RU73072U1 - DENSITY-FLOW METER OF LIQUID OR GAS MEDIA - Google Patents

Abstract

The proposed utility model relates to the field of measurement technology, can be used to measure the parameters of a liquid or gas directly in the pipeline, and can find application in the oil and gas industry. The objective of the claimed device is the expansion of operational capabilities, improving the accuracy of measuring the parameters of a liquid or gas in a pipeline with a low flow rate. This problem is solved by the fact that in the densitometer-flowmeter of liquid or gas media containing a measuring column mounted on the pipeline, two pressure difference sensors located at the bottom of the measuring column, a temperature sensor for the working medium, an absolute pressure sensor, impulse tubes with a “reference” the fluid that receives the pressure of the working medium directly by the contact method, the temperature sensor of the "reference" fluid, poured into impulse tubes located at the same level at the bottom of the meter The first column has two pressure taps, and a pressure tester mounted on the housing for the thermometer, as well as a recording unit; in which: the measuring column is made in the form of two coaxial vertical pipes installed with an annular gap - with an external pipe and an internal pipe, which is a bend of the pipe bend with a measured medium, the upper part of which is equipped with a constricting device with a nozzle, while the cross-sectional area of the inner pipe is the cross-sectional area of the annular gap, and the nozzle length is selected from the condition: l = (3 ÷ 4) d, where: l is the nozzle length, mm, ad is the nozzle diameter, mm, while

the measuring column is equipped with a third pressure difference sensor in the upper part of the specified column and a pressure sensor mounted on the outer wall of the pipe outlet in the lower part of the specified column, in addition, two pressure sensors are installed at right angles to the wall of the external pipe, while these pressure sensors are located on one level in the lower part of the vertical column from the level, which at a distance of H ₂ there is an additional pressure sensor connected by a pulse tube with a "reference" liquid with a pressure sensor pressure on the wall of the external pipe, and at a distance of H ₁ - another additional pressure sensor connected by a pulse tube to a pressure sensor on the thermometer body, while two pressure sensors located at right angles to the wall of the external pipe are connected by pulse tubes to the “reference” liquid with one of the indicated pressure difference sensors, and the pressure sensor mounted on the thermometer body is connected by a pulse tube to the “reference" liquid with the plus chamber of another pressure difference sensor, the pressure sensor is installed embedded on the outer wall of the pipe outlet, is similarly connected to the negative chamber of the same differential pressure sensor, in addition, the outer pipe of the measuring column is provided with a cover in which a hole is made for installing the positive chamber of the third pressure differential sensor, the negative chamber of which is connected to the measuring pulse tube, placed in the flow of the working medium upon exiting the nozzle of the constricting device of the inner pipe, and the density measurement of the medium is carried out according to the formulas: where: ρ _g - the density of the measured gas medium, kg / m ³ ; ρ _W - the density of the measured fluid, kg / m ³ ;

p _t.et. - the density of the "reference" fluid, at ambient temperature, kg / m ³ ; ΔP ₁ is the pressure difference between the column of the "reference" fluid and the sum of the hydrostatic pressure of the working medium and friction losses in the area of height H ₁ in the pipeline in an upward flow, Pa; ΔP ₂ is the pressure difference between the column of the "reference" liquid and the difference of the hydrostatic pressure of the working medium and friction losses at the site of height H ₂ in the measuring column in the downward flow, Pa; g is the acceleration of gravity, m / s ² ; H ₁ - the distance between the additional pressure sensor and the pressure sensor mounted on the wall of the outer pipe, m; H ₂ - the distance between the other additional pressure sensor and the pressure sensor mounted on the wall of the pipeline, m; H _1k and H _2k - the compensation height of the column of "reference" liquid to select the range of measurement of gas density, m, and the measurement of volumetric gas flow is determined by the following formula: where: Q _g - volumetric gas flow, m ³ / s; d is the nozzle diameter, m; d _m is the diameter of the membrane of the third pressure difference sensor, m; ΔР - pressure drop, measured by the third pressure difference sensor, Pa; ρ _g - the density of the measured gas, kg / m ³ and the measurement of the mass flow rate of the fluid flow is determined by the formula: where: M _W - mass flow rate of the liquid, kg / s; ρ _W - the density of the measured fluid, kg / m ³ ;

d is the nozzle diameter, m; d _m is the diameter of the membrane of the third pressure difference sensor, m; ΔР - pressure drop measured by the third pressure difference sensor, Pa. In addition, in the densitometer-flowmeter, a medium temperature sensor, an absolute pressure sensor, two pressure difference sensors located in the lower part of the measuring column, and a pressure difference sensor in the upper part of the specified column are connected to the recording unit.

Description

The proposed utility model relates to the field of measurement technology, can be used to measure the flow rate and parameters of liquid or gaseous media directly in the pipeline and can be used in oil and gas, oil refining and other industries.

Known methods and means of measuring flow using narrowing devices. They are widely used as the main industrial instruments for measuring the flow of liquid, gas and steam. All of them work on the principle of measuring differential pressure. (Kremlin P. P. "Flowmeters and counters of quantity." - L.: Mechanical Engineering, 1998)

A known method of measuring the flow of liquids, gas and steam by pressure drop, which involves the use of flow meters with a narrowing device installed in a special measuring chamber with a diameter larger than the diameter of the pipeline. (Pat. RU No. 2111457 “Method for measuring the flow rate”, priority 12.10.1993, publ. 05.20.1998)

When measuring the flow rate, the nominal bore of the pipeline does not decrease, and the pressure drop is comparable to the pressure loss in the pipeline in a section whose length is equal to the length of the measuring chamber. Knowing the pressure and parameters of the medium in the main pipeline according to standard methods for calculating narrowing devices, they determine, by solving from the opposite, the main geometric dimensions of the measuring chamber, and then select the type and characteristics of the differential meter.

The disadvantage of the device according to the known method is that in each case when measuring liquid or gas environments

the measuring chamber is selected according to previously known operating conditions and the properties of the medium being measured: density, viscosity, temperature, pressure. It is understood that these parameters are constant. In practice, depending on the ambient temperature and flow rate, a change in density, viscosity, temperature and pressure occurs, which increases the measurement error.

Known densitometer of liquid or gaseous media containing -shaped (loop-shaped) pipe of equal cross-section, consisting of ascending, horizontal and descending branches, three pressure selectors installed respectively on the ascending, horizontal and descending branches of the loop-shaped pipe, two pressure difference sensors, absolute pressure sensor, medium temperature sensor, impulse tubes with a "reference" fluid that perceives the pressure of the working medium directly by the contact method, and a recording unit. (Utility Model Patent No. 67263, publ. 10.10.2007, bull. No. 28)

To increase the accuracy of measurement, it is equipped with an additional temperature sensor of the "reference" liquid, poured into the pulse tubes and an additional pressure selector located on the housing for the thermometer. As the "reference" fluid used fluid in contact with the working medium, but not miscible with it. In addition, the pressure sampling devices installed on the ascending, descending branches of the loop-shaped pipe and the pressure sampling device located on the housing for the thermometer are at the same level in the lower part of the loop.

A disadvantage of the known densitometer with the ability to measure working media is that at low gas flow rates and low pressure in the pipeline (flow velocities V <10 m / s and pressure P <1.0 MPa), it goes into the category of a flow presence indicator. For wells with a low gas flow rate, a densitometer-flowmeter of a different principle of operation is needed, for example, using a constricting device that increases the flow velocity to about 20-30 m / s.

The objective of the claimed device is the expansion of operational capabilities, improving the accuracy of measuring the parameters of liquid or gas environments in the pipeline with a small flow rate.

The problem is solved in that in a densitometer-flowmeter of liquid or gaseous media containing a measuring column mounted on the pipeline, two pressure difference sensors located at the bottom of the measuring column, a medium temperature sensor, an absolute pressure sensor, impulse tubes with a “reference” liquid , perceiving the pressure of the working medium directly by the contact method, the temperature sensor of the "reference" fluid, poured into impulse tubes located at the same level in the lower part of the measuring column there are two pressure taps, and a pressure tester mounted on the housing for the thermometer, as well as a recording unit; the measuring column is made in the form of two coaxial vertical pipes installed with an annular gap - with an external pipe and an internal pipe, which is a pipe elbow with a measured medium, the upper part of which is equipped with a narrowing device with a nozzle, while the cross-sectional area of the inner pipe is equal to the cross-sectional area of the annular the gap, and the nozzle length is selected from the condition: l = (3 ÷ 4) d, where: l is the nozzle length, mm and d is the nozzle diameter, mm, while the measuring column is equipped with a third pressure difference sensor in the upper part the specified column and the pressure sensor mounted on the outer wall of the pipe outlet in the lower part of the specified column, in addition, two pressure sensors are installed at right angles to the wall of the external pipe, while these pressure sensors are located at the same level in the lower part of the vertical column from the level , which distance H ₂ placed additional pressure sensor connected to the pulse tube "reference" fluid with a pressure sensor on the wall of the outer tube, and at a distance H ₁ - other additional sensor ION coupled to the pulse tube to the pressure sensor on the housing of the thermometer, while the two

pressure sensors located at right angles to the wall of the outer pipe are connected by impulse tubes with a “reference” liquid to one of these pressure difference sensors, and a pressure sensor mounted on the thermometer body is connected by a pulse tube with a “reference” liquid with a plus chamber of another sensor pressure difference, a pressure sensor mounted on the outer wall of the pipe outlet is similarly connected to the negative chamber of the same pressure difference sensor, in addition, the outer pipe of the measuring column is supplied with and a cover in which a hole is made to install a third pressure difference sensor, the positive chamber of which is directly in contact with the working medium, and the negative chamber is connected to a measuring impulse tube placed in the working fluid stream when the nozzle of the narrowing device of the inner pipe exits, and the measurement the density of the medium is carried out according to the formulas:

where: ρ _g - the density of the measured gas medium, kg / m ³ ;

ρ _W - the density of the measured fluid, kg / m ³ ;

ρ _t.et. - the density of the "reference" fluid, at ambient temperature, kg / m ³ ;

ΔP ₁ is the pressure difference between the column of the "reference" fluid and the sum of the hydrostatic pressure of the working medium and friction losses in the area of height H ₁ in the pipeline in an upward flow, Pa;

ΔP ₂ is the pressure difference between the column of the "reference" liquid and the difference of the hydrostatic pressure of the working medium and friction losses at the site of height H ₂ in the measuring column in the downward flow, Pa;

g is the acceleration of gravity, m / s ² ;

H ₁ - the distance between the additional pressure sensor and the pressure sensor mounted on the wall of the outer pipe, m;

H ₂ - the distance between the other additional pressure sensor and the pressure sensor mounted on the wall of the pipeline, m;

H _1k and H _2k - compensation height of the column of "reference" liquid for selecting the range of measurement of gas density, m,

and the measurement of volumetric gas flow is determined by the following formula:

where: Q _g - volumetric gas flow, m ³ / s;

d is the nozzle diameter, m;

d _m is the diameter of the membrane of the third pressure difference sensor, m;

ΔР - pressure drop, measured by the third pressure difference sensor, Pa;

ρ _g - the density of the measured gas, kg / m ³ ,

and the measurement of the mass flow rate of the fluid flow is determined by the formula:

where: M _W - mass flow rate of the liquid, kg / s;

ρ _W - the density of the measured fluid, kg / m ³ ;

d is the nozzle diameter, m;

d _m is the diameter of the membrane of the third pressure difference sensor, m;

ΔР - pressure drop measured by the third pressure difference sensor, Pa.

In addition, in the densitometer-flowmeter, the absolute pressure sensor, the temperature sensor of the working medium, the temperature sensor of the "reference" liquid, and the pressure difference sensors are connected to the recording unit.

Figure 1 shows the inventive device.

The densitometer-flowmeter of gaseous media contains: inlet pipe 1, a measuring column, made in the form of two coaxial,

installed with an annular gap of vertical pipes, an outer pipe 2 and with an inner pipe 3, which is a bend of a pipe bend with a measured medium with a diameter D, the upper part of which is equipped with a narrowing device 4 with a nozzle 5, while the cross-sectional area of the pipeline is equal to the cross-sectional area of the annular gap ( passage ring) between the pipes, and the nozzle length is selected from the condition:

where: l is the length of the nozzle, mm;

d - nozzle diameter, mm.

An opening 7 is made in the cover 6 of the measuring column for mounting a pressure difference sensor 8, the positive chamber of which with a diameter of d _{m is} directly in contact with the medium to be measured, and its negative chamber is connected to the measuring pulse tube 9 installed in the immediate vicinity of the outlet of the medium flow from the nozzle 5 narrowing devices.

In the lower part, the measuring column of the densitometer-flowmeter contains: two pressure difference sensors 10 and 11, a pressure sensor 12 mounted on the housing of the thermometer 13 with a "reference" liquid, which is connected by a pulse tube 14 with a "reference" liquid with a plus chamber of the pressure difference sensor 11 , another pressure sensor 15, mounted on the outer wall of the pipe outlet 3, is connected by a pulse tube 16 with a “reference” liquid to the negative chamber of the specified differential pressure sensor, two pressure sensors 17 and 18, located directly angle at the wall of the outer pipe of the measuring column 2, are connected by impulse tubes 19 and 20 with a “reference” liquid with a pressure difference sensor 10. These pressure sensors are located at the same level in the lower part of the vertical column, from the level of which an additional sensor is located at a distance of Н ₂ pressure 21, connected by a pulse tube 22 with a "reference" liquid with a pressure sensor 18 on the outer wall of the measuring column

(external pipe), and at a distance of H ₁ - another additional pressure sensor 23, connected by a pulse tube 24 with a "reference" liquid to the body of the thermometer 13, filled with a "reference" liquid, where a temperature sensor 25 is installed. As a "reference" liquid is used liquid directly in contact with the medium being measured, but not miscible with it, for example, organosilicon, having known coefficients of volume expansion and compression. The pressure sensors 15, 17, 18, 21 and 23 have sealed entries into the measured volumes. An absolute pressure sensor 26 is installed on the inlet pipe, and a medium temperature sensor 27 is placed on the output pipe. An absolute pressure sensor 26, a medium temperature sensor 27, a "reference" liquid temperature sensor 25, pressure difference sensors 10, 11, and 8 are connected to the recording block 28, which, according to the program laid down in it, calculates the density of the working medium and volumetric or mass flow rate, and provides a visualization tool, for example, a computer (not shown in Fig.). After passing the measuring column, the medium exits the pipe 29.

Density meter-flow of gas or liquid media works as follows.

The measured medium (working medium "Q") enters the inlet of the pipe 1, passes through a vertical column 3 of a pipe with a diameter D, then the medium is poured out of the narrowing hole 5 with a diameter d and enters the annulus of the measuring column 2 with an annular cross-section with a diameter D _k . At the same time, pressure taps 17, 18, and 21, having sealed inlets into the annulus, transfer the pressure by means of impulse tubes 19, 20, 22 to the pressure difference sensor 10, and pressure tester 15, having a sealed inlet into the pipe 3, takes pressure from the pipeline and through the impulse tube 16 transfers it to the negative chamber of the pressure difference sensor 11, the positive chamber of which is connected by the impulse tube 14 with

a pressure sensor 12 on the housing of the thermometer 13, where the temperature sensor 25 measures the ambient temperature. At the same time, the pressure sensor 23, having a sealed input into the pipeline 3, takes pressure and transfers the pressure difference sensor 11 through the impulse tube 24 and 14 to the positive chamber. Having passed through the annular space, the working medium exits the pipe 29. Absolute pressure sensor 26, sensor the temperature of the "reference" liquid 25, the temperature sensor of the working medium 27, the pressure difference sensors: 10, 11, 8 are connected to the recording unit 28.

In the measurement process, a method is used to compare the static indicators of the "reference" fluid with the changing parameters of the working medium.

The essence of the measurement is disclosed in the following example of calculating the parameters of gas and liquid.

The studied flow of the working medium Q passes through a vertical branch pipe 3, where the differential pressure sensor 11 measures the pressure drop ΔP ₁ as the pressure difference between the column of the “reference” liquid and the sum of the hydrostatic pressure of the medium and friction losses in the height section H ₁ in the pipeline:

where: _{ΔР 1 et} - pressure created by a column of "reference"liquid;

ΔP _1g. - hydrostatic pressure of the gas column, determined by the distance H ₁ ;

ΔP _1TP - pressure loss on friction between the points of pressure selection 15 and 23;

ΔP _1k - compensation differential pressure at a height of H _1k .

After passing through the narrowing hole 5, the gas flow Q passes in the annulus of diameter D _k , the cross-sectional area of which is equal to the cross-sectional area of the pipeline, while the differential pressure sensor 10 measures the pressure drop ΔP ₂ as the pressure difference between the column

The "reference" fluid and the difference in the hydrostatic pressure of the gas and friction losses in the area of height H ₂ in the measuring column 2:

where: _{ΔР 2et} - pressure created by a column of "reference"fluid;

ΔP _2g - hydrostatic pressure of the gas column, determined by the distance H ₂ ;

ΔP _2tr - pressure loss on friction between the points of pressure selection 18 and 21;

ΔP _2k - compensation differential pressure at a height of H _2k .

By the condition of solving the problem: ΔP _1TP ≈ΔP _2tr. , since the gas flows in the opposite direction.

After transformations of equations 2 and 3 we get:

where: ρ _t.et - density of the "reference" fluid at operating temperature, kg / m ³ ;

ρ _g - gas density under operating conditions (at pressure P and temperature T = 273 + t ° C);

g is the acceleration of gravity, m / s ² ;

H ₁ and H ₂ - the height of the pressure sampling points, m;

H _1k and H _{2k the} height of the compensation column of the "reference" liquid to select the range of measurement of gas density, m;

The compensation heights H _1k and H _2k are selected from the condition:

where: ΔP _1k = ΔP _1et -ΔP _1g and ΔP _2k = ΔP _2et -ΔP _2g , while ΔP _1g = ρ _g gH ₁ , and ΔP _2g = ρ _g gH ₂

After transformations of equation 5, we get:

When exiting the constricting device with the nozzle 5, the dynamic pressure of the working medium is perceived by the membrane of the positive chamber of the pressure difference sensor 8, and the static pressure is transmitted through the impulse tube 9 to its negative chamber, while the pressure force of the cylindrical jet passing through the nozzle is equal to:

where: ϑ ₀ = V ₀ · W ₀ ,

or

where: W ₀ - cross-sectional area of the nozzle, m ² ,

V ₀ - flow rate in the nozzle, m / s.

According to the source [4]

where: ΔР is the pressure drop measured by the third pressure difference sensor 8, Pa;

S _eff - effective area perceiving high-speed flow, m ² .

Substituting F from formula 8, we obtain:

or from where:

Where:

d _m is the diameter of the membrane, m;

d is the diameter of the nozzle, m, then:

where: Q _g - volumetric gas flow, m ³ / s;

d is the nozzle diameter, m;

d _m is the diameter of the membrane of the third pressure difference sensor, m;

ΔР - pressure drop, measured by the third pressure difference sensor, Pa;

ρ _g - the density of the measured gas, kg / m ³ 4

and the measurement of the mass flow rate of the fluid flow is determined by the formula:

where: M _W - mass flow rate of the liquid, kg / s;

ρ _W - the density of the measured fluid, kg / m ³ ;

d is the nozzle diameter, m;

d _m is the diameter of the membrane of the third pressure difference sensor, m;

ΔР - pressure drop measured by the third pressure difference sensor, Pa.

The density and mass flow rate of liquid flows is determined by the formulas:

wherein:

where: _{ΔР 1 et} - pressure created by a column of "reference"liquid;

ΔP _1g. - hydrostatic pressure of the liquid column, determined by the distance H ₁ ;

ΔP _1TP - pressure loss on friction between the points of pressure selection 15 and 23.

where: _{ΔР 2et} - pressure created by a column of "reference"fluid;

ΔP _2zh - hydrostatic pressure of the liquid column, determined by the distance H ₂ ;

ΔP _2tr - pressure loss _due to friction between the points of pressure selection 18 and 21.

By the condition of solving the problem: ΔP _1TP ≈ΔP _2tr. , since the fluid flows are in the opposite direction.

After transformations of equations 11 and 12, we get:

and mass flow rate obtained in the same way as for gas:

Using the proposed product allows you to measure the density of gas or liquid, to determine their amount in pipelines with low flow.

