【Abstract】 Aiming at the problems of the traditional valve internal leakage detection method is complex and inaccurate detection, based on the principle of heat transfer, a quantitative calculation method for the leakage rate of the steam valve is proposed. This method can accurately and conveniently calculate the valve leakage speed, has the advantages of online, real-time measurement and simple instrument, which can provide reference for the on-line leak detection of steam valve.
【Keywords】 valve; steam medium; heat transfer principle; leakage rate calculation 【CLC】 TK01 + 2 【Citation Index】 A 【Article-No.】 1674-0688 (2016) 07-0110-03
In recent years, the rapid development of China's industry, the role of the valve in the industrial field more and more can not be underestimated. Valve as an important pipeline components, plays a crucial role in the pipeline. In the industrial field use, the leakage is one of the main forms of damage to the valve, but also affect the safety of the valve the most important issue. When the valve is put into use, due to changes in pressure and corrosion of the media inside the pipeline resulting in defects, leading to leakage. Leakage of the valve is mainly divided into internal leakage and external leakage, of which more serious leakage is the internal leakage, due to the internal valve, not easy to be detected, so the detection and repair methods are more complicated. In the high temperature and high pressure steam pipeline, the steam medium has the characteristics of large pressure, high temperature and small density, and the internal leakage of the valve is more likely to occur. This has led to the piping system pressure loss and energy dissipation and other issues [1]. Leakage of the valve not only reduces plant benefits, but traditional methods of detecting steam valves can result in plant shutdowns, resulting in unnecessary economic losses. Therefore, many people concerned about the steam valve on-line leakage detection methods were studied.
At present, the main principle of most of the traditional in-line leakage detection methods of steam valves is to arrange various sensors and measuring elements around the pipelines and detect the leakage of the valves by measuring the values of relevant parameters. However, most of these methods are limited to the introduction of the principle and the qualitative verification and analysis, but there are few researches on the problems of valve leakage and leakage speed. Therefore, the on-line inspection of steam valve in-depth study has great significance [2]. In this paper, aiming at the on-line leak detection of steam valve, based on the method of heat transfer, relevant models are established to quantitatively analyze and calculate the leakage rate of steam valve.
1 based on heat transfer analysis of the steam valve leak detection method According to the principle of heat transfer, heat transfer is due to the temperature difference between objects or objects caused by the heat is always automatically transferred from the higher temperature objects to temperature Low object. There are 3 ways to achieve this transfer: heat conduction, convection and heat radiation.
(1) Heat conduction refers to the transfer of heat energy by the thermal motion of molecules such as molecules, atoms and free electrons when no relative displacement occurs between the various parts of the object.
(2) Convection refers to the relative displacement between the parts of the fluid due to the macroscopic movement of the fluid, and the heat transfer process caused by the intermixing of the cold fluid and the hot fluid. Thermal convection occurs only in the fluid, and since the molecules in the fluid undergo an irregular thermal motion at the same time, thermal convection must be accompanied by the phenomenon of heat conduction.
(3) Thermal radiation refers to the way in which an object transmits energy through electromagnetic waves. Objects emit radiant energy for a variety of reasons, of which radiant energy due to heat is called thermal radiation.
This article is based on the above methods and principles of research. When the valve leaks, there will be higher than the ambient temperature inside the pipeline steam media. According to the law of conservation of energy, we know that the total heat transferred Q is conserved whether it is the convection of the fluid and the inner wall of the pipe, the heat conduction between the inner wall and the outer wall, or the convection and heat radiation of the outer wall and the surrounding medium [3 -4].
1.1 The outer wall of the insulation layer and the natural convection of air Natural convection heat is divided into large space natural convection and limited space natural convection, also known as external natural convection and internal convection. The so-called large space natural convection means that the development of the thermal boundary layer is not disturbed or obstructed by natural convection, and is not limited to being geometrically large or infinity. However, in the natural convection of limited space, or the development of the boundary layer is disturbed, or the flow of the fluid is restricted, so that the heat transfer law is different from the situation of large space. The heat exchange between the outer wall of the insulation layer and the air belongs to the external natural convection.
Pipe wall insulation and the environment of the natural convection heat Q is as follows:
Q = α1 (t2-t3) (1)
In formula (1): α1 is the heat coefficient of the outer wall of the air and the insulation layer; t2 is the temperature of the outer wall of the insulation layer; and t3 is the temperature of the air in the environment.
α1 = λ1Nu1 / l (2)
In the formula (2): Nu1 is Nusselt number; L is the size of equipment stereotypes.
External air and pipe insulation between the outer wall belong to the natural convection heat, so:
Nu1 = C (GrPr) n (3)
For the steam pipeline, C in the formula (3) is a coefficient (generally 0.1 to 0.13); n is a coefficient (generally 1/3); Gr is the Gramesche number; Pr is the Prandtl number.
Gr = (4)
Pr = (5)
Where: ρ is the density of air; Β is the expansion coefficient; G is the local gravity acceleration. △ t is t2-t3; Cp1 is the specific heat capacity of the fluid; μ1 is the fluid dynamic viscosity; λ1 is the air thermal conductivity.
1.2 The heat transfer between the pipe wall and the insulation layer (insulation layer) The internal heat transfer through the pipe wall (insulation layer) to the lower part of its external temperature is the phenomenon of thermal conductivity, through a large number of practical experience in the problem of thermal conductivity, thermal conductivity of the law It has been summarized as Fourier law.
