US2699041A - System for controlling the supply of heating gases to fluid heat exchange apparatus - Google Patents

Description

Jan. 11, 1955 w. H. ROWAND ETAL SYSTEM FOR CONTROLLING THE SUPPLY OF HEATING GASES T0 FLUID HEAT EXCHANGE APPARATUS 3 Sheets-Sheet 1 Filed Nov, 27. 1948 mm b n m a o W OT 01m R V 0 m s 9 I. 4 Z a l t i J m y m F F WOW. I 2

ATTORNEY Jan. 11, 1955 w. H. ROWAND EI'AL SYSTEM FOR CONTROLLING THE SUPPLY OF HEATING GASES TO FLUID HEAT EXCHANGE APPARATUS 3 sheets-sheet 2 1 Filed Nov. 27. 1948 SUPERHEA TE)? 74 16' M45752 CONTROLLER Will- Eowand A? Paul BL oughin INVENTO ATTORNEY Jan. 11, 1955 w. H. ROWAND ETAL SYSTEM FOR CONTROLLING THE SUPPLY OF HEATING GASES TO FLUID HEAT EXCHANGE APPARATUS 3 Sheets-Sheet 3 Filed Nov. 27. 1948 n A 0 Wm w o o H H O L T m H i w m wfix m W mm WP m R R 63 2%. 8328 T ccc Mm cc 26 9 Q .mu: 3 i H H L My AW g R L mmwhwmflfiz u mm $35. 3328" WWL A a on $1,

ATTORNEY SYSTEM FOR CONTROLLING THE SUPPLY OF HEATEVG GASES TO FLUID HEAT EXCHANGE APPARATUS Will H. Rowand, Madison, and Paul R. Loughin, Westfield, N. J., assignors to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application November 27, 1948, Serial No. 62,402

6 Claims. (Cl. 60-70) This invention relates to a novel method and control system for effecting rapid reduction in the firing rate of vapor generators, particularly steam boilers of the type including combustion gas heated reheaters, upon a sudden loss of load by the prime movers serviced by the boiler; while still maintaining the firing of the boiler at a minimum rate so that the load may be rapidly picked up when the trouble has been cleared. Specifically, upon a sudden loss of load, all of the boiler firing equipment is rapidly cut back to minimum rate and substantially half the firing equipment is cut out completely. It should be understood that the terms boiler or steam boiler as used herein are exemplary only, as the principles of the invention are applicable to vapor generators in general, whether the fluid used is Water or some other liquid.

The rising trend of the cost of fuel has emphasized the desirability, or even necessity, for improving the overall fuel efficiency of power generating plants to reduce operating costs. To this end, there is an increasing tendency toward the use of high temperature and high pressure steam powered generating equipment, with many newer installations being designed for use with steam temperatures of 1000 F. or higher. These plants generally utilize a single boiler unit to supply each turbine set, due to the high steam generating capacities of modern boilers, and the boiler units include both steam superheaters and reheaters heated by the high temperature combustion products from the furnace. The reheat temperature, for the steam entering the low pressure turbine, may be of the order of 1000 F. or higher in order to obtain the highest operating efiiciencies.

The use of such high initial and reheat steam temperatures introduces design and operating requirements which must be given careful consideration in order to make adequate provision for safe and efficient handling of the steam under all conditions, and to obtain the maximum possible overall fuel efficiency. For example, careful consideration must be given to the controls and operating procedure in starting up the generating plant, in shutting the plant down for a planned extended outage or in shutting down to a hot bank condition, in maintaining elements of the plant in stand-by condition for immediate resumption of load after an emergency shut-down due to a loss of load, and in complete emergency shut-down due to failure of a plant element, such as an essential boiler auxiliary, or due to sudden removal of the boiler from service.

Another factor of primary influence on the overall fuel efliciency is the continuity of operation. For example, in starting up, or re-starting, the plant, several hours normally are required to bring the prime movers to their operating temperature. This is necessary because of the different expansion rates of the several parts of the prime mover and of the connected piping, and to prevent damage due to thermal shock. The relatively thick-walled steam drums used with high pressure installations require ex treme care in changing temperatures due to the accentuated expansion and contraction problems involved. Thus, this starting time is correspondingly longer in the case of high temperature, high pressure installations.

