Device for controlling opening of an on-off valve of a steam turbine system with a bypass line

Abstract

 A device (20) for controlling opening of an on-off valve (IV) of a steam turbine system (1) having a bypass line parallel to a high-pressure turbine section; the device has a speed relay (22) for controlling opening of the regulating and on-off valves (CV, IV) by means of lever mechanisms (21) which correlate opening of the on-off valve (IV) with opening of the regulating valve (CV); a secondary relay (24) is provided to transmit control from a first portion (25) of the lever mechanisms (21), activated by the speed relay (22), to a second portion (26) of the lever mechanisms (21); and the secondary relay has a mechanical feedback cam (50).

Description

[0001] The present invention relates to a device for controlling opening of an on-off valve of a steam turbine system with a bypass line parallel to a high-pressure turbine section.

[0002] In a conventional-cycle system with no so-called RH bypass, i.e. with a heater between the high- and medium-pressure sections of the turbine, the turbine is rotated by an incoming stream of so-called SH steam fed to the high-pressure section via so-called CV regulating or control valves. The outgoing stream of so-called RH steam from the high-pressure section is heated and fed to the medium-pressure section via so-called IV feed or on-off valves.

[0003] During startup, the IV valves are opened before the CV valves : more specifically, when the CV valves begin opening, the IV valves are already open roughly 20 mm to allow the steam from the high-pressure section to flow into the RH steam line.

[0004] Unlike conventional-cycle systems, combination-cycle systems are not normally expected to operate at the rated load for prolonged periods of time, and so call for a high degree of flexibility.

[0005] In other words, the output load of a combination cycle depends on the "energy market", so the system must operate, not continuously, but at predetermined times according to market demand.

[0006] Given the necessity to start and stop the system fairly frequently, a bypass line parallel to the high-pressure section is employed to facilitate startup and stoppage.

[0007] When starting up a system with a bypass line, the RH steam line is pressurized by the bypass line itself, which means the CV and IV valves cannot be opened using the same timing as for systems with no bypass lines. In fact, assuming the same timing is used, and bearing in mind that a stream of pressurized steam is fed into the medium-pressure section, speed and load buildup would be practically impossible to control. More specifically, during startup, the pressure at the high-pressure section exhaust would not correspond to that of the SH steam flow inside the high-pressure section.

[0008] When switching from a non-bypass to a bypass system, therefore, opening of the CV and IV valves must be retimed to permit startup while the RH steam line is pressurized.

[0009] The CV and IV valves must be retimed to avoid ventilation problems in the final stages of the high-pressure section. Ventilation is the phenomenon whereby the steam in said stages is heated as a result of poor efficiency of the high-pressure section caused by very low steam flow. Ventilation increases the temperature of the steam at the high-pressure section exhaust with respect to nominal values, thus resulting in turbine problems, and is directly proportional to the pressure at the high-pressure section exhaust.

[0010] In known combination-cycle systems, the CV and IV valves are opened by means of an electrohydraulic (so-called DEHC) system, i.e. which regulates the opening pattern of the CV and IV valves during startup by means of electrohydraulically controlled actuators.

[0011] This system, however, is fairly complex, and is relatively expensive and takes a relatively long time to install and calibrate when converting a "non-bypass system" to a "bypass system".

[0012] It is an object of the present invention to provide a device for controlling opening of an on-off valve of a steam turbine system with a bypass line, designed to provide a straightforward, low-cost solution to the above problems.

[0013] According to the present invention, there is provided a device for controlling opening of an on-off valve of a steam turbine system with a bypass line, as claimed in Claim 1.

[0014] A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows, schematically, a steam turbine system with a bypass line;

Figure 2 shows a partial schematic view of a device for controlling opening of an on-off valve of the Figure 1 system, in accordance with the present invention;

Figure 3 shows a graph illustrating the degree of opening of the Figure 1 system valves;

Figure 4 shows a component part of the Figure 2 device.

