GB2281967A - A steam wetness optical probe - Google Patents

Abstract

An optical probe for the measurement of the wetness of steam in a model steam turbine. The probe is required to capture optical extinction spectra for the measurement of wetness due to line droplets down to 0.05 microns. The probe comprises a chamber 5 which can be open and closed to the steam environment and which has a beam of light transmitted through it in the two cases when it is open to steam and when it is purged of steam. The optical path is folded by employing a quartz corner reflector 9 at the remote end and cassegrain projection and reception optics 7 at the near end. A substantial path length is thus obtained without an unduly long probe. The steam chamber is closed by flat optical surfaces so simplifying any cleaning that may be necessary. Light is conveyed from the lamp 19 to the chamber 5 and then to the spectrometer 23 by means of optical fibres 15 and 17 respectively. <IMAGE>

Description

STEAM WETNESS OPTICAL PROBE ARRANGEMENT This invention relates to a steam wetness optical probe, particularly but not exclusively for use in determining the wetness of steam in the low pressure stages of a steam turbine. "Wet steam" is steam which is below its critical temperature so that, at sufficiently high pressure condensation occurs and water droplets, albeit extremely small ones, form. The occurrence of such droplets is very important in the efficient performance of steam turbines and it is very desirable to be able to measure the 'wetness fraction' of the steam in such condition. Both the number and size of droplets are of importance in determining this parameter.

Investigation of the steam wetness may be required to be performed in a working model (eg a one-fifth scale model) of the machine in question and this reduction in size increases the measurement problems many fold.

Optical probes for the measurement of steam wetness are known, there being two particular methods. One is the "extinction method" in which light is transmitted through the steam atmosphere, the transmitted and received flux magnitudes are measured and the ratio used to calculate the wetness fraction. In the other method the light scattered by the water particles is collected and measured and compared with the transmitted light. This is the "scatter method".

The present invention is particularly concerned with the extinction method although in certain embodiments it is anticipated that both methods will be incorporated.

The frequency of the light employed is influenced by the size of water droplets that are to be accommodated. The present invention is concerned with particle diameters in the region 0.05Hm to 2.0,um.

An object of the present invention is to provide an optical extinction probe arrangement which has an improved performance in the above context.

According to the present invention, a steam wetness optical probe arrangement for measuring the wetness fraction of steam, comprises a tubular probe containing projection/reception means directed towards but spaced from a reflex reflector, the projection/reception means comprising, for each of projection and reception means, an optical fibre termination mounted in a cassegrain reflector arrangement to project and receive respectively a collimated beam of light to and from the reflex reflector, the projection and reception means being fixed together side by side in an integral unit to prevent their relative movement, the tubular probe also comprising shutter means controllable to open and close selectively an aperture in the probe wall to admit steam to the space between the projection/reception means and the reflex reflector, the probe arrangement further comprising a light source coupled to the projection fibre and spectrometer means coupled to the reception fibre, and the arrangement being such as to permit measurement of light received after transmission through the space in the absence of steam and in the presence of steam and calculation of the steam wetness fraction from the ratio of the two measurements.

The cassegrain reflector arrangement may comprise a concave primary reflector and a plane secondary reflector, the secondary reflectors for the projection and reception means being provided by local coatings on a common quartz plate which isolates the cassegrain optics from the steam space.

The reflex reflector is preferably a quartz corner reflector presenting a plane face to the steam space.

The tubular probe may comprise double outer walls fixed in relation to the projection/reception means and adapted to be fixed to an access port in a steam turbine, a shutter sleeve being arranged to slide axially between the double walls and open or close the aperture to the steam space between the projection/reception means and the reflect reflector.

Alternatively, the tubular probe may comprise an outer shutter sleeve which encloses the aperture, the outer shutter sleeve being adapted to be fixed to an access port in a steam turbine and the probe components being movable en bloc axially to open or close the aperture to the steam space between the projection/reception means and the reflex reflector.

There is preferably included gas duct means for supplying dry gas to the steam space while the aperture is closed.

Preferably, the gas duct means may supply dry gas to the steam space at all times in operation, at a pressure marginally in excess of the steam pressure.

The spectrometer means may comprise a ruled-grating scannable spectrometer having two output slits accessing respectively, a photodetector responsive to ultra-violet and infra-red light and a photomultiplier responsive to visible light, movable mirror means being provided to select between the two output slits.

The ruled grating spectrometer may be the second of two similar spectrometers, the first spectrometer providing a partial suppression of scattered light frequencies outside the selected frequency.

