The detailed dynamic modelling and simulation of the rapid depressurisation ("blowdown") of high-pressure vessels is a key element of the safety analysis of oil & gas production plant and other high-pressure installations.
Depressuring gas
Nucleating new liquid phase in the form of droplets
Entrained liquid
Pooling and boiling liquid
Typical phenomena occurring on rapid depressurisation of a vessel. Droplets form as a result of the pressure reduction; some of the drops leave with the gas stream and others pool on the vessel floor.
Challenges
Depressurisation of a vessel results in a relief load imposed on the pressure relief system. It may also result in significantly reduced temperatures within the vessel and throughout the relief system, which may lead to embrittlement and high thermal stresses.
High-fidelity modelling of the vessel depressurisation can provide the following information:
- accurate calculation of relief loads entering the flare network, to furnish essential information for CAPEX decisions
- the temperature throughout the metal vessel walls, identifying areas of thermal stress and potential embrittlement
- the temperature of the relieving 'gas' stream (which may contain evaporating entrained liquids) to provide accurate information for choosing the appropriate material of construction.
Phenomena
The depressurisation of a vessel involves complex physical phenomena. A typical scenario is:
- The sudden decrease in pressure in a gas-filled vessel results in a rapid change in the thermodynamic state of the gas. This results in nucleation of liquids within the gas bulk.
- Some of this liquid leaves as entrained droplets in the high-velocity gas exit stream. It is important to understand the proportion of liquid in order to be able to specify the nozzle correctly.
- The exiting entrained liquid evaporates into the bulk gas stream, lowering the temperature of the exiting stream even further and presenting a risk of brittle fracture of the flare system pipework.
- The liquid remaining in the vessel may pool on the vessel floor, where it instantly starts boiling because of the relatively warm metal temperature it encounters.
- Through evaporation the pool of liquid can cool significantly as it shrinks, leading to significant temperature differences between the metal below the pool and its surrounds. This presents a very real threat of brittle fracture and rupture from the base of the vessel.
- Because of the rapid change of conditions the three phases co-existing in the vessel (gas, droplet and the pool of liquid) and the vessel wall are in a non-equilibrium state throughout most of the blowdown event.
- Other scenarios that may develop depending on the initial inventory and state of the material in the vessel – for example bubblet rather than droplet nucleation – can also be predicted.
All of these phenomena need to be taken into account to a high degree of fidelity within any tool used for providing decision support information for safety design.
PSE depressurisation tools
PSE has created a sophisticated vessel blowdown capability that takes into account all the phenomena above using a detailed 3-phase non-equilibrium high-pressure vessel model with 3-dimensional wall representation under blowdown conditions.
Example
Blowdown of vessel filled with gas of typical composition and pressure
Fluid and wall temperatures over time
Vessel wall temperature
Vessel wall stress
Key characteristics are:
- vertical or horizontal vessel orientations with different ends (torispherical, hemispherical or ellipsoidal)
- any number of vessels linked by pipework
- global mass and energy balance between the phases present and the vessel wall, at every stage of the blowdown
- rigorous calculation of non-equilibrium interactions among the various phases
- 3-dimensional model of the metal walls, including heat transfer between regions of the wall in contact with different phases
- axial, radial and tangential stress calculation based on temperature map
- blowdown of fluid containing an arbitrary mixture of hydrocarbons including deposit of water phase where applicable.
The model has been validated against the set of experimental data obtained from a full-scale vessel, as reported by Haque et al. (1992) and Szczepanski (1994).
Because the depressurisation model is implemented in gPROMS, it can take advantage of gPROMS computational power and efficiency for rapid and robust solution.
It also links seamlessly to gFLARE for downstream flare system dynamic analysis, including wall temperature modelling.
Supply
PSE currently supplies gFLARE for flare system design.
The depressurisation library will become an advanced option to the gFLARE suite in future. In the meantime PSE provides expert blowdown modelling services as part of our ModelCare consulting service.
References
- M.A. Haque, S.M. Richardson, G. Saville, Blowdown of Pressure Vessels. I – Computer Model, Transactions of the Institute of Chemical Engineers Part B: Process Safety Environmental Protection, 70(BI), 1 (1992).
- H. Mafgerefteh, S.M.A. Wong, A numerical blowdown simulation incorporating cubic equations of state, Comput. chem. Engng., 23, 1309 (1999).
- A. Speranza, A. Terenzi, Blowdown of Hydrocarbons pressure vessel with partial phase separation, Series of Advances in Mathematics, available from http://www.i2t3.unifi.it/upload/file/Articoli/animp_2004.pdf (2005).
- R. Szczepanski, Simulation programs for blowdown of pressure vessels. IChemE SONG Meeting (1994).
gFLARE can be used to design the flare system based on the relief flows generated by depressurisation models.
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