Our team, consisting of 1 local engineering consultant from Inner Mongolia and 2 engineers specializing in cold-resistant materials, conducted on-site inspections at 15 gas stations. Based on local soil reports (soil moisture content in grassland areas ranges from 18% to 22%, and soil pH values around mining areas are between 8.5 and 9.2) and meteorological data (an average of 35 days of strong winds per year), we developed the following plan for implementation:
1. Product Design (Design drawing numbers: DM-NM-2024-001 to DM-NM-2024-020)
Structure and Cold Resistance Design: The tank adopts a "double-layer steel wall + polyurethane insulation layer" structure. The inner wall is made of Q345R low-temperature toughness steel (8 mm thick, with an impact energy of ≥34 J at -40°C), while the outer wall is 6 mm thick. A 50-mm-thick high-density polyurethane insulation layer with a thermal conductivity of ≤0.024 W/(m·K) is inserted between the two layers to prevent diesel from freezing inside the tank in winter. The top of the tank is equipped with three DN100 connectors for the oil inlet pipe, oil outlet pipe, and level gauge, all of which use low-temperature resistant gaskets (made of nitrile rubber and silicone rubber composite, which do not harden at -40°C) to prevent leaks at low temperatures.
Anti-corrosion Design(for saline-alkali or slag soils): The inner wall is coated with food-grade epoxy resin (compliant with GB/T 17219 standards, with a dry film thickness of ≥150 μm) to prevent direct contact between the medium and the steel. The outer wall uses a three-layer anti-corrosion system consisting of "epoxy coal tar pitch primer + glass fiber cloth + epoxy coal tar pitch topcoat" (with a dry film thickness of ≥400 μm). Before painting, the surface is sandblasted and derusted to reach Sa2.5 standard. A 5-mm-thick galvanized steel plate is also welded to the bottom of the tank to protect it from damage caused by sharp impurities in the soil. Additionally, a 2-mm-thick HDPE anti-seepage film is wrapped around the outside of the tank to further prevent erosion by saline-alkali soils.
Safety Design: A 50-mm inspection channel is reserved between the double-layer tank walls, and fiber-optic leak sensors are installed (with a detection accuracy of 0.1 mL/h). The sensor signals are transmitted in real-time to the gas station's central control system and also connected to the customer's headquarters' smart management platform, enabling dual monitoring at both the site and the headquarters level. The tank foundation is constructed using a "reinforced concrete pad + crushed stone cushion" structure (with a pad thickness of 300 mm and a reinforcement ratio of 0.8%). A 200-mm-thick graded crushed stone layer is laid at the bottom of the pad to effectively compensate for seasonal settlement in grassland areas. Calculations show that the settlement of a single tank foundation can be controlled within 3 mm.
2. Production and Transportation (adapted for long-distance transportation in Inner Mongolia)
Production Process: Steel plates are rolled into cylindrical shapes using CNC plate rolling machines (with a rolling accuracy of ±1 mm). The double-layer tank walls are welded using fully automatic circumferential welders (with H08MnMoA welding wire suitable for low-temperature steel welding). After welding, 100% non-destructive testing is conducted (UT testing for the internal welds and RT testing at a 30% ratio; test report numbers: NDT-NM-2024-001 to NDT-NM-2024-060). All tanks must undergo "low-temperature sealing tests" before leaving the factory-they are placed in an environment chamber at -35°C for 48 hours, and compressed air at 0.1 MPa is introduced into the tanks. A pressure drop of ≤0.002 MPa is considered acceptable.
Transportation Plan: A single 50-m³ buried tank (including the insulation layer) weighs approximately 12 tons and has dimensions of Φ2.8m × 8.2m. Fifteen flatbed semi-trailers (with license plate numbers such as Meng AXXXXX and Meng BXXXXX) are used for transportation in five batches (one batch in April, May, and June, and two batches in July and August). To address the issues of strong winds and complex road conditions in some areas of Inner Mongolia, the tanks are wrapped in three layers of waterproof canvas, and a 100-mm-thick rubber cushion is placed at the bottom. Each vehicle is equipped with two local drivers familiar with the routes. The entire transportation process is monitored using a GPS positioning system to ensure that there are no collisions or corrosion during transport, and all 15 gas stations received their tanks on time.
3. Installation and Commissioning (avoiding the permafrost period to complete the work quickly)
Installation Team: Five professional installation teams, each consisting of 8 people (including 2 welders, 1 electrician, and 1 safety officer), were deployed to work simultaneously at the 15 gas stations. The installation process strictly follows the sequence of "site excavation → foundation pouring → tank hoisting → pipeline connection → leak detection → backfilling". The depth of site excavation is adjusted according to the local permafrost thickness (1.2 mm in Hohhot and Baotou, 1.5 mm in Ulanqab and Tongliao) to prevent damage to the tank walls caused by frost heave in winter. 25-ton truck cranes are used for hoisting, and special lifting equipment with rubber pads is employed to avoid damaging the anti-corrosion coating on the tank walls.
Commissioning and Acceptance: After the installation of each gas station is completed, three core tests are conducted: ① Double-layer tank wall leak testing (introducing compressed air at 0.05 MPa into the interlayer and maintaining the pressure for 24 hours without any pressure drop); ② Pipeline sealing testing (introducing gasoline at 0.1 MPa into the oil inlet and outlet pipes and maintaining the pressure for 12 hours without any leaks); ③ Low-temperature performance testing (filling the tank with 5 m³ of 35# diesel, allowing it to stand for 48 hours, and checking that the level gauge and valves do not freeze). Local emergency management departments and fire rescue teams are invited to participate in the acceptance process. All 20 tanks passed the acceptance tests on the first attempt and obtained the "Special Equipment Use Registration Certificate".