Manju,
You are quoting from only one project or engineering company's procedures.
Critical line selection is a matter for the owner and often the engineering contractor. There is no global agreement on this or when computer analysis is required, and client's requirements differ dramatically. Some codes give guidance, but generally not too specific.
Pegah,
Your stress range is 5 to 180 degress, regardless of the installation temp, so calcs should be based on that range. You might for instance do 21 to 5 and 21 to 180 deg. loadcases then add stresses, and use 21 to 180 deg. for your restraint / equipment loads.