Alternatively, the spectrometer means may comprise a spectrometer adapted to output a band of light frequencies to a photodetector array responsive to the band of frequencies simultaneously.

A steam wetness optical probe arrangement will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic block diagram of the probe and its associated apparatus; Figure 2 is a diagrammatic axial section of the steam region of the probe and its associated light paths; Figure 3 is an axial section of the probe showing the mechanical construction; Figure 4(a) is a diagram of a spectrometer arrangement switched for detection of infra red and ultra-violet; Figure 4(b) is a similar diagram arranged for visible light detection; and Figure 4(c) is a diagram of a spectrometer arranged for simultaneous detection of a band of frequencies.

The underlying principles of the optical extinction are as follows.

A known light flux fo(X) of wavelength i, is launched through a known distance in the steam to be tested. The transmitted flux (X), ie. that part of db (il) which is neither scattered nor absorbed by the steam, is measured with a photodetector so that the ratio #(#)/#0(#) can be derived. This is related to the extinction coefficient through the Bouguer or Beer-Lambert law: # (#) = exp[- #(#) L ] (1) where L = length of the path through steam T (X) = extinction coefficient.

For the degree of wetness normally encountered in the LP stages of steam turbines each droplet of suspended water scatters light independently so that the extinction coefficient is given by:

where D = droplet diameter Nr(D) = number density of droplets in the range D to D+dD Qxt(Xs D) = extinction efficiency of light of wavelength # by drops of diameter D.

A well established theory, usually known as the Mie Theory, shows that Qcxt is a function of the particle size parameter, a, where a = 2sD/R, and enables accurate values to be calculated.

The integral equation can therefore be inverted to yield N (D) which in turn can be used to calculate the wetness coefficient of the steam.

Unfortunately the process of inverting equation (2) suffers from two problems: The equation is ill-conditioned, particularly when the particle size parameter is very small, a situation which is unavoidable when droplets of less than 0. lem are to be measured; The spectral range over which T (X) is measured is limited by the situation and the equipment used.

The function Qxt(a) becomes a simple power law at small a, a situation known as Rayleigh scattering, and the spectral extinction data cannot be uniquely fitted to it.

The range of particle size parameters defined by measured data can be made larger by reducing the wavelengths used but absorption in the atmosphere in question limits the usable wavelengths to those above 180nm. In practical terms the generation and transportation of wavelengths below 250nm imposes restraints on the design of the probe.

Furthermore, it is known from experience and from theoretical predictions, that the fine droplet sizes in the model turbine are very small, about 0. lem or less. The droplet sizes in full scale turbines are larger, 0.5calm or more. It is therefore essential to use wavelengths down to the shortest limit practical in the probe for the model turbine, and in particular, from infra-red down to ultra-violet.

It is desirable that measurements should be made as quickly as possible and, with frequencies encompassing the whole visible range plus infra-red and ultra-violet, it would be particularly desirable if simultaneous measurements could be made over the whole band. One embodiment to be described does cater for this possibility but problems of scattered light do arise and an alternative scanned system is found satisfactory for most purposes.

Referring now to Figure 1, the probe 1 comprises a long supporting tube portion 3 ending in a steam chamber 5 which has at one end a projection/reception unit 7 and at the other end a reflex reflector 9. A slot 11 in the wall of the steam chamber can be opened and closed by a shutter sleeve 13 shown in Figure 3.

Substantially white light is provided for the probe on a quartz optical fibre 15, the remnant light from the probe being carried by a further quartz optical fibre 17.

The white light is provided by a xenon arc lamp 19 coupled to the fibre 15 by a lens focusing arrangement 21. The spectrum of the xenon lamp extends from about 0.2 microns to about 1.1 microns, ie from ultra-violet to infra-red. The UV end of this spectrum is of sufficiently high frequency, from the considerations described above, to respond adequately to droplets at the lower diameter end of the range of interest, ie 0.05 microns to 2.0 microns.

Remnant light from the probe, carried on fibre 17, is applied to a spectrometer arrangement 23, which may comprise one or two spectrometers as will be explained with reference to Figure 4. The spectrometer has a detector 25, a silicon-photodetector/ photomultiplier combination, on its output and in the case of a scanning arrangement a steppermotor control 27. The detector output is then applied to a computer 29 for computation, analysis, display and recording.

