Safety Moment #13: Common Process Safety Hazards (Part 2)

Process safety common hazards

Common Process Safety Hazards

One of the philosophies of 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 hazards associated with their facility, and then implement corrective actions based on a risk-ranking methodology. For this reason, facilities covered by PSM standards generally conduct a series of Process Hazards Analyses (PHAs), often using the Hazard and Operability (HAZOP) methodology.

Yet many hazards, particularly to do with utilities, 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. Our first post in this series was Safety Moment #10: Common Process Safety Hazards (Part 1) in which we discussed hazards associated with utilities, including common cause failures, process contamination, electrical power failure and the use of nitrogen.

We continue the discussion in this post with some additional thoughts to do with the hazards of utilities and emergency response equipment.

Reverse Flow to a Utility Header

The hazards associated with (unexpected) reverse flow from the process into a utility header can be very serious. This scenario is illustrated in the sketch, 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, which is at a higher pressure than the process, flows into the process through a check valve (with block valves on either side of it). 

Reverse Flow to a Utility Header

The hazard scenario is as follows:

  • The pressure in the utility header falls due to an operating upset so that its pressure 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

Process safety utilitiesMany utilities must survive a catastrophic event such as a major explosion or fire so that emergency response systems continue to function. For example, during a fire it is often important to keep cooling water flowing (to cool process operations), to maintain electrical power for critical pumps and compressors, and keep the steam header pressure up — once more to keep critical turbines running. Yet such an incident can destroy critical utilities header containing electrical cables, cooling water lines, and steam pipes. This is a common cause effect that can trigger domino events that make the original problem much worse.

Survivability of Emergency Systems

Common cause effect process hazards analysisIf an explosion or large fire occurs it is even more important that the emergency systems, particularly firewater and backup electrical power, remain operable. (For this reason firewater headers are often buried.)

The common cause effect that destroyed both the operating equipment and the backup systems was a major factor in making the Fukushima-Daiichi event so serious, as discussed in Safety Moment #10.

Blog

Safety moment process safety managementFor the latest information to do with common process safety hazards please check out our blog Plant Design and Operations.



 

Reference Material

Common process safety hazardsThe 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.