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 compressed,
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 compressors 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-compressor 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 triggered,
if a difference between the opening of the steam control valve and a target opening
is detected, 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] 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.
[0011] 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
[0012] The present disclosure provides a method, in which a primary servo valve system is
provided 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.
[0013] Therefore, 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.
[0014] 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.
[0015] 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.
[0016] In some embodiments, the servo piston of the secondary servo valve system is continuously
moved between the first position and the second position.
[0017] In some embodiments, the servo piston is moved from the first position to the second
position within a first period t
1.
[0018] In some embodiments, the servo piston is moved from the second position to the first
position within a second period t
2.
[0019] In some embodiments, t
x may be t
1+t
2. Additionally or alternatively t
1 and t
2 may be equal.
[0020] 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 secondary 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.
[0021] 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 pressure value within a period
t
z, wherein t
z is equal to the sum of t
1 and x and/or t
z is equal to t
2 and x, with x representing a freely selectable waiting period.
[0022] 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.
[0023] 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 pressure value does not reach a maximum target pressure value and/or a minimum
target pressure value.
[0024] 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.
[0025] 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 t
z.
[0026] In some embodiments, the device is operated in a petrochemical installation. In particular,
the device may be operated in a cracker or in a power plant.
[0027] 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.
[0028] The combination of features shown and described in the individual exemplary embodiments
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
[0029]
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 buildup and the pressure drop which are generated by the cyclical movement
of the servo piston in the secondary servo valve system.
Figure 2 shows a schematic structure of the system according to the disclosure comprising
- 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-compressor
- 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
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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 accumulated 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
movement 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.
[0034] In some embodiments, the servo piston of the secondary servo valve system is continually
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. Surprisingly, 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.
[0035] 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 pressure value and a minimum
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.
[0036] 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.
[0037] 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 t
x. In a further exemplary embodiment t
x may be characterized by the following relationship:

[0038] The time intervals t
1 and t
2 can be chosen freely and can be adjusted depending on the application. In an exemplary
embodiment, it further holds that t
1 = t
2.
[0039] 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 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 = t
1 + x and/or t
2 + x, where x is a freely selectable waiting period. More preferably, it holds that:
60 seconds ≤ t
z ≤ 600 seconds, more preferably 120 seconds ≤ t
z ≤ 300 seconds.
[0040] 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.
[0041] 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 secondary 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.
[0042] The control system of a steam turbine generally comprises a number of control and
actuation 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 actuator 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.
[0043] The method according to the disclosure can be used to control any optional steam
turbine. 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.
[0044] 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,
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 pressure value does not reach a maximum target pressure value and/or a minimum
target pressure value.
[0045] In an exemplary embodiment, 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 t
z. In this manner, it is provided that false alarms are caused due to noncritical delays
during pressure adjustment.
[0046] In some embodiments, the actuator is the main steam control valve that controls the
steam flow supplied to the steam turbine.
[0047] In some embodiment, the device according to the present disclosure is operated in
a petrochemical installation, preferably a cracker, or in a power plant.
[0048] 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.
[0049] 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).
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 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.
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 position 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 system (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 tz, wherein tz = t1 + x and/or t2 + x, with x representing 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 measuring 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.