A method and system for measuring a vertical density profile of a fluid

Abstract: In a method and system for measuring a vertical density profile of a fluid (2) in a fluid storage tank (1) an array of open-ended pressure pipes (4) extends downward into said fluid. The top end of each pressure pipe is connectable with a common pressurizing gas supply (9) and the bottom end incorporates a fluid level sensor head (3). The following steps are carried out for each pipe: - connecting said pipe with the common gas supply, - conducting the pressurizing gas into said pipe thereby forcing the fluid downward into said fluid level sensor head, comprising a lower end part in which the fluid passes an outlet opening forming an abrupt transition between said lower end part and an open ended tube, - determining a level indicating signal from the passing fluid, - determining a pressure indicating signal from the pressure on the fluid, and - deriving from said level and pressure indicating signals information on the said vertical density profile.

Description: The present invention relates to a method for measuring a vertical density profile of a fluid in a fluid storage tank. Particularly the present invention relates to a method, a system and a fluid level sensor head for measuring vertical density profiles in an LNG tank. In an LNG tank the phenomenon of "roll-over" is a potential cause of damage to such tanks. The phenomenon of "roll-over" should be understood as an uncontrolled rising of a fluid layer at the bottom of different density, present in such a tank, accompanied by rapid vaporization which may overwhelm the tank pressure relief systems. Hereinafter this phenomenon will be described in more detail. When an LNG cargo, which has a greater density than the already stored LNG, is loaded in a tank, the underneath LNG can cause stratification of the contents. Over a period of time, heat leaking into the tank bottom and the boiling-off of the lighter components at the surface, may cause the densities of the two layers to become equal before the temperatures of the layers come into equilibrium. Convection currents, previously restricted to the separate strata because of the density difference, can now occur throughout the whole tank. As stated above the bottom layer can rise to the surface, accompanied by rapid vaporisation which may overwhelm the tank pressure relief systems with hazardous consequences. Knowledge of the vertical temperature and density profiles can provide operators with information on whether the prevailing conditions may lead to roll-over, so that appropriate operational procedures can be applied. A conventional method applied in this field is the "bubbler" or gas purge method which comprises the step of measuring hydrostatic pressure, commonly used in the determination of liquid level and density in ambient temperature products. Gas is allowed to bubble through an open-ended tube at a known height in the liquid and a pressure transducer mounted outside the tank measures the gas pressure required to produce the bubbles. This conventional method is not very accurate, because the vertical position corresponding to the measured pressure is ill-defined and depends on the variable mechanism of bubble release. Thus it is an object of the invention to provide an improved method, a system and a fluid level sensor head for measuring a vertical density profile wherein the measurements are made with high accuracy. Another object of this invention is to provide a method, a system and a fluid level sensor head for measuring a vertical density profile which does not disturb the conditions present in the fluid to be investigated, for example LNG. The invention therefore provides a method for measuring a vertical density profile of a fluid in a fluid storage tank as claimed in claim 1. The invention further provides a system for measuring a vertical density profile of a fluid in a fluid storage tank as claimed in claim 2. Advantageously the lower end part of the fluid level sensor head is a tube with a diameter substantially greater than the diameter of the open ended tube. Particularly the outlet opening is comprised in a flat bottom connecting the lower end part with the open ended tube and normal to the axes of said tubes. In a further detail the lower end part is a tubular room the inlet opening of which has a diameter smaller than the diameter of the tubular room. It is remarked that FR-A-1,082,066 discloses a device for measuring oil or fuel in an automotive engine wherein a fluid level sensor head is connected to a pressure pipe, the head having a shoulder portion and an open ended tube. Further, US-A-3,399,573 and DE-A-395,063 disclose density measuring arrangements comprising a common supply means, means for conducting pressurized gas, and pressure determining means. However, the specific arrangement of the present invention has not been disclosed. The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Fig. 1 schematically shows a system for measuring a vertical density profile of a fluid in a fluid storage tank in accordance with the invention; Fig. 2 is a detail II of Fig. 1 drawn on a scale larger than the scale of Fig. 1 and gives a cross-sectional view of an advantageous embodiment of a fluid level sensor head in accordance with the invention; and Fig. 3 is a plot showing a curve as measured for LNG using the fluid level sensor head of the present invention during an operating cycle. In the figures the same reference numerals are used to indicate the same or similar parts or components. Referring to Fig. 1 a schematic overview of the system is shown in accordance with the present invention. In a fluid storage tank 1 a fluid 2 is contained of which a vertical density profile has to be determined. If for example the fluid 2 is LNG there may be a potential risk, as stated above, of the phenomenon of "roll-over" occurring during storage. For reasons of clarity only an array of two fluid level sensor heads 3 is shown. It is noted that an array may consist of many more sensor heads 3 when a complete profile has to be determined. Each sensor head 3 is mounted in any suitable manner on a pressure pipe 4 through which a pressurizing gas, for example He (helium) or N₂ (nitrogen), flows to force the fluid down thereby