Common Anti-surge recycle Across multi-stage CO2 compressor

[Sawsan311]

Dear All,

In view of different designs of CO2 compressors employing one common anti-surge recycle line, I have critically analyzed through ready Compressor control vendors manuals that an ideal design should have independent anti-surge control loop across each stage since each stage has different suction conditions and hence would have different surge characteristics. Although common recycle is cheaper from CAPEX, however, from lifecycle operating cost I believe it is not efficient. Additionally, surge protection of the lower pressure stages would be compromised due to the delay in surge recycle. This would be severed for CO2 compressor with high MW gas (more deflected performance curves including its surge limit line).

Hence, I need your views on:
- for VSD motor driven compressor, do you agree that independent anti-surge recycle across each stage. When do we opt for a common recycle across a 6 stage machine!
- would an integrally geared compressor decision with variable inlet guide vane dictate by default having a common recycle loop because the anti-surge controller should be integrated with the common inlet guide vane as a mean of performance controlling.

Thanks

Regards,

 

[georgeverghese]

With a high mol wt gas like CO2 and a 6 stage compressor, a single antisurge recycle line from stage 6D to 1S may cause low temp on stage 1S during recycle, hence 2 coolers may be required on stage 6D. One for recycle gas, and another for gas that goes further downstream ( after capacity recycle takeoff).
With individual antisurge recycle lines, low temps at lower stages are avoided. So cooling schemes are more complicated with a common AS line.

A common recycle line, on the other hand, would be much more stable, since control interactions amongst many antisurge controllers is avoided.

These are amongst some pros and cons. I would go for the common recycle line if (a) gas composition remains constant (b)cooling scheme remains simple

The IGV only affects stage 1 surge line location, so its position should be fed into the antisurge controller for stage 1 only. The surge line location for the other stages would remain unchanged.

 

[Sawsan311]

Thanks Mr.George, actually with CO2 the cryogenic behavior and hydrate formation becomes extremely problematic with a common anti-surge recycle, this is why designs tend to deploy individual recycle lines. Additionally, the interactions between the stage AS loops can be tackled by loop decoupling features that can be introduced by the compressor controller manufacturer (advanced process control).

 

[georgeverghese]

Agreed, with wet CO2 and the risk of hydrates in recycle lines, cooling schemes can become complicated with fewer recycles. But inherently, the fewer the controllers - recycle valves, the less the risk of control instability. Its better not to rely on loop decoupling - I have heard of instances where operators have had to switch back to simpler controls because loop decoupling doesnt work as intended.