When a prime mover is cut out due to any reason, such as loss of load, accidental tripping, or other reasons, it has been found that there is usually such a delay in locating and rectifying the trouble that the firing of the boiler is customarily interrupted. Frequently, these accidental outages require only a short time to locate the trouble and place the unit back on the line. If the firing United States Patent (:e

2,699,041 Patented Jan. 11, 1955 of the boiler has been completely interrupted, considerable delay is involved in re-starting operation. This is particularly true in the case of oil or pulverized fuel fired boilers, wherein the furnace should be purged before being relit. In the event of a very brief outage, the time needed to purge and relight the firing equipment may be the controlling factor in determining the duration of the outage. Consequently, it is desirable to provide some combustion control system or method of operation wherein the firing of the boiler will not be completely interrupted immediately upon occurrence of an accidental cutting out of a prime mover or steam user. Specifically, any reduction in the amount of time that the plant is kept out of service will result in a corresponding increase in the overall fuel efliciency.

Of course, the above mentioned relatively long time required to bring the prime movers up to the Working temperature and pressure is a factor which must be considered in such a control system or method of operation. If the firing of the boiler is interrupted completely, the temperature of the parts of the prime movers drops rather rapidly. In putting these units back in operation, great care must be exercised in raising their temperatures in order to prevent damage due to unequal expansion of parts or due to thermal shocks. Additionally, care must be taken to avoid too rapid temperature changes in the metal of the steam drum.

Continued full rate firing of the boiler unit following interruption of steam flow to the prime mover involves a hazard to the steam reheater of the unit inasmuch as it is still subject to heating by the furnace gas while it receives no low pressure steam from the exhaust of the high pressure turbine. If such a condition is maintained for an appreciable time the metal of the reheater tubes may be overheated to the extent that the latter are damaged. The higher the operating reheat steam temperatures, the greater the hazard involved.

The cusomary superheater is supplied with saturated steam irrespective of whether the prime mover is taking steam from the superheater or not, and a relief or safety valve is actuated by increases in pressure to cause a flow of steam through the superheater to the atmosphere, and this flow is sufficient to avoid overheating of superheating tubes if the customary subsequent reduction in fuel firing is accomplished.

However, once the high pressure turbine inlet valves close, the steam flow through the reheater is rapidly reduced to zero, and heat input to the reheater must be absorbed by the metal thereof. Consequently, the reheater temperature rapidly increases, and if the increase is maintained beyond a predetermined time, the reheater parts are damaged.

To avoid this condition, the present invention provides for an immediate cutting back of all the boiler firing equipment to a predetermined rate, on occurrence of an outage, with substantially one-half the firing equipment cut out of service completely. This cutting back and cutting out of firing equipment is efiected at such a rate that firing of the boiler is reduced to substantially onefourth the full load rate before there is an excessive temperature rise in the reheater.

In one embodiment of the invention, substantially onehalf the firing equipment of the boiler is cut out immediately upon occurrence of an outage persisting for more than a few seconds, and the remaining firing equipment is cut back to its predetermined rate over a longer period and in response to the drop in steam pressure as a result of the outage. In another embodiment, the cutting back of the firing equipment is begun immediately upon occurrence of a persisting outage and, when all the firing equipment has been reduced to the predetermined rate, substantially one-half the firing equipment is cut out completely. As a consequence of either of these procedures, the boiler firing rate is reduced sufiiciently rapidly to prevent an excessive temperature rise in the reheater, while the boiler firing is maintained at a reduced rate to reduce the outage time.

With the foregoing in mind, the main object of this invention is the coordination of the fuel supply system of a steam reheating boiler directly associated with a prime mover from which steam flows to the reheater, whereby protection of the reheater against overheating by gaseous products of combustion is accomplished by inltiat ng modification of the rate of fuel firing upon interruption of steam fiow to the reheater.

