Velocities of stationary curved flames propagating in tubes with ideally adiabatic and slip walls are studied in the model of a thin flame front. It is found that for a tube width larger than some critical value a flame front acquires a curved shape and propagates with the velocity exceeding the velocity of a planar front. The curved flame velocity depends on the expansion coefficient and on the tube width. However, there is a maximal possible velocity of a curved stationary two-dimensional flame of a fixed expansion coefficient, which cannot be exceeded with the increase of the tube width.