Vessel blowdown

State-of-the-art high-fidelity vessel blowdown modelling

The detailed dynamic modelling and simulation of the rapid depressurisation (“blowdown”) of high-pressure process equipment and piping is a key element of the safety analysis of Oil & Gas production plant and other high-pressure installations.

PSE’s sophisticated, high-fidelity depressurisation models have led the field in model-based blowdown analysis. We apply these as part of our consulting services to generate accurate blowdown temperatures and flowrates. Read our case studies to see the impact of high-fidelity modelling on real projects.

Challenges

Depressurisation of process equipment results in a flaring load imposed on the flare system. It may also result in significantly reduced temperatures within the process equipment and piping and through the flare system, which may lead to embrittlement and high thermal stresses. In most cases in Oil & Gas facilities, the blowdown operation sets the minimum metal temperature for the process equipment and so has critical implications on metal selection and facility cost.

Phenomena

The depressurization of process equipment and piping involves complex physical and thermodynamic phenomena. Consider the behaviour of a single vessel as it depressurizes:

• The rapid decrease in pressure in a gas-filled vessel results in a rapid change in the thermodynamic state of the super-critical gas. Typically, as the pressure reduces, the fluid crosses the vapour-liquid phase envelope; which results in the nucleation of liquids within the gas bulk.
• Some of this liquid leaves in the gas exit stream. The higher velocity of the gas in the outlet line and nozzle will typically lead to a higher rate of heat transfer than in the bulk of the vessel leading to colder pipework.
• The cold liquid will pool on the bottom of the vessel, where it instantly starts boiling because of the relatively warm metal temperature it encounters.
• Mass and heat transfer between the bulk gas and liquid phases continues with liquid condensing from the gas and lighter components re-evaporating from the liquid.
• The direct contact with the liquid leads to significant temperature differences between the metal below the pool and the vessel walls that are next to the gas phase. 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, liquid drops 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. More over, most blowdown segments of practical interest are much more complex than a single vessel: involving multiple vessels, with internals and piping. This can lead to specific problems with multiple low points in the systems such that “the [condensate] liquid may accumulate in the low points (e.g. bottom of vessels, drain connections).” API 521 6th edition. As described in the example above, the places where liquid can accumulate can experience much colder temperatures.

Limitations of current approaches

The plots below show various temperatures seen during the depressurisation of a vessel when calculated using traditional equilibrium approaches:

When liquid condensation occurs, conventional equilibrium approaches show poor predictions that are not suitable for safe design. They:

• under-predict metal temperature in contact with gas (left-hand plot)
• provide quantitatively and qualitatively incorrect predictions for the wall temperature in contact with the liquid (right-hand plot).

These predictions are not necessarily conservative.

The plots below show the same temperatures resulting from a rigorous depressurisation calculation using PSE’s Advanced Depressurisation models:

Experimental validation¹ shows that the detailed modelling approach with rate-based thermodynamics and 3D distributed system representation provides:

• accurate tracking of wall temperature in contact with gas (left-hand plot)
• accurate tracking of wall temperature in contact with liquid (right-hand plot)

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.

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 gFLARE, it can take advantage of the computational power and efficiency of the gPROMS platform. It also links seamlessly to other models for downstream flare system dynamic analysis, including wall temperature modelling.

Supply

PSE currently supplies gFLARE^® and depressurisation models as an optional deliverable for Oil & Gas consulting projects.

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).

Apply high-fidelity dynamic modelling to your blowdown analysis

The use of a rigorous model-based safety approach based on our gFLARE technology and coupled process and flare network models can result not only in improved process safety but also an improvement in operations and a considerable reduction in CAPEX. Read our case studies to find out how Oil & Gas companies are saving billions.