P-Trap Requirements on Clothes Washer Drain?

[Mitch H.]

I recently had to do some research (read: cutting into the wall) on my clothes washer drain.  I found that there is no p-trap, either in the wall or below grade in the crawl space.  The washer plumbing drains straight down in the crawlspace, and then does a 90 degree turn into the main line.  

What would the plumbers here recommend in this situation and what might be the code requirements?

[William Morrison]

Interesting read.  How old is the construction?  I ask because if you have no smell you might have a running trap.  They are illegal in most places now along with S traps and certainly in the national codes the local jurisdictions use as models.  (some local exceptions)
The distance of vent from the trap maybe limited by code.  But the function is limited by the diameter of the drain line, the slope and the length of the drop (added velocity=greater siphon).  The depth of the trap (a deep seal trap) can help.  Like the trap on your AC's condensate drain, the depth determines how much negative or positive pressure it can handle before it siphons completely.
Bottom line you should have a trap.