US3820756A - Sequentially operable control valve for a steam turbine - Google Patents

Abstract

A sequentially operable control valve for an axial flow steam turbine having a plurality of plugs which successively lift from their seats, the plugs and seats having surfaces which cooperatively produce consecutive throttling stages as steam passes therebetween, the first plug to lift off its seat having the greatest number of consecutive throttling stages and plugs which successively lift off their seats having a smaller number of consecutive throttling stages, the velocity through the consecutive throttling stages being slightly less than the speed of sound as the plug lifts, thus keeping the intensity of vibration and noise produced by this valve at a low level.

Description

United States Patent [191 Meyer June 28, 1974 SEQUENTIALLY OPERABLE CONTROL VALVE FOR A STEAM TURBINE Inventor: Charles A. Meyer, 2970 N.

Providence Rd., Media, Pa. 19063 Filed: Mar. 12, 1973 Appl. No.: 340,127

Related US. Application Data Division of Ser. No. 153,503, June 16, 1971, Pat. No. 3,763,894.

Int. Cl. Fl 6k 1/52 Field of Search 251/121, 127, 333

1 References Cited UNITED STATES PATENTS 1,919,233 7/1933 Lee 2,114,858 4/1938 Rosch FOREIGN PATENTS OR APPLICATIONS 1,334,387 7/1963 France 251/121 u.s. Cl. 251/121, 251/127 Primary Examiner-Robert G. Nilson Attorney, Agent, or Firm-F. .1 Baehr, Jr.

[ ABSTRACT A sequentially operable control valve for an axial flow steam turbine having a plurality of plugs which successively lift from their seats, the plugs and seats having surfaces which cooperatively produce consecutive throttling stages as steam passes therebetween, the first plug to lift off its seat having the greatest number of consecutive throttling stages and plugs which successively lift off their seats having a smaller number of consecutive throttling stages, the velocity through the consecutive throttling stages being slightly less than the speed of sound as the plug lit'ts, thus keeping the intensity of vibration and noise produced by this valve at a low level.

3 Claims, 7 Drawing Figures PATENTEnJun2e mm SHEET 1 BF 4 FIG.2

VlSd HBFISSHHd SEQUENTIALLY OPERABLE CONTROL VALVE FOR A STEAM TURBINE This is a division of application Ser. No. 153,503 filed June 16, 1971, now US. Pat. No. 3,763,894.

' BACKGROUND OF THE INVENTION This invention relates to sequentially operable control valves for axial flow steam turbines and more particularly to such a valve having seats and plugs which cooperate to form consecutive throttling stages across the mating seats and plugs.

With advances in technology the pressure and temperature of steam supplied to the turbine has increased beyond the critical pressure and temperature causing extremely high pressure drops across the control valves during start-up and during low load conditions. Sequentially operable control valves have been used for several decades'and provide a group of relatively small diameter valves requiring relatively small lift forces to raise each individual valve plug from its seat, compared to a single valve size for full load steam flow. However, as the first valve plug lifts off its seat it must throttle the pressure from supercritical pressure to partial vacuum, and such pressure drops produce supersonic steam velocities which cause high intensity noises and vibrations.

SUMMARY OF THE INVENTION In general, sequentially operable control valves made in accordance with this invention, comprise a plurality of mating plugs and seats, the mating plugs and seats being disposed to successively separate and thereby to allow fluid to flow therebetween. The mating plugs and seats have surfaces, which cooperate to produce consecutive throttling stages as fluid passes therebetween. The number of consecutive throttling stages is greatest in the first plug and seat to separate, the mating plugs and seats to successively separate having a lesser number of consecutive throttling stages.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detaileddescription in connectionwith the accompanying drawings, in which:

FIG. 1 is a partial sectional view of an axial flow steam turbine showing a typical steam chest and sequentially operable steam control valve;

FIG. 2 is an enlarged partial sectional view showing a portion of a seat and plug of a sequentially operable steam control valve made in accordance with this invention;

FIG. 3 is similar to FIG. 2, but shows the plug in its fully open position;

