RU2296983C1 - Method for heat vision control of heat isolation of vessels and pipelines - Google Patents

Abstract

FIELD: non-destructive control.

SUBSTANCE: in the method heating of wall of vessel (pipeline) under isolation for creating a temperature drop transversely to isolation layer is performed by method of blowing gas through internal hollow, with appropriate adiabatic gas temperature increase, while blowing is performed up to optimal pressure ρ_opt, at which adiabatic increase of gas temperature is maximal.

EFFECT: possible heat vision control of non-separable vessels and extensive pipelines, decreased energy costs of control.

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Description

The proposed method relates to the field of non-destructive testing, namely to infrared diagnostics and methods of thermal non-destructive testing.

Known methods for determining the quality of building envelopes using thermal imagers are described in the guidance documents (GOST 26629-85 "Method of thermal imaging quality control. Thermal insulation of building envelopes." International standard ISO 6781-83 "Thermal insulation. Qualitative detection of heat engineering violations in building envelopes. Infrared method ", Non-destructive testing: Reference: In 7 vol. V.5: In 2 book Kn.1: Thermal control. Kn.2: Electrical control. - M .: Mechanical Engineering, 2004. - p.279).

In these methods, the temperature head, which ensures the formation of temperature fields on the surfaces controlled by a thermal imager, is created due to heat generation inside buildings and structures due to the operation of heating, ventilation, heating and household heat-generating devices.

A known method of controlling the quality of power lines (Kanarchuk V.E., Chigrinets A.D. Non-contact thermal diagnostics of machines. M: Mechanical Engineering, 1987. - p.153), as well as a method of thermal imaging control of electronic circuit boards (Cesbury B. Reducing time costs and labor when finding damages on the circuit board of system X of the Plessis company // Infrared Observer. 1987, No. 15; Non-Destructive Testing: Reference: In 7 vol. T.5: In 2 book Kn.1: Thermal control. .2: Electrical control. - M.: Mechanical Engineering, 2004. - p. 298, 333).

In these methods, the temperature head is created due to the release of Joule-Lenz heat in conductors and consumers in which electric current flows.

The disadvantage of these methods is that in the absence of heat release inside the studied objects, the temperature difference between the observed surface and the environment disappears, which makes it impossible to conduct thermal imaging control.

There is a method of thermal imaging quality control of asphalt concrete coatings of motorways (Kanarchuk V.E., Chigrinets A.D. Contactless thermal diagnostics of machines. M.: Mechanical Engineering, 1987. - p.146). The method is based on the external (from the location of the thermal imager) heating of the controlled surface due to solar radiation.

The disadvantage of this method is the dependence of the measurement results on weather conditions.

The listed methods relate to passive control methods.

Active control methods are known in which the test surface is subjected to artificial heating. With external heating, the heat source is placed on the side of the thermal imager (Dragun V.L., Filatov S.A. Thermal imaging systems in studies of thermal processes. Minsk: Nauka i Tekhnika, 1989. - p. 94). External heating is practically not applicable for the surfaces of spherical and cylindrical objects (for example, vessels and pipelines). Internal heating can be achieved by placing heat sources on the back side or introducing heat sources (electric heaters, coolants, etc.) inside the controlled object. However, this is not always possible and leads to high energy costs.

The closest in technical essence (prototype) to the proposed method is a method of thermal imaging non-destructive quality control of multilayer compositions (Dragun V.L., Filatov S.A. Thermal imaging systems in the study of thermal processes. Minsk: Science and Technology, 1989. - p. 95 ) The method includes placing halogen emitters inside the controlled object, creating a heat flux with them to the inside of the surface to be monitored, controlling the temperature field on the outside of the surface to be monitored using a thermal imager.

The disadvantage of this method is the impossibility of its use in non-separable vessels and long pipelines, as well as high energy costs for its implementation.

The objective of the invention is to provide a method of thermal imaging control of thermal insulation of vessels and pipelines, providing a technical result, consisting in providing the ability to control the quality of thermal insulation of non-separable vessels and extended pipelines, as well as to reduce energy costs for monitoring.

This technical result is achieved by the fact that the internal cavity of the vessel (pipeline) is pressurized by gas from an external source to a pressure corresponding to the optimal value p _opt , at which the adiabatic increase in the temperature of the boost gas is maximal. The indicated set of features (gas pressurization of the internal cavity and the choice of pressurization pressure equal to the optimum value of p _opt , ensuring the maximum temperature increase) will make it possible to exclude the installation of internal heat sources and the corresponding disassembly of vessels (pipelines), combine thermal imaging control operations with mandatory periodic crimping and checking operations tightness and strength of vessels (pipelines), and also limit the amount of gas supplied to the internal cavity for pressurization.