Another object is to provide a control system and method in which, upon occurrence of an accidental outage of steam using equipment, firing of a boiler is maintained at a reduced rate for a predetermined time interval.

A further object is to provide such a control system in which the firing of a boiler is automatically rapidly -reduced to a predetermined rate upon occurrence of an accidental outage of a steam user.

Another object of the invention is to provide an automatic control system for a power generating plant m which, upon occurrence of an outage, the firing rate of the burners of a boiler is reduced to a predetermined rate upon the occurrence of an outage, and a proportion of the burners are cutout.

These and other objects, advantages, and novel features of the invention will be apparent from the following description and the accompanying drawings. In the drawings:

Fig. 1 is a vertical sectional view through a vapor generator embodying a vapor reheater and with which the control system and method of operation of the invention may be used;

Fig. 2 is a horizontal sectional view of the generator on the line 2-2 of Fig. 1;

Fig. 3 is a schematic diagram illustrating one embodiment of the control system of the invention; and

Fig. 4 is a view of a portion of the power plant elements shown schematically in Fig. 3, and illustrating another embodiment of the invention control system.

The combustion control system and operating method of the present invention are particularly applicableto power generating plants having steam boilers including combustion gas heated reheaters which reheat the exhaust steam from a high pressure unit for application to the low pressure unit and in which each unit including a pair of high pressure and low pressure turbines is supplied with steam from a single boiler rather than several boilers. However, while the invention is particularly adaptable to such plants, it is not limited in application thereto and may be used in plants not incorporating reheaters, or to plants in which the steam is reheated in a separately fired unit.

In accordance with the present invention, the control valves which are provided at the inlets to the high pressure and low pressure prime movers are provided with interlocking arrangements for controlling the firing equipment of the boiler. For example, should the admission valve to either the high pressure or low pressure prime mover or turbine close for more than a predetermined time, due to overspeeding of either unit or loss of load, the interlocking arrangements operate through the boiler controller to reduce the firing rate to a low predetermined rate. Typically, substantially half the burners of the boiler are cut-off, while those maintained in firing condition are reduced to a low predetermined operating rate. Should the outage or other condition persist for a longer predetermined time interval, the electrical generator equipment is cut-ofi the line in the usual manner. Provision is also made, by pressure relief valves, for relieving all or a portion of boiler capacity in advance of the high pressure prime mover, in advance of the reheater, and in advance of the low pressure prime mover to avoid excessive heating of these units when the load is reduced or dropped with the boiler still being fired.

The firing rate of the boiler is reduced sufliciently fast to prevent damage of the reheater by overheating. The relatively large mass of metal in the reheater permits of a fairly substantial heat absorption thereby without excessive temperature rise during the time the firing of the boiler is being reduced, with the firing rate being reduced to a safe minimum within a limited time period. This may be eifected either by immediately cutting out substantially half the firing equipment, followed by cutting back of the remainder to a predetermined rate responsive to the load reduction, or all the firing equipment may be rapidly reduced to the predetermined rate after which half the firing equipment is cut out. In the first instance, at least, it may be desirable to provide ignition stabilizing means or to provide, additionally, reduction of the secondary air fiow to the boiler.

In order to provide a clear understanding of the principles of the invention, the invention will be described as applied to a typical steam boiler delivering high temperature, high pressure steam from a superheater to a high pressure turbine, and including a combustion gas heated reheater capable of elevating the temperature of the exhaust steam from the high pressure turbine to substantially its initial temperature leaving the superheater before delivering the reheated steam to a low pressure turbine, although the invention is applicable to separately fired reheaters. Also, for purposes of illustration only, the invention will be described as applied to a boiler utilizing pulverized fuel firing equipment, although the invention principles are equally applicable to an oil or gas fired generator.

