Define realistic HAZOP consequences based on degree of overpressure.

Anyone who has taken part in a HAZOP will know the discussion about the consequences of overpressure. Under the Deviation ‘MORE PRESSURE’, causes such as ‘Valve inadvertently closed’, ‘Blocked in section subject to thermal radiation’ or even ‘Higher pressure from upstream system’ will be identified. The HAZOP Team then starts to discuss the potential consequences, firstly without any safeguards that may already be provided in the design. With a properly qualified HAZOP Team, there should be experienced operators present, who may well be able to describe the consequences of overpressure based on a previous event at their plant. This first-hand knowledge is then used as the basis for the further analysis of safeguards, recommendations, etc.

On the other hand, if the Team does not have personal experience, or is unable to reference clear industry precedents, the analysis of overpressure becomes more subjective. While the consequences of a loss of containment should not be underestimated, it is also clear that minor overpressure (say 20% over design pressure) does not lead immediately to bursting pipes and vessels, massive release of fluid, flying debris, etc. When leading HAZOPs, we have found it beneficial to refer to ‘rule-of-thumb’ values to guide the Team on their discussions. A useful table given in the literature is shown below.

Typical Consequence of Overpressure (Source: Fluor)

Of course, even minor flange leakage due to overpressure (Cat. 2 in the table) may have serious consequences, such as groundwater contamination, local flash or pool fire, with potential harm to personnel. However, our experience indicates that a clear, unexaggerated understanding of potential consequences of overpressure allows a better analysis of the necessary safeguards, without overdesign. The table can be a useful guideline in this regard.

PSC has provided independent Chairman and Scribe services for over forty HAZID, HAZOP (including HAZOP revalidation) and LOPA studies. PSC experts are certified according to IChemE and the Exida CFSE program. Please download our HAZOP-SIL-LOPA-References.