GB2312957A

GB2312957A - Alternating current spark discharge probe

Info

Publication number: GB2312957A
Application number: GB9609676A
Authority: GB
Inventors: John Taylor; Brian Hamilton Edmundson
Original assignee: ROTADATA Ltd
Current assignee: ROTADATA Ltd
Priority date: 1996-05-09
Filing date: 1996-05-09
Publication date: 1997-11-12
Also published as: GB9609676D0

Abstract

A spark discharge probe arrangement for measuring the clearance of a turbine blade B is energised by a high voltage AC power supply 1 which is connected between a probe electrode 8 and the turbine casing 9. The frequency of the high voltage is suitably at least 1MHz, which, owing to the skin effect, minimises the danger of electric shock and further ensures a low impedance circuit through the turbine bearings, which may be electrically insulating.

Description

SPARK DISCHARGE PROBE ARRANGEMENT AND METHOD The present invention relates to a spark discharge probe arrangement and to a method of determining the distance between a datum and a member (such as a turbine blade for example) which in use functions as an electrode.

Such arrangements are used for determining the clearance between the outermost blade tip of a turbine rotary assembly and the turbine casing and for calibrating other apparatus such as capacitive probes for example which are used to measure such clearances by measuring the capacitance to the turbine blade tip from a probe electrode.

Particularly at the high pressures found in gas turbines, the minimum gap required for sparking, even at the high voltage of e.g. 200V to 500V normally employed, is only a few pm and hence the clearance can be determined quite accurately from a knowledge of the position of the probe electrode relative to a datum, even if there is a significant uncertainty, in percentage terms, of the minimum gap required for sparking.

Hitherto, the spark has always been generated by a DC high voltage power supply, which represents a potential safety hazard.

A further problem arises from the difficulty of establishing a good electrical connection to a rotating assembly, since in some cases hydrodynamic bearings are employed, which are electrically insulating and must be electrically bypassed with a brush arrangement. In order to solve the above technical problems, the invention provides a spark discharge probe arrangement characterised by having an AC power supply, as defined in Claim 1.

In another aspect the invention provides a method as defined in Claim 5, of determining the distance between a datum and a member which in use functions as an electrode, characterised in that the power supply is an AC power supply.

The invention has the advantages that even insulating bearings such as hydrodynamic bearings will normally have sufficient capacitance to present a low impedance to AC, and that the danger of electric shock due to stored charge is much reduced.

Preferably the frequency of the high voltage which generates the spark is greater than 1 MHz, since at this frequency the skin effect minimises any current conduction through the human body in the event of an electric shock. Preferably the frequency is in the range 5 to 15 MHz.

Further preferred features are defined in the dependent Claims.

A preferred embodiment of the invention is described below by way of example only with reference to the accompanying drawing, Figure 1.

Figure 1 is an elevation, partly in section, showing a spark discharge probe arrangement in accordance with the invention.

Refening to Figure 1 a high voltage oscillator circuit 1 is arranged to generate a sinewave output voltage of RMS voltage of 200V to 500V frequency 500kHz to 2 MHz.

The frequency chosen is selected to be low enough to avoid interference with other electrical equipment in the system and to be high enough to avoid giving spurious readings. One output terminal of this high voltage AC power supply is connected to a probe P carrying a sparking electrode at its distal tip, and the other terminal is connected to the casing 9 of a turbine which includes a rotary turbine assembly having blades B.

The bearings (not shown) of the turbine assembly present a low impedance to the AC from the circuit 1 and hence the blades B are effectively electrically connected to casing 9 without the need for brushes or other bridging means. The probe P is insulated from the turbine by an insulating bushing 2 which may be of PTFE for example.

The probe P and its associated bushing 2 can be incrementally advanced towards blade B by a stepper motor (not shown) which sends a count signal to a counter input 13 of an indicator module 12. The distance of probe P relative to a rear datum 3 is displayed by display 15. The stroke of probe P, indicated by double-headed arrow 18, is suitably 6 mm.

The oscillator 1 also has output leads 11 which are connected to module 12 and output a predetermined signal as soon as a spark is generated across the gap between electrode 8 and blade B. As soon as a spark is generated, the advancement of probe P by the stepper motor is stopped and the known value of the minimum gap G required for sparking is added to the position of the tip of electrode 8 to give the position of the outermost tip of the turbine blades B.

In order to ensure that the probe P is kept at a moderately low, constant temperature, cooling air from an inlet 4 in mounting 16 is passed over the probe within a tubular housing 16 (as indicated by arrows 5) and then enters the hollow interior of the probe via apertures 7 near its distal end (as indicated by arrows 6) whence the air retums up the interior of the probe to an exhaust (not shown).

The stationary mounting M for the probe bushing 3 is held in position by a compression spring 10 within housing 16. The distance L from the interior of the turbine casing to the tip of electrode 8 is suitably 100 mm.

If blade B is of ceramic or other insulating material it can be metallised, e.g. with vapourdeposited silver or aluminium.

Claims

1.A spark discharge probe arrangement for detecting the distance between a member and a datum, the member being arranged in use to function as an electrode, the arrangement comprising a probe electrode which is movable relative to the datum towards said member, means for indicating the position of the probe electrode relative to the datum, and a high voltage power supply connectable across said electrodes, characterised in that said high voltage power supply is an AC power supply.

2. A spark discharge probe arrangement according to Claim 1, wherein said power supply is arranged to apply a high voltage of frequency greater than 150 kHz to said electrodes.

3. A spark discharge probe arrangement according to Claim 2, wherein said frequency is in the range 5 MHz to 15 MHz.

4. A spark discharge probe arrangement according to any preceding Claim, comprising means for incrementally advancing the probe electrode towards said member.

5. A method of determining the distance between a datum and a member which in use functions as an electrode, a high voltage power supply being connected across said electrode and a probe electrode, the probe electrode being advanced relative to a datum towards said member and the position of the probe electrode relative to the datum being determined in order to determine said clearance, characterised in that said power supply is on AC power supply.

6. A method according to claim 5, wherein said member is initially non-electrically conductive and wherein electrically conductive material is applied thereon and electrically coupled to an output terminal of said power supply.

7. A method according to Claim 6, wherein said conductive material is deposited from the vapour phase.

8. A method according to any of Claims 5 to 7, wherein said member rotates past said probe electrode.

9. A method according to Claim 8, wherein said member is a turbine blade.

10. A method according to any of Claims 5 to 9, wherein said power supply applies a high voltage of frequency greater than 150 kHz to said electrode.

11. A method according to Claim 10, wherein said frequency is in the range 5 MHz to 15MHz.

12. A method according to any of Claims 5 to 11, wherein said probe electrode is incrementally advanced towards said member and said clearance is determined from the position of the probe electrode relative to the datum and the known minimum gap required for sparking.

13. A spark discharge probe arrangement substantially as described hereinabove with reference to the accompanying drawing, Figure 1.

14. A method of determining the distance between a member and a datum, substantially as described hereinabove with regard to the accompanying drawing, Figure 1.