GB2186317A - Clamp used in tank contents measurement - Google Patents

Abstract

A clamp for securing a vertically slidable rod 36 relative to a base member 34 comprises a tilting plate 37 resting on the base member at one end, and urged to pivot downwards by a spring 44 on a bolt 41 passing through an aperture in the plate 37, a second aperture gripping the rod 36 as the plate tilts. As shown, a plurality of rods 36 are secured by a plurality of tilting plates 37 on a common base member 34. A method is described of obtaining data for transmission to a data processor and for calculating the depth and specific gravity of a liquid in a tank from four sensors, three of which are arranged at different known heights from the bottom of the tank and the fourth above the liquid level in the tank to measure air or gas pressure above the liquid. By using a switching mechanism the sensors are connected in turn to a pressure transducer to establish the highest sensor for which a positive differential with the fourth sensor reading indicates liquid above that sensor. This differential pressure is stored in a memory. If this reading indicates that there are two or more sensors in the liquid, two sensors are selected and the pressure differential between them is established and stored. The depth and specific gravity can then be calculated from the known height parameters and the two stored differential pressures. In the case where the only sensor in the liquid is the lowest sensor the depth of liquid is calculated using the last stored specific gravity value. <IMAGE>

Description

SPECIFICATION Improvements in Tank Contents Measurement The present invention relates to the measurement of the contents of liquid in a tank and particularly, but not exclusively to a method of compiling data and calculating liquid level and density from the compiled data. The invention also includes a device for locating and holding a substantially vertical tube or rod.

Our European Patent Application No. 81304233.0 describes a tank contents gauge which uses a singie transducer connected sequentially to three pressure sensors to detect pressure above the liquid in the tank, and enough data from two probes at known separation in the tank determine, by means of a data processor properly programmed, the tank depth, specific gravity and contents.

The accuracy of calculation depends upon the accuracy of the transducer and accuracy of +0.1% is normal. This accuracy is acceptable in tanks of the order of 3 metres depth, but for large tanks, of for example 20 metres depth this is not precise enough.

At present large tank depth is measured using servo-assisted float gauges which are expensive, complicated and require regular servicing.

It is the object of the present invention to provide a system for measuring depth using a simple device.

According to one aspect of the present invention there is provided a method of obtaining data for transmission to a data processor for calculating the depth and specific gravity of liquid in a tank, the method comprising the steps of locating in the tank at different known depths, at least three pressure sensors, providing a further sensor for giving a reading (PA) of pressure in the tank above the liquid, connecting the pressure sensors to a pressure transducer via a switching mechanism, providing a controller for controlling for any one reading which sensor or sensors are connected to the pressure transducer, and taking the following readings:: 1) by connecting the first sensors sequentially and the further sensor to the transducer, determine the highest first sensor for which there is a positive differential (PX-PA) indicating a head of liquid above that sensor, and storing the value (PX-PA) in a memory, 2) if the said highest first sensor is not also the lowest first sensor, establishing a differential pressure reading between two first sensors that are at that time in the liquid and storing the value (Py-Px) so obtained in the memory then 3) calculating from the known height Hx of the said highest first sensor and the known distance Dxy between the said two first sensors the depth from the equation (PxPA) X Dxy depth=[ ]+Hx (in metres) (PyPx and the (P,-P,)x1.0197x10-4 specific gravity= Dxy and storing the specific gravity value so calculated, 4) if the said highest first sensor is also the lowest first sensor, calculating the depth from the equation (PXPA)X100197X1O-4 depth = (in metres) Specific Gravity using the last stored specific gravity value for the liquid. (P is pressure in pascals and D is depth in metres).

The equations could obviously be modified if depths other than in metres are required. Also they could include a factor S representing the sensitivity of the transducer which can be determined by taking the transducer reading R of a known reference pressure Pr. The sensitivity is then P,/R.

Preferably the transducer is a differential transducer having both sides capable of connection to the further sensor and at least two of the said first sensors whereby the required pressures differences may be established by delivering the respective pressures to the opposite sides of the transducer.

