RU2087871C1 - Method of measurement of flow rate of multiphase stream - Google Patents

Abstract

FIELD: measurement of flow rate of multiphase streams with the aid of thermal flowmeters. SUBSTANCE: temperature difference is measured in two sections of measurement length between which fixed quantity of heat is fed, overheating of temperature-sensitive resistors during operation under mode of hot-wire anemometer is additionally measured in the course of fixed time interval as well as time during which each temperature-sensitive resistor has overheating value with in bounds of range characteristic of each substance included in stream and ratio of this to fixed time interval is taken into account for determination of flow rate. EFFECT: improved authenticity of method. 3 dwg

Description

 The invention relates to the field of flow measurement using heat flow meters and is intended to control multiphase flows, mainly in the oil industry.

A known method of measuring flow using a heat flow meter, which consists in measuring the temperature difference with thermistors in two sections of the measuring section, between which a fixed amount of heat is applied [1]
The disadvantage of this method is the low accuracy when measuring the flow of multiphase flows.

 The technical result from the use of the invention is to improve the accuracy when measuring the flow rate, consisting of substances of different density and thermal conductivity. This is achieved by the fact that, over a fixed period of time, the superheat of the thermistors is measured during operation in the hot-wire anemometer mode, the time during which each of the thermistors has a superheat value within the range characteristic of each substance entering the flow, and the ratio of this time to everything a fixed period of time is taken into account when determining the flow rate.

 In FIG. 1 shows a device that implements the method; in FIG. 2 change in the temperature of the superheat of the thermistor in a multiphase flow from time to time; in FIG. 3 dependence of the superheat temperature on the flow rate.

The flow meter contains a measuring section, thermistors T ₁ and T ₂ and a heater H between them (Fig. 1).

где М массовый расход; N подводимое тепло от нагревателя; Δt = t₂-t₁ - разность температур между терморезисторами T₂ и T₂; С - теплоемкость контролируемого вещества; K₀ поправочный коэффициент, учитывающий неравномерность распределения температур.The invention consists in the following. As you know, the flow rate in the calorimetric flowmeter, which is considered in this case, is limited by the formula

where M is the mass flow rate; N heat input from the heater; Δt = t ₂ -t ₁ is the temperature difference between the thermistors T ₂ and T ₂ ; C is the heat capacity of the controlled substance; K ₀ correction factor, taking into account the uneven distribution of temperatures.

где С_i массовая теплоемкость отдельного вещества; X_i - массовая доля; m число веществ, входящих в состав потока.When controlling a multiphase flow, the heat capacity depending on the composition can be determined by the formula

where C _{i is the} mass heat capacity of an individual substance; X _i - mass fraction; m is the number of substances that make up the stream.

Thus, the problem reduces to the definition of X _i , i.e. composition of the stream. The composition of the stream is determined as follows. A current greater than the value that was in the thermometer mode is passed through the thermistor T ₂ , i.e. transferred to the hot-wire anemometer mode, and its overheating is measured relative to the first resistor operating in the thermometer mode. The amount of overheating depends on what substance is being washed at the given moment in time by the resistor and its speed (Fig. 2). The greater the thermal conductivity of a substance, the less its overheating at the same speed. In FIG. 2 this is shown by the example of a mixture of water, oil, gas, where the thermal conductivity of water is greatest, oil is less, and gas is even less.

число Нуссельта;

число Рейнольдса;

число Прандтля; a коэффициент теплопередачи; d диаметр корпуса терморезистора; l,ν
соответственно коэффициенты теплопроводности и вязкости жидкости; С - теплоемкость жидкости (т.е. вещества, омывающего резистор); r плотность жидкости.For a more accurate estimate, we use the criterion heat transfer equation
N _u = AR $\genfrac{}{}{0ex}{}{\text{k}}{\text{e}}$ P $\genfrac{}{}{0ex}{}{\text{e}}{\text{r}}$ (3)
Where

Nusselt number;

Reynolds number;

Prandtl number; a heat transfer coefficient; d diameter of the thermistor housing; l, ν
respectively, the coefficients of thermal conductivity and viscosity of the liquid; C is the heat capacity of the liquid (i.e., the substance washing the resistor); r fluid density.

Heat transfer equation
q = α (t _p -t _o ) (4)
where q is the density of the heat flux generated by the thermistor; t _n superheat temperature; t ₀ flow temperature.

Continuity equation
Q = V • S (5)
where Q is the volumetric flow rate; V is the average flow rate.

Так как в уравнении (3) в зависимости от режима потока и формы обтекаемого тела K ≈ 0,5 0,8; e ≈ 0,35 0,45; то видно, что перегрев (t_n t₀) зависит в основном от плотности теплового потока, теплопроводности и плотности жидкости, а также ее скорости.From equations (4), (3) follows the dependence of overheating on the main parameters

Since in equation (3), depending on the flow regime and the shape of the streamlined body, K ≈ 0.5 0.8; e ≈ 0.35 0.45; it can be seen that overheating (t _n t ₀ ) depends mainly on the density of the heat flux, thermal conductivity and density of the liquid, as well as its speed.

где V_max максимальная скорость, определяемая из графика на фиг.3.In FIG. Figure 3 shows a general view of this velocity dependence for various substances. It can be seen that at a sufficiently low speed, it is possible to make the superheat uniquely associated with a specific substance. However, at high speed, this dependence becomes ambiguous. To ensure this speed, it is necessary, based on the maximum flow rate, to make the measuring section of the flowmeter of such an area that, according to equation (5), the condition

where V _{max is the} maximum speed determined from the graph in figure 3.

It is rational to place the measuring section horizontally, as In this case, the separation of substances entering the flow occurs and the thermistors T ₁ and T _{2 are} made up of discretely located and independently connected sections T ₁₁ T _1n and T ₂₁ T _2n , which allows to increase the residence time of each of the thermistors in a certain substance. This increases the accuracy of determining the composition, as reduced transient times of thermistors when changing their various flow substances (see Fig. 2).

 As a result, the proportion of each substance can be determined by the ratio of the time spent by a given section of a thermistor in a given substance to the entire period of measurement and by the number of these sections of a thermistor in a given substance.

 After that, the average heat capacity is determined by the formula (2) and then the flow rate by the formula (1).

Claims (1)

 A method of measuring the flow rate of a multiphase flow, which consists in measuring the temperature difference between the thermistors in two sections of the measuring section, between which a fixed amount of heat is supplied, characterized in that they additionally measure overheating of the thermistors during a fixed period of time when operating in the anemometer mode, the time during which each of thermistors has an overheating value within the range characteristic of each substance entering the stream, and the ratio of this time to about the entire fixed period of time is taken into account when determining the flow rate.

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