Safety Moment #17: Safe Limits

In this Safety Moment we discuss the topic of “safe limits” as used in process hazards analyses.

During hazard analyses one sometimes hears statements on the following lines.

  • Could high temperature cause an incident?
  • What is high temperature?
  • It is that temperature that could cause an incident.

In order to break circular reasoning of this type it is necessary to clearly and accurately define what is meant by safe limits. In the above discussion investigation may show that a chemical process becomes unstable above 210 C. We have assigned a number to the qualitative term “high temperature”.

Safe and operating limits for process hazards analyses.

But there can be more than one value for “high temperature”. The above sketch shows that high temperature can be:

  • The temperature that is outside the normal operating range.
  • The temperature that causes “trouble”, i.e., above which production problems such as off-spec materials may occur but the operation is still safe.
  • The temperature which creates an unsafe situation.
  • The temperature which is considered to be an emergency.

Therefore, before starting a hazards analysis it is important to define the operating and safe limits for all key variables. If that information is not available then it may be found that the discussions during the analysis help improve knowledge as to what constitutes safe limits.

discussion to do with the important topic of safe limits is provided in Chapter 1 of the book Process Risk and Reliability Management.