Patent Application
20180306634
Disclosed embodiments relate to electromechanical liquid level gauges that use the servo principle.
Electromechanical liquid level servo gauges (ESGs) are used for the accurate measurement of product level and the interface level in bulk storage tanks used for typical hydrocarbons (often referred to as fuel and oil) and a variety of other liquid chemicals. These products range from very light chemicals, like so-called LPG's (mixtures of propane and butane or even liquefied natural gas (LNG)) to all types of refined products such as naphtha, gasoline, diesel, jet fuels, lubricants and all types of chemicals, both pure and mixed.
The servo principle is based on the measurement of the apparent weight of a displacer that is within the tank. The displacer is a mechanical body suspended on a strong thin measuring wire, where the displacer material has a higher density than the liquid to be level measured. The measurement wire is wound on a high accuracy machined grooved drum with a calibrated circumference. The apparent weight resulting from the weight of the displacer minus the weight of the displaced liquid product is measured as a torque which is then used by a computing device such as a microcontroller with the servo motor used to rotate drum in order to position the displacer at a different height in the tank.
By rotating the drum the wire is spooled up or ‘paid’ out into the tank and the displacer is raised or lowered until the measured apparent weight equals the programmed set point. For safety reasons typically a magnetic coupling (using pole pairs) may be located between drum and electronics (motor, microcontroller, electronics, etc.) as many of the liquids products which are commonly stored in bulk storage tanks are flammable and typically need an explosion-safe design. The displacer being denser as compared to the density of the product in the tank is basically kept at the same level using Archimedes law which indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
The apparent weight resulting from the displaced liquid is dependent on the density of the displaced liquid and the amount of the displaced liquid. The amount of the displaced liquid depends again on the shape of the displacer, and the set point (i.e. how much weight there needs to be displaced).
Vapor influence caused by dense vapors, especially on products with low dielectric constant and a relative high dipole moment result in accuracy limiting physics, generally making radar unsuitable and unacceptable for legal metrology use. The large variation in saturation which are not predictable also makes it generally not possible to compensate for these vapor effects, which especially occur with light hydrocarbons and chemicals, where ESGs do not have these limitations. Some examples are LPGs, ethanol and multiple industrial solvents. Also foam is an example where an ESG still can detect the liquid surface while radar will generally not find any reflection. This means that ESGs are still an important and much relied upon accurate measurement technology, especially when high and certified accuracy is a need, such as for custody transfer applications.
This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.
Disclosed embodiments recognize for known ESGs the displacer will actively follow the level in the tank with a measurement control loop involving the force transducer and servo motor. This results in small movements of the displacer to overcome the static and dynamic friction which consumes electric power that heats the mechanics inside the ESG. This friction introduces a hysteresis effect between the force and position of the displacer which the control loop tries to reduce in a series of correction steps that typically degrades the level measurement accuracy of the ESG. The hysteresis as known in the art also varies for each ESG and over its operational lifetime.
Disclosed embodiments solve the above-described problems by providing ESGs which include automatic hysteresis corrected level measurement for liquid product(s) in bulk storage tanks. Disclosed level sensing performs an ‘entire down-dip’ (defined below) when the liquid level is essentially unchanging (defined herein as changing ≤0.1 mm/sec) which is used along with a measured (Frequency (F), Displacer position (S)) FS profile including a ‘move-down curve’ (defined below) and a ‘move-up curve’ (defined below) to calculate a current liquid level. For clarity as used herein an FS profile includes both a move-down FS curve and move-up FS curve (see
The method also includes calculating a correction to the level reading responsive to pumping in/expanding or pumping out/shrinking of liquid in a mode referred to herein as a ‘hold and correct mode’. Disclosed methods automatically compensate for the hysteresis caused by static and dynamic friction by keeping track of the location on the move-down and move-up curve (which spans up the entire hysteresis curve) under changing conditions. There is thus no need to explicitly measure the hysteresis, and disclosed methods have essentially no drift in the measured liquid level.
