US2912962A - Two-speed hydraulic servomotor system - Google Patents

Description

Nov. 17, 1959 F. H. KINDL TWO-SPEED. HYDRAULIC SERVOMOTOR SYSTEM lie Filed Feb. 21, 1957 H/S A TTORNE)" TWO-SPEED HYDRAULIC SERVOMOTOR SYSTEM Fred HflKindl, Ballston Spa, N.Y., assignor to General Electric Company, a corporation of Nevv'York Application February 21, 1957, Serial No. 41,626 4cm. Cl. 121-41 This invention relates to a hydraulic servomotor mechanism, particularly to such a mechanism which is used for positioning the stem of a steam valve in 'a prime mover such as an elastic fluid turbine.

' The servo mechanism described particularly herein is illustrated ascontrolling the'operation of an intercept valve which is used tostop the flow of motive fluid from the reheater section of a steam generator to the succeeding stages of a reheat type turbine.

In the operation of a steam turbine it is desired to have quick opening and closing times of the intercept valve. The conventional way of controlling the intercept valves is by a separate oil pressure system which actuates a single acting servomotor to move the valves to the open or closed position. The servomotor is normally springbiased in one direction and actuated in the other direction by oil under pressure, the supply of which is regulated by a pilot valve.

The intercept valve is normally of the balanced type =which balances out the effect of the steam flowing through the valve to prevent the steam from having any effect on the opening and closing movement of the valve. With this arrangement, the valve is under the control of the hydraulic servomotor system employed in such cases. While this arrangement has been generally satisfactory, experience has shown that the effective pressure over the downstream face of the valve disk can be much higher than pressures further downstream and also higher than the balance chamber pressure which in most instances tends to be only slightly above downstream pressure.'

,trol disk 1d controlling the flow between inlet 1h and outlet-1i has a piston portion Ie, the upper surface of 2,912,962 Patented Nov. 17,

Generally stated, the invention is practiced byproviding a hydraulic servo mechanism for controlling a steam turbine valve, which servo mechanism includes a regulator for controlling the movement of the servo mechanism; and means interposed between the regulator and the servo mechanism to limit the operating speed of the servo'mechanism in one direction while at the same timemaintaining a rapid movement of the servo mechanism in the opposite direction.

Referring now to the drawing, the intercept valve 1 is illustrated diagrammatically as comprising a housing 1a supporting the bushing assembly 1b, in which is slidably disposed the valve stem 10. Because it must perform accurate throttling functions, the valve 1 illustrated is of a pressure-balanced type. In such a valve the flow conwhich is exposed to the downstream pressure communicated to chamber 1 by a pressure balancing conduit 1g. Thus, the pressure on the bottom surface of disk 1d jis'etfectively balanced by the pressure on piston 1e and'reduces to a minimum the force required to move the valve to the fully open position. The operation of the intercept valve 1 is controlled by a servomotor 2 which is connected through rod 2a to stem 10 of intercept valve 1.

Rod 2a defines at its upper portion a valve stem sealing shoulder identified 2g which engages with a cooperating series of openings 3a, 3b, 3c, 3d, 32, and 3} adapted to When this responsive to turbine speed (not shown).

-into chambers 2d, 2e.

seat in bushing 1b to limit the upward movement of valve stem 1c. The servomotor is controlled by a pilot valve 3 which is under the control of some automatic device The pilot valve is-so arranged that upon a predetermined overspeed condition it is operated in a manner to be described hereinafter to close the' intercept valve. A control mechanism of this type is described more particularly in the U'.S. Patent 2,747,373, issued May 29, 1956, on an "application of M. A. Eggenberger et al., and assigned to the same assignee as the present invention.

Referring now to the servomotor 2 in detail, it comprises a cylindrical housing 2b containing piston 20 which is secured to piston rod 2a and which divides housing -2b Piston 2c is biased in a downward direction by compression spring 2 located in chamber 2d. The flow of fluid to servomotor 2 is controlled by pilot valve 3 which is a conventional piston valve having a receive inlet conduit 5, outlet conduits 6, 7 and drain conduits 9a, 9b respectively. Within the pilot valve 3' is a double piston arrangement 3g, 3h which are axially dis- .posed on rod 31'.

Rod 3i is connected to lever 15a which movement is controlled by link 15b which in turn is con- Accordingly, it is an object of this invention to provide an improved hydraulic servomotor system for actuating a valve, which incorporates special flow controlling means to limit the opening movement of the valve to prevent the imposition of excessive stresses on the valve stem when the valve reaches its maximum open position Y while permitting rapid movement of the valve in the closing direction.

A further object is to provide a novel servo mechanism specially adapted for use witha balanced valve which provides for rapid closing movement of the valve but which prevents rapid opening movement and thus tends to. prevent excessive loading of the valve stem which smay result in excessive stretching or breakage of the valve stem.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawing, which diagrammatically illustrates a hydraulic servomotor incorporating the invention and arranged to operate the intercept valve of a -stcamturbinee nected up to a device (not shown) responsive to turbine speed. The pistons 3g, 3h are located to control the flow of oil at ports 3b, 3c, respectively. Conduit 6 interconnects ports 3b and chamber 2e in servomotor 2, and conduit 7 connects up port 30 and chamber 2d. Thus, when rod 3i is moved downwardly, inlet conduit 5 which contains orifice 5a to limit the flow therethrough, is connected up with conduit 7 and conduit 6 is connected up with drain conduit9a to drain chamber 2e to a sump (not shown). When the pilot valve is in this position, the oil from inlet 5 acts on the piston 20 to move the intercept valve 1 in the closing direction. Downward movement into chamber 2d past the rod 2a.

