US3027137A

US3027137A - Control mechanism for operating steam turbines under partial load with full arc admission

Info

Publication number: US3027137A
Application number: US843585A
Authority: US
Inventors: Markus A Eggenberger
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1958-09-30
Filing date: 1959-09-30
Publication date: 1962-03-27
Anticipated expiration: 1979-03-27

Description

March 1952 M. A. EGGENBERGER 3,027,137

CONTROL. MECHANISM FOR OPERATING STEAM TURBINES UNDER PARTIAL LOAD WITH FULL ARC ADMISSION Filed Sept. 50, 1959 F: G. l. /7 5% 2a w L Lowo 22 if H/G/f 0 7/ Z 27 51.71 n

AUX- o MAIN L0AD U /4 t'Mfflfif/VC) I SYNC. A? m/ mzvs LOW- HT AUX. 32 L SYNCH.

25 Z4 Fl 6. 2. LOW- Z 22 H/El-l- 0 Z! 27 a a a JJZ/ a? Y 74 39' a 2a VJ W AUX. T N 7 MA/NLUAD EMERGENCY m/P VALVE INVENTOR wem HIS ATTORNEY uwr United States Patent C) M 3,027,137 CONTROL MECHANESM 150R ()PERATILNG STEAM TURBINES UNDER PARTIAL LQAD WITH FULL ARC ADMISSEGN Markus A. Eggenherger, Schenectady, N.Y., assiguor to General Electric Company, a corporation of New York Filed Sept. 36, 1959, Eier. No. 843,585 4 Claims. (Cl. 253--59) This invention relates to an arrangement for starting and loading a steam turbine with full arc admission, and more particularly it relates to an improved mechanism for positioning the turbine emergency stop valve to control the speed of the turbine under partial load.

in modern high temperature, high pressure steam turbine powerplants, the conventional arrangement for controlling the admission of steam to the high pressure turbine is by the use of a number of control valves operated in sequence from a common actuator to admit steam to separate nozzles. The nozzles discharge through arcuate openings placed on circumferentially spaced arcs or sectors arranged ahead of the first stage turbine blading. Each control valve, therefore, controls the steam flow to a different location on the casing circumference. Such an arrangement is well known in the art but an example of a nozzlebox showing the general shape of the arcuate openings on discharge side of the nozzleboxes is shown in Patent No. 2,380,237, issued to E. E. Harris on July 10, 1945, and assigned to the assignee of the present application.

A longstanding problem with this arrangement has been that, when the first control valve is opened, admitting steam to only one circumferential portion of the casing served by that control valve, severe thermal stresses may be created in the massive casing. As steam temperatures have increased, and as heavier casings have been employed to contain the higher pressures, this condition has become of greater consequence, and it has been suggested to initiate turbine startup using full arc admission or, in other words, by admitting steam to all the arcs at once while the turbine is brought up to full speed.

In addition to the control valves mentioned above, the conventional turbine valve arrangement also includes an emergency stop valve, placed upstream of the control valves, and which has a capacity sufficient to pass the steam flow required for full load. This stop valve is generally designed for open-shut rather than throttling" operation and is ordinarily operated by a hydraulic cylinder, Emergency overspeed governors are employed to trip the stop valve so that it will close in the event of overspeed or malfunction of the control valves. rrangements have also been used previously for bringing the turbine up to full speed at no load by manually manipulating the stop valve while all the control valves are open so as to introduce steam to all of the nozzles simultaneously.

In the arrangements for bringing the turbine to rated speed at no load using full arc admission. where the single stop valve has been used to control the admission to all the arcs at one time, the problems of speed control through manipulation of the valve are not severe, since the valve opening must merely provide for friction and energy losses in the turbine itself. Upon placing a load 3,027,137 Patented Mar. 27, 1962 on the turbine, however, the control problems become more difficult.

