US3361108A - Stand-by control for steam turbines - Google Patents

Description

M. M. HOBBS Jan..2, I968 STAND-BY CONTROL FOR STEAM TURBINES Filed April 6, 1966 mwmzmozoo .sues. This uneven heatin United States Patent 3,361,198 STAND-BY CONTRQL FOR STEAM TURBlNEfi Milton M. Hobbs, Marple, Springfield, Pa, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Apr. 6, 1966, Ser. No. 540,567 12 Claims. (Cl. 115--34) ABSTRACT OF THE DIS'CLGSURE Apparatus disclosed includes a steam turbine driven reversible ship propulsion system having respective ahead and astern control valves driven by separate servo systems, which are in turn driven by an operational amplifier Whose input responds to a summation of signals representing actual and desired propeller speeds. A speed controller in the stop position applies a low value ahead signal to the operational amplifier, and the ahead valve is alternately opened and closed in response to see-saw of dominance between the ahead and the speed feedback signals. Thus the turbine is cyclically started-stopped-started-stopped, and so on to prevent bowing of the turbine rotor. With the speed controller in the stop position, the operational amplifier has a negative feedback circuit that is operative when the input to the amplifier is in the ahead sense and inoperative when the input to the amplifier is in the opposite sense.

This invention relates to control systems for steam turbines and the like, and more particularly to a stand-by control for marine steam turbines wherein the turbine is stopped and started periodically while the ship is not moving to maintain turbine rotor straightness.

When steam turbines used in marine propulsion systems are at rest, uneven heating of the turbine parts enif not prevented, will cause the turbine rotor to become bowed, a condition which obviously cannot be tolerated. Accordingly, it becomes necessary to start and stop the turbine every few moments to intermittently rotate the rotor and maintain its straightness.

As an overall object, the present invention seeks to provide apparatus for automatically starting and stopping a steam turbine periodically in order to maintain straightness of the turbine rotor.

More specifically, an object of the invention is to provide stand-by control for a marine propulsion system wherein a steam valve controlling admission of steam to a turbine is periodically opened just wide enough and long enough to start the turbine rotor turning, and then shut long enough for the rotor to coast to a stop and remain stopped until the cycle is repeated in a short time.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single figure drawing which schematically illustrates one embodiment of the invention.

The steam propulsion control system as shown herein is described in detail in copending application Ser. No. 540,566 (Westinghouse Electric Corporation) filed concurrently herewith and assigned to the assi nee of the present application. For a full and detailed description of the overall control system, reference may be had to that application, the present description being limited to those elements which effect stand-by operation of the turbine to maintain rotor straightness.

With reference now to the drawing, a ship propeller is driven through a gear reducer 12 by means of high pressure and low pressure steam turbines 1d and 16, respectively. Turbine 14 is connected to gear reducer 12 and, hence, to the propeller 10 through rotatable shaft 18.

3,3511% Patented Jan. 2, 1968 In a similar manner, the low pressure turbine 16 is connected to the gear reducer 12 through shaft 20 having conventional reversing turbine blades 22 thereon.

Steam for the turbines '14, 16 and 22 is supplied by means of a boiler, not shown, connected to a common' header 28. From header 28, the steam can flow through a main ahead valve 30 to the high pressure side of turbine 14; thence from the low pressure side of turbine 14 and through header 32 to the high pressure side of turbine 16. Finally, the steam at low pressure passes from the low pressure side of turbine 16 through header 34 to a condenser 36. Under the circumstances just described, the ship is moving in the ahead direction.

In order to reverse the propeller 10 and cause the ship to move in the astern direction, the valve 30 is closed and a main astern valve 38 opened, whereupon steam will flow to the reversing blades 22 on shaft 28 and thence through header 34 to the condenser 36. The arrangement just described is, of course, a common. and well-known marine propulsion system.

The main ahead valve 30, for example, is controlled by means of a servomotor, schematically illustrated at 39 and described in detail in the aforesaid copending application Serial No. 540,566. For purposes of the present application, however, it will be sufficient to state that the servomotor includes a magnetically permeable core, schematically illustrated by the arrow 40, which can be rotated about a center point in one direction or the other by means of two windings 42 and 44 connected to the output of an ahead servo amplifier 46 in a push-pull arrangement. The input to the servo amplifier 46 on lead 48, comprising a speed control, will cause the servo amplifier to rotate the core ill in one direction and open valve 30 when the control voltage on lead 48 is of negative polarity and increasing magnitude. When the control voltage on lead 43 is still of negative polarity but decreasing in magnitude, the core will rotate in the opposite direction to close valve 3%. Whenever the control voltage on lead 48 is not changing, the output from the servo amplifier .46 will remain constant while the core 40 and valve 35) remain in their previously-established position by virtue of a negative feedback path Stl around the amplifier 46 providing a conventional servo loop. The servomotor 39 is mechanically connected to the valve 3th by means of a conventional servomechanism, schematically illustrated by the broken line 41.