FIG. 4 is an enlarged partial sectional view of a seat v andplug showinganother embodiment of this inven- FIG. 6 is a diagram showing steam flow versus pressure as the steam passes through a typical seven-stage sequentially operable control valve and showing steam flow versus pressure as steam flows through a sequen- FIG. 7 is a Mollier diagram showing the dissipation of energy across a plug and seat arrangement which has five throttling stages.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, FIG. 1 shows a sequentially operable control valve C for an axial flow turbine T having seven separate plugs, l, 2, 3, 4, 5, 6, and 7 and mating seats 1, 2, 3, 4, 5, 6'. and 7, which sequentially lift to admit fluid, in this case steam, to separate segments of an annular array of circumfer' entially spaced nozzles 9. The plug indicated by 1 being the first plug to lift off its seat, the plug indicatedby 2 being the second plug to lift off its seat, thus the numbers indicating the sequence in which the individual plugs lift from their seat as the control valve C opens to admit more steam to the nozzles 9.

When the velocity of the fluid flowing across the seat of a valve exceeds the velocity of sound, Mach 1, i.e., the Mach number which is the ratio of the fluid velocity to the velocity of sound in the fluid is greater than 1.0,

objectionably high intensity noises and vibrations are produced, therefore it is desirable to provide throttling valves which will not produce velocities in excess of Mach 1. i

To provide pressure drops which produce velocities less than Mach 1 in a sequentially operable control valve mating plugs and seats as shown in FIGS. 2, 3, 4

and 5, each have surfaces which cooperate to provideannular openings which increase in size in downstream direction to produce consecutive throttling stages so disposed that the velocity across each throttling stage remains generally equal to each other as separation between the plugs and seats increases. The number of throttling stages is greater in the first plug 1 to separate from its seat 1', as the first plug 1 to separate from its seat 1' handles the greatest pressure drop. Plugs and seats which successively separate have a lesser number of consecutive throttling stages as they are required to provide for smaller pressure drops. The last valve to separate from its seat may have only a single throttling stage as the pressure drop across this plug and seat is normally low enough to produce velocity less than Mach 1.

To minimize the number of consecutive throttling stages, the pressure drop across each stage at the instant the plug begins to separate from its seat should be such as to produce a velocity generally equal to the velocity of sound in the fluid, a. velocity of Mach 1. The

velocities across the individual stages should also remain equal to each other as the plug rises even though the velocities, on a whole, decrease due to a reduction in the pressure drop. 7

To provide equal velocities across the stages the area of the throat or opening of the throttling stages must tially operable control valve made in accordance with this invention; and

increase in a downstream direction, since the specific volume of the fluid, in this case steam, increases as the pressure decreases. To provide equal velocities across the consecutive throttling stages the ratio of the open area of the throats of the throttling stages must remain generally equal as the plug separates from its seat, since the pounds of steam flowing across the stages must be equal.

FIGS. 2 through 5 show three embodiments which will provide increaseddownstream open throat area for consecutive throttling stages which maintains the same ratio of open area as the plug lifts.

FIGS. 2 and 3 show a partial enlarged sectional view of the plug 1 and seat 1, wherein the seat ll has two annular grooves 21 and 23. The grooves 21l and 23 each have rounded protuberances or lips 25 which extend inwardly from the upper edges of the groove. The plug 1 has three annular skirts or collars 27, 28 and 29. The inner skirt 27 seats on an upper surface 30 of the seat 1' and the skirts 28 and 29 register with the grooves 21 and 23 respectively. The skirts 28 and 29 are formed from frusto- conicalsurfaces 31 and 32 and 33 and 34 each of which form an angle a with a cylindrical plane represented by the lines 35, 36, 37 and 38 as shown in FIG. 3. The frustoconical surfaces are so disposed that the angle a increases in downstream direction so that the ratio of the open area of the throttling stages remains generally equal as the plug lifts from the seat to provide generally equal velocities through the consecutive throttling stages. To eliminate the carryover of velocity from one throttling stage to the adjacent downstream throttling stage it may be de sirable to provide a step as indicated at 39.

