Gas Turbine Starting Process Explained: Are They Self-Starting?

Gas turbines, despite their impressive efficiency and reliability, are not typically self-starting. Initiating the startup process often requires external assistance. This article delves into the intricacies of the starting process for gas turbines, including the initial power source, fuel ignition, acceleration, and the self-sustaining operation. We will also discuss the differences in starting methods for large and small gas turbines.

Initial Power Source and Fuel Ignition

For gas turbines to commence operations, an external power source is critical. Common methods of starting include the use of an electric starter motor or a gas generator. Once the turbine reaches a certain speed, usually between 10-15% of its operational speed, fuel is injected into the combustion chamber and ignited. This ignition and subsequent acceleration to the operational speed marks the beginning of the startup sequence.

Accelerating to Operational Speed

After ignition, the turbine accelerates to its operating speed. At this speed, the gas turbine can sustain itself and generate power. The turbine then enters a self-sustaining operation phase, relying on the combustion of fuel to maintain its operation. It is during this phase that the turbine becomes fully operational and capable of producing power.

Large Vs. Small Gas Turbines: Starting Methods

The starting method for large gas turbines often involves the use of a hydraulic motor or an electric starter. In contrast, small gas turbines might use a different approach. A typical method for starting small gas turbines is to blow high-pressure natural gas (NG) through a nozzle. This causes the high-pressure turbine to spin rapidly. Once the natural gas is shut off, the turbine continues to spin by inertia but at a lower speed. At this point, the igniters are activated, and fuel is introduced into the combustors. The turbine accelerates, generating enough hot gas to spin the low-pressure turbine coupled to a load.

Dependence on External Power During Startup

It is important to note that, regardless of the size of the gas turbine, an external driver is necessary to rotate the turbine until it achieves sufficient rotational speed. This external driver generates the initial power required to drive the compressor and power the auxiliary systems. If the turbine is used to power a generator, it is designed as a single-shaft system, where the same turbine operates both the compressor and the load.

Conclusion

While gas turbines are efficient and reliable once running, they require assistance to start up. The startup process involves an initial power source, fuel ignition, and acceleration to operational speed. Understanding these steps is crucial for maintaining and optimizing the performance of gas turbines.