US3908686A

US3908686A - Pressure control for variable pressure monotube boiler

Info

Publication number: US3908686A
Application number: US444918A
Authority: US
Inventors: J Warne Carter; Jr J Warne Carter
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-02-22
Filing date: 1974-02-22
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

An apparatus for regulating the pressure in a monotube boiler in accordance with the changes in the pressure drop across the associated throttling valve. A reversible motor which drives a rotary air-and-fuel control member is reversibly energized in response to movement of a piston in a cylinder connected across the throttling valve. The piston actuates a lever extending between a pair of respective motorenergizing microswitches carried on a swinging bracket which is coupled to the air-andfuel control member. The cylinder has a fixed second piston, relative to which the cylinder is movable in response to excessive boiler tube pressure, this cylinder movement acting to energize the motor in a direction to reduce the supply of fuel and air and thereby bring down the boiler tube pressure.

Description

United States Patent Carter et al.

1 51 Sept. 30, 1975 Primary E.\'aminer-Robert G. Nilson ABSTRACT An apparatus for regulating the pressure in a monotube boiler in accordance with the changes in the pressure drop across the associated throttling valve. A reversible motor which drives a rotary air-and-fuel control member is reversibly energized in response to movement of a piston in a cylinder connected across the throttling valve. The piston actuates a lever ex tending between a pair of respective motorenergizing microswitches carried on a swinging bracket which is coupled to the air-and-fuel control member. The cylinder has a fixed second piston, relative to which the cylinder is movable in response to excessive boiler tube pressure, this cylinder movement acting to energize the motor in a direction to reduce the supply of fuel and air and thereby bring down the boiler tube 12 Claims, -6 Drawing Figures [5 PRESSURE CONTROL FOR VARIABLE PRESSURE MONOTUBE BOILER [76] Inventors: J. Warne Carter, 2206 Weeks Pk. Ln., Wichita Falls, Tex. 76308; J. Warne Carter, Jr., Neville Apt. No. [571 8, Burkburnett, Tex. 76354 [22] Filed: Feb. 22, 1974 [2]] Appl. N0.: 444,918

[52] US. Cl 137/94; 60/664; 122/448 R [51] Int. Cl. ..F01D 17/08 [58] Field of Search 122/448 R, 448 S; 126/351; 137/94; 236/25 A; 60/664 [56] References Cited UNITED STATES PATENTS 2,079,165 5/1937 Gorrie 122/448 12' x 2,081,948 6/1937 Michel et al 1 1 122/448 S 2,143,820 l/l939 Payn 1 1 137/94 X 2,777,513 1/1957 Cooper 137/94 X pressure.

FOREIGN PATENTS OR APPLICATIONS 686,483 l/l940 Germany 137/94 724077-25 Muygf 68 ExPn/vae'e U.S. Patent Sept. 30,1975 Sheet 2 of2 3,908,686

PRESSURE CONTROL FOR VARIABLE PRESSURE MONOTUBE BOILER This invention relates to control apparatus for steam power systems, and more particularly to apparatus for regulating the pressure in a water tube steam boiler of the variable pressure type.

A main object of the invention is to provide a novel and improved control system for regulating the pressure in a monotube boiler of the variable pressure type, the system involving relatively simple components, being reliable in operation, and providing a high degree of overall thermal efficiency by reducing throttling of the steam across the main throttle valve of the system.

A further object of the invention is to provide an improved fuel-and-air control apparatus for a monotube boiler of the variable pressure type, the apparatus operating to provide a relatively narrowly controlled definite pressure drop across the throttle valve associated with the boiler by utilizing the pressure drop across the throttle valve to control the fuel firing rate of the boiler, the apparatus involving relatively inexpensive components, being easy to install, being operable over relatively long periods of time with a minimum amount of human supervision, and providing a substantial improvement in overall thermal efficiency of the system by minimizing the amount of throttling of the steam across the throttle valve.

A still further object of the invention is to provide an improved pressure control system for a monotube boiler of the variable pressure type, the apparatus acting to reduce the stress on the parts of the system such as the boiler tube, water pump and packing, and the like, by preventing the boiler from operating continuously at maximum pressure, the apparatus being highly sensitive to changes in pressure across the throttle valve of the system, and operating rapidly to control the pressure in response to such changes, whereby to maintain the pressure drop across the throttle valve within specified limits over a wide range of load conditions, and the system being arranged to shut off the fuel flow automatically when the boiler pressure becomes excessive.