Referring to Figs. 1, 2 and 3, a steam boiler 10 is shown as comprising a unit of the radiant boiler type having its walls lined with tubes 11 converging, at their lower portions, to form a hopper outlet 12. Steam generated by the unit is delivered to steam drum 13 from which the steam is conducted by tubes 14 to the first section 16 of a superheater. The superheated steam from section 16 may pass through an attemperator 9 (Fig. 3) and is delivered through piping (not shown) to a second superheater section 17, from which it is delivered to a steam main 18. Main 18 delivers superheated steam to the inlet of a high pressure prime mover, such as a steam turbine 40 (Fig. 3). Disposed between superheater sections 16 and 17, and in the convection section 15 of generator 10, is a reheater 20 comprising two sections 21 and 22. Reheater 20 receives steam exhausted from the high pressure unit through an attemperator 45 (Fig. 3) and a main 23, reheats the steam to substantially its initial superheated temperature and delivers it to a main 24. From main 24, the reheated steam is delivered to a low pressure prime mover, such as a second steam turbine 50 (Fig. 3).

In the illustrative arrangement, the firing equipment for vapor generator or steam boiler 10 includes a plurality of fuel pulverizers 25, shown as four in number, and each delivering pulverized fuel to three pulverized fuel burners 26 through fuel lines indicated at 27. The primary and secondary air for pulverizers 25 and for fuel burners 26 is provided from airheater 30 having an air inlet 31 and delivering air through conduits 32, 32 to windbox 33. Other conduits 34 take the primary air from conduits 32 and deliver it to pulverizers 25.

The hot combustion gases provided by the burning of the fuel in combustion space 36 of boiler 10 pass upwardly to convection section 15, where the very high temperature gases first flow over the tubes of the second superheater section 17. From section 17, these very hot gases flow over the second section 22 of reheater 20, the first section 21 of the reheater, and then into a downward flow passage 37. In flowing through passage 37, the hot gases first pass over the first superheater section 16 and then over economizer elements 38. The gases, somewhat cooled by heat extraction therefrom, then pass through the air heater 30, where more heat is extracted fronli the gases, and into an outlet 39 leading to the stac The operation of the invention as applied to a boiler 10 of the type shown in Figs. 1 and 2 will be best understood by reference to the schematic diagram of Fig. 3. Referring to the figure, the steam drum 13 is shown as provided with safety valves A having a relieving capacity of of the capacity of boiler 10, and these valves are preferably mounted on top of drum 13. As stated, the steam from drum 13 is delivered through conduits or tubes 14 to the two superheater sections 16 and 17 shown as having spray attemperator 9 therebetween. At the outlet of section 17 are two spring loaded safety valves B and a power control valve C, having a combined relieving capacity of 30% of the capacity of boiler 10. From these valves, main 18 delivers the high pressure, high temperature steam to the high pressure turbine 40.

There are two valves immediately preceding the high pressure turbine. Valve D is a manually reset stop valve, which is arranged to trip on predetermined overspeeding, usually 10%, of the turbine and which remains either wide open or tightly shut. The second valve, E, is a variable opening control valve, regulated by the turbine governor. On a sudden decrease of the electrical load coupled to turbine 40, valve E may close completely, but will immediately reopen a suflicient extent to pass the steam required by the reduced electrical generator load.

From the outlet of high pressure turbine 40, the exhaust steam passes through main 23 and spray attemperator 45 to reheater 20, where the temperature is raised to approximately the temperature of the steam entering turbine 40. In main 23 are valves F having a relieving capacity of substantially 75% of the output of boiler 10 at substantially one-half the designated operating pressure. The reheated steam from reheater 20 is delivered through main 24 to low pressure turbine 50, and a valve G, having a relieving capacity of substantially 25% of the output of boiler 10 at substantially one-half the rated pressure, is located in main 24.

Immediately preceding low pressure turbine 50 is an interceptor valve H, which is a manually reset stop valve arranged to trip on predetermined overspeeding of turbine 50 (such as due to loss of electrical load) or upon loss of vacuum in condenser 55 receiving the steam exhausted from turbine 50.

It should be noted at this point that tripping of the high pressure turbine stop valve D on loss of load on both machines will not protect low pressure turbine 50 from overspeeding. This is due to the energy of the steam stored under pressure in high pressure turbine 40, in the steam piping and in reheater 20. In a particular installation, the energy of this stored steam is sufiicient to run low pressure turbine 50 for about four minutes after closure of high pressure turbine stop valve D.

