EP4532902A1

EP4532902A1 - Method and device for controlling a steam turbine

Info

Publication number: EP4532902A1
Application number: EP23730024.9A
Authority: EP
Inventors: Hans-Martin SCHELL; Cornelia GROHS; Sascha WILLEMS; Andrei Gonioukh; Andreas Schicht; Gunter HARMENS
Original assignee: Basell Polyolefine GmbH
Current assignee: Basell Polyolefine GmbH
Priority date: 2022-05-31
Filing date: 2023-05-26
Publication date: 2025-04-09
Anticipated expiration: 2043-05-26
Also published as: WO2023232704A1; EP4532902B1; EP4286661A1

Abstract

A method and device for controlling a steam turbine, in which the steam turbine is controlled via a primary servo valve system as a main control circuit and a secondary servo valve system as a backup control circuit, the servo piston of the secondary servo valve system being kept in constant motion.

Description

METHOD AND DEVICE FOR CONTROLLING A STEAM TURBINE FIELD OF THE DISCLOSURE [0001] The present disclosure relates to a method and a device for controlling a steam turbine, in which the steam turbine is controlled via a primary servo valve system as a main control circuit and a secondary servo valve system as a backup control circuit, the servo piston of the secondary servo valve system being kept in constant motion. BACKGROUND OF THE DISCLOSURE [0002] Turbo-compressors are generally used when large gas volume flows have to be com- pressed, for example in compression installations for increasing the pressure in gas pipelines, as blowers in blast furnaces or steel mills, in air or gas liquefaction plants, as air or nitrous gas com- pressors in nitric acid plants, in petrochemical plants and refineries or as vacuum blowers in the paper industry. The choice of the drive unit for the turbo-compressor depends on the application. Whilst gas turbines are employed in gas pipeline and offshore applications, electric motors with frequency inverters are mostly used as a drive for small and medium power requirements. In plants in which sufficient and reliable amounts of steam is available, as is generally the case in chemical plants, steel mills or ironworks, a steam turbine is preferably used as the drive. [0003] Generally, the steam turbine is controlled by using a steam control valve to regulate the amount of steam supplied to the turbine, so as to guarantee the rotational speed of the turbo-com- pressor necessary for the production process. [0004] The steam control valves are mostly equipped with a hydraulic control system, in which the steam flow, and thus the rotational speed of the turbo-compressor, are controlled by varying the flow rate of the hydraulic liquid. Oil is often used as the hydraulic liquid. [0005] Such control systems are described in various references. [0006] CN105545842 describes a control system and a control method for synchronizing a static TRT blade actuator, the synchronization being achieved with the use of two servo valves, a manual reversing valve and an electromagnetic reversing valve. [0007] CN 109268347 discloses a steam turbine interruption early warning system for a set of generators, which system comprises a main oil pump, a servo switch valve, a cartridge valve, a pilot valve, a hydraulic pressure controller, a DCS control and an oil drive. The early warning system is to early detect interruptions, for example due to an oil leak. [0008] JP2022027090 relates to a method for monitoring the opening of a steam control valve for increasing or reducing the amount of steam supplied to a turbine, wherein a first alarm is trig- gered, if a difference between the opening of the steam control valve and a target opening is de- tected, and a second alarm is triggered, if the deflection of a hydraulic actuator does not match a predefined target value. [0009] It is the object of JP 2019031941 to provide a steam valve drive device and a stem valve that can be operated continuously, even if a non-conformity occurs in the control valve. This object is intended to be achieved by the steam valve drive device comprising a control device, a first control valve for controlling an amount of working fluid, a second control valve arranged parallel to the first control valve, a first and a third stop valve for interrupting the supply of a working fluid to the control valve and for discharging the working fluid from the control valve, a second and a fourth stop valve for interrupting the supply of the working fluid to a hydraulic cylinder device from the control valve and for discharging the working fluid to the control valve from the hydraulic cylinder device, and a bypass flow channel. The steam valve drive device continually controls the steam valve through the second control valve on the basis of a signal from the control device, closes the first and second stop valves and discharges the working fluid in the first control valve between the first and second stop valves and the first control valve via a bypass flow channel, if a malfunction occurs in the first control valve. However, the system described has the drawback that the amount of steam has to be reduced significantly during the inspection. The instabilities and fluctuations caused thereby are a