lowering the fluid level. In the case of LNG helium was chosen to be the working gas because of its low solubility in LNG, although nitrogen would be acceptable and more economical for normal use. When said level reaches the respective sensor head an accurate level value will be obtained as will be explained below. To calculate a mean density value data is needed from a pair of sensor heads. The measurements are made separately with each sensor head, a selector valve 5 selecting the appropriate sensor head from said array. The system further comprises a vent pipe 6 extending from the top of the storage tank 1, a vent valve 7 to reduce the pressure of the gas in a gas supply pipe 9, and a pressure transducer 8 conventionally used in order to measure differential pressure. The direction of the pressurizing gas flow in the supply pipe 9 is indicated by the arrow g. As can be seen in Fig. 1 the height difference between such a pair of sensor heads 3 has a set value Δh. Furthermore the heights (or depths) of the sensor heads, particularly of the level indicating components, are preset exactly. Then by means of equations for hydrostatic equilibrium which are known to those skilled in the art and will not be described in detail the mean density value of such a Δh layer can be calculated using gas and gravity characteristics as required. Fig. 2 as a detail II of Fig. 1, drawn on a scale larger than the scale of Fig. 1, shows a cross-sectional view of an advantageous embodiment of a fluid level sensor head 3 in accordance with the present invention. The sensor head 3 is connected with a gas pressure pipe and extends into the fluid filled storage tank. The fluid 2 is forced downward into the pressure pipe 4 by the pressurizing gas as indicated by the arrow g, the fluid having a fluid level or meniscus M. Said level M will be lowered further by said pressurizing gas in that the fluid level M will pass four successive levels, indicated respectively L1-L4. As shown in this figure the sensor head 3 comprises a chamber 10 with an inlet opening 11 at level L1 and provided with an outlet opening 13 at level L3, the outlet opening 13 forming an abrupt transition between said chamber 10 and an open ended tube 12. At level L3 the cross-sectional area of the sensor head is reduced substantially. Advantageously the chamber 10 is a tube with a diameter substantially greater than the diameter of the open ended tube 12. In the embodiment of the fluid level sensor head 3 as shown in this fig. the outlet opening 13 is comprised in a flat bottom 14 connecting the chamber 10 with the open ended tube 12 and normal to the axes of said tubes. Furthermore the inlet opening 11 of the chamber 10 has a diameter smaller than the diameter of said lower end part, thus forming a tubular room 15 between the pressure pipe 4 and the open ended tube 12 thus allowing a more abrupt transition at level L3. As can be seen also in Fig. 2 the inlet opening 11 is comprised in a flat top 16 connecting the tubular room 15 with the pressure pipe 4 and normal to the axes of said room and pipe. In said embodiment the pressure pipe 4, the chamber 10 and the open ended tube 12 are arranged coaxially. However, such an arrangement is not necessary as will be clear to those skilled in the art. In an advantageous embodiment the chamber 10 and the flat bottom 14 can be connected by a rounded corner 17 in order to prevent fluid from remaining transiently in the corner since a right angled corner could obstruct all the fluid is forced downward throughout said outlet opening 13 at one go. Referring to Fig. 3 a plot is shown illustrating the pressure behaviour of LNG within the sensor head in accordance with the invention during the lowering of the meniscus M. Pressure readings from the pressure transducer 8 are measured at known time intervals and are plotted, with pressure p on the vertical axis and time t on the horizontal axis, respectively in units 10⁵ Pa and s. A pressure unit is marked by p corresponding with a pressure difference of 2 mm LNG when the density is 460 kg/m³. Also indicated on the plot are the said four sensor head levels, L1, L2, L3 and L4. As shown in the plot the passing of a cross-sectional area transition is significantly resolved as a sharp discontinuity in the rate of change of pressure. Particularly passing of level L3, at the outlet opening 13 as shown in Fig. 2 is marked clearly. Thus a precise measurement of pressure at each level is obtained. Consequently accurate average density values as described hereinbefore can be calculated. It is noticed that the sharpness of the discontinuity will be influenced by various factors. Among these, flow rate of the downward pressurized fluid, viscosity of the fluid, and ratio of diameters (or cross-sectional areas) of the lower end part and the outlet opening to the open ended tube will be of great importance. Therefore, the choosing of suitably adapting dimensions, magnitudes and properties will be the result of engineering practice. The measurement sequence is as follows: a sensor head is selected and the mean pressure required to produce bubbles at the open end of the sensor head determined. This pressure value is used as a reference to preset the interface within the sensor head. As an alternative the detection of level L1 can be used as a source of reference pressure to preset the interface and in this case no bubbles need be evolved into the main body of the fluid. Pressure readings are then taken at equal time intervals, and for example plotted on a computer screen and stored. Advantageously at the same time the pressure rate is calculated in order to determine when the meniscus is traversing the restriction. A fixed number of data points are collected after the detection of the discontinuity. The sequences of pressure measurements are then analysed, to determine the precise pressure at the discontinuity L3 in the plot, i.e. the abrupt change of gradient. For example, the following algorithm is used. A window, 80 samples wide, is selected, a straight line is fitted to the points by a least squares method and the standard deviation of the pressure readings about the line is determined. The window is stepped through the data sequence, a new line being determined at each step, until a position is found where