A still further object of the invention is to provide an improved control device for use with a variable pressure monotube boiler of a type intended for use as the power plant of a vehicle, the control apparatus being relatively compact in size, having relatively fewparts, and acting to automatically allow changes of pressure differential across the throttle valve of the system at different power settings, thereby providing good throttle response for low pressure settings, for example, in urban driving, and also providing good efficiency for cruise power settings.

Further objects and advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings, wherein:

FIG. 1 is an end elevational view, partly in cross section, of a fuel-and-air regulating apparatus for a monotube boiler constructed in accordance with the present invention, said view being taken substantially on the line 1-1 of FIG. 2.

FIG. 2 is a front elevational view, partly in vertical cross-section, of the apparatus of FIG. 1, said view being taken substantially on the line 22 of FIG. 1.

FIG. 3 is an enlarged longitudinal cross-sectional view taken through the pressure-responsive cylinder assembly employed with the apparatus of FIGS. 1 and 2, said view being taken substantially on the line 3-3 of FIG. 2.

FIG. 4 is an enlarged fragmentary front elevational view of an end portion of the pressure-responsive cylinder assembly showing the securement through the main supporting bracket of one of the piston rods employed in the pressure-responsive view assembly, said view being taken substantially on the line 4-4 of FIG. 1.

FIG. 5 is a schematic diagram, diagrammatically illustrating the locations of the conduit connections associated with the control apparatus with respect to the main throttle valve of the system with which the apparatus of FIGS. 1 to 4 is employed.

FIG. 6 is a schematic diagram showing the electrical connections in a typical installation involving the apparatus of FIGS. 1 to 4.

A steam power system employing a variable pressure boiler, for example, a boiler of the monotube type, has several important advantages over a constant pressure boiler, but also has problems which must be overcome if it is to be successfully used for an application requiring the ability to quickly and effectively respond to a wide range of changes in loading, for example, when the system is employed as the power plant for an automobile. The major advantage of a variable pressure boiler are the following:

1. There is an increase in overall thermal efficiency.

In a constant pressure boiler it is necessary to operate continuously at the maximum pressure that would be ever required by the steam expander, and therefore when the expander is operating at a low power output, the main control valve must throttle a relatively large pressure drop, or a control means must be provided to vary the engine cut-off, neither of which is very efficient.

2. There is less stress on the boiler tube, the water pump and the associated packing, because the boiler is not operated continuously at the maximum pressure.

The main disadvantage heretofore encountered with the use of a variable pressure boiler is the delay in throttle response. There is a delay between the time the throttle or fuel firing rate is increased and the time that the boiler pressure will start to increase. Incomparison, in a constant pressure boiler the pressure isalready present, and therefore a certain amount of stored energy is available in the boiler, although not a very great amount since only a relatively small amount of water is present in the entire boiler, for example, of the order of one quart in a typical installation of the scale contemplated by the present invention.

The primary concept of the present invention is to narrowly control the pressure drop across the throttle valve in a manner such that substantially instantaneous throttle response is provided, this pressure drop being employed to control the fuel firing rate.

To increase the pressure, the fuel flow rate may be increased, and at the same time, the waterflow rate may be increased. For example, in a typical embodiment of the present invention illustrated diagrammatically in FIG. 5, there may be a pressure drop of the order of 250 pounds as? square inch across the throttle valve 11 when the fab] flow is low, namely, when the expander; showri ill l2, is in an idling condition. This pressure al b proximately llBBllhds per square inch when the'fuel flow is at a ill'tlillllilllh: In the apparatus, presently to be tll'ibfe ases substantially linearly to apdescribed, as the throttle valve 11 is opened. the pressure across this valve decreases and this decrease in pressure is employed to operate a spring-loaded piston, which in turn is suitably linked to increase the fuel firing rate. The apparatus includes a feed back" mechanism which senses and controls the rate at which the fuel firing rate is increased or decreased. A small decrease in pressure differential across the throttle valve 11 results in a small increase in the fuel firing rate, whereas a large decrease in said pressure differential will result in a large increase in the fuel firing rate. The reverse action occurs for increase in said differential pressure.

