Recuperating the LM2500 Gas Turbine

Abstract

The LM2500 Gas Turbine is a reliable simple cycle gas turbine that has been in US Navy service for more than 15 years. For a simple cycle gas turbine its design point fuel efficiency is quite good, approximately 0.400 (lb/hp-hr) at 26,250 BHP under US Navy rating conditions. Off design it is not quite as efficient, although its efficiency does not start to degrade significantly until operation below 10,000 BHP is reached. Operational experience with the LM2500 gas turbine in the US Navy has shown that at least 90% of its operational life will be spent at horsepowers that are less than 10,000. Therefore, in an effort to increase the range and the ability to remain on station of LM2500 powered US Navy ships, methods to improve the LM2500’s low power fuel efficiency were investigated. One area that had been discounted in the past was recuperating the LM2500. On the surface recuperating the LM2500 does not appear to provide much. The primary reason for this is that the design point compressor discharge temperature is within 200 degrees F of the gas turbine exhaust temperature. But off-design the situation changes, particularly if variable area turbine nozzles (VATNs) are introduced to maintain cycle temperatures. This paper will discuss the initial concept design process that was performed by the gas turbine division in the Naval Sea Systems Command (NAVSEA), the activity that is responsible for ship design in the US Navy. This will include the initial assumptions and the gas turbine cycle modeling that was undertaken to determine the potential benefits of recuperating the LM2500. Based on the success of these preliminary efforts, General Electric Company was then tasked to help perform additional cycle analysis. GE, working together with NAVSEA, proceeded to determine the optimum configuration of a Recuperated LM2500 (or LM2500-R). The guiding philosophy behind this effort is “maximum gain with minimal change.” The direction of this effort was to provide a cost effective, retrofitable package, in a reasonable amount of time, to upgrade the LM2500 for improved low power fuel efficiency.