Elementary considerations of turbine governing dictate that, as the turbine comes up to speed at no load, the valve opening will be substantially constant corresponding to the energy and friction losses in the turbine at rated speed under no load. As the turbine is loaded, however, it will have a tendency to slow down, which fact will be sensed by the governor and translated to a wider valve opening. Thus the speed may be maintained substantially at the set speed as the load is applied. The degree of governor movement required to effect a given valve movement is termed the regulation of the system, a broad regulation representing a relatively large governor movement and a narrow regulation representing a relatively small governor movement for a given change in load. This rep resents a greatly simplified explanation of how the turbine is normally loaded by means of the sequentially operated control valves.

it full arc admission is contemplated through the use of the stop valve as a throttling device, the progressive addition of a load on the turbine dictates that the stop valve be progressively opened as load is applied in order to maintain the rated speed. When the turbine casing has come up to temperature, however, and it is desired to revert to the use of the control valves, the control system used on the stop valve must not interfere with normal control by means of the control valves.

In this respect, it is desirable that the stop valve be positioned with broad speed regulation in order to give stable governing during the time that it is controlling the turbine. On the other hand, if turbine control is to reside in the control valve governing or primary system, broad regulation of the stop valve is not desired since this requires a larger governor travel or similar actuating movement to open the stop valve wide so as to pass full steam flow. Moreover, a broad regulation is unnecessary as being wasteful of the total governor travel available. Therefore, While the primary control system is functioning, only a narrow speed regulation of the stop valve is necessary, occupying only a small portion of the stop valve governor travel.

Accordingly, one object of the present invention is to provide improved throttling valve means for loading a steam turbine in a manner to reduce large temperature gradients in the high pressure turbine casing.

Another object is to provide improved means for starting and partially loading a steam turbine with all of the control valves open so as to accomplish full arc admission.

Still another object is to provide an improved valve governing mechanism having differing degrees of regulation in various ranges of valve position.

A further object is to provide an improved governing mechanism for a steam turbine admission valve giving broad regulation at reduced conditions of loading and narrow regulation with greater loading.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a diagrammatic representation of the steam turbine control mechanism in the broad regulation phase of operation while under small turbine load; and

PEG. 2 is a diagrammatic representation of the same control mechanism showing the narrow regulation phase of operation while under greater turbine load.

Generally stated, the invention is practiced by providing the stop valve with an internal bypass valve to be operated by a hydraulic servo mechanism in accordance with the dictates of a separate auxiliary governor. The restoring linkage of the servo mechanism inwrporates a two-ratio linkage to provide a restoring action for the hydraulic servo pilot valve in order to create the broad and narrow regulation phases. Control may be shifted easily from the auxiliary governing system using the stop valve to the main governing system using the control valves.

Referring now to FIG. 1 of the drawing, a high pressure turbine 1 is controlled during operation at a substantial load by a main governing mechanism 2 which controls the admission of steam through

similar control valves

3, 3a, etc. Control valve 3 incorporates a valve disk 4 which is opened and closed by a common actuator 3b operated by an hydraulic relay 5 in response to the movement of input rod 6. It will be appreciated that relay 5 is only a simple diagrammatic showing of a rather complicated hydraulic servo system which serves to multiply the stroke and power of input rod 6. Although an understanding of the operation of relay 5 is not material to the present invention, an example of such a hydraulic relay may be seen by referring to Patent No. 2,811,837, issued to M. A. Eggenberger on November 5, 1957, and assigned to the assignee of the present application. It will also be appreciated that control valve 3, shown in cross-section, is only one of a number of similar valves illustrated diagrammatically by the single valve 3a. These several valves are connected in parallel and are operated by cams from a common actuator 3b shown symbolically as a liftbar operating valve stems of varying lengths, so that the control valves are sequentially opened in a predetermined sequence to admit steam to the associated nozzle boxes discharging into the turbine buckets. Common actuator 3b can also be a cam arrangement or any suitable device for sequentially opening

valves

3, 3a, etc. It will also be understood by those skilled in the art that steam discharged from high pressure turbine 1 may be reheated by reheat coils 7 and discharged to an intermediate pressure turbine 8 through a reheat stop valve 9 and a reheat control valve 10, thereafter to be further reheated or expanded in additional turbine stages (not shown). Reheat control valve 10 may also be operated by linkage or hydraulic means controlled by the main governing mechanism 2 but, for the sake of simplicity, this connection is represented only by a dot-dash line 11.

A simple diagrammatic representation of the main governing mechanism 2 is illustrated by a lever 12 positioned by a main synchronizing hand wheel 13, a main load limit hand wheel 14, and a centrifugal governor 15. A breakdown link 16 permits the main load limit hand wheel 14 to override the dictates of governor 15 to position the input rod 6 when it is not desired that governor 15 position the rod 6. Governor 15 is, of course, connected to the turbine shaft by associated gearing (not shown) so as to rotate at a speed proportional to shaft speed.

