Abstract
The design of blowdown systems involves performing engineering calculations for setting equipment Minimum Design Metal Temperatures (MDMT), by utilizing the adiabatic cold depressurization dynamic simulation tools. Minimum critical exposure temperatures (CET), also obtained using the aforementioned simulation tools, are generally evaluated jointly by Processand Mechanical engineers in order to select a suitable material of construction. The selected material's MDMT must also be compatible with the predicted cryogenic exposure and risk of ductile-brittle transitions of the system and its associatedblowdown lines and flare system.
A conventional approach practiced in many Engineering, Procurement, and Construction EPC and FEED consultant companies is to set over-conservative assumptions and an unrealistic design basis when conducting adiabatic depressurization calculations and setting the MDMT of the various system components. One of the major assumptions set by these companies is that upon initiating adiabatic blowdown, the system temperature would have stabilized to the minimum winter ambient temperature. In essence, such an over-conservative basis ultimately dictates the selection of a highly exotic metallurgy for the system and flare headers, drastically affecting project economics.
As part of a completed FEED project for gas compression station and considering the warm weather conditions in the United Arab Emirates (UAE), a detailed MDMT study was performed utilizing state-of-the-art engineering and embedding practical bases and assumptions when determining the minimum exposure temperatures for equipment and flare system involved in a gas compression station. The optimized approach developed stems from using practical integrity requirements of compression systems as well as iterative simulations for determining the safe initial blowdown temperature set point. As the system mainly comprises of compression units, the practical basis used for cold depressurization is driven by compressors vendor's integrity requirements of preventing compressors prolonged settle out & shutdown conditions and mandating automatic depressurization after a specific settle out time. Accounting for this mandatory requirement, the initial temperature for adiabatic depressurization of compressors is calculated considering minimum ambient conditions and a specific safe compressor settle out duration after which adiabatic depressurization is carried out for determining the CET. For system components which do not involve compressor units, iterative simulation was used to identify the safe initial temperature for adiabatic cold blowdown, which in turn was used for determining the set point for auto-depressurization temperature based ESD blowdown non-programmable solid state logic and a SIL assessed blowdown safety function. The adequacy of the assessment of the system's depressurization with this above approach was validated with respect to hydrate formation risks in system equipment, blowdown vent lines and flare headers. With the optimized FEED approach, pragmatic MDMT results were obtained with associatedless exotic metallurgy compared to the usual conservative assumption for MDMT calculations. Additionally, an additional benefit of this techno-economic approach also involves optimizing the number of the flare headers in terms of eliminating the potential for hydrate formation.
This paper explains the FEED optimization MDMT study approach, and demonstrates the initiatives considered to optimize the design of the flare and blowdown system in terms of metallurgy and number of flare headers. Engineering judgement and administrative logic controls were used to safeguard the system while optimizing its Capital Expenditure (CAPEX). The tools and justifications used in this FEED study are innovatively different from the typical design approaches most FEED and EPC contractors tend to follow, which usually result in highly expensive and unnecessarily exotic designs.