There are several displacer movements used in disclosed level sensing referred to herein as “dips” that are defined below, where as noted above S stands for the displacer position and F for the measured force:
1. An ‘entire down-dip’ starts with the displacer suspended completely (entirely) above liquid level. The displacer is moved down to a position so that it is completely below the liquid level. The displacer is moved up again to a position passing the calculated liquid level Su (Su being the middle of the height of the displacer while moving the displacer up (u)) entirely above the liquid. The displacer is again moved down to the calculated liquid level Sd (being the middle of the height of the displacer while moving the displacer down (d)), and stops moving so that the displacer is then on the move-down FS curve.
2. An ‘entire up-dip’ starts with the displacer at a position immersed entirely below the liquid level. The displacer is moved up to a position entirely above the liquid level. The displacer is moved down again at a position passing the calculated liquid level Sd entirely below the liquid. The displacer is moved up to this calculated liquid level Su and stops moving. The displacer is then on the move-up FS curve.
3. A ‘partial down-dip’ moves down the displacer to a position that is not entirely below the liquid level. The displacer is moved up again passing the calculated liquid level Su but not entirely above the liquid. The displacer is moved down to the calculated liquid level Sd and the displacer is stopped moving. The displacer is then on the move-down FS curve.
4. A ‘partial up-dip’ moves up the displacer at a position that is not entirely above the liquid level. The displacer is moved down again passing the calculated liquid level Sd but not entirely below the liquid. The displacer is moved up to the calculated liquid level Su and is stopped moving. The displacer is then on the move-up FS curve.
Disclosed embodiments include an automatic hysteresis compensated method of level measuring a liquid in a storage tank. An ESG is provided including a controller having a processor, a displacer suspended on a measuring wire from a measuring drum for causing a torque on the drum having a servo motor coupled to rotate the drum arranged to balance a weight of the displacer, where a change in liquid level causes a change in a counterforce to move the ESG out of balance. The processor monitors an output of a sensor that senses the torque and then in response controls a movement of the motor. The processor includes an associated memory storing a disclosed level gauging algorithm.
A measured FS profile is provided including a move-down FS curve obtained from moving the displacer to entirely down into the liquid to determine Fd, Sd set points on the move-down curve corresponding to a center of the displacer when moving down and a move-up FS curve from moving the displacer entirely up out of the liquid to determine Fu, Su set points on the move-up curve corresponding to a center of the displacer when moving up. The algorithm implements provided a time derivative of F is essentially zero performing a move down-dip of the displacer (including starting with the displacer suspended completely the liquid level and moving the displacer down to entirely down into the liquid level) or a move up-dip of the displacer (including starting with the displacer suspended below the liquid level and moving the displacer up to completely above the liquid level), then for the move down-dip moving the displacer up passing Su or for the move up-dip moving the displacer down passing Sd, and for the move down-dip then moving the displacer to return to Sd or for the move up-dip then moving the displacer to return to Su. A current liquid level is then determined from Fd upon the return to Sd or from Fu upon the return to Su depending on the calibrated (reference) curve used.
Thus for the determining step using the move-down curve as the reference curve for S to be measured, the current liquid level obtained from Fd using a move down-dip equals Sd. If the current liquid level is obtained from Fu (using a move up-dip), it equals Su−ΔSdu. (See
Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate certain disclosed aspects. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments.
One having ordinary skill in the relevant art, however, will readily recognize that the subject matter disclosed herein can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring certain aspects. This Disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the embodiments disclosed herein.
ESG 100 includes a displacer 235 within a tank 202 that has a flange 204. The displacer 235 is suspended on a measuring wire 238 from a drum 240 that extends through the flange 204 for causing a torque on the drum 240. The displacer shape and displacer dimensions are generally known. A servo motor with a gear (servo motor) 245 is coupled by a drive shaft 249 to rotate the drum 240 to balance a weight of the displacer 235 in the tank 202 having a liquid therein (not shown). An equilibrium condition exists when the displacer 235 is at a top surface of the liquid, wherein a change in the liquid level causes a change in a counterforce to move the ESG 100 out of balance. As noted above, although not shown, any force which acts via the measuring wire 238 on the drum 240 sensed by force transducer 225 can be transferred as a torque to processor side of the ESG 100 using a magnetic coupling 247.
After moving down the displacer 235 entirely under the level, (a move down FS curve), the displacer's middle needs to be placed essentially exactly on the interface level for an accurate level measurement. However, because of the change of direction by moving the displacer 235 up, a hysteresis effect is caused by friction which needs correction for level measurement accuracy. This hysteresis effect is known to vary between individual ESG instruments because of production variation and over an ESG's lifetime. It is recognized the hysteresis effect is fortunately identical with the direction change of the displacer moving down/up or moving up/down.
Because the behavior of F is different for rise and fall of the liquid, there is a disclosed process referred to herein as “hysteresis equalization”. After disclosed hysteresis equalization, the displacer will be held at that position. The force F will now only change if the level of the liquid will rise or fall, referred to herein as “hold and correct”.
The drive compartment 245a includes a motor 245 including a drive train 246, which imparts rotation to the drum 240 via a shaft 249. For example, the drive train 246 or shaft 249 could generate a magnetic field, and a magnetic coupling 247 can be used to convey torque between the shaft 249 and the drum 240. In these embodiments, no direct connection may be needed between the drum compartment 240a and the other compartments 245a, 222a.
However, other techniques for causing rotation of the drum 240 can be used, such as when the shaft 249 is physically connected to the drum 240. The drive train 246 includes any suitable structure for imparting rotation to the drum 240. In particular embodiments, the drive train 246 comprises a stepper motor that causes the drum 240 to rotate in specified steps, meaning the drum 240 does not rotate freely but instead in defined amounts or “steps.” Each step of the motor 245 should therefore impart a known amount of rotation to the drum 240. In these embodiments, since the drum 240 has a known diameter or circumference, the length of connector the wire 238 that is dispensed or collected during a single step rotation can be known with a high degree of certainty.
The drive compartment 245a also includes a force transducer 225 which identifies the torque induced on the drum 240 by the weight of displacer 235. When the displacer 235 is dangling from the wire 238, the measured torque is higher. When the displacer 235 is completely or partially submerged in the material in the tank, the measured torque is lower. The force transducer 225 generally identifies the torque on the drum 240 by measuring the torque on the shaft 249. The drive compartment 245a is also shown including a user interface 218 and network interface 220.
The power supply compartment 222a includes a power supply 222, which provides operating power for the ESG 100. The power supply 222 can provide power to various components of the drive compartment 245a. Depending on the implementation, the power supply 222 may or may not supply power to the drum compartment 240a. The power supply 222 can include any suitable structure for providing power, such as a battery, fuel cell, or solar cell.
As described above, a significant advantage of disclosed ESGs is the increase of accuracy of the liquid level measurement by elimination of the hysteresis independent of the specific ESG or/and its stage in its lifetime. Additional advantages include automatic density measurement of the liquids.
Regarding the measurement of the density of the liquid(s) by an entirely submerged displacer 235, the densities ρ of the liquid(s) can be calculated from the positions where the displacer is entirely submerged in the liquid.
Regarding calculation of accuracy increase using other displacer shapes, the accuracy of Scorrected is recognized to depend on the radius of the middle assumed cylindrical shaped part of the displacer because:
The displacer's radius (r) can be increased with a displacer that has a density closer to the densities of the liquid(s) in the tank in combination with the length of the displacer. A generally good combination can be the displacer material being aluminum with a density of 2.7 g/cm3. The density can be reduced by a factor of 2 using a hollow displacer since the liquid densities are usually below 1.0 g/cm3 (water). A displacer length of 20 mm and 55 mm radius results in an impressive accuracy increase of 7.5. The absolute accuracy is usually in Newton per mm, but this is unit-less because it compares two accuracy rates of which one is x times higher than the other compared to 250 gram displacers with a radius of 25 mm for all interface exchanges.