In accordance with a further aspect of the invention there is provided a device for locating and holding a vertical or substantially vertical tube or rod, the device comprising a locking plate having opposed first and second sides and a base plate comprising a support for said first side, means, receiving an upwardly directed screw threaded bolt and means allowing passage therethrough of the tube or rod to be held, the locking plate having on a line between the first and second sides an aperture allowing passage of the tube of rod with a close fit on at least two sides, and spaced therefrom between the aperture and the second side, a second aperture to receive the screw threaded bolt, the device including a resilient member on the bolt which urges the locking plate to a position which reduces the horizontal cross section of the first aperture and hence grasps the tube or rod to prevent its falling under its own weight, the locking action being releasable by returning the locking plate to a substantially horizontal position against the resilient bias.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of apparatus for performing a method in accordance with the invention, Figure 2 is a flow diagram of a method similar to the method described with reference to Figure 1, and in accordance with the invention, Figure 3 is a plan view of a device for supporting tubes in accordance with a second aspect of the invention, and Figure 4 is a cross-section view of the device of Figure 3.

The method will hereinafter be described with reference to the apparatus of Figure 1. The method using three sensors and a differential transducer switchable on both sides to the sensors is illustrated in the flow diagram of Figure 2. Catering for transducer offset and sensitivity is not specifically included in that diagram, but could be incorporated if required as described below.

The tank contents gauge illustrated in Figure 1 is designed for use in locations where it is acceptable to use pressure sensors in the form of Hydrostatic probes, such as with petrol in a ventilated tank. In many ways it is an extension of the subject mattersof European Patent Application E0048589 to which reference should be made for details of components and mathematics. In the present arrangement four pressure sensors, 1,2,3,4, each comprise a hollow tube 1 of about 22 mm outer diameter, of progressively longer length. Each probe 11 has a probe head fitted at the top of the probe outside the tank incorporating a special connector and nonreturn valve. This prevents any escape of liquid or gas should one of the air tubes be damaged.

Each probe has two air lines (14,15:14,16:14, 17: 14, 18:) connected to the probe head 12 in the form of 6 mm outerdiametertubes. One tube 14 carries air at low pressure from a compressor 19 via a regulator (not shown) and restrictor 20 to the probe 11, whilst the other 15, 16, 17, 18 carries an air pressure reading back to a differential pressure transducer 21. While embodiments of the invention and their operation are described with air as the operating medium, it will be appreciated that other suitable gases could be used.

The lines 15, 16, 17,18 are connected to respective solenoid valves 26,25,24,23. The valve 23 is connected to one side of the pressure transducer 21 and switches the pressure applied to the transducer terminal between line 18 and a line 27 to the solenoid valve 24. The valve 26 switches between line 15 and an outlet port 28 which is connected to ambient pressure in the tank (or any ambient pressure if the tank is not pressurised). The valves 24 and 25 are connected in series between the valves 23 and 26 and can switch between the next valve and lines 16,17 respectively. Ambient pressure is also transmitted from the outlet port 28 to the other side of the pressure transducer 21. Thus by switching the solenoids 23,24, 25, 26 in turn the following differential pressure readings can be taken.

i) with settings as shown in Figure 1, the reading Z=the offset of the transducer, as there is no applied pressure across the transducer.

ii) with solenoid 26 energised, a reading A=P1 PAZ where P1 is the pressure at the sensor 1, and PA is the pressure above the liquid in the tank.

iii) with solenoid 25 energised, a reading B=P2PA-Z where P2 is the pressure at the sensor 2.

iv) with solenoid 24 energised a reading C=P3-PA-Z where P3 is the pressure at sensor 3.

v) with solenoid 23 energised a reading D=P4-PA-Z where P4 is the pressure at sensor 4.

Thus subtracting the offset Z from the readings A, B, C, and D the hydrostatic pressure head of liquid at the four probes is established. By providing an extra solenoid (not shown) in the series, the transducer can also be connected to a reference pressure whereby the sensitivity S of the transducer can be determined.

