WO2008060164A1

WO2008060164A1 - A method and a device for detection of rubbing between a stator and a rotor of a turbo-machine

Info

Publication number: WO2008060164A1
Application number: PCT/NO2007/000403
Authority: WO
Inventors: Lyder Moen
Original assignee: Dynatrend As
Priority date: 2006-11-17
Filing date: 2007-11-16
Publication date: 2008-05-22
Also published as: NO20065282L

Abstract

A method for detection of rubbing between a stator (2) and a rotor (4) of a turbo -machine (1), comprising: placing a transmitter (16, 18) in the form of an accelerometer, or a sensor for measuring vibration at ultra-high frequencies, or a microphone at the turbo-machine (1); then analyzing the signal from the transmitter (16, 18) with respect to at least frequency or amplitude and comparing it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns; and determining, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present.

Description

A METHOD AND A DEVICE FOR DETECTION OF RUBBING BETWEEN A STATOR AND A ROTOR OF A TURBO-MACHINE

This invention concerns a method for detection of rubbing between a stator and a rotor of a turbo-machine. More particularly, it concerns a method for detection of rubbing between a stator and a rotor of a turbo-machine, comprising:

- connecting at least one transmitter in the form of an accelerometer, or a sensor for measuring vibration at ultrahigh frequencies, to the turbo-machine, or placing a microphone at the turbo-machine;

- then at least demodulating or analyzing, with respect to frequency or amplitude, the signal from the at least one transmitter and comparing it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns. It is determined, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present.

The term rubbing (friction) implies that mechanical contact arises between the stator and the rotor of the turbo-machine when operational. The term "adaptive values" represents values that change automatically relative to the operating conditions of the turbo-machine, for example due to instantaneous power consumption.

The efficiency of a turbo-machine requires the fluid, which flows through the turbine steps of the turbo-machine, to flow between the blades and only insignificantly via other openings. Turbo-machines are therefore formed with a relatively small clearance between the rotor steps and the associated stator portion. Typically, pressure differences across a step are maintained by means of labyrinth seals or adjacent surfaces having a relatively small clearance.

Due to relatively large temperature differences existing in a turbo-machine, it may be necessary to reduce said clearance subsequent to having started the turbo-machine in order to achieve a higher efficiency. Thus, it is known to blow air against the outside of the stator in order to reduce the diameter of the stator. Other known methods of reducing the clearance between the rotor and the stator comprise hydraulic and mechanical compression of the stator housing.

Should interruptions in the operation of the turbo-machine arise, it may be found that the rotor is rubbing against the stator in one or several of the steps of the turbo-machine. By so doing, material may be worn off from all or parts of the circumference of the rotor, and also to a lesser extent from the stator, whereby the clearance between the rotor and the stator increases.

Several circumstances may cause rubbing. The stator housing of a turbo-machine being disconnected when fully operational may, for example, experience uneven cooling, causing the stator housing to assume a "banana shape" in the longitudinal direction and ovality in cross section. Rotor steps located midways between the bearings of the rotor are most exposed to rubbing .

In the compressor steps of the turbo-machine, so-called "stalling" or "surging" may occur, in which the fluid does not flow as predicted, but in which it stops, or possibly flows in the wrong direction. The English terms stall and surge are used given that they are incorporated in the turbo- machine community of Norway. Stalling and surging may cause the rotor to enter into heavy imbalance, which results in a portion of the blades of the rotor rubbing against the stator. When such instability arises, the blades may reach onto the stator housing and lose some metal from the blades. A permanently increased imbalance is thus introduced, which may limit the output capacity of the turbo-machine.

Wear and tear of this type, in which dissimilar clearances exist along the circumference of the rotor, will generally aid in introducing stalling and surging more easily than for the turbo-machine having normal clearances. This causes the possible working range of the turbo-machine to be reduced, whereby further damage may arise due" to these relatively large clearances.

An increased clearance between the rotor and the stator of a turbo-machine generally reduces the efficiency of the turbo- machine significantly. This decline in efficiency must be compensated with an increased fuel supply, which results in increased operational temperatures and a reduced operating time. With respect to steam turbines, the amount of steam to the turbo-machine must be increased to be able to produce the desired power. In some cases, rubbing may cause such great damage that the turbo-machine must cease operating immediately.

The object of the invention is to remedy or reduce at least one of the disadvantages of the prior art.

The object is achieved according to the invention and by means of the features disclosed in the following description and in the subsequent claims .

According to the invention, a method for detection of rubbing between a stator and a rotor is characterized in connecting at least one transmitter in the form of an accelerometer , or a sensor for measuring vibrations at ultra-high frequencies, to the turbo-machine, or placing a microphone at the turbo- machine;

- then analyzing the signal from the at least one transmitter with respect to at least frequency or amplitude and comparing it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns; and

- determining, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present.

