Thermal elongation of steel-in-steel direct-buried insulated steel pipe is generally compensated with bellows compensators. Compensator capacity shall be 1.1 to 1.2 times the thermal elongation of the pipe segment.
(2) When steel-in-steel direct-buried prefabricated steam pipe is arranged in an L- or Z-shape, natural compensation may be considered, but it is necessary to calculate whether the clearance inside the pipe meets requirements.
(3) Bellows compensators should preferably be of the direct-buried pressure-balanced type to reduce loads on fixed supports. Compensator material shall be SUS316L stainless steel, resistant to chloride-ion corrosion.
(4) Bellows compensators are best arranged symmetrically on both sides of a fixed support to balance the loads on the support and to facilitate drain devices.
(5) Fixed supports may be internal or external type according to calculated load magnitude.
(6) When the bearing capacity of the soil at the trench bottom is ≤80 kPa, a C15 plain-concrete bedding layer shall be added at the trench bottom to prevent uneven settlement of the buried pipe.
(7) The outer wall of the outer steel pipe of steel-in-steel steam insulated steel pipe is coated for anti-corrosion. From an economic standpoint, if high-temperature pipelines are not insulated, large heat losses will reduce the thermal efficiency of boilers, steam turbines, and the entire plant, lowering the economic performance of thermal power plants and being detrimental to effective energy utilization. Therefore the national electric power industry technical management regulations clearly stipulate: “Steam pipes, water pipes, and flanges or stop valves on pipelines with temperatures above 50°C shall all be insulated.” To save energy and reduce heat dissipated to the surroundings by thermal equipment, pipelines, and their accessories, national standard GB4272-84 stipulates that the surface temperature of insulated objects in thermal power plants shall not exceed 50°C. Most heat-source plants in the provincial capital have begun ignition and boiler warming, and steam-pipeline warming and steam supply are also being carried out in succession. These preparations are to ensure that boilers and pipelines can operate normally when formal heating begins.
4. Installation Precautions
(1) Joint insulation and outer-jacket welding may proceed only after carrier-pipe welding is complete and 100% X-ray inspection has been passed.
(2) The outer steel pipe of steel-in-steel insulated steel pipe shall be a product that has passed hydrostatic testing. After installation, an airtightness test shall be performed, typically at 0.2 MPa, holding pressure for 5 minutes with no leakage of the outer steel pipe as the acceptance criterion.
(3) For drain devices on steel-in-steel steam insulated pipe, the branch connection should be as close as possible to the fixed support to prevent damage from excessive thermal displacement. Drain pipes shall be provided with insulation and anti-corrosion measures.
