Anti surge Valve for Capacity control is acceptable 1

[Mech199]

Hello,

Is it recommended to use ASV valve to regulate compressor throughput in addition to Anti-surge service? Please advise.

Compressor train Configuration as follows:

- Two stage centrifugal compressor train (LP+ MP) working in parallel (2X50%)configuration.
- Driven by electric motor + (VFD/Fluid Coupling)

 

[georgeverghese]

This description is too brief, can you elaborate further on system configuration.

 

[Mech199]

Ok Let me share further details. If you are looking for specific data Pls inform.
Application: Sour gas export compressor
Two Compressor Trains working in parallel (2x50%) each train parameters as follows:
LP+MP Configuration
Capacity: 77mmscf/d
Turndown capability to 45mmscf/d
Pressure Suction/ final discharger: ~1.6 barg / ~42barg
1st stage Suction Temp.: ~59°C
MW: 29.7
Driver: Motor with VFD
I don’t have any Compressor selection in hand and no Curves. I have following questions:
1). with using VFD 40% turndown achievable?
2). Can ASV be regulated to maintain Compressor throughput. Do we foresee any complexity in compressor control? Is it safe to use? Can machine safety still be ensured without any compromise?

 

[georgeverghese]

Okay, this is clearer now.
1)We must distinguish between turndown within the compressor train and external turndown. External turndown can be down to 0% by combined action of VFD and the capacity recycle valve. Internal turndown is influenced by the performance curves on these 2 stages. If you can derive a combined performance curve for these 2 stages, you can find out what min speed ( at close to the surge line) is required to achieve the overall dp of 40.4bar. Someone has claimed that 40% turndown on flow is possible - you can check from this combined performance curve.
2)Since you have 2 parallel trains, each train will most likely have an antisurge valve for protection of each stage (so 2 antisurge valves for each train). There are 2 possibilities for capacity control
Option(a) a conventional approach where these antisurge valves are dedicated to antisurge function, and an overall (master) capacity recycle valve is installed to recycle gas from the combined discharge of the 2 compressors back to the common LP feed of the 2 compressors
Option(b) either the LP or the MP stage antisurge recycle valve on both the trains acts as the overall capacity recycle valves. In this case the master capacity controller goes through a signal selector( which chooses either of the signals from the master capacity controller or the stage antisurge controller) to act on the stage recycle valve.
At the moment, in theory, it seems to me that either of these schemes will work. But there may be concerns with low temperatures and / or hydrates in the LP side of the recycle line in option (a) which may be worse when stage 2 cooler exit gas is colder than normal( which may be due to a combination of turndown and winter time coolant temps). On the other hand, option (a) may be simpler and more stable / more responsive from a process control perspective. Detailed simulations should be carried out for each of these options and engineering solutions to problems found should be worked out for each problem encountered. Finally, weigh up these 2 options, developed in detail to the extent necessary for concept selection, and see which way to go - a team of process, rotating and process controls engineers, together with compressor vendor should ideally be part of this evaluation exercise.

 

[georgeverghese]

Just realised this overall compression ratio of 16.6 can only be achieved with 3 compression stages over each LP - MP compression train, so we are probably looking at 3 antisurge valves per train, not 2. Either the LP or the MP section has 2 stages within a single barrel??

 

[rotw]

- I doubt your configuration is 2 x 50%. If you have LP and MP, the train should be configured to switch from serial to parallel (often the case) and even if not, the likelihood is that the flow breakdown typically 70% (LP) and 30% (MP) or close. Feasibility depends on how your operating points are spread. If your points can be split in two groups combination as [low head, highflow] and [high head, low flow] it probably can be done, provided:
1/ The serial and parallel envelops are preferably contiguous (if applicable)
2/ A set of changeover valves is included together with capacity control and VFD control
- The relatively high MW makes this scenario more stringent to achieve.

I have developed a software to make such kind of simulation.
You need to provide more details (complete datasheet) and I can check it out for you.

 

[georgeverghese]

Yet another set of options would be, for each train (3) independant capacity recycle valve for LP casing or (4) independant capacity recycle valve for MP casing. Am assuming you wouldn't have low temp issues/ hydrate issues during recycle for either of these options.