What happens when sga and pga reach limits?
SGA: nothing serious, it is a fix sized memory area, once it gets full, the database reuses memory that is rarely accessed using LRU (least recently used) algorithm

Memory Architecture

PGA: Once PGA_AGGREGATE_LIMIT is reached, the database aborts calls of sessions having the most untunable (like PL/SQL collections) PGA memory, and those sessions receive ORA-04036 error. If that does not help and PGA usage is still above the limit, the database may even kill those sessions:

PGA_AGGREGATE_LIMIT


  Actions Taken When PGA_AGGREGATE_LIMIT is Exceeded
  
  Parallel queries will be treated as a unit. First, the sessions that
  are using the most untunable memory will have their calls aborted.
  Then, if the total PGA memory usage is still over the limit, the
  sessions that are using the most untunable memory will be terminated.
  
  SYS processes and background processes other than job queue processes
  will not be subjected to any of the actions described in this section.
  Instead, if they are using the most untunable memory, they will
  periodically write a brief summary of their PGA usage to a trace file.


(19327352832 + 12884901888)/1024/1024/1024 = 30

You have 32 GB memory in your server, you have allocated 30 GB memory to the database, and there is nothing else running on it, as you said. With proper configuration and sizing, you should not see swapping under normal circumstances:


hugepages for decreasing page tables 
filesystemio_options=setall for direct I/O and avoiding filesystem caching
vm.swappiness set to 10 or even lower
sizing: reasonable amount of database sessions/processes


This does not mean the system will never swap. For example:


SYS sessions and background processes are not limited by PGA_AGGREGATE_LIMIT. You can still have rogue procesesses consuming PGA memory above the limit causing swapping.
If you use and unreasonably sized connection pool, like 100s or even 1000s of database processes, those processes have some private memory that is not part of PGA, so they add up on top of the SGA + PGA limit. This is especially significant on AIX, fortunately not so much on Linux.