- what is high, pressure ?
- you did not specify tertiary seal? labyrinth or other type (close clearance)? for instance this determines whether the supply is controlled via flow control or pressure control.

nitrogen supply pressure into tertiary seal (carbon ring type) .
supply was controlled by PCV into drive end and non drive end.
One side of the seal is experiencing high pressure.

There is usually a restriction orifice ( or some other fixed throttling device) that maintains a backpressure on the N2 side of this DGS.

Either this RO is not sized correctly for this N2 flowrate, or there is high backpressure from the LP flare header ( if this gas is vented to LP flare), or there may be a blockage on some flame arrestor in the N2 vent line discharge( if this gas is vented locally) ??

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:
http://turbolab.tamu.edu/proc/turboproc/T31/t31pg1...