In accordance with the invention, the drain conduit 9 is formedintheshape of 'an inverted'U so'thatapressure head 71 is set up to provide a back pressure in conduits 9a, 9b. Interconnecting chamber 2d and conduit 9a through pilot valve is conduit 8. Located in conduit 8 is check valve 12 which permits flow in the direction leading :from conduit 9 to chamber Zdbut prevents flow from taking place in the opposite direction. Thus, when the pressure in chamber 2d drops below that in conduit 9 oil will flow through check valve 12, conduit 8, and into chamber 2d. Conduit 7 which interfollows.

Upon receipt of a signal from the pre-emergency speed --relay (not shown) calling for movement of the valve 1 in a closing direction, link b, and leveriSa are moved downwardly to move the valve control rod 3i to interconnect inlet conduit 5 with conduit 7, and conduit 6 with drain conduit 9a to admit high pressure oil to chamber 2d and drain fluid from chamber 2e respectively. It ,the signal requires only a small motion, the valve stem 1c and piston rod 2a moves in the closing direction only until the return linkage 15c, 15d, 15c restores the pilot valve to its on-port position, at which time the valve closing movement of rod 2a stops. If, however, the turbine loses full load, the floating lever 15a and pilot valve rod 31' drops far enough so that the pilot valve 3 will stay off port until valve 1 is closed. When this occurs, the fluid from chamber 2e is rapidly drained through conduits 6, and 9a; and then spring 2 combined with the fluid pressure in chamber 2d forces valve 1 closed very rapidly. It can be appreciated that this will require oil in chamber 211 at a rate much greater than can be supplied through conduit 7 and flow limiting orifice 13, with the result that a partial vacuum could exist in chamber 2d. However, at this time check valve 12 opens to permit oil to flow directly from the chamber 2e through conduit 6, pilot valve 3, conduit 8 and check valve 12 to chamber 2d. This flow takes place due to the back pressure existing in conduit 9 resulting from the configuration thereof. The provision of this supplemen tary fluid prevents a partial vacuum from occurring which, if permitted would result in actually lengthening the closing time of the intercept valve which would be undesirable. It can be appreciated by'one skilled in the art of steam turbines that it is important that the control valves 'be closed as quick as possible on overspeed to prevent damage to the turbine.

It remains to note that the oil seal 11 prevents air from leaking into chamber Zn! in the event a partial vacuum exists in chamber 2d. Thus,

since the'oil in the servomotor system is recirculated a large flow of high pressure oil from the fluid pressure source (not shown) is not required to obtain the desired open and closing times of the valve'l.

When it is desired to open the intercept valve 1, the

"pilot valve rod 3i is moved upwardly through actuation tion, the upward movement of intercept valve 1 is retarded by the orifice 13 which limits the rate at which fluid can flow from chamber 2d to drain conduit 9b. Thus,

pressure-i8 built up in chamberld to produce a downward force sufficient to overcome any steam forces which may exist tending to blow the valve open. It should be noted that no fluid can flow through check valve 12 at this time. The reduction in the upward speed of the valve 1 prevents the valve stem 10 from being stretched or broken when the sealing shoulder 2g engages bushing 11). l

Thus it can be seen that with the aid of this invention, the intercept valve 1 will be closed by the servo mechanism in the desired minimum time, but will be prevented from rapidly jerking open by the temporary cushioning effect of the fluid contained in the servo mechanism, which cushioning effect is the result of restricting the flow of fluid out from the chamber 2d when the valve 1 is moved in the opening direction.

While a single embodiment of the invention has been described in detail herein, it will be obvious to those skilled in the art that many changes and substitutions of equivalents might be made. For example, it may be desirable to connect conduit 8 to a separate source of fluid under pressure and not rely on the configuration of drain conduit 9 to provide a back pressure in conduit 8 so that'fluid is available in conduit 8 when thevalve 1 is moved'to a closed position. In addition, a relief valve could be located in conduit 9 to maintain a back pressure therein. Also, while the invention is illustrated as being applied to a balanced valve,.it will be obvious to those skilled in the art thatthe inventionis not so limited.

