sub vibration on turbo expander/compressor

[FredMachado]

thread384-197619
Does anybody could help me with this problem? We have this same problem here in Brazil and we didn´t find the main cause. Does anybody found the solution or the cause of this sub vibration on turbo expander? Our sub is 0,22 X rpm and anplitude 0,2 mils. It is causing premature failure on the expander side bearings (tilting pad).

 

[MikeHalloran]

Can you answer the questions that were asked in the referenced thread?


Mike Halloran
Pembroke Pines, FL, USA

 

[FredMachado]

The fluid is natural gas on expander side and residual gas on compressor side. We notice that the sub vibration appeared in 22000 rpm. The nominal speed is 35000 rpm. Inclusive addition to this we ask the unit operator to increase the gas temperature on expander side and the sub vanished but the unit efficience went down and with a little bit of process variation the sub come back. The data Sheet is attached.

 

[turb1]

This type of vibration, especially when it appears and disappears with temp and pressure changes of the expander suction separator, is likely caused by seal and impeller shroud cross coupling. Because a cryogenic expander is two phase at the discharge, and typically two phase at the nozzle exit, swirling liquid forms at the backside of the impeller by the wheel seal. This swirling liquid imparts sub synchronous forces on the rotor which can in turn cause vibration if the bearing stiffness is low enough. A swirl break can be added to stop this from occurring, or bearing changes could increase the stiffness of the bearings to reduce the shaft displacement (vibration). Please feel free to contact me by email at contact@blair-engineering.com.