However, interface exchanges between liquids have lower accuracy compared to air/liquid exchanges measured with an identical shaped displacer, because of the differences between the densities. There use of displacer's density as an ESG design parameter for the displacer is believed to be another new feature. A further advantage is the ability for a continuously measured liquid density to be correlated with the shape of the displacer to provide enhanced diagnostic information enabling preventive maintenance (detecting displacer contamination), and increased safety.
Reducing the need for ESG movements provided by electronic level correction as described above has several advantages, including longer lifetime for the ESG as automatic level adjustments reduce motor wear, and lower power consumption which can be important when for example by supplied power by solar power. Instead of conventionally continuously recalculating the level set point and trying to keep the immersion depth of the displacer constant (using the servo motor 245 to rotate the measuring drum 240), it is also possible perform a virtual (electronic) correction to the level reading. The same method can even be used to reduce normal servo movements as result of normal level changes.
Regarding a physical analysis for operation of an ESG to which disclosed embodiments can be applied, the displacer 235 for an ESG such as ESG 100 has a mass Mdis which as described above is mounted on a measuring wire 238 (also called a cord) in a tank 202 having at least one liquid therein. The force of gravity Fdis on the displacer 235 will be Mdis·g where g is the acceleration constant (gravity) at the earth's surface (g=9.8 m/s′2).
F
dis
=M
dis
g
Archimedes law states that the displacer 235 with a volume Vdis and a mass Mdis surrounded with a liquid with density ρ in kg/m2 is forced downward toward the ground with a force F which is smaller than the force of gravity applied on the displacer 235 by a factor ρ·Vdis·g.
F=(M
dis−ρ·Vdis)g
The displacer's 235 density that is selected (typically >2.0 g/cm3) to be significantly higher than the density of most liquids (typically <0.8 g/cm3) stored in the storage tank, so that the displacer 235 will always be forced toward the ground (i.e., bottom of the tank 202). The liquid's density ρ can thus be found from the measurement of F if the displacer 235 is entirely immersed in the liquid as all other parameters in the F equation are known parameters. Alternatively,—the density can be measured with an entire dip. Both an up-dip and down-dip are possible as both of these dips places the displacer entire above and in the liquid, only the sequence being different. The entire dip is typically conducted frequently and only in a steady-state tank (i.e., very low rate of change in the level, noted above to be ≤0.1 mm/sec), so there is essentially no pumping in/out, but because of daily temperature changes may shrink or expand the tank and the liquid.
Regarding disclosed level correction, as noted above, disclosed displacers can optionally be symmetrically-shaped to make the FS profile linear and thus easier to analyze. The displacer 235 is moved through the liquid level (full dip or only a partially dip) and the servo motor 245 will control displacer movements to follow the move-up or move-down curve and stop moving the displacer 235 if the reduced force is becoming stable (i.e., not changing, except for the density increase of the liquid itself which can be neglected). In that case the displacer 235 is entirely under the interface level. The interface can be between air and a liquid, or between different liquids that have different densities.
To obtain a straight line between the corner points in the move-up curve and move-down curve of the FS profile (except for the corners), the shape of the displacer 235 is recognized to need to be symmetrical. A symmetrical shape is however not required if a straight line is not needed. An example displacer 235′ having a symmetrical shape is shown in
The level measurement accuracy of a conventional current ESG is ½ΔSdu while disclosed ESGs implementing disclosed methods which closely follow the FS curves are much more accurate (by a factor 10 or more). The position of the displacer 235 can be calculated by a disclosed algorithm (taking the corner points Smin, Smax and calculating the middle) where as described above, Sd is the middle of the displacer when moving down, and Su being the middle of the displacer when moving up. As noted above, the position of the displacer is calibrated based on either the calculated Sd or Su value, which being the center of the displacer essentially exactly corresponds with the interface level of the liquid in the tank.
