Safety Moment #17: Safe Limits

Safe limits for process safety management

The material shown here has been extracted from the ebook 52 Process Safety Moments and from the book Process Risk and Reliability Management .

During hazard analyses statements such as the following are commonly heard.

  • 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 210C. 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, which can be used for any process variable such as level, pressure or flow rate, 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.

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Copyright © Ian Sutton. 2018. All Rights Reserved.