By definition, drain valve(s) are located at the lowest point of the installation. If there are multiple sections with low elevation, drain valves need to be provided for each section.
Definition and location of flushing connections depend on the size and layout of piping, and need to be with a sufficiently large diameter to ensure the required flushing velocities (general figure is around 5 fps, but this may vary depending on the expected type and size of particles in the system and difficulty of removal - see also attached PDF). Fittings that can accumulate dirt and corrosion products and cause blockage during flushing, should be removed prior to flushing activities.
Depending on cleanliness of the flushing fluid, a temporary strainer may need to be fitted in the filling line.
