The Use of LOPA Method to Demonstrate Correct Installation of PSVs

In 2001, the LOPA (Layer of Protection Analysis) method was published by CCPS; and nowadays is the most used method to a) evaluate the reliability and safeties of the safeguards by comparing the mitigated frequency with a target called acceptable risk or acceptable frequency; b) to evaluate the risk in terms of money (Financial Risk). Another option is to evaluate if the safeguard is installed in the correct location; so it can work as intended and with required reliability/failure frequency.

When API 520 was a recommended practice (RP) and LOPA was not already established; some plant designs considered the installation of PSVs in the “most convenient location” instead of the “most useful location”. Most convenient location considered that some flow restrictions to PSVs are very unlikely to happen so it will not impact the reliability of the safeguards. Examples like: PSV to protect a fractionation column installed 1) downstream of overhead exchanger, 2) downstream of a non-CSO valve, etc are common in old plants.

However, when there are potential barriers between the PSV and the source of energy, the Probability of Failure on Demand (PFD) increases by the addition of the PFD of the barriers. Real world examples indicate how serious this problem can become –and the solution - if the PSV cannot protect the vessel/equipment because there is a form of isolation between the PSV and the source of pressure.