Cantclimbtom said:
Down jumaring any more than the very shortest section is a horrible task.
I had to do it in an icing-sugar silo at Bassett's sweet factory, abseiling in through the loading hopper. The risk assessment hadn't taken sugar, friction and heat into consideration, and we found after only three metres that our descenders no longer worked - in either direction. Essentially we were glued to the ropes. The silo was 30 metres high, and the only way out was at the bottom, currently blocked with a drift of icing-sugar. So we had to reverse-prussik the entire thing, cleaning sugar off the walls with plastic brooms, and then dig our way out at the bottom, shoveling it through the outlet to a guy in a cherry-picker with a load of bin-bags. Hilarious.
Our misery was relieved when we found out we'd qualified for discount rates at the factory shop due to our heroic performance (they thought us insane), and after a quick shower, staggered out with a giant crate of almost-free chocolate, Liquorice Allsorts and Jelly Babies.
So yes, learn to reverse-prussik whenever possible