Credible relief scenarios

If the decision has been made to install an emergency relief system, then further information will be required about the reacting system, both under normal and 

Is there scope for obtaining a smaller relief system size by designing out maloperations or system failures by means of inherent safety or otherwise (see Annex 1) 

Is relief system design for the worst case scenario acceptable in terms of confidence in the design, cost and reliability  

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At this stage, a number of credible maloperations will have been defined that can lead to vessel over-pressurisation. In order to cope with all the credible runaway scenarios, the relief system will need to be sized for the "worst case runaway" reaction that can occur, and this is normally the maloperation that will give rise to the highest rate of temperature and/or pressure rise over the relief range. 

Now we have two related failures occurring at the exart same time. The power failure stops the pump and thus stops the cooling water to the column condenser. This causes the column to go into relief, which then causes the operator to react, initiating the second failure directly related to the first failure. This is a perfectly credible relieving scenario, and the calculation of relieving capacity should be based on the amount of vapour generated by the heat provided by a wide-open steam valve without taking into account the amount of vapour that can be condensed ... 

Containment within the vessel for the credible worst-case scenario reducing the design requirements for the emergency relief system this step is frequently too expensive and impractical in a multipurpose facility. 

For the purposes of the Workbook, the worst case scenario is the credible combination of equipment failures and maloperations that gives rise to the largest calculated relief size compared with other credible scenarios. See Chapter 3. The worst case scenario is the basis for the relief system design. 

Singh, J., Batch Runaway Reaction Relief Re-evaluation of credible Scenario, Process Safety Prog., Vol. 16, No. 4, pp. 255-261, 1997. 

New process equipment installed within 7.6 meters from the grade/ground level will require pressure relief devices due to fire scenario. For revamp cases, it may be advantageous to place vessels on platforms at >7.6 meters if fire case relief load is most credible and bottlenecks the existing flare system. Fire circle or zone is defined as the maximum affected area during any equipment fire in the facility. API 521 (2014) defines its area as 230 to 460 m. Addition of process equipment inside an existing fire circle may increase the fire circle size. Hence, care shall be taken to review the fire circle size with each equipment addition. It will impact the peak relief load during the fire scenario. 

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