at present, at home and abroad, the development of thermal insulation pipeline directly buried laying technology has a history of more than 60 years. thermal pipeline directly buried laying generally adopts three laying methods: uncompensated laying, compensated laying and one-time compensated laying. thermal pipeline transport media can generally be divided into two types: high temperature steam and hot water.
non-compensated direct burial laying method: in the direct burial laying of heating pipeline, the friction force between soil and external surface of insulation pipe is used to fix the pipeline. the whole pipeline does not need to install compensator or compensating pipe fittings, so as to give full play to the plastic potential of steel and make the pipeline form a self-balanced state. pipeline does not need to be preheated before ditch backfilling.
compensated direct burial laying method: when the medium temperature of pipeline transportation is greater than or equal to 120 c, it is difficult for steel plasticity to make the pipeline form its own equilibrium state, and it is impossible to preheat the pipeline. therefore, compensated laying method should be adopted, and compensators and fixed piers should be installed on the pipeline.
one-time compensation direct burial laying method: this direct burial laying method uses one-time compensator to absorb a part of the expansion of pipeline in the preheating state during installation, thereby reducing the thermal stress at the highest service temperature. one-time compensator refers to the compensator which compensates once when directly buried pipeline is laid, and becomes a part of pipeline after welding and fixing. the one-time compensator should be selected according to the length of pipeline, the spacing of fixed piers, the thermal displacement of pipeline and the maximum compression of compensator. the compensation amount should be greater than the expansion of the friction length of the pipeline. two compensators can be connected in series on the straight line segment, and no fixed piers need to be installed in the middle. the disposable compensator needs to be insulated.
practice has proved that for thermal pipelines with specifications less than or equal to dn 500, direct burial without compensation can be adopted. for thermal pipes with specifications larger than dn 500, non-compensated cold installation can also be achieved by increasing pipe wall thickness. for insulation pipes with specifications greater than or equal to dn 200, spiral seam welded pipes with q235b material are generally used, and the thermal stress generated by heating of pipes should not be greater than the allowable stress. it is necessary to calculate soil friction, elastic force and maximum installation length.