For your immediate review, please download the latest free resource paper from PII, titled Understanding IPL Boundaries, written by Arthur M. Dowell, III, P.E. – PII Principal Engineer & Instructor, and being presented at the 14th Global Congress on Process Safety in Orlando, Florida.

From the Abstract:

Layer of protection analysis (LOPA) is a simplified risk assessment tool that has been in use for almost three decades. The technique has improved the focus on independent protection layers (IPLs) that can prevent the progression of an initiating cause to an undesired consequence (a scenario). An IPL must be capable of preventing the scenario from reaching the consequence. To execute the simplified LOPA approach, the IPL must be independent of the initiating cause and other IPLs.

A potential pitfall in LOPA and in the management of IPLs identified in LOPA is misunderstanding the boundary of the IPL.

For example, a pressure relief valve is intended as an IPL to prevent the consequence of catastrophic rupture of the vessel with potential for fatality. In reality, for the pressure relief valve to prevent vessel rupture, the inlet piping from the vessel to the relief valve and the outlet piping from the relief valve to the ultimate destination must provide sufficient flow capability. The IPL boundary must include any block valves in the inlet and outlet piping, and any devices such as flame arrestors or back pressure controllers. The PFD (probability of failure on demand) must include everything in the expanded IPL boundary.

If the IPL boundaries are not correctly understood, the LOPA is not correct and the organization is deluding itself on the risk reduction.

The paper provides examples and illustrations for several types of IPLs: safety instrumented functions, dikes, relief device with fire-resistant insulation and cladding on the vessel, operator response to alarm, and deflagration arrester. The paper includes diagrams to illustrate the concepts.

For other valuable free resource papers available from Process Improvement Institute, please visit the Free Resources section of the PII website at