problem and may have adverse effects on the process. [0010] EP 0128284 A1 discloses a control and monitoring device for turbo-engine control valves for industrial turbines. The turbines have a servo piston cylinder system for controlling the control valve and an electro-hydraulic transducer for producing a hydraulic correcting variable for the servo piston cylinder system as a function of an electrical correcting variable. [0011] It is a further problem with such control systems that the control and lubricating oil circuits of the turbo-compressor and of the steam turbine receive the oil from the same reservoir so that a mixing of the oil flows from the two applications occurs. Due to thermal stress and contact with process media, the oil ages, forming solid particles in the process. These contaminations may cause an obstruction of the valves, in particular of the sensitive servo valves, which in the worst case may result in a failure of the installation. This problem is exacerbated in the components in which there is no constant flow or in which the flow rate is low, as for example in emergency systems which are intended to take over control of the steam turbine in case of a malfunction or a failure of the main control system. Thus, there is a risk that the emergency system does not operate and the installation comes to a standstill which generally comes with a great financial loss. [0012] Against this backdrop, there is thus a need to provide a method that enables a reliable control of a steam turbine. SUMMARY OF THE DISCLOSURE [0013] The present disclosure provides a method, in which a primary servo valve system is pro- vided as a main control circuit and a secondary servo valve system is provided as a backup control circuit for controlling a steam turbine, and in which the secondary servo valve system is continually kept in motion and flown through. [0014] Therefore, a first subject matter of the present disclosure relates to a method for control- ling a steam turbine, the method comprising the following steps: i) providing a primary servo valve system as a main control circuit for controlling the steam flow entering the steam turbine; ii) providing a secondary servo valve system as a backup control circuit for controlling the steam flow entering the steam turbine; wherein the servo piston of the secondary servo valve system is freely movable between a first position and a second position; iii) generating a control oil return flow from the secondary servo valve system; iv) cyclically moving the servo piston between the first position and the second position within a period tx and simultaneously sensing the pressure in the control oil return flow; v) recording the sensed pressure values while forming a maximum and a minimum pressure value vi) triggering an alarm signal, if the measured pressure fails to reach a minimum and/or maximum target pressure. [0015] The method of the present disclosure keeps the secondary servo valve system, which serves as a backup in case of a failure of the primary servo valve system, is kept in motion und is constantly flown through by the control oil flow, so that a seizure of the servo piston is prevented. Thus, it is ensured that also the secondary servo valve system is always operational and can take over the control of the steam turbine should the primary servo valve system fail. Costly failures of the installation and an associated production stop can be avoided in this manner. [0016] The maximum and minimum pressure value are measured values formed from the re- cording of the sensed pressure values. As the servo piston cyclically moves between the first posi- tion and the second position within a period tx the pressure in the control oil flow return varies within a certain range, in other words the pressure goes up and down depending on the position of the servo piston. Thus, a maximum and minimum pressure, i.e. the end points of the range, can be measured, for example for each iteration of the cyclical movement of the servo piston. Said meas- ured maximum and minimum pressure may then be compared to the target pressure, which may be preset. If the measured pressure does not reach the target pressure, the alarm signal will be triggered. [0017] In some embodiments, the first position is a discharge position of the secondary servo valve system and the second position is a supply position of the secondary servo valve system. [0018] In some embodiments, the servo piston of the secondary servo valve system is continu- ously moved between the first position and the second position. [0019] In some embodiments, the servo piston is moved from the first position to the second position within a first period t1. [0020] In some embodiments, the servo piston is moved from the second position to the first position within a second period t2. [0021] In some embodiments, tx may be t1+t2. Additionally or alternatively t1 and t2 may be equal. [0022] In some embodiments, a primary control oil flow may be supplied to the primary servo valve system from an oil receptacle and may be supplied from there to an actuator via a magnetic switch valve, which actuator controls the steam supply