both (i) the next five points beyond the window lie above the fitted line by more than two standard deviations and (ii) the sixth point by more than four standard deviations. The last point in the window is then taken to be the one nearest to the transition. A second line is fitted to the next 25 points following the transition and the exact pressure at the discontinuity is determined as the intersection of the two fitted lines. Subsequent sensor heads from said array are then selected in turn and the pressures at their discontinuities determined. The density profile is then calculated. Advantageously, for the measurements of the fig. 3-plot, the sensor head 3 has internal diameters of 12.0 mm, 22.10 mm and 0.5 mm for respectively the pressure pipe 4, the lower end part 10, and the open ended tube 12, and an internal height for said end part of 31 mm. It will be clear to those skilled in the art that various refinements can be incorporated in effecting the measurement sequence. For example the pressure transducer readings can be compensated for temperature drift by periodically connecting both pressure ports to the tank vapour space and then measuring the output. Such reference values can be used for calibration corrections. Furthermore two gas flow rates can be used, a high flow to quickly move the gas/fluid level within the selected tube and a low flow for use during the measurements. Also the position of the fluid level within the tube can be preset by using a combination of venting to the vapour space to raise it and the gas flow to depress it. It will be appreciated that the measurement and control system will consist of advanced measuring and control means, such as programmable switch units, digital multimeters and solenoid valves, controlled by means of a computer system enabling automated operation. Various modifications of the present 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: A method for measuring a vertical density profile of a fluid in a fluid storage tank (1) by means of an array of open-ended upright pressure pipes (4) which extend downward into said fluid, each said pipe having at its upper end means (5) for connecting to a common pressurizing gas supply (9) and said pipe having at its lower end a fluid level sensor head (3), said sensor heads being at respectively different levels in said tank, characterized by the following steps, (a) for each pipe: connecting said pipe with the common gas supply, conducting the pressurizing gas into said pipe thereby forcing the fluid downward into said fluid level sensor head mounted at the lower end of said pipe, said head comprising a chamber (10) having a horizontal cross-sectional area larger than said pipe and a lower end part in which the fluid passes through an outlet (13) having an opening of smaller horizontal cross-sectional area than said chamber, thereby forming an abrupt transition in cross section of said sensor head, said abrupt transition being at a precisely known level in said tank, said head also comprising a tube (12), open at both ends with one open end attached to the lower end part of said chamber to receive pressurized gas passing through said outlet, sensing the pressure being applied to said gas as it is passed through said sensor head and in particular as the gas-fluid interface passes through said abrupt transition, to develop pressure indicating signals, and (b) deriving from said pressure indicating signals for at least two sensor heads and from information as to the levels within said tank of at least said abrupt transitions, information on said vertical density profile A system arranged to carry out the method of claim 1 for measuring a vertical density profile of a fluid in a fluid storage tank (1) comprising an array of open-ended upright pressure pipes (4), each pipe extending downward to a different depth in said fluid, the top end of each said pressure pipe having means (5) for connecting with a common pressurizing gas supply (9), and the bottom end of said pressure pipe having a fluid level sensor head (3), characterized by said sensor heads being at respectively different levels in said tank each said sensor head comprising a chamber (10) having a horizontal cross-sectional area larger than said pipe and a lower end part in which the fluid passes through an outlet (13) having an opening of smaller horizontal cross-sectional area than said chamber, thereby forming an abrupt transition in cross-section of said sensor head, said abrupt transition being at a precisely known level in said tank, said head also comprising a tube (12), open at both ends with one open end attached to the lower end part of said chamber to receive pressurized gas passing through said outlet, means (8) for sensing the pressure being applied to said gas as it is passed through a said sensor head and in particular as the gas-fluid interface passes through said abrupt transition, to develop pressure indicating signals, means arranged to derive from said pressure indicating signals for at least two sensor heads and from information as to the levels within said tank of at least said abrupt transitions, information on said vertical density profile. A system as claimed in claim 2, characterized in that the chamber (10) is a tube with a diameter substantially greater than the diameter of the open ended tube. A system as claimed in claim 2, or 3, characterized in that the outlet opening is comprised in a flat bottom connecting the lower end part of said chamber with the open ended tube and normal to the axes of said tubes. A system as claimed in claim 3 or 4, characterized in that the chamber is a tubular room, the inlet opening (11) of which has a diameter smaller than the diameter of the tubular room. A system as claimed in claim 5, characterized in that the inlet opening is comprised in a flat top connecting the tubular room with the pressure pipe (4) and normal to the axes of said room and pipe. A system as claimed in anyone of the claims 2-6, characterized in that the pressure pipe (4), the chamber (10) and the open ended tube (12) are arranged coaxially.

Inventors: COLE JOHN BERNARD, EVANS PETER GWILYM, COLE, JOHN BERNARD, EVANS, PETER GWILYM
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: G01F23
IPC codes: G01F23, G01N9, F17C13, G01L1