High-fidelity system depressurisation

The importance of representing the blowdown segment accurately

Process blowdown usually determines the minimum design temperature and thus the material of construction (e.g. carbon steel, low temperature carbon steel or stainless steel) for each part of an Oil & Gas facility and so can have a very significant impact on capital cost expenditure. Equally, if wrongly specified, the safety of the facility is at risk. Read our case studies to see the impact of high-fidelity modelling on real projects.

A typical, traditional ‘engineering’ approach is to represent each blowdown segment entire system by a single vessel (the so-called ‘lumping’ approach), as show below.

The dimensions of this notional vessel are adjusted so that its volume matches the total sub-system volume. An appropriate aspect ratio is chosen to provide either a suitable heat transfer area or a representative diameter for heat transfer calculations.

Vessel wall thickness is chosen to approximate the entire metal mass of the original blowdown segment. The approach is subjective, and the precise approach employed varies from engineer to engineer.

Traditional system blowdown approach
Traditional approach – whole process is 'lumped' to a single-vessel representation

Comparison of traditional vs. rigorous approach

The only conservative way to make a screening calculation using the above approach is to take no credit for metal temperatures (the extreme case); this is equivalent to a simple isentropic flash calculation to atmospheric pressure. For a typical natural gas mixture starting for example at 60 bar and 5°C results in a minimum fluid temperature of -140°C.

By contrast, a rigorous, system-level analysis approach acknowledges that elevations and aspect ratios vary hugely in many systems and it is essential to capture this in the model.

The approach thus involves creating a process flowsheet that represents the geometry of the blowdown segment much more realistically. A typical gFLARE model is shown below.

Rigorous system blowdown modellingPSE's approach using gFLARE to represent the distributed process

Creation of such a model involves reviewing P&IDs, equipment data sheets and isometrics to determine pipe and equipment dimensions, and noting whether there are low spots where condensate liquid can form and accumulate, and then creating a flowsheet representing these appropriately.

Results

The results of a run using such a model show (below) that different parts of the system metalwork will experience significantly different temperature profiles.

Distributed system blowdown approach
Distributed system blowdown approach: different parts of the system metalwork experience significantly different temperature profiles

The advantages of a rigorous approach

Generally the fluid and metal temperatures observed during system blowdown are the minimum observable temperatures in an Oil & Gas process facility.

It is clear that any attempt to lump units and consider some ‘average’ behaviour is missing key information and is fundamentally flawed when used to specify minimum metal temperatures.

It is also important to consider that the fluid temperatures in the system also vary greatly so the temperature upstream of the blowdown point (as connected to the flare network) is significantly different to that in the vessel; so an accurate assessment of the blowdown segment is important when specifying the minimum design temperature for the flare network.

Apply high-fidelity dynamic modelling to your depressurisation 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.

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