What action required if N2 supply failure in Dry Gas Secondary seal ?

[PP007]

We have the Cycle gas compressor in Polymer plant with DGS. Recently face problem of N2 supply failed from source. So in such condition N2 failure at secondary seal also. in such case what should be the our action ? Immediately compressor need to stop or we can run the compressor for 1 to 2 hrs for taking the safe plant shutdown. If compressor stop immediately, which lead to chunk formation in reactor which is also not desirable.

 

[JJPellin]

Is the seal a tandem configuration with both seals facing the same direction or a double configuration with the two seals facing opposite directions. For most tandem arrangements, there would still be process gas or buffer gas present to pass between the seal faces. The process gas (potentially dirty) may foul the faces and cause a seal failure. But, it may be reasonable to continue running. For a double seal, there may be no gas flow to pass between the seal faces. This can result in catastrophic seal face failure. We have had this occur on a double dry gas seal on two occasions. We kept running. The rotating seal face fractured and pieces dropped down onto the shaft. When we did shut the machine down, the pieces of Tungsten Carbide had machined through the sleeve and about 1/4" into the shaft. I do not know the hazards of your process. If the seal is leaking to atmosphere and mechanical failure of the seal faces is potentially damaging the shaft, would you want to continue running?

Johnny Pellin

 

[PP007]

Dear Johnny Pellin

Thanks for the reply. We have the tandem seal arrangement , both seal facing same direction & seal leak diverted to flare line. In our case buffer gas ( using fresh ethylene)in primary seal & N2 in secondary seal as well as in barrier seal also. In the failure of N2 , plant continued the machine & there is no problem in seal after start up.

 

[georgeverghese]

Typically, the action is to shutdown and depressure the compressor on secondary seal N2 supply failure. If not, flammable gas from the primary seal area could continously leak to the area just outside the shaft seal and you would have a Zone 1 (or Zone 0 ??) hazardous area fire risk.

 

[Nicolas13]

is the chamber between the seals connected to a flare line with a non return valve? If this chamber may stay pressurized during the N2 source failure, then you can run for some mn. The operatin time after N2 failure depends of the material, design of the seals, the actual worn out, fouling. One hour is too long.

 

[PP007]

Thanks Nicolas,
In secondary Seal vent to directed to atmosphere & there is no check valve in between.

 

[rotw]

I agree with what JJPellin pointed out, in addition you risk bearing oil contamination to DGS. You also risk seal gas migration in secondary vent (with its safety implications).
Typically shutdown is recommended. Depressurization is something to evaluate.
If reactor is endangered by a condition such as no feed forward flow or runaway reaction can occur because compressor is stopped, your process guys should know. But any scenario should have been accounted for upfront, I mean in the validation of the dry gas seal system design before construction and based on some sort of HAZOP assessment.

 

[PP007]

Thanks Rotw

I am totally agree with.