FIG. 4 shows a partial sectional view of a plug 3 and seat 3 wherein the plug 3 has three annular skirts 430, 41 and 42 which extend downwardly and register with three frusto- conical surfaces 44, 45 and 46 disposed on the seat 3'. The conical surfaces 44, 45 and 46 form an angle B with a series of insersecting cylindrical planes represented by the lines 47, 48 and 49. The conical surfaces are so disposed that the angle B increases on consecutive conical surface in downstream directions to provide increased downstream throat area in the consecutive throttling stages and to maintain a constant ratio between throat areas as the plug 3 lifts from its seat 3. Thus, the velocities through the consecutive throttling stages remains generally equal to each other as the plug separates from its seat.

FIG. shows a partial sectional view of a plug 4 and seat 4', wherein the plug has two annular skirts 50 and 51, which extend downwardly and register with two toroidal surfaces 52 and 53. The outer skirt 51 registers with the upper convex toroidal surface adjacent a plane through the center of the toroid which divides the toroid into two equal size rings. The inner skirt 50 registers with the convex portion of the toroid adjacent the top of the toroid. The skirts thus cooperate with the toroidal surfaces to provide consecutive throttling stages having throat areas which increase in downstream direction and maintain the same ratio of throat area as the plug 4 separates from its seat 4. Thus, the arrangement of skirts and toroidal surfaces provide velocities through the consecutive throttling stages which are generally equal to each other as the plug 4 separates from its seat 4. The toroidal surfaces are so disposed to provide a step 55 between the throttling stages to prevent carryover of velocity from one stage to the adjacent downstream stage.

FIG. 6 shows a graphical representation of pressure versus flow for each plug as it lifts from its seat until it is fully opened. The solid lines 1'', 2", 3", 4", 5", 6", and 7" represent the pressure versus flow across plugs l, 2, 3, 4, 5, 6, and 7, and seats 1, 2, 3, 4, 5, 6', and 7, respectively, as the plugs separate from the mating seats of a sequentially operable control valve herebefore made. The dotted curved lines la, lb, 10, 1d, and 1e; 2a, 2b, 2c, 2d; 3a, 312, c; 30, 4a, 4b; 5a, 5b; 6a; and

7a represent the values of pressure and flow of steam downstream of each consecutive throttling stage of the plugs l, 2, 3, 4, 5,6, and 7 and seats I, 2', 3', 4', 5', 5, and 7 made in accordance with this invention. a represents the first throttling stage; b represents the second consecutive downstream throttling stage, c the third stage and so on for each plug and seat. As noted hereinbefore, the plugs and seats are numbered in the sequence in which the plugs lift. Thus a sequentially operable control valve made in accordance with the invention and controlling a turbine T supplied with 3,690 psi steam may have, as shown in FIG. 6, five consecutive throttling stages in the first plug l and seat l to separate, for consecutive throttling stages in the second plug 2 and seat 2' to separate, three consecutive throttling stages in the third plug 3 to separate from its seat 3 and two consecutive throttling stages in the fourth and fifth plugs 4 and 5 to separate from their seats 4 and 5". The sixth and seventh plug 6 and 7 to separate from their seats 6' and 7' may only provide a single throttling. A solid line 611 at the top of FIG. 6 represents the steam inlet pressure of 3,690 pounds per square inch. Line 63 angled downwardly from line 61 represents the steam pressure to the plugs and seats and shows the reduction of pressure with increased flow past the plugs and seats due to the pressure drop in the inlet piping. A dotted line 64 represents a critical pressure drop, that is the pressure drop across the plug and seat which will produce velocities generally equal to the velocity of sound in the fluid, Mach 1. It should be noted that the pressure drop across the first stage (1" of each plug is generally equal to or less than the critical pressure drop, therefore the velocity across the first stage is equal to or less than Mach 1. Since the velocities across the consecutive stages are generally equal, none of the velocities produced by a valve made in accordance with this invention are greater than Mach 1. Thus, such a sequentially operable control valve will not produce high intensity noises and vibration.

