Seal gas flow back to the compressor

[lahori]

Hi,
I am working on a Dry Gas Seal Project for Centrifugal compressor. I have a book on Compressor Dry Gas Seal In which following is written

"It is also desirable to minimize seal gas consumption. The majority of
the injected seal gas flows across the inner labyrinth seal and back into the
compressor (i.e., the flow is recycled), and very little flow is actually required
for the gas seal. This recycled flow into the compressor is inefficient and uses
more energy at a cost to the user"

Now here he is referring to after passing through inner labyrinth seal and back into the compressor (i.e, the flow is recycled). I cannot understand this one. In my understanding once seal gas is injected it flows through primary seal( where it ac as working fluid for mating and stationary ring) and then goes to primary vent.

I am still very new to Dry Gas Seal System so may be I still do not have full grasp of the topic. Please help me out here.
Regards
Yasir

 

[rotw]

"In my understanding once seal gas is injected it flows through primary seal( where it ac as working fluid for mating and stationary ring) and then goes to primary vent."

Part (X) flows through the primary seal and goes to primary vent. Another part (Y) flows through the process labyrinth towards the compressor suction (if it is suction side) or through the back of the balance piston if it is discharge side and goes back to suction via recycle line. So the impeller will "see" the total of leakages through both process labyrinths (2*Y) (drive end and non-drive end); this flow adds up to main process flow and counts towards duty (absorbed gas power) of compression. At final discharge this flow (2*Y) is not recirculated by some means so it will leak to the downstream process (discharge line). Typically this flow counts for ~97% of the total seal flow, so what is going to the vent is very minor when compared to what goes into the compressor. And even the total seal flow (vent + process labyrinth) is generally very little amount - relatively speaking - when compared to the main flow of the compressor. As far as I know it is often neglected for compressor power calculations, while as comparison - the flow through balance drum labyrinth for instance is usually quite tangible.
The above is what would happen for a straight-through impeller arrangement casing, but the principle works quite the same for back to back arrangement too.

 

[JJPellin]

What is the configuration of the seal? It will be quite a bit different for a tandem arrangement versus a double arrangement. What is the source of the seal gas? If may be a very valuable product such as clean make-up hydrogen. It may be a stream of compressor discharge gas which has been cleaned up in a gas conditioning system before injection into the seal. I would normally not be concerned about the energy aspect of this gas usage. I would be more concerned about other things: What is the value of the gas I am using? If the system is downgrading very valuable, clean, pure gas into a lower valued stream, this can be very, very expensive. Is this gas flow backing out machine capacity? If my process unit is limited by the capacity of the compressor, adding any gas into the compressor could be backing out machine capacity that could be used to increase the unit capacity. Does the compressor also have a buffer gas on the process side of the seals? What is the source of that buffer gas? The flow of buffer gas is probably much higher than the flow of seal gas.

Johnny Pellin

 

[mighoser]

Using abradable polymer seals will lower consumption. Fluorosint 500 or Duratron PAI are commonly used.

________________________________________
Mitsubishi Chemical Advanced Materials

http://www.mcam.com

 

[rotw]

To my knowledge abradable seals (such as PEEK) are used for the labyrinths that are on the front and back side of impellers, to minimize recirculation leakages thereby gaining an edge in terms of efficiency. Their use is subject to particular engineering assessment. I am not aware of their application as material for dry gas seals (process labyrinths). And it is custom for "conservative" vendors to choose between aluminium and stainless steel as the main options.

The choise of material for seals depends on the conditions of operations, gas composition, etc.
On high temperature applications, for example, even the use of Aluminium becomes not recommanded.

 

[georgeverghese]

Seal gas in dynamic gas seals is ideally some preconditioned dry gas and its flowrate is miniscule in comparison to the compressor handling rate. Check that the seal gas pressure is adequate to enable forward flow even at max normal compressor settleout pressure.