Gas 2025 Issue

PCV

PIT

Backup fuel gas from pipeline

Fuel gas to are

PCV

Fuel gas pre-heater

Steam Cond.

Flue gas to atmosphere

LNG boil-o gas from LNG storage

PIT

Compressor gearbox

BOG compressors

Fuel gas mixing drum

Gas turbine A

LNG boil-o gas to other trains

PCV

Compressor gearbox

Excess LNG boil-o gas to other users

Gas turbine B

Figure 1 LNG facility fuel gas (FG) system

releases excess FG to a flare header. This valve, along with the backup NG valve and pressure control on the com - pressors, makes up the control scheme for the FG system depicted in Figure 1 . Upset scenario modelling In line with process engineering design best practice, the previously described FG system must be tested against various upset scenarios to ensure system reliability. A dynamic simulation model was developed according to the configuration shown in Figure 1. The dynamic simulation accounts for transitional changes in the system, reflect - ing changes in composition, pressure, and temperature during upsets while taking credit for system volumes and elevations. The dynamic simulation also simulates control - ler actions and effects, which are ignored by steady-state models. Such rigorous simulations offer accurate predic - tions of system response and provide a reliable basis for engineering design. Two scenarios are considered in the FG system dynamic simulation. The first considers how backup NG will balance the system if the BOG supply to the gas turbines is lost. The second considers how the system will handle a pressure increase when a gas turbine trips. In this scenario, the FG system uses valves to send excess FG to the flare header while the FG system adjusts. Scenario 1: BOG compressors trip Gas turbines are designed to operate using BOG because it is typically the more economical fuel source available at LNG facilities. However, as outlined, there are several sce - narios where BOG may not be available. In these scenarios, NG is used as a substitute, which has a higher energy con - tent and allows the gas turbines to operate more efficiently. Since BOG is the saturated vapour fraction of the LNG stored in the LNG storage tanks, it typically contains ~16% nitrogen gas. Due to its composition, BOG’s heating value is less than that of the NG used to balance the supply when

BOG is not available. A reason for this is NG typically has a small composition of natural gas liquids (NGLs). NGLs are not present in any meaningful quantity within the BOG. Therefore, when switching from BOG to NG, a lower FG flow rate will be required in the gas turbine due to the higher heating value. If the flow rate to the gas turbine is not reduced, the temperature in the turbine increases, causing it to trip. MWI is used to evaluate both the compo - sitional and temperature impacts on the gas turbines. The trip of the BOG compressors is simulated with the following assumptions: • BOG has an MWI of 40. • NG has an MWI of 50. • BOG compressors have an outlet pressure of 1,073 psig. • Backup FG supply has a pressure of 1,044 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. • The balancing FG line will open automatically and intro - duce backup FG to the system once the BOG compressor trips. The gas turbine design dictates that the MWI must not change in either direction by more than 5% per minute and that the pressure change must be kept below 10 psi per second. The dynamic simulation will help evaluate whether the FG system design can meet these design conditions. The results of the dynamic simulation are shown in Figures 2 and 3 . The FG mixing drum’s main purpose is to sufficiently mix the FG, from various possible sources, such that the MWI at the outlet of the drum meets gas turbine specifications. Dynamic simulation of the FG mixing drum shows the MWI change increases by 3% per minute with the current design, well within the 5% per minute limit set by the gas turbines. Figure 3 shows the pressure profile at the mixing drum and gas turbine inlets. A sudden drop in pressure is observed throughout the system when tripping the BOG

Gas 2025