A relatively high-frequent vibration is generated when rubbing occurs . The phenomena may be compared to a barrier vibration in a turning lathe or to squealing brakes.

In the time domain, the measuring signal, which may be an unprocessed signal, an integrated signal, or a demodulated signal, exhibits a significant amplitude increase when rubbing occurs. However, in the frequency domain, in which the signal is processed by means of for example FFT (Fast Fourier Transform) , peaks will arise at frequencies being equal to or larger than the blade turning-frequency for each turbine step, and normally at significantly higher frequencies than that of the blade turning-frequency. Experiments show that the peaks oftentimes are in the region of 20-100 kHz when the blade turning-frequency is in the region of 8-20 kHz. In this connection, frequencies in the region of 20-100 kHz may be referred to as ultra-high frequencies .

In order to gather the signals in question, and in addition to selecting transmitters capable of operating in the frequency region in question, it is appropriate to set so- called high-pass and low-pass filters at the respective two and 4 times the blade turning-frequency, i.e. at a substantially higher frequency region than that analyzed by means of, for example, bearing monitoring.

The result of the comparison may be presented in the form of a machine-readable signal. Advantageously, the machine- readable signal may be used to control the amount of a cooling agent being supplied to the turbo-machine .

Typically, the signal analytical equipment comprises a computer or an analyser formed with the required signal input and output gates, and which is programmed to be able to analyze the signal from the transmitter with respect to at least frequency or amplitude and to compare it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns, and in which it is determined, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present. Signal processing, signal analysis, signal integration, signal modulation and signal comparison as well as generation of output signals from the signal processing, all represent prior art well-known to the skilled person and hence is not described in further detail.

If desirable, the output gates from the analytical equipment may be connected, for example via a control system, to a valve or an actuator arranged so as to be able to adjust the clearance between the rotor and the stator of the turbo- machine, for example by controlling the amount of a cooling agent being supplied to the cooling- or shrinking system of the turbo-machine, or a hydraulic system or a stator clamp.

The cooling agent, which may be comprised of air, generally flows towards the stator, whilst the cooling agent flows from the compressor section of the turbo-machine and via cooling ducts surrounding the stator, or from adjoining components. The valve is connected to a flow path of the cooling agent.

In the following, an example of a preferred embodiment is described and is depicted on the accompanying drawings, in which:

Fig. 1 shows, schematically and in section, a number of turbine steps in a turbo-machine, in which the clearance between the rotor and the stator of the turbo-machine is indicated, in one of the steps, with the letter T, and in which transmitters are connected to the turbo-machine;

Fig. 2 shows, schematically and in smaller scale, a side view of a turbo-machine provided with ducts for a cooling agent, the ducts being shown partially in section; Fig. 3 shows, schematically, a graph of a signal from the transmitter, in which time is shown along the abscissa, while the ordinate depicts the amplitude of the signal; and

Fig. 4 shows, schematically, a graph of a signal from the transmitter, in which the abscissa depicts signal frequency.

In the drawing, reference numeral 1 denotes a turbo-machine comprising a stator 2 and a rotor 4. Both the stator 2 and the rotor 4 comprise a number of stator steps 6 and rotor steps 8, respectively.

The turbo-machine 1 includes, in a manner known per se, a compressor section 10, a high-pressure turbine section 12 driving the compressor of the turbo-machine, and a low- pressure power turbine section 14, in which a power turbine (not shown) is connected to a gear box (not shown) or other type of driven component.

A first transmitter 16 in the form of an accelerometer, or a sensor for measuring vibration at ultra-high frequencies, is connected to the stator 2 close to the rotor steps 8, see fig. 2. Due to the ambient temperatures, the transmitter 16 must be placed at a certain distance from the stator 2. A second transmitter 18 in the form of a microphone, preferably an ultrasonic microphone, is placed at the side of the turbo- machine 1, and preferably close to the power turbine sections 12, 14. Both of the transmitters 16 and 18 are connected to signal processing- and analytical equipment known per se (not shown) by means of wires 20.

The first transmitter 16 is suitable for producing signals from which turbine steps causing potential rubbing may be derived, while the second transmitter 18 is suitable for being able to produce satisfactory signal quality also at relatively high signal frequencies, at which high signal frequencies the first transmitter possibly may produce signals of unsatisfactory quality.

The high-pressure turbine section 12 of the turbo-machine is surrounded by a number of first cooling ducts 22, while the low-pressure turbine section 14 is surrounded by a number of second cooling ducts 24. The cooling ducts 22, 24 are provided with nozzle orifices 26 conducting the cooling agent towards the stator 2 at corresponding rotor steps 8, see fig. 1.

