fell off the back of a truck hey!?...Anywho, if you want to clean a 50ltr keg it's a bit tricky especially if your homebrew leaves residue in the bottom like most tend to do, some tend to turn harder and cake on the bottom of the keg more than others to. My family runs a cider house and they use a high preassure hot water cleaner specially made for the job. The idea is the fast flow of hot water breaks down the scum and it sterilises it all at the same time. I assume you don't want to purchase one of those? So the other alternative is to fill the keg with boiling water or better still, boil some water in it, let it boil for a bit and then tip out the water (on your neighbours lawn if their giving you the shits
). other than that, not much else you can do. I wouldn't get to concerned about it being super clean as long as there's no mold in it, it should be fine and your beer should keep.
As for filling it. Depends on what gear you have, there's a few ways you can do it.
1st method. (should mention I've not done it this way before) get a 19ltr corney keg and fill that with your brew, put the lid on and do a liquid transfer through your beer out valve (make sure you have your coupler attached) it's kinda like filling a bottle from a keg, same principle. Why through the beer out valve? because it has a tube that runs all the way to the bottom and this will prevent the beer from splashing and mixing with any O2 that maybe inside the keg. or you can purge the keg before filling.
2nd method, is to remove the spear and cap completely and fill using a hose, this method requires you know how to remove the spear and cap, tools you'll need...a keg top remover which you can buy online or a pipe wrench and a small multi grip. here's what you do, with the multi grips push down on the rubber seal till you can grab the metal bit of the spear, lock the multi grip to the head and then lift and turn 'CLOCKWISE" Yes I said CLOCKWISE!! you should feel it turn a tiny bit and I mean a tiny bit (5mm) once you've done this remove the multi grips and the spear will fall away into the keg. The next step is to use the pipe wrench and open the cap. this is done in the normal fashion by turning ANTI CLOCKWISE. once you have this done you can clean and fill till your hearts content.
Also when you have the spear out you will notice the spear has a 'hook' like lock on it to lock it into the cap, if you grind this off you won't need to remove the spear first in the future. but note this is to prevent the spear from shooting through your face if for some reason there's a terminal failure of the spear, should be fine though. I've done it and never had an issue.
good luck with it and persist. took me a while to figure out how to do it but it's actually pretty simple once you know how.
