In the process of natural gas extraction, gathering and transportation, in order to prevent the hydrate from condensing into solid due to the low temperature, it is often necessary to use a heating furnace to heat the natural gas. The heating furnace is an indispensable and important equipment in the natural gas gathering and transportation system, and is usually used in the wellhead, metering station, transfer station, etc. It uses fuel to burn in the furnace to generate high-temperature flame and flue gas as a heat source, and heat the natural gas flowing in the furnace tube through heat conduction, radiation heat transfer and convection heat transfer, so that it can reach the temperature required for production for transportation and separation. , is the device that consumes the most energy. In order to reduce the energy consumption of the heating furnace, it needs to be insulated to achieve low energy consumption and high-efficiency operation of the heating furnace.

 

Compared with felt insulation materials, thermal insulation coatings have the advantages of easy construction, can be coated on any space-limited and special-shaped components, stable thermal conductivity, and the formation of a complete closed structure can effectively prevent corrosion under insulation (CUI). In recent years, it has been widely studied and applied to the thermal insulation of heating furnaces. After more than ten years of rapid development of thermal insulation coatings, its research and development has entered a bottleneck period. The performance of coating products is similar, and the competition in the industry has intensified. Although the silicate thermal insulation coating is low in price and excellent in thermal insulation performance, its mechanical strength is poor, and problems such as cracking will occur during the service period. With the continuous development of aerogel materials, new aerogel thermal insulation coatings have emerged at home and abroad. Chen Hao et al. used ceramic microbeads as insulating fillers to prepare thermal insulation coatings, and sprayed them on the surface of the heating furnace. The test results showed that after spraying the coatings, the surface temperature of the heating furnace was reduced by about 56.7 °C, and the thermal insulation effect was very good. Wang Ning et al. conducted thermal insulation treatment on the outer wall of the heating furnace by spraying energy-saving paint. The results show that the energy-saving paint can significantly reduce the temperature of the furnace wall, and the temperature distribution is uniform and has no obvious fluctuation; after the use of the thermal insulation paint, the heat dissipation loss of the heating furnace is reduced by 1.87%. Sun Xuedong has developed a thermal insulation coating for the thermal insulation requirements of heating furnaces. The coating formula adds strong radiation absorbers, high temperature resistant materials, combustion catalysts, etc. The prepared coatings can increase the energy saving effect by about 7%. Chen Baolan applied the "T-2" thermal insulation coating containing hollow ceramic microbeads to the surface of the heating furnace and the external floating roof tank. The results showed that the outer surface temperature decreased from 117 °C to 73 °C and the heat dissipation loss was reduced by applying 2mm coating. .

 

1 Investigation and condition evaluation of thermal insulation layer

 

On-site investigation of the application of thermal insulation of a heating furnace in a natural gas transmission station. The basic situation of the heating furnace is as follows: it consists of four parts: the heating furnace body, the burner and the control system, the instrument automation system and the PLC system. During operation, the outer surface temperature of the furnace body is 80°C when the insulation layer is not applied. In the original design, a commercially available thermal insulation coating of a certain brand (the performance parameters are shown in Table 1), the thickness of the coating is 6mm, the bottom of the thermal insulation coating is epoxy anti-corrosion primer, and the outer surface of the thermal insulation coating is coated with yellow acrylic polyurethane topcoat .

 

Table 1 Performance parameters of the original thermal insulation coating

 

 

At present, the heating furnace has been running for 4a, and the appearance inspection of thermal insulation and the detection of thermal insulation and thermal insulation effect of the heating furnace are carried out on site. The results are shown in Figure 1.

 

 

Figure 1 Current status of heating furnace insulation

 

It can be seen from Figure 1 that there are large areas of bubbling and cracking on the top of the heating furnace tank. After removing the protective surface layer, it is found that the thermal insulation coating is hollow, and the strength of the thermal insulation coating is low, which can be easily cleaned with a spatula. The epoxy anti-corrosion primer under the thermal insulation coating is in good condition, and there is no damage, cracking, bubbling and other phenomena. This is mainly because the thermal insulation properties and mechanical strength of thermal insulation coatings are contradictory. In the formulation development of thermal insulation coatings, the addition of thermal insulation fillers leads to excellent thermal insulation properties, but the reduction in the amount of film-forming binders leads to mechanical strength. decline. Therefore, it is necessary to find a balance between the two properties in formula design, and adjust the amount of fiber in the formula according to the requirements of easy construction and surface flatness, and determine a coating formula with good comprehensive performance, good workability and smooth surface to meet the reliability of application. and longevity.

 

2 Repair plan

 

2.1 Insulation coating selection

The defects of the original thermal insulation coating were repaired by self-developed aerogel thermal insulation coating. The self-developed aerogel thermal insulation coating uses SiO2 aerogel and hollow glass microbeads as thermal insulation fillers, acrylic resin as film-forming base material, and metal oxides with high reflection and scattering rates such as ZrO2 and TiO2 as auxiliary functional fillers. Combined with mixed particle size fibers to enhance the high and low temperature cracking resistance of the coating, the overall performance is good. The thermal insulation coating combines the dual characteristics of anti-corrosion coatings and thermal insulation materials, and forms a thermal insulation layer with a certain strength after drying. Its performance parameters are shown in Table 2.

