500C Steam Pipe Wall Thickness

For a quick estimate, use Barlow's formula: P = 2St/D, where P = pressure, S = allowable stress, t = thickness, and D = inside diameter. If your material is 90,000 psi, and the safety factor is 4:1, the allowable stress will be 22,500 psi. A fully correct formula has a few other terms, which are usually minor except for extreme conditions, but this will give you a start.

Beware that any flat or other noncylindrical surfaces will require a different calculation, so you must consider the total geometry of the pressure-containing parts. You will also need to check how the temperature affects the strength of the chosen material; this will typically decrease its allowable stress.