Method and an apparatus for determining the clearance between a turbine casing and the tip of a moving turbine blade

1. A method of determining the clearance between a tip of a moving turbine blade and a turbine casing comprising the steps of

(a) emitting a low-coherence light from a light source,

(b) directing the low-coherence light through a collimator lens to the blade tip wherein the back surface of the collimator lens is placed at the level of the surface of the turbine casing,

(c) directing light reflected from the collimator lens and from the blade tip to a beam splitter and directing a first resulting beam to a reference beam and a second resulting beam to a delay arm,

(d) shifting the frequency of the reference beam by means of an acousto-optical modulator to a predetermined frequency,

(e) scanning the time delay of the delay beam by means of a variable delay line,

(f) combining the reference beam and the delay beam by means of a beam combiner,

(g) detecting the power spectrum due to the interference of the light reflected from the collimator lens and from the moving blade tip at the frequency determined by the acousto-optical modulator,

(h) calculating from the obtained power spectrum the distance between the turbine casing and the moving turbine blade tip.

2. The method according to claim 1 , comprising the step of adjusting the ratio of intensity of the reference beam to intensity of the delay beam by means of a variable attenuator placed in the reference beam.

3. The method according to claim 1 , comprising the step of directing the low-coherence light through a fiber optical isolator prior to directing it to the collimator lens( 13 ).

4. The method according to claim 1 , comprising the step of filtering the beam emerging from the beam combiner prior to detecting the power spectrum.

5. The method according to claim 1 , wherein the low-coherence light to and from the collimator lens and the turbine blade tip is directed by means of a circulator.

6. The method according to claim 1 , wherein the low-coherence light is directed to the turbine blade tip and from the turbine blade tip by means of two separate fibers.

7. The method according to claim 1 , wherein the distance between the turbine casing and the turbine blade tip is determined during the operation of the turbine.

8. The method according to claim 7 , wherein the distance between the turbine casing and the turbine blade tip is determined during start-up or shut-down of the turbine.

9. The method according to claim 8 , wherein the distance between the turbine casing and a plurality of turbine blade tips is determined.

10. The method according to claim 8 , wherein the point in time of each turbine blade passage is determined.

11. An apparatus for determining the distance between a turbine casing and a tip of a moving turbine blade comprising

(a) a low-coherence light source,

(b) a collimator lens placed at the level of the turbine casing, the collimator lens axis extending essentially perpendicularly to the surface of the blade tip,

(c) a beam splitter placed in the light beam reflected from the collimator lens and from the object blade tip,

(d) an acousto-optical modulator placed in a reference beam emerging from the beam splitter,

(e) a variable delay line placed in a delay beam emerging from the beam splitter,

(f) a beam combiner placed in the reference beam and delay beam,

(g) a detector placed in a light beam emerging from the beam combiner,

(h) a processing unit connected to the detector.

12. The apparatus according to claim 11 , further comprising a circulator between the low-coherence light source and the collimator lens.

13. The apparatus according to claim 12 , further comprising a variable attenuator in the reference beam.

14. The apparatus according to claim 12 , further comprising a fiber optical isolator between the low-coherence light source and the collimator lens.

15. The apparatus according to claim 12 , further comprising an optical filter between the beam combiner and the detector.