From the view point of common industry practice, N2 separation gas is typically connected to vent, unless the secondary vent is connected again to flare but this would be then more of an exception (it would anyway subject NDE /DE sides to equal levels in term of back pressure). Unlike instrument air, N2 as separation seal is applied in a rather straight forward manner when it comes to explosive mixture considerations.
- I would look at the following points (which may be connected one another):
1/ health of the secondary seal; failure of this seal would normally be detected through adequate instrumentation. Yet it is good to understand what is the result of the routine inspection of the low point drain on the secondary vent piping? first, has this been done and is there anything that points out to oil migration?
Possibly, a high secondary seal flow would induce extra back pressure which would be seen on the N2 separation line still at levels not high enough to trigger operator via alarm.
2/ mechanical integrity of the carbon ring; it is not specified by OP whether this is a contacting or no contacting type. Close clearance tend to require extra maintenance, have selection guidelines which more restrictive and to some extent are dependent upon dryness of N2.
3/ Control valve dysfunction. High pressure N2 to the seals is typically avoided by installing a relief valve upstream the PCV in order not to exceed the piping class design pressure in case of control valve failure / malfunction. High pressure - here as a cause and not as a consequence - would pose a mechanical risk on operation of the seal.
For more insight and implications, I suggest this reference: