RU2188396C2 - Method of measurement of level, density, interface and temperature of liquid in reservoir and device for its embodiment - Google Patents

Abstract

FIELD: monitoring and measurement technology; tank fields of oil pipe lines. SUBSTANCE: method consists in forcing liquid out of tube immersed in medium under test by means of air till operation of nearest sensitive element and at said moment pressure is measured. Then, liquid is forced out of said tube till operation of next sensitive element and pressure is measured at said moment which is determined by magnitude of signal of converter to which sensitive elements is determined directly by magnitude of signal and for even sensitive elements, number is determined by operation of previous or subsequent sensitive element. At the moment of operation of second and subsequent sensitive elements,, forcing-out the liquid is discontinued and pressure being measured is compensated to zero and process is repeated till complete forcing-out of liquid. Number of layer where density changes by amount exceeding the preset value is determined through comparison of densities in each layer and interfaces are calculated by definite formula; at the moment of operation of sensitive elements having even numbers, temperature of respective layer is determined by magnitude of temperature-sensitive resistor found in output circuit of sensitive element. Device proposed for realization of said method includes tube submerged in medium under test which hold equidistant sensitive elements provided with break contact connected with resistor in series and all output circuits of sensitive elements are connected in parallel and are connected to converter which controls valves through controller; plus chamber of differential pressure gauge is connected via valve with air supply until; adjustable pressure differential setter is connected to pneumatic line in parallel with valve; it contains valves connected in series with pneumatic reservoir between them. EFFECT: enhanced accuracy of measurements. 3 cl, 1 dwg

Description

 The invention relates to instrumentation, in particular to methods and devices for measuring parameters of liquid media, such as level, density, density gradient, liquid temperature, as well as the phase boundary in two-phase media, and can be used in various industries, in particularly in tank farms of oil pipelines.

 A known method of measuring the liquid level, which consists in measuring the pressures on two piezometric tubes - movable and stationary when alternately sparging air through them and comparing the measured pressures with each other. The liquid level is judged by the movement of the movable tube relatively stationary, and before the start of the movement, the pressure drop across the fixed tube is measured, compensated for it to a fixed value, and while maintaining a fixed pressure value on the fixed tube, the movable tube is moved, bringing the pressure in it to the previously measured pressure fixed tube (A.S. USSR 1499876).

 The disadvantage of this method is the difficulty of implementation when measuring large level values. With large values of the measured level, it is required to move the movable tube by a significant amount (up to half the height of the liquid level), which in many real cases is 5-10 meters and cannot be realized by simple methods with acceptable accuracy.

There is also known a method of measuring the level of liquids closest to the proposed invention and selected as the prototype of the method, which consists in alternately sparging air for three measurement cycles through the main lower (first measure), additional upper (second measure) tube, and also through both of these tubes simultaneously (third beat), and measuring the pressure drop across these tubes and between them. The value of the liquid level is judged by the results of three measurement steps according to the formula:
L = l + h • (1+ ( y ₁ -y ₃₎ / (y ₁ -y _2)),
where l is the distance between the lower cut of the main tube and the bottom,
h is the distance between the lower sections of the tubes,
y ₁ , y ₂ , y ₃ - measurement results in 1-3 cycles, respectively (A.S. USSR 1649290).

 The disadvantage of this method is the difficulty of implementation in the case when high measurement accuracy is required. In fact, the accuracy of this method is determined by the accuracy of the differential pressure gauge. For the best samples of differential pressure gauges, the measurement error is 0.08-0.1%, therefore, for a level measuring range of 10 m, the error cannot be less than 1 cm, which is unacceptable for many practical purposes.

 A device for measuring the level of liquids is known that is closest to the invention and selected as a prototype of the device, comprising an air power supply unit supplying positive and negative independent pneumatic lines between which a differential pressure gauge is connected. A positive line through a normally open pneumatic valve feeds the main measuring tube immersed in a liquid to a depth of H, and through a normally closed pneumatic valve - an additional tube immersed in a liquid to a depth of H-h. The negative pneumatic line through the normally open pneumatic valve and pneumatic throttle is connected to the atmosphere, and through the normally closed pneumatic valve it feeds an additional tube. The output of the differential pressure gauge is connected to the input of the control and computing unit (controller), the outputs of which are connected to the control inputs of the pneumatic valves (A.S. USSR 1649290).

 The disadvantage of this device is the low accuracy of the measurement, especially when you want to measure in a wide range of levels. In fact, the accuracy of this device is determined by the accuracy of the differential pressure gauge. For the best samples of differential pressure gauges, the measurement error is 0.08-0.1%, therefore, for a level measuring range of 10 m, the error cannot be less than 1 cm, which is unacceptable for many practical purposes. In addition, at large dynamic ranges, the density gradient begins to influence the error of the measurement result - the liquid density in the lower part of the tank is higher than in the upper one, and this can lead to an increase in the measurement error.

The meter contains a tube 1 immersed in a controlled environment 2, which is a two-phase mixture with an interface. On the tube 1, at equal distances h, k sensing elements 3 _k , k = 7.2, are located. . . , n is the sequence number of the sensitive element 3 _k . The upper sensitive element 3 _k is located below the upper limit of measurement by h, and the lower sensitive element 3 _n is at the level of the lower limit of level measurement. The output circuits of each sensor element 3 _k contain a normally open contact 17 _k connected in series with the resistor 18 _k , and all output circuits of the sensor elements 3 _{k are} connected in parallel and form a two-terminal device that is connected to the converter 15. The sensor element 3 _k is located so that contact 17 _k closes when the air - liquid interface in the tube 1 is at a distance k • h from the cut of the tube 1.

 The tube 1 is connected through a positive pneumatic line 4 with a positive chamber of the differential pressure gauge 6, which is also connected through the second valve 9 by a negative pneumatic line 5 to the liquid-free space of the tank. The minus chamber of the differential pressure gauge 6 is connected to the tank-free space of the tank by a negative pneumatic line 5, through a third valve 10 and an adjustable differential pressure regulator 11 connected in parallel with it.

 The air supply unit 7 supplies air to the positive pneumatic line 4 through the first valve 8, and the positive pneumatic line 5 through the third valve 10 and differential pressure regulator 11. The regulator 11 is a pulsed pneumatic resistance containing a fourth valve 12 connected in series, an air capacity 14 and a fifth valve 13. Inputs the control valves 8, 9, 10, 12 and 13 are connected respectively with the first, second, third, fourth and fifth outputs of the controller 16, the first and second inputs of which receive signals from the converter 15 and the differential pressure gauge 6, respectively of course.

Resistor values 18 are determined by the number _k k _k sensing element 3, for example, the first sensing element resistance is 100 ohms, the third - 110 ohms, the fifth - 120 ohms, etc. for all odd k. In contrast, all 18 _k resistors with an even k number are the same and are thermistor converters in contact with a controlled fluid. Converter 15 converts the amount of resistance connected to it into voltage. Thus, at the moment of closing the contact 17 _{k, the} voltage at the output of the transducer 15 determines the number k of the sensitive element 3 _k , near which there is an air-liquid interface in the tube, or the temperature of the liquid in the region of the sensitive element 3 _k .

The following elements can be used as components of the meter:
- Sensitive elements 3 - end switches series BPT - 211, TU 16-526.442-78, which uses a DC relay as a load, normally open contacts of the latter are used as contact 17.

 - Differential gauge 6 - a device of the SAPFIR-22DD type, measurement limit 6.3 kPa, accuracy class 0.5 (basic error not more than 0.03 kPa).

 - Air supply unit 7 - type БПВЩ - 2А complete with F416 compressor.

 - Valves 8, 9, 10, 12 and 13 - elements of the Unified System of Industrial Pneumatic Automation (USEPPA) type П1.ПР5.

 - Pneumatic capacity 14 - element USEPPA type POE.50.

 - Converter 15 is a current source operating on a resistive load (resistor 18).

 - Controller 16 - any industrial controller that has 2 inputs for inputting analog signals and 5 output signals for controlling relay loads, for example, type KPS 19-06 with corresponding input-output modules.

 - Resistors 18 for even sensitive elements 3 - copper thermometers of resistance of graduation 50M (50 Ohms at zero temperature) like TSM-0879.

LEVEL MEASUREMENT
In the initial state, the liquid is in the tube 1 at the liquid level in the controlled reservoir (at a height H relative to the sensing element 3 _k ), valves 8 and 10 are open, and valve 9 is closed. Air from the power supply unit with air 7 enters through the valve 8 and pneumatic line 4 into the tube 1, and the liquid from the tube begins to be displaced, and the interface drops down. The pressure in the positive chamber of the differential pressure gauge 6 begins to increase and at the moment when the interface reaches the sensitive element 3 _k and the contact 17 _{k closes} , it will be:
P ₁ = ρ ₁ • g • H + P _l1 + P ₀ ,
where ρ ₁ is the average density of the liquid in the area from the surface to the sensing element 3 _k ,
g is the gravitational constant
P _l1 - pressure loss in the pneumatic line 4,
P ₀ - pressure in the free space of the tank.

At the same time, the air from the power supply unit by air 7 through the valve 10 and the negative pneumatic line 5 enters the space free of liquid and the pressure in the negative chamber will be equal to:
P ₂ = P _l2 + P ₀ ,
where R _l2 - pressure loss in the negative pneumatic line 5.

Since the plus and minus pneumatic lines 4 and 5 are identical, then with equal air flow rates, the pressure loss in them is also equal, therefore, at the moment of contact 17 _k closing, the signal at the output of differential pressure gauge 6 will be equal to:
U ₁ = P ₁ -P ₂ = ρ ₁ • g • H.

With further liquid displacement, the interface will fall down, the contact 17 _{k will} open, and at some point in time it will reach the next sensitive element 3 _{k + 1} and close the contact 17 _{k + 1 in it} . Since the distance between two adjacent sensitive elements 3 _k and 3 _{k + 1} is equal to h, the signal at the output of the differential pressure gauge 6 will be equal to:
U ₂ = ρ ₁ • g • H + ρ ₂ • g • h,
where ρ ₂ is the average density of the liquid in the area between adjacent sensitive elements 3.

For small values of h, we can assume that ρ ₁ = ρ ₂ and then we have:
U ₂ = ρ ₁ • g • (H + h).

At this moment, the controller 16 closes the valve 8 and opens the valve 9, the air from the tube 1 begins to be displaced into the free space, the interface begins to rise, the contact 17 _{k + 1} opens and at some point in time reaches the sensing element 3 _k . At the moment of closing contact 17 _k on this sensitive element, the value of the output signal of the differential pressure gauge will be:
U ₁ = ρ ₁ • g • H,
the controller 16 will open the valve 8 and close the valve 9 and the cycle of the device will be repeated.

Therefore, the value of H can be determined from the results of two adjacent measurements of U ₁ and U ₂ according to the formula:
H = h • U ₁ / (U ₂ -U ₁ ).

The value of k is also uniquely determined by the value of the signal from the differential pressure gauge 6, because either in the first (U ₁ ) or second (U ₂ ) measurement, a resistor 18 with an odd serial number A will be connected to the input of the transducer 15, the value of which is uniquely associated with the serial number k.

The liquid level is determined by the results of two adjacent measurements U ₁ and U ₂ and serial number k according to the formula:
L = h ₀ + h • (k + U ₁ / (U ₂ -U ₁ )),
where h ₀ is the distance from the bottom to the lower sensing element 3.

 The error in determining the level includes the error in setting and maintaining the value of h, the error in the operation of the sensing elements 3 and the error in measuring pressure. The last component is included only in the error in determining the value of N. Since the value of H is at large values of k a small part of the measured level, then the relative error of the differential pressure gauge 6 is actually included in the error of the level measurement result, weakened by a factor of k. Since the first two components of the error in determining the level are small, we can assume that the relative error of the measurement result is equal to the relative error of the differential pressure gauge 6 divided by k.

For example, let you want to measure a level that varies in the range of 3-11 m (dynamic range of 8 m), with an error of 5 mm (reduced error of 0.025%) in an oil tank with a density of 800 kg / m ³ .

1. We choose a differential pressure gauge 6 with a measuring range of 6.3 kPa and a basic error of 0.5%, while for oil with a density of 800 kg / m ^{3 the} range of levels from 0 to 0.8 m with a maximum absolute error of 4 mm will be covered.

 2. We select the number of sensing elements 3 equal to 20 and set them evenly from 3 m to 10.6 m. For sensitive elements 3 of the BPT-211 type, the response error does not exceed 0.2 mm.

 3. At the level of 10.6 m in the sensitive element 3, a resistor with a nominal value of 100 Ohms is used, at the levels of 10.2, 9.4, 8.6, .., 3.0 m are thermistors TCM, and at levels of 9.8 , 9.0, 8.2, ..., 3.4 m (i.e. for the other odd sensitive elements 3) resistors are installed with a nominal value of 110, 120, ..., 190 Ohms, respectively.

4. In the range of oil temperature changes from 0 to 100 ^o With the resistance of the thermistor TCM graduation 50M varies from 50 Ohms to 71.4 Ohms, i.e. by the voltage at the output of the Converter 15, you can obviously determine the number of the sensor 3, near which there is a float 2. For this, the Converter 15 must have a resolution of less than 10 Ohms or 7.1% of the range of 50-190 Ohms. For odd sensitive elements 3, the number is determined directly by the output voltage, for even - the number of the previous odd is incremented or the number of the subsequent odd is decremented.

 5. The maximum basic error in determining the level will not exceed the sum of the error in operation of the sensing element 3 plus the maximum error in the differential pressure gauge 6, i.e. 4.2 mm. This is equivalent to the given measurement error of the level of 0.02%, although the proposed method uses a differential pressure gauge of class 0.5. Actually, the main level measurement error is reduced to 0.005% (1 mm) when using a differential pressure gauge of class 0.25 and 40 sensing elements 3.

DENSITY MEASUREMENT
Let ρ ₁ be the average density of the liquid in the region from the surface to the first sensor 3, ρ _{k the} average density in the kth region between the sensors 3 _k-1 and 3 _k , k = 2, 3, ... n.

Suppose that in the initial state, the liquid – air interface is in the tube 1 at the liquid level in the controlled reservoir (at a height H relative to the first sensor 3), valves 8 and 10 are open, and valve 9 is closed. The air from the power supply unit 7 enters through the valve 8 and the pneumatic line 4 into the tube 1 and the liquid from the tube begins to be forced out, and the interface drops down. The pressure in the positive chamber of the differential pressure gauge 6 begins to increase and at the moment when the interface reaches the sensing element 3 and contact 17 is closed, it will be equal to:
P ₁ = ρ ₁ • g • H + P _l1 + P ₀ .

With further movement of the liquid displacement from the tube, the interface will sequentially pass by the following 3 _k sensitive elements and the pressure in the plus chamber of the differential pressure gauge 6 at these moments will be:
P ₂ = P ₁ + ρ ₂ • g • h,
P ₃ = P ₂ + ρ ₃ • g • h,
...

P _k = P _k-1 + ρ _k • g • h.

 Upon reaching the interface of the sensing element 3 with an even serial number, the controller 16 closes the valves 8 and 10 and connects the differential pressure switch 11 to the negative chamber of the differential pressure gauge 11. At the same time, the controller controls the valves 12 and 13, so that such pressure is created in the negative chamber of the differential pressure gauge 6, when which the signal at the output of the differential pressure gauge 6 is zero. For this, the controller for some time opens the valve 12 and closes the valve 13, the air enters the pneumatic reservoir 14 and the pressure in it begins to increase. At some point in time, the controller closes the valve 12 and opens the valve 13 and the air from the tank 14 enters the pneumatic line 5. By setting the switching frequency of the valves 12 and 13, the average pressure in the pneumatic tank 14 can be changed over a wide range.

Indeed, let P _A be the pressure in the pneumatic line at valve 13 at point A, P _{at the} pressure in the pneumatic line at valve 12 at point B. When the pneumatic containers of volume V are connected alternately to points A and B, the mass of gas in it according to the basic gas law will be:
when connected to point A - M _A = R • V • P _A
when connected to point B - M _B = R • V • P _B ,
where R is the coefficient of proportionality.

If, in turn, using pneumatic valves with a frequency of F, the pneumatic capacity is connected to points A and B, then the mass of gas in it will change and this change will be:
M _A- M _B = R • V • (P _A -P _B ),
And the mass flow through it will be:
Q = F • (M _A -M _B ) = F • R • V • (P _A -P _B ).

 Therefore, by changing the switching frequency of the pneumatic valves 12 and 13 with a constant flow of air in the pneumatic line, it is possible to control the pressure drop in it over a wide range. Thus, the differential pressure controller 11 sets the pressure, the value of which is determined by the switching frequency of the valves 12 and 13.

After reaching zero level, the controller 16 opens the valve 8 and the displacement of fluid from the tube 1 continues. Therefore, the signals at the output of the differential pressure gauge 6 at the time of reaching the interface of the sensitive elements 3 will be equal to:
U ₁ = ρ ₁ • g • H,
U ₂ = ρ ₂ • g • h,
...

U _k = ρ _k • g • h.

In this case, the pressure drop on the chambers of the differential pressure gauge 6 does not exceed the limit of its measurement. Therefore, according to the measurement results of U ₁ , U ₂ , ..., U _k you can find the density of the liquid in the layers:
ρ ₁ = U ₁ / g / H,
ρ ₂ = U ₂ / g / h,
...

ρ _k = U _k / g / h
MEASUREMENT OF THE BORDER LEVEL OF THE MEDIUM SECTION
Let on the k-th plot at level l with respect to the sensitive element 3 _k pass the interface between the media with densities ρ _k and ρ _{k + 1} . Using the measurement procedure with pressure compensation in the negative chamber of the differential pressure gauge 6 in the same way as when determining the density, we have:
U _k-1 = ρ _k • g • h,
U _k = ρ _k • g • (h-1) + ρ _{k + 1} • g • l,
U _{k + 1} = ρ _{k + 1} • g • h.

Hence, according to the results of three consecutive measurements of U _k-1 , U _k and U _{k + 1,} one can find the value l:
l = h • (U _k -U _k-1 ) / (U _{k + 1} -U _k-1 ).

Thus, to determine the level of media separation S, it is necessary, by measuring the density, to determine the number k of the layer in which the density jump occurred, and according to the results of three neighboring measurements, calculate:
S = h ₀ + h • (k + (U _k -U _k-1 ) / (U _{k + 1} -U _k-1 )).

TEMPERATURE MEASUREMENT
The temperature is measured in parallel with the measurement of the level, density or interface of media, because in these measurements, a temperature-sensitive resistor 18 (for the sensitive element 3 with an even serial number) is connected at least once to the input of the converter 15 and the voltage at the output of the converter 15 is proportional to the average temperature in the region of the temperature-sensitive resistor 18 of the sensor 3.

 When determining the density, successively displacing the liquid from the tube 1, you can determine the temperature in all layers of the tank.

Claims (2)

1. A method of measuring the level, density, interface and temperature of a liquid in a tank, including air displacing a liquid from a tube immersed in a controlled medium, and measuring the pressure in it, while at the same time the sensing elements are uniformly installed on the tube and triggered, when the air-liquid interface in the tube is at a distance k • h from the tube cut (k = 1 ... n, k is the number, n is the number of sensing elements), the measured pressure in the tube is recorded and the numbers are sensitive elements, moreover, for odd sensitive elements, the number is determined directly by the value of the converter signal, to which all sensitive elements are connected, and for even ones, as the increment of the previous number or the decrement of the number of the subsequent sensitive element, for which resistors with odd number are included in the output circuits whose ratings are determined by the number of the sensitive element, thermistor converters are included in the output circuits of sensitive elements with an even number The contactors in contact with the controlled fluid, in addition, at the moment the second and all subsequent sensing elements are triggered, they stop the fluid displacement and compensate the measured pressure to zero, and then the displacement process continues until the last sensing element operates, and the level and density layers are determined by the formulas
L = h ₀ + h • (k + U ₁ / (U ₂ -U ₁ )) - liquid level,
ρ ₂ = (U ₂ -U ₁ ) / g / h is the average density of the upper liquid layer between the first and second sensitive elements,
ρ _i = U _i / g / h is the average density of the ith liquid layer, i = 3, ..., n, n is the number of sensitive elements,
where h ₀ is the distance from the bottom to the lower sensing element;
h is the distance between the sensing elements;
U ₁ , U ₂ - pressure measured at the moment of operation of the first and second after the beginning of the displacement of liquid sensing elements;
k is the serial number of the first of the triggered sensing elements;
U _i - measured pressure at the moment of operation of the i-th sensitive elements, i-3 ... n;
i is the serial number of the layer in which the average density is measured;
g is the gravitational constant
the average temperature of the corresponding liquid layer is determined at the moment of operation of the sensor with an even number by the value of the resistor in the output circuit of the triggered sensor, and the medium level is found by comparing the calculated densities in each of the layers between the sensors and determining the number of the layer in which the density changed to a value greater than the specified value and calculated by the formula
S = h ₀ + h • (j + (U _j -U _j-1 ) / (U _{j + i} -U _j-1 )) - section level,
where j is the sequence number of the layer in which the density change is greater than the specified value;
U _j - measured pressure at the moment of operation of the j-th sensitive elements, j = 2 ... n.  2. A device for measuring the level, density, interface and temperature of a liquid in a tank, containing a tube immersed in a controlled environment, sensitive elements evenly mounted on the tube and triggered when the air-liquid interface in the tube is at a distance of k • h from tube cut (k = 1 ... n, k is the number, n is the number of sensitive elements), the output circuits of each of which contain a normally open contact connected in series with a resistor, and for sensitive elements with odd n By means of measures, the nominal values of the resistors are determined by the number of the sensitive element, and all resistors with even serial numbers are thermoresistive converters, while all the output circuits of the sensitive elements are connected in parallel and form a two-terminal connected to the input of the converter, a positive pneumatic line through which air is supplied to the tube, a differential pressure gauge, the positive chamber of which is connected to the positive pneumatic line and through the first valve to the first output of the power supply unit by air, the negative pneumatic line, connected through a second valve with a positive pneumatic line, and through a third valve - with a negative chamber of the differential pressure gauge and the second output of the air supply unit, while a cut of the negative pneumatic line is located above the surface of the controlled medium, an adjustable pressure regulator connected in parallel with the third valve and containing the fourth valve in series, pneumatic capacity and a fifth valve, a controller, the first input of which is connected to the output of the resistance-voltage converter, the second input to the output of the differential pressure gauge, and he first, second, third, fourth and fifth outputs - respectively with first control inputs of the second, third, fourth and fifth valves.