to the steam turbine. Furthermore, a sec- ondary control oil flow may be supplied to the secondary servo valve system from an oil receptacle and is returned into the oil receptacle via the magnetic switch valve, wherein the magnetic switch valve is controlled such that the control oil flow from the secondary servo valve system is supplied to the actuator if the primary servo valve system fails. [0023] In some embodiments, the alarm is triggered if the pressure in the control oil return flow does not reach the minimum and/or the maximum target pressure value within a period tz, wherein tz is equal to the sum of t1 and x and/or tz is equal to t2 and x, with x representing a freely selectable waiting period. [0024] In some embodiments, the steam turbine drives a turbo-compressor in a petrochemical installation, in particular a cracker, or a generator in a power plant. [0025] A further subject matter of the present disclosure relates to a device for controlling a steam turbine, comprising i) an oil tank containing a control oil; ii) a primary control circuit comprising a primary servo valve system; iii) a secondary control circuit comprising a secondary servo valve system; iv) a magnetic switch valve; v) a control unit for controlling the steam flow to the steam turbine and vi) an alarm system, wherein the primary control circuit and the secondary control circuit are connected to the oil tank, wherein the primary servo valve system and the secondary servo valve system are configured to guide the control oil flow to the magnetic switch valve, wherein the magnetic switch valve is controlled such that it switches from the primary servo valve system to the secondary servo valve system if the primary servo valve system is not operational; wherein the secondary control circuit comprises a limiting orifice, a valve and a measuring unit that are arranged between the oil tank and the secondary servo valve system; wherein the measuring unit is designed to measure a minimum pressure value and/or a maximum pressure value in the control oil flow, which is generated by cyclically moving a servo piston in the secondary servo valve system between a first position and a second position, wherein the alarm system is configured such that an alarm signal is triggered if the measured pres- sure value does not reach a maximum target pressure value and/or a minimum target pressure value. [0026] In some embodiments, the alarm system is triggered if the measured pressure value does not reach a maximum target pressure value and/or a minimum target pressure value. [0027] In some embodiments the alarm system is configured such that an alarm is triggered if the maximum target pressure value and/or the minimum target pressure value is not reached within a time period tz. [0028] In some embodiments, the device is operated in a petrochemical installation. In particu- lar, the device may be operated in a cracker or in a power plant. [0029] The present disclosure is illustrated with reference to the following Figures which should by no means be understood as limiting the idea of the disclosure. [0030] The combination of features shown and described in the individual exemplary embodi- ments serves solely the purposes of explanation. According to the statements above, it is possible to dispense with a feature of an exemplary embodiment if its technical effect is of no importance in a particular application. Conversely, according to the above statements, a further feature can be added in an exemplary embodiment if its technical effect is meant to be advantageous or necessary for a particular application. BRIEF DESCRIPTION OF THE DRAWINGS [0031] Figure 1 shows an exemplary pressure recording of the control oil return flow within the scope of the method according to the disclosure, wherein the recording shows the pressure build- up and the pressure drop which are generated by the cyclical movement of the servo piston in the secondary servo valve system. [0032] Figure 2 shows a schematic structure of the system according to the disclosure com- prising 1. oil reservoir 2. limiting orifice and adjusting valve 3. bearing lubrication and sealing oil system of the steam turbine and the crude gas turbo-com- pressor 4. pressure measurement secondary servo valve system 5. primary servo valve system 6. secondary servo valve system 7. switch valve 8. steam supply 9. steam control valve 10. steam turbine DETAILED DESCRIPTION OF THE DISCLOSURE [0033] A first subject matter of the present disclosure relates to a method for controlling a steam turbine, the method comprising the following steps: i) providing a primary servo valve system as a main control circuit for controlling the steam flow entering the steam turbine; ii) providing a secondary servo valve system as a backup control circuit for controlling the steam flow entering the steam turbine; wherein the servo piston of the secondary servo valve system is freely movable between a first position and a second position; iii) generating a control oil return flow from the secondary servo valve system; iv) cyclically moving the servo piston between the first position and the second position within a period