RF water level measurement

Abstract: A water level measurement system for use with a pressurised power plant boiler comprises a tubular pressure vessel for attachment to one end of the boiler such that the water level in the pressure vessel is the same as in the boiler. The pressure vessel has a central rod-like conductor which together with the vessel forms a coaxial RF waveguide. A pair of spaced reference discontinuities are provided in the conductor above the maximum level of the water, so that an RF pulse injected into the waveguide is reflected successively by both discontinuities and the surface of the water. The level of the water can then be determined independently of changes in the dielectric constant of the vapour above it, from the ratio between the time intervals between the first and second reflected pulses and between the first and third reflected pulses. The conductor is mounted at the top of the pressure vessel by way of a pressure seal and a ceramic insulator.

Description: The present invention relates to the measurement of liquid level, for example in a tank or pressure vessel, and is particularly, though not exclusively, concerned with the problems of measurement of liquid level under conditions of high temperature and pressure. Changes in water level in power plant boilers are commonly monitored indirectly by means of devices which are affected by the density and temperature of the water and which are thus prone to error. The existing devices are reliable but do not give adequate resolution, especially when a wide range of levels may occur. The present invention proposes the measurement of the liquid level directly by means of time-domain reflectometry using a short r.f. pulse which is reflected from the surface of the liquid. The method involved comprises determining the time of travel of a pulse between a reference position and the point at which it is reflected from the surface of the liquid. The reference position can be a discontinuity from which the pulse is also reflected so that the distance from the reference position to the surface is measured by the time interval between the reflected pulses. German Patent Specification DE-A-2440402 describes a liquid level measurement device using this measurement principle. The reference position is established at a point near the input to the centre conductor of a transmission line by connecting that point to earth through a switch. The transmission line extends through the liquid surface and pulses are reflected alternately from the reference position and from the liquid surface. The reflected pulses are used to establish oscillations whose frequency gives a measure of the position of the liquid surface relative to the reference position. U.S. Patent Specification US-A-3703829 uses a similar system in which pulses are reflected from the surface of liquid in a tank and from the end of a coaxial probe which is immersed in the liquid. The time delay for the receipt of the reflected pulse from the surface serves as a measure of the height and hence the volume of the liquid while the time delay for the reflected pulse from the end of the probe, relative to that from the surface, is a measure of the density of the liquid. Hence the weight of liquid in the container can be calculated. Where changes in the gas or vapour over the liquid are likely to cause significant variations in the speed of travel of the pulses it is necessary to provide for measurement of the speed of travel so that the distance can be accurately determined. Accordingly, in accordance with the present invention, there is provided a liquid level measurement device having inner and outer probe conductors separated by a dielectric gap which is filled by the liquid up to the level to be measured and above that level by the gas or vapour over the liquid, a coaxial feeder for applying R.F. pulses to the upper end of the probe for reflection at the liquid surface, the probe having a first discontinuity near its upper end for producing a first reference reflected pulse in response to each R.F. pulse and a second discontinuity below the first but in the gas or vapour zone and above the normal maximum level of the liquid for producing a second reference reflected pulse in response to each R.F. pulse, and circuitry coupled to the probe for calculating from the reference reflected pulses and the pulse reflected from the liquid surface and the distance between the surface and the first discontinuity. Since the reflected pulse from the second discontinuity is travelling under the same conditions as that from the liquid surface the distances from the first discontinuity are directly proportional to the measured time of travel. In the application of the invention to the measurement of liquid levels in a boiler, a column is provided which is coupled to the boiler at two different levels so that the liquid enters the column and rises to the same level as in the boiler. The pressure-retaining shell of the column forms the outer conductor of the probe and the inner conductor is supported along the axis of the column and is connected through a pressure seal at the upper end of the column to the coaxial feeder. The invention will now be described in more detail with the aid of an example illustrated in the accompanying drawings, in which: Figure 1 is a vertical section of a measuring device in accordance with the invention for monitoring liquid level in a pressurized boiler; and Figure 2 is a detail on a larger scale of the upper end of the device showing the construction of the pressure seal. As seen in Figure 1 the device comprises a column which is mounted vertically and has an outer, tubular, pressure-retaining steel shell 10 which is connected by couplings 11 and 12 with the interior of the boiler so that the water 13 rises to the same level in the column as in the boiler and the space 14 above the water is filled with pressurized steam. A steel rod 15 is mounted coaxially within the tubular shell 10, being supported at its lower end by a locating ring 16 and attached at its upper end to a titanium rod 17. The rod 15 is a sliding fit in the locating ring 16, which constitutes an RF short circuit between the rod 15 and the shell 10, and is a screw fit into the bottom of the rod 17 to facilitate disassembly for transportation. As shown in Figure 2 the upper end of the rod 17 is coupled to a titanium tube 18 by an aluminium oxide ceramic collar 19 which is brazed to the rod 17 and tube 18, e.g. as described in United kingdom Patent Application No. 2,173,138. A stainless steel pin 20 passing through the collar 19 and the tube 18 forms the electrical connection from the rod 17 to the inner conductor of a coaxial feeder 21. The tube 18 is coupled by a Wadelock pressure seal 22 to the shell 10 and at its upper end is connected by a cap 23 to the outer conductor of the coaxial feeder 21. A Wadelock seal is a well known form of dual-ferrule metal-to-metal seal between two suitably tapered surfaces (i.e. no separate sealing washer is required): another form of such seal is a swage-lock seal, which could also be used if desired. The tube 18 and the pin 20 with the air gap between them are dimensioned to have the same coaxial impedance as the feeder 21. However at the ceramic collar 19 there is a change of impedance which results in reflection of a transmitted pulse. A second reflecting discontinuity is at the junction of the rod 17 and the rod 15 (Figure 1). The next reflection is at the surface of the water 13 and a bottom reflection is obtained from the ring 16 which provides a virtual short-circuit for R.F. pulses. As shown in Figure 1, when an R.F. pulse is transmitted down the column the reflection from the first discontinuity at time tO is a positive pulse with a reflection coefficient of approximately 18%. The second discontinuity at a distance L from the first will normally give a similar reflection at time t1. The water surface at height h gives a negative reflected pulse with a reflection coefficient of 47.5% at time t2. If there is no water in the column a large negative pulse is received from the bottom ring 16. Conversely if the water rises above the steam branch 11 and swamps the reference reflection from the second discontinuity the second reflection will be large and negative instead of small and positive. It will be seen that the height of the water level can be calculated from the equation h = H - t2 - t0 t1 - t0 L The result is therefore independent of the velocity of the pulses providing that they all travel in a region where the dielectric is the same steam or vapour under the same conditions of temperature and pressure. The transmitted pulses are R.F. pulses of approximately 1 nanosecond pulse width. The echo waveform does not consist purely of the reflected pulses described but requires analysis by appropriate circuits to extract the reflected pulses of interest before carrying out the calculation of the liquid level.