[0015] Number 1 in Figure 1 indicates a combination-cycle steam turbine system comprising a high-pressure first turbine section 2; a medium-pressure second turbine section 3; and a low-pressure third turbine section 4.

[0016] Section 2 is supplied by a line 7 with so-called SH steam, which is flow-regulated by a CV valve.

[0017] The outlet of section 2 communicates with the inlet of section 3 over a line 8 fitted with an RH heater. The flow of so-called RH steam to section 3 is regulated by an IV valve downstream from the RH heater.

[0018] The outlet of section 3 communicates with the inlet of section 4 over a line 9; and a condenser 10 is provided at the outlet of section 4.

[0019] System 1 comprises a first bypass line 11, which connects line 8 to condenser 10, is parallel to the IV valve and to sections 3 and 4, and has a valve 12 for regulating the steam flow fed directly to condenser 10 without going through sections 3 and 4. System 1 also comprises a second bypass line 13, which connects lines 7 and 8, is parallel to section 2 and to the CV valve, terminates upstream from the RH heater, and has a bypass valve 14 for regulating the steam flow fed directly to the RH heater and to section 3 without going through section 2.

[0020] Opening of the CV and IV valves during startup is controlled by a control device 20 shown partly in Figure 2.

[0021] Device 10 provides for mechanically regulating speed and load buildup by controlling opening of the CV and IV valves, in time with each other, by means of lever mechanisms 21 (shown partly). By virtue of lever mechanisms 21, the opening geometry and mode of the CV and IV valves are predetermined, rigidly defined, and mutually correlated, i.e. timing of the CV and IV valves is defined at the design stage.

[0022] Device 20 comprises a control member or so-called speed relay 22, the position of which determines the opening of the CV and IV valves. More specifically, speed relay 22 can be set as a function of the speed and load buildup requirements of system 1.

[0023] When starting up system 1, opening of the CV and IV valves is controlled in a definite sequence. The established opening pattern of the CV and IV valves is shown in the Figure 3 graph, in which the y axis shows the degree of opening or lift of the valves expressed as a percentage, and the x axis shows displacement of speed relay 22 in millimetres.

[0024] Curve (c) shows the CV valve opening or lift pattern, and curve (b) the IV opening or lift pattern. In other words, regulating device 20 produces predetermined valve opening patterns defined by curves (b) and (c).

[0025] With reference to Figure 2, to control the IV valves, device 20 comprises a transmission member or so-called secondary relay 24, which transmits control from a first portion 25 of lever mechanisms 21, activated by speed relay 22, to a second portion 26 of lever mechanisms 21, with controls opening of the IV valves in a manner not shown.

[0026] Secondary relay 24 determines the way in which opening of the CV and IV valves is correlated, and comprises: a single-acting actuator 27 with a rod 28, translation of which activates portion 26 of lever mechanisms 21; a control valve 29 with a piston 30 for regulating oil flow to actuator 27; a mechanical feedback system 31 activated by rod 28; and a lever 32 which activates piston 30.

[0027] The ends 34, 35 of lever 32 are hinged, about movable axes, to respective substantially vertical top rods 36, 37 forming part of portion 25 of lever mechanisms 21 and system 31 respectively.

[0028] An intermediate portion 38 of lever 32 is hinged about a movable axis to a bottom rod 39, which moves piston 30.

[0029] Rod 37 activates end 35 vertically in opposition to an elastic member 40, and is hinged at its top end to an intermediate portion 41 of a further lever 42 forming part of system 31.

[0030] One end 43 of lever 42 is hinged to oscillate about a fixed axis, and the opposite end is fitted with a hinged cam-follower roller 48, which rolls along the outer periphery of a disk cam 50 also forming part of system 31. Cam 50 is connected to rod 28 by a rack-and-pinion transmission, not shown in the drawings, and rotates about a respective fixed axis 51 in response to translation of rod 28.