In actual practice not all these readings are required for every calculation. The readings that are required for a calculation are a pressure at the highest probe which enters the liquid, ambient, or above liquid pressure in the tank, and a pressure difference between any two of the probes which are in the liquid. To obtain these pressure readings it is possible to use either the apparatus of Figure 1 or an equivalent apparatus in which a further set of solenoids enables probes 1,2, and 3 also to be switched to the other side of the transducer.

It will be appreciated that if the liquid in the tank is above probe 2 but below probe 1 the following readings will be taken; i) above to establish the offset, ii) above which will give no positive pressure difference once the offset has been deducted, iii) above will give P2-Pwhich will be sent to a memory 29 iv) above will give combined with iii) above P3-P2 and v) above will not be taken.

The readings are relayed to the memory 29 and thence to a data processor 30 programmed to carry out the required calculations from the gathered information. A controller 32 energises the required solenoids in accordance with instructions on a program in the data processor.

Once the required readings have been taken the depth and S.G. can be calculated from the following equations:- PXxSxDxy depth of liquid=[ ]+Hx in metres (PyXS)(PxXS) (PYPX)X1-0197X10-4 specific gravity= Dxy where Px is the pressure reading at the highest probe at which there is a positive reading, in pascals Py is the pressure reading at the probe below that for which the reading Px was taken in pascals Dxy is the physical vertical distance in metres between the probes x and y in metres.

S is the sensitivity.

Hx is the height in metres of the probe x above the floor of the tank is metres.

In fact to provide the pressure difference for density measurement (Py-Px) any two probes under the liquid may be used, although in practice it is likely that the probes x and y will be used as defined because it relieves the necessity of taking further readings.

Where only the bottom probe is covered it will not be possible to use a calculated S.G. value. However by storing in the memory values for S.G. as they are calculated, the last calculated S.G. value can be used as described in European Patent Specification No.

E0048589.

An advantage of the multi-probe arrangement described above is that the density and therefore S.G. of the liquid can be determined for different levels in the tank. By using the differential pressure and known physical distance between any two probes in the liquid the avergage S.G. value between those probes can be determined.

In other respects the operation and modifications of the tank contents gauge can readily be adapted from the disclosure of the above mentioned European Patent Specification.

When assembling more than two probes in a tank where it is necessary to know the exact height of each probe from the bottom there are problems particularly where the tanks are deep ones requiring long unmanageable lengths of tubing.

The following arrangement may be found to be satisfactory. Figures 3 and 4 illustrate a flange plate 31 which is bolted to the tank through, for example eight, bolt holes 33 around the edge of the flange plate. Towards the centre of the flange plate the material of the plate is thicker below the plate at 34.

In the same area there is a raised platform 35 above the flange. The illustrated device is designed to support four tubes 36. For each tube 36 there is provided a separate locking plate 37. Each locking plate 37 is substantially rectangular with a substantially triangular piece at one end which piece is supported on the platform 35 in such a way as to form a fulcrum for movement of the locking plate 37. The locking plates each have two apertures 38, 39 spaced from one another on a line running longitudinally from the fulcrum triangular end of the plate. The aperture nearer the triangular end has a length along the line which is substantially the same as the equivalent dimension of the tube (i.e. the diameter of a cylindrical tube) and is designed to receive the tube passing therethrough.The second aperture 39 receives a bolt 41 or stainless steel stud, one end of which bolt is screw threaded into a threaded hole 42 in the flange plate 31. The locking plate 37 is retained on the bolt by a self retaining nut 43. Between the plate 37 and the nut 43 a spring loaded action is provided by a compression spring 44 abutting the nut 43 at one end and the locking plate at the other end, the movement of the nut against the spring being limited by a spacer 45 on the bolt.

