Dry Gas Seal Failures

One of our centrifugal compressor 2 BCL 306/A of make GE houses the 3rd and 4th compression stages and is being used at our Urea Plant for feeding CO2 gas into the Urea Reactor at 160 kg/cm2. Compressor is equipped with John Crane make Dry Gas Seals of type 28AT at its drive and non-drive ends. In the last two and a half months, we have experienced twice the failure of non-drive end dry gas seal of aforesaid compressor; details are narrated below:

 

  1. The compressor was overhauled in September 2016 and both drive end and non-drive end seal cartridges were replaced. Since then the compressor had been in service No anomaly had been observed in any of the dry gas seals, i.e., no rise in the seal vent pressures till February 2018.

 

  1. In February 2018, during annual turnaround of our Urea plant, only bearings of the compressor were replaced and seal cartridges were not removed.  After turnaround, when machine was taken in service, we started experiencing the rise in the vent pressures of only non-drive end dry gas seal; it went up to 7 kg/cm2 against the normal value of 0.2 kg/cm2.

 

  1. Moreover, a sharp rise in the replacement frequency of dry gas seal filters was observed; every time when we have dismantled the filter housing for replacing the filter, we have found the cotton fiber filters (5 micron size) covered with brownish dust. Lab analysis revealed the presence of Iron in the process CO2 Pertinent to mention that as the pressure difference of 0.5 kg/cm2 has been maintained between filtered and non-filtered gas all the time of operation, it is therefore confirmed that Iron is passing through the filters

 

  1. On April 04, 2018 availing the opportunity of Urea plant outage, the compressor was off lined and both seals were replaced. Upon dismantling the cartridge of non-drive end seal, all internals were found covered with layer of iron rust. Seat and balanced face was in good condition; only balanced face O-ring and thrust ring were found damaged / dislodged.

 

  1. The cartridge of drive end seal was found in clean condition and no Iron dust was found on its internals.

 

  1. After the maintenance, vent pressure of drive end dry gas seal dropped to normal values of 0.15 to 0.20 kg/cm2. However, after operation of 5 to 6 weeks, again the vent pressure of only non-drive seal started creeping up and as of to date the vent pressure is reached up to 3 kg/cm2.

 

  1. Can anyone shed light on this phenomenon. Questions that come to mind:
    1. Is the fine Iron dust responsible for repeated failure of non-drive end dry gas seal; if so how Iron is impairing the functionality of dry gas seal.
    2. Why Iron dust is only going into the non-drive end dry gas seal and not in the drive end seal. Be noted that d/s of filter the same line splits into two lines leading to each seal Picture IPicture II[1)Picture II[2)Picture III[1)Picture III[2)Picture III[3)Picture IV[1)Picture IV[2).
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