 having pressure drums 10, 10' are provided with feed water from a common feed supply 5, feeding drums 10, 10' via feed water regulators, indicated schematically in FIG. 1. The steam output from water tubes '7, 7 returns to the steam drums 10, 10 of each generator as is conventional, and is discharged thereform to superheaters 9 and 9' and then to steam header 11 and out for use via line 13. Suitable non-return valves prevent cross flow between generators. In the usual system, it is conventional practice to monitor, for regulation purposes of each generator the following conditions:

A-A-Drum water level B-B-Drum pressure C-C-Steam flow E-E-Furnace pressure (combustion chamber pressure) as shown and at the points illustrated in FIG. l.

These conditions are converted into `signal pulses by suitable means, which pulses are normally directed to a feed water regulator to supply water thereto in response to these signal pulses.

FIG. 2a, then, illustrates schematically the system used for converting these monitored conditions for a steam generator into a control system incorporating the present invention.

As seen in FIG. 2a, the generator control system may include a plurality of condition sensing transducers 5, 15, 25, 35, and 45 each selected, as is conventional in the art, to sense one `of the given operating conditions as for example at A, B, C, or D of FIG. 1. As shown, transducer 5 senses drum water level, i.e., the level of water in the steam drum. Transducer senses the pressure in the steam drum. Transducers 35 and 45, respectively, sense the fiow or rate of fiow of feed water input and steam output, respectively. Transducer senses furnace or firebox pressure, that is suction or draft in the combustion chamber developed by the usual Iblower systems used to control rate of combustion.

Each of the sensed conditions may be translated by a signal generator, designated generally as SG, into air pressure; the signal generators being supplied with air from a suitable source at approximately 0-70 pounds per square inch gauge, which pressure is regulated by the signal generators in the range of 0 4() p.s.i.g. so as to generate a variable pressure signal in proportion to the variations in the conditions sensed by the individual transducers. As is customary in the art, the individual pressure signals developed by the signal generators are, in turn, used to drive recorders and/ or regulators, the latter of which when used automatically control the operation of the steam generator within the given operating parameters.

The system, thus far described, is known in the art and need not be amplified in detail herein. It suffices to say that steam generators manufactured and installed by such entities as Babcock and Wilcox, among others utilize such air control systems which in turn operate from and by instruments and regulators manufactured by such concerns as Foxboro Instrument Co., Beckman Instrument Co., and others.

Other systems utilize electrical signal generators, which though not specifically shown, perform the same function, converting the sensed condition to electrical rather than air pulse signals. It should be understood that the invention is equally applicable to electrically operated systems.

Considering now the schematic diagram of FIG. 2b, along with FIG. 2a, it will be seen that, in addition to the conventional arrangement wherein control signals are utilized in a feed-water regulation arrangement, the air signals are also transmitted from the drum pressure (B), .furnace pressure (E), feed water inlet (D) and steam flow C (outlet) transducers 15, 25, 3S and 45 via lines 12, 13, 16 and 18 to an emergency shut-off system 104) which forms -the basis of the present invention.

The novel safety system, see FIG. 2b, is comprised of a series of pressure lsensitive switches 70, 80 and 90. One of these switches, 70, is la normally open, pressure closed switch. On the other hand, switch 9) is a normally closed, pressure opened switch; while the switch 80 is a differential pressure switch, sometimes called a double Bouden switch, which is normally held open by balanced pressure and is closed upon an unbalance of the sensed ressures by which it is operated. Each of these switches is purely conventional in the art and may be purchased on the open market, at least one manufacturer being the Mercoid Corporation of Milwaukee, Wisconsin. In all cases, the required pulse or signal pressure necessary to operate these switches can be preselected or adjusted. In the present system, since signal pressure is directly related to the steam generator condition sensed, the operating pressure for each switch is directly related to the condition sensed. For example, switch 7tl-furnace pressurecan be set to close at about .3 inch of mercury; the drum pressure switch 90 can be set to close if drum pressure falls below x pounds-less than 600 p.s.i.g. for example, in a steam generator operating at 600 p.s.i. normally; and the switch Si) can be set to close when the ratio of feed water flow to steam output liow, as indicated by the respective signal generators influencing same exceeds the ratio 1 to 1 for which the switch is set.

In other words, so long as the pulse signals from the feed water flow and the steam output iiow are balanced, switch 80 Iemns open.

Signal line 13 is connected so as to close switch 70 in the event furnace pressure exceeds the preset value and will remain open at all values lower than that selected. Switch 90, pulsed through line 12, on the other hand, will close when the drum pressure falls below, for example, 599 p.s.i. which is below the normal operating pressure of the generators 1 and 3. Switch 80, finally, is connected, via lines 16feedwater flow-and 18steam flowto close when the steam iiow or generator output falls below the selected ratio of feed-water input to steam output.

As shown in FIG. 2b, switches 76 and 90 are connected in series. Switch is connected in parallel with the pressure switch 7G and in series with switch 90 whereby when both switches 70 land 90 are closed, or alternatively when switch 80 and 90 are closed the circuit is completed to a latching relay LR via a two position, manually operated Automatic-Manual switch (not numbered) the latter also being purely a conventional item, well known in the art.

