AN APPARATUS FOR PRODUCT/WATER INTERFACE MEASUREMENT IN A PRODUCT STORAGE TANK

Abstract: A transmission line open at one end, is used to determine the level of water, if any, at the bottom of product storage tanks. Changes in capacitance that result from the interface, are transformed by proper matching, into much larger voltage changes at the line input. The voltage measurements are converted to level, by a microprocessor.

Description: AN APPARATUS FOR PRODUCT/WATER INTERFACE MEASUREMENT IN A PRODUCT STORAGE TANK The invention relates to an apparatus for product/water interface measurement in a product storage tank. In particular, the invention relates to an apparatus for continuously monitoring the water level at the bottom of a product storage tank, e.g. an oil product storage tank. When storing a product in oil industry, e.g. in a refinery, distribution terminal or depot, careful measurement of the (oil) product (e.g. crude oil, refined products and the like) within clearly defined limits of accuracy is essential not only to ensure that the purchaser of a quantity of (oil) product receives the volume or weight which is invoiced to him and that the seller does not over-deliver, but also to enable a constant check to be kept on stocks, and on storage, transport and handling losses. In product storage tanks, any free water may accumulate on the tank bottom, due to several causes. For example, water may accompany the (oil) product during transport and when filling the storage tank with (oil) product, water enters the storage tank. Further, generally the supply line to the storage tank used is water washed prior to each delivery to the storage tank and thus there is a risk of water contamination when water is entering the tank ahead of the (oil) product. Generally, there is also any residual water left when filling and emptying storage tanks. Therefore, in (oil) product storage tanks, a water bottom may accumulate and a knowledge of the amount of water, if any, at the bottom of the tank is essential. This ensures that calculation of tank contents deals only with (oil) product stored, not water. This is in particular relevant in countries wherein duty is levied on (oil) products. Consequently, any miscalculation or loss in product storage and handling now involves product on which duty has been paid. This, coupled with increased environmental requirements has shown the need for more accurate stock accounting systems. The above is particularly relevant when transfer of custody and/or duty is involved. At present, testing for water and determination of water level in (oil) product storage tanks, is carried out manually (dipping techniques) with the usual disadvantages regarding personal interpretation (accuracy), safety and measurement frequency. In tanks where water does accumulate, it is normally maintained below 80 mm, with a maximum expected level of 150 mm and thus there is a need for an instrument which is able to measure the expected water bottom range. It is remarked that already many methods and techniques exist to detect product/water interfaces, but due to the abrupt change in dielectric constant, capacitance techniques are most commonly employed. The drawback with using capacitance is that to provide adequate resolution, precise measurements are often necessary. This necessitates either careful circuit design or sophisticated signal processing which can be expensive and complex to implement. Further, it is already known to apply a co-axial transmission line to determine liquid-level in a tank. Electrical pulses are applied at the line input. Changes in dielectric constant that result from the interface, cause the pulses to be reflected back to the line-input and from the measured reduction in pulse length, the liquid level is calculated. However, the measurement accuracy of this known system is limited by the need to make precision high speed measurements. It is therefore an object of the invention to provide a cheap and simple apparatus for measuring the water level at the bottom of a storage tank and providing water-bottom information. The invention therefore provides an apparatus for measuring the water level at the bottom of a product storage tank, comprising a sensor assembly adapted to be located at the tank bottom, said assembly comprising a plurality of measurement sensors arranged in a housing provided with one or more openings to enable product/water entering or leaving the sensing area, each measurement sensor comprising an open-circuited transmission line, comprising at its open end a monopole antenna or probe, said probe being surrounded with an insulated material Λ and further comprising means for exciting the transmission line. The apparatus of the invention makes use of the properties of an open-circuited transmission line (e.g. a co-axial transmission line) to act as a resonant transformer to produce a voltage proportional to the depth of water at the tank bottom. Small changes in capacitance which result from the interface, are transformed into much larger voltage changes at the sending end which are more easily measured. Advantageously, a quarter wave (co-axial) transmission line is applied. More advantageously, the probe length is less than 1/10 of the wavelength used to excite the transmission line. A microprocessor compares the voltage measurements with a preprogrammed look-up table to determine water level. The invention will now be described in more detail by way of example by reference to the accompanying drawings, in which: fig. 1 represents schematically a system for measuring water level at the bottom of a product storage tank wherein the apparatus of the invention is applied; fig. 2 represents the sensor assembly of the system of fig. 1; and fig. 3 represents part of a sensor of the sensor assembly of fig. 2. Referring now to fig. 1, a. product storage tank 1 has been shown. As already discussed in the foregoing, due to several reasons, a water bottom may accumulate in the tank. The apparatus for measuring water level at the bottom of the tank 1 comprises a sensor assembly 2 connected in any manner suitable for the purpose to signal processing electronics 2a. The sensor assembly 2 comprises a plurality of sensors and is designed to be located in any manner suitable for the purpose at the tank bottom e.g. by means of a winch and guide assembly (not shown for reasons of clarity) and communicates by any suitable connection 3 e.g. via a stainless steel tube with the processing electronics 2a mounted at the tank top. The processing electronics 2a is connected in any suitable manner (e.g. an armoured cable 4) to an intrinsically safe power supply and serial interface 5. Information from supply 5 can be transmitted via any suitable line 6 to a host computer (not shown for reasons of clarity) . In fig. 2 the sensor assembly 2 has been shown in a partially transparent longitudinal section. A sensor body 7 is provided with one or more slots (not shown) to enable product/water entering or leaving the sensing area. The sensor body 7 is connected in any mechanical way suitable for the purpose to the connection 3 shown in fig. 1. Such mechanical details are known to those skilled in the art and will not be described in detail. Advantageously, there are four sensors (8, 9, 10, 11), each having a length of 100 mm, the lower three (8, 9, 10) being measurement sensors spaced vertically 50 mm apart (distances A and B in fig. 2). The measurement range, advantageously 200 mm, is represented by C. The fourth sensor 11 is a reference sensor, located above the water interface. Further, a top cover 7a and a base unit 7b are shown. Each sensor (8, 9, 10, 11) is connected to its own co-axial cable 12 (only two being shown for reasons of clarity) which via the connection 3 in any suitable manner are connected to the electronics on top of the tank. For reasons of clarity, mechanical connection details are not represented fully. Advantageously, the number of measurement sensors is three. However, if the water composition is reasonably consistent, then the number of measurement sensors could be reduced to two. This would reduce the manufacturing complexity and subsequent cost. Fig. 3 represents a longitudinal section of part of a sensor. A co-axial transmission line 12 comprises a centre conductor (13), an inner protective covering 14, an outer protective covering 15 and a braided screen 16. In order to increase the capacitive coupling and hence sensitivity the centre conductor 13 is extended by attaching a monopole antenna or probe 17. The outer braid of the co-axial line is removed from the end section of the line, the monopole antenna or probe is attached and the inner conductor and probe are covered by a non-conductive coating. The active area is insulated e.g. by a glass pocket 18. Further, a suitable seal 19 is represented. This is attached to the glass pocket by a glass to metal seal 20. Advantageously, the length of the transmission line is an odd multiple of the wavelength divided by four. Advantageously, the braid is removed over a length much smaller (e.g. less than 1/10) than the wavelength used to excite the transmission line. Provided the probe length is less than 1/10 of the wavelength used to excite the transmission line, no appreciable radiation field exists. in fulfilling the above, only localized measurements are made as the field is contained within a relatively small sample volume. The operation of the apparatus of the invention is as follows. As already indicated hereinbefore, the apparatus of the invention is based upon the properties of an open-circuited transmission line. Advantageously, the apparatus of the invention is operated in the frequency range of 10 MHz to 100 MHz. More advantageously, the length of the transmission line is adjusted electronically by optimizing the operating frequency, rather than by choosing the nearest quarter-wavelength multiple to suit the installation. Transmission lines are known as such to those skilled in the art and the general operation thereof will therefore not be described in detail. Generally, it can be said that signals are transmitted along the transmission line and are reflected at the surface of the fluid interface, due to a change in dielectric constant. The reflected signals are processed further in order to obtain voltage measurements indicating level. Calibration data is obtained in any suitable manner. For example, by lowering the sensor assembly 2 through the product interface during installation, recording the sensor outputs and copying them into an appropriate look-up table. When the antenna or probe is immersed in a fluid, its impedance is related to the complex permittivity of that fluid. If it is assumed that the impedance is reactive, which is a reasonable assumption considering its construction, then the phase shift induced on the transmission line is proportional to the reactance (which is capacitive) appearing at the open-circuit. The equivalent circuit is that of a transmission line terminated by a capacitor, the value of the capacitor being determined by the probes radiating geometry, and the complex permittivity of the medium surrounding the probe. Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

