Guide Vane in Turbo-expander

A little of both, but mostly a controller.

They're both control and energy-saving devices.

For control purposes, they modulate flow.

For energy-saving, they guide flow (hence their name) so that it enters turboexpanders in a swirl rather than head-on.

Inlet guide vanes serve both purposes in centrifugal compressors, too (including gas compressors).

Cheers! DZ

The IGV in a turboexpander is not a safety device. In fact, they are not tight-closing, so some gas will always leak by them.

Typically, they are controlled by an actuator, similar (if not the same) as what would be used on a control valve. They can be configured to control pressure or flow and there are variations of these (upstream, downstream, recompressor discharge, etc) depending on the exact process that the turboexpander is being used for.

In most applications, the IGV are used a a backpressure regulator to control the upstream pressure to the plant (in conjunction with a JT valve).

The only turboexpanders that would have a speed governor are those that are driving generators, which are not common in cryogenic gas processing plants.

In an oil-bearing turboexpander, the overspeed protection is typically programmed in a dynalco speed monitor (alarm high, trip high high).

Turboexpanders are different from other rotating equipment in that they are process driven. Other equipment (for example a compressor) drives the process. A turboexpander operates as a function of the process that it is operated in.

Typically for an expander to overspeed, the process would have to change from the original design conditions. For example if the expander is in a seperation process with a subcooler (where the flow is split between the expander and a subcooler at the inlet of the expander) and the "split" changes from the original design such that more flow goes to the expander and less flow goes to the subcooler. With the same total plant inlet flow and composition, the tower overheads (which is the typical expander-recompressor stream) will be roughly the same. Therefore the ratio of flow between the expander and recompressor will be different from the original design (more flow on the expander for the same recompressor flow) and the machine would overspeed. In this scenario, the fix would be to re-wheel the recompressor side of the turboexpander to increase the head (and diameter) of the recompressor wheel.

It is my opinion that pressure control for turboexpanders is better. The reason is that the expander tends to be more steady in pressure control (less speed fluctuation, etc).

Exactly!