Model-based Safety

Blowdown & depressurisation

The study enabled us to save $1.5m on the cost of this vessel.

— Stephen Leng, BP Operational Excellence

Depressurisation (blowdown) modelling is an essential activity in safety design.

Key activities are to determine peak pressure and flowrates on failure, as well as resulting vessel and pipe wall temperatures followig Joule-Thomson expansion of vapour.

Accurate quantification is necessary to verify that safety systems are adequately designed, as well as to avoid unnecessary capital expenditure resulting from overdesign.

The modelling challenge

Until recently there has been a lack of software on the market capable of handling the complexity of the calculations in a format that is easily accessible to engineers:

Depressurisation is inherently dynamic, with process behaviour conditions varying dramatically over time. "Pseudo-steady-state" techniques have given poor approximations in the past.
During depressurisation there may be flow reversal in areas of the process at times. Most dynamic simulation packages cannot handle this adequately.
It is often necessary to model temperature profiles in pipework and vessel walls, using two or three-dimensional distributions.
If liquid is present, it is necessary to include accurate phase modelling. In many cases assumptions of equilibrium do not hold, and it is necessary to model the temperature distribution within the liquid.
The rate of change of key variables during blowdown leads to fast transients, that require a robust and powerful solution techniques.

Dynamic simulation provides an alternative method to better define the relief load and improves the understanding of what happens during relief.

API Standard 521 (Clause 5.22)

In particular there has been a lack of integration between blowdown/depressurisation calculations and downstream flare system design. Typically these are separate activities involving cumbersome, time-consuming and error-prone transfer of data between software.

PSE's gPROMS software and the state-of-the-art Advanced Model Library for Pressure Relief Systems (AML:PRS) address all these issues by providing a comprehensive and integrated capability for the design, analysis and optimisation of blowdown operations and flare networks.

PSE provides

PSE provides a combination of software tools and services:

The gPROMS Advanced Process Modelling environment, which provides all the essential capabilities – dynamic simulation and rigorous thermodynamics within a flowsheeting environment – to deal with the phenomena described above.

The gPROMS Advanced Model Library for Pressure Relief Systems (AML:PRS), executing within the gPROMS framework.
The AML:PRS not only provides detailed 3-D vessel models with rigorous thermodynamics and wall temperature modelling, but also a full capability for downstream flare modelling to analyse the impact of time-vaying relief loads.

PSE's expert ModelCare configuration and consulting services

The challenge: feed temperatures of below –100°C

Model formulation

3-D discretisation and heat fluxes – click for more detail

Decision: Inconel vs. carbon steel?

Scenario and results

Worst-case scenario – two successive blowdown events

Tube wall temperatures at inlet and outlet for worst case

Azimuthal temperature profile over time

Temperature color map through knock-out vessel wall thickness

Example: BP Angola Block 18 knock-out vessel

PSE's depressurisation models were applied to the safety relief system of a BP offshore platform.

The diagram on the right (click for detail) shows the areas modelled.

In constructing the relief system for the Angola Block 18 offshore development, BP urgently required high-accuracy information to support a decision on material of construction for the system knock-out vessel.

The vessel is 15m in length, and has to handle incoming gas at around -100°C. If it could be shown that temperatures in the vessel wall remain above 230 K at all times during depressurisation, the vessel could be constructed from carbon steel with an Inconel lining.

However steel may become brittle and fail at very low temperatures. Thus if the temperature approached or exceeded the limit, it would be necessary to construct the vessel out of the expensive Inconel alloy.

The tank usually contains some residual liquid, which is instrumental in reducing extremes of temperature.

Modelling approach

The model used the vessel model from PSE's AML:PRS Level-2 library. This included:

3-dimensional (axial, radial and azimuthal) modelling of heat conduction through the steel vessel walls and Inconel liner.
3-dimensional modelling of the residual liquid in the tank.
1-D energy model for gas phase, allowing for axial variation.
Predictive heat transfer correlations for:

solid-to-ambient heat transfer coefficient
gas-to-liquid/solid heat transfer coefficient

The model was executed for various scenarios, including different amounts of residual liquid and two blowdown events in sequence.

Results

The results of the simulation are an accurate steel wall temperature map that shows temperatures at any point in the vessel walls over the elapsed time of the depressurisation. Examples are shown on the right.

The results showed that, despite the extreme temperature of the gas entering the vessel, at nowhere does the steel wall temperature ever approach the 230 K limit for carbon steel.

Conclusions

The study verified lower-cost design, showing conclusively that it was unnecessary for the vessel to be constructed from Inconel, and the less expensive carbon steel option could be used with an Inconel lining.

This saved $1.5m on the vessel construction, and allowed the vessel to be constructed and delivered much earlier than it would otherwise have been.

Modelling timescale was of the order of 15 days elapsed, well within the purchasing decision timeline.

Following this study, the same procedure was also applied to the upstream pipework.

With thanks to BP