High Back Pressure on Pressure Safety Valves (PSVs) in a Flare System. Developing the Simulation model, Identifying and analyzing the back-pressure build-up
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Process safety is a disciplined framework for managing the integrity of operating systems and processes handling hazardous substances. It is achieved by applying good design principles, engineering, and operating and maintenance practices. Flare systems play an important role in the safety of Oil and Gas installations by serving as outlets for emergency pressure relief in case of process upsets. Accurate and reliable estimation of system thermo-hydraulic parameters, especially system back-pressure is critical to the integrity of a flare system design. Accurate design of the flare system plays a key role in containing possible process safety hazards on the oil and gas installation, especially oil and gas offshore platforms. In order to enable uniformity and consistency, design guidelines and constraints are provided within the industry, both national and international standards - NORSOK, API and ISO - which serve as recommended practice in process and flare system design. This thesis is focused on analyzing the back-pressure build-up in the high pressure flare system at Kollsnes gas processing plant. The relief scenario considered in this thesis is pool fire case in condensate system in Kollsnes gas processing plant. The simulation tool used to model the flare system in this case is Aspen Tech's Flare system analyzer (known as FLARENET), is a steady state simulation tool. The FLARENET model includes the pressure safety valves (PSV), downstream tail pipes, flare header, flare knock out drum and flare stack. All the actual plant data are given as input to the model so as to get the more practical result. After running the simulation model, it emerges that for a total relieving rate of 108.33 kg/sec (Vapor flow) in pool fire scenario, the back-pressure generated at some of the PSV's in the relief network is 10.6 Barg against their set pressure of 10 Barg. This raises serious process safety concern as the relieving rate from the PSVs is drastically reduced due to very high back-pressure, which in turn will increase the pressure inside the process equipment exposed to fire. This concern has been conveyed to the Kollsnes plant operations group. To verify the results obtained from FLARENET simulation, I had undertaken actual plant verification. This was carried out in co-ordination with the Kollsnes plant operations group during September 2012, just before annual maintenance shutdown of the plant. The back-pressure results obtained as a result of controlled blow-down from the plant matched well with the FLARENET simulation results. Further follow up tasks is being under taken by Statoil ASA to alleviate the back-pressure problem. This thesis suggests two options for solving the problems. Further evaluation of the suggestion and its implementation in the plant is going on in the company. This thesis also opens door for further research on high back-pressure in flare system and analyze the problem dynamically. This will reduce the conservative steady state assumptions and will have much wider industrial acceptability with respect to cost savings potential.