The astern valve 33 is controlled by a similar servomotor 52 connected to the output of an .astern servo amplifier 54, also provided with a negative feedback path 5'6. Whereas the ahead servo amplifier 4-6 and servomotor 39 respond to negative control voltages on lead 48, the astern servo amplifier 54 and servomotor 52 respond to positive control voltages. In this respect, when a positive control voltage on lead 48 is increasing in magnitude, the servomotor 52 will cause the valve 38 to open further; whereas a positive control voltage on lead 48 of decreasing magnitude will cause the servomotor 52 to close the astern valve 38.

Connected to the main ahead valve 30 is a limit switch 58 which is closed when the ahead valve is completely closed (i.e., shut). In a similar manner, a second limit switch 60 becomes closed when the astern valve 38 is completely closed. When valve 30 is completely closed and limit switch 58 closed, relay H becomes energized to close its normally open contacts H1. Similarly, when the astern valve 38 is completely closed, closure of limit switch 60 causes energization of relay I to close its contacts I1.

The main control for the propulsion system comprises a throttle control lever 62 movable from a central or stop position to an ahead position where the ship moves ahead; and movable backwardly from the stop position for the astern direction. The throttle lever 62 is connected to a reference divider or potentiometer 64 such that the output of a movable tap on the potentiometer appearing on lead so will have a polarity dependent upon whether the throttle lever 62, is moved to an ahcad no n or at astern position, and a magnitude dc; upon the amount of movement of the throttle lexcr in either direction from its center or stop position. in the particular control system shown herein, the output on lead as will be positive with respect to ground when the throttle lever moved to an ahead position and negative w'th respect to ground when it is moved to an astern posi more, its rnagnituce will increase in the positive din when the throttle lever is moved further in a clocmvisc direction from the stop position as shown, and will increase in magnitude in the negative direction it is moved in a counterclockwise direction from the stop positoin. The output on lead 66 is proportional to desired speed and its polarity indicative of the desired direction of move ment of the Ship. Hence, the signal on lead es can be referred to as a desired speed signal.

The desired speed signal on lead is added in series with the output of a tachometer generator which is mechanically connected to the propeller of the st 'p by suitable means, not shown herein. Hence, the output of the tachometer generator 67 is proportional to the actual speed of the propeller, and the polarity of the signal across the output terminals of generator 67 is indicative of the direction of actual movement, either astern or ahead. Hence, the output of tachometer generator 67 can be referred to as the actual speed signal.

The actual speed signal across tachometer gene ator 6'7 normally has a polarity opposing that on lead on whenever the lever 62 is moved from a previously established position, Whenever the actual Speed of the ship coincides with the desired speed as determined by the pos ion of throttle lever 62, the output of reference divider cornprising the desired speed signal will be equal to, and pposite in polarity to, the actual speed signal produced by tachometer generator 67 such that the voltage between lead 63 and ground will be zero. However, whenever the desired speed signal dillers from the actual speed si nal, a speed error signal or voltage will be produced between lead and ground. This voltge is applied across :1 voltage divider comprising resistors 76, '72 and in series. The resistor 72 is provided with a movable tap 76 which is connected through resistor 723 to the input of an operational amplifier 30, the output of ampl fier fill being applied through resistor to the lead cc, the output of the operational amplifier comprises the aforesaid speed control voltage which is applied to amplifiers and and which is effective, when negative, to actuate the servomotor 39 and effective, when positive, to actuate the scrvomotor 52'.

The operational amplifier is provided with a first feedback path including a capacitor which filters out any ripple appearing in the input direct current control voltage. A second feedback path includes capacitor and resistor 83 in series with normally closed contacts C3; of a relay C. This second feedback path including capacitor 30 and resistor provides proportional plus integral control action. That is, in response to an abrupt change in the voltage on lead 63, there will be a step output or abrupt increase in voltage on lead 58, followed by a gradual change. As an example, assume a sudden speed error of one propeller revolution per minute. This will produce an error voltage on lead 68; and almost immediately after the appearance of the error voltage, the output of amplifier as on lead will change abruptiy. However, after this abrupt change in output, the integral action of the amplifier will cause the output to continue to change after the transient step change resulting from the proportional response to the sudden error.

