(78b) High Integrity Protective System Design Using A Risk Based Approach

With the update of the High Integrity Protective System (HIPS) methodology by API in 2007, Dow Chemical updated its internal work process to apply Dow's risk based work processes to HIPS design, application and evaluation. Rather than just focusing on the vessel related consequences of substituting a HIPS for a pressure relief device (PRD), the work process calls for stepping back and looking at the overall scenario and all the potential consequence outcomes. Using a risk based methodology requires looking at a much wider consequence potential than what API has traditionally focused on.

By using Layer Of Protection Analysis (LOPA) to evaluate the consequences of the overpressure scenarios and determine the consequence severity factor or LOPA Target Factor for each scenario, the wider consequence potential is covered. The High Integrity Protection System (HIPS) safety integrity level (SIL) is determined by the highest risk scenario, which is typically the scenario with the greatest LOPA consequence severity factor. The resulting SIL level for the HIPS is adjusted to close the HIPS scenario risk gap. Other applicable LOPA independent Protection Layers (IPLs) are included in the LOPA risk evaluation. The HIPS design is verified by calculation to meet the required SIL level.

There are pressure relief device (PRD) services for which PRDs can't deliver the over-pressure protection that we need and expect. The updated work process also identified these as key opportunities for using a HIPS:

- a conventional PRD is not practical or possible to use - a conventional PRD will not be reliable - a conventional PRD will work but will result in high treatment cost Each of these cases is discussed in the paper.

The overall result is an efficient process that links together the existing work processes for conventional relief design, Layers of Protection Analysis (LOPA) and Safety Instrumented Systems (SIS). This produces a risk based method for applying and designing protection layers for use in a HIPS. This paper gives an overview of this work process with some application examples and describes how it is used for improving safety while reducing risks, and potentially reducing costs.