Information sources:

1. Kremlin P.P. "Flow meters and quantity counters." - L .: Mechanical engineering, 1998.

2. Patent RU No. 2111457 "Method for measuring the flow rate", priority 12.10.1993, publ. 05/20/1998

3. Patent for utility model No. 67263, publ. 10/10/2007, bull. No. 28.

4. A.D. Altshul, L.S. Zhivotovsky, L.P. Ivanov, Hydraulics and aerodynamics. - M .: Stroyizdat, 1985

Claims (2)

1. A densitometer-flowmeter of liquid or gaseous media, containing a measuring column mounted on the pipeline, two pressure difference sensors located in the lower part of the measuring column, a medium temperature sensor, an absolute pressure sensor, impulse tubes with a “reference” liquid that receives working pressure the medium directly by the contact method, the temperature sensor of the "reference" liquid, which is poured into the impulse tubes located at the same level in the lower part of the measuring column two pressure samplers, and a pressure selector mounted on the housing for the thermometer, as well as a recording unit, characterized in that the measuring column is made in the form of two coaxial vertical pipes installed with an annular gap - with an external pipe and an internal pipe, which is the elbow of the pipe bend with the measured medium, the upper part of which is equipped with a narrowing device with a nozzle, while the cross-sectional area of the inner pipe is equal to the cross-sectional area of the annular gap, and the length of the nozzle is selected from the condition l = (3 ÷ 4) d, where l is the length of the nozzle, mm, ad - nozzle diameter, mm, while the measuring column is equipped with a third pressure difference sensor in the upper part of the specified column and a pressure sensor mounted on the outer wall of the pipe outlet in the lower part of the specified column, in addition, two pressure sensors are installed at right angles to the wall of the external pipe, and wherein said pressure sensors are located on the same level as the bottom of the vertical column, the level of which is at a distance H ₂ placed additional pressure sensor connected to the pulse tube "etalo hydrochloric 'liquid to the pressure sensor on the wall of the outer tube, and at a distance H ₁ - other additional pressure sensor connected to the pulse tube to the pressure sensor on the thermometer body, the two pressure sensors arranged at right angles to the wall of the outer tube, connected to pulse tube with a “reference” liquid with one of the indicated pressure difference sensors, and a pressure sensor mounted on the thermometer body is connected by a pulse tube to a “reference” liquid with a plus chamber of another pressure difference sensor In addition, the pressure sensor mounted on the outer wall of the pipe outlet is likewise connected to the negative chamber of the same pressure difference sensor, in addition, the outer pipe of the measuring column is provided with a cover in which a hole is made for installing a third pressure difference sensor, whose positive camera is in direct contact with working medium, and the minus chamber is connected to a measuring impulse tube placed in the working medium stream at the outlet of the nozzle of the constricting device of the inner pipe, and measured the density of the medium is carried out according to the formulas

where ρ _g - the density of the measured gas medium, kg / m ³ ; ρ _W - the density of the measured fluid, kg / m ³ ; ρ _t.et. - the density of the "reference" fluid at ambient temperature, kg / m ³ ; ΔP ₁ is the pressure difference between the column of the "reference" fluid and the sum of the hydrostatic pressure of the working medium and friction losses in the area of height H ₁ in the pipeline in an upward flow, Pa; ΔP ₂ is the pressure difference between the column of the "reference" liquid and the difference of the hydrostatic pressure of the working medium and friction losses at the site of height H ₂ in the measuring column in the downward flow, Pa; g is the acceleration of gravity, m / s ² ; H ₁ - the distance between the additional pressure sensor and the pressure sensor mounted on the wall of the outer pipe, m; N ₂ - the distance between the other additional pressure sensor and the pressure sensor mounted on the wall of the pipeline, m; H _1k and H _2k - compensation height of the column of "reference" liquid for selecting the range of measurement of gas density, m, and the measurement of volumetric gas flow is determined by the following formula:

where Q _g is the volumetric flow rate of gas, m ³ / s; d is the nozzle diameter, m; d _m is the diameter of the membrane of the third pressure difference sensor, m; ΔР - pressure drop, measured by the third pressure difference sensor, Pa; ρ _g - the density of the measured gas, kg / m ³ , and the measurement of the mass flow rate of the fluid flow is determined by the formula

where M _W is the mass flow rate of the liquid, kg / s; ρ _W - the density of the measured fluid, kg / m ³ ; d is the nozzle diameter, m; d _m is the diameter of the membrane of the third pressure difference sensor, m; ΔР - pressure drop measured by the third pressure difference sensor, Pa. 2. The densitometer-flowmeter according to claim 1, characterized in that the medium temperature sensor, absolute pressure sensor, two pressure difference sensors located in the lower part of the measuring column, and a pressure difference sensor in the upper part of the specified column are connected to the recording unit.