turboexpander 1

[palmtree1]

this is first time with turboexpanders

1. usually when shaft comes out of casing, say centrifugal compressor, first is the sealing arramgement and then the bearings radial and thrust. i thnk i saw a turboexpander illustration showing magnetic bearings first and then the seals why is this so? after the seals there are the regular hyrodynamic bearings are these back ups?
2. do the magnetic bearings also carry thrust loads?
3. there also seems to be a fairly elaborate hydraulic thrust control system would this be because there is no other device in the equipment that would be suited to absorb thrust loading?

thank you very much
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[dcasto]

Expanders use labyrinth seals and a seal gas system that keeps the oil from flowing toward the process.

expanders have huge thrust issues. The expander can have 700 psi differential and the compressor only 50 psi. they typically have similar areas where the gas pressure is acting on.

 

[turb1]

Not all turboexpanders are Magnetic bearing types. Most in operation today are of the hydrodynamic bearing type. Both types have the ability (usually) to control thrust of the rotor. In both types the shaft seals are used to seperate the bearing cavity from the process.

AMB machines have a thrust "disc" of which two axial bearings act upon. The oil bearing type typically have combination radial and thrust bearings, with the thrust bearing usually of the tapered land type.

On an oil bearing machine there is a valve (auto thrust equalizer or auto thrust balancer) that typically connects the inlet of the compressor to a cavity behind the compressor wheel. The compressor wheel has a labyrinth seal on its back which creates a "piston" to thrust the rotor back and forth.

When the valve is closed, the pressure behind the wheel raises to the wheel tip pressure (some value a little lower than discharge pressure). When the valve is open the pressure bleeds to roughly the compressor inlet pressure. As this pressure acts on this "piston" the rotor is thrusted either towards the compressor bearing (valve closed) or towards the expander bearing (valve open).

This valve is usually controlled by a hydraulic actuator. The actuator is connected to two oil pressure ports which come directly from the thrust bearings. One from the compressor bearing to the bottom of the hydraulic actuator and one from the expander bearing to the top of the hydraulic actuator. As the pressure builds on either one of the thrust faces, the valve opens or closes accordingly and balances the thrust on the rotor automatically.

The function of the ATE seems complicated at first, but it is a relatively simple and reliable device.