Process-related hazards are often initiated by problems with utility systems because a failure in the utility system can create multiple problems simultaneously at many different locations. Hence utility failures are often an example of a common cause effect. For example, if a nitrogen header may become contaminated with oxygen, thus creating a danger in many parts of the facility at the same time.
Examples of hazards that can be created by utility systems are provided below.
If process materials enter a utility header, serious problems can be created. On one refinery, for example, the highly toxic and corrosive chemical hydrogen fluoride (HF), which is also discussed in Safety Moment #40: The Missing Operating Procedure, leaked into the instrument air system. This had the effect of spreading HF all around the refinery; it was even being vented from instrument lines in the control rooms.
When a leak occurs between the process and one of the utility systems it is often difficult to track down the source of the leak. For example, it is quite common to place a hydrocarbon detector in the plume from a cooling tower. Then, if one of the process coolers or condensers leaks, the detector will detect the presence of hydrocarbons. The difficulty lies in knowing which of the equipment items is leaking.
Electrical Power Failure
Failure of the electrical system can lead to "High Pressure" in those cases where the utility is removing energy from the process. For example, an overhead fin-fan on a distillation column may serve to condense the overhead vapors from that column. Loss of power will cause the vapors to pass through the condenser without being cooled or condensed, thus creating a high pressure situation.
Many facilities have forced draft cooling towers. Once more, should there be a power failure these cooling towers will not operate, leading to the potential for high temperatures and pressures in the operating units.
Loss of power can also cause critical instruments to shut down. These instruments should be backed up with an Uninterruptable Power Supply (UPS).
Reverse Flow to a Utility Header
Related to the Process Contamination problem just discussed is the potential for reverse flow from the process into the utility system, as illustrated in the sketch. The top line is a utility such as nitrogen, steam, or service air. The lower line shows a process stream containing a hazardous chemical. In normal operation, the utility flows into the process through a check valve (with block valves on either side of it).
The hazard scenario is as follows:
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