Slow-Roll of Natural Gas Compressors in a 420 MW Cogeneration Plant

Abstract

This Paper describes the efforts required to operate two steam turbine-driven centrifugal natural gas compressors at significantly reduced rotative speeds. Natural gas is used to power a 420 MW combined cycle cogeneration plant. Normally, the plant uses natural gas fuel at supply line pressure and the two compressors are on idled standby. The goal of the project is to reduce motive steam consumption in the steam turbine driving the natural gas compressors. The present operating mode, or idle mode, is where both natural gas compressors recycle compressed gas via dedicated intercoolers. This mode of operation occurs about 95% of the time. The remaining 5% of the time is when natural gas compressors boost the trunk line pressure in order to deliver fuel to the gas turbine combustors. Both natural gas compressor trains are designed for rapid increase of rotative speed in case one natural gas compressor is shut down. In order to put a natural gas compressor on hot standby slow-roll condition, a special, dedicated slow roll line is proposed for the fine control of the rotative speed.