We had this same issue with our steam/water mix valve.....which is why we had to bail on using it. The open demand for steam.....meaning.....the boiler was allowed to flow through the water wide open, created a "venturi" effect that draws the boiler condensate through the pipe and into our mix valve. We were getting the same water look and smell as you.
What you'll probably have to do is this:
1.) Ensure the steam supply line coming off the steam main form the boiler is coming from the TOP of the steam main. Make sure it goes vertical for a foot or so at least. Many condensate cary-ove rissues are due to this alone.
2.) Make sure the pipe diameter is large enough that water isn't completely filling the pipe, and allows gas to pass an doesn't create the venturi.
3.) Barring the above, you may have to put a steam trap in the system just ahead of or at the point where the steam supply breaks off the main to go out to the keg washer. The closer to the keg washer the better. Condensate going through the pipe will fall through the trap, and should return to the boiler.
4.) REALLY throttle down the steam pressure going into the keg if there's a steam regulator somewhere on the washer or you may have to install one. If it is releasing the steam to the keg at a full 15 psi, it is dumping the entire line pressure all at once.
I think your problem is compounded by the fact that your boiler is probably like ours..............it heats really quickly but has no reservoir....no buffer, as it were.....and the steam line pressure drops considerably (almost to zero?) when the washer calls for steam. It drops very, very fast.
Anywho, bounce these things around and hit me back on my home E-mail for more discussion.........