Reciprocating compressor - rod packing seal leakage 1

[ENG7345]

Hi all,
I am working on a HAZOP scenario regarding the rod packing seal failure of our reciprocating compressors.
We have a few recips is our LNG plant, stabilization unit. The compressors have lubricated rod packing seal. The HAZOP scenario is failure of the lubricator, loss of lubrication, accelerated packing failure to the point of excessive leakage to the distance piece.
API 618 requires a DN 40 vent connected to each distance piece, also requires the vendor to confirm this is sufficient for the seal failure (which we don’t have, the compressors are ~40 years old, probably vendor never asked at the time).
API 521 recommends modelling the clearance between the stuffing box and rod as an orifice plate. This turned out to be extremely high. To vent such a huge amount of gas a 6” vent connection is required on the distance piece. I have never seen such a huge vent connected to a recip. It seems to be overly conservative.

Now the question is that how much leakage should we expect from a failed seal? Is there any diagram to show the packing seal leakage over the time? I understand that API 521 method is correct if the entire packing rings inside the stuffing box is wiped out, but that is very unlikely, it may happen over a very long time that the seal is not inspected or maintained, but nothing that happens over the night.
Is anyone aware of a study on the rod packing seal leakage and failure in different modes?
Thank you

 

[georgeverghese]

Though I dont have answers to your query on quantity of leak gas, it is unsafe to operate these gas compressors without adequate venting of the rod packing leak gas. Talk to the recip compressor and get a quote for replacement / modification of the rod packing assembly to enable remote venting of leak gas. The compressors I have come across all had small bore vent lines ( DN25 or DN50 at most), even for the HP stages.

 

[JJPellin]

I believe that is excessively conservative. I cannot provide you with estimates for gas leakage. Our recip compressors are set up to operate non-lube. We use lubrication to extend the life of the packing, rings and rider bands. But, on a loss of lube, they can run for a very long time. We require that they get the lubricator back into service within four hours or plan to shut the compressor down. We have lost lubricators many times and have never had a packing leak result from these events. Add instruments to alert you on the loss of lubrication. You can take credit for response to critical instruments. You can take credit for field operator response and intervention. I have never seen a line as large as 6 inches for venting the packing leakage and do not believe that is necessary. I know I did not answer your specific questions.

Johnny Pellin

 

[ENG7345]

Thank you Johnny.
I totally agree with you. Actually I am inclined to advise our team against changing the vent size and location and like what you said my experience with the packing seals is that they are very forgiving and reliable.
Can I ask you a bit more about your compressors, what size they are and their intent? Are they methane or NG compressors?

 

[ENG7345]

Hi georgeverghese,

Thank you for your reply. We do have a DN40 vent on each distance piece, but as I said it is not a fit for total seal failure as described in API 521.
API 521 requires to model the leakage with a square edge orifice plate equal to the clearance between the piston rod and packing case. We did it and ended up with a huge leakae rate that requires a 6" pipe, otherwise the distance piece will be overpressured.

 

[georgeverghese]

"model the leakage with a square edge orifice plate equal to the clearance between the piston rod and packing case"

Would imagine no more than an annular gap of say 2-3mm between piston rod and periphery of damaged packing case to get the cross sectional area of the equivalent orifice plate.

 

[ENG7345]

The annular gap is 76mm OD (Packing case hole) and 70mm ID (piston rod). the equivalent orifice plate is 33mm in diameter. in 55 bar (dP) flowrate would be very high.

 

[georgeverghese]

That would be equivalent to completely blown out rod packing? Seems unrealistic for a credible failure scenario to me.
Also, the length of the rod packing along the axis of the piston rod also adds to the resistance to gas flow.

 

[JJPellin]

We have about 40 reciprocating compressors ranging in size from 1000 HP to just under 10000 HP. They are in make-up hydrogen service at pressures up to 2000 psi. We have two, three and four cylinder machines with one, two or three stages of compression.

Johnny Pellin