Lost of seal gas supply

[rQuestionEngineering]

How long can a dry gas seal run without seal gas supply before it fails ?
Without dismantling the seal to check, can the damage be assessed by boroscope and static test?

 

[rotw]

w.r.t the 1st question, guess the answer is "it depends".
The question is way too general. At first, there are many DGS configurations (tandem, single, double, etc.).
Also what is meant by failure? is it total absence of seal gas, insufficient supply pressure and/or flow?

As example, for tandem DGS - in case primary seal gas source fails (meaning "no flow" to the seal) process gas (unfiltered) would run through the DGS primary seal, leaking to flare (and possibly vent). This is a highly not recommended scenario. Unfiltered seal gas could damage the primary seal faces whose integrity is very sensitive to presence of particle and/or condensate beyond admissible spec. (e.g. see API 614) as the seal faces are separated with an extremely thin gap. I would say that dirtiness of process gas (clean NG vs. heavy C5+ compounds to give an example), speed of rotation, type of seal geometry (grooves), duration of exposure, among other things, are several factors to affect the magnitude of the damage.

In addition, process gas MW could be quite different from the seal gas one and as such, the lift off forces might be inadequate to ensure proper gap separation of the seal faces (DGS OEM is normally involved and DGS selection is done considering the seal gas composition and operating conditions). In the event of compressor depressurization, while compressor being at stand still, absence of primary seal gas supply may be "tolerated" but subject to conditions and the duration shall be "limited" (consult the compressor/DGS OEM) ; not a recommended procedure generally speaking.

Second question, if seal faces are damaged, how are you going to assess that condition with boroscopic inspection...?
When seal gas faces are damaged, the dry gas seal SHALL be replaced (expect impact on production when done out of "normal" maintenance context, process safety risk depending on the service).
Edit: Dry gas seal cartridges can normally be removed for inspection/replaced, without opening the compressor casing (i.e. part of minor overhaul procedure).

Sorry for the many brackets..