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Mixing drum inlet Mixing drum outlet Gas turbine inlet

Mixing drum inlet Gas turbine inlet

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compressors. The pressure at the inlet to the gas turbines slowly returns to its normal level as the backup FG valve opens to balance the loss of BOG supply. The change in FG system pressure is much faster than the change in MWI. The design of the FG mixing drum must allow for sufficient mixing for the MWI while sustaining sufficient pressure within the system. Dynamic simulations show that the maximum change in pressure observed by the gas turbines is -3 psi per second. This is below the ±10 psi per second limit for the gas turbines, meaning the FG system design satisfies both MWI and pressure conditions for the turbines. Scenario 2: Single gas turbine trip In a case where there is an FG demand disruption, such as a single gas turbine trip, there will be an excess of FG and pressure within the FG system. To handle excesses of FG and pressure, a pressure control valve (PCV) will open, sending the excess FG to a flare header. The trip of a sin - gle gas turbine scenario is simulated with the following assumptions: • Assume one of the two gas turbines trips. • BOG compressor has an outlet pressure of 1,073 psig. • The gas turbine main header has a normal operating pressure of 967 psig. • Gas turbine sub-headers have a normal operating pres- sure of 962 psig. • PCV for bleeding excess FG to the flare header is assumed to respond accordingly to mitigate the pressure surge and to avoid the high-high pressure trip (1,000 psig) at the gas turbine main header. An automatic control scheme was initially used for the PCV to respond to the pressure surge caused by a single turbine trip, but this type of control is unable to respond fast enough to the pressure surge. As a result, the pressure at the main header and sub-header exceeds the set point of 1,000 psig for three minutes, reaching a peak pressure of 1,010 psig. Therefore, a manual control logic is imple- mented to open the PCV fully in 10 seconds when the main header pressure reaches 995 psig. Once pressure falls Figure 2 Change of Modified Wobbe Index with BOG com - pressor trip

below this manual setpoint, the control logic switches back to automatic control. With the manual control logic implemented, the pressure surge is mitigated and kept below the set point, as shown by the yellow and green lines in Figure 4 . The maximum pres- sure reached in this case is 996 psig, comfortably below the set point of 1,000 psig. The 4 psi margin between the max- imum pressure and the set point is well above the typical measurement accuracy of a pressure transmitter (<±0.1%) to avoid an erroneous trip. Dynamic model results validate the effectiveness of the modified pressure control scheme and control parameters used. Conclusion Dynamic process simulations are a powerful tool, allow - ing engineers to evaluate processes and control systems in a way that emulates how they operate in a real-world environment. This tool allowed for the validation of the Figure 3 Pressure profile upon trip of the boil-off gas compressors

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P at sub-header-auto P at sub-header-10s open

P at main header-auto P at main header-10s open High-high pressure trip

Figure 4 Pressure profile of FG main header and sub- header to gas turbines

Gas 2025