In 2007, API 521/ISO 23251 [AANSI/API Standard 521, Petroleum and Natural Gas Industries Pressure-Relieving and Depressuring Systems, 2007] published guidance on the use of high integrity protective systems (HIPS). The Dow Chemical Company (Dow) 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 Dow has traditionally focused on for the design of relief systems.
By using a very robust layer of protection analysis (LOPA) process to evaluate the consequences of the overpressure scenarios and to determine the consequence severity factor or LOPA target factor for each scenario (Dow has seven levels), the wider range of consequences is covered. The 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) included in the LOPA HIPS scenario risk evaluation are validated. The HIPS design is verified by calculation to meet the required SIL level.
There are PRD services for which PRDs cannot deliver the overpressure protection that Dow needs and expects. The updated work process also identifies key opportunities for using a HIPS when: a conventional PRD is not practical or possible, a conventional PRD will not be reliable or a conventional PRD will work but will result in high treatment cost. Each of these three opportunities is discussed.
The overall result is an efficient process that links together the existing work processes for conventional relief design, LOPA, and safety instrumented systems. This produces a risk-based method for applying and designing protection layers into a HIPS. This article gives an overview of this work process with some application examples and describes how this work process is used to improve safety while reducing risks and potentially reducing costs. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011