(A Siemens V94 2A combustion turbine plant)
Total-flooding mode for gas turbines
Minimum retention time
A system that discharges a predetermined quantity of CO2 and then counts on an initial higher concentration to decrease gradually (hopefully not less than the minimum concentration at the end of the required duration) is adequate for short retention times (i.e. 10 minutes or less). This is referred to as a “single-shot”. However, experience has shown that most turbine enclosures do not offer the level of “tightness” required for a single-shot concentration to last 20 minutes or more. Therefore, an extended discharge is a prudent method. This consists of a smaller secondary CO2 system that continues to “trickle” a smaller quantity of CO2 into the enclosure after the initial discharge is exhausted. The quantity of CO2 provided for the extended discharge should take into account the required duration.
As mentioned above, CO2 carries a unique personnel hazard – the potential for asphyxiation, or death due to lack of oxygen. NFPA 12 has taken an aggressive stance, beginning with the 2005 edition, by making stringent personnel safeguards mandatory for both new and for existing systems. These include a proliferation of ANSI Z535-compliant warning signs, manual lockout valves, and audible and visual alarms.
(More in November-December 2012 issue of Turbomachinery International magazine)