The operation of the device is as follows: The spring urges the outer end of the locking plate downwardly while the inner triangular end sits on the piatform 35. This causes the locking plate 37 to be in a non-horizontal position and hence the horizontal cross section of the first aperture is smaller than that of the tube so that the tube cannot freely pass through. To fit the tube the outer end of the plate 37 must be lifted against the spring bias, for example by a screw driver, until the plate is horizontal so that the tube can be lowered through the plate and the flange plate into the tank. As soon as the locking plate is released the spring bias comes into action which returns the plate 37 to a position in which it grips the sides of the tube to hold it in position. Using this system and by marking the tube where it is to be fixed, the tube can be lowered into the tank until the mark is reached with the locking plate held up. When the mark on the tube has been reached the locking plate is released to lock the chosen position. The strength of the spring bias will be determined by the position of the nut 43. This can be increased by tightening the nut once the correct position has been achieved.

The spring could equally well be located between the flange plate 31 and the locking plate 37. In this case the locking action would be released by downward pressure on the plate.

Using this arrangement the components can be very easily assembled on site using no special tools and with little risk of losing a tube in the tank. Thus four tubes can be accurately set at their different heights by marking the tubes with the required heights before assembly.

Claims (8)

1. A device for locating and holding a vertical or substantially vertical tube or rod, the device comprising a locking plate having opposed first and second ends and a base plate comprising a support for the said first end, means for receiving a screw threaded bolt and means allowing passage through the base plate of the tube or rod to be held, the locking plate having on a line between the first and second ends a first aperture allowing passage of the tube or rod to be held with a close fit on at least two sides, and, spaced therefrom, a second aperture to receive the screw threaded bolt, the device including a resilient member on the bolt which urges the locking plate to a position which reduces the horizontal cross section of the first aperture and hence grasps the tube or rod to prevent it falling under its own weight, the locking action being releasable by returning the locking plate to a substantially horizontal position against the resilient bias.

2. A device according to claim 1 in which the first aperture is located between the first end and the second aperture.

3. A device according to claim 1 or 2 wherein the base plate is arranged to hold a plurality of rods or tubes and comprises a plurality of rod or tube holding apertures arranged around a central platform, the device including a locking plate for each rod or tube, and the locking plates each having the said first end suppported by the platform.

4. A device for locating and holding a vertical or substantially vertical rod or tube, substantially as herein described with reference to Figures 3 and 4 of -the accompanying drawings.

5. A method of obtaining data for transmission to a data processor and for calculating the depth and specific gravity of a liquid in a tank, the method comprising the steps of: 1) locating in the tank at different known heights from the bottom of the tank at least three pressure sensors and providing a further sensor for giving a reading of pressure in the tank above the liquid, 2) connecting the pressure sensors to a pressure transducer means via a switching mechanism, and providing a controller for controlling, for any one reading, which sensor is or sensors are connected to the transducer means, and taking the following readings:: i) by connecting the first sensors sequentially and the further sensor to the transducer means, determine the highest first sensor for which there is a positive differential pressure from the further sensor indicating a head of liquid above that sensor, and storing that value of differential pressure in a memory, ii) if the said highest first sensor is not also the lowest first sensor, establishing a differential pressure reading between any two first sensors that are at that time in the liquid, and storing the reading obtained in the memory then 3) calculating from the known height of the said highest first sensor, the known distance between the said any two first sensors, and the two differential pressures established at i) and ii) above a depth and a specific gravity and storing the specific gravity so calculated;; 4) if the said highest first sensor is also the lowest first sensor calculating the depth of liquid from the differential pressure established at i) above and the last stored specific gravity value forthe liquid.

6. A method according to claim 5 wherein the transducer means includes a differential transducer and the method includes connecting the said further sensor to one side of the transducer and the said first sensors sequentially to the other side of the transducer to obtain the necessary readings.

7. A method according to claim 5 wherein the transducer means includes a differential transducer and the method includes connecting the said further sensor to one side of the transducer and the said first sensors to whichever side of the transducer is required to obtain the necessary values in as few readings as possible.

8. A method of obtaining data for transmission to a data processor and for calculating the depth and specific gravity of a liquid in a tank substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.