Claims: A liquid level measurement device having inner and outer probe conductors separated by a dielectric gap which is filled by the liquid up to the level to be measured and above that level by the gas or vapour over the liquid, a coaxial feeder for applying R. F. pulses to the upper end of the probe for reflection at the liquid surface, the probe having a first discontinuity near its upper end for producing a first reference reflected pulse in response to each R.F. pulse and a second discontinuity below the first but in the gas or vapour zone and above the normal maximum level of the liquid for producing a second reference reflected pulse in response to each R.F. pulse, and circuitry coupled to the probe for calculating from the reference reflected pulses and the pulse reflected from the liquid surface the distance between the surface and the first discontinuity. A device as claimed in claim 1, in which the outer probe conductor is in the form of a pressure-retaining shell with couplings at two levels for connection to a container for liquid whose level is to be monitored so that the liquid level in the shell is the same as that in the container, and the inner conductor is connected to the coaxial feeder through a pressure seal at the upper end of the probe. A device as claimed in claim 2, in which the inner conductor is supported within the shell by a locating ring at its lower end. A device as claimed in claim 2 or claim 3, in which the inner conductor is insulated from the outer conductor by an insulating collar at its upper end and is connected to the inner conductor of the coaxial feeder by means of a pin passing through the collar and the pressure seal with an air gap around the pin.

Inventors: RICHARDS PAUL NIGEL, WILLSON JOLYON PETER, RICHARDS, PAUL NIGEL, WILLSON, JOLYON PETER
Applicants: SOLARTRON GROUP LTD, SOLARTRON GROUP LIMITED
Application year: 1992
Main classification: G01F23
IPC codes: G01F23