For a gas turbine driving a Generator, we define the load as being the power (in Watts) that the generator is supplying to the grid. I want to know how the load is defined for a gas turbine driving a Compressor (for gas compression facility). Also I want to know how load sharing is performed for the system (Turbine-Compressor). In other words the analogy of Droop & Isochronous in case of Turbine – Generator, which is widely discussed in Control.com site).

Thanks.

 

While it's not very common on older machines, it is possible to measure the amount of torque being transmitted by a prime mover to a driven device. As you mention, for generators it's simple: amps are directly proportional to torque. They are generally operated at a constant speed.

For compressors, they are not necessarily operated at a constant speed. The speed can vary with the load, and the load can expressed in terms of flow-rate and/or pressure. In my experience, there is no analogy for droop and isochronous control.

 

Isoc and droop control modes are only relevent when controlling a generator. The control modes have inputs and outputs for both the turbine AND the generator. In ISOC the machine is being asked to maintain frequency; power output varies based on the connected load. In droop mode the frequency is determined by the grid and power output is the controlled variable.

The only comparison I could make for a compressor application would be if the CT-compressor was to run at constatnt speed and you wanted a flow rate control mode and a discharge pressure control mode. Both would involve controls on both the compressor and CT.

 

Droop control applies only to generator drives. Mechanical drive gas turbines operate on Isochronous control. For mechanical-drive applications, two-shaft turbines are generally prefered to single-shaft units. LP turbine speed is controlled to equal the set point TNR, which is set by the process.

Speed set point can be controlled by one of the following of the process compressor:

* Discharge pressure controller
* Suction pressure controller
* Discharge flow controller.

These controllers can be implemented in a DCS or in the turbine control system itself.

Now on your question of quantifying the performance:

For generator-drives, it is straight forward. For compressor-drives, you can use mass flow rate or volumetric flow rate of gas pumped. Use of polytropic head is used in surge control algorithms.