Latching relay LR, when energized, trips a normally open-manually reset switch LRS, allowing same to close to energize the solenoids SV, SVZ and/or SV3 which, in turn, close feed water valve and fuel cut-off 12% and the fan dampers via switch 130 where the latter is used` Again, the reset switch and the solenoid operated valves 110, are all commercial items, readily available on the market and need not be described or shown in detail herein.

Having thus described the signal generating system, consideration will now be given to the operation of the overall safety control arrangement.

Under normal conditions, all pressure switches 70, 8G and 90 are open. Except in abnormal conditions, when manual control is desirable as, for example, during steam generator start up or shut down, the Automatic-Manual switch will be set on Automatic The manual reset switch LRS is open and since latching relay LR is not energized, same remains open during all normal operation of the generator. In the usual control systems the feed water regulator acts to regulate feed to the generator in direct proportion to demand as reflected by the steam iiow signal. Valves 110 and 120, then, remain open.

In the event that a superheater tube should burst, steam ow drops because of the steam loss in the superheater. In conventional systems, the loss in steam output would also act to regulate feed water to lessen introduction of water into the generator because the two are interrelated as has been set forth above. However, discharge of steam, internally of the furnace, raises the furnace or combustion pressure above the normally prevailing pressure of .3 to .5 inch of mercury, to a pressure almost or slightly above atmospheric, which closes switch 70.

Simultaneously, the steam loss will reduce drum pressure in drum 10 below the normal operating pressure. Switch 9i) closes; thus the circuit latching to relay LR is closed, tripping the latch to reset switch LRS, releasing switch LRS which closes to cut off the feed water and fuel by the operation of solenoid valves 110 and 120. Also switch opens to cut olf furnace suction to reduce the rate of combustion in the combustion chamber. This abnormal condition is alleviated by the complete shut down of the generator. If the rupture is relatively minor and the generator must of necessity be operated for several hours before it can be shut down completely, then the Automatic-Manual switch can be moved to manual, the switch LRS manually reset and the generator operation carried out by manual control. If the rupture is serious, however, a potentially dangerous situation is immediately controlled, and, in either case, the plant personnel are aware of the existing condition and can act accordingly.

Let it again be supposed that in the midst of normal operation a water tube, as distinguished from a superheater tube, ruptures. Under these circumstances, steam ow from the superheater will drop off. However, feed water fiow will increase due to the fact that the usual feed water regulator will be infiuenced by the feed water level signal which is dropping as water pours into the combustion chamber of the furnace. Thus, pressure differential switch 8f) would close due to the discrepancy between the feed water ow and steam output and the associated signals from generators SG. At the same time, furnace pressure again would rise due to steam generation in the combustion chamber. Switch 70 would close with the result that latching relay LR is energized completing the electric circuit and again feed water and fuel are cut off.

The particular series-parallel arrangement of the switches '70, 80 and 90 is such that a combination of conditions must be in existence before the system acts. It will be appreciated that at times sudden shut down of a plant, i.e., rapid decrease in steam demand or a sudden demand, the reverse situation, conditions such as low feed water ow-high output, high furnace pressure could be created in various momentary combinations that will trip one or more of the several switches.- However, these momentary swings in boiler operation will not act to cause shut down because, unless the right combination of variables exists at the precise moment, the combination of closing paired switch 79 and 90 or 80 and 90 will occur, only when the conditions brought about by a tube rupture exist, be it water or superheater tube in the usual steam generator.

The importance of cutting off feed water to a solid fuel fired boiler or generator cannot be over emphasized. As the feed water from a ruptured tube is dumped into the combination chamber, steam is fiash generated resulting in pressures which will blow the generator, if not stopped almost immediately. On the other hand, where fuel can be cut off instantly, as in a gas or oil fired generator, continued excessive discharge of water into the combustion chamber can result in structural distortions, warpage, cracking and the like, which require extensive repairs to the generator if not complete replacement thereof.

In this connection, it should be noted, however, that it is desirable to keep some minimal water ow in a gas or oil fired steam generator so that the tubes do not heat harden before the generator cools off. To this end, valve 110 may include some means, by-pass, bleed port or an equivalent, or by pressure regulation, i.e., closing to a minimal flow-through so as to permit only a small regulated flow to the generator during the cooling off period. In a solid or waste fuel fired generator, such an arrangement is not practical. However, in the latter case, since there is no fuel fiow cut-off which can instantly cut off the heat of combustion, the solenoid operated switch 130 is operated to cut off the power to the blower fans thus reducing combustion rate in the chamber and permitting gradual cooling of the entire structure.

In the case of multiple boilers operating from a single source of feed Water supply as illustrated in FIG. 1, it is most important to cut feed water promptly, when a water tube breaks in one generator, because increased feed water supply to the disabled generator will rob the companion generator. This occurs because usual feed water systems are so co-related so as to supply a volume of water sufficient for peak operating conditions of both generators and very little more. Thus, if a tube blows in one generator, lthe drain on the feed-water system would be more than the system could supply even though the usual feed water regulator would open the supply system to full flow to keep the level in steam drum 10 or 10 at a safe condition.

