Little story.

Was around 14-15 years old (maybe weighed 110 lbs), putting on NH3 with chisel plow, but the hitch pin came out on NH3 wagon= thus unhooked the quick coupler at rear of chisel plow.

Backed up and got the pin back in and #9 wire in pin hole. 
Next, released any NH3 on chisel plow and its lines/hoses = open valve and left open.
Shutoff valve at tank, then open little bleed valve on "jumper hose" between tank and chisel plow quick coupler(using leather gloves). Let it air out and empty.  

Now, here are my mistakes: 1: Didn't have a face shield or proper gloves.
                                       2. After all the NH3 was out of hose(or so I thought) I shut the bleed screw, BEFORE I tried to reinstall the hose into quick coupler.
 


NOTE: sun was shining good that day, too=constantly warming up that black, jumper hose.



 Next, I get on the ground and try to pull back on chisel plow quick coupler and at the same thing, try and push inward on big, black NH3 hose, to connect them. I couldn't get it, so I tried again and again, finally I got my shoulder down and right arm pushing on hose and left arm pulling on chisel plow coupler = my face was right at quick coupler. I pushed and pulled with everything I had (had my eyes closed=I was straining a lot), AND it was enough to open the quick coupler tip , on the black hose, and some NH3 gas came out!

Ran back to water tank and rinsed off, quickly. 
 I was ok, BUT I figured out, that I shouldn't have closed that bleed valve, as the sun was warming up the black hose and the fumes/gas that was still in there, had created some pressure and that is what got me.

Released/opened bleeder valve, wiggled the heck out of the black NH3 hose(trying to remove any gas) and finally got it reattached.

Note: This Quick coupler had never been used before, because we normally, just shutoff at the NH3 tank, at open the valve on the chisel plow(to empty the lines), then unhook/unscrew at the tank and wrap the big black jumper hose, down and on top of the chisel plow; to swap out tanks, so I didn't really know the correct procedure, UNTIL this happened.

It only takes once, with this stuff. I was lucky, Hopefully this story will help you understand a little bit of things that can happen.

Other times, Have had lower 3 point hitch break, pulling COOP rented applicator, and it twisted and the toolbar crawled up the outer dual and thus that puts that side off the applicator knives, up in the air, with NH3 going everywhere=I got it shut off, quickly(pull rope shutoff) and left the cab quickly. I did get stopped before the toolbar hit the cab/fender, luckily.