It is, of course, desired to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention. A i

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

l. in a hydraulic servo mechanism for positioning an output member by actuating means including a servomotor comprising a housing, an output piston connected to said output member and dividing the housing into first and second chambers, a source of operating fluid under pressure, a pilot valve for controlling the flow of fluid from said source to the servomotor, and conduit means interconnecting the source, pilot valve,-and first and second chambers whereby upon movement of the pilot valve to direct the flow of fluid from the source to the first chamber the output piston is movedin a first direction tending to reduce the size of the second chamber, and upon movement of the pilot valve to-direct the flow of fluid to the second chamber the piston is moved in the opposite direction tending to reduce the size of the first chamber, means biasing said piston in the first direction, and means dependent on the setting 'of the pilot valve for moving the output member at a first speed in the first directionand at a second slower speed in the opposite direction, said means including a first-conduit connecting the pilot valve and first chamber, means in the first conduit for restricting the flow therethrough,a second conduit connecting said second chamber and said first chamber, first valve means in said second conduit for permitting flow in the direction from the second chamber to said first chamber, and second valve means between said first valve means and the second chamber arranged to be positively controlled by said pilot valve movement to admit fluid to the first valve means.

2. Apparatus of the character described which includes a source for supplying fluid under pressure to a servo mechanism, the servo mechanism comprising a reduce the size of'the second chamber, a pilot valve for-regulating the flow of fluid between said source-and said first and second chambers, first conduit means interconuecting said source and the pilot-valve, second conduit means interconnecting the pilot valve and the first chamber, third conduit means interconnecting the pilot valve and the second chamber, fourth conduit means interconnecting SaKlSECOlld chamber and the first chamber, first valve means in said fourth conduit for permitting flow in the direction between said second chamber and first chamber, flow limiting means in said second conduit for retarding the fiow of fluid between the pilot and the first chamber, and second valve means arranged to be positively controlled by the pilot valve movement to admit fluid from the second chamber to the fourth conduit, whereby upon movement of the pilot valve to direct the flow of fluid from the source to the first chamber and drain the fluid from the second chamber, the piston is moved in the first direction at a faster rate than when the piston is moved in the opposite direction when the pilot valve is moved to a position to supply fluid to the second chamber and drain fluid from the first chamber.

3. A hydraulic system for actuating a control valve for supplying motive fluid to a turbine, the combination of a hydraulic servomotor comprising a housing containing a piston dividing the housing into first and second chambers and having an output member secured thereto, spring means in said first chamber for biasing the piston in a first direction, a source of fluid under pressure for supplying fluid to said servomotor, a pilot valve for controlling the flow of fluid between said source of pressure and servomotor, the pilot valve including a housing defining an axial bore and a pressure fluid inlet port, drain ports spaced axially on either side of the inlet port, outlet ports defined between the inlet port and each of said drain ports, and a reciprocating flow control member disposed in said bore and comprising an axially slidable member with at least two axially spaced lands disposed in said bore in cooperation with said outlet ports, a first conduit means connecting said source with said inlet port, second and third conduit means connecting said outlet ports with the first and second chambers of said housing respectively, flow limiting means in said second conduit for restricting the flow of fluid therethrough, fourth conduit means connecting said second chamber and said first chamber, check valve means in said fourth conduit means for limiting'flow through said fourth conduit means in the direction from said second chamber to said first chamber, whereby upon movement of the flow control member to a position which interconnects the first conduit means with the second conduit means, and the third conduit means to drain, the piston will. be moved in a first direction at a first speed in response to the spring means in said first chamber and the fluid supplied from said inlet, and said second chamber through said second and fourth conduit means respectively, and upon movement of the flow control member to a position which interconnects the first conduit means with the third conduit means and the second conduit means to drain, the piston will be moved in the opposite direction at a second speed slower than said first speed due to the means in said second conduit which restricts the flow of fluid from said first chamber.

4. In a hydraulic servo mechanism for positioning an output member by actuating means including a servomotor comprising a housing, an output piston connected to the output member and dividing the housing into first and second chambers, resilient means in said first chamber for biasing the piston in a first direction, a source of fluid under pressure for supplying fluid to said servomotor, a pilot valve for controlling the flow of fluid between said source of pressure and servomotor, the pilot valve including a housing defining an axial bore and a pressure fluid inlet port, drain ports spaced axially on either side of the inlet port, outlet ports defined between the inlet port and each of said drain ports, and a reciprocating flow control member disposed in said bore and comprising an axially slidable member with at least two axially spaced lands disposed in said bore in cooperation with said outlet ports, a first conduit means connecting said source with said inlet port, sec ond and third conduit means connecting said outlet ports with the first and second chambers of said housing respectively, flow limiting means in said second conduit for restricting the flow therethrough, fourth conduit means for draining the fluid from said drain ports, means for providing a positive pressure in said fourth conduit means, fifth conduit means connecting said fourth conduit means with said first chamber, check valve means in said fifth conduit means for permitting flow in the direction between said fourth conduit means and said first chamber, whereby upon movement of said pilot valve in a direction to interconnect the source of fluid under pressure and said first chamber the piston is moved in the first direction at a first speed and upon movement of the pilot valve to interconnect the source of fluid under pressure and said second chamber the piston is moved in the opposite direction at a second speed substantially less than said first speed.

References Cited in the file of this patent UNITED STATES PATENTS 563,304 Ongley July 7, 1896 1,912,184 Ferris May 30, 1933 2,191,792 Hill Feb. 27, 1940 2,622,866 Johnsen Dec. 23, 1952 2,628,731 Reuter Feb. 17, 1953 2,646,025 Deardorff July 21, 1953

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