What will be the best thermal insulation material for insulation of pipe lines having temperature of 500℃. Mineral wool and Calcium Silicate can be used but here we have a constraint of both weight and space.
There is a well known but counter-intuitive result that for a given temperature, below a certain diameter, any insulation at all is worse than no insulation, because it conducts heat (poorly but better than air) to a larger external radiating surface.
Details should be in any good book on heat flow, but for common steam pipe temperatures, the critical diameter is of the order of 5mm.
So, below a certain pipe diameter, leave the pipe exposed to keep the radiating area small. We don't know from your question if your pipe dia can be small enough to allow this.