Is the Gas Processing Industry Wasting Billions of Dollars in the Construction of Processing Facilities? a Design Case Study

In mid to high pressure gas processing facilities, the minimum metal temperatures in process equipment and piping are observed during highly transient depressurization operations (“blowdown”). The minimum metal temperature usually sets the material of construction: if metal temperatures below the Low Temperature Carbon Steel limit of -46oC (-50oF) are possible then the usual requirement is to select materials that exhibit ductile behavior below this point even if normal operating temperatures are much warmer. Such choices have a huge impact on project costs and order times and ultimately project viability.

To explore this issue we use as basis a recent case study that considers the design from Pre-fFEED through to Final Investment Decision of a gas processing facility. During pre-FEED, the design team applied a widely used methodology for depressurization system design, based on idealized equilibrium volumes. This predicted extremely low metal temperatures in nearly all parts of the processing facility during a depressurization operation. Violations of the LTCS limit were predicted even when depressurization was initiated immediately from typical operating conditions. To avoid material embrittlement, large vessels and piping would have required expensive stainless steel construction for approximately 80% of the facility; this was considered to have significant impact on the feasibility of the project.

As a result, the design team decided to apply a more detailed methodology to assess minimum metal temperatures during depressurization. This methodology involves a detailed representation of the processing facility using accurate dynamic models and requires a greater level of analysis and effort. The design team accounted for different system pipe and vessel dimensions, wall thickness and materials of construction, system elevations, points where condensate liquid may accumulate, drainage and the number and location of blowdown valves for each isolatable segment.

The results from the detailed methodology predicted much more accurate metal temperatures at each location throughout the processing facility. Having identified specific locations of concern, the design team developed targeted solutions addressing all predicted issues whilst minimizing capital expense. Savings were achieved by optimizing material selection and other critical design specifications including wall thickness, line schedules, blowdown valve locations and blowdown rates.

For this case study, by utilizing the detailed blowdown methodology throughout the design cycle only 30% of the facility is being constructed from stainless steel - saving the operator up to $1.8bn through reduced material costs.