Detection of thermal insulation materials for large LNG storage tanks and pipelines
Large LNG storage tanks are important equipment for LNG receiving stations, and their capacity is generally above 4w cubic meters. At present, the maximum capacity of domestic LNG receiving station storage tanks is 200,000 cubic meters, and the common one is 160,000 cubic meters. Unlike pressure vessels, large LNG storage tanks are large-pressure, low-temperature vessels, which reduce the temperature inside the tank to below -162 °C under normal pressure to achieve the function of storing LNG. Large LNG storage tanks were originally designed to achieve peak shaving storage of natural gas transmission and distribution through this “large cryogenic atmospheric pressure vessel”.
Large LNG storage tanks are mainly single-contained tanks, double-contained tanks and all-contained tanks. They are double-layer tanks. The inner and outer tanks are filled with insulation materials. The main insulation materials are foam glass brick, expanded perlite, and elastic glass fiber felt. , glass wool, etc. The thermal insulation materials used in the LNG conveying pipeline mainly include: rubber, polyisocyanurate PIR, polyurethane PU, diene, aerogel felt and the like.
Obviously, the performance evaluation of thermal insulation materials is especially necessary and important.
The National Glass Fiber Product Quality Supervision and Inspection Center (CSF Quality Inspection Center) has undertaken the testing of insulation materials for many LNG receiving stations in China and has extensive experience in testing the insulation materials in this field. According to the testing experience of the CSG Quality Inspection Center and the performance of various cold insulation materials used in large LNG storage tanks, the main performance and evaluation methods are: thermal insulation performance (thermal conductivity at low temperature), mechanical properties (resistance Compressive strength, tensile strength, flexural strength, especially at low temperatures, moisture permeability (moisture permeability coefficient, wet resistance factor, wet flow density), water content, water absorption, etc., the main test methods are:
I. Determination of thermal conductivity of thermal insulation materials at low temperature
The thermal conductivity of the cold insulation material at low temperatures (-165 ° C, -150 ° C, -120 ° C, -100 ° C, -50 ° C, -40 ° C) was tested according to methods such as GB/T 10294-2008 and ASTM C177. Thermal conductivity is the most basic performance of thermal insulation materials. The minimum requirements for thermal conductivity are also specified in the product standards of various thermal insulation materials, such as foam glass ASTM C552, expanded perlite JC/T 209, rubber and plastic GB/T 17794, Isocyanurate GB/T 25997-2010, diene foam product JC/T 2495-2018, polyurethane GB/T 21558-2008, and the like.
GB/T 10294, ASTM C177, ISO 8302 and BS EN12667 are the most common methods for testing the thermal conductivity of materials, namely the Guarded hot plate apparatus. The test process establishes a one-dimensional steady-state transfer between hot and cold plates. The thermal process, which measures the thermal conductivity by the Fourier one-dimensional steady-state heat transfer model by measuring the energy, temperature gradient, and sample thickness applied to the hot plate. Because it directly measures the energy in the heat transfer process, the protective hot plate method is an absolute method and is often used as an arbitration method.
The Heat flow meter apparatus is also one of the commonly used methods for testing the thermal conductivity of materials. It simulates a one-dimensional steady-state heat transfer process by measuring the heat flux density, the hot and cold surface temperatures of the sample, and the sample. Thickness, a test method for calculating the thermal conductivity by Fourier one-dimensional steady-state heat transfer model. Testing standards: GB/T 10295-2008, ASTM C518-15, BS EN 12667-2001, ISO 8301-1991.
2, the evaluation method of mechanical properties of thermal insulation materials
The cold-preserving materials that need to be load-bearing are mainly foam glass bricks, asphalt linoleum and the like used for cold preservation at the bottom of the tank. The compressive strength of the foamed glass under normal temperature and low temperature conditions (e.g., -165 ° C) can be tested according to ASTM C165, and the flexural strength of the foamed glass at normal temperature is tested according to GB/T 5486 and ASTM C203. Glass fiber mats and elastic felt materials for tank walls and tank tops also need to have certain tensile strength and elasticity. The tensile strength and compression strength of glass fiber mats and elastic felts at normal temperature and low temperature can be tested according to GB/T 17911. Bomb rate. For the environment in which the insulating material for LNG is used, the compressive strength and tensile strength tested at low temperatures can better reflect the performance of the material. In the current test method, the low temperature environment is mainly achieved by liquid nitrogen (-196 ° C), so The low temperature environment can be as low as -196 °C during the test.
3, the evaluation method of moisture permeability of thermal insulation materials
There are a large number of cells in the heat insulating material. At low temperatures, the ambient moisture in the open cell structure is frozen into ice, causing the material to harden, which reduces or loses the thermal insulation properties, while the closed cell material (such as foam glass, rubber, polyurethane) PU, polyisocyanurate PIR, diene, etc.) well block the transmission of moisture. Therefore, the moisture permeability of the material is also important for the cold retention performance of the LNG storage tank. The test standards for moisture permeability are GB/T 17146-2015 and ASTM E96/E96M-16. During the test, the test piece is sealed on the test cup opening equipped with a dryer or a saturated solution to form a test component. In a constant temperature and humidity laboratory environment, water vapor will flow through the test piece due to the difference in water vapor pressure between the test cup and the laboratory environment. By periodically weighing the weight of the component, the water vapor in a steady state can be measured. The test results of flow and moisture permeability have several expressions, usually three types: moisture permeability coefficient, moisture resistance factor and wet flow density.
4. Test method for moisture content and water absorption rate of thermal insulation materials
“Water” needs to be avoided as much as possible in the use of thermal insulation materials. All insulation materials used in large LNG storage tanks have tests for moisture content and water absorption. The water content is generally tested by the drying method. The test standard GB/T 20313-2006/ISO 12570:2000. For example, the expanded perlite JC/T 209 refers to GB/T 20313 to test its mass moisture content, the index is ≤2.0%. . There are various methods for water absorption. For example, the water absorption of foam glass is tested by ASTM C552, and the vacuum water absorption of rubber and plastic is tested by GB/T 17794-2008.
V. Other performance: density and granularity
Density is the basic parameter of various materials. For example, expanded perlite JC/T 209 specifies the bulk density and density uniformity of different types of products. The density of foam glass is generally tested according to ASTM C303; particle size, generally for expanded perlite JC /T 209, the test results of the particle size were obtained by the sieving method, and the test results were expressed as: 4.75 mm mesh sieve residue and 0.150 mm mesh throughput.
The performance testing of materials is particularly important and necessary, but in engineering projects, the sampling plan and inspection plan of materials are also an important part of engineering quality control. To a certain extent, it is more important than the testing itself.