The apparatus presently to be described also incorporates a novel fail-safe device, being a second spring'loaded piston which starts to move when the boiler pressure becomes excessive, for example, at approximately 1,900 pounds per square inch in the typical embodiment considered herein, and which acts to shut off the fuel flow. By the time the boiler pressure has reached an upper safe limit, (for example, 2,100 pounds per square inch in the typical embodiment herein illustrated and described,) the fuel flow is reduced to its minimum amount and is then shut off. This fail-safe (or auxiliary pressure response device) is arranged and interlinked with the main piston device in a unique manner such that it allows a predetermined, measurable linear movement (stroke) of the main piston while allowing identical linear travel for the auxiliary pressure response device independently one with respect to the other, but not allowing simultaneous movement of both.

As will be presently seen, with the control system of the present invention a variable pressure boiler system of inherent reliability can be successfully employed, the system including the required pressure differential across the throttle valve necessary for instant throttle response. Furthermore, as will be presently explained, the apparatus may be adjusted to provide a larger pressure differential across the throttle valve for urban and stop-and-go driving, and a lower pressure differential across the throttle valve for highway cruise conditions.

Referring to the drawings, 13 generally designates a boiler suitably mounted on a vehicle, said boiler including a boiler tube 14. Secured to one end of the boiler 13 is a supporting plate 15 on which is mounted a rotary air damper member 16 for controlling the supply of combustion air to the boiler. For example, the member 16 may comprise a drum formed with an aperture in its periphery, shown at 17, which cooperates with a concentrically arranged inner fixed drum member 18 having an aperture 19 arranged so that it can register with the aperture 17 and also arranged so that its degree of registering may be varied as member 16 rotates. Thus, with the configuration illustrated in FIGS. 1 and 2, the supply of combustion air to the boiler decreases when the member 16 rotates in a clockwise direction, as viewed in FIG. 2, whereas the air supply increases when the member 16 rotates in a counterclockwise direction.

Mounted on the peripheral portion of the member 16 is a fuel control cam 20 having a suitably contoured edge 21, which is engaged by a roller 22 journaled on the end of a follower arm 23 which is rotatably mounted on a shaft 24 and which is provided with a cable guide pulley 25 around which extends a fuel con-- to the arm 23 and the other end being connected to a movable fuel control valve element contained in the boiler fuel valve assembly, shown at 28. The fuel supply rate control mechanism thus described is arranged to increase the fuel rate simultaneously with an increase of boiler combustion air, namely, with counterclockwise rotation of the member 16, and conversely to decrease the fuel supply rate simultaneously with clockwise rotation of air damper member 16. The damper member for purposes herein is described as being operated electrically. However, any equivalent operating means may be employed for this purpose, such as an electro-mechanical means, a hydraulic means, a pneumatic means or a direct mechanical means, within the contemplation of the present invention.

In the typical example disclosed herein the damper member 16 is operated by a reversible electric motor 29, whose shaft is drivingly connected to a pulley element 30 around which is engaged an endless belt 31 which is drivingly engaged in a peripheral groove 32 provided on air damper member 16, as shown in FIG. 1. Motor 29 may be energized from a suitable source of current, such as a battery 33, respective directional windings of the motor being connected to-said battery through opposite forward and

reverse microswitches

34 and 35, which are operated in a manner presently to be described.

Generally designated at 36 is a pressure-responsive valve assembly comprising a cylindrical main body 37 which is slidably engaged with a conformably shaped guide member 38 secured on the plate member 15. The left end portion of the cylindrical body 37, as viewed in FIG. 2, is provided with a cup-shaped end plug element 39 which is threadably secured in the left-end portion of the cylinder body 37 and which is formed with a central aperture 40 through which slidably extends a piston rod 41 whose outer end portion is adjustably secured to an upstanding lug 42 formed integrally with the plate member 15 and extending perpendicularly thereto, as is clearly shown in FIG. 4. The rod 41 extends through an aperture in the lug 42 and is adjustably secured to the lug by

opposite clamping nuts

43, 43 threadably engaged on the end portion of rod 41 on opposite sides of lug 42.

The right-end portion of the cylinder assembly 36, as viewed in FIG. 2, comprises a cylindrical segment 44 having a reduced left-end portion 45, as viewed in FIG. 2, which is threadably secured in the right-end portion of the main cylinder body 37. The segment 44 is generally cup-shaped and has a relatively thick left-end wall portion 46 and is provided with a cover plug 47 threadably engaged in its right-end portion, as viewed in FIG. 2. The left-end portion of the segment 44 has a main axial bore 48 in which is sealingly and slidably disposed the main piston element 49.