In Fig. 3, the boiler 10 is indicated as being fired with pulverized fuel from pulverizers 25. The construction of pulverizers 25 forms no part of the present invention, and these pulverizers may be of any well-known type including a mill for pulverizing the coal and a primary air fan for supplying primary air from conduits 34 to the mill to carry the pulverized coal particles in suspension to burners 26. In accordance with customary practice, each pulverizer includes a main control which, in the oil position, effects deenergization of both the mill and the fan driving arrangements. Also in accordance with customary practice, the primary air fans are provided with dampers which regulate the air flow to the burners and thus the fuel delivery rate, the variations in primary air flow also controlling the coal feed to the mill in accordance with the air flow.

The operation of boiler 10 is controlled by a steam pressure responsive control system similar to the type shown and described in U. S. Patent No. 2,098,914, issued November 9, 1937, to H. H. Gorrie for Control System. In the Gorrie system, particularly that shown in Fig. 3 of his patent, the steam pressure is applied to a pilot valve (shown in Fig. 5 of Gorrie) which controls the application of pressure air to a standardizing relay (shown in Fig. 4 of Gorrie) which, in turn, controls the operation of a selector valve (shown in Fig. 6 of Gorrie). The selector valve is provided with suitable manually-operated valves and mechanisms whereby control of one or more boilers may be eifected in response to manual operation of the selector valve or in response to the steam pressure. The selector valve governs the application of pressure air to individual controls for each boiler.

In the present invention, the master unit including the pilot valve, relay and selector valve have been schematically indicated at 60 as included in a unit designated master controller and connected by conduit 61 to steam main 18 and by conduit 62 to a source of air under controlled pressure. Reference is made to the Gorrie patent for further details of construction and interconnection of the elements constituting unit 60, although a similar relay is schematically indicated at 75 and similar selector valves are schematically shown at 70.

The steam pressure responsive control elements of the Gorrie patent control the application of pressure air to individual control elements for each of a number of boilers. In the present case, such steam pressure responsive control elements, schematically included as unit 60, control the application of pressure air to control elements associated with each pulverizer 25. As shown, a pressure air conduit 63 connects controller 60 to the pressure air inlets of four primary air selector valves 70, each associated with a pulverizer 25. Each selector valve 70 individually controls the application of pressure air to its associated relay 75, and such application may be governed automatically in response to steam pressure in main 18, or manually, dependent upon the setting of the selector valves 70. Also, dependent upon the setting of controller 60, all the selector valves 70 may be placed under manual control or may be made responsive to the steam pressure in main 18.

Each selector valve 70 controls the flow of pressure air to the space above a flexible diaphragm 71 of relay 75. A fixed diaphragm 72 is below diaphragm 71 and has a flexible bellows arrangement 73 to which is secured a rod 74 connecting diaphragm 72 to a second flexible diaphragm '76. The chamber beneath diaphragm 76 is in communication with a pilot-valve assembly schematically illustrated at 77 as controlling flow of pressure air to the cylinder 78 of an operator 80 having its piston 79 connected to the damper (not shown) controlling the flow of primary air to pulverizer 25. Pilot valve assembly 77 includes a pilot valve of the type shown in Fig. 5 of the Gorrie patent and operated by a pressure responsive device, connected to relay 75, in the manner shown in Fig. 3 of Johnson Patent No. 2,169,150, for example.

Normally, the space between diaphragms 71 and 72 of relays 75 is connected to atmosphere, as shown at 81 for the two right hand relays in Fig. 3. In the present case, for a purpose to be described, the corresponding spaces of the two left hand relays 75 are connected to a solenoid operated valve 85 by a conduit 82 having an adjustable bleed to atmosphere, 83. In its non-operated or non-energized position, valve 85 vents conduit 82 to atmosphere as at 84. However, when valve 85 is energized, it connects conduit 82 to a source of pressure air regulated by a pressure regulator 86.

