Pressure to Part of a Circle

[ColStudent]

Hey guys I'm currently modeling the cross section of a pipe . I am trying to apply a pressure load to only part of the pipe (1/8 of it), but I can't seem to just select a quarter of the pipe, it selects the entire circumference. How can I apply the load to only the eightI want?

Thanks for the help.

 

[TGS4]

Partition the circle first.