Piston element 49 is formed with the axial hollow boss to which is rigidly connected an axially extending rod 50 which extends sealingly and slidably -through the central portion of end plug 47. The boss 100 is received in a flanged bearing cup 101. Plug 47 is formed with an annular seat 102. A relatively heavy coiled spring 61 is interposed between end plug 47 and cup 101, bearing at its right end on the seat 102 and at its left end on the flange of cup 101, as viewed in FIG. 2, biasing the piston element 49 leftwardly.

Rigidly secured on rod 41 is a relatively small piston 53 which extends slidably and sealingly through a central bore 52 in wall portion 46 and which is loosely received in the axial bore of piston element 49. Wall portion 46 is formed with an integral collar element 103 defining an annular seat 104 therearound. A flanged annular bearing cup member 105 is provided on the rod 41, the flange portion: thereof being received in the seat 104. A spherical bearing washer 106 is provided between the cup member 105 and the left end of piston element 53, as viewed in FIG. 2.

A coiled spring 57 surrounds rod 41 and bears between end plug 39 and the flange of cup member 105, exerting leftward biasing force on cylinder body 37 relative to the fixed rod 41.

Piston element 49 is formed with an annular collar element 107 in which is diametrically secured a transverse pin 108 which extends through a longitudinal slot 109 formed in piston 53, as shown in FIG. 3, thus limiting the axial movement of piston 49 relative to piston 53.

The spaces on the opposite sides of the main piston element 49 are connected across the throttle valve 11. Thus, a conduit 65 connects the outlet end of the boiler tube 14 to the space at the left-side of piston element 49, as viewed in FIG. 2, and another conduit 67 connects the conduit 68 leading from the throttle valve 11 to the expander 12 to the space at the right side of the piston element 49, namely, to the interior of the hollow segment 44. Thus, the pressure differential across the

conduits

65 and 67 acts on the piston element 49 in a manner tending to move it rightwardly in the bore 48 against the opposing force exerted by coil spring 61. Likewise. the boiler pressure acting through the conduit 65 is exerted between the main piston 49 and the smaller piston element 53 and acts to move the entire cylinder assembly rightwardly relative to the fixed rod member 41 against the biasing force of the coil spring 57.

Rigidly secured to the main supporting plate upwardly and rightwardly adjacent the cylinder segment 44 and extending transverse thereto is a post member 70. Thus, as shown in FIG. 1, the post member 70 may be provided with a reduced end stud 71 which extends through an aperture in plate 15 and is clamped thereto by a clamping nut 72. Rotatably and supportingly mounted on post member 70 is a sleeve member 73 to which is rigidly secured a generally triangular bracket member 74 to the opposite lower corner portion of which are secured the opposing

microswitches

34 and 35. Rotatably mounted on the sleeve member 73 is .another sleeve member 75 to which is rigidly secured a depending spring arm 76 which extends between the inwardly facing operating plunger elements of the

microswitches

34 and 35 and which depends sufficiently downwardly so as to be engaged by the outer end portion of the rod element 50, as shown in FIG. 2, the arm 76 being biased against the outer end portion of the rod 50 by a coil spring 77. One end of spring 77 is anchored to plate 15 and the other end of said spring comprises an arm 78 which bears against arm 76 and biases arm 76 in a clockwise direction, as viewed in FIG. 2.

Axially secured to the left-end portion of post member 70 is a pivot bolt 79 on which is rotatably engaged a collar element 80 which is received in the left-end portion of sleeve member 73. A cup-shaped member 81 receives the left-end portion of sleeve member 73 and collar element 80 and is rigidly secured thereto by a fastening screw 82. Rigidly secured to the cupshaped member 81 is an arm 83. Arm 83 is thus rigidly connected to sleeve member 73 and is rotatable relative to the fixed post member 70.

A follower extension arm element 84 is adjustably secured to the arm 83 by the provision of a longitudinal slot 85 in arm 83 and a pair of clamping screws 86, 86 engaged through slot 85 and threadably engaged with arm 84, whereby arm 84 may be adjustably secured longitudinally relative to arm 83. The upper end of arm element 84 is provided with a pin 88 slidably and rotatably engaging in a radial slot 89 provided in the circular face of member 16.