The throttle control lever 62 has connected tnereto a wiper brush 9% which energizes lead 92 when, and only when, the throttle lever 62 is in its central or stop position. Thus, whenever the ship is at rest, the lead 92 will be energized. Assuming that both the ahead valve 3t and astern valve 33 are closed, relays H and I will be enered to close contacts H1 and ii. Consequently, relay C will now become energized and will remain energized through contacts C12, even though the ahead valve 3% opens and relay H becomes decncrgized. Thus, once the relay C becomes energized, it Will remain energized until the throttle LCVEF is moved from its central or stop position, until the astern valve 33 opens to deenergize relay 1.

When .eluy becomes energized, contacts Cl open to thereby interrupt the feedback path comprising capacitor and resistor 88 in series. At the same time, closure of contacts C3 short out capacitor es and remove any residual charge thereon. This insures that once the throttle lever is moved from its stop position and relay C becomes deenergized, no residual charge will exist On the capacitor 86 to impede the response characteristics of the system.

in addition to the two back paths previously described, there is a bacl; path 94 comprising capa itor in s vim r crs and When the throttle lever s43 is not in its central or sto position and the relay C is deen ed, contacts will be closed, thereby grounding lecdb path 94. When, however, the relay C becomes cnerg with the ship at rest and valves and c sed, the contacts C-t will open and contacts C5 will ciao, thereby connecting the junction of capacitor 5'6 and resistor to ground through diode 103.

When the throttle lever 62 is placed in the stop position, the maneuvering valve 30 or which has been open will close. Once both the ahead and astern valves 3 and :1, respectively, are closed as sensed by limit switches El (l lil l' lu L, 3llTQlZPCl and remains energized SO lOrQ 35;

the throttle lever is left in the stop position. ed above, relay C disconnects the normal feedck circuit around the amp tier by virtue of contacts which now open, and connects the higher gain feedbetween the n rand output of amplifier 38. the throttle lever at; is in its center or stop position, ht positive vol e is produced on lead 66 as is more fully explained in the aforesaid copending application Ser. No. 540,566. That is, at this time, the lead 66 is not grounded but, rather, is slightly positive with re spect to ground. This positive voltage, when applied to the amplifier 83, causes its output on lead 6 to integrate slowly from positi e to negative polarity. When the voltage on lead is sutiiciently negative, the servornotor opens steam valve During this process, capacitor changes its charge from the polarity shown to opposite polarity.

Once the valve is opened, the propeller 18 rotates in the forward direction, thereby causing the tachometer generator -37 to produce a voltage with the polarity bown on the drawing. This voltage, applied to amplifier 52*, ca sea the negative output on lead 4?,- to swing fully positive abruptly, because diode fludischarges capacitor to ground (via co ict C5) and thereby et'li'ectively prevents t e high g2 feedback path through resistors 93 and from acti a as a result the ahead valve is closed. This positive voltage on lead 48, however, cannot actuate the astern servomotor 52 for the reason that the output of amplifier 5 2 is now grounded via contacts C6 which are now closed. After the valve closes, the propeller 18' stops, and the voltage output of the tachometer G] drops to zero, whereupon the slight positive voltage on lead 66 will cause the aforesaid cycle to repeat.

Thus, by the action of the circuit, valve 38 is opened just long enough for the propeller lid to be accelerated to some value ordii vrily less than three revolutions per minute. At this point, the speed of the propeller is quickly sensed by the tachometer with the output of amplifier on lead 48 quickly changing from a negative voltage necessary for opening the ahead valve 30 to a positive voltage causing hard closing of the ahead valve. If the ship has no headway, the turbine speed will gradually reduce to zero and the amplifier 80 with feedback path 94 will begin to integrate from the positive hard closed region back into the negative region necessary for the next steam pulse.

It can be seen, therefore, that the present invention provides a means for periodically rotating the rotor of a steam turbine while it is at rest to prevent uneven heating of the rotor and to maintain its straightness. Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements Without departing from the spirit and scope of the invention.

