Usually the surface blow pipe is perforated with holes along its top surface; however, when a scum pan is also employed, the holes are located along the bottom of the pipe surface.

The centerline of the pipe is normally situated at a distance from the bottom of the steam drum equal to approximately one fourth the diameter of the drum.

To ensure adequate blowdown, the aggregate cross-sectional area of these perforated holes must be equal to approximately twice the cross-sectional area of the pipe.

All of the above.