Figure 2 shows, diagrammatically, the optical path arrangement of the probe. As mentioned previously, this part of the probe comprises two basic units, the projection/reception unit 7 at the left hand end in Figure 2 (the 'near' end of the probe in operation in a turbine), and the reflex reflector unit 9 at the right hand end (the remote end of the probe in operation). Each of these units is very rugged and self-integrated. The projection/reception unit 7 comprises a cylindrical housing 31 which contains the optical fibre terminations 32 and 34, each mounted within a block 33, 35 having a concave forward face 37, 39, constituting a primary mirror in a cassegrain arrangement. The cylindrical housing 31 also forms a spacer for a quartz plate 41 which has very small silvered mirror patches 43 and 45 confronting the fibre ends 32 and 34 respectively.The quartz plate is otherwise transparent.

The reflex reflector 9 consists of a solid quartz corner piece having a flat face 47 presented to the steam space 8, reflection being by triple internal reflection in known manner.

The arrangement by which the flat faces of the quartz plate 41 and the reflector 9 confront and contain the steam space 8 permits ease of cleaning since no other optical surface is exposed to the steam atmosphere and these two flat surfaces are easily accessible.

The spacing of the fibre terminations, the spot reflectors 43 and 45, and the concave mirrors 37 and 39 is such as to produce a collimated beam of white light which is directed to one side of the reflector 9 so as to be reflected parallel to the incident path but displaced by the pitch of the fibres. Substantially all of the light from the projection fibre 32 is therefore returned to the reception fibre 34 in the absence of any obstructing atmosphere in the steam space 8.

While the two end units, the projection/reception unit 7 and the reflector 9 are individually very rugged and rigid, they are located relative to each other by the steam chamber 5. This chamber and its surrounding shutter sleeve 13 are fairly stiff but do permit small transverse movement of one end relative to the other. The use of a reflex reflector 9 accommodates such small movements by still returning the reflected beam parallel to the incident beam.

Referring now to Figure 3 this shows more detail of the mechanical construction of the probe. In the particular embodiment shown, the steam chamber 5, and the units 7 and 9 slide in unison inside a shutter sleeve 13 which is fixed to the supporting tube 3 which is fixed to the access port in the turbine wall. The fibres are fixedly mounted in a shutter tube 49 which moves as one with the steam chamber 5 inside the shutter sleeve 13 and supporting tube 3.

The steam chamber is opened to the ambient steam atmosphere by way of a slot 11 in the steam chamber wall. This slot is exposed by driving the whole inner body inwards into the turbine, ie to the right in Figure 3, leaving the shutter sleeve 13 in its position fixed to the turbine.

A gas tube extends alongside the fibres in the shutter tube and is connected to several passages 51 in the wall of the steam chamber 5. These passages are connected to the steam space 8 and are constantly supplied with dry nitrogen or other relatively inert gas at a pressure slightly above the pressure of steam in the chamber. When the steam access slot 11 is closed, the nitrogen purges the steam chamber through leakage paths between steam chamber 5 and shutter sleeve 13 although if need be, venting passages could be incorporated. When the steam access slot 11 is open, the nitrogen supply is still maintained. Since the entry points are at the ends the nitrogen helps to keep the faces of the optical surfaces 41 and 47 clean. The presence of nitrogen otherwise has negligible effect on the absorption or scattering of light.

In an alternative design of shutter operation the probe body is double walled and a shutter sleeve slides between the two to expose or close the steam access slot which extends through both walls'-. The advantage of this design is that there is no movement of the optical fibre cables between the measurements with and without steam. Such movement imposes very slight stresses on the optical fibres which can introduce a slight level of uncertainty in the light flux measurement.

The folded geometry of the design, as shown in Figure 2, provides adequate spatial resolution for use in a model turbine and at the same time a sufficiently long light path for measuring very small water droplets. The use of reflecting rather than refracting optics for focusing and collimating the beams avoids introducing chromatic aberration into the system.

Referring now to Figure 4(a) and (b) two spectrometers 53 and 55 are employed in tandem. The first spectrometer has an input slit IS on which light from fibre 17 (Figure 1) is focused. The light is then intercepted by an order sorting filter OSF which selects only the required spectrum from the series present. This is followed by a series of mirrors M and an intervening 1200 lines/mm, 70mm x 70mm ruled grating blazed at 250nm. The grating G is steerable by a stepper motor to step through the frequency band from Lw to IR. The selected frequency is passed to a second, similar, spectrometer 55 which has a grating synchronised to output the same frequency. In this spectrometer there are two output slits, one directed at a silicon photodetector 57 and one at a photomultiplier 59.A 'swing away' mirror is operable to select between the two output detectors by permitting the beam to pass to the silicon photodetector 57 or deflecting it to the photomultiplier 59. The former is used for the UV and IR frequencies and the latter for visible frequencies. Figure 4(a) shows the arrangement with the photodetector 57 in use and Figure 4(b) with the photomultiplier in use.

