GB2055475A - Electronic sight glass gauging - Google Patents

Abstract

Sight glasses for observing levels of media at high temperatures and/or high pressures are well-known for boilers and the like. The invention provides an electrical level sensor probe (72) within a wall of a normally transparent "sight-glass" gauge. The material of the wall and the placement therein of the probe (72) affords electrical insulation and sensitivity of the probe to capacitance variations according to the level of the medium to be gauged. The probe (72) is entirely separated from the interior of the device and ends (74,76) thereof protrude exteriorly for connection to suitable electronic sensing circuitry. Preferably, the probe (72) is moulded into the wall at making thereof. <IMAGE>

Description

SPECIFICATION Electronic sight glass gauging The invention relates to level sensing and is particularly concerned with systems and apparatus where the sensing device is subjected to high pressures and/or temperatures as in boilers and steam generating plant.

Traditionally, it is customary to use a so-called sight-glass arrangement in such circumstances and to rely upon visual reading against a scale or datum marking. Conventional designs of sightglass arrangements devote much attention to seal integrity over substantial ranges of temperature and pressure cycling. However, these arrangements do not provide an electrical signal that can be used by process control equipment, usually of an electric nature, and this is a substantial disadvantage, as is the absence of a measurement that is accurate beyond the possibilities of operator sighting.

It is, of course, well known to provide level sensors for storage bins, tanks and the like that work electrically according to variations of an electrical parameter such as capacitance or conductance. Usually, however, such sensors employ a sensing probe inserted into the relevant container via a metal mounting boss from which the probe is electrically insulated. Conventional sealing between the probe, insulation means and the mounting boss has been of a compression clamping type that is highly effective even at high pressures and ambient temperatures, but higher working temperatures require de-rating of the sensor to much lower temperatures. For example a probe-type sensor that is suitable for operation at 200 atmospheres with ambient temperature has to be derated as low as atmospheric pressure at temperatures of the order of 2000 C.

We have therefore been led to investigate the possibilities of applying probe-type sensing to sight-glass type arrangements, but the seal problems have remained difficult to solve even for proposals where the sensor probe has been laid down on, or otherwise attached to, an interior surface of the sight glass arrangement as electrical connections must be taken out through the device as a whole and require sealing.

What we now propose is a level sensing device for a high pressure and/or high temperature medium as in boilers or steam generating plant to replace or serve as a "sight-glass", wherein an electrical sensor probe is located in permanent separation from an interior of the device to be occupied by said medium, such separation being by material that is suitable for pressure/temperature-sealing arrangements and is both an electrical insulator and permits of sensing electrical capacitance affects on the sensor probe by said medium according to its level.

Advantageously the permanent separation of the sensor probe from the interior of the device is achieved by embedment in a wall member. Such embedment can be at a predetermined depth from its surface that is to be interior and in contact with the material or medium to be sensed, and ends, or at least one end, of the probe taken out through the opposite surface, or an end or ends, of the wall member. Such embedment is most conveniently done as a moulding-in at formation of the wall member, but, if the latter is a composite member, may be in or as an intermediate sandwiched layer, even laid down in the innermost layer.

Normal sighting may be provided for in a sensor device hereof and the probe may itself be in a transparent wall member through which such sighting is possible.

Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1A and 1 B are part-sectionai side and end views of one well-known sight-glass device; Figure 2 shows modification of one wall member of the device of Figures 1 A and 1 B to embody the present invention; and Figure 3 shows how this invention can be embodied in a different kind of sight-glass device.

In Figures 1 A and 1 B, a sight-glass device 10 comprises a rectangular central metal structure 1 2 that is centrally apertured at 14 after the manner of a frame that, on both sides, is relieved or rebated rearwardly at 1 6, 18 in order to accommodate sealing gaskets 20, 22 and glass plates 24, 26. Exterior edge parts of the latter also seat via gaskets 28, 30 on rebates 32, 34 of outer metal frames 36, 38 that are secured together by tie-bolting 40 in order to ensure the desired pressure and temperature tolerant sealing of the device 10.

