Higher DP than design in R/G centrifugal compressor

[1906]

I'm running VRDS, VACUUM RESIDUE DESULFURIZING UNIT. And, as you guys know well, there is big and important centrifugal recycle gas compressor driven by high pressure steam. However, recently compressor shows higher DP than that of E.O.R condition presumably caused by high recycle gas molecular weight by high cracking effect in catalyst. My question is this, If we are operating with higher comp. DP than design, what parts of the compressor should be the main concerns and ?
You guys' in-depth comments would be appreciated.

 

[JJPellin]

I am a compressor guy, so I cannot speak to all of your concerns. However, running a centrifugal compressor with gas that is heavier than normal at DP's that are higher than normal can have some concerns. First, depending on how your system is controlled, I would be concerned about surging the compressor. At higher DP, the compressor can be forced back on the curve and go in the surge region. The actual operating point of the machine should be compared to the curves and predicted surge line. If the compressor is running closer to surge, it might be necessary to open a spill-back line to run the compressor out to higher flow to get further from the surge region. Secondly, heavier gas will require more power. The increase in power could place you at risk of coupling issues (check the design service factor for the coupling using the new power requirements), motor overload (if it is a motor driver), turbine control problems (if the turbine power is exceeded, the turbine may loose speed control and start to slow down). Slowing down the turbine would be a bigger concern if the machine is running above critical speed for the driver or compressor. If there is any chance that the speed could drop down into a critical speed range, extremely high vibration could result. The last concern I would have is regarding the compressor seal system. Depending on the seal design and control scheme, maintaining seal oil differential pressure (for an oil pressurized seal) could be a problem. This is not likely since the suction pressure to the compressor probably didn't change much. But it is worth checking to make sure your control system is capable of adjusting to the new conditions and still meeting the DP requirements of the seals. This could be an issue with gas seals as well as oil pressurized Iso-Carbon or Iso-Sleeve seals but I would be most concerned about Iso-Carbon seals.