The consecutive throttling stages in each plug are generally labyrinth type throttling stages which provide increased throat area in the downstream direction to produce generally equal velocities across each stage and generally equal dissipation of energy across each stage as shown by plotting line 65 the pressure drop across the plug 1 and seat 1 shown in FIGS. 2 and 3 on the Mollier diagram shown in FIG. 7. The initial pressure is 3,690 psi and the first stage of throttling between the frustoconical surface 34 and the lip 25 reduces the pressure to 2,050 psi, producing a velocity approximately equal to Mach 11 as the plug begins to lift. However, as soon as the flow rate across the plug and seat increases the velocity decreases below Mach 1. The second stage of throttling between the frustoconical surface 33 and the lip 25 reduces the pressure to 1,140 psi and produces a velocity approximately equal to Mach 1, as the plug begins to lift, but as before, as the flow rate across the second throttling stage increases the velocity decreases below Mach 1. The third and fourth throttling stages between the frustoconical surfaces 32 and 31 and lips 25 reduce the pressure to 635 psi and 353 psi, respectively, and each throttling stage produces velocities approximately equal to Mach 1, as the plug begins to lift. As noted earlier, as rate of steam flowing past the stages increases the velocity decreases below Mach 1. The fifth throttling stage between the inner skirt 27 and the surface reduces the pressure of the steam to 196 psi and also produces velocity approximately equal to Mach 1 as the plug separates from the seat. As steam begins to flow past the plug and seat the velocity reduces to a velocity below Mach 1.

As indicated by the line 65 on the Mollier chart in FIG. 7 the dissipation of energy, change in entropy, is generally equal for each throttling stage so that consecutive throttling provides efficient throttling with a minimum number of stages to handle the pressure drop as the plug begins to separate from its seat and yet provides minimal pressure drop across the plug and seat when the plug is fully opened to optimize the flow of steam over the plugs and seats at each extreme in the operation of the sequentially operable control valve. Thus providing a sequentially operable control valve which is free for high intensity noises and vibrations when operating at low loads and which has a low pressure drop when operating near its fully open position.

What is claimed is: l. A valve for controlling the flow of an expandable fluid, said valve comprising a plug and a seat, said plug and seat having means disposed thereon for producing consecutive throttling stages, said consecutive throttling stages comprising a plurality of annular grooves and a plurality of collars which register with said grooves, said collars having frustoconical surfaces, said frusto-conical surfaces being disposed at different angles and being so disposed that the angle of the downstream frusto-conical surfaces are greater than the angle of the upstream frusto-conical surfaces to produce consecutive throttling stages in which the velocity of the fluid flowing through the stages remainsgenerally equal to each other as the plug separates from the seat.

2. A valve as set forth in claim 1, wherein grooves have annular rounded protuberances which extend inwardly from the edges of said grooves.

3. A valve as set forth in claim 1, and further comprising a step disposed between adjacent frusto-conical surfaces to eliminate carryover of velocity from one stage to an adjacent stage.

Claims (3)

1. A valve for controlling the flow of an expandable fluid, said valve comprising a plug and a seat, said plug and seat having means disposed thereon for producing consecutive throttling stages, said consecutive throttling stages comprising a plurality of annular grooves and a plurality of collars which register with said grooves, said collars having frustoconical surfaces, said frusto-conical surfaces being disposed at different angles and being so disposed that the angle of the downstream frusto-conical surfaces are greater than the angle of the upstream frustoconical surfaces to produce consecutive throttling stages in which the velocity of the fluid flowing through the stages remains generally equal to each other as the plug separates from the seat.

2. A valve as set forth in claim 1, wherein grooves have annular rounded protuberances which extend inwardly from the edges of said grooves.

3. A valve as set forth in claim 1, and further comprising a step disposed between adjacent frusto-conical surfaces to eliminate carryover of velocity from one stage to an adjacent stage.

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