The assessment of the consequences of high-pressure releases of flammable gases is a fundamental requirement for the safe design and operation of industrial installations, plants, and pipework.
A scenario of particular interest is related to a high-pressure jet-fire developing after ignition of a gas jet release which results in a thermal loading to the surroundings (domino effect, potentially leading to escalation scenarios).
Transient calculations of the thermal evolution of pipes and vessels containing high pressure natural gas, and subject to jet fire, have been carried out using the simulation tool OLGA. The simultaneous depressurization of affected plant components has been included, to emulate the intervention of emergency shutdown procedures.
Scope of the above simulations was to investigate the risk for integrity of the analysed pipes and vessels during the timeframe in which they are potentially exposed to fire attack, according to the requirements of last edition of American Petroleum Institute Standard RP 521 (Ed. 2014) [1] for pressure relieving and depressurizing systems.
According to this Standard, a rupture is assumed to occur if, during the depressurization time, the total (Von Mises) stress on the piping is above the (derated for temperature) Ultimate Tensile Strength (UTS) of the metal.
This paper describes the model basis and characteristics and shows the results of some selected simulations with or without a Passive Fire Protection (PFP) system on the affected components and taking into account of a staggered depressurization philosophy.
The calculation method is applicable to all kind of natural gas processing plants, as topside offshore installations, or onshore systems.
API RP 521 specifies that the integrity of pipes and vessels potentially exposed to fire attack in a gas plant shall be analysed, and an analytical method is there presented as alternative approach to an empirical method based on past fire tests on tanks.
Since the empirical method applicability is limited by the specific conditions of the background tests, it is preferable to adopt the analytical method for a best fit to a design case of interest. During a fire impact on a gas plant component, a rupture is assumed to occur if the total (Von Mises) stress on the piping overcomes the (derated for temperature) Ultimate Tensile Strength (UTS) of the metal; furthermore, it is considered that the considered plant component will be depressurized because of Emergency Shut Down (ESD) procedures.
