API 617 centrifugal compressor Seal gas pressure 1

[DieguitoI]

I have a centrifugal compressor with tandem dry gas seal.
API 617 requires that maximum sealing pressure shall be at least equal to the settling-out pressure. What are the issues in restarting the compressor from settled out conditions if seal gas source has a pressure lower than settle out pressure? I can only thing about of unfiltered process gas migrating through primary gas seal leading to potential damage of seal mating surfaces due to solid particles.

 

[gerhardl]

Are you asking the probable reason for not getting the full pressure of sealing gas in one of the two sealing chambers, and that you think this could be caused by wear/dirt?

If yes, the answer is then 'yes, likely', but also a multitude of other reasons: Incorrect operation, incorrect compatibility, lacking maintenance, quality of sealing materials, corrosion, abrasion and growth on surfaces, lacking cleaning and drying of gas etc, mechanical faults,etc.

Manual or supplier could normally help.

 

[Rha257]

Having a seal gas supply pressure lower than the settle-out pressure will lead to contamination of the dry gas seal due to the migration of the contaminated process gas from the compressor through the inner process labyrinth since the supplied seal gas in the dry gas seal inboard cavity cannot overcome the pressure of the process gas on the compressor side. You need to ensure a positive differential pressure of 3-5 Bar between the seal gas supply pressure and the settle-out pressure.

Hope this answers your question.

 

[georgeverghese]

You wont be able to restart the compressor if your seal gas pressure is lower than settleout pressure, due to start inhibit from the FSLL on the seal gas supply line at the compressor. You will have to blow down the compressor first to enable seal gas flow to clear the start inhibit from the seal gas supply FSLL.
You may get away with disarming this start inhibit to enable a pressurised restart, but sooner or later, the labyrinth seals in the shaft seal assembly will fail. The more fouling the process gas, the sooner the seals will fail. If the labyrinths at the outboard end are damaged, process gas leakage may be too much for the buffer air - you will then have a higher risk of process gas leak to atm at the outboard end of the shaft seal.
Another common consequence is the dissolution of some of the chemical components in the process gas in to the recirculating lube oil at the nearby bearings. Gradual accumulation of these heavier components leads to a flammable gas atmosphere in the lube oil tank and tank vent exhaust, lube oil contamination and loss of lubrication efficiency at the shaft bearings. In the past, there have been many fires at lube oil tank vents resulting from this. We may concede that the probability of this occuring in your case is less but not zero,since this operating safeguard is compromised only during the limited duration of shutdown-settleout.

 

[Airsporter1st]

Assuming seal gas is derived from discharge, the usual way around this issue is the incorporation of a booster into the seal gas system. Regardless of casing presuure, the booster will add sufficient pressure to ensure a positive flow of conditioned seal gas through the seal chamber laby at start-up, shut down and any other time that there is insufficient deltaP across the compressor. If seal gas is derived from an external source, then consideration could be given to taking an alternative supply from a suitable compressor stage, with a high select provided by check valves.

Lots of 'ifs' and 'buts' of course, but a possible solution.

 

[SB1976]

Starting a Centrifugal Compressor from settled-out condition with Seal gas pressure lower than settle-out pressure is a potentially dangerous operation. Why so? This is because with lower seal gas pressure, there is risk of air ingress into the seal chamber. Not all components of the seals are non-sparking type. Hence, in the event of spark generation, that air in combination with HC gas will form an explosive mixture ready for causing catastrophic damage at elevated temperature.

Hence, such an operation is absolutely not recommended even if all usual start permissives are bypassed.