A short circuit developed in the wiring of a fire eye system on one of two 600 psig boilers serving a refinery. This triggered a rapid sequence of events that resulted in a one week shutdown of much of the refinery. Fortunately no one was injured and equipment damage was negligible.When the hydrocracker make-up compressor started slowing down, the control system at the reformer released the excess hydrogen to the flare. However, no increase flame was observed at the flare and the crude unit pressure went over 50 psig. The trouble was quickly diagnosed as a blockage in the flare system. The hydrocracker units were shutdown. Quick action by operating personnel prevented potentially catastrophic failures to the flare system and other operating equipment.No major leak occurred and within two hours, it was concluded that the molecular seal was plugged. Witnesses at the time of the upset observed debris flying out of the flare. A search revealed five or six ice boulders, each weighing 50 to 60 pounds at the base of the flare. Also the 3 inch centre steam pipe and nozzle was found in the area.