Abstract:According to thermodynamics, the constraint conditions of the maximal allowance excavation length under shut down was confirmed and the mathematical model for the first time was established. The analysis results showed that the allowance excavation length of shutdown pipeline depended on several factors including repairing season, shutdown repairing time, the oil temperature at the outlet of heating station. Some simulation and calculation were did taking China east pipeline from Tai Yang Sheng station to Lin Yuan station, whose distance was 47.8km, as research object, allowance excavation length was largest in May and October, and when maintenance time was less than 6 hours, the whole line can be excavated in theory. November was not fit for excavation to maintenance under shut down. It needed to enhance the oil temperature at Tai Yang Sheng station in April. At the same time, the maximal allowance excavation length was restricted by the starting point of excavation.