The load limit 14 is represented by a screw that overpowers the speed governor. The breakdown link 16 contains a spring in order to allow the speed governor 15 to maintain its position. By screwing the load limit 14 down, the valve opening (load) can be decreased. If the load limit 14 is turned back up, it will reopen

valves

3, 3a to the position in which the breakdown link 16 will again be solid and from then on the speed governor 15 will cona trol the position of the

control valves

3, 3a and further movement of the load limit 14 will have no effect.

The load limit 14 is used usually in starting up the turbine. By means of this the control valves can be gradually opened until the turbine is almost up to speed, where the speed governor 15 takes over. Under load, the load limit 14 can be used, as its name implies, to limit the load to some maximum value, regardless of a low speed condition. If, however, the speed should rise, the speed governor 15 will be able to close the valves all the way if necessary, regardless of the position of the load limit 14. This feature is necessary for overspeed protection.

As will be appreciated by those skilled in the art, centrifugal governor 15 will position the particular control valve which is controlling at the time, represented by valve disk 4- to hold speed substantially constant, moving upward to open the valve as the speed decreases. The speed maintained by governor 15 corresponding to a given load on the turbine may be adjusted by the main synchronizing hand wheel 13.

The

various control valves

3, 3a, etc. are supplied with motive fluid by the steam generating and superheating coils shown diagrammatically at 17. The steam flows through a single stop valve 13 which is normally set to remain wide open during operation and to be closed quickly in the event of turbine overspeed or malfunction of the control valves 3 to prevent damage to the turbine. Stop valve 18 comprises a main valve disk 18a bypassing steam through internal bypass ducts 135. A bypass valve disk is arranged to cooperate with main valve disk 18a so as to close 05 the flow of steam through the bypass ducts 18b in a manner which will be obvious from the drawing, although in some cases a separate bypass valve might be desirable. The bypass ducts 18b are of such a size that they will allow the flow of sufiicient high pressure steam to operate the turbine at approximately 20% to 40% load at rated speed. Thus the partial loading of the turbine may be controlled entirely by the movement of the bypass valve disk 180 when the

control valves

3, 3a are full open. Both

disks

18a, and 18b of valve 18 are actuated by a common valve stem 19 which, in turn, is positioned by a spring-loaded servo motor 20. An emergency trip valve 20a is controlled by an emergency governor (not shown) to discharge hydraulic fluid and close valve 18 in the event of overspeed.

Stop valve 18 is positioned, in acordance with the invention, by auxiliary control mechanism 21 which is represented diagrammatically as a lever 22 controlled by an auxiliary synchronizing hand wheel 23, an auxiliary load limit hand wheel 24, and an auxiliary centrifugal speed governor 25. A break-down link 26 is interposed between speed governor 25 and lever 22 to permit auxiliary load limit 24 to override the centrifugal governor 25, in the manner previously mentioned in connection with the main governing mechanism 2. Auxiliary governing mechanism 21 operates much the same as main governing mechanism 2 insofar as the action of the governor, the synchronizing hand wheel and the load limit hand wheel are concerned. The right-hand end of lever 22 is connected to an output link 27 which in turn is connected to a floating lever 28. Lever 28 acts to operate a pilot valve 29 connected to a source of oil under pressure (not shown) by conduit 30 and to then restore pilot valve 29 to its on-port position by the movement of valve stem 19.

The linkage supplying the feed-back or restoring action to pilot valve 29* from the movement of valve stem 19 will now be described in detail. This restoring linkage comprises a two-ratio restoring mechanism 3 1. A lever 32 is connected to receive the movement of valve stem 19 at its right-hand end by a restoring arm 33 and a link 34. Thus the right-hand end of lever 32 is rigidly fixed according to the position of valve stem 19. At its left-hand end, lever 32 transmits the restoring force through link 35 to the lever 28 and from there to pilot valve 2?. Lever 32 is supported by a special lever 36 which in turn is supported from a stationary fulcrum on pivot point 37. Special lever 36 is pivoted to lever 32 at its left-hand end by a connection shown as 38. The right-hand end of lever 36 is shaped to form an abutment surface 39 upon which lever 32 can bear. Lever 36 is biased about ful crum 37 in the counterclockwise direction by a spring 4%) and is prevented from rotating beyond a given point by a stationary stop 41 as seen from the drawing. Thus lever 32 operates as a two-ratio lever" having two separate mechanical advantages depending upon its position. FIG. 1 shows lever 32 pivoting about fulcrum 37 when the right-hand end of lever 32 has been depressed below the horizontal position to bear on special lever so. It will readily be appreciated that as lever 32 pivots about point 37 a small movement of the right-hand end of lever 32 will result in a large movement of the left-hand end of lever 32.

