Approximating conditions within a multistage axial compressor and assessing the possibility of hot surface ignition on the heated surfaces

Abstract

Aero-derivative gas turbines are deployed on offshore oil and gas production platforms, among other engine types and locations relevant to the industry. These gas turbines continuously ingest large amounts of air from their surroundings, presenting the possibility that leaking flammable gas may be drawn into the air intakes and the gas turbine ingests the leaking flammable gas. When leaking flammable gas is detected on a platform, gas turbines initiate wind down by rapidly turning off their fuel supply. As the gas turbine winds down, the ingested flammable gas and air arrives at the last stages of the compressor. The mixture of flammable gas and air is now slowing down over and being heated by the hot internal surfaces found at the rear of the compressor and subsequently in the combustor and in the turbine. The present study develops a stage un-stacking method that approximates the conditions within the multistage axial compressor of a gas turbine. Under the conditions approximated above, the present study continues to examine the possibility of hot surface ignition taking place on the heated surfaces within the compressor.

After unstacking the axial compressor of the LM2500 aero-derivative gas turbine, only the last 2 stages attain a mean temperature of about 680 K and 723 K respectively. On the heated surfaces, a 2-step reaction mechanism for methane and oxygen implemented with finite rate chemistry, numerically models the reaction between methane gas and oxygen. The Van’t Hoff ignition criterion indicates ignition on the surface when the local temperature gradient crosses zero. The chemistry model is validated using the Smyth and Bryner [1] experiment and achieves acceptable agreement with an ignition temperature between 1300 K and 1310 K (compared to 1252 K to 1367 K) for low speed flow at about 0.16 m s-1. The temperature difference between the normally operating axial compressor and ignition temperature is at least 577 K. Given forced convection flows within the axial compressor during wind down, the metal surfaces will cool down and the require ignition temperature will rise. The temperature difference to overcome for ignition will widen. The probability that an ignition scenario develops into a hot surface ignition is considered very low.