This invention relates generally to turbine engines and, more particularly, to fuel systems for turbine engines.
Dual fuel turbine engines typically utilize a gas and a liquid fuel, along with steam, during engine operation. Premix gas fuel and steam nozzles are configured such that under maximum fuel flow conditions, where product requirements are met, the fuel pressure within the fuel nozzle is within an acceptable range. The fuel pressure within the fuel nozzle sometimes is referred to herein as the fuel nozzle pressure.
Setting the fuel nozzle so that the fuel will have an acceptable pressure during maximum fuel flow conditions can cause engine flame outs at engine start conditions. That is, the flame necessary for fuel combustion dies out due to an unacceptably low fuel nozzle pressure at the fuel nozzle tip during relatively low fuel flow rates.
Dual fuel engines configured with gas and liquid fuels, and steam, customarily exhibit reduced operation performance during normal engine start and low power conditions. Specifically, undesirable flame outs may occur, even under steady state fuel flow conditions, if there is a relatively low pressure ratio at the fuel nozzle tip while operating primarily on gas fuel. Unfortunately, raising the fuel nozzle pressure during engine start conditions can cause operational problems at the maximum fuel flow end where the engine operates most of the time.
Accordingly, it would be desirable to improve the functionality of a dual fuel engine during normal engine start and operating conditions. Additionally, it would be desirable if the improved functionality during engine start did not adversely affect the operation of the engine during maximum fuel flow conditions.
These and other objects may be attained by a fuel delivery system and methods for delivering fuel to a dual fuel turbine engine which is configured to start and operate, at least partially, on gas fuel. The fuel delivery system includes at least one gas fuel circuit configured to deliver gas fuel, at least one liquid fuel circuit configured to deliver liquid fuel, a steam circuit configured to deliver steam, and a dual fuel nozzle. The dual fuel nozzle includes a plurality of fuel inlets, a steam inlet, a fuel and steam premix chamber, and a fuel nozzle tip.
More specifically, the fuel delivery system has at least one connecting line that connects the gas fuel circuit to the liquid fuel circuit. The connecting line is in flow communication with the gas fuel circuit and the liquid fuel circuit. During initial operation of the engine at least a portion of the gas fuel is diverted from the gas fuel circuit to at least one liquid fuel circuit to increase fuel pressure at the fuel nozzle tip during normal engine start and operation conditions.
Once the engine has operated a sufficient length of time, a boiler generates steam that is utilized to raise the fuel nozzle pressure above the combustor flammability deficiency range. The flow of gas fuel is diverted from the liquid fuel circuit to the gas fuel circuit and into the nozzle. The gas fuel purge is cleared from the liquid fuel circuit and liquid fuel is then supplied through the liquid fuel circuit.
The combined use of liquid and gas fuel circuits allows operation of the turbine engine even when steam is not available. The system can operate under steady state fuel flow conditions at relatively low pressure, and under maximum flow conditions, where the engine operates most of the time.