An increase in the temperature of the boost gas leads to an increase in the temperature of the vessel wall (pipeline) due to heat transfer and the formation of a temperature difference across the thermal insulation layer, providing the possibility of thermal imaging control of the outer surface of the insulation.

The value of p _opt is determined by the formula

p _opt = π _opt p ₀ ,

where π _opt is the optimal degree of increase in boost pressure, p ₀ is the initial pressure in the vessel (pipeline).

1 <n≤k,

where n is the polytropic index, k is the adiabatic index of the boost gas.

The physical meaning of having a maximum of the dependence of the final temperature of the boost gas on the boost pressure can be explained as follows. At small degrees of increase in boost pressure, the increase in the temperature of the initial gas as a result of adiabatic compression is not large enough. At high degrees of pressure increase, the increase in the temperature of the initial gas is significant, however, it is leveled by a large mass of boost gas. The optimum value of π corresponds to a situation where the increase in the temperature of an ideal gas as a result of adiabatic compression is already large enough, and the mass of the boost gas is still small enough to significantly reduce the temperature of adiabatic compression.

Let us justify the value of the optimal pressure p _{opt of} pressurization of the inner cavity of the vessel (pipeline) at which the increase in gas temperature will be maximum.

When pressurizing thermally insulated containers, the gas temperature rises as a result of adiabatic compression of the gas that was in the tank before pressurization.

A feature of the gas’s behavior under boost is that the increase in the internal energy of the gas as a result of the compression work is evenly distributed between the initial gas and the boost gas.

Let us analyze the process of increasing the gas temperature when pressurizing a thermally insulated vessel of volume V.

We assume that the process of boosting occurs in two stages. The initial state of the gas in the vessel before pressurization is shown in Figure 1. The intermediate state of the gas in the vessel when pressure p _n is shown in Figure 2. The final state of the gas in the vessel after temperature equalization is shown in Figure 3. At the first stage, the initial gas is compressed from the initial pressure P ₀ to boost gas pressure P _n without mixing with it (conditionally separated from it by a weightless hermetic piston). In this case, the compressed gas in the tank will occupy the volume V ₁ , its temperature will increase to the value of T _I , determined from the equation of the adiabatic process

where T ₀ is the initial temperature of the gas in the tank, k is the adiabatic index,

- степень повышения давления при наддуве.

- the degree of increase in boost pressure.

The boost gas will take volume V _II . Wherein

The change in the pressure and temperature of the boost gas in the process of filling the tank will be neglected and considered equal to the pressure P _n and temperature T _n in the source of the boost gas.

At the second stage, the temperature field is equalized as a result of mixing of the boost gas from the volume V _II with the initial gas from the volume V _I. As a result, the temperature of the boost gas is reduced, and the temperature of the boost gas rises. The process ends with the establishment of the final temperature T _to the entire volume of the tank. The value of T _to determine the equation of energy balance

where U _k , U _I , U _II - the value of the internal energy of the gas after equalizing the temperature in the volume of the tank, as well as before leveling in the volumes U _I and U _II, respectively

- удельные изохорные теплоемкости газа при температурах Т_к, T_I, Т_н, m_к - суммарная масса газа в емкости после наддува, m_I и m_II - масса начального газа и масса газа наддува в емкости.In equations (4, 5, 6)

- specific isochoric heat capacities of the gas at temperatures T _k , T _I , T _n , m _k - the total mass of gas in the tank after boosting, m _I and m _II - the mass of the initial gas and the mass of the boost gas in the tank.

With small changes in gas temperature, the dependence of C _υ on T can be ignored, so that

Note that the type of boost gas and the initial gas are one and the same.

From the equation of mass balance

Based on expressions (3-8), we can write

Define the values of m _I and m _II .

The mass m _I is equal to the mass m _{0 of the} initial gas, which filled the tank to boost.

With small degrees of pressure increase, the compressibility of the gas can be neglected, and then, in accordance with the Clapeyron-Mendeleev equation

Дж/(кмоль К) - универсальная газовая постоянная.where μ is the molar mass of the gas;

J / (kmol K) is the universal gas constant.

The mass m _II is equal to the mass of the boost gas filling the volume V _II at pressure p _n and temperature T _n :

From expression (2)

Since the volume V _I is formed by compression of the initial gas mass m ₀ = m _I to a pressure p _n , it can be found from the expression

After substituting the values of T _I , m _I , m _II from expressions (1), (10), (11) taking into account (12) and (13) into formula (9), after some transformations we get

In the formula (14) a = (k-1) / k, and it is also accepted that the temperature of the initial gas and boost gas coincides: T ₀ = T _n .