In
Step 402 comprises providing a measured FS profile including a move-down FS curve obtained from moving the displacer to entirely down into the liquid down to determine Fd, Sd corresponding to a center of the displacer when moving down and a move-up FS curve from moving the displacer entirely up out of the liquid to determine Fu, Su corresponding to a center of the displacer when moving up. The FS profile is used to measure the level of the liquid by measuring the changes in the frequency to be converted to a force F. After an FS profile measurement, all previous profile measurements are not included in the determination of the last Fd, Sd point as the FS profile measurement resets all measurement results and provides an accurate new level set point Sd (or Su) which corresponds to the level of the liquid directly after the FS profile determination.
Step 403 comprises provided a time derivative of F is essentially zero, performing an at least a partial down-dip of the displacer including starting with the displacer suspended completely above the liquid level, moving the displacer down to below the liquid level, then moving the displacer up passing Su, and then moving the displacer to return to Sd. Step 404 comprises determining a last interface level of the liquid from Fd upon its return to Sd.
Responsive to a predetermined minimum rise in the liquid level wherein the displacer is fixed at set point Sd and the level follows the hold and correct region mirrored at the move-down FS curve from set point (Fd, Sd), a partial move-down of the displacer is performed to determine an updated Fd value to provide an even further updated liquid level. The predetermined minimum rise in liquid level can correspond to Fd−F equal to an ΔFmax value equal to Fd−Fmin, wherein Fmin is the F value when S equals an Smax value which is a corner point on the FS profile. (See Smax in
The displacer can be a symmetrically-shaped displacer (see displacer 235′ in
If the liquid level rises (pumping in or volume expansion of liquid) the level of the liquid will follow the mirrored move-down curve until Fd−F>ΔFmax. Then the method performs a partial down-dip 515 to move from location B to location A back on the move-down curve obtain a new Sd value. The current liquid level is now equal to the new Sd value where Sd is the calculated set point at the new location A. A dips results in a new Sd and a new Su. This will become the new set point where the displacer's middle is moved to Sd=Smin+(Smax−Smin)/2 (for a symmetrical shaped displacer such as displacer 235′ shown in
If the liquid level drops (pumping out or shrink of the liquid) the level of the liquid will follow the mirrored move-up curve. A partial up-dip 520 is then performed to get the displacer solidly on the move-up curve shown as being at location C. The level of the liquid Lc will follow the move-up curve until F−Fu>ΔFmax which will move location D to location C by performing a partial move-up dip reaching location D on the move-up curve. Then a partial up-dip 525 is used for making a transition from location D to location C is then performed to obtain a new Su value, where the current liquid level is now equal to the new Su value. The real liquid level is always on the mirrored move-down curve because the real level as described herein is calibrated for this. Accordingly, the corrected liquid level Lc=Su−ΔSdu+ΔS. However, as described above choice to calibrate on move-down curve is a design choice, so that one can also calibrate using the move-up curve.
Significant disclosed features shown include the x minutes condition shown while at location A, B, C or D where an entire up-dip or down-dip can be used to provide updated FS curves to provide an updated ΔFdu and updated ΔSdu. This overcomes the effect of increasing static friction along time which would otherwise result in hysteresis-based errors which is recognized to be generally important in steady state level changes. The x minute condition for location A and C will end up in location A and C itself, while the x minute condition for location B and D will result in location A and C, respectively.
Partial dips above may be repeated if sensed level changes from changes in F result in when a ΔF value is >a predetermined ΔFmax value. Trigger partial dips when Fd−F>ΔFmax for level rising (partial move-down) and Fd−F>ΔF max for level drops (partial move-up).
Disclosed embodiments are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way.
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the subject matter disclosed herein can be made in accordance with this Disclosure without departing from the spirit or scope of this Disclosure. In addition, while a particular feature may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
