Processing facilities use a variety of power sources: electric motors drive pumps, compressors, aerial cooler fans, reactor stirrers, and remotely actuated valves. Steam turbines drive many of the same equipment items. A loss of these power sources can sometimes cause an overpressure. For example, loss of electricity to a motor-driven cooling water pump can cause loss of cooling to a condenser; loss of power to a reflux pump can cause reflux failure. The Power Failure contingency is different from the contingencies (such as Loss of Reflux) it may initiate in that a single power failure may affect many power users simultaneously. Depending on the power distribution system, a single failure might cause loss of cooling to a partial condensing system and the shut-down of a compressor that takes suction from the uncondensed column overhead stream. This contingency may thereby generate a larger required relief load than either the loss of cooling or the blocked overhead outlet would alone.
The required relief flow rate in the power failure contingency is dependent on the particular effects of the potential failure. Therefore, no generally applicable method can be described.