Design Case Study for 450 MW CCGT Unit – 2 options

Old steam turbine co-generation plants are presently undergoing modernization and retrofitting with combined-cycle gas turbine units of various power capacities. The aim of these modernization projects is to replace outdated equipment and also to increase the power and heat generating capacity.

Power generating plants have a strategic importance for urban communities and major industrial centers since the shutdown of such a plant could trigger a domino effect in the power grid with a rolling blackout for the connected consumers. Since demand is growing ahead of power generation, there is virtually no spare capacity and in turn this increases the risk of emergency shutdowns. However, it is generally accepted that the reliability of gas and steam turbines per se is sufficiently high, therefore there is, in turn, a need for highly reliable booster compressors for supply of fuel gas to the gas turbines.

Specific Aspects of BCS Design for CCGT Applications

The key issues and tasks addressed in the design of such facilities are dictated by the requirement that BCS should provide continuous gas supply to GTU fuel feed system (in accordance with GTU manufacturer’s specifications) over the whole range of variations in the operating environment (inlet gas pressure and temperature, ambient air temperature, GTU load variations, compressor emergency shutdown).

It is obvious that minimum suction pressure and the minimum suction temperature should be assumed as the worst design case, considering that the maximum mass gas flowrate is required under these conditions. These conditions correspond to the winter season, when the maximum gas volumes are drawn from the pipeline, and the turbines require the greatest power input.

In order to maintain uninterrupted gas supply standby compressor units must be provided. These units may be maintained either in a "hot" backup state (which requires "oversize" compressors and significantly adds to the project costs) or in "quick-startup" mode (in the latter case a group of receivers should be provided at the discharge line in order to maintain the system operating during the startup of standby compressor).

Compressor equipment will be located in a machine hall measuring 18 m x 20 m and originally designed to accommodate three screw compressor units. Auxiliary equipment, such as an air compressor plant for instrument air generation, a nitrogen plant, and a low-voltage control cabinet will be installed in adjacent rooms. As such, the total floor area occupied by the main and auxiliary equipment is 18 m x 24 m. Storage tanks and air coolers (or cooling towers depending on the type of cooling system) are located in an outdoor area near the building.

Compressor units delivered as completely prefabricated, factory-built packages will be mounted on individual foundation blocks.

Lube oil facilities are entirely omitted for centrifugal compressors because, there is in this instance no consumption of oil, while a complete oil change in lube oil sumps will be done only once in 2-3 years. Oil lubricated reciprocating compressors require day tanks for lubrication which have to be refilled periodically, depending on the reservoir size. Lube oil facilities are entirely omitted for centrifugal compressors, as no oil will be consumed in this case, while a complete oil change in lube oil tanks will be done only once in 2-3 years. Reciprocating compressors require daytanks for lubrication which have to be refilled once in two weeks.

The compressor house will be equipped with fire and gas detection and automatic fire extinguishing systems.

As can be seen from the presented data, a compressor station based on centrifugal units has a number of key advantages:

• Compact design of the equipment
• No need for additional cooling system or coalescing filters to separate oil from the gas at compressor discharge
• No lube oil facilities (oil in the drive casings will be changed once in 2-3 years without refilling).

It should also be noted that centrifugal compressors:

• are faster to start up from the "hot" backup state
• require less man-hours for scheduled maintenance and less operating costs
• can run continuously for two years without loss of efficiency.

These benefits may only be set off by a slightly better power performance of reciprocating compressors at higher inlet gas pressures (this requires re-adjustment of reducing gear and piston clearance spaces) and lower initial equipment cost (which eventually equals due to the higher operating costs).