FIG. 2 shows the lever 32 employing a different mechanical advantage as it pivots about point 38, since the lever 36 is now restrained from further counterclockwise movement by stop 41. Spring 4% prevents the left-hand end of lever 36 from rising due to any upward force from link 34. In this latter mode of operation, with the righthand end of lever 32 elevated above the horizontal, a given movement of the right-hand end of lever 32 will result in a much smaller displacement of the left-hand end of lever 32.

Since the movements of the right-hand end of lever 32 follow the movements of the valve stem 19, the degree of opening of valve 18 will, therefore, determine the rate of feed-back? which restores the pilot valve 29 through rotation'of lever 32. The linkage is preferably proportioned and arranged so that lever 32 shifts from pivot point 37 to pivot point 38 just before the bypass valve disk 13c engages the main valve disk 18a. In FIG. 1, the linkage is pictured with the right-hand end of lever 32 depressed, pivoting about point 37. As bypass valve disk 180 is operated through the action of the governor, a high degree of feed-back is achieved by the action of the two-ratio restoring linkage 31. Conversely, FIG. 2 illustrates how, when the load of the turbine reaches such an extent that it is necessary for the control valves to take over the control and for the stop valve disk 18a to go wide open, a smaller degree of feed-back is provided by the restoring linkage to pilot valve 29, since lever 32 is now pivoting about point 38. This means that broad regulation, i.e. change in speed with respect to change in valve position, will be obtained when valve disk 180 is controlling, and narrow regulation will be obtained when valve disk 18a is being actuated. What this means in practical terms is that the majority of the stroke available in centrifugal governor 25 is efiectively utilized in operating valve disk 180 during the period of partial loading when speed control of the turbine resides in the stop valve 18. Only a very small part of the stroke of centrifugal governor 25, on the other hand, is used to move the main valve disk 18a from a closed to an open position. This results in a very effective utilization of the governor 25 to control speed while the turbine is under partial loading conditions and control is being accomplished by the stop valve 18.

The operation of my improved control mechanism for starting steam turbines will now be described. With the turbine at a standstill, the synchronizing and load limit hand wheels are adjusted as follows. As seen in FIG. 1, the auxiliary synchronizing hand wheel 23 is adjusted to a low speed setting and the main synchronizing hand wheel 13 is adjusted to a high speed setting. The high and low speed settings of these synchronizing hand wheels are only relative and refer to a range about the rated speed taken as 100%. For example, in the embodiment shown, the auxiliary synchronizing hand wheel 23 would have a range from a low of 95% to a high of 110% of rated speed while the main synchronizing hand wheel 13 would have a range 5 from a low of to a high of 107% of rated speed. Thus, since the auxiliary control 21 is set at a lower speed than main control 2, it will take over speed control first.

The load limit

hand wheels

14, 24 are positioned as follows. Load limit hand wheel 14 is all the way up as seen in FIG. 1, allowing all control valves 3 to open wide. The auxiliary load limit hand wheel 24 is closed all the way (shown open in FIG. 1) overpowering the control position of centrifugal governor 25 by means of the breakdown link 26. The manner in which breakdown link 26 allows the load limit 24 to overpower the governor 25 can be readily visualized from the figure.