Having thus described the invention in detail, it will be apparent that various modifications will occur to those skilled in the art, all within the scope and spirit of the invention which is limited only as defined in the claims, wherein what is claimed is:

1. A steam generator control system comprising a plurality of transducers for sensing conditions existing within the generator during operation thereof, a control system including signal generating means associated with said transducers and operative to automatically regulate the conditions of operation of said generator, valve means actuated by said control means, said control system including further a plurality of switch means responsive to the output of said signal generating means, said plurality of switches comprising a pressure closed switch, a pressure opened switch, and a differential pressure switch operatively connected in a series-parallel circuit responsive to an abnormal sensed condition of operation of said generator such that operation of either said pressure opened switch or said differential pressure switch when coupled with operation of said pressure closed switch in response to the sensed abnormal condition will operate said valve means to terminate operation of said generator.

2. A wet tube steam generator comprising sources of fuel and water supply and means for regulating air flow through the combustion chamber, control means for regulating said fuel and water supply, said control means including further means monitoring the operation of the generator and providing a pressure control signal; means for terminating operation of the generator in the event of a tube rupture, said means comprising a plurality of pressure responsive switches, one of said switches being responsive to air pressure in the combustion chamber, the second of said switches operating in response to variations in the ratio feed water flow to steam output, the third switch responding to pressure on the output side of the generator, said first and third switches being connected in series and said second and third switches also being connected in series, said second-mentioned switch being connected in a parallel circuit with respect to said first-mentioned switch, means regulating flow from said water source to and the rate of fuel combustion in said generator, said means being operable to a closed position when either said first and said .third-mentioned switch or said second and third mentioned switch is closed due to abnormal conditions sensed in the generator whereby tube rupture therein will result in cut-off of the supply of fuel and water to the generator.

3. The combination as defined in claim 2, wherein the means for regulating water to the generator is comprised of a normally open, solenoid closed valve.

4. The combination as defined in claim 3, including the means for controlling the rate of combustion in the generator.

5. The combination as defined in claim 4 wherein the means to regulate the rate of combustion comprises a blower, said blower being rendered inoperative by closure of aforesaid first and third or second and third mentioned switch means.

6. The combination defined in claim 3 including further a second steam generator supplied from said fuel and water sources, said second generation being identically controlled by separate control means as defined whereby failure of one generator results in termination of solely the operation of that generator.

7. The combination as defined in claim 3 including a further manually set, solenoid released latching switch operated in response to closure of said first and third or said second and third mentioned pressure responsive switch means.

References Cited by the Examiner UNITED STATES PATENTS 2,258,719 10/1941 Saathoff 122-448 FOREIGN PATENTS 732,723 6/1955 Great Britain.

KENNETH W. SPRAGUE, Primary Examiner.

Claims (1)

1. 2. A WET TUBE STEAM GENERATOR COMPRISING SOURCES OF FUEL AND WATER SUPPLY AND MEANS FOR REGULATING AIR FLOW THROUGH THE COMBUSTION CHAMBER, CONTROL MEANS FOR REGULATING SAID FUEL AND WATER SUPPLY, SAID CONTROL MEANS INCLUDING FURTHER MEANS MONITORING THE OPERATION OF THE GENERATOR AND PROVIDING A PRESSURE CONTROL SIGNAL; MEANS FOR TERMINATING OPERATION OF THE GENERATOR IN THE EVENT OF A TUBE RUPTURE, SAID MEANS COMPRISING A PLURALITY OF PRESSURE RESPONSIVE SWITCHES, ONE OF SAID SWITCHES BEING RESPONSIVE TO AIR PRESSURE IN THE COMBUSTION CHAMBER, THE SECOND OF SAID SWITCHES OPERATING IN RESPONSE TO VARIATIONS IN THE RATIO FEED WATER FLOW TO STEAM OUTPUT, THE THIRD SWITCH RESPONDING TO PRESSURE ON THE OUTPUT SIDE OF THE GENERATOR, SAID FIRST AND THIRD SWITCHES BEING CONNECTED IN SERIES AND SAID SECOND AND THIRD SWITCHES ALSO BEING CONNECTED IN SERIES, SAID SECOND-MENTIONED SWITCH BEING CONNECTED IN A PARALLEL CIRCUIT WITH RESPECT TO SAID FIRST-MENTIONED SWITCH, MEANS REGULATING FLOW FROM SAID WATER SOURCE TO AND THE RATE OF FUEL COMBUSTION IN SAID GENERATOR, SAID MEANS BEING OPERABLE TO A CLOSED POSITION WHEN EITHER SAID FIRST AND SAID THIRD-MENTIONED SWITCH OR SAID SECOND AND THIRD MENTIONED SWITCH IS CLOSED DUE TO ABNORMAL CONDITIONS SENSED IN THE GENERATOR WHEREBY TUBE RUPTURE THEREIN WILL RESULT IN CUT-OFF OF THE SUPPLY OF FUEL AND WATER TO THE GENERATOR.

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