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Hazards of Utilities

May 20, 2014

Steam Leak

The material in this post is extracted from Chapter 17 – Common Hazards – of the book Plant Design and Operations.

Utility systems can create many difficult-to-predict hazards because they connect many different sections of a process. 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. They generate common cause effects. 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.

Process Contamination

A second, and more subtle, 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), which is discussed in Chapter 5, 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.,

Loss of power can also cause critical instruments to shut down. These instruments should be backed up with an Uninterruptable Power Supply (UPS).

Nitrogen

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 must be avoided as such contamination could create a mixture that is no longer an inert medium.

Its availability in large volumes in many facilities also allows for its use as an emergency source of instrument air. However, this practice 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, and signs and barriers installed.
  • Once the problem with the instrument air system has been resolved, the nitrogen to instrument air cross connection must be removed.

Reverse Flow to a Utility Header

The hazards associated with (unexpected) reverse flow can be very serious. One particularly troublesome reverse flow scenario is ‘Reverse flow to a utility header’ as illustrated in Figure 17.3, which shows two lines. 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).

Figure 17.3
Reverse Flow to a Utility Header

Reverse Flow to a Utility

The hazard scenario is as follows:

  • The pressure in the utility header falls (say, due to an operating upset) so that it is lower than the pressure in the process line.
  • The check valve fails to fully close.
  • Process chemicals flow into the utility header from the process line.
  • Process chemicals are then distributed to many other locations in the facility via the utility header.

To make matters worse, this scenario is ‘memory-less’, i.e., once the pressures revert to normal there is no indication as to what happened. Identification of the source of the contamination can be particularly difficult if the process chemical that has entered the utility header is used in many parts of the overall process.

Survivability of Utilities

Many utilities must survive a catastrophic event such as a major explosion or fire. Frequently, such an incident can destroy critical utilities header containing electrical cables, cooling water lines, and steam pipes. Such an incident is a very serious common cause effect. The incident can trigger a domino event whereby a small incident eventually results in a major catastrophe. The effect is magnified if emergency systems such as the firewater header are also damaged.

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