Via a first valve 28 and a first supply pipe 30, the first cooling ducts 22 are supplied cooling agent in the form of air from the compressor section 10. Via a second valve 32 and a second supply pipe 34, the second cooling ducts 24 are supplied cooling agent from the compressor section 10, but at a somewhat lower pressure than for the first cooling ducts 22. The components 22, 24, 30 and 34 form the flow path of the cooling agent.

During normal operation, the signal level from the transmitter 16 and 18 will yield an average amplitude, which is indicated by "a" in fig. 3, whereas the amplitude of the signal will rise to a higher level, which may exceed a predetermined reference value "b" , when rubbing takes place.

Correspondingly, the frequency spectrum of the measuring values shown in fig. 4 will generally show the blade turning- frequency "c" during normal operation, whereas a composite frequency pattern, which will occur at higher frequencies as indicated at "d" in fig. 4, will occur when rubbing takes place .

When a comparison of at least measuring values or frequency patterns, which have expected measuring values or frequency patterns, indicate that rubbing takes place, the output gates (not shown) from said analytical equipment will at least yield signals indicating that rubbing is present, or will control the valve 28, 32 for the particular cooling ducts 22, 24 towards reduced supply of cooling agent, thereby increasing the temperature of the stator 2. The diameter of the stator 2 is thus increased simultaneous with increasing the aperture T between the stator 2 and the rotor 4, see fig. 1. Moreover, the result of the comparison may be stored in a register (not shown) for subsequent evaluation.

In alternative embodiments (not shown) , the first and the second cooling ducts 22, 24 may be supplied a cooling agent individually in order to adapt the supply to the turbo- machine 1, which for example is provided with more than one high-pressure turbine section 12.

Claims

C l a i m s

1. A method for detection of rubbing between a stator (2) and a rotor (4) of a turbo-machine (1) , c h a r a c t e r i z e d i n connecting at least one transmitter (16, 18) in the form of an accelerometer, or a sensor for measuring vibration at ultra-high frequencies, to the turbo-machine (1) , or placing a microphone at the turbo-machine (1);

- then analyzing the signal from the at least one transmitter (16, 18) with respect to at least frequency or amplitude and comparing it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns; and

- determining, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present .

2. The method according to claim 1, c h a r a c t e r i z e d i n that the signal is an unprocessed signal.

3. The method according to claim 1, c h a r a c t e r i z e d i n that the signal is an integrated signal.

4. The method according to claim 1, c h a r a c t e r i z e d i n that the signal is a demodulated signal.

5. The method according to any one of claims 1-4, c h a r a c t e r i z e d i n presenting the result of the comparison in the form of a machine-readable signal.

6. The method according to claim 5, c h a r a c t e r i z e d i n using the machine-readable signal to control the amount of a cooling agent being supplied to the turbo-machine (1) .

7. A device for detection of rubbing between a stator (2) and a rotor (4) of a turbo-machine (1), c ha r a c t e r i z e d i n that the turbo-machine (1) is provided with a transmitter (16, 18) in the form of an accelerometer, or a sensor for measuring vibration at ultra-high frequencies, or a microphone;

- wherein the transmitter (16, 18) is connected to analytical equipment.

8. The device according to claim 7, c ha r a c t e r i z e d i n that the analytical equipment comprises a computer or an analyser programmed to be able to analyze the signal from the transmitter (16, 18) with respect to at least frequency or amplitude and to compare it to at least predetermined or adaptive values from the group comprising measuring values and frequency patterns; and

- wherein it is determined, on the basis of said comparison and predetermined limit values, whether rubbing is present or has been present.

9. The device according to claim 7, c h a r a c t e r i z e d i n that the analytical equipment, via an output gate and a control system, is connected to a valve (28, 32) or an actuator arranged so as to be able to adjust the clearance between the rotor (4) and the stator (2) of the turbo-machine (1).

10. The device according to claim 9, c ha r a c t e r i z e d i n that the valve (28, 32) or the actuator is arranged so as to be able to adjust the amount of a cooling agent being supplied to the turbo- machine ( 1) .

11. The device according to claim 10, c h a r a c t e r i z e d i n that the cooling agent is comprised of air.

12. The device according to claim 10 or 11, c h a r a c t e r i z e d i n that the cooling agent flows towards the stator (2).

13. The device according to claim 12, c ha r a c t e r i z e d i n that the cooling agent flows from a compressor section (10) of the turbo-machine (1) and via cooling ducts (22, 24) surrounding the stator (2), the valve (28, 32) being connected to a flow path (22, 24, 30, 34) of the cooling agent.