tx and simultaneously sensing the pressure in the control oil return flow; v) recording the sensed pressure values while forming a maximum and a minimum pressure value; vi) triggering an alarm signal, if the measured pressure fails to reach a minimum and/or maximum target pressure. [0034] The method of the present disclosure keeps the secondary servo valve system, which serves as a backup in case of a failure of the primary servo valve system, is kept in motion und is constantly flown through by the control oil flow, so that a seizure of the servo piston is prevented. Thus, it is ensured that also the secondary servo valve system is always operational and can take over the control of the steam turbine should the primary servo valve system fail. Costly failures of the installation and an associated production stop can be avoided in this manner. [0035] The maximum and minimum pressure values are measured values formed from the re- cording of the sensed pressure values. As the sensed pressure values vary within a certain range depending on the position of the servo piston, it is possible to measure the maximum and minimum pressure values while recording the sensed pressure values. In particular, a new maximum and minimum pressure value may be measured for each iteration of the cyclical movement of the servo piston. The measured pressure may then be compared to target pressure values and if those target pressure values are not reached an alarm will be triggered. [0036] In some embodiments of the method according to the disclosure a primary control oil flow is supplied from an oil receptacle to the primary servo valve system from which the control oil flow is supplied on through a magnetic switch valve to an actuator that controls the steam supply to the steam turbine. A secondary control oil flow is supplied from the oil receptacle to the secondary servo valve system and is returned into the oil receptacle. In a further embodiment, the magnetic valve is controlled such that the control oil flow is supplied from the secondary servo valve system to the actuator if the primary servo valve system fails. [0037] In conventional installations, a combined lubrication oil and control oil system is used, whereby obstructions of the control valve can occur, for example due to abraded material accumu- lated in the oil. According to the method of the disclosure, the control oil flow is returned from an oil receptacle back into the oil receptacle through the secondary servo valve system, if the secondary servo valve system is in the backup mode. The regular flushing of the secondary servo valve system creates a flushing effect, by which small contaminations that would otherwise accumulate in the valve are flushed away. Furthermore, the accretion of solids is prevented by the continuous move- ment of the servo piston. Thus, upon a failure of the main control circuit, the secondary servo valve system can be used without delay to control the steam turbine, in which the control oil flow is then supplied by the secondary servo valve system to the actuator which controls the steam supply to the steam turbine. Moreover, a negative impact on the steam flow in regular operation, which is partly common in conventional methods, is prevented. In some embodiments, as part of the method of the method of the disclosure, control oil and lubricating oil are taken from the same oil tank. [0038] In some embodiments, the servo piston of the secondary servo valve system is continu- ally moved between the first position and the second position. This continuous movement of the servo piston causes a cleaning effect on the running surfaces of the cylinder and the piston, so that an accumulation of an accretion that would cause a blocking of the cylinder is prevented. Surpris- ingly, it has been observed that the cyclical and continuous movement of the piston causes a certain heat input that prevents oil in the system from cooling, which would cause flocculation in the oil and would again result in a blocking of the system. [0039] The continuous movement of the servo piston causes a rising and falling pressure in the control oil return flow, which pressure can be used as a control function of the system. If specific target values, such as a maximum target pressure value and a minimum target pressure value, are not reached, one can conclude on a malfunction of the system and an alarm is triggered. In some embodiments of the method according to the disclosure, the servo piston is moved from the first position to the second position within a first period t1, and thereby a pressure change is caused in the control oil return flow. This pressure change may in an exemplary embodiment be monitored based on target values defined in advance. [0040] In a further exemplary embodiment, the servo piston is moved from the second position to the first position within a second period t2, and thereby a pressure change is caused in the control oil return flow. The pressure change thus caused can be monitored on the basis of target values defined in advance, and thus the correct functioning of the secondary servo valve system can be monitored and maintained.

