Ghamsa,
What I have done previously is to take the vapor phase of the reboiler outlet stream at relieving pressure as the relief rate.
Upon reflux failure, what happens is that the liquid-vapor traffic inside column is disturbed. So the vapors from reboiler rising inside the can not condense against a cold reflux and hence build up at top.
Another way could be taking the vapor load right above the feed tray. That vapor will see no reflux to condense against, so it builds up at top.
regards