It will thus be seen that rotation of the member 16 will generate a camming action between slot 89 and pin 88 transmitted by the

connected arm segments

84 and 83 to the sleeve member 73, which will in turn swing the bracket member 74 around the pivot post in the same direction as the

arm segments

83 and 84. This changes the position of the opposing

microswitches

34 and 35, but the swinging movement of the depending arm 76 extending therebetween is produced only by the action of the piston rod element 50, which is in turn produced by the pressure differential changes across the throttle valve 11 acting on piston element 49 against the biasing force of the spring 61.

In operation, when the pressure differential across the throttle valve 11, namely, the differential across the

conduits

65 and 67 becomes great enough to overcome the'force exerted by the coil spring 61, the piston 49 will move rightwardly, as viewed in FIG. 2, and compress spring 61 further until the force exerted by the spring and the force on the piston are equal. This piston movement causes rod 50 to move arm 76 rightwardly,

as viewed in FIG. 2, and thereby close microswitch 35. This energizes motor 29 in a direction to rotate member 16 clockwise, as viewed in FIG. 1, namely, in a direction to reduce the area of the air damper aperture defined by the overlapping

openings

17 and 19 and which also reduces the fuel input rate to the boiler. This clockwise rotation of member 16 acts to rotate the arm 84-83 in a counterclockwise direction, as viewed in FIG. 2, and thus moves the bracket element 74 in a direction away from arm 76 until microswitch 35 is allowed to open and thereby deenergize motor 29. A reverse action occurs when the pressure differential decreases, allowing spring 61 to push piston 49 leftwardly, the arm 76 following the leftward movement of the rod 50 by the action of the spring 77, whereby switch 34 closes and energizes motor 29 in a direction to rotate member 16 counterclockwise, as viewed in FIG. 2, to thereby increase the air damper aperture and the fuel input rate. The apparatus will, therefore, act to stabilize the pressure differential across the throttle valve 11 under various loading conditions and over a predetermined pressure differential range.

It will be noted that a small movement of piston 49 produces a relatively small change in the damper opening, whereas a large movement of said piston produces a large change in the damper opening. The change in the damper opening is thus proportioned in accordance with the amount of movement of the piston element 49.

It will be noted that the feed-back ratio, namely, the magnitude of response to changes in pressure differential across throttle valve 11, can be regulated by adjusting the position of the extension element 84 with respect to the arm 83, namely, by adjusting the effective length of the composite arm 84-83.

The apparatus also operates to prevent an excessive pressure build-up in the boiler tube 14. Thus, when the steam pressure in the conduit 65, connected to the outlet end of boiler tube 14, becomes high enough (for example, about 1,900 pounds per square inch in a typical embodiment) to overcome the force of the coil spring 57, the cylinder body 37 moves rightwardly, as viewed in FIG. 2, relative to the piston 53, and compresses spring 57 further until the force exerted by the spring and the force on the piston 53 are equal. This movement of the cylinder 36 causes rod 50 to swing arm 76 counterclockwise, as viewed in FIG. 2, to close microswitch 35 and to produce the action described above, namely, movement of member 16 in a direction to close the air damper aperture and reduce the fuel input rate. The apparatus responds rapidly enough so that by the time the boiler pressure reaches its upper safe limiting value of pressure, for example, 2,100 pounds per square inch in the typical embodiment above mentioned, the air damper aperture becomes completely closed and the fuel input rate is reduced sufficiently to allow the boiler pressure to decrease to a safe value. When the boiler pressure has returned substantially to its normal operating pressure value, the apparatus will stabilize at a normal operating pressure.

From the above description, it will therefore be seen that the automatic control apparatus above described acts to substantially maintain a specified pressure differential across the throttle valve 11 over a wide range of operating conditions and also acts to prevent an excessive build-up of boiler pressure. At different power settings, the pressure differential allowed across the throttle valve is automatically changed so that the apparatus provides good throttle response for low pres sure settings (for example, in urban driving) and good efficiency for cruise power settings.

While a specific embodiment of an improved system for regulating the pressure in a water tube steam boiler has been disclosed in the foregoing description, it will be understood that various modifications within the spirit of the invention may occur to those skilled in the art. Therefore, it is intended that no limitations be placed on the invention except as defined by the scope of the appended claims.

What is claimed is:

1. In combination, a variable pressure steam boiler, combustion fuel control valve means connected to the boiler to supply a controlled rate of combustible fuel thereto, variable control throttle valve means having different power settings connected to the output of the boiler to control the pressure on a utilizing device supplied by the boiler, pressure differential-sensing means connected across said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means.

2. The structural combination of claim 1, and an adjustable air damper member, and means simultaneously operating said fuel control valve means and said air damper member.