An analysis of the dependence T _k = T _k (π) shows that for π = 1 T _k = T ₀ , and as π → + ∞ T _k → T ₀ . This circumstance indicates the presence of a maximum of T _to for some value of π = π _opt .

We find π _{opt by the} standard way of differentiating the function T _to (π) with respect to π and equating the derivative to zero:

To simplify the differentiation procedure, we transform expression (14) by dividing the numerator and denominator by π:

Since the maximum value of T _to (π) corresponds to the minimum of the function in the denominator

Then we replace formula (15) with the expression

Differentiation (17) leads to the following result:

and taking into account (18), we obtain

Or, substituting instead of a its value:

The corresponding value of the maximum possible temperature increase will be

So, for monatomic gases (helium, argon), for which k = 1.67, the values of π _opt and T _{to max} will be:

π _opt = 3.59;

T _{to max} = 1.23T ₀ .

For diatomic gases (air, nitrogen, oxygen, hydrogen), for which k = 1.4, the values of π _opt and T _{to max} will be:

π _opt = 3.25;

T _{to max} = 1,14T ₀ .

Finally, for triatomic gases (water vapor, carbon dioxide), for which k = 1.33, the values of π _opt and T _{to max} will be:

π _opt = 3.16;

T _{to max} = 1,12T ₀ .

The nature of the dependence of the relative temperature increase T _c / T ₀ (denoted by Z) on the degree of pressure increase π (denoted by Y) under boost (for different k) is shown in Fig. 4 (for a monatomic gas; k = 1.67), 5 (for diatomic gas; k = 1.4), 6 (for a triatomic gas; k = 1.33).

Expressions for calculating the maximum temperature increase were obtained for the case of adiabatic compression. However, in real conditions, the vessel or pipeline in which the compression is carried out is covered with thermal insulation with a finite thermal resistance. As a result of this, in the process of pressurization, there is a loss of heat from the gas through thermal insulation into the environment. In the limiting case, when the entire increase in the internal energy of the gas obtained as a result of compression is parried off by the equivalent heat removal, the boost process will have an isothermal character.

Thus, the actual process of pressurizing a tank (pipeline) coated with thermal insulation with finite thermal resistance will be in the range between isothermal and adiabatic processes.

If we neglect the change in the heat capacity of the gas in the process of boosting, then this process can be considered polytropic. In this case, in formulas (21, 22), the value of the adiabatic exponent k should be replaced by the value of the polytropic exponent n. Moreover, 1 <n≤k:

Thus, the optimal value of the boost pressure of the inner cavity of the vessel (pipeline) to provide thermal imaging control of thermal insulation is

where π _{opt is} determined from formulas (21) or (23).

The pressurization of vessels and pipelines to pressures less than p _opt leads to a decrease in the sensitivity of the thermal imaging control method due to underheating of the gas. The pressurization to pressures greater than p _opt , in addition to reducing the sensitivity of the method, leads to an additional consumption of boost gas and corresponding economic costs.

An example of the implementation of the proposed method is illustrated on the basis of the installation shown in Fig.7. The installation includes sources of boost gas: compressor 1 and cylinder 2, distribution shut-off valves 3, 4, 5, gearbox 6, pressure gauge 7, a controlled vessel 8 with thermal insulation 9, and also a thermal imager 10.

The installation is as follows. The vessel 8 is pressurized with a boost gas from the initial pressure p ₀ in the vessel 8 to the pressure p _opt . To do this, gas from the compressor 1 through the valve 3 or from the cylinder 2 through the valve 4 is supplied to a pressure reducer 6 adjusted to the outlet pressure, and then through the open valve 5 it is supplied to the internal cavity of the vessel 8. The pressure in the vessel 8 is controlled by a pressure gauge 7. when the pressure p _opt in the vessel 8 is reached, the valve 5 closes. The adiabatic increase in pressure in the inner cavity of the vessel 8 leads to an increase in the temperature of the gas in the vessel, as a result of which the wall of the vessel is heated. The heat flux caused by the temperature difference between the vessel wall and the environment is transmitted through thermal insulation 9 to its outer surface, forming temperature fields of various configurations on it, determined by the quality and current technical state of thermal insulation 9.

Temperature fields on the outer surface of thermal insulation 9 are controlled using a thermal imager 10.

Claims (1)

A method of thermal imaging control of the insulation of vessels and pipelines, including pre-heating the walls of the vessels and pipelines under insulation to create a temperature difference across the insulation layer and subsequent thermal imaging control of temperature fields on the outer surface of the insulation, characterized in that the pre-heating is carried out by gas pressurization of the inner cavity of the vessels and pipelines and the corresponding adiabatic increase in the temperature of the boost gas, and the boost is carried out to optimal pressure p _opt , at which the adiabatic increase in the temperature of the boost gas is maximum.

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