Auxiliary load limit 24 is then slowly opened and lever 22 will follow the hand wheel upward under the influence of the compressed break-down link 26. This will raise link 27 and the left-hand end of lever 28 to move pilot valve 29 from its on-port position and allow hydraulic fiuid to be admitted from supply conduit 30 to the hydrau lic cylinder 20. As the hydraulic fluid causes the valve stem 19 and bypass valve disk to rise, steam is admitted from steam coils 17 through the stopvalve bypass holes 13b. As previously mentioned, since control valves 3 are wide open, steam will be admitted to all of the nozzles in the high pressure turbine at once, allowing for even heating around the periphery of the high pressure casing. The movement of valve stem 19 is transmitted through arm 33 and link 34 to lever 32 as its pivots around point 37 as seen in FIG. 1. The movement of the lefthand end of lever 32 restores pilot valve 2? to its on-port position.

The speed of the turbine increases until it reaches the low speed setting of the auxiliary synchronizing hand wheel 23, whereupon the auxiliary load limit 24 becomes inactive and may be completely opened as pictured in FIG. 1 since control resides in the auxiliary speed governor 25. Since the main synchronizing hand wheel 13 is set at a higher speed setting than the auxiliary syn chronizing hand wheel 23, main governor 15 does not yet have control through the control valves 3. Governor 25 now causes the valve disk 180 to open as partial load is applied, broad speed regulation, therefore very stable governing, being afforded through the high ratio feedback restoring action of lever 32.

FIG. 2 illustrates the second phase of control with the auxiliary components pictured in the narrow speed regulation zone as they control the main stop valve disk 18a, while main speed governor 15 exercises the primary control over the turbine through control valves 3. To shift from auxiliary start-up control to main control, it is only necessary to manipulate the auxiliary and main synchronizing

hand wheels

13, 23. Control is shifted by first decreasing the speed setting of the main synchonizing hand wheel 13 and then by increasing the speed setting of the auxiliary synchronizing hand wheel 23 above the setting of main synchronizing wheel 13 so that the main governing mechanism 2 takes control.

While in this phase of operation, the two-ratio linkage 31 is as shown in FIG. 2 so that lever 32 pivots about point 33 on the special lever 36. This results in a small restoring displacement for a given movement of the valve em 1?, resulting in a narrow regulation. of valve disk 18a as it is controlled by governor 25. This control over stop valve 18 by the auxiliary governor in no way affects the conventional use of stop valve 18 to shut off the flow of steam in an emergency when actuated by the emergency trip valve 20a.

In addition to the start-up function of the auxiliary control system, it will be appreciated that, in the event it is desired to operate the turbine under partial loading for an extended period of time, it would be desirable to operate on the auxiliary control 21 without ever shifting over to main control 2. Thus the advantages of full arc admission in reducing localized heating of the high pressure turbine casing can be realized at partial loading.

' The stop valve positioning servo mechanism with the two-ratio linkage 31 can also be used to advantage without the use of the auxiliary governor control 21. Link 27 may be replaced by a handle for manual operation of the pilot servo 29. in this method of operation, the operator watches a tachometer and moves the left-hand end of lever 23 to control the speed. The two-ratio linkage will provide more stable control during start-up and partial loading with faster responding control when the valves can be fully opened after the main control 2 has taken over.

Thus it will be seen that a very effective method of preventing localized heating of the turbine casing is achieved through full arc admission by means of the novel auxiliary control system shown which positions the stop valve Whi e all the control valves are wide open, yet without interfering with the conventional functions of the stop valve. By means of the two-ratio restoring linkage, broad speed regulation of the stop valve is achieved under partia load, while narrow regulation of the stop valve is achieved under higher conditions of loading when the control valves are exercising primary control.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a steam turbine having a plurality of steam admission nozzle means circumferentially spaced around a casing, a plurality of control valves connected in parallel fiow relationship and arranged to open sequentially to admit steam to the turbine casing, stop valve means serially connected to supply steam to the control valves, main. control means connected to sequentially open said control valves as the load increases and adjustable over a first speed range, auxiliary control means responsive to turbine speed connected to move said stop valve means to throttling positions intermediate its closed and open positions, said speed responsive auxiliary control means being adjustable over a second speed range in overlapping relation with said first speed range, and means causing said main control means to hold all control valves wide open while said auxiliary control means positions the stop valve mean under partial load, whereby steam will be admitted uniformly through all said admission nozzle means 2. In combination, a steam turbine having a plurality of steam admission nozzle means circumferentially spaced around the casing, a plurality of control valves connected in parallel flow relationship to admit steam to the turbine casing, main control means connected to sequentially open said control valves for controlling the admission of steam flow to the turbine as the load increases, stop valve means serially connected to supply steam to the control valves, hydraulic servo means connected to move said stop valve means to throttling positions intermediate the closed and open positions comprising hydraulic motor means connected to the stop valve, a pilot valve connected to control said hydraulic motor means for positioning the stop valve, and restoring linkage means, said linkage means comprising an input member actuated by the movement of the hydraulic motor means, an output member for restoring the pilot valve to its neutral position, and a two-ratio linkage connecting said input and output members for transmitting restoring movement at two separate ratios to the pilot valve for a given movement of the hydraulic motor means, said two-ratio linkage providing a relatively large restoring movement ratio to the pilot valve over an initial range of movement of the stop valve and a relatively small restoring movement ratio to the pilot valve over a second range of movement of the stop valve, and means causing all control valves to remain open during said initial range of movement of the stop valve, whereby K; a large restoring movement is transmitted to the pilot valve during partial load operation with the control valves full open for admission to all turbine nozzles while a smaller restoring movement is transmitted to the pilot valve when the main control means is controlling the turbine.

