[Haichuan Chemical Sharing] Application of Silica Aerogel Felt in Direct-Buried Steam Pipes
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1 Overview: Aerogels were first synthesized in 1931 by Kister at Stanford University in the United States through the hydrolysis of water glass [1]. It was not until 1993 that aerogels began to be applied in fields such as aerospace and the chemical industry [2-4]. In recent years, aerogels have been increasingly used as a new type of insulating material in the heating sector. The commonly available aerogel insulation material on the market is silica aerogel foam (hereinafter referred to as aerogel foam). Aerogel foam (a flexible material that can be bent and wrapped to fit the shape of the object to be insulated) has an extremely low thermal conductivity, making it suitable for insulation, heat retention, and heat isolation in temperature ranges from -200 to 800 °C. It is one of the most ideal insulating materials for industrial pipelines, storage tanks, equipment, and more. A comparison of the technical and economic performance of aerogel foam and other insulation materials is shown in Table 1 [5-6]. Table 1 Comparison of technical and economic performance between aerogel blankets and traditional insulation materials [5-6]
As can be seen from Table 1, the properties of traditional insulation materials (those in Table 1 other than aerogel blankets) vary. Except for high-temperature resistant glass wool, the seismic resistance, service life, moisture absorption per unit volume, strength, fiber uniformity, water resistance, difficulty of installation, installation losses, impact on human health, and environmental impact of the other materials are all unsatisfactory. Therefore, in recent years, high-temperature resistant glass wool has been widely used in directly buried steam pipes. Compared to traditional insulation materials, apart from price, aerogel foam has advantages in the vast majority of performance indicators. This paper conducts a comparative analysis of the technical and economic aspects of directly buried steam pipes with insulation layers consisting solely of high-temperature resistant glass wool, or those with aerogel foam combined with high-temperature resistant glass wool.
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2 Insulation structure ① Insulation structure 1: Insulation structure 1 is one that uses only high-temperature resistant glass wool for insulation, as shown in Figure 1. As can be seen from Figure 1, for the directly-buried steam pipes using only high-temperature resistant glass wool, 3 layers of such glass wool are employed, and the reflective layer is made of aluminum foil glass fiber cloth. The insulation layer shall be tied in layers using binding straps (steel strips or wire), with the spacing between these straps not exceeding 400 mm.
In multi-layer insulation structures, it is advisable to place insulation materials with low thermal conductivity on the high-temperature side, and those with high thermal conductivity on the low-temperature side [7]. Therefore, the innermost insulation layer is made of aerogel foam, while the outer layer is made of high-temperature resistant glass wool, supplemented by aluminum foil fiberglass cloth. In this way, the thermal insulation advantages of aerogel foam in high-temperature environments can be fully utilized, allowing the thickness of the outer layer of high-temperature resistant glass wool to be significantly reduced. This prevents the overly thick glass wool from sagging due to its own weight, which could affect the insulation performance.
3 Technical and Economic Comparison 3.1 Project Overview A certain steam pipeline project uses directly buried steam pipes with a working steel pipe specification of D630×10; the total length of these pipes is 1,220 m. The design pressure is 2.5 MPa, the design temperature is 300 ℃, the depth at which the pipes are buried is 1.8 m, and they operate throughout the year. Based on the calculation of the thickness of each insulation layer and the specifications of the outer protective steel pipe in the insulation structures, the economic efficiency of the two types of insulation structures is compared. 3.2 Calculation Methods and Known Parameters: The thickness of the insulation layer and the specifications of the outer protective steel pipe are calculated using the method of controlling the surface temperature of the outer protective steel pipe. The specific calculation methods are based on CJJ/T 104—2014 \"Technical Specifications for Direct-Buried Steam Pipes in Urban Heating Systems\" and reference [7]. Since the directly buried steam pipes are laid beneath the road surface, the temperature of the outer surface of the protective steel pipe is set to 50 °C, as specified in Article 6.1.5 of CJJ/T 104—2014; this value of 50 °C is therefore adopted. The heat transfer coefficient of the ground surface above the directly buried steam pipes is taken as 12 W/(m2·K). The thermal conductivity of the air in the air layer at operating temperature is 0.03 W/(m·K), while the thermal conductivity of the soil is 2.78 W/(m·K). The atmospheric temperature is taken as the average temperature in the hottest month during pipeline operation, which is 28.7 ℃. The natural soil temperature is taken as the average temperature in the hottest month at the depth where the pipeline is buried, which is 23.9 ℃.
The formula for calculating the thermal conductivity λ1 of high-temperature resistant glass wool (applicable temperature range: 70–350 °C) is [6]:
The price of spiral-welded steel pipes is 4,600 yuan per ton; the price of high-temperature resistant glass wool is 600 yuan per m3; the price of aerogel insulation is 17,000 yuan per m3; the price of the extra-strong epoxy coal tar anti-corrosion coating is 110 yuan per m2; and the unit cost for trench excavation and backfilling is 30 yuan per m3.
3.3 Calculation Results For the convenience of calculation and analysis, the thermal resistances of the working steel pipe, aluminum foil glass fiber cloth, outer protective steel pipe, and anti-corrosion layer are ignored, and the cost of the working steel pipe is not taken into account. When calculating the costs for trench excavation and backfilling, the trench width is taken as 250 mm, based on the distance between the two sides of the trench and the widest part of the outer protective steel pipe of the directly buried steam pipe. Based on the aforementioned calculation methods and known parameters, the thickness of each insulation layer as well as the specifications of the outer protective steel pipes for the two types of insulation structures can be calculated, as shown in Figures 3 and 4 respectively. Based on the calculation results of the insulation layer thickness and the specifications of the outer protective steel pipe, the mass of the outer protective steel pipe and the volume of the insulation layer can be determined (see Table 2); thereafter, the economic parameters for the two insulation structures can be calculated (see Table 3). As can be seen from Tables 2 and 3, under the same thermal insulation performance, the overall cost of Insulation Structure 2 is slightly higher than that of Insulation Structure 1. However, Insulation Structure 2 has a thinner total insulation layer, making it more stable; this facilitates the long-term, efficient, and stable operation of steam pipes. Additionally, it reduces the required specifications for the outer protective tube, thereby decreasing the space occupied by buried pipes and contributing to better urban planning. Therefore, the thermal insulation structure 2 has stronger overall advantages.
Figure 3 Thicknesses of each insulation layer and specifications of the outer protective steel pipe in insulation structure 1
Figure 4 Thicknesses of various insulation layers and specifications of the outer protective steel pipe in insulation structure 2