Turo-expander Control

Turbo-expanders are widely used by the natural gas processing industry to recover heavier hydrocarbons from natural gas streams or natural gas liquids (NGLs). Thanks to high efficiency and reliability, turbo-expanders are the workhorses of many cryogenic industries.

Turbo-expander in conjunction with the J-T valve

The typical turbo-expander train is shown in the figure below. A J-T (Joule Thompson) valve is used to bypass the flow around the turbine wheel. The J-T valve is used when the turbo-expander is offline or unable to meet the flow demands. Turbo-expanders are Usually controlled by manipulating the IGV position while maintaining the upstream and the downstream pressures constant.

The typical control schemes for turboexpander trains are based on a split range control between the expander guide vanes and the J-T valve with an addition of a low signal selector to allow for independent closure of the expander guide vanes.

The speed of the train is governed by the torque developed by the turbine wheel. The greater the torque delivered by the expander compared to that of that absorbed by the compressor, the greater the rotating speed. Control systems limit the speed by limiting the opening of the IGV.

For the compressor, increasing the flow rate leads to increasing the power needed to drive the compressor. Since opening the recycle valve increases the flow rate through the compressor, the recycle valve can be used to vary the rotational speed of the turbo-expander train. Therefore, the speed of the train can be first limited by opening the compressor’s recycle valve. This will result in an increase in torque delivered by the compressor while the torque from the expander remains the same and in turn will slow down the turboexpander train.

Controlling the speed using the compressor recycle valve allows for limiting the train speed while keeping the turbine IGV in a more open position. This results in an increase in condensate production, due to the fact that the gas expansion in the expander is an isentropic process whereas the expansion across the J-T valve is an isenthalpic process.

Turbo-expanders are not usually a speed controlled machines. Actually, the speed is determined by the power balance between the turbine wheel and the compressor wheel. So, the turbo-expander can operate on a wide range of speeds. Depending on the process conditions i.e. flow rates, pressure ratios across the turbine and compressor wheels, turbine inlet temperature, gas composition... etc. the speed of the turbo-expander is set.

Compressor surge control

The turbo-expander compressor, like any other dynamic type compressr must be protected against surge. Compressor operation in the surging area can lead to catastrophic events i.e. flow reversals, speed fluctuations and thrust bearing damage. The compressor is protected against surge by means of a bypass valve that recycle the gas from the discharge side to the suction side to increase the flow and decrease the pressure difference. The aim of the anti-surge controller is to keep the compressor operating point within a safe margin from the surge line.

References

1- Control for Cryogenic Turboexpanders, compressor control corporation whitepaper.

2- Fundamentals of turboexpander design and operation, Gas Processing News.