Assuming that master controller 60 and all four selector valves 70 are set for automatic" operation, variations of the steam pressure in main 18 effect operation of controller 60 to vary the control air flow to valve 70. These valves vary the pressure above diaphragm 71 of relays 75, causing a corresponding variation in the pressure below diaphragms 76. This is due to the interconnection of diaphragms 71 and 76 by rods 74. As explained, the space between diaphragms 71 and 72 is at atmospheric pressure and, in the present case, the space between diaphragms 72 and 76 is also at atmospheric pressure.

The pressure variation beneath diaphragms 76 is communicated to the pressure responsive devices of assemblies '77, which operate the pilot valves of the assemblies to control the application of pressure air to operators 80. The latter vary the positions of the primary air flow dampers to vary the air flow to pulverizers 25, in turn effecting a variation in the coal feed thereto. Thus, the fuel supply to burners 26 is varied in accordance with the steam pressure in main 18.

If any of the valves D, E or H close for some reason, the resulting variation in steam pressure in main 18 causes the pulverizers 25 to cut back to a lower rate, or to their minimum stabilized rate. However, at high temperatures of 1000 F. or greater, and high pressures, such cutting back may not take place rapidly enough to prevent damage to the boiler parts, such as reheater 20, due to the continuing heat input thereto without heat extraction therefrom. Furthermore, even at the minimum stabilized firing rate of the pulverizers, the gas temperatures may still be sufliciently high to damage the boiler elements, particularly the reheater 20. The present invention provides a very rapid reduction of the firing rate to a predetermined low value and a cutting out of substantially fifty per cent (50%) of the pulverizers upon loss of load by either turbine.

The operation of the invention can be explained best by reference to a typical steam boiler, for example, one having a boiler delivering 937,000 lbs/hr. of high pressure steam at 2080 p. s. i. and 1050 F. with 390 p. s. i. and 1000" F. at the reheater outlet, the boiler 10 being supplied with feed water at 441 F. This equipment will give a heat rate of about 9300 B. t. u. per net kilowatt.

With the specific pressures, temperatures and steam rates mentioned above, the valves B and C have a combined relieving capacity of 285,000 lbs./hr., and the above mentioned relieving capacity of valves F and G is eflective at a pressure of 450 p. s. i. Substantially, 30,000 to 40,000 lbs/hr. of steam are required at no load.

Valves D, E and H are equipped with electrical interlock contacts, such as D-l, D-2, E-l, H1 and 1-1-2. Contacts E1 control the pick-up of a variable delay pickup relay set, in the embodiment shown in Fig. 3, to close its front contact 91 five seconds after energization of the relay, although any desired time setting may be used. Thus, relay 90 will not operate upon the usual fluctuations of valve E in controlling the speed of turbine 40. However, if valve E closes and remains closed for more than the preset time, relay 90 will pick up and close contact 91.

Contacts D1, 91 and H-1 control the energization of a control circuit 95 which, when energized, operates solenoid valve 85. Circuit 95 also controls energization of solenoid operators, magnetic contactors, or the like, indicated at 92 on the two right hand pulverizers, to open the main control switches of these two pulverizers and thus cut out the two right hand pulverizers. Operation of solenoid valve 85 connects pressure air, regulated at about 20#, to conduit 82 and thus to the space between diaphragms 71 and 72 of the two left hand relays 75. This effects actuation of relays 75 to decrease the pressure in the spaces below diaphragms 76, in turn effecting actuation of pilot valve assemblies 77 in such a way as to control operators 80 to move the primary air dampers to the minimum stop positions of the two left hand pulverizers 25.

Thus, if valves D or H close, upon a predetermined over-speeding of their associated turbines, or if valve E remains closed for longer than a predetermined time, for example five seconds, control circuit 95 is energized. This immediately cuts out the two right hand pulverizers 25 and initiates an immediate reduction, to a predetermined low value, of the firing rates of the other two pulverizers. Accordingly, the firing rate of boiler is reduced, in direct response to closure of valves D, E or H, to about one-fourth its full rate or, in the present example, to less than 280,000 lbs./ hr. Thus overheating of the reheater is prevented, while the boiler is maintained in firing condition for immediate resumption of load.

