On the basis of considering steam density variation

a model of optimal steam velocity and thickness of thermal insulation layer has been established. The target function of the model is the annual overall cost of the pipeline

including pressure reduction due to friction

temperature drop due to heat dissipation

depreciation charge of the pipeline proper and thermal insulation materials

as well as expenses of mainlenance etc. At the same time

corresponding solution-finding software has been developed by considering variation of steam density. The practical calculation results show

there is an optimal steam velocity

which is decreasing along with pressure-and density-increasing. Under Present economic conditions

the optimal flow velocity of medium-pressure steam is only 10～20 m/s. If the pipeline is much longer

it is necessary to consider the variation of steam density during the time of design and calculation.