3. The structural combination of claim 1, and wherein said fuel control valve means is provided with a rotary control member and the means operatively coupling said pressure differential-sensing means to said fuel control valve means comprises interengaging cam means driven by said sensing means and acting on said valve means.

4. The structural combination of claim 1, and wherein said pressure differential-sensing means comprises a cylinder having a movable piston, biasing means acting on said piston, and conduit means connecting the spaces on opposite sides of the piston across said throttle valve means.

5. The structural combination of claim 1, and wherein said fuel control valve means includes an adjustable air damper member.

6. The structural combination of claim 1, and wherein the means operatively coupling said pressure differential-sensing means to said fuel control valve means comprises reversible electric motor means drivingly coupled to said fuel control valve means, and means toreversibly energize said electric motor means in accordance with changes in the pressure differential across the throttle valve means sensed by said sensing means.

7. The structural combination of claim 1, and auxil iary pressure-responsive means connected to the boiler, and means operatively coupling said auxiliary pressure-responsive means to said fuel control valve means to proportionally reduce the fuel flow as the boiler pressure approaches a predetermined safe value.

8. In combination, a steam boiler, combustion fuel control valve means connected to the boiler to supply a controlled rate of combustible fuel thereto, throttle valve means connected to the output of the boiler, pressure differential-sensing means connected across said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means, wherein said pressure differential-sensing means comprises a cylinder having a movable piston, biasing means acting on said piston, and conduit means connecting the spaces on opposite sides of the piston across said throttle valve means, means movably axially supporting said cylinder, and means moving said cylinder in a direction to reduce fuel flow when the boiler pressure rises above a predetermined safe value.

9. The structural combination of claim 8, and wherein the means axially supporting said cylinder includes a fixed piston rod having a small piston axially aligned with and loosely received in an axial bore in said first-named piston, the space on one side of the first-named piston including the space between said small piston and said axial bore.

10. The structural combination of claim 9, and wherein said first-named piston has an annular collar surrounding said small piston which is provided with a transverse pin, said small piston having a longitudinal slot receiving said transverse pin, whereby to limit axial movement of said first-named piston relative to said small piston.

11. In combination, a steam boiler, combustion fuel control valve means connected to the boiler to supply a controlled rate of combustible fuel thereto, throttle valve means connected to the output of the boiler, pressure differential-sensing means connected across said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means, wherein said pressure differential-sensing means comprises a cylinder having a movable piston, biasing means acting on said piston, and conduit means connecting the spaces on opposite sides of the piston across said throttle valve means, and means supporting said cylinder for axial movement,

said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means, and wherein said fuel control valve means comprises a rotatably mounted adjustable air damper member provided with a fuel control cam and a stationary fuel valve having a rotary control arm engaging said control cam.

l= i =l

Claims

6. The structural combination of claim 1, and wherein the means operatively coupling said pressure differential-sensing means to said fuel control valve means comprises reversible electric motor means drivingly coupled to said fuel control valve means, and means to reversibly energize said electric motor means in accordance with changes in the pressure differential across the throttle valve means sensed by said sensing means.

7. The structural combination of claim 1, and auxiliary pressure-responsive means connected to the boiler, and means operatively coupling said auxiliary pressure-responsive means to said fuel control valve means to proportionally reduce the fuel flow as the boiler pressure approaches a predetermined safe value.

11. In combination, a steam boiler, combustion fuel control valve means connected to the boiler to supply a controlled rate of combustible fuel thereto, throttle valve means connected to the output of the boiler, pressure differential-sensing means connected across said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means, wherein said pressure differential-sensing means comprises a cylinder having a movable piston, biasing means acting on said piston, and conduit means connecting the spaces on opposite sides of the piston across said throttle valve means, and means supporting said cylinder for axial movement, means drivingly coupling said piston to said fuel control valve means, and means moving said cylinder in a direction to reduce fuel flow when the boiler pressure rises above a predetermined safe value.

12. In combination, a steam boiler, combustion fuel control valve means connected to the boiler to supply a controlled rate of combustible fuel thereto, throttle valve means connected to the output of the boiler, pressure differential-sensing means connected across said throttle valve means, and means operatively coupling said pressure differential-sensing means to said fuel control valve means, and wherein said fuel control valve means comprises a rotatably mounted adjustable air damper member provided with a fuel control cam and a stationary fuel valve having a rotary control arm engaging said control cam.