 

Table 2 Performance test results of self-developed aerogel thermal insulation coatings

 

 

In order to ensure the coating construction efficiency and prevent excessive consumption loss due to coating sagging, if it is found that the viscosity of the thermal insulation coating is not conducive to construction, 5%~10% deionization can be added according to the on-site trial coating conditions before use. Water or acrylic resin adjust the viscosity of the coating to improve the workability of the thermal insulation coating. It should be noted that if the amount of acrylic resin added is too large, the thermal insulation performance of the coating will decrease.

 

In addition, the interior of the thermal insulation coating is filled with a large number of open-pore structures, and the infiltration of water vapor will affect the thermal insulation performance of the coating and cause problems such as corrosion under the thermal insulation layer. Therefore, the thermal insulation coating needs to be matched with a protective surface layer. Different from hard surfaces such as steel structures, the protective surface layer has a certain permeability on the surface of the thermal insulation coating after brushing. If the difference is too large, under the influence of thermal expansion and contraction, the protective surface layer will be pulled and cracked, resulting in cracks. In this paper, referring to GB/T1733-1993 and GB/T1771-2007, test panels for water resistance and neutral salt spray resistance were prepared respectively to test the anti-corrosion properties of self-developed thermal insulation coatings and matching acrylic polyurethane topcoats. The supporting scheme is shown in Table 3. For each protection scheme, 2 test plates are prepared, one for measuring water resistance and one for measuring salt spray resistance. Among them, in the supporting protection scheme B, the interface agent is a kind of adhesive with two-way affinity, which has strong adhesion and can be used to improve the adhesion between the thermal insulation coating and the topcoat.

 

Table 3 Design scheme of supporting protective surface layer of thermal insulation coating

 

 

After a certain experimental period, take out the test plate and observe whether there are bubbling, cracking and other phenomena on the surface. The test results are shown in Table 4.

 

Table 4 Test results of water resistance and salt spray resistance of the supporting protection scheme of thermal insulation coatings

 

 

It can be seen from Table 4 that in Scheme A, the surface of the dry thermal insulation coating was directly painted with acrylic protective topcoat. After 24 hours of the water resistance test, dense small blisters appeared on the surface, and large blisters formed in local areas. This is because the thermal insulation coating is in a loose and porous state, the protective topcoat and the base layer are not firmly bonded, and the pressure generated by the evaporation of water causes the coating film to bubble. Based on the above problems, Scheme B and Scheme C add a bidirectional adhesive interface agent between the thermal insulation coating and the protective surface layer. One is to improve the bonding strength of the protective surface layer, and the other is to prevent the protective surface layer from penetrating the thermal insulation coating. Excessive permeability leads to a decrease in thermal insulation performance. It can be seen that after the coatings of the two schemes are resistant to water for 96h and salt spray for 2000h, there are no bubbling and cracking on the coating surface, indicating the feasibility of the matching scheme.

 

2.2 Calculation of thermal insulation layer thickness

Calculate the thickness of the thermal insulation coating for repair according to the on-site working conditions. Generally, the thickness of the insulation layer and the outer surface temperature are calculated according to GB50264-2013. The calculation formulas are shown in formulas (1) and (2). Here, according to the designed thickness of the insulation layer, the outer surface temperature ts of the insulation layer and the heat dissipation loss Q can be reversed. .

 

 

In the formula: D0—outer diameter of equipment or pipeline, m; D1—outer diameter of insulation layer, m; t0—outer surface temperature of equipment or pipeline, ℃; ts—outer surface temperature of insulation layer, ℃; ta—ambient temperature, ℃ ;λ—thermal conductivity of thermal insulation material, W/(m·K); as—heat transfer coefficient (also known as heat release coefficient) between the outer surface of the insulation layer and air, W/(m2·K); Q—insulation per square meter Maximum allowable heat loss from the outer surface of the layer, W/m2.

 

According to the on-site working conditions and environment, in order to ensure the aesthetics after repair, the thickness of the original insulation layer should be kept consistent. The specific parameters are shown in Table 5.

 

Table 5 Specific parameters

 

 

According to the above formula, the outer surface temperature ts of the thermal insulation layer is calculated to be 47.02°C, and the heat dissipation loss Q is 197.9W/m2. The calculation results show that the temperature of the outer surface of the thermal insulation coating is reduced to 47℃ after repair, which meets the design requirement of preventing the surface temperature from being scalded below 60℃.

 

2.3 Design of thermal insulation layer

The following issues need to be considered in the design of the thermal insulation layer: (1) If the original thermal insulation layer primer has been partially damaged, a low surface treatment epoxy coating (MC-STE-1, CNOOC Changzhou Coating Chemical Research Institute Co., Ltd.) is planned to be used as the repair. Primer; (2) In order to make the original thermal insulation coating have good compatibility with the repaired thermal insulation coating, and to prevent problems such as cracking at the repair boundary due to differences in coatings, remove the loose old thermal insulation coating, and apply 2 coats to the junction. Interface agent; (3) In order to improve the adhesion of the thermal insulation coating, 80g/m2 medium alkali glass fiber mesh cloth is used to fix the thermal insulation coating through the mesh structure; (4) In order to improve the waterproof performance of the thermal insulation layer, in the thermal insulation The interface agent is brushed between the coating and the protective surface layer to increase the adhesion between the thermal insulation coating and the protective surface layer, while preventing the penetration of the protective surface layer into the thermal insulation coating. The protective topcoat should be applied as thinly as possible under the premise of meeting the requirements. To sum up, the repairing thermal insulation coating supporting system is shown in Figure 2.

 

 

Figure 2 Schematic diagram of the structure of the repairing thermal insulation coating