[0031] For example, downward movement of rod 36 opens control valve 29, and so raises rod 28 to open the IV valve; cam 50 therefore rotates anticlockwise, with reference to the attached drawings, to raise roller 48 and therefore end 35 of lever 32; which upward movement defines the amount of mechanical feedback controlling control valve 29, and closes control valve 29 to maintain the degree of opening of the IV valve set by speed relay 22.

[0032] Cam 50, which provides for mechanical position feedback of the IV valve, is designed to obtain the IV valve lift curve (b) shown in Figure 3.

[0033] With reference to Figure 4, the profile of cam 50 has a steeply sloping straight portion 52 (Figure 4).

[0034] Portion 52 is connected, on one side, to a circular initial portion 53 by a connecting surface 54 of 16 mm in radius, equal to the radius of roller 48, and, on the other side, to a circular end portion 55 by a connecting surface 56.

[0035] The profile of cam 50 is defined by the following movement of roller 48 (with respect to circular portion 53) as a function of the rotation angle of cam 50 (with respect to an initial angular position in which rod 28 is fully lowered) :

Roller movement [mm]	Cam rotation [°]
0	22
1,0	24
1,9	26
2, 9	28
4, 8	32
7, 6	37
11, 4	45
17, 1	85
22,8	148
28, 6	210
34,3	270

[0036] The profile of cam 50 so defined provides the necessary mechanical feedback to achieve optimum correlation between the opening of the CV and IV valves. At the first opening stage, a more gradual IV valve opening curve than in curve (b) in Figure 3 is theoretically possible, provided the pressure angle A between cam 50 and roller 48 at the steepest point along the profile is less than 60°, to prevent roller 48 from sticking (pressure angle A is defined at the angle between the line perpendicular to the profile of cam 50, and the line tangent to the rotation axis of lever 42).

[0037] More specifically, with the chosen profile defined in the above Table, pressure angle A at the end of profile portion 52, i.e. the point at which pressure angle A is maximum, equals 56.4°.

[0038] As will be clear from the foregoing description, device 20 ensures correct operation of a steam turbine by means of strictly preconfigured mechanical adjustment, thus enabling fast easy conversion of a system originally designed with no bypass.

[0039] Clearly, changes may be made to device 20 as described herein without, however, departing from the scope of the present invention.

Claims

1. A control device for a steam turbine system comprising:

- a first and second turbine section;

- a first line for feeding steam to the first section;

- a second line extending from the outlet of said first section to the inlet of said second section;

- a controlled regulating valve for regulating steam flow to said first section;

- an on-off valve for regulating steam flow to said second section;

- a bypass line between the first and second line and parallel to said first section;

the control device being characterized by comprising:

- a speed relay (22) controlling opening of said regulating and on-off valves (CV, IV);

- lever means (21) for transmitting control to said on-off valve (IV) and correlating opening of said on-off valve (IV) with opening of said regulating valve (CV);

- a secondary relay (24), which transmits control from a first portion (25) of said lever means (21), activated by said speed relay (22), to a second portion (26) of said lever means (21), and comprises:

a) a fluidic actuator (27);

b) a control valve (29) activated to regulate fluid flow to said actuator (27);

c) a mechanical feedback system (31) activated by said actuator (27) and comprising:

(1) a cam rotating about a fixed axis;

(2) a lever hinged about a fixed axis and fitted, at one end, with a cam-follower connected to said cam;

the profile of said cam defining a movement of said cam-follower (48) as a function of the angle of rotation of said cam (50), with respect to an initial position, of at least the following values:

Movement [mm]	Cam rotation [°]
0	22
1, 0	24
1,9	26
2,9	28
4,8	32
7,6	37
11,4	45
17,1	85
22,8	148
28,6	210
34,3	270

2. A device as claimed in Claim 1, characterized in that the pressure angle (A) between the cam (50) and the cam-follower (48), defined as the angle between the line perpendicular to the cam profile and the line tangent to the axis of rotation of said lever, is less than 60°.

Drawing