Common Process Safety Hazards
Hazards of Utilities. One of the philosophies behind Process Safety Management (PSM) is that each chemical process is unique. Therefore it is not possible to have a prescriptive standard that tells operating companies what to do. Instead, companies have to identify the unique hazards associated with their facility, and then implement corrective actions based on a risk-ranking methodology. For this reason, facilities covered by PSM standards have to conduct a series of Process Hazards Analyses (PHAs), often using the Hazard and Operability (HAZOP) methodology.
Yet many hazards, particularly to do with utilities, piping, valves and hoses, are really not all that different from one facility to another. Therefore, in order to save time during the PHA and also to improve the quality of the analysis, it is useful to list and evaluate some of these common hazards before the PHA meetings start.
This Safety Moment is the first in a series that describes some of these common hazards. The topic of this one is “Hazards of Utilities”.
Hazards of Utilities
Utility systems require special scrutiny during hazards analyses because they connect many different sections of a process. Some of the issues to do with utility-related hazards are discussed below.
Common Cause Failure
The first and most obvious problem, to do with utilities is that their failure will create simultaneous problems throughout the facility. A utility failure is an example of a common cause effect.
The catastrophe at Fukushima-Daiichi illustrates this phenomenon; a single event (a subsea seismic movement) caused the earthquake that wrecked the reactors and then caused the tsunami that knocked out the safety systems whose purpose was to protect against the effects of an earthquake.
A second problem to do with utilities is their potential for process contamination. On one refinery, for example, the highly toxic and corrosive chemical hydrogen fluoride (HF) 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 indicate 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,
Loss of power can also cause critical instruments to shut down. These instruments should be backed up with an Uninterruptable Power Supply (UPS).
Nitrogen has many uses on process facilities, including the inert-gas blanketing of tanks, equipment purging, and as a carrier for catalyst regeneration. Oxygen contamination of the nitrogen system could render it ineffective as an inert-purge medium, thus creating flammable mixtures.
The availability of nitrogen in large volumes in many facilities allows for its to be used as an emergency source of instrument air. But this strategy can have serious consequences. Instrument air systems often vent or leak into confined areas — the presence of nitrogen could create a serious breathing hazard. It is suggested that the following guidelines be considered when using nitrogen to back up the instrument air supply.
- Do not allow permanent connections between the nitrogen system and either the plant or instrument air systems.
- Utility nitrogen stations should be clearly marked and have special connectors and hoses which are not common to any other system. Universal air hose connections (crow’s foot) should not be used in nitrogen service.
- Locations where backup nitrogen is being used should be monitored and alarmed for low oxygen concentration; signs and barriers should be installed.
- Once the problem with the instrument air system has been resolved, the nitrogen to instrument air cross connection must be removed.
For the latest information to do with common process safety hazards as part of your Process Safety Management program, please check out our blog Plant Design and Operations.
The material provided here is extracted from Chapter 19 of the 2nd edition of Plant Design and Operations. Subsequent safety moments will discuss other common process safety hazards.