In accordance with standard practice, the two generators 41 and 51 are connected to main bus 52 through circuit breakers CB-i and CB2. Each of the circuit breakers is controlled by a delayed action relay 93 which relays, in the usual case, are arranged to open the circuit breakers substantially five minutes after energization of the relays. Such energization is effected through a control circuit 96, which is energized by contacts D-2 or 1-1-2, closed when either valve D or H closes upon turbine overspeeding or, in the case of valve H, by loss of vacuum in condenser 55.

Fig. 4 schematically illustrates another manner in which the control system and method of the invention may operate to rapidly reduce the firing rate of boiler 10 in response to closure of valves D, E and H. In this arrangement, contacts D-1 and H-1 control the energization of delayed action relays 100 and 105, respectively, and contacts D-2 and H-2 control circuit 96 in the same manner as in Fig. 3. Relays 100, 105, as well as relay 90, are adjustable delayed action relays set, in the arrangement of Fig. 4, to close their front contacts 101, 106 and 91 substantially fifteen seconds after energization of the relays, although other time settings may be used for different conditions. Contacts 91, 101, 106 control energization of control circuit 95 to operate devices 92 to cut out the two right hand pulverizers 25.

Operation of the system of Fig. 4 is as follows. Upon closure of valves D, E or H, the resultant change in the steam pressure in main 18 effects operation of controller 60 and its associated elements to reduce the firing rate of all four pulverizers to a predetermined low rate in the same manner as previously described. This reduction takes a predeterminable time interval, and relays 90, 100 and 105 are set to close their contacts 91, 101 and 106 at the end of this time interval. Thus, after the controller 60 has reduced all pulverizers to such predetermined rate, half the pulverizers are cut out by the action of the relays through control circuit 95.

If the reduction in firing rate is not effected at the desired rate by operation of the controller 60 in the usual manner, the control arrangement including solenoid valve 85, shown for the two left hand pulverizers in Fig. 3, may be applied to all four relays 75. In such case, valve 85 would be energized over a control circuit closed directly by contacts D-1 and H-1 and, through an adjustable time delay relay, by contacts E1. Such arrangement effects a more rapid reduction in the firing rate of all pulverizers, followed by cutting out of substantially half the pulverizers.

It will thus be understood that the novel control system and operating of the invention provides for substantially increased continuity of operation which in turn results in an increased overall efiiciency of the heat cycle. Particularly, boiler 10 is not shut-down upon loss of electrical load, but is reduced to a firing rate after a predetermined time interval following closure of valves due to loss of electrical load. The boiler may be maintained at this minimum firing rate until the condition is found and corrected, if correction is possible, without the necessity of restarting the boiler from a substantially cold condition. Should the trouble persist for a period longer than five minutes, for example, the electrical load is cut-off the plant. While specific time adjustments for relays 90, and have been mentioned by way of example, variations in such time settings are possible and within the scope of the invention.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles thereof, it will be understood that the invention may be otherwise embodied without departing from such principles.

We claim:

1. In combination with a vapor generator, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said vapor generator and reheating, by the heating gases of said vapor generator, exhaust vapor received from said first prime mover, and a second prime mover operated by reheated vapor from said reheater, said vapor generator being supplied with heating gases from a plurality of combustion devices, a control system comprising, in combination, control means governing admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; control means governing admission of reheated vapor to said second prime mover from said reheater and operable to interrupt the supply of vapor thereto; and control mechanism directly operable by either of said control means, responsive to a vapor flow interruption by the latter, to interrupt the operation of a preselected proportion less than the whole number, of said combustion devices.

2. In combination with a vapor generator, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said vapor generator and reheating, by the heating gases of said vapor generator, exhaust vapor received from said first prime mover, and a second prime mover operated by reheated vapor from said reheater, said vapor generator being supplied with heating gases from a plurality of adjustable combustion devices, a control system comprising, in combination, control means governing admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; control means governing admission of reheated vapor to said second prime mover from said reheater and operable to interrupt the supply of vapor thereto; and control mechanism directly operable by either of said control means, responsive to a vapor fiow interruption by the latter to interrupt the operation of a preselected proportion, less than the whole number, of said combustion devices and to adjust the rate of 1operation of the remaining devices to a predetermined va ue.

