Effects of a Water-mist Fire Protection System on Integrity and Operation of a Heavy-duty Gas Turbine

This paper describes the steps performed by the authors’ company to validate and qualify the introduction of a new fire protection system for heavy-duty gas turbines based on water mist technology. Initially, a finite element model (FEM) of a newly developed 32 MW-class industrial gas turbine for mechanical drive and power generation was generated to simulate gas turbine (GT) casings deformations in case of fire and subsequent water mist discharge for various operating conditions and ventilation setups. Secondly, a test was conducted according to NFPA 750 (2003) at the supplier’s facilities to check extinguishing efficiency; a mockup was used to simulate the engine. Finally, tests were performed on the first engine to test (FETT) operating the fire-extinguishing system during the full load testing campaign. Engine conditions were replicated within and outside normal operating conditions and analytical predictions have been matched with the test data. An extrapolation from the gas turbine results to various heavy-duty gas turbine sizes was conducted. INTRODUCTION The fire suppression system technology based on water nebulization (water mist) has gone through a terrific development in the last 10 years. The main reasons for this are the necessity to reduce the use of substances harmful to the stratospheric ozone (traditional gases to extinguish fires are made up of fluorine) and the International Marine Organization (IMO) regulations, which prescribe the installation of water mist systems on all naval vessels. Nowadays, modern water mist systems have reached a level of efficiency such that their adoption has also often been considered in the field of rotating machines. Paradoxically, when such systems are used, greater is the chance of fire and greater is the capability of extinguishing it. Heavy-duty gas turbines are designed in order to operate in a very controlled microclimate since they are very sensitive to sudden and/or nonuniform thermal changes of the surrounding atmosphere; therefore, gas turbines are generally contained in pressurized or depressurized enclosures. It is then of primary importance to understand if discharging water mist onto a hot turbine casing will result in thermal shock to the casings’ material. Thermal shock can be the origin of material cracks or warps and potentially cause failure of the casings. In order to determine the potential for thermal shock to turbine casing material, it is necessary to know the cooling rate of the casing’s steel. Also, a second risk introduced by an abrupt cooling is to have an excessive distortion (out of roundness) of the casings and the statoric components and consequent contact with the rotoric parts and engine damage. The main objective (and the novelty) of this investigation is to fully understand the effects of a water-mist fire protection system on the integrity and operation of a heavy-duty gas turbine. The validation process of the water-mist system has been, therefore, rather laborious and articulated in many steps, mainly due to the lack of previous such evaluations. The first part of the investigation was dedicated completely to characterize, through finite element model techniques (FEM analysis), the engine acceptability limits in terms of temperature and thermal exchange coefficients between turbine casings and the enclosure air compartment. Subsequently, different suppliers of water mist systems were contacted and features of each alternative were analyzed and benchmarked. Three suppliers were finally selected and each system was tested not only to estimate its functionality (in agreement with NFPA 750, 2003) but, above all, to estimate the “cooling” effect. 73 EFFECTS OF A WATER-MIST FIRE PROTECTION SYSTEM ON INTEGRITY AND OPERATION OF A HEAVY-DUTY GAS TURBINE