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The fuel gas mixing drum is an integral part of many plants, especially LNG plants. Any change in fuel gas composition, properties or pressure can lead to operational problems in fuel-using systems, such as gas-fired turbines. These systems are designed to accept a certain degree of change of fuel gas properties; however, their burners are sensitive to the rate of this change. To limit this, fuel gas mixing drums are typically used.
Fig. 1A shows a typical sketch of a fuel gas mixing drum. It consists of two separate sections, with a knockout drum (KOD) in the bottom section and a central riser pipe with a number of baffles in the top section. Fuel gas enters from the bottom section and flows to the top section, or vice versa.
Fig. 1A. Typical sketch of a fuel gas mixing drum.
The riser contains a series of holes in each baffle section, so gas from the riser holes mixes with gas from the baffle sections. Baffles are sloped to allow the free draining of liquid. This arrangement ensures proper mixing and gradual changes in any process parameters. The KOD section ( Fig. 1 ) is designed as a normal two-phase separator, such as a compressor suction drum. Here, the mixing drum design procedure is reviewed for operational optimization.
Number of mixing stages in top section
The fuel gas mixing drum ensures gradual change in fuel gas properties, such as the Wobbe Index, and also acts as a buffer vessel to limit pressure variation. The Wobbe Index is an indicator of the interchangeability of fuel gases. Some gas turbine vendors use different terminologies for this parameter, such as the Modified Wobbe Index and Gas Index, and to include temperature as an additional variable; however, these are all derivatives of the Wobbe Index. In the following discussion, the equipment is designed to limit the rate of change of the Modified Wobbe Index (MWI), defined as:
(1)
If the feed gas composition changes, i.e., from heavier to lighter, the MWI at the outlet of the drum will change gradually from value MWI[subscript]OLD (heavy gas) to value MWI[subscript]NEW (light gas).
(2)
If the maximum allowable rate of change of the MWI is R[subscript]Allowable [%/sec] as specified by the gas turbine...