[0041] In some embodiments of the method according to the disclosure, the servo piston of the secondary servo valve system may be moved cyclically from the first position to the second position and back within a period tx. In a further exemplary embodiment tx may be characterized by the following relationship: tx = t1 + t2.

[0042] The time intervals t1 and t2 can be chosen freely and can be adjusted depending on the application. In an exemplary embodiment, it further holds that t1 = t2.

[0043] The method according to the disclosure provides that an alarm is triggered if the pressure in the control oil return flow does not reach the defined target values. This alarm can be triggered with a delay in time, so that an embodiment is in which the alarm is triggered if the pressure in the control oil return flow has not reached the minimum and/or the maximum target pressure value within a period t_z. In this manner, false alarms can be prevented that may be triggered by slight pressure variations in the control oil return flow. The time interval t_z can be chosen freely and can be adjusted. In one embodiment, it holds that: t_z = t1 + x and/or t2 + x, where x is a freely selectable waiting period. More preferably, it holds that:

60 seconds ≤ tz ≤ 600 seconds, more preferably 120 seconds ≤t z ≤ 300 seconds.

[0044] For the time that the main control circuit comprising the primary servo valve system operates properly, no interference by the backup control circuit has to be provided and its influence on the operation of the steam turbine should be kept as little as possible. In an exemplary embodiment, the control oil flow is therefore returned into the oil receptacle when the servo piston of the secondary servo valve system is in the first position or between the first position and the second position. In addition, a regular flushing of the secondary servo valve system is achieved by this guiding of the control oil flow, so that a clogging of the valve is prevented.

[0045] The control method according to the disclosure is characterized in particular in that it provides for an instantaneous operational readiness of the backup control circuit if the main control circuit should fail. In the context of this protective function, an embodiment is envisioned in which in case of a failure of the primary servo valve system, the control oil flow is supplied by the second- ary servo valve system to an actuator, for example a hydraulic main steam valve, which controls the steam flow to the steam turbine. In this manner, the secondary control circuit can assume the control of the steam turbine, without a failure of the installation occurring. [0046] The control system of a steam turbine generally comprises a number of control and ac- tuation units that control the steam flow supplied to the turbine. In an exemplary embodiment of the method according to the disclosure, the servo valve systems supply the control oil flow to an actu- ator via a magnetic switch valve, the actuator operating a valve that controls the supply of the steam flow to the steam turbine. Should the primary servo valve system fail, the control oil flow can be supplied to the actuator from the secondary servo valve system via the magnetic switch valve and the control oil flow can thus be maintained. [0047] The method according to the disclosure can be used to control any optional steam tur- bine. In one embodiment, the steam turbine drives a turbo-compressor, particularly one used in a petrochemical plant, such as a cracker. In an alternative embodiment, the steam turbine drives a generator in a power plant. [0048] A further subject matter of the present disclosure relates to a device for controlling a steam turbine, comprising i) an oil tank containing a control oil; ii) a primary control circuit comprising a primary servo valve system; iii) a secondary control circuit comprising a secondary servo valve system; iv) a magnetic switch valve; v) an actuator for controlling the steam flow to the steam turbine and vi) an alarm system, [0049] wherein the primary control circuit and the secondary control circuit are connected to the oil tank, wherein the primary servo valve system and the secondary servo valve system are configured to guide the control oil flow to the magnetic switch valve, wherein the magnetic switch valve is controlled such that it switches from the primary servo valve system to the secondary servo valve system if the primary servo valve system is not operational; wherein the secondary control circuit comprises a limiting orifice, a valve and a measuring unit that are arranged between the oil tank and the secondary servo valve system; wherein the measuring unit is designed to measure a minimum pressure value and/or a maximum pressure value in the control oil flow, which is generated by cyclically moving a servo piston in the secondary servo valve system between a first position and a second position, wherein the alarm system is configured such that an alarm signal is triggered if the measured pres- sure value does not reach a maximum target pressure value and/or a minimum target pressure value. [0050] In an exemplary embodiment, the alarm system is configured such that an alarm is trig- gered if the maximum target pressure value and/or the minimum target pressure value is not reached within a period tz. In this manner, it is provided that false alarms are caused due to non- critical delays during pressure adjustment. [0051] In some embodiments, the actuator is the main steam control valve that controls the steam flow supplied to the steam turbine. [0052] In some embodiment, the device according to the present disclosure is operated in a petrochemical installation, preferably a cracker, or in a power plant. [0053] A control oil flow is directed from a reservoir (1), from which also the oil for the bearing lubrication and the sealing oil system of the steam turbine and the crude gas turbo-compressor (3) is taken, into the control system comprising a primary servo valve system (5) and a secondary servo valve system (6). In regular operation, after passing the secondary servo valve system (6), a first part of the control oil flow is returned into the oil reservoir (1) via the switch valve (7), wherein the pressure in this return flow is monitored using a limiting orifice and an adjusting valve (2) as well as a pressure gauge (4). Should the pressure in the secondary servo valve system (6) not reach the predetermined target values within a defined period, an alarm will be triggered. [0054] After passing the primary servo valve system (5), a second part of the control oil flow is supplied via the switch valve (7) to the steam control valve (9) which controls the steam supply (8) to the steam turbine (10). Should the primary servo valve system (5) fail, the control oil flow of the secondary servo valve system (6) can be supplied to the steam control valve (9) via the switch valve (7) and assume the control of the steam flow (8).