According to Fourier's law, the heat transferred by the cylinder wall is as follows:
Q = (6)
D1 is the inner diameter of the pipe; d1 is the outer diameter of the pipe (inner diameter of the thermal insulation layer); d2 is the outer diameter of the thermal insulation layer; l is the length of the pipe; λ2 is the length of the pipe; Λ3 thermal conductivity of the insulation layer; Rs1, Rs2 for the pipe wall and the outer wall of the fouling thermal resistance.
1.3 pipe steam medium forced convection heat transfer [5]
The main difference between internal flow and external flow is that the relative relationship between the flow boundary layer and the flow channel wall is different: in the external flow, the fluid boundary layer on the heat exchange wall can freely develop without being hindered by the flow channel wall or limit. As a result, there is often a region outside the boundary layer where external flow can be ignored wherever the velocity gradient or the temperature gradient is negligible. In the internal flow, the development of the boundary layer on the heat transfer wall is limited by the flow channel wall, so the heat transfer law is obviously different from the external flow.
As the steam inside the pipe between the media, the heat transfer mode should be forced convection internal heat exchanger, then: Q = α2 (t0-t1) (7)
α2 = λ4 Nu / l (8)
Where: α2 for the steam and pipe wall heat transfer coefficient; λ4 for the steam thermal conductivity.
Between the steam medium and the pipe wall belongs to forced convection heat transfer, and the temperature of the steam medium will actually decline, so:
Nu2 = 0.023Re0.8Pr0.3 (9)
Re = (10)
Pr = (11)
Where: Nu2 Nusselt number; u is the steam leakage rate; ρ2 is the density of steam; μ2 is the dynamic viscosity of steam; Cp2 is the specific heat capacity of steam at constant pressure.
The above all kinds of legislation, we can get the steam flow in the pipeline speed, that is, the leakage rate of the valve u.
2 Detection Element Selection and Arrangement Installation 2.1 Detection Element Selection To accurately grasp the valve leakage point and leakage rate, the best way is to use more accurate temperature measurement instruments and temperature measurement. Temperature measurement instruments are divided by temperature measurement methods are mainly two types of contact and non-contact. Contact measuring instrument measuring principle is relatively simple and reliable, high precision. However, due to temperature measurement devices and the measured medium need to be fully heat exchange, so there is the phenomenon of temperature measurement delay. At the same time, by the use of measuring instruments temperature limit, but can not be measured in very high temperature medium. The principle of non-contact temperature measurement instrument is mainly thermal radiation, so it does not require direct contact with the measured medium and a wide temperature range, not subject to the upper limit temperature, the reaction speed. But the emissivity of the object, the measurement distance and the water vapor and other factors will affect the measurement results [6]. Taking into account the nature of the steam medium, decided to use contact temperature measuring element [7].
2.2 Detection of components installed Temperature measurement devices should play a role in data collection. According to the working pressure of steam, temperature and other characteristics, a reasonable choice of temperature measurement device is to ensure accurate measurement results.
(1) The installation of temperature measuring element shall be convenient for the maintenance, verification and disassembly of the instrument maintenance staff.
(2) In the installation of protective jacket, in order to reduce the lag when measuring temperature, can be installed between the two casing heat transfer good filler.
(3) Where the temperature components installed pressure, must ensure that the seal. When the pressure is too high, should be installed insulation cover.
(4) As the steam is a high temperature medium, when installing the temperature measuring element, it should be kept as vertical as possible to prevent the temperature element protecting tube from being deformed under high temperature.
(5) If the steam medium contains impurities, in order to protect the temperature measuring element from abrasion, a protective cover should be installed to ensure the accuracy of the measurement result.
(6) When the temperature measuring device is installed in the negative pressure pipeline or equipment, it is necessary to ensure the sealing of the mounting hole and the temperature measuring device so as to avoid lowering the indicated value of the temperature measuring point when the cold air is inhaled.
(7) When installing the temperature measuring element in the pipe with large steam flow rate, the temperature measuring element must be installed obliquely so as to avoid the excessive erosion.
(8) to avoid thermal radiation. Since the calculation of the leakage rate of the steam valve described in this article is based on the neglect of thermal radiation, a radiation shield [8] should be installed between the temperature measurement element and the wall of the device.
3 Conclusion In this paper, the leakage rate of the steam valve were analyzed and calculated, the conclusions are as follows.
(1) The theory of heat transfer and thermodynamics are fully combined with the actual project, and the internal leakage rate of the valve is calculated quantitatively by the related knowledge of energy conservation law, heat conduction and heat convection.
(2) By reasonably selecting and arranging the measuring elements, the valve with internal leakage can be determined more accurately, saving time and providing maintenance personnel with convenience, and to some extent, the economic benefits of the factory are guaranteed.
(3) The calculation method described in this paper is based on the theoretical basis of heat transfer and thermodynamics. When the temperature difference between medium and environment is large, the calculation method can calculate the valve leakage rate more accurately.
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[Editor: Chen Zeqi]
[About the author] Lei Yajun, male, Xi'an, Shaanxi, PetroChina Yunnan Petrochemical Co., Ltd. engineers, equipment management.
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