End-access to the interior 14 of the device is provided via counter bores 42, 44 the outer of which is of a standard form for coupling at the pressure/temperatures involved.

The whole device 10 is exceptionally robust and very well suited to its use as a sight glass.

Figure 2 shows how the advantages of that structure are maintained in their entirety whilst at the same time providing for the desired sensing for electronic indication, monitoring and/or control purposes. Thus, in Figure 2, the glass plate 26 is shown having embedded therein an electrical conductor wire 50 at a depth 52 that is adequate to provide electrical insulation and desired capacitance sensitivity. Ends 54, 56 of the moulded-in probe wire 50 are shown taken out through the exterior surface for electrical connection purposes to electronic signal derivation and/or utilisation means.

It will be appreciated that the conductor wire ends 54, 56 could be taken through ends of the glass plate. Also, the plate 26 could be of any mouldable material that is compatible with pressure/temperature requirements and with the nature of the fluid material to be sensed, for example plastics materials such as polypropylene, if necessary fibre-reinforced. Transparency is clearly not essential as electronic detection is envisaged, but may well be preferred even though the possibility clearly exists of relying upon the other plate 24 for sighting purposes.

Composite clamped or connected plate members are also feasible, though generally less advantageous compared with the illustrated integral structure. For composite plate structures, the probe may be one layer of a sandwich, or a deposition on the inner layer, or let into the inner, perhaps even another layer.

Figure 3 shows a different type of sight-glass utilising a glass tube 60 sandwiched by tiebolting 62 via gasket rings 64, 66 between metal plates 68, 70 that may, with advantage, be grooved to seat the gaskets or/and the tube ends. A wire sensor probe 72 is shown moulded into the tube 60 with exterior end connections 74, 76.

It will be appreciated that the probe wires 50, 72 and the embedding material constitute a capacitive sensor that is sensitive to excursion therealong or level of a substance or medium to be sensed to produce a proportional capacitance change and thus a proportional electrical signal when connected to appropriate electronic circuitry. Changes of such excursion or level will produce proportionate changes of capacitance and electrical signal.

Level gauge devices hereof may be of any desired length but a standard size is obviously an advantage for production purposes, and application to particular desired positions or to cover greater lengths is readily achieved by interconnection of adjacent or spaced devices via their exterior probe sensor connection terminals.

Control may also be provided relative to discrete datum positions or ranges by dividing the sensor wire into discrete sections each with protrusive end connections.

Whilst our investigations of existing sight glasses indicates that embedment is necessary in order to achieve desired spacing from the device's interior, it may be that the use of other wall materials, or particular applications, will allow total wall thicknesses equal to or less than that considered to be optimum embedment depth.

Then, exterior deposition of a conductor may be used, if desired with a laid-down insulating substrate layer to build up to a desired optimum for capacitance sensing in a given situation.

It will be appreciated that, especially where a liquid to be sensed is a dielectric two sensors that oppose each other may be used with one of them characterised in a manner for a purpose analogous to that in our patent application No. 14983/76.

Claims (8)

1. A level sensing device for a high pressure and/or high temperature medium as in boilers or steam generating plant to replace or serve as a "sight-glass", wherein an electrical sensor probe is located in permanent separation from an interior of the device to be occupied by said medium, such separation being by material that is suitable for pressure/temperature-sealing arrangements and is both an electrical insulator and permits of sensing electrical capacitance affects on the sensor probe by said medium according to its level.

2. A device according to claim 1, wherein the probe is embedded in a wall member defining at least part of said interior.

3. A device according to claim 2, wherein ends of the probe protrude exteriorly from the wall member.

4. A device according to claim 2 or claim 3, wherein the probe is moulded into said wall member at making thereof.

5. A device according to claim 2, 3 or 4, wherein said wall member is a tube.

6. A device according to claim 2, 3 or 4, wherein said wall member is a plate.

7. A device according to any one of claims 2 to 6, wherein said wall member and/or another wall member is or are transparent.

8. A level sensing device substantially as herein described with reference to and as shown in Figures 1 and 2 or Figure 3 of the accompanying drawings.