Claims: C L A I M S 1. An apparatus for measuring the water level at the bottom of a product storage tank, comprising a sensor assembly adapted to be located at the tank bottom, said assembly comprising a plurality of measurement sensors arranged in -a housing provided with one or more openings to enable product/water entering or leaving the sensing area, each measurement sensor comprising an open-circuited transmission line, comprising at its open end a monopole antenna or probe, said probe being surrounded with an insulated material and further comprising means for exciting the transmission line. 2. The apparatus as claimed in claim 1, wherein the transmission line is a co-axial transmission line. 3. The apparatus as claimed in claim 1 or 2, wherein the transmission line is a quarter wave transmission line. 4. The apparatus as claimed in any one of claims 1-3, wherein the measurement sensors are arranged at a different height to cover the measurement range. 5. The apparatus as claimed in any one of claims 1-4 comprising 2 or 3 measurement sensors. 6. The apparatus as claimed in any one of claims 1-5 comprising a reference sensor. 7. The apparatus as claimed in claim 2, wherein the outer braid of the co-axial line is removed from the end section of the line, and the monopole antenna or probe attached, and wherein the inner conductor and probe are covered by a non-conductive coating. 8. The apparatus as claimed in claim 7, wherein the length of the braid removed is less than 1/10 of the wavelength used to excite the transmission line. 9. The apparatus as claimed in any one of claims 1-8, wherein the length of the transmission line is an odd multiple of the wavelength divided by four. 10. The apparatus as claimed in any one of claims 1-9, wherein an operating frequency of 10-100 MHz is applied. 11. The apparatus as claimed in any one of claims 1-10, wherein the probe length is less than 1/10 of the wavelength used to excite the transmission line.

Inventors: WELCH MICHAEL, WELCH, MICHAEL
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1993
Main classification: G01F23
IPC codes: G01F23