With each step position of the gratings G, light flux measurements are taken with the steam chamber 5 closed and purged of steam, and again with the steam chamber open and the projected light beam partially extinguished by scattering and absorption. The whole spectrum from 0.2 microns to 1.1 microns is traversed in this way. Calculations are then performed by the computer as indicated above, and the wetness fraction determined.

The arrangement of Figures 4(a) and 4(b), using two spectrometers in tandem is used to select the wanted frequency and suppress scattered light which inevitably enters the system in practice. This selection of a single frequency (or very narrow frequency band) does however mean that a scanned system, stepping through the spectrum, has to be employed. This takes rather longer than is ideally desirable.

Figure 4(c) shows a basically similar spectrometer arrangement using a single spectrometer of the above type but with a fixed grid and consequently a frequency dispersed output beam. This is accommodated by a photodetector array 61 which is responsive to the whole spectrum simultaneously. Parallel signal channels or an electronically scanned arrangement then provides the signals to the computer. This system is of course much faster, since a complete assessment can be made with a single operation of the shutter. However, the scattering of light can present difficulties.

Claims (11)

1. A steam wetness optical probe arrangement for measuring the wetness fraction of steam, comprising a tubular probe containing projection/reception means directed towards but spaced from a reflex reflector, the projection/reception means comprising, for each of projection and reception means, an optical fibre termination mounted in a cassegrain reflector arrangement to project and receive respectively a collimated beam of light to and from the reflex reflector, the projection and reception means being fixed together side by side in an integral unit to prevent their relative movement, the tubular probe also comprising shutter means controllable to open and close selectively an aperture in the probe wall to admit steam to the space between the projection/reception means and the reflex reflector, the probe arrangement further comprising a light source coupled to the projection fibre and spectrometer means coupled to the reception fibre, and the arrangement being such as to permit measurement of light received after transmission through said space in the absence of steam and in the presence of steam and calculation of the steam wetness fraction from the ratio of the two measurements.

2. A probe arrangement according to Claim 1, wherein said cassegrain reflector arrangement comprises a concave primary reflector and a plane secondary reflector, the secondary reflectors for the projection and reception means being provided by local coatings on a common quartz plate which isolates the cassegrain optics from the steam space.

3. A probe arrangement according to Claim 1 or Claim 2, wherein said reflex reflector is a quartz corner reflector presenting a plane face to the steam space.

4. A probe arrangement according to any preceding claim, wherein the tubular probe comprises double outer walls fixed in relation to said projection/reception means and adapted to be fixed to an access port in a steam turbine, a shutter sleeve being arranged to slide axially between said double walls and open or close said aperture to the steam space between the projection/reception means and the reflect reflector.

5. A probe arrangement according to any of Claims 1-3, wherein the tubular probe comprises an outer shutter sleeve which encloses said aperture, said outer shutter sleeve being adapted to be fixed to an access port in a steam turbine and the probe components being movable en bloc axially to open or close said aperture to the steam space between the projection/reception means and the reflex reflector.

6. A probe arrangement according to any preceding claim, including gas duct means for supplying dry gas to the steam space while said aperture is closed.

7. A probe arrangement according to any preceding claim, including gas duct means for supplying dry gas to the steam space at all times in operation, at a pressure marginally in excess of the steam pressure.

8. A probe arrangement according to any preceding claim, wherein said spectrometer means comprises a ruled-grating scannable spectrometer having two output slits accessing respectively, a photodetector responsive to ultra-violet and infra-red light and a photomultiplier responsive to visible~ light, movable mirror means being provided to select between the two output slits.

9. A probe arrangement according to Claim 8, wherein said ruled grating spectrometer is the second of two similar spectrometers, the first spectrometer providing a partial suppression of scattered light frequencies outside the selected frequency.

10. A probe arrangement according to any of Claims 1 - 7, wherein said spectrometer means comprises a spectrometer adapted to output a band of light frequencies to a photodetector array responsive to said band of frequencies simultaneously.

11. A steam wetness optical probe arrangement substantially as hereinbefore described with reference to the accompanying drawings.