Claims

CLAIMS 1. A method for controlling a steam turbine (10), the method comprising: i) providing a primary servo valve system (5) as a main control circuit for controlling the steam flow (8) entering the steam turbine (10); ii) providing a secondary servo valve system (6) as a backup control circuit for controlling the steam flow (8) entering the steam turbine (10); wherein the secondary servo valve system (6) comprises a servo piston that is freely movable between a first position and a second position; iii) generating a control oil return flow from the secondary servo valve system (6); iv) cyclically moving the servo piston between the first position and the second position within a period tx and simultaneously sensing the pressure in the control oil return flow (4); v) recording the sensed pressure values while forming a measured maximum and a meas- ured minimum pressure value from the recording of the sensed pressure values; vi) triggering an alarm signal, if the measured maximum and/or minimum pressure values fail to reach a minimum and/or maximum target pressure.

2. The method according to claim 1, characterized in that the first position is a discharge position and the second position is a supply position.

3. The method according to at least one of the preceding claims, characterized in that the servo piston of the secondary servo valve system (6) is continuously moved between the first po- sition and the second position.

4. The method according to at least one of the preceding claims, characterized in that the servo piston is moved from the first position to the second position within a first period t1.

5. The method according to at least one of the preceding claims, characterized in that the servo piston is moved from the second position to the first position within a second period t2.

6. The method according to at least one of the preceding claims, characterized in that tx = t1 + t2 and/or t1 = t2.

7. The method according to at least one of the preceding claims, characterized in that a primary control oil flow is supplied to the primary servo valve system (5) from an oil receptacle (1) and is supplied from there to an actuator (9) via a magnetic switch valve (7), which actuator controls the steam supply (8) to the steam turbine (10), and a secondary control oil flow is supplied to the secondary servo valve system (6) from an oil receptacle (1) and is returned into the oil receptacle (1) via the magnetic switch valve (7), wherein the magnetic switch valve (7) is optionally controlled such that the control oil flow from the secondary servo valve sys- tem (6) is supplied to the actuator (9) if the primary servo valve system (5) fails.

8. The method according to at least one of the preceding claims, characterized in that the alarm is triggered if the pressure in the control oil return flow does not reach the minimum and/or the maximum pressure value within a period t_z, wherein t_z = t₁ + x and/or t₂ + x, with x repre- senting a freely selectable waiting period.

9. The method according to at least one of the preceding claims, characterized in that the steam turbine (10) drives a turbo-compressor in a petrochemical installation, particularly a cracker, or a generator in a power plant.

10. A device for controlling a steam turbine (10), comprising: i) an oil tank (1) containing a control oil; ii) a primary control circuit comprising a primary servo valve system (5); iii) a secondary control circuit comprising a secondary servo valve system (6); iv) a magnetic switch valve (7); v) a control unit (9) for controlling the steam supply (8) to the steam turbine (10), and vi) an alarm system, wherein the primary control circuit and the secondary control circuit are connected to the oil tank (1), wherein the primary servo valve system (5) and the secondary servo valve system (6) are configured to guide the control oil flow to the magnetic switch valve (7), wherein the magnetic switch valve (7) is controlled such that it switches from the primary servo valve system (5) to the secondary servo valve system (6) if the primary servo valve system (5) is not operational; wherein the secondary control circuit comprises a limiting orifice (2), a valve (2) and a meas- uring unit (4) that are arranged between the oil tank (1) and the secondary servo valve system (6); wherein the measuring unit (4) is designed to measure a minimum pressure value and/or a maximum pressure value in the control oil flow, which is generated by cyclically moving a servo piston in the secondary servo valve system (6) between a first position and a second position, wherein the alarm system is configured such that an alarm signal is triggered if the measured pressure value does not reach a maximum target pressure value and/or a minimum target pressure value.

11. The device according to claim 10, characterized in that the alarm system is configured such that an alarm is triggered if the maximum target pressure value and/or the minimum target pressure value is not reached within a period tz.

12. The device according to at least one of claims 10 or 11, characterized in that the device is operated in a petrochemical installation, preferably a cracker, or in a power plant.