US3763894A

US3763894A - Sequentially operable control valve for a steam turbine

Info

Publication number: US3763894A
Authority: US
Inventors: C Meyer
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1971-06-16
Filing date: 1971-06-16
Publication date: 1973-10-09
Anticipated expiration: 1990-10-09

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 51 Oct. 9, 1973 SEQUENTIALLY OPERABLE CONTROL VALVE FOR A STEAM TURBINE Charles A. Meyer, Media, Pa.

Westinghouse Electric Corporation, Pittsburgh, Calif.

June 16, 1971 Inventor:

Assignee:

Filed:

Appl. No.:

References Cited UNITED STATES PATENTS 5/1967 Rankin.... 137/630.19

4/1938 Rosch 251/333 X 7/1933 Lee 251/333 X Primary Examiner-Robert G. Nilson Attorney-A. T. Stratton, et al.

5 7] 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 stagesbeing 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.

SHEF 1UP 4 PATENTED OCT 9 I973 FIG.!

1C l m m FIG.2

1 SEQUENTIALLY OPERABLE CONTROL VALVE FOR A STEAM TURBINE 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 vibratrons.

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 OFTHE DRAWINGS 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 and plug showing another embodiment of this invention;

. FIG. 5 is an enlarged partial section view of a seat and plug showing a third embodiment of this invention;

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 sequentially operable control valve made in accordance with this invention; and

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 circumferentially spaced nozzles 9. The plug indicated by 1 being the first plug to lift off its seat, the plug indicated by 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.

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 provide annular 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.

To provide equal velocities across the stages the area of the throat or opening of the throttling stages must 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 acrossthe consecutive throttling stages the ratio of the open

annular grooves

21 and 23. The

grooves

21 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

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

frustoconical surfaces

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 or 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 desirable 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

40, 41 and 42 which extend downwardly and register with three

frustoconical 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 [3 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. 5 shows a partial sectional view ofa 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 tor oid 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 t0- roidal 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 1a, lb, 10, 1d, and 1e; 20, 2b, 2c, 2d; 30, 3b, 36; 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

1, 2', 3, 4', 5', 6', and 7 made in accordance with this invention. a

represents the first throttling stage; b represents the second consecutive downstream throttling stage, 0 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 1 and seat 1' 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 61 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 a 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 1 as the plug begins to lift. However, as soon as the flow rate acrossthe 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 l, 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 30 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:

1. A sequentially operable control valve comprising a plurality of mating plugs and seats, actuating means common to said plugs and disposed to successively separate said plugs from said seats to allow fluid to flow thcrebetween, said mating plugs and seats having means disposed thereon for producing consecutive throttling stages as fluid passes therebetween, the number of consecutive throttling stages being greatest on the first plug and seat to separate, the mating plugs and seats to successively separate having a lesser number of consecutive throttling stages.

2. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages are formed by registering surfaces on said mating plugs and seats cooperating to provide annular openings which increase in size in downstream direction.

3. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages are formed by registering surfaces cooperatively associated to produce annular openings between plugs and seats and the openings are so disposed that the velocities therethrough remain generally equal to each other as the separation between said plugs and seats increases.

4. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of annular skirts disposed on the plug, said skirts registering with generally conical surfaces on the mating seat, the slope of the outer conical surface being greater than the slope of the inner conical surface thus providing generally equal velocities between the skirts and conical surfaces as the separation between the mating plug and seat increases.

5. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of annular skirts disposed on the plug, at least one skirt having sloping sides which converge adjacent the mating seat, the mating seat having at least one annular groove, said skirt and groove cooperating to provide a pair of .consecutive throttling stages which have openings which increase in size in downstream direction and which are so disposed that velocities across the openings remain generally equal to each other as the separation between the mating plug and seat increases.

6. A control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of annular skirts disposed on at least one plug, said skirts register with generally toroidal surfaces on the mating seat, said skirts and toroidal surfaces being so arranged to provide annular openings through which the velocities generally remain equal to each other as the separation between the mating plugs and seats increases.

7. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of concentric annular skirts disposed on the plug, the outer skirts being flared outwardly, the skirts registering with generaly toroidal surfaces, said toroidal surface which registers with said outer skirt being an upper convex portion adjacent a plane through the center of a toroid and said toroidal surface which registers with the inner skirt being a convex surface adjacent the top of a toroid, said toroidal surfaces being so disposed to form steps, each step being a throttling stage and so arranged to provide generally equal velocities through the consecutive throttling stages as the separation between the mating plug and seat increases.

8. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of concentric skirts disposed on the plug, said skirts registering with surfaces on the mating seat in such a manner to provide labyrinth type consecutive throttling stages which increase in area in downstream direction and the ratio of the areas remains essentially constant as the plug and mating seat separate.

Claims

1. A sequentially operable control valve comprising a plurality of mating plugs and seats, actuating means common to said plugs and disposed to successively separate said plugs from said seats to allow fluid to flow therebetween, said mating plugs and seats having means disposed thereon for producing consecutive throttling stages as fluid passes therebetween, the number of consecutive throttling stages being greatest on the first plug and seat to separate, the mating plugs and seats to successively separate having a lesser number of consecutive throttling stages.

5. A sequentially operable control valve as set forth in claim 1, wherein the means for producing consecutive throttling stages comprises a plurality of annular skirts disposed on the plug, at least one skirt having sloping sides which converge adjacent the mating seat, the mating seat having at least one annular groove, said skirt and groove cooperating to provide a pair of consecutive throttling stages which have openings which increase in size in downstream direction and which are so disposed that velocities across the openings remain generally equal to each other as the separation between the mating plug and seat increases.