A total electrical power failure can affect both process equipment and utilities, resulting in multiple discharges to the relief header.
Determination of Credibility
A detailed evaluation of all electrical power supplies to the facility should be performed to determine the possibility of a total power failure. Large process facilities typically have multiple electrical power supplies to increase reliability. However, one should consider the possibility that a single event could affect multiple electrical power supplies. Experience indicates that most operating facilities have suffered a total loss of electrical power at some time.
Determination of Impact on the Collection and Disposal System
All electrically-driven machinery such as pumps, compressors, and air coolers will shutdown. Consequently, other utility systems such as cooling water, instrument air, and shutdown systems may be affected.
In many installations, electrically driven machinery has an installed spare unit using a different energy source. Note that API Recommended Practice 521 states that such spares are not considered 100% reliable; therefore, credit should not normally be taken for their operation. However, where two or more units are in parallel, have different energy sources, and normally operate simultaneously, credit may be taken for a partial failure (e.g., the electric pump fails, but the steam driven pump continues to operate).
Care is required when taking credit for the continued operation of machinery driven by alternate energy sources, because the power failure may affect auxiliary equipment (e.g., lube oil pumps) that would cause the machinery to shut down. In addition, equipment driven by an alternate energy source may shut down due to consequences of the power failure. For example, the power failure may cause a high level in a compressor suction drum, causing the compressor to shut down.
Finally, careful consideration should be given to collateral failures that may mitigate the effects of a total power failure. Typical examples include loss of feed, loss of heat input via forced circulation reboilers, and loss of heat medium circulation.