3. In combination with a vapor generator, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said vapor generator and reheating, by the heating gases of said vapor generator, exhaust vapor received from said first prime mover, and a second prime mover operated by reheated vapor from saidreheater, said vapor generator being supplied with heating gases from a plurality of adjustable combustion devices, a control system comprising, in combination, control means governing admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; control means governing admission of reheated vapor to said second prime mover from said reheater and operable to interrupt the supply of vapor thereto; and control mechanism directly operable by either of said control means, responsive to a vapor flow interruption by the latter, to immediately interrupt the operation of a preselected proportion, less than the whole number, of said combustion devices and to simultaneously adjust the rate of operation of the remaining devices to a predetermined value.

4. In combination with a vapor generator, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said vapor generator and reheating, by the heating gases of said vapor generator, exhaust vapor received from said first prime mover, and

a second prime mover operated by reheated vapor from said reheater, said vapor generator being supplied with heating gases from a plurality of adjustable combustion devices, a control system comprising, in combination, first control means controlling admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; second control means governing admission of reheated vapor to said second prime mover from said reheater and operable to interrupt the supply of vapor thereto; control mechanism directly operable by either of said control means, responsive to a vapor flow interruption by the latter, to adjust the rate of operation of said combustion devices to a predetermined value and, after such adjustment to interrupt the operation of a preselected proportion, less than the whole number, of said devices.

5. In combination with a vapor generator, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said vapor generator and reheating, by the heating gases of said vapor generator, exhaust vapor received from said first prime mover, a second prime mover operated by reheated vapor from said reheater, and vapor pressure responsive means controlling the firing rate of said vapor generator, said vapor generator being supplied with heating gases from a plurality of adjustable combustion devices, a rapid override control system comprising, in combination, an interceptor valve and a speed control valve serially controlling admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; a second interceptor valve governing admission of reheated vapor to said second prime mover from said reheater and operable to interrupt the supply of vapor thereto; and control mechanism directly operable by any of said valves, responsive to a vapor flow interruption by the latter, to interrupt the operation of a preselected proportion, less than the whole number, of said combustion devices and to effect operation of said pressure responsive means to adjust the rate of operation of the remaining devices to a predetermined value.

6. In combination with a vapor generator, said vapor generator being supplied with heating gases from a plurality of adjustable combustion devices, a first prime mover operated by vapor generated by said vapor generator, a reheater included in said generator and utilizing such heating gases to reheat exhaust vapor from said said first prime mover, a second prime mover operated by reheated vapor from said reheater, control devices governing the adjustments of said combustion devices, and a vapor pressure responsive controller governing operation of said control devices; an overriding rapid action control system comprising, in combination, an interceptor valve and a speed control valve serially controlling admission of vapor to said first prime mover from said vapor generator and operable to interrupt the supply of vapor thereto; a second interceptor valve governing admission of reheated vapor to said second prime mover and operable to interrupt the supply of vapor thereto; control mechanism directly operable by any of said valves, responsive to a vapor flow interruption thereby, to interrupt the operation of a preselected proportion, less than the whole number, of said combustion devices and to effect operation of said control devices, independently of said vapor pressure responsive controller, to rapidly adjust the rate of operation of the remaining combustion devices to a predetermined value to reduce the supply of heating gases sufiiciently fast to avoid excessive temperature rises in said reheater when the flow of exhaust vapor therethrough is interrupted.

References Cited in the file of this patent UNITED STATES PATENTS 853,808 Lemp May 14, 1907 1,610,034 Bowerbank Dec. 7, 1926 1,648,343 Hartmann Nov. 8, 1927 1,769,457 Powell July 1, 1930 2,116,587 Toensfeldt May 10, 1938 2,184,224 Lucke Dec. 19, 1939 2,335,655 Dickey Nov. 30, 1943 2,467,092 Ostermann Apr. 12, 1949

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