I almost killed myself today - I think!. I needed to strip on the grain. So yesterday in preparation for a full day stripping today, I duly filled my thumper (full SS beer keg) with slop and put clear wort in the pot (full SS beer keg). Turned on my pot today. Left it heat up. I was in and out of the barn (where I still) keeping an eye on the still. One hour went by with the gas burner full swing under the pot, and for some bizarre reason the thumper never started thumping. Fuck me, the next thing the steam started bursting out the gasket where the copper pipe is attached to the pot. I realised there was a blockage some where in the pipes, so I quickly turned off the gas and I got the spanner and opened up the nut holding the copper pipe onto the pot to release the pressure. The thing exploded with gas and liquid all over the place. There was some pressure built up in the pot.

I did not mind as I was glad the SS keg pot itself did not explode. If this happened I would not be typing this tonight.
Anyway I went away for a while and let things cool down. Came back, opened up my copper piping over the pot and loosened the nut holding the piping to the thumper. Kneeled down and blew into the pipe from the pot end - not a friggin budge. Realised the grain/slop in the thumper must have really settled too well over night and the pressure coming from the pot was just not strong enough to clear the blockage at the bottom of the pipe going into the bottom of the thumper. So I got a stick, opened the 4 inch port hole in the thumper and stirred the slop. Then I started blowing down the pipe, stirred some more, and after a few goes I managed to blow through the pipe and the thumper started bubbling. Put it all back together and let her do her job.
I am never going to let the slop settle in the thumper again over night. Instead as the pot is heating up I will put the slop into the thumper.
My kids were around the barn (where I still) today so I am feeling lucky that no one was injured. There was some blow out of the pot when I opened the nut. Anyway we live and learn.
Moral of the story - dont let the slop in the thumper at all, put it in just before you run your still.
Also, if running a pot / thumper make sure to use your ears - a thumping thumper is good, whereas a silent thumper is bad !
Hope this saves others from the same mistake.
Jimy