3. In combination, a steam turbine having a plurality of steam admission nozzle means circumferentially spaced around the casing, a plurality of control valves connected in parallel flow relationship and arrangedto open sequentially to admit steam to the turbine casing, main control means connected to sequentially open said control valves for controlling the admission of steam how to the turbine as the load increases, stop valve means serially connected to supply steam to the control valves, said stop valve means comprising a main valve disk and a bypass valve for bypassing said main valve disk with steam sufficient to accommodate a substantial partial turbine load, hydraulic servo means connected to move said stop valve means to throttling positions intermediate the closed and open positions comprising hydraulic motor means connected to open first the bypass valve and then the main valve disk, a pilot valve connected to control said hydraulic motor means for positioning the stop valve means, and restoring linkage means, said linkage means comprising an input member actuated by the movement of the hydraulic motor means, an output member for restoring the pilot valve to its neutral position, and a two-ratio linkage connecting said input and output members for transmitting restoring movement at two separate ratios to the pilot valve for a given movement of the hydraulic motor means, said two-ratio linkage providing a relatively large restoring movement ratio to the pilot valve as the bypass valve opens and a relatively small restoring movement ratio to the pilot valve as the main valve disk opens, auxiliary control means responsive to turbine speed connected to position said pilot valve, whereby broad regulation will be achieved while the stop valve bypass is controlling and narrower regulation will be achieved while the stop valve main disk is controlling, and means to shift control of the turbine between the main control means and said auxiliary control means.

4. In combination, a steam turbine having a plurality of steam admission nozzle means circumferentially spaced around a casing, a plurality of control valves connected in parallel flow relationship to admit steam to the turbine casing, main control means connected to sequentially open said control valves as the load increases and adjustable over a first speed range, stop valve means serially connected to supply steam to the control valves comprising a main valve disk and a bypass valve for bypassing said main valve disk with steam sufficient to accommodate a substantial partial turbine load, hydraulic servo means connected to move said stop valve means to throttling positions intermediate the closed and open positions comprising hydraulic motor means connected to open first the bypass valve and then the main valve disk, a pilot valve connected to control said hydraulic motor means for positioning the stop valve means, and restoring linkage means, said linkage means comprising an input member actuated by the movement of said hydraulic motor means, an output member for restoring the pilot valve to its neutral position, and a two-ratio linkage connecting said input and output members for transmitting. restoring movement at two separate ratios to the pilot valve for a given move ment of the hydraulic motor means, said tworatio linkage providing a relatively large restoring movement ratio to the pilot valve as the stop valve bypass opens and a relatively small restoring movement ratio to the pilot valve as the stop valve main disk opens, and auxiliary control means connected to position said pilot valve and adjustable over a second speed range in overlapping relation with said first speed range whereby control over the turbine can be shifted between the main and auxiliary control means so that the turbine may be partially loaded by positioning the stop valve under broad speed regulation with all of the control valves wide open.

References Cited in the file of this patent UNITED STATES PATENTS Keller June 5, 1906 Wilkinson July 17, 1906 Davis Dec. 26, 1911 Dake May 29, 1923 Fletcher Dec. 30, 1930 Caughey June 2, 1931 Dickinson et a1. Apr. 9, 1935