I claim as my invention:

1. In control apparatus for a ship propulsion system including a valve for admitting fluid under pressure to turbine means having a rotor mechanically coupled to the ships propeller; the combination of means for intermittently rotating said rotor and propeller while the ship is stopped, comprising servomotor means for opening and closing said valve, tachometer generator means coupled to said propeller and adapted to produce a direct current voltage when said rotor and propeller rotate, a source of direct current voltage in series with said tachometer generator means and having a polarity opposing the direct current voltage produced by the tachometer generator means when said valve is open, operational amplifier means operable in response to the voltage resulting from the series addition of said direct current voltages and having its output connected to said servomotor means, and a negative feedback path for said operational amplifier means such that said source of directcurrent voltage will cause the operational amplifier means to produce an output voltage which opens said valve, the valve remaining open until the rotor and propeller accelerate to the point where the voltage produced by the tachometer generator means overrides said source of direct current voltage and causes said valve to close, and means operable only when the tachometer generator voltage overrides said source of direct current voltage for reducing the effectiveness of said negative feedback path whereby to increase the gain of the amplifier means and thus accelerate the closing of said valve.

2. The combination of claim 1 wherein the output of said operational amplifier means is applied to said servomotor means through a servo amplifier having a negative feedbackpath.

3. The combination of claim 2 wherein said servomotor means will open said valve when the output of the operational amplifier means is of one polarity and increasing in magnitude, and wherein the servomotor means will cause said valve to close when the output of the operational amplifier means is of said one polarity and decreasing in magnitude.

4. The combination of claim 1 wherein said valve comprises an ahead valve which causes the ship to move in the ahead direction when it is opened, the ship propulsion system also including an astern valve for admitting fluid under pressure to the turbine means to rotate the propeller such that the ship moves in the astern direction, servomotor means for actuating said astern valve, and means initially responsive to said ahead valve being in closed position for disabling said latter-mentioned servomotor means when the ship is stopped and for maintaining said disablement even after said ahead valve is opened to initiate said intermittent operation.

5. The combination of claim 1 wherein said operational amplifier means includes a second feedback path including a capacitor and a resistance element, which second feedback path causes the amplifier to operate as a proportional plus integral controller, means for interrupting said second feedback path when the ship is stopped, and means for disabling said first-mentioned feedback path when the ship is moving.

6. The combination of claim 1 wherein said feedback path includes a capacitor in series with a resistance element, and a unidirectional curent device connecting the junction of said capacitor and resistance element to ground.

7. In control apparatus for a ship propulsion system including a valve for admitting fluid under pressure to turbine means having a rotor mechanically coupled to the ships propeller; the combination of means for intermittently rotating said rotor and propeller while the ship is stopped comprising operational amplifier means having a negative feedback path, means responsive to the output of said amplifier means for controlling said valve, means for applying a voltage of one polarity to said amplifier means to cause said valve to open, means operable when said propeller rotates for applying 21 voltage of the other polarity to said amplifier means to cause said valve to close, and means operable only when the voltage of said other polarity is applied to said amplifier means for reducing the effectiveness of said negative feedback path whereby to increase the gain of the amplifier means thus to accelerate the closing of said valve.

8. The combination of claim 7 wherein said feedback path includes a capacitor, and said means for reducing the effectiveness of said feedback path comprises means for discharging said capacitor.

9. The combination of claim 7 wherein said valve comprises an ahead valve which causes the ship to move in the ahead direction when it is opened, the ship propulsion system also including an astern valve for admitting fluid under pressure to the turbine means to rotate the propeller such that the ship moves in the astern direction, servomotor means for actuating said astern valve, and means initially responsive to said ahead valve being in closed position for disabling said latter-mentioned servomotor means when the ship is stopped and for maintaining said disablement even after said ahead valve is opened to initiate said intermittent operation.

10. The combination of claim 7 wherein said operational amplifier means includes a second feedback path including a capacitor and a resistance element, which second feedback path causes the amplifier to operate as a proportional plus integral controller, means for interrupting said second feedback path when the ship is stopped, and means for disabling said first-mentioned feedback path when the ship is moving.

11. The combination of claim 7 wherein said feedback path includes a capacitor in series with a resistance element, and said means for reducing the effectiveness of said feedback path comprises a unidirectional current device connecting the junction of said capacitor and resistance element to ground.

12. The combination of claim 1 wherein said feedback path includes a capactior, and said means for reducing the effectiveness of said feedback path comprises means for discharging said capacitor.

References Cited UNITED STATES PATENTS ANDREW H. FARRELL, Primary Examiner.

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