Safe Operating Limits – Part 2
In a previous blog, Safe Operating Limits – Part 1, we provided a heuristic for defining the safe operating limits:
- Obtain the operating envelopes for all modes of operation
- Identify key limit values for the process parameter
- Add limit values commensurate with complexity and risk
- Drive the Safe Operating Limits as close to the operating envelope as reasonably practical
- Evaluate consequences of deviations
In Part I, we used a simple example illustrating temperature limits; however, readily admitting our bias, we find pressures to be much more interesting and pertinent. During design, the maximum design pressure is specified using a ‘margin’ above the maximum expected operating pressure. After the vessel is constructed, the fabrication and testing process establishes the Maximum Allowable Working Pressure (MAWP) at a value greater than or equal to the design pressure. Again, the typical approach then says to set the upper safe operating pressure as the MAWP, after all the ‘working’ pressure is the ‘operating’ pressure… Note that this assumes the most common situation where a pressure relief device is set to open at the MAWP, so it may be more appropriate to say that the upper safe operating pressure is taken to be the relief device set pressure.
For the case of the upper pressure limit, there is also something happening implicitly that most of us do not give much thought to unless performing a detailed risk analysis. For most equipment, a specific passive mechanical system is required by the code of construction to ensure the upper limit of a safe mechanical integrity pressure is not exceeded – namely, a pressure relief device. For a pressure vessel, the code of construction establishes a corresponding Maximum Allowable Accumulation Pressure(s) (MAAP) for pressure excursions. The pressure relief device is sized (based on potential overpressure scenarios selected) to ensure the relief pressure does not exceed the MAAP. We would therefore suggest that the MAAP (not the MAWP as is conventionally assumed) is then implicitly taken as the safe upper mechanical integrity pressure. If the MAWP was established as the safe upper mechanical integrity pressure, then every time a pressure relief device opened, one would initiate procedures to shut down and inspect the equipment since the pressure excursion exceeded the safe mechanical integrity pressure. In our experience, this usually doesn’t happen, particularly for reclosing pressure relief valves. In fact, we often find that the relief device opening is not even recorded and acted upon later, for example as input to relief device inspection or PSM improvement efforts.
Even for low risk systems where the pressure relief device may be the only line of defense against exceeding the maximum safe design pressure, one should recognize the ‘operating ratio’ for the installed pressure relief device. An operating ratio for a pressure relief device is defined in API Standard 520 Part 1 as the ratio of the maximum system operating pressure to the set pressure of the relief device (or some adjusted pressure, as for the marked burst pressures of rupture disks)1, and a limit is commonly specified by the relief device manufacturer. Operation at pressures above this operating ratio limit can affect the operation and reliability of that relief device – for pressure relief valves, simmering, spring weakening, seat damage, and internal fouling may occur.2 As a result, the relief device’s maximum allowable operating pressure becomes one of those key limiting values for pressure: the upper safe operating pressure should be less than or equal to the relief device’s maximum allowable operating pressure.
In the case of the simple, low risk system outlined above, the consequences of exceeding the upper safe operating limit can be stated that “continued operation above the safe operating pressure may affect the operability and reliability of the relief device and if the pressure continues to rise to the set pressure, the relief device is expected to activate”. The relief device should be on a regular inspection (or replacement) plan, and so the effects of these deviations can be identified and mitigated.
Example simple upper pressure limits
The figure above simply illustrates the impact of the relief device operation on the safe operating envelope, and does not reflect any further efforts to drive the safe operating limit closer to the operating envelope. Up next in Part III, we will look at the impact of pressure relief system design on the definition of safe operating limits.
 American Petroleum Institute. “API Standard 520: Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries; Part I—Sizing and Selection”. 9th Edition, July 2014.
 Parr IM. “The Reliability of Safety Valves and Relief Valves”. In International Symposium on Runaway Reactions and Pressure Relief Design. AIChE, 1998: 285-317.